GEOLOGICAL SURVEY CIRCULAR 800

Geological Studies of the COST GE-1 Well, United States South Atlantic Outer Continental Shelf Area Geological Studies of the COST GE-1 Well, United States South Atlantic Outer Continental Shelf Area

By Peter A. Scholle, Editor

GEOLOGICAL SURVEY CIRCULAR 800

1979 United States Department of the Interior CECIL D. ANDRUS, Secretary

Geological Survey H. William Menard, Director

Use of brand names in this report is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202 CONTENTS

Page Abstract 1 Introduction 1 Acknowledgments 3 Geologic setting, by William P. Dillon, Kirn D. Klitgord, and Charles K. Paull 4 Regional stratigraphy and structure of the Southeast Georgia Embayment, by Page C. Valentine 7 Data summary and petroleum potential, by Peter A. Scholle 18 Lithologic descriptions, by E. C. Rhodehamel 24 Petrophysical summary, by E. K. Simonis 37 Petrographic summary, by Robert B. Halley ,- 42 Foraminiferal biostratigraphy, paleoecology, and sediment accumulation rates, by C. Wylie Poag and Raymond E. Hall 49 Calcareous nannofossil biostratigraphy and paleoenvironmental interpretation, by Page C. Valentine 64 Radiometric age determinations, by E. K. Simonis 71 Geothermal gradients, by E. I. Robbins 72 Organic geochemistry, by R. E. Miller, D. M. Schultz, G. E. Claypool, M. A. Smith, H. E. Lerch, D. Ligon, C. Gary, and D. K. Owings 74 Geophysical studies, by R. C. Anderson and D. J. Taylor 93 Structure of the Continental Margin near the COST No. GE-1 drill site from a common-depth- point seismic-reflection profile, by William P. Dillon, Charles K. Paull, Alfred G. Dahl, and William C. Patterson 97 Conclusions 108 References cited 109

ILLUSTRATIONS

Page Figure 1, Locality map 2 2. Regional geologic and bathymetric features in relation to the COST No. GE-1 well, other wells, and USGS seismic line TD-5 5 3- Location of wells, cross-sections lines, and structural features of the Southeast Georgia Embayment ° 4. Section XX' through offshore Southeast Georgia Embayment 9 5. Section YX' through Southeast Georgia Embayment 12 6. Sections YY" and ZZ' through Southeast Georgia Embaymentr 14 7. Generalized lithologies and depositional environments for COST No. GE-1 well 19 8. Porosity and permeability data as a function of depth 21 9. Comparison of various measures of thermal maturity as a function of depth 22 10. Lithologic log 25 11. Porosities of conventional and sidewall cores 39 12. Permeabilities of conventional and sidewall cores 40 13. Core porosities as related to core permeabilities 41 14- Photomicrograph of chalk 45 15. Photomicrograph of chalk 45 16. Photomicrograph of calcareous shale 45 17. Porosity depth relations of chalks in COST No. GE-1 well compared with other chalks 46 Page

18-25. Photomicrographs: 18. Oolite 47 19. Oolite 47 20. Moldic porosity in blocky calcite cement 47 21. Oomoldic porosity 47 22. Fine-grained arkose 48 23. Medium-grained arkose 48 24. Quartz ite 48 25. Volcanic rock (trachyte) 48 26. Biostratigraphic column for Albian through Pleistocene rocks 50 27. Paleoecological dip section of Cenozoic rocks, Southeast Georgia Embayment 52 28. Paleobathymetry, accumulation rates, and subsidence rates of rocks in the COST No. GE-1 well 58 29. Sediment accumulation rate curve for COST No. GE-1 well 59 30. Comparison of Cenozoic sediment accumulation rates at COST No. GE-1 well and nearby core holes 60 31. Diagrammatic examples of subsidence rate calculations 61 32. Temperature data for the COST No. GE-1 well 73 33. Gas-chromatographic analyses of saturated hydrocarbons of Tertiary rocks 79 34. Gas-chromatographic analysis of saturated hydrocarbons of Tertiary and Upper Cretaceous rocks 80 35. Gas-chromatographic analyses of saturated hydrocarbons of Upper and Lower Cretaceous rocks 82 36. Summary and comparison of organic-richness analyses of samples from the COST No. GE-1 well 84 37- Gas-chromatographic analyses of mud drilling additives, COST No. GE-1 well 85 38. Gas-chromatographic analyses of unfractionated sediment samples, COST No. GE-1 well 86 39. Summary profiles of types of organic matter present in the COST No. GE-1 well 88 40. Maturity profiles of organic matter from the COST No. GE-1 well 89 41. Summary of light-hydrocarbon analyses from the COST No. GE-1 well 90 42. One-dimensional synthetic seismograms derived from the COST No. GE-1 well transit- t ime curve 94 43. Two-way vertical travel time as a function of depth in the COST No. GE-1 well 96 44. Interval velocity, average velocity, and root-mean-square velocity as a function of vertical two-way travel time, COST No. GE-1 well 96 45. Line drawing interpretation and depth section of USGS seismic profile TD5 across Southeast Georgia Embayment 98 46-49. Photographs: 46. Section of seismic profile through COST No. GE-1 well drillsite to upper Florida-Hatteras Slope 100 47. Section of seismic profile across lower Florida-Hatteras Slope and inner Blake Plateau 101 48. Section of seismic profile showing outer Blake Plateau and inner Blake Spur 103 49. Section of seismic profile showing outer Blake Spur and western Blake Basin 104

TABLES

Table 1. Comparison of biostratigraphic analyses from the COST No. GE-1 well 20 2. Porosity and permeability values from conventional cores of the COST No. GE-1 well- 37 3. Porosity and permeability values from sidewall cores of the COST No. GE-1 well 38 4. Petrography of carbonate rocks from the COST No. GE-1 well 43 5. Petrography of clastic terrigenous, metasedimentary, metamorphic, and metamorphosed volcanic rocks from the COST No. GE-1 well 44 6. Calcareous nannofossil biostratigraphy of the COST No. GE-1 well 65 7. Rb-Sr whole-rock analyses from the COST No. GE-1 well 71 8. Circum-Atlantic offshore geothermal gradients 72 9. Lithologic descriptions of samples used for organic geochemical analyses 76 10. Organic-carbon and thermal-evolution analyses for the COST No. GE-1 well 77 11. Organic carbon and extractable organic matter, COST No. GE-1 well 78 12. Velocity data from the COST No. GE-1 well- 95 Geological Studies of the COST No. GE-1 Well, United States South-Atlantic Outer Continental Shelf area

By Peter A. Scholle, Editor

ABSTRACT hydrocarbons, is thermally immature, and has a very poor source-rock potential. The interval The COST No. GE-1 well is the first deep from 3,600 to 5,900 ft (1,100 to 1,800 m) has a stratigraphic test to be drilled in the southern high content of marine organic matter but part of the U.S. Atlantic Outer Continental appears to be thermally immature. Where buried Shelf (AOCS) area. The well was drilled within more deeply, this interval may have significant the Southeast Georgia Embayment to a total depth potential as an oil source. The section from of 13,254 ft (4,040 m). It penetrated a section 5,900 to 8,850 ft (1,800 to 2,700 m) has composed largely of chalky limestones to a depth geochemical characteristics indicative of a poor of about 3,300 ft (1,000 m) below the drill oil source rock and is thermally immature. platform. Limestones and calcareous shales with Rocks below this depth, although they may be some dolomite predominate between 3,300 and marginally to fully mature, are virtually barren 7,200 ft (1,000 and 2,200 m), whereas of organic matter and thus have little or no interbedded sandstones and shales are dominant source-rock potential. Therefore, despite the from 7,200 to 11,000 ft (2,200 to 3,350 m). thermal immaturity of the overall section, the From 11,000 ft (3,350 m) to the bottom, the upper half of the sedimentary section penetrated section consists of highly indurated to weakly in the well shows the greatest petroleum source metamorphosed pelitic sedimentary rocks and potential. meta-igneous flows or intrusives. Biostrati- graphic examination has shown that the section INTRODUCTION down to approximately 3,500 ft (1,060 m) is Tertiary, the interval from 3,500 to 5,900 ft Until the completion of the COST No. GE-1 (1,060 to 1,800 m) is Upper Cretaceous, and the well, all information on the stratigraphic section from 5,900 to 11,000 ft (1,800 to 3,350 framework of the Southeast Georgia Embayment had m) is apparently Lower Cretaceous. The come from onshore wells, offshore bottom indurated to weakly metamorphosed section below sampling or shallow coring, geophysical surveys, 11,000 ft (3,350 m) is barren of fauna or flora or extrapolation from the COST No. B-2 well but is presumed to be Paleozoic based on (Scholle, 1977b) located more than 750 mi (1,200 radiometric age determinations. Rocks deposited km) north of the GE-1 drill site. Between at upper-slope water depths were encountered in February 22 and May 31, 1977, however, the first the Upper Cretaceous, Oligocene, and Miocene deep stratigraphic test was drilled in the parts of the section. All other units were southern portion of the U.S. Atlantic Outer deposited in outer-shelf to terrestrial environ­ Continental Shelf (AOCS) by the Ocean Production ments . Company acting as operator for a group of 25 Examination of cores, well cuttings, and original participating companies, the Conti­ electric logs shows that potential hydrocarbon- nental Offshore Stratigraphic Test (COST) reservoir units are present within the chalks in Group. This hole, designated the COST No. GE-1 the uppermost part of the section as well as in well, was drilled on the outer part of the sandstone beds to a depth of at least 10,000 ft Continental Shelf about 74 mi (119 km) east of (3,000 m). Sandstones below that depth, and the Jacksonville, Fla., at lat 30° 37' 07.6892" N. metamorphic section between 11,000 and 13,250 ft long 80° 17' 59.1451" W. (fig. 1). The well was (3,350 and 4,040 m), have extremely low drilled using the ODECO jack-up drill barge, the permeabilities and are unlikely to contain "Ocean Star." Water depth at the site is 136 ft potential reservoir rock. (41.5 m), and drilling continued to a depth of Studies of organic geochemistry, vitrinite 13,254 ft (4,040 m) below the Kelly Bushing (KB) reflectance, and color alteration of visible or to 13,175 ft (4,016 m) below sea level. The organic matter indicate that the chalk section well is located off-structure but directly down to approximately 3,600 ft (1,100 m) adjacent to the area offered for leasing on contains low concentrations of indigenous March 28, 1978, as part of Lease Sale No. 43

1 28°N 30°N 32°N 34°N 36°N 38°N

86°W

26°N

84°W

FEDERAL LEASE BLOCKS (SALE No. 43)

WATER DEPTH IN METERS

2000 m

OUTLINE OF SOUTHEAST GEORGIA EMBAYMENT

76°W 74°W 72°W 70°W 68°W

Figure 1. Locality map showing site of COST No. GE-1 well in relation to the Southeast Georgia Embayment, Lease Sale No. 43 blocks, and coastal cities.

(fig. 1). The granting of Federal leases within and geophysical topics. Environmental or 50 mi (80 km) of the GE-1 drill site on May 1, operational details are not included as these 1978, has made possible the publication of have been amply described by Amato and Bebout detailed information about the geological (1978). A brief synthesis of all available data findings from the COST No. GE-1 well. Leasing is presented in this paper in the section "Data stipulations provide for public disclosure by Summary and Petroleum Potential." Much of the the U.S. Geological Survey (USGS) of all basic data will also be available in chart form geological information of the well 60 days after (Scholle and others, in press). such leasing. All work reported in this paper has been Considerable basic lithologic, strati- done using rotary-drill cuttings rather than graphic, and geochemical data, most of it conventional or sidewall cores. Electric logs derived from industry reports, have been and core descriptions were available, however, released previously (Amato and Bebout, 1978). to aid in checking the accuracy of sample-depth This Circular summarizes some of that data and assignments. The cuttings, in conjunction with adds information from other studies conducted by electric logs, thus provided a useful picture of the USGS. It is by no means an exhaustive or the complete spectrum of lithologic variation completed program, but because the COST No. GE-1 within a given interval and were commonly more well has such immediate interest for both the useful than the spot samples provided by petroleum industry and academic workers, we sidewall coring. All depth references in this believe that publication of such preliminary report for electric-log, core, or cuttings data results is justified. are based on depth below the KB, which is 98 ft The publication is divided into separately (30 m) above mean sea level and 234 ft (71 m) authored sections on geological, geochemical, above the sediment-water interface. ACKNOWLEDGMENTS Klitgord, John Grow, and John Schlee were very valuable in the completion of the geophysical Technical assistance in sample preparation sections. Parts or all of the manuscript was was provided by Patricia Brady, W. D. Marks, and reviewed at various stages of preparation by L. Frederick Zihlman. Other technical help came M. Bybell, R. A. Christopher, 0. W. Girard, Jr., from Helen Colburn, Patricia Forrestel, J. L. R. E. Mattick, Peter Popenoe, and C. C. Smith. Hennessy, Kirn Schwab, and Linda Sylvester. We are indebted to them for their many helpful Discussions with Page Valentine, Wylie Poag, Kirn suggestions. GEOLOGIC SETTING

William P. Dillon, Kirn D. Klitgord, and Charles K. Paull

The COST No. GE-1 drill site is located on age (Applin, 1951; Rainwater, 1971). However, the Outer Continental Shelf, east of much of the onshore part of the Southeast Jacksonville, Fla. (fig. 2). The Continental Georgia Embayment apparently is underlain by Margin off the Southeastern United States does undeformed tuffaceous clastic deposits not consist of a simple shelf, slope, and rise intermixed with basaltic and rhyolitic flows as it does farther north. Instead, the abyssal (Milton and Hurst, 1965; Milton, 1972; Barnett, depths are separated from the shelf by a wide 1975; Popenoe and Zietz, 1977; Gohn and others, intermediate surface, the Blake Plateau (fig. 1978; T. M. Chowns, West Georgia College, 2), The gently sloping transitional surface written cotnmun., 1976). Some of these volcanic between shelf and plateau is called the Florida- rocks are associated with continental terrige­ Hatteras Slope. Seaward of the Blake Plateau, nous deposits, presumably of Triassic age, which south of lat 30 N., the ocean floor drops may have been deposited in grabens or half abruptly to form the Blake Escarpment, whereas, grabens produced during the early rifting stage to the north, the transition to abyssal depths of the opening of the Atlantic Ocean. is less abrupt. Rocks of Jurassic age do not crop out in The main zones of basement subsidence (fig. the Southeastern United States. Upper Jurassic 2) are the Blake Plateau Basin, Carolina Trough, or Lower Cretaceous deposits may have been and Southeast Georgia Embayment. The Blake encountered at depth below Cape Hatteras (ESSO- Plateau Basin and Carolina Trough (fig. 2) are Hatteras Light 1 well) and consist of finely regions of major subsidence, amounting to more crystalline, partly oolitic limestone and gray than 40,000 ft (12 km), and are believed to be shale beds that grade downward into conglomerate floored by transitional basement (Klitgord and and sandstone of possible continental origin Behrendt, 1979). Transitional basement, as (Spangler, 1950; Maher, 1971; Brown and others, defined here, indicates a modified oceanic 1972). The Lower Cretaceous-Jurassic section basement, formed of mantle-derived intrusive and drilled in Florida and the Bahamas consists of extrusive mafic rocks mixed with considerable shallow-marine, organic-rich carbonate rocks quantities of sediments and fragments of and, in the Bahamas (for example, in the Chevron continental basement. We estimate that the COST Great Isaac 1 well), has increasing amounts of No. GE-1 well was drilled on continental evaporite deposits at depth (Tator and Hatfield, basement, but close to the western boundary of 1975; Jacobs, 1977). An onlapping wedge of transitional basement underlying the Blake sedimentary rocks, inferred from regional Plateau Basin. stratigraphy and geophysical studies to be of The COST No. GE-1 well was drilled above a Jurassic age, thickens rapidly seaward and in basement high at the seaward margin of the the Blake Plateau Basin and Carolina Trough, its Southeast Georgia Embayment, a gently subsided, thicknesses reaches 20,000-23,000 ft (7-8 km). eastward-plunging depression recessed into the These inferred Jurassic deposits, however, continent between two platform areas, the probably do not extend landward into the Carolina Platform on the north (Klitgord and Southeast Georgia Embayment nor even to the COST Behrendt, 1979) and the Florida Platform on the No. GE-1 drill site (Dillon and others, 1979). south (Eardley, 1951); the embayment is probably The Cretaceous and Cenozoic sedimentary floored by continental basement. The Paleozoic rocks of the southeastern U.S. Continental and Precambrian basement of the platforms Margin occur in a transitional zone between a exposed in the Piedmont is formed of igneous and predominantly clastic depositional province metamorphic rocks (Overstreet and Bell, 1965), north of Cape Hatteras and a carbonate province whereas pre-Cretaceous "basement" beneath the to the south that includes Florida and the Coastal Plain of Georgia and northern Florida is Bahamas. More than 400 wells have been drilled sandstone and shale of Ordovician to Devonian in Georgia and Florida, some of which reached 81C 80°

Seismic Profile TD5 © Drillsite

Zones of basement subsidence

_____ Bathymetric contour in meters

SOUTHEAST GEORGIA dwj M f^. /.m ft *m I Mtf^m m

PLMFORt

28° -

27° -

26° -

Figure 2. Regional geologic and bathymetric features in relation to the COST No. GE-1 well, other wells, and DSGS seismic line TD-5. basement rocks. The stratigraphy and structure, drilled off the Suwannee Saddle, a depression in therefore, are fairly well known for the onshore the Cretaceous top, where Maestrichtian deposits area (regional stratigraphy and structure are are thinned or absent (Toulmin, 1955; Chen, discussed by Valentine, this volume). On the 1965; Applin and Applin, 1967). other hand, only a few wells have been drilled Post-Paleocene deposition was concentrated offshore (Bunce and others, 1965; Hathaway and on the present Continental Shelf, probably others, 1976; Benson and others, 1976). restricted on the seaward side by the Cretaceous rocks were penetrated in one of these development of Gulf Stream flow across the inner wells located on the Florida-Hatteras Slope off Blake Plateau. The Gulf Stream not only limited Charleston (USGS 6004, Hathaway and others, shelf development and caused complicated cut- 1976) and in two of the wells located on the and-fill deposition against its western flank at Blake Spur (DSDP 390 and 392, Benson and others, the Florida-Hatteras Slope, but also scoured and 1976). Rocks inferred to be of Cretaceous and eroded the sea floor of the inner Blake Paleocene age underlie the Continental Margin Plateau. Seaward of the Gulf Stream, a slightly and Coastal Plain in a broad blanket thickening thickened pod of Cenozoic deposits is present. to over 13,000 ft (4 km) in the Blake Plateau The COST No. GE-1 well was drilled at the center Basin and Carolina Trough (Dillon and others, of a lens of Eocene deposits which fill the 1979). Because these sediments are inferred to seaward extension of the Southeast Georgia be of shallow-marine origin, the Cretaceous- Embayment and account for most of the upbuilding Paleocene shelf apparently was very wide, and outbuilding of the shelf off the Georgia extending from the present Coastal Plain to the Coast (Paull and others, 1978; Paull, 1978). Blake Escarpment. The COST No. GE-1 well was REGIONAL STRATIGRAPHY AND STRUCTURE OF THE SOUTHEAST GEORGIA EMBAYMENT

Page C. Valentine

The stratigraphy and structure of the that dips seaward beneath the Florida panhandle Southeast Georgia Embayment are relatively well and the Gulf of Mexico (Applin and Applin, 1967, known beneath the Coastal Plain from studies of pis. 2, 4, and 6). The Upper Cretaceous and the many wells drilled in Georgia and northern Cenozoic strata that fill the Southeast Georgia Florida during the last 40 years. Strata in the Embayment are exposed southeast of the Fall Line basin, however, dip and thicken seaward, and the along the northwestern margin of the Georgia and basin's offshore extent is not well South Carolina Coastal Plain. The outcrop delineated. Until 1977, offshore drilling in pattern of the Upper Cretaceous is illustrated the embayment had been limited to eight sites in figure 3. (maximum penetration of 1,000 ft (305 m)) on the The stratigraphic and lithofacies Continental Shelf and inner Blake Plateau; and, relationships depicted in the cross sections until recently, deep-penetration seismic- (figs. 4-6) draw heavily on the following reflection profiling in this region also had not investigations, outlined below by area. In some been extensive. The COST No. GE-1 well cases, a drill hole treated in more than one penetrated Paleozoic basement beneath the shelf study may not bear the same numerical in a deep part of the Southeast Georgia designation in each report, and a cross-check of Embayment, and study of this section now allows the driller and landowner is necessary to the offshore stratigraphy of the basin to be identify the well. In this study, the outlined. Four stratigraphic sections have been designations of Florida wells are those of constructed across the basin to illustrate its Applin and Applin (1967); other sources of shape, to identify the distribution of carbonate information on these wells include Applin and and clastic lithofacies, and to interpret the Applin (1965), Goodell and Yon (1960), Puri and geologic development of the region (fig. 3). Vernon (1964), Chen (1965), and Maher (1971). The axis of subsidence of the Southeast Georgia wells bear Georgia Geological Georgia Embayment dips seaward beneath the Survey (GGS) numbers, and interpretations are Georgia Coastal Plain and adjacent Continental based on reports by Herrick (1961), Herrick and Shelf. At the GE-1 site, on the outer part of Vorhis (1963), Chen (1965), Applin and Applin the Continental Shelf, approximately 11,000 ft (1967), Marsalis (1970; with paleontology by S. (3,350 m) of Cretaceous and Cenozoic sedimentary M Herrick and E. R. Applin), Maher (1971, with strata have accumulated on Paleozoic basement. paleontology by E. R. Applin), Cramer (1974), The basin opens seaward beneath the Blake Vorhis (1974), and USGS unpublished information, Plateau into a larger and deeper trough, the 1978 (for GGS 1197). Blake Plateau Basin, a north-south-trending South Carolina well sections are from Zupan feature that contains at least 40,000 ft (12 km) and Abbott (1976) and, in the case of the of post-Paleozoic strata (Dillon, Paul, Dahl, Plantersville well, W. H. Abbott (South Carolina and Patterson, this volume). The landward Division of Geology, oral commun., 1977). The margins of the embayment are bounded by Upper Cretaceous stratigraphy of the Fripp structural highs: the Cape Fear Arch on the Island well (fig. 5) is based on unpublished northeast, the Georgia and Carolina Piedmont studies of calcareous nannofossils by the province on the northwest, and the Peninsular author. Well designations are as follows: Arch on the southwest. The Cape Fear Arch Savannah (S), Hilton Head Island (HH), Fripp extends far to seaward and is expressed in the Island (FI), Kiawah Island (KI), Plantersville basement and overlying rocks beyond the shelf (PV), and Myrtle Beach (MB). In North Carolina, edge (fig. 3; Popenoe and Zietz, 1977, fig. the Calabash (C) well section, located south of 2). Between the Peninsular Arch and the Cape Fear, is from Zupan and Abbott (1976), and Piedmont, a minor low, the Suwanee Saddle, well NC-NH-OT-15, located north of Cape Fear, is separates the more deeply subsided Southeast from Brown and others (1972). Georgia Embayment from the Southwest Georgia Offshore, subsurface stratigraphic infor­ Embayment, a structurally similar depression mation is comparatively meager and is based BLAKE PLATEAU

Figure 3. Map of the Continental Margin showing locations of wells and cross sections, and structural features related to the Southeast Georgia Embayment. FC7, BT4, and FC8 indicate intersections of seismic profiles with cross section. Data base for structure contours also includes onshore wells not shown here (Herrick and Vorhis, 1963; Applin and Applin, 1967; Maher, 1971; Cramer, 1974). Well designations are as follows: Savannah (S), Hilton Head Island (HH), Fripp Island (FI), Kiawah Island (KI), Plantersville (PV), Myrtle Beach (MB), and Calabash (C). PENINSULAR ARCH SOUTHEAST GEORGIA EMBAYMENT CAPE FEAR ARCH SW X X' NE F170 COST GE-1 FC7 NC-NH-OT-15 I I _____I Sea u. ^^£^^^^^^£^~ L6V0I " Upper :r__^L-- _, Post Oligocene Nay. age Tayjge Oligocene 9 _ ? "" _ ------"Aust. age Eglford^age _ 500-

1000-

1500-

2000-

2500- Marine Carbonate (and Evaporite) Facies

;-;j Marine Clastic Facies Marginal and Nonmarine Clastic Facies

3000- 0 100KM 0 50 Ml ±-fi.-fW + V+ * + * PALEOZOIC V VERT EXAG. X I28 ,:BA^SEMENT{:

Figure 4. Section XX' through offshore Southeast Georgia Embayment, from F170 in northeastern Florida through the COST No. GE- 1 well to the NC-NH-OT-15 well at Cape Fear, N.C. FC8, BT4, and FC7 represent intersections with seismic profiles, a source of basement depths (Dillon and Paull, 1978). Magnetic profile is from Klitgord and Behrendt (1977). Datum is sea level. Well designations are as follows: Savannah (S), Hilton Head Island (HH), Fripp Island (FI), Kiawah Island (KI), Plantersville (PV), Myrtle Beach (MB), and Calabash (C). primarily on core holes at JOIDES sites Jl, J2, others, 1968; Pickering and others, 1977; P. and J5 (Bunce and others, 1965; Charm and Popenoe, written commun., 1978). others, 1969; Schlee, 1977) and core holes at Lower Cretaceous rocks are well represented sites 6002, 6004, and 6005 drilled by the USGS beneath Florida and the outer part of the (Hathaway and others, 1976). The locations of Continental Shelf, but they thin toward the Cape these drill holes are shown in figure 3. Fear Arch and may be absent altogether at the Depth to basement, where not known from crest. Approximately 184 ft (56 m) of Lower to drilling, has been determined from a structure- Upper Cretaceous rocks, chiefly marine elastics, contour map of the basement surface compiled by have been reported to overlie basement at well Popenoe and Zietz (1977) and also from NC-NH-OT-15 (Brown and others, 1972; unit F, interpretations of offshore seismic reflection Washita and Fredericksburg age equivalents). profiles (Dillon and Paull, 1978) that intersect The stratigraphic units of Brown and others stratigraphic section XX' (figs. 3, 4). The (1972) that represent strata at or just above composition of basement rocks varies widely the Lower-Upper Cretaceous boundary, however, throughout this region and includes felsic and are not yet firmly dated and are presently under mafic igneous rocks, metamorphic and study (J. E. Hazel, USGS, written commun., metasedimentary rocks, and in some cases clastic 1976). There is some evidence that the Lower sedimentary strata of Triassic(?) age (Popenoe Cretaceous may be absent beneath the South and Zietz, 1977, their table 1, and references Carolina Coastal Plain (Gohn and others, 1978) therein). and, herein, the lowermost beds in NC-NH-OT-15 are questionably assigned to the Upper SECTION XX': This section (figs. 3, 4) is drawn Cretaceous. In the south, beneath Florida, the from Cape Fear south-westward across the Lower Cretaceous rocks of the Comanchean Series Continental Shelf through the COST No. GE-1 well (Trinity, Fredericksburg, and Washita age) in to eastern Florida. It depicts the well F170 represent a transgressive sequence configuration of the southern flank of the Cape culminating in a shallow-marine carbonate Fear Arch and the Southeast Georgia Embayment interval. A lithologically similar sequence of beneath the shelf edge off Georgia. The eastern equivalent age occurs in the GE-1 well. flank of the Peninsular Arch appears at well An almost complete sequence of Upper F170 on the Florida coast. Pre-Cretaceous Cretaceous strata is present in the Southeast basement dips steeply into the basin from the Georgia Embayment and Peninsular Arch region. northeast and southwest and lies approximately In the COST No. GE-1 well, Turonian through 10,800 ft (3,300 m) below sea level at GE-1. Maestrichtian units are represented; the The shape of the basement surface must be Cenomanian, however, is not well developed; and regarded as schematic, as it is based on the a poorly dated, thin, shallow-marine interval drilling records of three wells and on below the Turonian may mark an unconformity interpretations of three seismic reflection between the Cenomanian (or underlying Albian) profiles (FC7, BT4, and FC8) that cross the and the Turonian. section (Dillon and Paull, 1978); one point is On the Cape Fear Arch, the Upper Cretaceous derived from the basement map of Popenoe and is thinner than it is in the basin, and there Zietz (1977). The pre-Cretaceous basement in are probably some hiatuses in the rock record the vicinity of GE-1 appears in seismic records there. A major unconformity is thought to exist to be an irregular, block-faulted surface in coastal South Carolina where the oldest Late containing local sedimentary basins of probable Cretaceous beds, dated as Cenomanian, underlie Jurassic and Triassic age (Dillon, Paull, Dahl, strata of Santonian age; rocks of Turonian and and Patterson, this volume). The COST No. GE-1 Coniacian Age have not been recognized in this well was drilled on an isolated magnetic high region (Hazel and others, 1977; Gohn and others, (fig. 4) that correlates with a faulted basement 1978). However, a recent unpublished study by block. Seismic-reflection evidence indicates this author of the Late Cretaceous calcareous that, in fact, acoustic basement drops down nannofossils in the Fripp Island, S.C. well between GE-1 and the coast. The magnetic indicates that the "Cenomanian" beds in this profile in figure 4 is from a recently published region are at least in part Turonian (fig. 5). aeromagnetic study of the Atlantic Continental Offshore at GE-1, Turonian beds are well Margin by Klitgord and Behrendt (1977). The represented in the section, and the Cenomanian prominent magnetic low northeast of the GE-1 may be absent. The "Cenomanian" strata beneath site is part of a linear magnetic high and low the South Carolina Coastal Plain have been dated pair (Brunswick Anomaly) that trends chiefly on the occurrences of spores and pollen, southwestward from the East Coast Magnetic whereas the ages of the offshore units are based Anomaly and intersects the coast near Brunswick, on marine calcareous nannofossils and planktic Ga. The Brunswick Anomaly is believed to foraminifers. Inconsistencies in stratigraphic originate in a susceptibility contrast within interpretation will persist until the zonation the pre-Cretaceous basement rocks of the schemes of marine fossils and palynomorphs can Southeast Georgia Embayment and to reflect a be more precisely correlated. The configuration probable intrusive complex onshore and a of the Upper Cretaceous surface in figure 4 is Mesozoic structural edge offshore (Taylor and based on four wells and on an unpublished

10 structure-contour map, part of which is Older sedimentary rocks undoubtedly occur in illustrated in figure 3. local basins on the faulted basement surface. The entire Upper Cretaceous sequence in the To the northeast of GE-1, on the southern flank Southeast Georgia Embayraent and beneath ^lorida of the Cape Fear Arch, the acoustic basement is composed of marine carbonate strata, surface is smooth and probably represents a excepting a short: interval of marine elastics in volcanic layer of Jurassic age which overlies a the upper part of the Atkinson Formation in well deeper faulted surface (Dillon and Paull, 1978; F170. Strata on the Peninsular Arch are seismic reflection profiles FC8, BT4, and shallow-water, carbonate-platform deposits FC7). The Upper Cretaceous and Cenozoic characteristically composed of limestone and sequence beneath Florida is essentially a chalk associated with evaporites. Tn the carbonate bank, whereas the geologic section Southeast Georgia Embayment, at GE-1, across the Southeast Georgia Embayment argillaceous chalks that have a deeper-water represents the distal portion of a thick aspect indicative of modern outer-shelf and carbonate-shelf wedge that thins both to the upper-slope environments persist throughout the north against the Cape Fear Arch and seaward Upper Cretaceous section. Dark-gray carbonate beneath the Florida-Hatteras Slope and the Blake silts of Maestrichtian and Campanian Age, Plateau. representing a similar environment, have been SECTION YX' ; Cross section YX' , which cored at site 6004 (fig. 3) on the south flank extends from Cape Fear southwestward along the of the Cape Fear Arch. In contrast, the section coasts of South Carolina and Georgia and into on the Cape Fear Arch at NC-NH-OT-15 is a marine northern Florida, is shown in figure 5 (location clastic sequence containing several thin of section is shown in fig. 3) . The overall limestone beds. The location of the transition configuration of rocks of Late Cretaceous and from carbonate to clastic sedimentary facies has younger age closely resembles that observed not yet been determined and is therefore offshore in section XX' (fig. 4). Pre- illustrated schematically in figure 4. Cretaceous basement beneath Florida and Cenozoic sedimentary strata vary greatly in southeastern Georgia in section YX' was drawn thickness from Florida to the Cape Fear Arch using information from drill holes; but the six along line XX' (fig. 4) . A fairly constant depth points from the Savannah well north along thickness is maintained from the Peninsular Arch the coast and including the Myrtle Beach well into the Southeast Georgia Embayment, but unlike are from the basement map of Popenoe and Zietz beds of the Upper Cretaceous, which remain (1977, fig. 2). The magnetic profile in figure relatively thick over the Cape Fear Arch, 5 (Klitgord and Behrendt, 1977) shows that the Cenozoic beds become very thin and hiatuses and Brunswick Anomaly, mentioned above, coincides overlaps are present in the section. The with the deepest part of the Southeast Georgia distribution of Tertiary units in the offshore Embayment. region is not well known in detail, and A major difference between this cross unpublished structure-contour maps have been section and section XX' is the change in shape used to construct the interpretation presented and depth of the pre-Cretaceous basement in figure 4. Cenozoic strata along XX' are surface. Onshore, beneath the coast, the shape carbonates, except for some Paleocene marine of the basement surface is closely reflected in clastic beds cored near the coast at site 6005 the Cretaceous surface, and basement is also on the south flank of the Cape Fear Arch, and much shallower than it is beneath the outer part post-Miocene carbonate and quartzose sands that of the Continental Shelf. The basement plunges are present throughout the region. The strata seaward from a depth of about 4,700 ft (1,430 m) beneath Florida are shallow-water limestones and below sea level in well 719 in coastal Georgia dolomites, whereas shelf limestones dominate the (fig. 5) to approximately 10,800 ft (3,300 m) at Cenozoic interval at GE-1 beneath the Conti­ the GE-1 site (fig. 4). In the deepest part of nental Shelf and also at NC-NH-OT-15 on the Cape the basin onshore, the base of the Atkinson Fear Arch. Eocene limestones are by far the Formation (in this region, the lowest recognized thickest and most widespread of the Cenozoic stratigraphic unit of the Upper Cretaceous) is units. The presence of middle Eocene limestones less than 330 ft (100 m) above basement (fig. on the Cape Fear Arch, a region of predominantly 5), whereas, at GE-1, more than 4,600 ft (1,400 mmarine clastic deposition, indicates the un­ m) of Lower Cretaceous beds are present above usually broad influence of the climatic regime basement. that promoted deposition on the Atlantic margin The unit that underlies the Atkinson of marine carbonates containing fossil Formation (fig. 5) is relatively thin (less than assemblages of Gulf Coast affinity at least as 330 ft (100 m) thick) throughout eastern and far north as Georges Bank (site 6019 of Hathaway southern Georgia (Herrick and Vorhis, 1963, fig. and others, 1976). 19; Cramer, 1974, fig. 3) and is composed of Section XX' depicts, then, a fractured and nonmarine, varicolored, clastic sedimentary faulted pre-Cretaceous basement at GE-1 overlain rocks. Herrick and Vorhis (1963) referred to by a thick Lower Cretaceous transgressive this unit as questionable Lower Cretaceous, sequence of nonmarine to marine elastics chiefly on the basis of its stratigraphic succeeded by shallow-marine carbonate rocks. equivalence with marine strata to the south in

11 PENINSULAR SOUTHEAST GEORGIA CAPE FEAR ARCH ARCH EMBAYMENT sw FlalGa. F5 876 153 7247191197 363

_____ ligocefie1 ///. /Nayarrb' :dg'e' /. './. '. Toylor ^jg_e "^ .-.-. .-.-. :..-.-ij ^e

1000-

| | Marine Carbonate (and Evaporite) Fades 1500-

fc'.-Sj Marine Clastic Facies

\//fy Marginal and Nonmarine Clastic Facies

Magnetic Profile 2000- 150 100 KM 2001 0 50MI 2501 VERT. EXAG. X I28 3001 350

Figure 5. Section YX' from the Peninsular Arch in northern Florida through the Southeast Georgia Erabayment in coastal Georgia and South Carolina to the Cape Fear Arch. Wells S, HH, FI, KI, PV, and MB do not reach basement; depths to basement are from Popenoe and Zietz (1977). Datum is sea level. Well designations are as follows: Savannah (S), Hilton Head Island (HH), Fripp Island (FI), Kiawah Island (KI), Plantersville (PV), Myrtle Beach (MB), and Calabash (C). Florida; Applin and Applin (1967) placed it in Georgia (fig. 6jO. Section ZZ' is located the Comanchean Series. The exact age of the somewhat east of the Suwannee Saddle, where the unit is unknown at present and it is designated basement becomes even shallower, about 3,600 ft Lower Cretaceous(?) in figure 5. (1,100 m) below sea level, before it dips into The Upper Cretaceous Series maintains a the Southwest Georgia Embayment. The Suwannee relatively uniform thickness along section YX' Saddle, a shallow depression terminating the (fig. 5). In contrast to the Tipper Cretaceous Peninsular Arch on the northwest, separates the marine carbonate sequence offshore, the marine Southeast Georgia Embayment and the Southwest clastic facies is well represented along the Georgia Embayment. The saddle can be observed coast, especially in the deeper part of the in structure-contour maps at all levels from the Southeast Georgia Embayment and on the Cape Fear Pre-Cretaceous basement surface (Popenoe and Arch. During Atkinson and early Austin time, Zietz, 1977) through the Eocene and perhaps the the entire region was a marine clastic Oligocene (Herrick and Vorhis, 1963; Chen, 1965; province. Subsequently, shallow-marine Applin and Applin, 1967). As the Southeast carbonate and evaporite beds were deposited Georgia Embayment narrows to the west, the offshore and in Florida and transgressed basement surface beneath the northern margin westward and northward into southeastern dips more steeply southward (figs. 5, 6). Georgia. The carbonate facies persisted here The pattern of facies change observed in throughout the Late Cretaceous, but only the section YX' (fig. 5) is repeated in the two youngest unit, the Lawson Limestone of profiles in figure 6. Marginal and nonmarine Maestrichtian Age, is present farther to the elastics are overlain by marine elastics that north where it grades into marine elastics are in turn succeeded by a thick sequence of between wells 1197 and 363 (fig. 5). marine carbonates. It is evident that, in the The Cenozoic sequence onshore in section western part of the basin (section ZZ', fig. YX' has a configuration similar to that of 6j^), the marginal and nonmarine Lower equivalent strata offshore. A thick sequence of Cretaceous(?) rocks are significantly thicker Paleocene through Oligocene beds occurs in than in the east and that the thickest northern Florida and southeastern Georgia but accumulation of Cretaceous deposits is located thins markedly to the north over the Cape Fear somewhat north of the greatest thickness of Arch. The Southeast Georgia Embayment is Tertiary strata. Marine elastics of the narrower here than offshore, and the Cretaceous- Atkinson Formation grade into the marginal and Tertiary boundary is only about 410 ft (125 m) nonmarine clastic beds of the Tuscaloosa higher beneath the coast than it is at GE-1, Formation updip along the northern margin of the indicating that the floor of the embayment at basin. Navarro-age strata are present this level is comparatively flat beneath the throughout the Southeast Georgia Embayment, but, Continental Shelf. The marine clastic facies is according to Applin and Applin (1967, pi. 6A) , limited to a thin interval of Paleocene age on they are absent both from the Suwannee Saddle the south flank of the Cape Fear Arch and to area and also from a broad region in the post-Oligocene strata that fill the uppermost Southwest Georgia Embayment. In section ZZ', part of the basin in eastern Georgia. Most of near the Suwannee Saddle, the Upper Cretaceous the Cenozoic strata are carbonates that include is markedly thinner in wells 150, 481, 144, and shallow-marine limestones, dolomites, and minor 496 than to the north, and beds of Navarro age amounts of evaporites in Florida, and chiefly are absent at well 144. Much of the thinning limestones of shelf origin in Georgia and South occurs in Navarro-age strata that lie near the Carolina. Eocene strata are again the thickest transition between the marine carbonate facies and most widespread of the Tertiary units. in the south and the marine elastics in the Section YX' (fig. 5) clearly illustrates the north. northward transgression of the marine carbonate province during the Late Cretaceous and Synthesis of stratigraphy and structure Tertiary, as well as the pronounced buildup of the carbonate facies in Florida and southeastern Beneath the Coastal Plain of North and Georgia. South Carolina and central Georgia, the acoustic SECTIONS YY' and ZZ': Two sections, YY' and basement dips southeastward and strikes parallel ZZ', that extend from the interior of the to the Fall Line, a regional trend that is Georgia Coastal Plain southward into northern interrupted in northern Florida and in the Florida are presented in figure 6 (section coastal and offshore areas of the southeastern locations shown in fig. 3). These sections Continental Margin (Popenoe and Zietz, 1977, exhibit stratigraphic and structural fig. 2) . The basement is depressed in relationships that are in many respects similar southeastern Georgia and beneath the Continental to those observed along the coast in section YX' Shelf, forming a basin the Southeast Georgia (fig. 5), although significant differences do Embayment that opens seaward into an even exist. The basement surface in the axis of the deeper basin beneath the Blake Plateau. The Southeast Georgia Embayment rises from Southeast Georgia Embayment is shallow beneath approximately 4,700 ft (1,430 m) at the coast to the Georgia Coastal Plain (4,600-4,900 ft about 3,950 ft (1,200 m) in south-central (1,400-1,500 m) below sea level; fig. 5), but

13 deepens rapidly seaward to about 10,800 ft blocks beneath the shelf; farther offshore, (3,300 m) beneath the outer part of the basement lies at a depth of at least 40,000 ft Continental Shelf at the COST No. GE-1 well (12 km) beneath the Blake Plateau Basin (Dillon, (fig. 4). The basin is probably considerably Paull, Dahl, and Patterson, this volume). The deeper locally in areas of down-faulted basement wide range of pre-Cretaceous rock types

PENINSULAR ARCH SOUTHEAST GEORGIA EMBAYMENT sw X NEJSE y Florida I Georgia F36 F5 876 153 119 148 95 789 480

I I i I ill Sea "I I Marine Carbonate Level (and Evaparite) Facies Marine Clastic Facies Marginal and Nonmarine Clastic Facies 500-

Atkinson 1000-

- 0 100KM II .Lower Cretf?) 0 60MI 1500- VERT. EXAG. X 1 28

Florida Georgia F36 F39 150481144 496107

Sea Level

500-

1000-

r-WASHITA CRETACEOUS BASEMENT -Atkinson Fm. B

Figure 6. Sections extending from the Peninsular Arch in northern Florida through the Southeast Georgia Embayment updip toward the Fall Line. Datum is sea level. _A, Section YY'; _B_, Section ZZ'. Well designations are as follows: Savannah (S), Hilton Head Island (HH), Fripp Island (FI), Kiawah Island (KI), Plantersville (PV), Myrtle Beach (MB), and Calabash (C).

14 represented in the basement beneath the limestones of Turonian age by a short interval Southeast Georgia Embayment suggests that this of undated shallow-water strata. There is no surface has a complex developmental history direct information regarding the nature of Lower (Barnett, 1975; Popenoe and Zietz, 1977). Cretaceous rocks seaward beneath the Blake Cretaceous and younger strata have been Plateau, but shallow-water carbonate bank and deposited on this eroded and, in part, block- reef deposits of Early Cretaceous age are faulted surface that has subsided between two present at the outer edge of the Blake Plateau, relatively stable features, the Cape Fear Arch on the Blake Spur, where they are overlain by on the northeast and the Peninsular Arch on the Barremian-Albian pelagic oozes (Benson and southwest. others, 1976; site 390). The basal sedimentary units beneath the In summary, during the Early Cretaceous, eastern Georgia Coastal Plain have not yet been the part of the present Southeast Georgia dated and are designated as Lower Cretaceous(?) Embayment that underlies the modern Continental in figures 5 and 6. Lower Cretaceous(?) strata Shelf was a depositional area characterized in are poorly represented in the Southeast Georgia part by a smooth volcanic basement of Embayment beneath the Coastal Plain; they are Jurassic(?) age and also by block-faulted apparently absent in northeastern Georgia, are Paleozoic basement containing numerous basins at most 330 ft (100 m) thick in eastern Georgia, filled with marine and nonmarine clastic and are thickest (660 ft. (200 m or less)) in the sedimentary rocks. Seaward, beneath the present western shallowest part of the basin (fig. 6; Blake Plateau, a shallow-water carbonate Herrick and Vorhis, 1963; Cramer, 1974). These platform developed that became a pelagic beds are nonmarine, varicolored elastics that province as the Continental Margin subsided have been assigned an Early Cretaceous(?) age on during Barremian-Albian time. At GE-1, this the basis of correlations with dated rocks in subsidence is marked by the occurrence of northern Florida that are inferred to be shallow-marine carbonates of Aptian-Albian stratigraphic equivalents (Herrick and Vorhis, Age. Beneath the Coastal Plain, the western 1963, p. 51). In South Carolina, near part of the present basin experienced subsidence Charleston, a deep core hole was recently on a much smaller scale during the Early drilled by the U.S. Geological Survey (Hazel and Cretaceous. The basement is much shallower than others, 1977; Gohn and others, 1977); and the it is offshore, and only a relatively thin oldest sedimentary strata were found to be interval of nonmarine clastic beds of Early Cenomanian in age. However, they overlie a Cretaceous(?) age is present. Certainly, no short interval (33 ft (10 m)) of unfossiliferous basin deposits developed beneath the present strata that rest on weathered and geochemically southeastern Georgia Coastal Plain, and the altered basalt (radiometric age of basalt Southeast Georgia Embayment did not exist approximately 95-105± 4 m.y., a minimum age). onshore; deposition was concentrated in the Gohn, Gottfried and others (1978) reported that subsiding region beneath the modern shelf and subsequent nearby core holes penetrated the seaward. basalt (843 ft (257 m) thick; radiometric ages, A comparison of the Coastal Plain cross Late Triassic and Early Jurassic) and drilled sections (figs. 4-6) reveals that the Upper into underlying clastic red beds of unknown age Cretaceous becomes thicker seaward and does not that resemble Triassic (or Jurassic) rocks vary radically in thickness along the regional exposed in the Piedmont (Gohn and others, strike of the Continental Margin in the coastal 1978). A recent study of a series of coastal regions of northern Florida, Georgia, South wells in South Carolina (Gohn and others, 1978) Carolina, and even over the Cape Fear Arch indicates that Lower Cretaceous beds are (section YX', fig. 5; Applin and Applin, 1967, probably absent and that, therefore, a major pi. 6B; Maher, 1971, pi. 17B; Brown and others, regional unconformity exists between the Upper 1972, pi. 49; and Cramer, 1974, fig. 4). The Cretaceous and underlying pre-Cretaceous distributional nature of the Upper Cretaceous is sedimentary rocks or crystalline basement in marked contrast to that of Cenozoic deposits beneath the South Carolina Coastal Plain. Even which are concentrated in the Southeast Georgia if true Lower Cretaceous is present beneath the Embayment. It appears that the Upper Cretaceous southeastern Coastal Plain, its thinness and strata were deposited in a sedimentary wedge on somewhat patchy occurrence represent the effect a regionally subsiding continental margin or in of a major erosional episode prior to deposition a broad marginal basin. Local subsidence did of Upper Cretaceous sediments. occur, however, northwest of the present Offshore, as basement dips beneath the Southeast Georgia Embayment in west-central Continental Shelf, the Lower Cretaceous becomes Georgia, and the Upper Cretaceous is thicker much thicker. It is about 5,000 ft (1,500 m) there than it is to the southeast. This thick at GE-1 and may be even thicker in thickening can be observed at the northern end adjacent, down-faulted areas beneath the of section ZZ' (fig. 6B, wells 508 and 472). shelf. At GE-1, the Lower Cretaceous consists South of this area of subsidence, the margin was of nonmarine and marine clastic rocks succeeded apparently stable (or uplifted) over the by shallow-water marine carbonate units that are northern end of the present Peninsular Arch; separated from overlying Upper Cretaceous shelf here, on the Suwannee Saddle, and to the west,

15 Navarro-age strata are absent (fig. 6ji), and the Blake Plateau (Bunce and others, 1965; Charm their absence is attributed by Applin and Applin and others, 1969, sites J3, J6; Benson and (1967, figs. 3, 4, pi. 5C) to lack of deposition others, 1976, site 390; Schlee, 1977). on an area of regional uplift. Earlier studies The thickest accumulation of Cenozoic beds have called upon a slow rate of deposition in a occurs beneath the coast in southeastern Georgia subsiding area (Hull, 1962; Chen, 1965) or on and offshore beneath the Continental Shelf; the post-Cretaceous erosion (Toulmin, 1955) to top of the Cretaceous dips very gently seaward explain the thinning and local absence of in this region, and the Cenozoic is 2,950-3,300 Navarro-age strata in this region. ft (900-1,000 m) thick (figs. 4, 5). The In a broad sense, the Upper Cretaceous section becomes somewhat thinner westward toward deposits represent a transgressive sequence. In the Suwannee Saddle (fig. 6). And, to the Florida and Georgia, sections YY' and ZZ' (fig. south, Paleocene and Eocene strata remain rather 6) illustrate the interfingering of the marine uniform in thickness over the Peninsular Arch, clastic Atkinson Formation with the marginal and whereas post-Eocene beds are considerably nonmarine clastic Tuscaloosa Formation that lies thinner and in part eroded away. On the updip to the north and west. Marine elastics northeastern margin of the basin, the Cenozoic are transgressed from the south and east by strata thin dramatically over the Cape Fear Arch shallow marine carbonate platform deposits of (fig. 5), and all the Tertiary units are to some Austin, Taylor, and Navarro age that extend from extent absent and overlapped by younger strata; Florida into southern and southeastern Georgia the stratigraphic relations are not unlike those (figs. 4-6). Offshore, carbonate shelf deposits observed among Cretaceous and Cenozoic units at are represented throughout the Upper Cretaceous the Fall Line to the west. sequence at GE-1 (fig. 4). Interpretations of The Cape Fear Arch, part of the Carolina seismic-reflection profiles of the shelf and Platform (Dillon, Kiltgord, and Paull, this Blake Plateau imply the presence of shelf volume), is the dominant positive feature carbonates seaward beneath the Blake Plateau beneath the southeastern Coastal Plain and (Dillon, Paull, Dahl, and Patterson, this Continental Shelf. Maher (1971), and earlier volume) that grade into pelagic oozes (Benson Siple (1959) and Bonini and Woollard (1960), and others, 1976, site 390) deposited in deeper reviewed previous studies of the stratigraphy water resulting from subsidence of the margin. and structural origin of the arch. Previous The Upper Cretaceous Series, therefore, was authors treated the Cape Fear Arch as an deposited on a broad continental margin and, uplifted basement feature associated with during the early part of the Late Cretaceous, downwarping on the northeast and southwest; and marine shelf elastics were deposited in northern the complex stratigraphy of the area, involving Florida and in the eastern regions of Georgia erosional hiatuses and the overlapping of and the Carolinas. A carbonate platform soon strata, has been interpreted as evidence for the became established in Florida and persisted occurrence of mutiple movements of the basement, throughout the Late Cretaceous. Marine elastics chiefly in the Tertiary. were transgressed by carbonates in southeastern The present study is an evaluation of the Georgia, but they remained dominant elsewhere regional stratigraphy and structure of the beneath the Georgia and South Carolina Coastal southeastern Continental Margin; and detailed Plain and northeastward over the present Cape stratigraphic relationships, especially in the Fear Arch. Carbonate shelf sediments Cape Fear Arch area, are not treated. accumulated beneath the present Continental Nevertheless, it appears that a somewhat Shelf and somewhat seaward beneath the present different view of the structural development of Blake Plateau, while pelagic carbonate oozes the Southeast Georgia Embayment and the Cape collected in deeper water at the seaward edge of Fear Arch is justified, one that emphasizes the subsiding margin. subsidence in the basin rather than uplift on Cenozoic sedimentary rocks in the Southeast its margins. Georgia Embayment are chiefly carbonates (figs. It is clear from published maps portraying 4-6) . In northern Florida, on the Peninsular the basement surface (Bonini and Woollard, 1960, Arch, these strata were deposited in shallow fig. 4; Maher, 1971, pi. 4; Popenoe and Zietz, water and represent the continued buildup of the 1977, fig. 2) that the basement bulges seaward carbonate platform that developed in the Late at the Cape Fear Arch and is recessed landward Cretaceous. To the north, in Georgia and the in the Southeast Georgia Embayment. This Carolinas and also offshore beneath the configuration is expressed in the overlying Continental Shelf, shelf limestones predominate units, including the Upper Cretaceous (fig. 3) and transgress Tertiary elastics updip on the and the Paleocene (P. Valentine, unpublished northern and western margins of the basin. structure-contour map). The thickness of the Miocene and Quaternary elastics, chiefly Upper Cretaceous in the Southeast Georgia calcareous quartz sands and sandy silts, occur Embayment and over the Cape Fear Arch is more beneath the Georgia and South Carolina Coastal uniform than that of the overlying Tertiary Plains, but are less significant offshore (fig. 5). Upper Cretaceous beds thicken beneath the Continental Shelf. Tertiary and somewhat in the Southeast Georgia Embayment, but Quaternary calcareous oozes are present beneath not markedly (fig. 5; Maher, 1971, pi. 17B), an

16 indication that during the Late Cretaceous coast, and the shelf edge; they exhibit no neither the embayment beneath the present seaward bulge attributable to uplift of the Coastal Plain nor the Cape Fear Arch was active arch. Subsidence during the Tertiary has in opposing senses, but that the entire margin occurred mainly in the Southeast Georgia subsided almost uniformly. The Paleocene does Embayment while the Cape Fear Arch has remained thicken somewhat in the basin beneath the relatively stable, and the shape of the Upper Georgia coast (fig. 5) and may signal the onset Cretaceous surface (fig. 3) is the result of of localized subsidence in this region. this differential downwarping. When further However, the Paleocene is very thin offshore subsidence, deposition, and compaction occur in (fig. 4), and interpretations of seismic the area, the structure contours of the Eocene profiles show that it is thin throughout the and younger beds will conform to those of the offshore region; like the Upper Cretaceous, it underlying older units in that region. The evidently was deposited on a gently dipping thinness of Tertiary beds on the arch and the continental margin that preceded the formation erosional hiatuses within them are chiefly the of the present Continental Shelf. As the result result of repeated oceanic transgressions and of subsequent subsidence, the deposition of regressions over a basement feature that is younger rocks, and compaction, the Upper basically stable, although minor isostatic Cretaceous and the Paleocene surfaces have adjustments undoubtedly occurred due to sediment assumed the shape of the underlying basement loading and continual upward adjustment of the surface. Appalachian Mountains. The Eocene, on the other hand, exhibits a Offshore, the Cenozoic strata of the different configuration. This unit is very southeastern Continental Margin have been thick in the Southeast Georgia Embayment beneath investigated by many workers. Most recently, the Coastal Plain (fig. 5) and even thicker Schlee (1977) studied the lithology of six offshore at GE-1 (fig. 4). Herrick and Vorhis offshore core holes in this region (Bunce and (1963, p. 55) postulated a middle Eocene age for others, 1965); he also reviewed the work of the onset of local subsidence in the Southeast previous authors and elucidated the Cenozoic Georgia Embayment, apparently because the middle geologic development of the Southeast Georgia Eocene is the oldest unit exhibiting local Embayment and the Blake Plateau Basin. The thickening in the embayment according to their present study supports the previous findings in isopach maps (Herrick and Vorhis, 1963, fig. most respects. The dip reversal on the Eocene 9). Evidence from offshore drilling (Bunce and surface beneath the Georgia coast and inner others, 1965; Schlee, 1977) indicates that the shelf, noted by Schlee (1977, fig. 8), is also modern Continental Shelf became established evident in the structure-contour map in figure during the Eocene; and, in fact, the 3; and a similar dip reversal in the same area progradational nature of this unit is visible in is present on the Oligocene surface (P. seismic profiles across the shelf (Dillon, Valentine, unpublished map). The structural low Paull, Dahl, and Patterson, this volume). on the Eocene (fig. 3) is overlain by only about Eocene and younger rocks, therefore, were laid 165 ft (50 m) of Oligocene beds. The Oligocene down on a continental margin upon which a is very thin beneath the Georgia Coastal Plain continental shelf was forming, and the deposits (Herrick and Vorhis, 1963, fig. 4; Cramer, 1974, became very thick in an actively subsiding fig. 9) and, unlike both older and younger Southeast Georgia Embayment while remaining thin strata, no depocenter is discernible. This lack on the Cape Fear Arch. The structure contours is the result of extensive erosion during an on the top of the Eocene over the arch (fig- 3) oceanic regression, probably in the early do not conform to the shape of the basement Miocene. The Miocene depocenter is located over surface; they cut across the structure-contour the structural low on the Eocene surface (fig. trends of the older units (the Upper Cretaceous 3; Cramer, 1974, fig. 10). Miocene strata thin in fig. 3) but parallel the general structural seaward but subsequently thicken beneath the trend of the modern shelf and retain the shape outer shelf at GE-1. of the original depositional surface. Oligocene The occurrence of pelagic carbonate oozes structure contours and post-Oligocene isopachs throughout the Cenozoic section beneath the (P. Valentine, unpublished maps) exhibit a Blake Plateau indicates that subsidence of this similar configuration. region, initiated in the Early Cretaceous at the present Blake Escarpment, has continued to the If the Cape Fear Arch had been uplifted present. The Cenozoic development of the numerous times during the Tertiary, the Southeast Georgia Embayment and the Continental configuration of the Upper Cretaceous and Shelf resulted from the deposition and Cenozoic beds over the arch would have been progradation of Eocene and younger sediments. affected similarly; structure contours of these These sediments were laid down at a much higher strata on the arch would have been displaced rate here than they were seaward beneath the seaward, and south of the arch their angle of Blake Plateau, where, during this time, incidence to the coast would have increased. As submarine currents inhibited deposition of shown in figure 3, Eocene structure contours are sediments and even caused extensive erosion of oriented subparallel to the Fall Line, the the thin Cenozoic beds now found there.

17 DATA SUMMARY AND PETROLEUM POTENTIAL

Peter A. Scholle

The COST No. GE-1 well penetrated 13,039 ft Biostratigraphic dating of the sedimentary (3,974 m) of Cenozoic to Paleozoic section. section below 9,000 ft (2,700 m), it should be Lithologic, biostratigraphic, and paleoenviron- emphasized again, is very uncertain. The mental studies, summarized in figure 7 and table section down to the metasediments at 11,000 ft I, indicate that the section to a depth of about (3,350 ft) is probably entirely Lower 3,300 ft (1,000 m) consists primarily of Eocene Cretaceous, but it may extend into the and Oligocene chalks deposited in middle-shelf Jurassic. To date, no age-definitive fauna has to upper-slope environments. The Oligocene and been recovered from the 9,000- 11,000-ft (2,700- Miocene portions of the section were deposited to 3,350-m) interval. in the deepest waters, probably in upper- Sedimentation rates of various strati- continental-slope conditions. The depth graphic units are discussed by Poag and Hall interval from 3,300 to 4,600 ft (1,000-1,400 m) (this volume). The highest rates of sedimenta­ consists of Upper Cretaceous, Paleocene, and tion occurred during the Albian to Senonian lower Eocene calcareous shales and subordinate interval, the middle and late Eocene, and the limestones. Estimates of depositional environ­ middle Miocene. The lowest depositional rates ments for these beds range from middle shelf to prevailed in the Late Cretaceous (Campanian and upper slope. From 4,600 to 7,200 ft (1,400- Maestrichtian), the Oligocene, and the 2,200 m), limestones and dolomites with inter- Pleistocene. The overall range of sedimentation bedded sandstones and shales predominate. The rates is not great, however. Maximum rates are section is Albian to Senonian in age, and about 2.5 in. (6.4 cm) per 1,000 years; minimum estimates of depositional environments range rates are about 0.5 in. (1.3 cm) per 1,000 from inner to outer shelf. Between 7,200 and years. II,000 ft (2,200-3,350 m) , interbedded sand­ The porosity and permeability relations of stones and shales predominate, although in­ conventional and sidewall core samples from the dividual beds of limestone, dolomite, anhydrite, GE-1 well are shown in figure 8. Very high and, in the lower part of the section, coal are porosites,in the 25-to 40-percent range, are common. These units, deposited in inner-shelf encountered in the chalks in the 1,000- to to terrestrial environments, are largely barren 3,000-ft (300- to 900-m)-depth range. Corre­ of fossils. They have been tentatively dated as sponding permeabilities for these fine-grained Early Cretaceous, however. Below 11,000 ft limestones are, however, predictably low. (3,350 m) to the bottom of the well, the section Nevertheless, both oil and gas production are is composed of green and gray-green, very fine known from chalks of comparable petrophysical grained, highly indurated to weakly metamor­ character from areas such as the North Sea, the phosed, pelitic sedimentary rocks, with in­ U.S. Gulf Coast, and the U.S. Western Interior trusive and (or) extrusive meta-igneous rocks (Scholle, 1977c). Significant gas shows and especially abundant in the lower part of this potential future gas production have also been interval. Dating of several samples of meta­ found in lithologically comparable, though morphosed igneous rocks from this interval older, chalks on the AOCS in the Scotian Shelf (Simonis, this volume) yielded values around 355 area of Canada (Scholle, 1977c). In areas where million years before present (m.y.B.P). This overpressuring and (or) fracturing have affected may reflect a thermal event in the Upper the chalks, even better reservoir character may Devonian and puts a minimum age on the meta- be expected. Loss of fluid circulation in the sedimentary section. chalk and calcareous shale interval from 2,800 Detailed biostratigraphic data on the GE-1 to 4,900 ft (850-1,500 m) during the drilling of well is shown in table 1. Reasonably good the GE-1 well indicates that fracturing may be agreement exists between the three studies significant in that part of the section. shown. The greatest disagreement occurs in the In the interval from 5,700 to 7,200 ft placement of the boundaries of the Upper (1,750-2,200 m), porosity values in the mixed Cretaceous stages; even there, however, the zone assemblage of limestones, dolomites, and of disagreement rarely exceeds 100 ft (30 m). sandstones are varied and unsystematic. The

18 PALEOBATHYMETRY POAG

LITHOLOGY & HALL VALENTINE AMATO and DEPTH INFEET DEPTH METERS this volumeithis volume BEBOUT(1978) SYSTEM/ IN _j -J SERIES DESCRIPTION 5 ,ft (/) 0) (/) (/) !i 1 1OOO- OLIGOCENE T+r 5OO- J LU =K Fossiliferous, chalky Z =K 2OOO- LU limestones O 0 ^T LU T^rr1 r3^ 3OOO PALEOCENE 1OOO- / -±>: ^ Calcareous shales with -TTC~. j_ j_ some limestone 4OOO UPPER CRETACEOUS HT i '-j 1 5OOO- 15OO- ^T1 Limestones with significant ?! ! '' interbedded sandstone, 6OOO- shale, and dolomite

2OOO- 7OOO- ?

CRETACEOUSLOWER 1 8OOO- Interbedded sandstone and 25OO- shales. Some beds of limestone, dolomite, 9OOO- ^rl anhydrite, and coal beds present ? 3OOO- H?_ 1O.OOO

^r-_ "S5 11.OOO

Highly indurated to weakly [ 3500- metamorphosed shales DEVONIAN? 12.OOO ^> with meta-igneous rocks

13,OOO 4OOO -

Figure 7. Generalized plot of lithologies and depositional environments of sediments in the COST No. GE-1 well. Environmental designations are as follows: NM, nonmarine; IS, inner shelf; OS, outer shelf; USL, upper slope; and MSL, middle slope. Modified from Amato and Bebout (1978). 19 Table 1. Comparison of biostratigraphic analyses from the COST No. GE-1 well

INTERNATIONAL POAG AND HALL VALENTINE

BIOSTRATIGRAPHERS (1977) (this volume) (this volume)

Pliocene Upper Pliocene Neogene Middle Miocene Middle Miocene Oligocene Middle-lower Oligocene Ol gocene - 1.O

Upper I V - 2O c Eocene V o Eocene o Middle UJ - 30 Lower f Upper Paleocene 1 Paleocene tr- Mae strichtian t 1 *=" Maestnc htian Campanian - 4O Campanian Campanian Cretaceous in in 0) D Santonian a i_ O i_ O a 0 V Santonian V V a o D Santonian a o a « a o w D o - 5O DC Coniacian Do Coniacian LU Coniacian 0 O Lower Turonian Turonian LU Turonian Cenomaman - 6.O

X 20 Albian a. - 7 O LU Q Albian 3 n O D 0) D o - 80 O D 1) a o J 0) a 0 Aptian 0) ) U Aptian 7 0 t3 0) - 9.O and » o $ _i o Valangmian j ' ? ? - 1O

- 1 1

? ? Devonian? ?

limestones contain both primary and secondary and 11,000 ft (3,050-3,350 m) have been greatly (leached) porosity; the dolomites are mostly reduced by diagenetic factors and no reservoir finely crystalline and nonporous. In some units are likely to be found in that interval. carbonate units, considerable amounts of primary Likewise, in the slightly metamorphosed section and secondary pore space have been obliterated below 11,000 ft (3,350 m), diagenetic alteration by anhydrite cement. has obliterated all significant porosity and no Arkosic sandstones and siltstones provide reservoir rocks are expected. possible reservoirs in the red-bed sequence Impermeable beds, which could act as seals below 7,200 ft (2,200 m). The original porosity for hydrocarbon entrapment, are present of these rocks has been reduced by compaction, throughout the section. The thick shales and pressure solution, and cementation by silica, calcareous shales between 3,600 and 5,700 ft calcite, and anhydrite (Halley, this volume). (1,100-1,750 m), as well as thinner shales and Despite these factors, porosities in the 15- to anhydrite beds in the deeper parts of the 30-percent range are common in the 7,200- to section, are the best potential seals. 10,000-ft (2,200- to 3,050-m)-depth range. The COST No. GE-1 well has a low geothermal Corresponding permeabilities are mostly in the gradient today (0.89°F/100 ft (16.2°C/km)), and 1- to 100-millidarcies (md) range, although a geochemical indicators apparently show that the few of the coarser sandstones have gradient has been that low or lower throughout permeabilities as high as 4,100 md (averaged in Cretaceous to Holocene time. Data on color with other values in fig. 8). Porosities and alteration of kerogen, temperature of maximum permeabilities of the sandstones between 10,000 pyrolysis yield, carbon preference index, and

20 primary vitrinite reflectance are summarized in napthenes, aromatics, and Nitrogen-Sulfur-Oxygen figure 9. compounds (NSO's) present in the well. Although In general, organic geochemical studies the upper half of this interval still shows have shown that the section in the COST No. GE-1 signs of contamination by drilling additives, well down to about 3,600 ft (1,100 m) contains there is considerable indication that the small amounts of organic carbon. Furthermore, extractable Cic+ hydrocarbons in this interval the interval shows very low concentrations of are, at least partly, indigenous and are derived indigenous hydrocarbons, indicating a lack of from the abundant amorphous marine kerogen significant biogenic or thermogenic generation present in this section. However, vitrinite- of hydrocarbons. Vitrinite-reflectance values reflectance values of 0.35 and a kerogen- average about 0.2 throughout the interval and, maturation rank of 2- strongly imply thermal coupled with visual-kerogen-alteration values of immaturity for this interval. In general, 1 to 1+, indicate thermal immaturity. Inter­ vitrinite-reflectance values of 0.45 to 0.5 and pretation of Cj5+ extractable hydrocarbons in kerogen-maturation values of 2 to 2+ are taken the lower part of this Tertiary interval is to mark the earliest onset of significant greatly hampered, however, by contamination from thermogenic liquid-hydrocarbon generation drilling additives. (Bartenstein and Teichmllller, 1974; Hood and The underlying interval of Upper Cretaceous others, 1975; Vassoyevich and others, 1970). shales (3,570-5,950 ft (1,090-1,810 m)) has the Such values are not encountered in the GE-1 well highest organic carbon content of any section in until depths in excess of 8,500 ft (2,600 m). the GE-1 well. Organic carbon averages 1.24 The peak rate of liquid-hydrocarbon generation percent (by weight) through this interval, with is apparently marked by vitrinite-reflectance individual beds containing in excess of 14 values in the range of 0.6 to 0.7 (kerogen- percent organic carbon. Visual-kerogen analyses maturation values of 2+ to 3), and such values have shown that much of this organic matter only occur below 9,000 ft (2,750-m) in the COST consists of hydrogen-rich, "oil-prone" kerogen No. GE-1 well. types. The shales in this interval also have The portion of the Lower Cretaceous section the highest total extractables, paraffin- from 5,950 to 8,900 ft (1,810-2,700 m) has a considerably lower average organic carbon content than the overlying section. The kerogen CORE POROSITY (percent) PERMEABILITY (milhdarcies) in this section also appears to consist primarily of the hydrogen-deficient, "gas-prone" O 25 5O 75 1OO types. This corresponds with the low-pyrolitic- hydrocarbon-to-organic-carbon ratios found in this interval and indicates a very poor source for liquid-petroleum hydrocarbons. The deepest part of this interval, from 8,500 to 8,900 ft (2,600-2,700 m), does, however, show thermal maturity. The Lower Cretaceous (?) and Devonian(?) sedimentary rocks in the COST No. GE-1 well below 8,900 ft (2,700 m) are largely nonmarine to coastal; and, although they are thermally mature, they must be viewed as having little or no potential as petroleum source rocks because of their very low organic carbon contents. Essentially five major factors are involved in the origin and entrapment of hydrocarbons: (1) source rocks, (2) temperatures sufficient to generate liquid or gaseous hydrocarbons, (3) porous reservoir rocks, (4) impermeable seals, and (5) structural and (or) stratigraphic traps. Indications from the GE-1 well are that rocks of high organic carbon content and "oil- prone" kerogen types are present in the 3,570- to 5,950-ft (1,090- to 1,810-m) interval. Rocks between 5,950 and 8,900 ft (1,810-2,700 m) have a poor source potential because of low contents Figure 8. Porosities and permeabilities meas­ of organic carbon and the more terrestrial ured on conventional and sidewall cores hydrogen-deficient nature of the kerogen in this from the GE-1 well as a function of depth interval. Sedimentary units below 8,900 ft (data from Amato and Bebout, 1978). Where (2,700 m) are virtually barren of organic matter numerous conventional core measurements and have essentially no source-rock potential. were available for short depth intervals, Temperatures appear to have been sufficient plotted values are averages. for liquid-hydrocarbon generation at depths

21 VISUAL TEMPERATURE CARBON PRIMARY KEROGEN OF MAXIMUM PREFERENCE VITRINITE MATURATION PYROLYSIS YIELD INDEX REFLECTANCE INDEX (°C) A AND B (R 0 percent) O O O O o o CO ID h- 0) T- CO 123 ^ < ^ ^ ID ID O 1 2 3 4 5 O.2 O.4 O.6 0.8 1.O

1.O

2.O

3.O 10

4.O

5.0 15 O O O O O 6.O CO DC 111 2O 111 7.O 111 111 LL -

8.O : T. 25 Q_ Q_ 111 a 9.0 Q

10 3O

11

35 12

13 4C 14

Figure 9. Comparison of various measures of thermal maturity as a function of depth in the COST No. GE-1 well. Visual-kerogen-maturation-index and primary-vitrinite-reflectance data are from Core Laboratories, Inc. (1977); temperature of maximum pyrolysis yield is from Miller and others (this volume); carbon-preference-index data above 10,500 ft (3,200 m) are from Miller and others (this volume), whereas data below that level come from Geochem Laboratories, Inc. (1977).

22 below about 8,500 ft (2,600 m), although some the hundreds of millidarcies are common. If indications of incipient generation are present thermally mature marine shales interfinger with at higher levels in the well. Unfortunately the these sandstones in a down-dip direction, the source-rock potential is low in the intervals potential exists for excellent oil and gas that have the optimum thermal-maturation and reservoirs within the 5,700- to 10,000-ft reservoir-rock characteristics. If the Upper (1,750- to 3,050-m) interval. Although Cretaceous shaly interval is encountered in sandstones are present below 10,000 ft (3,050 areas of higher thermal gradient or deeper m), they are tightly cemented and, in spite of burial, however, it could be an excellent source some gas shows in GE-1, must be considered as rock; and this interval must be considered as non-reservoir units in offshore hydrocarbon the one with the highest source potential for exploration. the Southeast Georgia Embayment. Seals, in the form of shales, are present Potential reservoir rocks are available in throughout the GE-1 section. In addition, two sections. Very low permeability (but high anhydrite beds, which would act as seals, are porosity) chalks are present in the 1,000- to present below about 6,000 ft (1,800 m). 3,500-ft (330-to 1,050-m) interval. Where Although the availability of structures for fractured or overpressured these could act as hydrocarbon trapping cannot be evaluated from a reservoir units, especially because of their single well that was intentionally drilled off- association with the directly underlying structure, regional geophysical studies do not potential-source sections. More traditional show abundant structures with significant sandstone and limestone reservoirs with high closure in the area of the COST No. GE-1 well. porosity and permeability are present throughout Thus, stratigraphic trapping through lateral the Lower Cretaceous part of the section from facies changes may be of greater exploration 5,700 ft (1,750 m) down to about 10,000 ft interest in this area than in other basins along (3,050 m). Individual sandstone beds reach 90 the Atlantic offshore margin. ft (28 m) in thickness, and permeabilities in

23 LITHOLOGIC DESCRIPTIONS

E. C. Rhodehamel

The 11 thologic log for the COST No. GE-1 (4,033-4,036 m) was determined by Halley (this well, shown in figure 10, is compiled from low- volume). powered microscopic examination in reflected Standard USGS rock-type abbreviations, light of all available samples (drill cuttings) shale (sh.), siltstone (sts.), sandstone (ss.), of the stratigraphic test; results of this conglomerate (cgl.), limestone (Is), dolomite examination, in general, are supported by (dol.), and quartzite (qtz.) are used throughout interpretations from the electrical-induction, the descriptive material. Standard rock-color spontaneous-potential, natural-gamma-ray, sonic, chart (National Research Council, 1948) colors density, and neutron geophysical logs. Avail­ are used. Their associated color numberings are able samples representing 30-ft (9-m) sections not supplied in the lithologic description in were taken at 90-ft (27-m) intervals from 420 to order to reduce the description to manageable 4,020 ft (128-1,225 m); and below that depth to size and to facilitate readability. 13,240 ft (4,036 m), 10-ft (3-m) sample sections Both lithologic percentages and rock- were taken at 30-ft (9-m) intervals to within 14 porosity evaluations are estimated from the ft (4 m) of the total depth of the hole. Sup­ sample material. The lithology presented is, at plemental x-ray powder diffraction data on three best, an estimate of the lithology encountered samples between 12,540 and 12,670 ft (3,822- and does not truly represent a "bed-by-bed" 3,862 m) were supplied by Mary Mrose, USGS, section. As such, a brief written description Reston, Va. Charles Milton, USGS, Reston, Va., of the natural lithologic units, largely (written commun., 1978) made a petrographic interpreted from geophysical logs, is provided examination of the altered trachyte rock noted and is considered a valuable adjunct to the from 12,780 to 12,790 ft (3,895-3,898 m), and graphic lithologic log shown in figure 10. the trachyte noted from 13,230 to 13,24'0 ft

24 EXPLANATION

(a)

(c) (a) Argillite (b) Quartzite (c) Volcanic rock

Abbreviations used in text fe Limonite(ic) + iron stain(ing) S Siderite Ch Chlorite(itic) Ca Carbonaceous Ig Lignite C Coal fragments Ep Epidote H Hematite fragments A Aragonite Gy Gypsum crystals M Mica, micaceous mf Abundant microfossilr sf Shell fragments P Phosphate(Ic) Py Pyrite G Glauconite(ic) Cgl. Conglomerate Dol. dolomite (a) Ls. limestone Qtz. quartzite Sh. shale Ss. sandstone Sts.siltstone

(a) (b)

(a) Dolomite (b) Dolomltic limestone

Figure 10. Lithologic log, COST No. GE-1 well: Explanation.

25 PERCENT OF SAMPLE DESCRIPTION 25 50 75

exl

.nrf c sf.. .- 420-600 Shell hash, loose, waterworn, brecciated; quartz sand, clear frosted, loose, angular to subrounded, fine to very coarse grains ranging to granule size. Oolite pellets, white to gray, loose; oomicrite, gray; biomicrite; sparite; calcareous mud; glauconite, green to brown; and phosphate pellets, brown. Sedimentary and volcanic rock fragments.

600-690 Shell hash, loose, waterworn, brecciated; biomicrite; many loose ooids, fine- to coarse-grained; quartz grains, some euhedral, ranging to pebble size (40-mm diam.). Ss., clear to frosted, poorly sorted, fine-grained, ranging to sts., gray, dense, noncalcareous, and cherty. Glauconite, green and brown; phosphate nodules and pellets.

690-780 A variegated, colored breccia of shells; chert, banded, milky; GO: micrite; biomicrite, gray, sandy; oolitic micrite, white, gray; P- o pelmicrite, gray; Is., gray, sucrosic, hard, low porosity; and dol., olive gray. Quartz sand, clear, subangular to rounded, fine to very coarse-grained (to pebble size). Ooids, gray, small, loose; green glauconite; brown phosphate pellets and nodules; heavy minerals and metamorphic rock fragments. L 780-1050 Micrite, gray with shell hash and loose ooids. Biomicrite; '.^'^-^i.-^-L.^-^. pelmicrite; phosphate pellets; glauconite, and hematite fragments. Euhedral and broken quartz crystals, clear to frosted, fine-grained to granule size (2.5-mm diam.). Gypsum, white, occasional twinned Spontaneous Resistivity pseudomorphs. potential Millivolts Ohms m"/m2, Gy 20 0.21.010 100

1050-1140 Mostly casing cement, with a white, sandy, and brown, siliceous, angular, nonfossiliferous fragment (chert?).

1140-1230 Lithology like at 1050 ft; casing cement with some chert and argillaceous micrite (chalk) and (or) white to gray lime mud.

-375 M "1230-1590 Biomicrite, fossil fragments (foraminifers), gray, moderately z soft, moderate to abundant intergranular porosity. Dol., brown to gray, crypto- to microcrystalline, dense, ccmchoidally fractured. £ Occasional beds oE calcareous dol. with ooids and eroded and broken micrite (chalky, earthy-textured material).

Figure 10. Lithologic log, COST No. GE-1 well: 0-1,500 ft (0-457 m)

26 Spontaneous Resistivity potential , Millivolts Ohms m /m PERCENT OF SAMPLE 20 0.2 1.0 10 100 0 25 50 75 __ 1__1 x 1500- 1500-1590 Llthology like at 1230 ft. r-457

1590-1625 Lithology like at 1230 to 1590 ft, but with some sand, quartzose, milky, smokey, clear, fine- to medium-grained, suba igular

1625-1680 Biomicrite, gray, mostly chalky textured, some oolitic, microcrystalline, or argillaceous. Some fragmental sparry Is., and thin chert beds, gray, very dense, cryptocrystalline, slightly dolomitic, with little intergranular porosity, and earthy luster. Trace of fine-grained, detrital, coaly fragments.

A & A A lg

1860-2220 Biomicrite, like at 1680 ft, but less fossiliferous. Some micrite and sparite at 2130 ft. Chert, increasing with depth, gray, dense, cryptocrystalline, carbonaceous, nondolomitic, with little porosity, becoming more argillaceous with depth. Some loose sand, clear, and very coarse grained at 1950 ft, but no quartz present at 2040 ft.

3 2100

-2220-2490 Biomicrite and micrite, argillaceous, slightly indurated, very porous, fine-grained biogenic fragments. Chert, gray to brownish-gray, translucent, dense, very low porosity, nodular, with conchoidal fracture and, at 2310 ft, finer textured than at 2220 ft. Some ss. with very fine grained pellets. No macrofossils at 2310 ft, but quartz sand, clear, crystalline, and very coarse.

mf-L -L J. J_ A A A A A A

A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A -2490-2850 Lithology like at 2220 ft, with more chert and less micrite. Some gray and white pellets; seed pods, resinous, brown (0.5-ram A A A A A A A diam.); calcarenite, fine-grained; and fine bituminous particles. A A A A A A A Claystone and sts. very fine grained, dense, thin-bedded (less A A A A A A A than 1/2-um thick), and with conchoidal fracture. Some glauconite, black and hematite staining; quartz sand, clear, medium-grained. A A A A A A A Subrounded to angular grains at 2670 ft. Less quartz sand at A A A A A A A 2760 ft. A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A

Figure 10. Lithologic log, COST No. GE-1 well: 1,500-2,700 ft (457-823 m)

27 Spontaneous Resistivity potential Millivolts Ohms m2 /m PERCENT OF SAMPLE 20 0.2 1.0 10 100 25 50 75 _i_i ^^____^___^

1- A A A A A A A A A '- A A A $ A A A A A LA A A ^ A A A A A m " A A A L L.A A A / i A A A A A "A A A i^ A A A A A *-.--- * ^x. 2800 T.-^.~.-^.?N!:^iv A Li A A A A A _t_ _i_*_i i A A A A A 2 850-2900 Calcarenite, fine- to medium-grained, somewhat sparry, '^N A A A A nonfossiliferous, and some glauconite, green, very fine grained. Muscovite, large (3-ram diam.) flakes with smaller mica flakes of i , 1 , , I ! l-l A brass and gold color; some as inclusions in the calcarenite. [. A i i i 1 1 1 1- 1- Some milky quartz sand and chert. Walnut hull contamination. T ' 1 i i- d 1 -1 1 1- 1 n r-U-T- . i . i . 1 1- IAI 1 c. -+ - - 4- +- + -£- -4* + ++ + -M __nf| -t- ^t- V 2900-3030 Micrite and fine calcarenite, gray, argillaceous, chalky, -1- H - -4- -t- -4- 4^-t--f- -i-\ -1- -4- H- -h -H -h -f- -+'-»- -t-\ microcrystalline, some sparry, with moderate to abundant moldic - -1- - - -t- + -t-4~V"4-+4-"t|. to interparticle porosity, slightly microfossiliferous, some cross 4-4-4- ^Fl -f- | -f- J bedding, and very micaceous, like at 2850 ft. Some quartz sand, 4- + -f- -| \ | J | clear, broken, angular to subangular grains, and microfoss'ils. - -4- - -.4-.-l-.4- -1- -1- -+- -+- Walnut hull contamination. -H- +- -l- 4- -4- + + -(-+-(- ~ ^^ ~~ -H-4- -t- -1- --J--1- 4- -H 4- -1- -H -1- + -1- -4-^ -4- -1- - -4- - - -4- -1- 4- 3030-3060 Micrite to fine calcarenite, gray, becoming increasingly J- M -"-_ 0- J- J- argillaceous, more dense, and with some black lignite. Mica, FT- like at 2850 and 2970 ft. Some quartz sand, clear, broken, angular to subangular grains, and microfossils. Walnut hull contamination. Err f ==1" "^j _J __ 1 3060-3210 Micrite, light-gray; becoming very argillaceous with depth; _l__ ' . limy sh. at 3300 ft, thin (less than 0.2-mra), horizontally bedded, -i_ ' _ microcrystalline, soft to moderately dense, moderate to low porosity, blocky; minute mica flakes; some very fine calcarenite. - _ ' ' ' Sh., light-gray, very calcareous, with wavy bedding at 3390 ft. £j-j~ i "^z^zz- Walnut hull contamination. 3210-3390 Lithology like at 3060 ft. Some chert, gray to brownish- gray, dense, with conchoidal fracturing beginning at 3300 ft; ^L-§5=i-±-±-" some green glauconite in small fractures and sparse mica.

I^JC^. ~ -^-J ?--^:l

=T7^ -~ J390-3480 Lithology like at 3060 ft, except some calcareous sh. - -^ L - /A 1 1 i>I-1 1 _

3480-3600 Sh., calcareous, as above at 3390 ft. Micrite, very _-M_ ...0...... 0.--..Q...--...I argillaceous, with thin horizontal bedding (like at 3120 ft), and _L __-L ..r^-.-.-t moderate to scarce intrapartlcle porosity, mica (both large and small flakes, like at 2850 ft) some brown pellets and ooids at 3570 ft, 3^2 and chert. Some quartz sand, clear, as much as 4-mm diam., and iz angular. Sparry Is. and large particles of lignite. _o ^- 3600-3660 Argillaceous Is., soft, and calcarenite

t-3750 Sh., gray, very calcareous, biomierite, gray, and argill Is., slightly oil stained. Glauconite, brown, some green, 0.5-mm-di0.5 mm diAm. npllets- weathered;weathprpHi andnd mica,mira. twotwn typesfvn*>c like1ilr*a at 28507R ft. _L _ir^_, Walnut hull ation.

0. _ .Tl __ _ _ CL _ - _ _

"TTjkrVy JiflLAfe

-.._!_ ..------^ ----L--

V , -5 , ^v '~ " '|"~

- , Ij ^. ^ is

Figure 10. Lithologic log, COST No. GE-1 well: 2,700-3,900 ft (823-1,189 m)

28 Spontaneous Resistivity potential Millivolts Ohms m2/m PERCENT OF SAMPLE 20 0.2 1.0 10 100 25 50 75

TTJi: .in*_-..

4010-4130 Micrite, argillaceous, and intercalated sh., calcareous, very fine (0.2-mm thick) horizontal, distinct bedding; with quartz grains, clear (to 4-mm diam.), and angular to subrounded. Small pelecypod shells, foraminifer, and broken fossils. Some chert, gray, and iron stained. Glass spheroids, numerous (like at 3750 ft). Gray sh. fragments like at 4020 ft. Quartz sand, medium-grained.

4130-4280 Micrite and biomicrite, gray; partly argillaceous ___ calcilutite; white pellets, foraminifers, macrofossils, and --1- Ig calcite(?) plates. Quartz sand, loose, yellow, clear, medium- grained to as much as 2-mm diam., and angular. More biomicrite, micrite, and iron staining with less foraminifers and glauconite " "* "' "* 1 ^ A re i ~*~ py than at 4010 ft. Glass spheroids, like at 3750 ft. Gray sh., --l- A soft, calcareous, fossiliferous, and argillaceous. Some calcite, pinkish-gray. No loose quartz at 4230 ft. At 4200 and 4230 ft, more white calcareous pellets and calcite crystals.

4280-4395 Biomicrite, white, argillaceous, cryptocrystalline, soft, low to moderate interparticle porosity. Some loose glass spheroids, calcareous sand, pellets, and ooids. More clay and sh. at 4380 ft. Micrite; calcarenite and biomicrite, clayey, fossiliferous, and iron stained. Clay, gray. Some calcite crystals, quartz sand, loose, and coarse-grained.

.4395-4580 Micrite; biomicrite; very fine calcarenite, argillaceous, white, moderate to low porosity, and fossiliferous. Sh., -^ Pyj mf A 5fi. G -^.-^ H gray, fossiliferous, moderately hard to soft, calcareous, some ±. fe _i_ II I *- -*-~-S open partings, and moderate inter-particle porosity. Some quartz ^A /-x G mf I lfl»l -Jo- j-Ji sand, medium-grained, angular to subrounded, and glass spheroids as above. At 4410 ft, micrite with fewer fossils, iron oxide, and aragonite. More sh., increasing with depth, calcareous, and silty, and loose calcarenite. Some calcite crystals. No quartz sand. Some pyritized foraminifers at 4470 ft. At 4500 ft, some fossiliferous gray sh.

4580-4940 Sh., gray, moderately hard to soft, easily lost by sample washing, very fine bedding (0.05-mm thick), moderate to low Interparticle and intergranular porosity, megafossils, calcareous, and silty. Calcarenite, and biocalcarenite, fine- to medium- grained fossiliferous; micrite and biomicrite, white to yellowish- gray, silty, very fine grained, argillaceous, and poorly consolidated. Some glass spheroids, calcite crystals, but no quartz grains. Some manganese and iron-oxide stain at 4710 ft. At 4860 ft, lithologies are especially soft and wash out easily.

.4940-5150 Lithology like from 4860 to 4940 ft, except more biomicrite and sh., as above, but more calcareous. Some large, calcite fragments; quartz sand, medium-grained; and glass spheroids. Sh., gray, soft, poor to fair porosity, silty, calcareous; biomicrite, semi-indurated, with fair porosity, and a bio­ calcarenite of foraminifers, aragonite crystals, and broken calcareous shell fragments. Thin soft Is. at about 5140 ft. Soft sh. as above, probably underlogged between 4370 to 5030 ft. Calcareous clay increasing; some biocalcarenite, largely of pyritized foraminifers, and iron oxide like at 4980 ft.

Figure 10.~Lithologic log, COST No. GE-1 well: 3,900-5,100 ft (1,189-1,554 m)

29 DEPTH IN METERS

iaaj NI raaaa vlty potential 2, Millivolts Ohms n /m 20 0.2 1.0 10 100 1000 PERCENT OF SAMPLE -H+ i i r- 25 50 75 [6320-6500 Ls., micrlte, partly sandy; blomicrlte; blosparlte; oosparlte; I pelmicrlte; and oolitic and pelsparite packstone; mottled yellowish- ffifc3i&ii gray, gray, and pink, wi iSg altered (weathered) bela 6500 ft and le at6540to6610 ft,hard .line texture, and ly ~i ' *u calcite recrystallization in raicrite. Sh. , gray, greenish-gray, dense -7 i « i 7 ' * ' I I--I o l~ l« silty, hard, somewhat calcareous to noncalcareous, micaceous, I 1 ! o carbonaceous, low porosity; pyritized fossil plants; intercalated sts. becoming dominant at 6410 to 6440 ft; fossiliferous at 6470 to ^Mjrrr^rp^^p-ig e?? 6500 ft, and dolomitic and calcitic below 6620 ft. Some ss. >i i »i <»T (subgraywacke to subarkose), and gray, fine- to coarse-grained, i. j-i. r.MHk: calcareous sts. Ss. is gray, very fine grained, silty, noncalcareous thlBr^ ^J^ to calcareous, dense with low to high porosity, angular to subrounded grains, very carbonaceous; interbedded with sh. and with Is. at 6360 to 6370 ft, and 6390 to 6400 ft. Quartz sand, clear, very coarse (1. 3-mm diam. ) , well-rounded, and iron-stained. Anhydrite^ ^-^a-v^-i. .1ifei white, -massive, non porous; and crystalline rosettes and gypsum i « ' r ' r ff crystals occurring in oolitic Is. vugs. Dol. , below 6500 ft, gray, vuggy, dense, sucrosic, conchoidally fractured; high intraparticle, interparticle, and intercrystalline porosit". Some white, mega- crystalline calcite, and amber and tar plate 1 ^ts below 6620 ft. [-6650-6800 Ls. like at 6540 to 6610 ft, altered, with fair to excellent T» i e i «i » TW r vuggy, interparticle, porosity; some dense, with low porosity, and more fossiliferous. Sh. and sts., gray, fine- to medium-grained, like at 6630 ft. Anhydrite and gypsum in crystals, stained, white, massive, fractured; and dense forms like between 6540 to 6610 ft. ttl^^^g; Some ss. , very fine grained, calcite, clear to railky, recrystallized and megacrystalline, some tar(?) and amber. n4~T^& 6800-6890 Ls., gray, mottled by pellets, biosparite, oosparite, pelsparite; pink and white micrite, microcrystalline to megacrystalline; recrystallized, white calcite, gray oomicrite, biomicrosparite; and pelmicrosparite, dense, low to high vuggy and intraparticle porosity. Where altered. Is. is more porous. Sh. , gray, very fine grained, soft to hard, nonmicaceous to micaceous, noncalcareous to calcareous. Greenish-gray sh. , and some calcareous greenish-gray claystone. Ss. , gray, very fine grained, and noncalcareous; and a very fine grained calcareously cemented sts. Most sh. and sts. are soft, have moderately low porosity, and have some brownish-gray sh. Anhydrite, like at 6750 ft intercalated with micro- crystalline and sucrosic, brown, dense dol. 6890-7020 Dol., yellowish-gray, finely crystalline to sucrosic to dolomicritic,

mottled; some biosparite, biomicrite, calcarenite, oosparite, and micrite. Sh. and sts., like at 6870 ft, very fine grained, micaceous, gray, and greenish-gray, very to slightly calcareous. Anhydrite, like at 6870 ft, and coarsely crystalline dol. Some loose quartz. 7020-7070 Ls., yellowish-gray, fossiliferous, biosparite with intra- granular porosity; biomicrite, oosparite with vuggy porosity; and oomicrite, pelsparite, and much micrite, with calcite, large crystals, recrystallized, and dolomitic. Dol., calcareous, finely crystalline, sucrosic, dense to porous, interparticle porosity, dull and sparry. Sh. and sts., gray, very fine grained, little to much calcareous cement, and some green claystone and sh. Ss. (subgraywacke), gray, mottled with bronze mica, very fine grained, some noncalcareous cement, fair to good porosity, with quartz. Some white, massive anhydrite, coal and dark greenish-gray sh. Walnut hull contamination. '7070-7160 Sh. and sts., gr gray, gr little much calcareous cement, and hard to soft. Ls. , oosparite, oomicrite, pelsparite, biosparite, biomicrite, micrite, and recrystallized calcite. very fine grained to fine grained; and sts. (subgraywacke), fair to good porosity, poor to fair sorting, angular to well-rounded grains, snd nonfossil- iferous. Some white, massive, and greasy- looking anhydrite, microcrystalline

7160-7200 Sh. and sts., gray, greenish-gray, and gray silty sh. , with little to much calcareous cement. Ls. , white to gray, some dolomitic, dense, oosparite,

porosity. Dol., gray, dense, low porosity, rryptocrystalline to coarsely- crystalline, sucrosic. Ss. (subgraywacke), gray, mottled, very fine-graine to fine-grained dolomitic, high porosity, clean to silty, moderate sorting, and nonfossiliferous. Anhydrite, white, pink, and dense, like at 7110 ft. Ls. and sh., thin beds, like at 7170 ft. Walnut hull contamination. 7200-7400 Sh., gray, some brown to greenish-gray; trace of gravish-red, partly

bedding 1-mm thick, Is., gray, mottled with pellets of biosparite, oosparite, biomicrite, oomicrite, micrite; yellow stained at 7380 ft, vuggv porosity, fossiliferous to nonfossiliferous. Fine- to medium-grained crystalline, dens gray dol., sucrosic to cryptocrystalline, with low porosity. Ss. and sts., subgraywacke, orthoquartzite, to subarkose, white, gray, some mottled, greenish-gray, fine grained to very fine grained, well to moderately sorted, nonfossiliferous, calcareous to dolomitic, some silty and high pornsity. Ss. grades to low maturity sts. Anhydrite, white, pink, dense, massive, in ~1_ sh. and dol. Walnut hull contamination. 7400-7490 Lithology like at 7380 ft, with decreasing sh. to 7460 ft, and more red and brownish-gray toward the base. Increasing dol., cryptocrystalline to coarsely crystalline to 7460 ft; less ss. and sts. to 7430 ft, and then increased with depth. Some micrite, limy grainstone, and oomicritic wackestone. More coal to 7470 ft, but less at the base. Clavstone, sh., and sts., with very fine grained, thin horizontal-bedded ss. intercalated. Notable amounts of anhydrite.

Figure 10.~Lithologic log, COST No. GE-1 well: 6,300-7,500 ft (1,920-2,286 m).

31 Spontaneous, Restivity potential Millivolts Ohms m2 /m PERCENT OF SAMPLE 20 0.2 1.0 10 100 1000 25 50 75 -H+ i i i i i -2286 7490-7910 Sh. and very f ine-gra.ined to fine-grained sts., grayish-red, brown, and gray, faintly ralc.ireous; and at,., tray, noncal careous and silty. Ss. and coarse-grained sts., like at 7530 ft. A little bioraicrite and dol., gray, dense, like at 7560 ft; some mega- crystalline and recrydtallized. Much loose, quartz sand with red clayey coatings. Walnut hull contamination.

< .*

'7910-8330 Sh., and very fine grained, shaley sts., brownish-gray, greenish-gray, to grayish-red, moderately soft to moderately hard, somewhat calcareous; and sh. and sts., very fine grained, gray, partly sandy, like at 7440 ft, with some claystone. Ss. , yellowist gray to gray; and coarse-grained sts., like at 7530 ft. Ss., well- sorted, angular to subangtilar grains, nonfoasiliferous, very fine- to medium-grained, very silty, dolomitic, calcareous to noncalcarec dense, and with moderate to low porosity. Much loose, fine to coai grained quartz sand. Much coal from 8040 to 8050 ft. Some coal;

pink and white anhydrite, sandy dolomicrite, and dol., Rray, dense, sucrosic, and cryptocrystalline to megacrystalline. Little to mud walnut hull contamination.

-!£_ .

Ir^ )30-8450 Sh., grayish-red and gray, soft to hard, slightly calcareous to noncalcareous, partly micaceous; and very fine grained sts., fossiliferous, very calcareous, grading to a shaley, sandy, and silty r micrite, like at 8220 ft, fossiliferous; some soft, slightly calcareou brownish-gray sh. Ss., gray (subarkose), clean, carbonate-cemented;

coarse-grained, very silty, angular, very poorly sorted to moderately well sorted, partly indurated, with moderate to high porosity, and 8400 J i^lispJ; grains

8450-8690 Sh., grayish-red, some mottled gray; with pink and gray :ar anhydrite, silty; silty to sandy, yellowish-gray biomicrite at 8640 ft; and very fine grained to coarse-grained sts., a slightly micaceous, and calcareous, soft, and nonfossiliferous. Sh. and very fine grained sts., gray, hard, and some calcareous cement. Ss., subgraywacke, brown grayish-red to pale-red, and gray at the base, fine- to medium- grained, very angular to rounded grains, poor to well sorted, with visible moderate to high porosity, being more dense with depth, very -|^4;;;Yf clayey to moderately clean, pnor to fair induration, and nonfossil­ iferous. Some megarrystalline and calcareous dol.; brown, and gray, megacrystalline and dense dol., with more at the base; and much iron oxide cement. Some dirty reddish-brown claystone; white and gray, V:^W:';:;-'' silty, micrite and pelmicrite; and loose, iron-oxide-stained quartz grains, clear, milky, pitted, very angular to subangular, with poor to fair sorting, nonfossiliferous, and little to no auxiliary minerals. Ss., quartzose (orthoquartEite ranging to subarkose) from 8540 to 8570 ft, gray, and grayish-red, very fine grained to coarse grained, moderate to high porosity, and somewhat clayey to clean.

Figure 10. Lithologic log, COST No. GE-1 well: 7,500-8,700 ft (2,286-2,652 m)

32 DEPTH IN METERS DEPTH IN METERS DEPTH IN METERS

OOOZ e 0001 /W'*««~X/w^ji^^

a

13 aJ MI Hiaaa Spontaneou: Restlvlty potential Ohms tn /m Millivolts oo PERCENT OF SAMPLE o oo 20 25 50 75 -H + 12,300-1 r __ u __ w __.__ <_-,__, 12,500-12,650 Argilllte, greenlsh-gr and gray, pyritUed like at 12,480 ft; cryptocrystalline, with increased alteration like from 12,150 to 12,490

rocks; some vein quartz, with little calcareous inclusions; soft, finely crystalline, some anhydritic cement;, and walnut hull and upper-hole con­ taminants. Pyritized siliceous argillite is harder, very fine grained, dense, quartzite-like with dark pyritic mottling of milky, clear, and 12,400 ^_ l_ r" "T" L ~~~*~~ translucent masses; approaches soft to hard metaquartzite. Metaquartzite is greenish-gray and gray, very fine grained to fine grained, dense, and hard to soft, with pink, red, and green soft anhydrite(?) cement, and pink and red feldspars; an unidentified black mineral in many chips.

12,650-12,770 Argillite, some anhydritic?, greenish-gray and dusky- purple, mediocrystalline to cryptocrystalline, pyrite cubes; interbedded with dusky-purple and greenish-gray anhydritic 12,500 argillites and an anhydritic?-cemented,.arkosic (feldspar) metaquartzite. Quartz, hard to soft, clear, very fine grained to medium-grained, grayish-purple to brown, mottled red, green and purple (by feldspars and anhydrite?); has opaque black mineral. Quartz grains become coarser with depth, reach cgl. size at 12,750 ft, and are clear, hematite red, and milky colored. X-ray diffraction shows metaquartzite is quartz and feldspar plagioclase. Complex sample lithology.

12,770-12,920 Trachyte, becoming highly altered with quartz vein in­ jections, very hematitic, brownish-black, with macrocrystalline recrystallized? lathes, 0.7-ram long, soft to hard, becoming harder with depth, vuggy at 12,870 ft, and mottled with earthy red and yellow specular hematite and chlorite. Some hematite blebs have .*, dark growth rinds, and (or) pyrite nuclei suggesting moderately 12,700 high-temperature alteration. Anhydritic cemented metaquartzite, some soft with red streaks, stained, and mottled dark braftn with

vein quartz, clear, broken, macrocrystalline; rhombic calcite; and pink, euhedral calcite showing sharp contact with siliceous, green, altered trachyte, and anhydritic argillite, and hematitic wall rock. Some anhydrite pinkish, earthy, silty, massive; 12,800 grayish-red; anhydritic argillite with microcrystalline texture; and chlorite; and grayish-red chert breccia with hematitic cement. In general, major mineral type is feldspar. Major rock types are trachyte and anhydritic?-cemented metaquartzite. Large (0.6-mm) feldspar crystals, specular hematite, and vein quartz in sharp contact with anhydrite argillite and tjrachyte masses suggest low- to moderate-grade metamorphic activity with hydrothermal solutions. Sample represents volcanic rocks, somewhat silicified, and metamorphosed sediments. 12,920-13,040 Trachyte, altered, brownish-black, moderately hard to very hard (at injected quartz-vein contacts). .A lesser amount at 13,010 to 13,040 ft. Metaquartzite, some conglomeratic, and possibly altered igneous volcanics, somewhat silicified, like at 12,780 ft. Grayish-red, microcrystalline, siliceously altered argillite, like at 12,770 to 12,920 ft, some brecciated or conglomeratic, mottled brownish-black, gray, 13,000 olive, white, green, brown, red, and purple, with hematite staining and cement, silica-injected, and hardened. Much broken vein quartz, milky to clear, with sharp siliceous vein contacts. Argillite is silica-impregnated near veins, forming a cryptocrystalline metaquartzite like at 12,870 ft. Anhydrite, earthy, pink, silty, and soft. Some K^PvJL Jl^ soft, very finely crystalline (sucrosic), grayish-green (epidote?) mineral vein calcite, and greenish-gray argillite. 13,040-13,190 Argillite, somewhat anhydritic?, metamorphosed, rCh_ J^- greenish-black, dusky-purple, to dark-red, moderately hard; pyritic, partly mottled red and green with feldspars and anhydrite? cement. Light, multicolored, and siliceously altered argillite like at 12,990 ft. Grayish-red to dusky-purple argillite, very siliceous, harder than greenish-black variety, more brecciated or conglomeratic, both with cryptocrystalline to sucrosic microcrystalline appearance and, in part, altered and hardened by siliceous injection. Trachyte, brownish- black, hematitic like from 12,780 ft downward; siliceous, metamorphic trachyte as at 12,870 ft. Earthy (clayey), light-gray to white, chalkv.f' very soft, multicolored, mottled, and thin-bedded anhydrite masses. \ 13,254- Vein quartz, milky white, and loose. Grayish-green, soft, unctuous,

purple argillite. Volcanic rocks, now very silicified. Rocks have siliceous appearance. Some hematitic, red, fine-grained fragments are brecciated, and cherty to jasperite looking (approaching hornfels) at 13,170 ft. 13,190-13,254 Metaquartzite, hornfels or iasperite-like; contact- metamorphosed argillite, greenish-black, dusky-purple, and red, completely silicified, and altered by much quartz; and chlorite and calcite vein injections. Probably very silicified, felspathic, volcanic rocks like at 12, 780 ft. Trachyte altered by quartz, calcite, and rhlorite vein intrusions (Charles Milton, written commun. , 1978), with morv epidote and feldspars. Vein quartz, white, milky to clear, and with pink and large red

conglomeratic rock, as in the last several hundred feet.

Figure 10. Lithologic log, COST No. GE-1 well: 12,300-13,254 ft (3,749-4,040 m).

36 PETROPHYSICAL SUMMARY -= '

E. K. Simonis

Ten conventional cores and 68 sidewall which core analyses are not available may cores recvered from the COST No. GE-1 well were contain additional favorable reservoir rocks. analyzed by Core Laboratories, Inc. (1977). Above 5,700 ft (1,700 m), no sandstones Results of the analyses are summarized in tables were recovered in either the conventional or 2 and 3, and in figures 11, 12, and 13. For the sidewall cores. Many of the limestones in this conventional cores, the analyses were performed section are highly porous chalks, but their on 1-in. (2.5-cm)- diameter plugs taken at permeabilities are very low; they plot outside approximately 1-ft (0.3-m) intervals, resulting the general trend on the porosity versus in a wide range of porosity and permeability permeability plot in figure 13. The chalks may values (table 2, figs. 11, 12). On the basis of prove to be potential reservoir rocks, how­ the results of the core analyses, rocks with the ever. For example, chalks with low perme­ best reservoir characteristics appear to be ability values are highly productive in the concentrated between 5,700 and 10,000 ft (1,750- North Sea. Between 5,700 and 10,000 ft (1,750- 3,050 m) (figs. 11, 12); however, intervals in 3,050 m), sandstone is the lithology that has the best reservoir characteristics. Porosities i'This section is reprinted, with minor from 25 to 30 percent are common and perme­ changes, from Amato and Bebout (1978). abilities are as much as 4,000 md.

Table 2. Record of conventional cores, their range of porosity and permeability, and their dominant lithology

[Footage designations can be converted to meters by multiplying the number of feet by 3.3. Leaders ( ) indicate no data available]

Core Cored Interval (feet) recovery Porosity (percent) Permeability (md) Core Hominant No. Top Bottom Feet Cut (feet) High Low Mean High Low Mean lithology

1 3,024 3,054 30 30.0 30.5 7.5 19.8 0.68 o.m 0.1 Limestone. 2 6,607 6,657 50 48.4 23.3 4.8 12.8 8.1 0.05 0.8 Do. 3 7,040 7,100 60 58.9 29.3 3.5 11.4 284.0 n.07 22.8 Do. 4 8,331 8,391 60 59.8 29.1 3.3 18.3 4,llf>.0 0.01 600.0 Sandstone. 5 9,453 9,513 60 53.1 20.9 2.3 10.5 203. f> n.ni 29.0 Do.

6 10,518 10,520 2 0.33 10.8 10.8 10.8 4.2 4.2 4.2 Do. 7 10,520 10,580 60 46.1 18.5 2.8 9.3 14.0 0.01 1.5 Do. 8 10,931 10,933 2 0.0 9 10,933 10,935 2 0.0 10 11,000 11,002 2 0.0 ~ -- -- ~

11 11,357 11,387 30 29.0 1.4 0.1 0.42 0.01 0.01 0.01 Argillite. 12 11,635 11,643 8 7.4 0.2 0.2 0.2 0.01 0.01 0.^1 Do. 13 12,624 18,627 3 1.0 0.6 0.6 0.6 0.01 0.01 0.01 Do. 14 13,247 13,251 4 0.1 Meta-basalt (?). 15 13,252 13,254 2 0.66 Do.

37 .and Most of the permeable sandstones are very nant lltholon les of sldewall cores from ' COST No. GE-1 well fine to medium grained, moderately to poorly sorted, and incompletely cemented by quartz [Da ic., 1977 Pernneablllty valijes marked by asterisk were determlned empl rlcally] overgrowths and hematite. Porosities and Calculated permeabilities of carbonate rocks are generally Depth LltholoR lower, but a sandy dolomite recovered in con­ (feet) (percent) (nilllldarcles) (g/cm3 ) ventional core No. 3, between 7,093 and 7,099 ft

1,867 569 36.2 0.5 2.56 Chalk. (2,161-2,163 m), has a porosity of 29 percent 2,244 684 39.5 2.3 2.72 no. and a permeability of 280 md. 2,288 697 33.9 2.0 2.55 no. 2,433 742 21.9 0.1* 2.71 no. The core analyses indicate a sharp 2,754 839 44.1 2.2 2.73 n0 . reduction of porosity and permeability below 2,790 850 36.9 2.7* 2.53 no. 10,000 ft (3,050 m). Between 10,000 and 11,050 2,924 891 22.8 1.9 2.64 n0 . 2,985 910 43.7 6.1 2.37 Do. ft (3,050-3,368 m), the porosities of analyzed 2,988 911 41.6 2.1* 2.61 no. sandstones drop below 20 percent and most of the 2,998 914 44.2 2.7 2.52 no. 3,010 917 33.2 1.7 2.56 no. permeability values are less than 10 md. Inten­ sified cementation by quartz overgrowth and 4,360 1,329 23.2 0.1 2.57 Do. 5,755 1,754 19-1 3.5* 2.64 Sandstone. compaction by pressure solution appear to be 5,804 1,769 25.6 447.0 2.63 Do. 5,850 1,783 23.8 10.0 2.63 Do. important factors in the reduction of porosity 6,028 1,837 30.1 3.8 2.67 Do. 31.2 46.0 2.60 Do. and permeability. 6,110 1,862 Below 11,050 ft (3,368 m), to the total 6,162 1,878 25.9 15.0 2.61 Do. 6,374 1,943 27.2 19.0 2.57 Do. depth of the well at 13,254 ft (4,039 m), the 6,398 1,950 27.3 14.0 2.60 Do. section consists of essentially impermeable, 6,546 1,995 23.0 0.1* 2.71 Limestone. 6,900 2,103 24.2 0.1* 2.77 no. metamorphosed or highly indurated sedimentary

6,940 2,115 20.2 O.I 2.59 no. and igneous rocks of Paleozoic age. 7,040 2,146 18.3 3.6 2.72 no. 7,114 2, 168 25.0 67.0 2.63 Sandstone. 7,210 2,198 27.7 0.4* 2.61 Slltstone. 7,306 2,227 19.4 O.I* 2.56 Limestone.

7,373 2,247 27.7 33.0 2.66 Sandstone. 7,456 2,273 28.3 55.0 2.65 no. 7,501 2,286 25.8 20.0* 2.66 no. 7,546 2,300 25.5 19.0* 2.63 n0 . 7,608 2,319 25.5 54.0 2.63 no.

7,648 2,331 28.2 7.0* 2.65 no. 7,660 2,335 32.7 255.0 2.68 Do. 7,753 2,363 25.8 6.3* 2.64 no. 7,791 2,375 26.8 54.0 2.63 n0 .

7,849 7.392 22.3 3.0* 2.63 no. 7,910 2,411 28.9 530.0* 2.63 Sandstone 7,954 2,424 27.0 56.0 2.66 Do. 8,032 2,448 27.1 5.7* 2.68 Do. 8,280 2,524 21.8 0.1* 2.67 Do. 8,343 2,543 24.3 7.0* 2.60 no.

8,388 2,557 22.8 3.2* 2.59 no. 8,414 2,565 33.4 700.0* 2.66 n0 . 8,484 2,586 26.6 7.2* 2.62 no. 8,566 2,611 29.4 28.0 ?.62 no. 8,594 2,619 31.1 181.0 2.65 no. 8,612 2,625 28.8 49.0* 2.64 n0 . 8,673 2,644 29.2 135.0* 2.67 no. 8,875 2,705 29.0 150.0* 2.64 n0 . 9,053 2,759 26.4 26.0* 2.63 no. 9,117 2,779 26.3 19.0 2.65 no. 9,172 2,796 28.1 23.0* 2.63 no. 9,294 2,833 24.0 20.0* ?.64 no. 9,560 2,914 23.9 5.1 2.61 no.

9,676 2,949 26.6 27.0 2.65 no. 9,770 2,998 26.7 49.0 2.67 no. 9,782 2,982 26.8 25.0* 2.65 no. 9,850 3,002 2R.7 280.0 2.63 no. 9,864 3,007 30.5 220.0* 2.66 Do. 9,970 3,039 25.3 18.0 2.63 no. 9,990 3,045 27.2 50.0 2.64 no.

10,114 3,083 18.9 0.9 2.60 Po. 10,357 3,157 16.5 0.5 2.61 Do. 10,363 3,159 15.1 0.7 2.61 no. 10,406 3,172 13.0 0.2 2.60 no. 10,706 3,263 16.3 1.1 2.61 no. 10,726 3,269 15.5 1.7 2.62 no.

38 I I I I I I I I T O.3

©

0 Core 1oo ooooooo oo oo oooooocoo ooooooo o © ©©

O O O Core 2 o ODD oooo o ocoooo oor> 'Core 3o o oo ooooooo oooo o oo ooooooo .j. O OOOO O 0 t- lU- ' + LLJ ^ fe + ^ LLJ Core 4« »

Q_ Core 5 LLJ Q 1O -l- + Core 6 CONVENTIONAL CORES 11 Sandstone X Core 11 o Limestone Core 12 X Argillite 12 SIDEWALL CORES XCore 13 + Sandstone 13 © Limestone Total Depth 13,254'

I I I I I I I I I I I_ I I I I l I I I l I I I I I I I I I I I I I I I I I I I I I I 1O 15 2O 25 3O 35 40 45 CORE POROSITY, IN PERCENT

Figure 11. Porosities of conventional cores and sidewall cores, COST No. GE-1 well. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 II I I I I I I II

2 r ° CONVENTIONAL CORES 1 e© <> Sandstone _ ® © o Limestone 3 < o oo»«o oCore 1 © ° © © x Argillite 1 SIDEWALL CORES 4 + Sandstone < ' © Limestone

5 - s . 0 0 6 + -I- + 01 0 + + ¥ So o oo ooo ooooooooo o oo o Core 2 2 £ ^, loo ooooooooo o oo o o oo o© o o ooo o + o ooooo Core 3 < DC h- ..-**** LLJ LLJ LLI 8 LLJ LL Core 4_

± I - 9 - h- Q. Q_ LLJ LLJ Q ' > » Core 5 Q 3 10 +"*"++ + + + + Core 6 t "*" -«- Core 7 11 -

> 4 : Core 11

\ t : Core 12 12 -

^ I Core 13 13 -_ / Total Depth 13,254' 4 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I II I I I I I I II O.1 O.5 1.0 5 1O 2O 5O 1OO 2OO 5OO 1OOO 2OOO 5OOO CORE PERMEABILITY, MILLIDARCIES

Figure 12. Permeabilities of conventional cores and sidewall cores, COST No. GE-1 well. 5OOO 4OOO 3OOO CONVENTIONAL CORES 2OOO Sandstone o Carbonate SIDEWALL CORES 1OOO + Sandstone O Limestone

5OO 4OO 3OO

CO 2OO UJ ++ O DC < 1OO Q

^ 5O + £ ++ 40 > 3O

=! 2O CD < UJ o cc 10 UJ Q_ UJ 5 DC 4 O O 3 2

1.0

O.5 O.4

0.3 0 0 O.2

O.1 15 2O 25 3O 35 4O 45 CORE POROSITY, IN PERCENT

Figure 13. Core porosities plotted against core permeabilities, COST No. GE-1 well.

41 PETROGRAPHIC SUMMARY

Robert B. Halley

INTRODUCTION chalky texture, and samples from 3,570 to 5,700 ft (1,090-1,750 m) are true argillaceous chalks Rock chips from drill cuttings at 56 and calcareous shales with globigerinid intervals were selected by hand and thin Foraminifera set in a matrix of clay and calcite sections were made to provide material for a mud containing coccoliths (figs. 14-16). brief petrographic overview of the lithologies Porosity in these fine-grained carbonate encountered in the COST No. GE-1 well. Each rocks is rarely visible in thin section because thin section contained between 10 and 30 rock pore size is small, on the order of 5 to 10 m or chips and was stained for potassium feldspar, less (Halley, 1978). Occasionally, pore space calcite, ferroan calcite, and ferroan is visible, as in the interior of Foraminifera, dolomite. Observations and classification of and in these cases the porosity is almost always these rocks, based on visual estimates, are primary. Most of the fine-grained porosity is tabulated in tables 4 and 5. The classification inferred to be primary as well. Scholle (1977a, procedure follows that of Folk (1968). Working c) has recently discussed chalk diagenesis and with cuttings tends to diffuse lithologic the importance of burial diagenesis in these boundaries by contaminating those cuttings from fine-grained carbonate rocks. In particular, the indicated sample depth with material from the role of pressure solution of original fine­ above. Nevertheless, cuttings provide valuable grained calcite with reprecipitation as cement information not only on composition, texture, appears to be a first-order control on porosity and depositional environment, but also on in chalks during burial. postdepositional alteration of the sediments. Porosity determined from well logs shows an Depositional environments are discussed irregular but discernible decrease with depth to elsewhere in this volume (Poag and Hall, this about 5,700 ft (1,750 m) in the COST No. GE-1 volume; Valentine, this volume). Composition well. A generalized plot of porosity versus and diagenesis of the rocks are reviewed here depth for the upper portion of the GE-1 well with particular reference to preservation, (fig. 17) shows that the fine-grained creation, or destruction of porosity. carbonates, some of which are not strictly true The stratigraphic section at GE-1 consists chalks because of their argillaceous matrix, predominantly of carbonate rocks above about appear to behave similarly to chalks with 7,150 ft (2,180 m) and of terrigenous clastic respect to porosity modification. The porosity rocks below this depth. Data from carbonate and decrease with depth is very similar to that in terrigenous clastic units in the well are listed chalks of the Gulf Coast and is perhaps separately in table 4 and are discussed intermediate between that of the Gulf Coast and separately below. Scotian Shelf when plotted with data from Scholle (1977a). CARBONATE ROCKS From about 5,700 to 7,200 ft (1,750-2,200 m), the GE-1 well contains a varied sequence of Beneath a blanket of shelf terrigenous generally medium grained calcarenites, dolomite, elastics and carbonates less than 800 ft (250 m) and anhydrite, with minor but significant thick, the GE-1 well encountered open-shelf and amounts of quartz sandstone, pyrite, and slope carbonate rocks to a depth of about 5,800 glauconite. Common rock types include oolites, ft (1,800 m). These rocks are primarily fine­ fossiliferous calcarenites, dolomite, micrite, grained, cherty and argillaceous limestones with and anhydrite. These rocks indicate a shallow- some calcareous shales. Most have a clayey marine, often restricted, carbonate-bank or carbonate-mud matrix with Foraminifera as the shelf setting. At about 5,800 ft (1,800 m), a most abundant allochem. Even the samples from conspicuous, carbonate-cemented, feldspathic, 1,230 ft (375 m), 1,620 ft (494 m), and 2,130 ft glauconitic sandstone suggests a major (649 m), which contain sand-sized fragments of regression, if not a hiatus, between the red and green algae, bryozoans, echinoderms, and shallow-water restricted-shelf carbonates and mollusks, are fine-grained, indicating a overlying fine-grained open-marine limestones. shallow-water, yet relatively low energy, This observation is supported by biostrati- environment. Most of these limestones have a graphic data (Poag and Hall, this volume).

42 Table 4. Petrography of carbonate rocks from the COST No. GE-1 well

[Depths, given In feet and meters, represent 30-ft (9-m) sampling intervals from stated depth downward: X = common; XX = very common; t= trace; ? = possible]

Explanation of abbreviations used in table: MM = micrite or clay matrix Grain size: Oo = ooids CSS = calcite-cemented sandstone F = fine Q « quartz FCS = fossiliferous calcareous sandstone M = medium Cement/Matrix: FOCC = fossiliferous oolitic calcareous C = coarse BC = blocky calcite OCC = oolitic calcarenite Constituent: FC = fibrous or bladed calcite Miscellaneous: Ar = arthropod skeletal fragments Si = chert or chalcedony CH = chert Al = algal D = dolomite T)ol = dolomite Br = bryozoan A = anhydrite GL = glauconite Mo = Mo Husk Porosity: PY = pyrite EC = echinoderm P = primary F = feldspar Fo = foraminifer S = secondary A = anhydrite Sp = sponge spicules Rock Name: FEC = ferroan calcite Pe = peloids FCL = fossiliferous calcilutite VC = calcite-filled fractures FCC = fossiliferous calcarenite

Depth Depth Grains Constituents Cements Porosity (ft) (m) MM F M C Ar Al Br Mo EC Fo Sp Pe Oo 0 BC FC Si D A PS Rock name Miscellaneous

600 183 x x XX XXX X Uncons. sand 1,050 320 x x XX X X FCL CH, GL. 1,230 375 x XXXX XX XX FCC CH. 1,680 512 x X XXXXX X X FCC CH. 2,130 649 x x X XX XX X FCL, FCC CH.

2,490 759 x x XXXX X X FCL, FCC CH, Dol. 2,850 869 x XXX XXX X FCC CH, Dol. 3,120 951 x x XX X FCL 3,570 1,088 x x XX XX FCL CH, GL. 3,660 1,116 x x XX XX X FCL CH.

4,020 1,225 x x XX X FCL 4,140 1,262 x x X X FCL CH. 4,350 1,326 x x X XX FCL 4,440 1,353 x XX XX FCL CH. 4,500 1,372 x x X FCL 4,830 1,472 x x X FCL

5,100 1,554 x x X FCL 5,370 1,332 xxx X X FCL-FCC PY, FEC, VC 5,700 1,737 x x X X FCL 5,760 1,756 x X XX FCC F, GL, FEC

5,790 1,765 x X X CSS F, GL 5,880 1,792 x x X X XXX FCSS GL, PY, FEC 6,030 1,838 x X XX FCSS GL, PY, FEC 6,090 1,856 x X XX X FCSS GL, PY 6,210 1,893 x X XXX XXX FOCC

6,540 1,993 x X XXXXX XXX FOCC PY, FEC 6,570 2,003 x XX XXX XX FOCC 6,600 2,012 x Dolomicrite A 6,720 2,048 x XXX X X OCC A 6,870 2,094 x Dolomicrite A 7,170 2,185 x x XX XX X X OCC Dol

Porosity varies widely and unsystematically 2,048-m) samples, but is also visible at with depth in the zone between 5,700 and 7,200 scattered intervals between 5,800 and 7,200 ft ft (1,750-2,200 m). Here, diagenesis has played (1,750-2,200 m), Dolomite, while common, is not a much more complex role in determinining abundant and is usually dense and nonporous. porosity distribution than in the overlying Significant intercrystalline dolomite porosity fine-grained carbonate rocks. In addition to was not observed, except where dolomite had some primary porosity in these rocks (figs. 18, partially occluded secondary porosity in 19), considerable secondary porosity is produced limestones. Three generations of calcite cement by dissolution of ooid grains and nuclei (figs. are distinguishable in the 5,700- to 7,200-ft 20, 21). Secondary porosity is particularly (1,750- to 2,200-m) interval. The earliest is a prominent in the 6,540- and 6,720-ft (1,993- and bladed to fibrous, isopachous, nonferroan cement

43 Table 5. Petrography of terrigenous clastic, metasedimentary. metamorphlc and metamorphosed volcanic rocks from the COST No. GE-1 well

[Depths, given in feet and meters, represent 30-ft (9-m) sampling intervals from stated depth downward; X = common; XX « very common; t = trace: ? = possible]

Explanation of abbreviations used in table:

Grain size: Cl - clay OA = quartzarenite F - fine C = calcite OT = quartizite M = medium D = dolomite AR = argillite C = coarse A » anhydrite Miscellaneous: Constituent: Porosity: Dol = dolomite Q quartz P = primary VRF = volcanic rock fragments K = potassium feldspar S = secondary MRF = metamorphic rock fragments P = plagioclase Rock Name: SLST siltstone R = rock fragments A = arkose SH = shale Mi = mica SA = subarkose P = primary porosity Cement/Matrix: FQA = feldspathic quartzarenite HC - hematite coatings Q " quartz VQ = vein quartz

Depth Depth Grain size Constituents (ft) (m) F M C Q K P R Mi 0 Cl C D A Rock name Miscellaneous

Terrigenous clastic rocks

7,500 2,286 X X X X X X X X X A-SA VRF, MRF, HC 7,740 2,359 X X XXX X XXX A-SA VRF, HC 8,340 2,542 X X X X X X A-SA HC, SLST 8,640 2,633 X X X X XX X X A Hr, SLST 8,930 2,722 X X X X X OA-SA HC, P 9,330 2,844 X X A HC, SLST

9,480 2,890 X X X X X X FOA-SA HC, SLST, SH 9,570 2,917 X X X X X FOA-SA HS, Dol, P 9,690 2,954 X X X X SA HC, P 9,830 2,996 X X X X X FOA-SA HC, Dol, SH 10,170 3,100 Dol, SLST, SH 10,380 3,164 X X X X FOA-SA Dol, SLST, SH 10,800 3,292 X X X X QA-FQA

Metasedimentary rocks

10,860 3,310 X X X X QT 10,920 3,328 X X X X QT 11,100 3,383 X X X X X AR 11,250 3,429 X X X X X AR 11,460 3,493 X X X X X AR HC

11,610 3,539 X X X X X AR HC 11,700 3,566 X X X X X X AR 12,150 3,703 X X X AR vo 12,600 3,840 X X X X X X AR,OT 12,720 3,877 X X X X X X AR,OT

Metamorphic and metamorphosed volcanic rocks

12,990 3,959 Hornfels, metamorphosed lavas 13,230 4,032 Hornfels, acidic lavas (trachyte).

around ooids, possibly an early submarine 6,030 ft (1,838 m). The fine-grained pyrite is cement. This is followed by a blocky calcite easily mistaken for a solid hydrocarbon until cement. Both generations are found in rocks viewed in reflected light. containing considerable primary pore space. The final cement generation is ferroan blocky TERRIGENOUS CLASTIC ROCKS calcite and occludes most remaining primary and some secondary pore space. Anhydrite cement Below 7,200 ft (2,200 m) lies a thick red- fills primary and secondary pore space in bed sequence consisting of red arkose, several samples, and extremely fine grained siltstone, and shale with minor amounts of dark pyrite is a final pore coating in samples from shale and quartzarenite. The angularity, poor

44 Figure 14. Chalk: partially silicified plank- Figure 16. Calcareous shale: planktonic Fora­ tonic Foraminifera and tabular mollusk(?) minifera (stained dark with calcite stain) fragments in a fine-grained calcitic and in fine-grained argillaceous matrix. siliceous matrix. Sample No. 3120; bar Sample No. 4830; bar scale is 300 ym. scale is 150 ym. sorting, and, in some samples, clay matrix of extensive. The base of this sequence at about these sandstones suggest that only a moderate 10,850 ft (3,307 m) marks another major break in amount of depositional porosity originally was the stratigraphic section of the COST No. GE-1 present. The primary porosity of these rocks well. has been diminished by compaction, pressure A sequence of slightly metamorphosed solution of quartz grains, quartz overgrowths, sedimentary rocks, now quartzites and and cementation by carbonates and anhydrite. argillites, occurs below about 10,850 ft (3,307 Diagenesis, together with the presumed moderate m) . Complete cementation in the quartzites and initial porosity of these sands, has left little pressure solution, chlorite cementation, and visible porosity. Nevertheless, some primary compaction in the argillites have obliterated porosity is preserved in samples from 8,130, porosity in these rocks (fig. 24). 9,570, and 9,690 ft (2,478, 2,917, and 2,954 m, The last two samples examined, 12,990 and respectively), as shown in figures 22 and 23. 13,230 ft (3,959 and 4,033 m) (fig. 25), contain The general lack of feldspar alteration through cuttings of true metamorphic and metamorphosed the 7,200- to 10,850-ft (2,194- to 3,307-m) igneous rocks, signifying another major break in interval suggests that moderate amounts of the sequence; these may represent the primary porosity will occur where cementation by basement(?) underlying the sedimentary sequence. quartz, carbonate, or anhydrite is not

Summary

The probability of reservoir rocks occurring below about 10,850 ft (3,300 m) is negligible owing to metamorphism evident in samples from below that level. Between about 10,850 and 7,200 ft (3,300-2,200 m), scattered m r I" intervals of significant porosity and permeability may be found in a red-bed sequence, where cementation and pressure solution/ compaction have not obliterated primary pore space. Significant secondary-porosity develop­ ment due to feldspar removal was not observed and does not seem likely in light of the fresh appearance of most feldspars in this interval. The best reservoir rocks are within restricted shelf carbonates occurring between 5,700 and Figure 15. Chalk: planktonic and benthonic 7,200 ft (1,750-2,200 m) , where high primary and Foraminifera in a fine-grained calcitic secondary porosity account for much of the best matrix. Note primary pore space in permeability encountered below 1,000 ft (304 m) foraminiferal chambers. Sample No. 4020; in the GE-1 well. Although possessing good bar scale is 300 ym. porosity, the fine-grained limestones above

45 about 5,700 ft (1,750 m) probably are so Im­ horizons. Data presented by Scholle (1977a, c) permeable as to make them unlikely candidates suggest that permeabilities of 3 md or less for reservoir rocks unless they are extensively should be expected throughout the interval from fractured or contain undetected permeable 1,000 to 5,700 ft (300-1,750 m).

DEPTH, IN FEET

1OOO 2OOO 3OOO 4OOO 5OOO 6OOO 7OOO 8OOO 9OOO 1O.OOO 11.OOO 12.OOO 13.OOO

LLJ O CC 40 LLJ

30

0.5 20

2OOO 3OOO

DEPTH, IN METERS

Figure 17. Generalized porosity-depth relation for upper 5,700 ft (1,730 m) of COST No. GE-1 well, compared to some typical chalks (modified from Scholle, 1977a, c).

46 Figure 18. Oolite: individual ooids with Figure 20. Moldic porosity (after ooids?) in first-stage fringe of small, iron-poor blocky calcite cement. Calcite is stained calcite crystals and second stage of dark; lighter dolomite partially fills large, scattered, iron-rich blocky calcite molds. Sample No. 6720; bar scale is 300 crystals. Dark material lining pores is ym. pyrite. Sample No. 6540; bar scale is 300 ym.

Figure 21. Oomoldic porosity in blocky calcite cement, stanined dark. Some porosity loss Figure 19. Same view as figure 18, but with has resulted from molds being secondarily crossed polarizers to emphasize extensive filled with anhydrite. Sample No. 6720; primary and secondary porosity (black). bar scale is 300 ym.

47 Figure 22. Arkose: fine-grained; feldspars Figure 24. Quartzite: pressure solution-modi- stained dark; minor amounts of rock fied grain contacts; no visible porosity, fragments and dolomite cement. Small Sample No. 10,860; bar scale is 300 m. amount of visible primary porosity labeled (P). Sample No. 8640; bar scale is 300 pm-

Figure 25. Volcanic rock: oriented feldspar laths in trachyte. Sample No. 13,230; bar scale is 300 Urn. Figure 23. Arkose: medium-grained; feldspars stained dark; minor rock fragments and opaque iron minerals (hematite?). Considerable primary pore space (P). Sample No, 9570; bar scale is 300 ym.

48 FORAMINIFERAL BIOSTRATIGRAPHY, PALEOECOLOGY, AND SEDIMENT ACCUMULATION RATES

C. Wylie Poag and Raymond E. Hall

INTRODUCTION interpreted from the generic composition of the benthic assemblages, from the relative abundance Very little geologic information has been of planktic specimens, and from the general published concerning the middle and outer parts diversity of species in each sample. The time of the Florida-Georgia Continental Shelf. The scales and biostratigraphic zones used here are only previous stratigraphic studies based (in those of Berggren and van Couvering (1974) for part) on foraminifers are those of Bunce and the Cenozoic, and van Hinte (1976) for the others (1965) and Schlee (1977), who studied Cretaceous. Interpretation of the data pre­ cores from holes drilled by the Joint Oceano- sented here is preliminary, and modifications graphic Institutions for Deep Earth Sampling can be expected as new data arise and further (JOIDES); and of Hathaway and others (1976), who analyses take place. briefly described the data from core hole 6002 of the USGS Atlantic Margin Coring Project STRATIGRAPHY AND CORRELATION OF STRATA (AMCOR). In addition to the analysis of the IN THE GE-1 WELL COST No. GE-1 well samples presented here, we reinterpreted the data from JOIDES core holes J- Upper Pliocene Rocks (^364 to ^544 ft; 1 and J-2 and more thoroughly discuss the data (^111 to ^166 m). The youngest sample available from AMCOR 6002. The biostratigraphic analysis from the GE-1 well is of late Pliocene age (fig. of the Neogene strata is a joint effort by Poag 25). We assume that the upper 131 ft (40 m) of and Hall. The other analyses and interpre­ sediment (down to 364 ft (111 m )) is tations presented herein are those of Poag. Pleistocene, based on comparison with samples Rotary-ditch samples from the GE-1 well from the other core holes (J-l, J-2, AMCOR were collected at 30 ft (9.1 m) intervals, 6002). At 386 ft (118 m), Globorotaloides beginning at about 364 ft (111 m) below the planispira and Globigerina incisa are present KB. Each sample is a composite of approximately along with Neogloboquadrina dutertrei and 30 ft (9.1 m) of drilled sedimentary rocks. We Sphaeroidinella dehiscens. Globigerinoides examined each sample in the Miocene through obliquus appears at 420 ft (128 m), and Pliocene interval and in the lower Eocene Globorotalia miocenica appears at 447 ft (136 through middle Coniacian interval; otherwise, we m), along with Globigerina apertura and examined samples at 90-ft (27.3-m) intervals. Globigerinoides conglobatus. Lower in this Samples below 8,840 ft (2,695 m) have been interval, other typical Pliocene species appear, reported to contain no foraminiferal assemblages such as Globigerina decoraperta, Globorotalia (Amato and Bebout, 1978), and we have not exilis, and Globorotaloides planispira. The examined that part of the section. We did, entire interval is assigned to Zone N. 21 of the however, examine foraminiferal assemblages from late Pliocene. Zone N. 21 is also present, but 77 sidewall cores .taken in the GE-1 well. (See thinner, in J-l and J-2 (on the southern rim of Amato and Bebout, 1978.) We sampled the JOIDES the embayment), but the entire Pliocene is J-l and J-2 core holes at 1 ft (30-cm) intervals missing in AMCOR 6002 (on the northern rim of and the AMCOR 6002 core hole at 30-ft (9-m) the embayment) (fig. 27). intervals. Depths cited for the JOIDES and Middle Miocene Rocks (^364 to ^544 ft (^166 AMCOR core holes are given as distance to ^219 m)). We observed no evidence of lower penetrated below the sea floor (bsf), but depths Pliocene Zones N. 19 to N. 20 or of upper for the GE-1 well are in distance below the Miocene Zones N. 13 to N. 18 in the GE-1 well. KB. For consistency, biostratigraphic changes A hiatus of 10 m.y. separates the upper Pliocene that occur between samples are placed halfway and middle Miocene rocks (fig. 25). At 561 ft between sample tops. (171 m), Globorotalia praefohsi, Orbulina In this analysis, the age of the rocks is suturalis, Turborotalia siakensis, and a host of based primarily on planktic foraminifers, but additional planktic species mark middle Miocene these determinations are supplemented by benthic Zone N. 11/12. At 589 ft (179 m), Globorotalia species when applicable. Paleoenvironments are peripheroronda and jG. peripheroacuta appear.

49 a to pt o N in t» o FEET BELOW KELLY BUSHING FI m PJ 3 -1 > > * 0 (D OQ Cn O (D 1-3 H O O O o 5 5 n to PJ PJ 1 1 1 1 1 1 ? » 1 1 1 1 i ' . 'T . ' -^ - i < SOQ S H. ro 8 FEET BELOW SE :A FLOOR 8 r £r , ° , . , J 1 1 1 1 1 1 1 1 1 i i i i i i LOWER MIDDLE UPPER fl> rr 3 (D UPPER EOCENE MIDDLE PLEIST )$#> CO ET (D OLIGOCENE OLIGOCENE! MIOCENE PLIOCENE w%$> (D (D PJ z I2 ! z N> ro BIOZONES 55 tD O ' ' « Pt O (D S fl> S o p> ET O 3 O, CO r 3 pt (D (D 3 W 33 ET H- r* |0||M.SHELFSLOPE1 Is Pt O R" O PALEOENVIRONMENT S (t> o H- O 3 :i CL ^-s O Q XX CL I M o N M Pt to o * 3 « 2 O (B (D Pt CO > pt to PJ ET Hi L b' 9 M (P O ? S 0 S M 3 § B PJ O- 400 3 DO 200 100 0 3 7$ r* ^r CL p )-u O CL (P p> 1 i i i i i i i i i 1 i i i i i 1 1 1 1 1 1 1 1 1 1 i i i 1 M 3 OQ CO (D CL . CO ? !?« METERS BELOW SEA FLOOR Ef W 03 H O O ET C O (P CO C M ETOQ H> H- ET (D o fl> - 3 3 3 £ 3 OQ *Td (D w . O M .. n> rt c o n> (D pj rt 3 H- (P CO n n rt PJ M r* O CO OQ O V ET (P LOWER AND MIDDLE EOCENE MIDDLE EOCENE UPPER EOCENE B g (P Hi 3 e- o a . fl> M M (P "D i -D Hi O D ! "D r<£ $ O ^ ? O M P- O CO « P> II ° ro n -o Sr, c§ ^-S n co "tfS o-o O H* S i M rt-O H- fl> « ro H- sr p- O CO 3 O fl> O (P 5 X n> (P 3 PJ Pt O O r, 3 pt co hj O S^ p> (D M (P tt> H. (B 3 PJ 3 rt OQ (P fl> f* CO O O W H"

ET rt (D O a 900 800 700 600 500 1 i i 1 i i i 1 i i 1 i « o n> oo to of 14 m.y.; fig. 25). Chiloguembelina cubensis, ouachitaens is , and Globigerinita unicava Globigerina officinalis. and members of the primitiva appear at 824 ft (251 m), marking Zone Globigerina ciperoensis group occur at 725 ft P. 20. At 915 ft (279 m), Turborotalia (221 m) and appear to represent Zone P. 21 of increbescens, Globoquadrina tapuriensis, and the middle Oligocene, because Turborotalia Pseudohastigerina barbadoensis mark Zone P. 19 kugleri, which is typical of Zone P. 22, is of the lower Oligocene; Turborotalia euapertura absent. Turborotalia ampliapertura. also was first observed in this sample. Zone P. Globoquadrina gortanii. Globigerina 19 continues in the sample at 1,184 ft (361 m),

PALEOENVIRONMENT PALEOENVIRONMENT i SHELF 1 M. SHELF! o SHELF SLOPE 1 1 i SHELF] M. SHELF o SHELF! SLOPE 30m ' 100 m ' 200m 500m 1 1 30 ml 100 m 200 m 1 500 m

- r-5000 £ < 5500- - 1- U(fl3 z TURONIA -350CT - - - UC4 J '- z ; < o - o - LOWER z -5500 f 2 PJ _ s o < - CJ o- o r 4000- ^> - C SG-* .

UC 10 - 6000- ^ o -

_^X^ o -4000 f - UC9 - ^*v '. - - -6000_ __-- - - - z < z 4500 e O " - 5 .^ "^^ rO z W5 ^ ~~ - < J - UC8 m _i _ - < o 6500 - r o>§-

-450Q _

o - - o_ - 5 - 500 o - o - o Z CM 5000- < fS \ CJ < i^ z 10 o o - CJ 13 o " 7000 o - r-* S~^ m - ^ X tf~^< ^

No. GE-1 well. Biozone designations for Cenozoic from Berggren and van Couvering (1974); for marks at left column under derrick symbol, location of top of examined cuttings samples: I. Shelf,

51 J-l AMCOR COST 6002 GE-I SEA LEVEL J-2 B

SEA FLOOR

500- -500

-1000

1500- -1500

-2000

2500- -2500

3000- 3OOO

1000 1000-

3500- 3500

Figure 27. Approximate dip section of Cenozoic rocks near axis of Southeast Georgia Embayment showing foraminiferal biostratigraphy and paleoecology. Line of section is drawn through the COST No. GE-1 well. J-l and J-2 wells near southern rim and AMCOR 6002 well near northern rim of embayment are projected onto line of section. This projection causes the reversed dip from GE-1 to J-2. Vertical exaggeration X 63. where Globoguadrina sellii accompanies an Maestrichtian Rocks (^3.514 to ^3.671 ft assemblage of small globigerinids. The middle (^1,071 to ^1.119 m)). The lowest Paleocene and lower Oligocene units of GE-1 are also zones (P. 1-P. 3) and uppermost Maestrichtian present at AMCOR 6002 and J-2, but only the zone (UC 17) seem to be missing in the GE-1 well middle Oligocene is represented at J-l (fig* (fig. 25). The hiatus represents a gap of 8 27). m.y. Zone UC 16 is represented at 3,530 ft (1,076 m) by a diverse assemblage of planktic Eocene Rocks (^1,217 to 3.300 ft (^371 to species including Globotruncana gansseri, _G. ^1,006 m)). A hiatus of 5 m.y. separates the patelliformis, G^. area, G_. stuartiformis, Q. lower Oligocene Zone P. 19 from the upper Eocene stuarti. G_. elevata, Rugotruncana rugosa, Zone P. 16 in the GE-1 well (fig. 25). The Globotruncanella petalloidea, Ps eudot extularia presence of Hantkenina primitiva and deformis, _P_. elegans, and Pseudoguembelina Globigerinatheka index tropicalis, along with costulata. At 3,619 ft (1,103 m), a similar numerous Lepidocyclina and nummulitids at 1,266 planktic assemblage is accompanied by the ft (386 m), marks the highest occurrence of highest occurrence of Bolivinoides miliaris. upper Eocene Zone P. 16. Globigerinatheka which signifies the lower part of Zone UC 14 mexicana mexicana. Turborotalia cerroazulensis Campanian Rocks (^3,671 to ^4,130 ft cerroazulensis, and JT. cerroazulensis (v/q.119 to ^1.259 m)). At 3,684 ft (1,123 m) cocoaensis. also typical of Zone P. 16, occur in Neoflabellina rugosa and Globotruncana various samples down to 1,873 ft (571 m). At ventricosa mark the middle part of Zone UC 12 1,972 ft (601 m), Zone P. 15 is indicated by (fig. 25); these species are accompanied by Globigerinatheka index rubriformis, Q. Rugoglobigerina mi lamensis, Archaeoglobigerina subconglobata luterbacheri, and Acarinina cretacea, A^ blowi, Globotruncana linneiana, and primitiva. A similar sequence of upper Eocene (J. contusa. Zone UC 13 (youngest Maestrich­ zones is present at J-l and J-2 on the southern tian), if present, is less than 30 ft (10 m) rim of the embayment. But AMCOR 6002 did not thick. At 4,072 ft (1,241 m), the highest penetrate deeply enough to reach the lowest part occurrence of Bolivinoides decoratus and of the upper Eocene (fig. 27). Kyphpyxa christneri indicate Zone UC 10. Middle Eocene Zone P. 14 is represented at 2,162 ft (659 m) in the GE-1 well by Santonian Rocks (^4.130 to ^4.760 ft Truncorotaloides topilensis, Acarinina senni, (-^1.259 to ^1,451 m)). The first indication of and _A. broedermanni. Zone P. 13 is indicated at species of Zone UC 9 occurs at 4,120 ft (1,256 2,431 ft (741 m) by Orbulinoides beckmani and m), where Marginotruncana angusticarenata and Globigerinatheka kugleri. Similar assemblages Globorotalites multiseptus first occur (fig. occur in the lowest parts of J-l and J-2 (fig. 26). Bolivinoides strigillatus occurs at 4,285 27). ft (1,306 m). At 4,383 ft (1,336 m) the first The interval from 2,568 to 3,300 ft (783- occurrence of Globotruncana concavata indicates 1,006 m) in the GE-1 well contains poorly Zone UC 8; Sigalia appears 90 ft (30 m) lower; preserved, sparse foraminiferal assemblages Whiteinella archaeocretacea and _W. aprica(?) which are difficult to interpret. Scattered occur at 4,668 ft (1,423 m). occurrences of species such as Acarinina quetra, Coniacian Rocks (^4,760 to ^5.433 ft _A. nitida, Morozovella wilcoxensis. M. aequa. (^1,451 to ^1,656 m)). The first evidence for and _M. subbotinae indicate that lower Eocene Zone UC 7 occurs at 4,776 ft (1,456 m), where rocks are probably present, but that the Marginotruncana sigali and _M. marginata specimens are too scarce to permit more refined appear. Clavihedbergella simplex occurs at zonation (fig. 25). 4,950 ft (1,509 m); Marginotruncana schneegansi occurs at 5,138 ft (1,566 m). Reworked spec­ Paleocene Rocks (^3.300 to ^3,513 ft imens of the Cenomanian markers Rotalipora (^1.006 to ^1,071 m)). The precise position of cushmani and _R. greenhornensis occur at 5,062 the youngest Paleocene rocks in GE-1 is also and 5,219 ft (1,543 and 1,591 m), respectively difficult to distinguish. Scattered specimens (fig. 26). of Planorotalites pseudomenardii, Subbotina Lower Turonian Rocks (^5.433 to ^5,810 ft triloculinoides, and Acarinina pusilia pusilia, (^1,656 to ^1,771 m)). Globotruncana helvetica which are lower upper Paleocene species, occur the marker for Zone UC 5 of the middle Turonian, as high as 3,055 ft (931 m), but sidewall cores appears at 5,482 ft (1,671 m), but it is at 3,211 ft (979 in) still contain lower Eocene accompanied by the Zone UC 4 markers species. The highest occurrence of apparently Praeglobotruncana stephani and _P_. turbinata. in situ Paleocene foraminifers is at 3,317 ft This faunal association and the evidence of (1,011 m), where Planorotalites pseudomenardii, shoaling water depth at 5,433 ft (1,656 m) (fig. Morozovella kolchidica, M. velascoensis, 26) suggest that upper and middle Turonian rocks Acarinia angulata, A_* pus ilia pus ilia, Subbotina have been eroded away. velascoensis, and _S_. triloculinoides occur Albian Rocks (^5.810 ft to ^7.405 ft together and indicate Zone P. 4 of the lower 6M..771 m to ^2.257 m)). Below 5,810 ft (1,771 upper Paleocene. Thus, a hiatus of 2-3 m.y. may m), the foraminiferal faunas of the GE-1 well separate the lower Eocene from the upper are largely sparse benthic assemblages that Paleocene (fig. 25). contain no distinct age-diagnostic species. NO

53 evidence of Cenomanian assemblages exists* The are unconformities caused by subaerial ero­ Albian interval was identified on the basis of sion: late Albian to early Turonian, late palynomorphs. Based on the lithology, paleo- Maestrichtian to late Paleocene, late Paleocene environment, and comparative depositional rates, to early Eocene, and late Pliocene to early discussed more fully below, we conclude that Pleistocene. The late Oligocene to middle Cenomanian rocks are missing from this well Miocene and the middle Miocene to late Pliocene (fig. 26). hiatuses were caused by nondeposition in a Aptian to Valanginian(?) Rocks (^7,405 to marine environment, and the late Eocene to early ^10.990 ft (^2.257 to <3.350 m)). Palynomorphs Oligocene hiatus is the result of submarine examined by R. Christopher and J. Bebout (USGS, erosion. written commun., 1978) suggest that Aptian rocks PALEOECOLOGY are present at 7,411 ft (2,259 m). At 9,525 ft (2,903 m), an Early Cretaceous flora is still The depositional environments of sediments present, but the species are long ranging, penetrated by the GE-1 well vary from terres­ indicating an age of Barremian to Aptian (but no trial (nonfossiliferous) biotopes to ocean older than Barremian). The remaining 1,467-ft depths equivalent to those of the modern upper (447-m) interval above basement in the GE-1 well continental slope (650-1,650 ft (200-500 m)). (9,525-10,758 ft (2,903-3,279 m)) is not fossil- Most of the Cenozoic and Upper Cretaceous rocks iferous, but one can estimate the age of the accumulated in marine continental-shelf bio- oldest sedimentary rocks as approximately topes, but the Lower Cretaceous rocks are Valanginian (Early Cretaceous) on the basis of largely nonmarine and marginal-marine an extrapolation of sediment accumulation deposits. The following paleoecologic dis­ rates. (See further discussion below.) cussion proceeds from the oldest to youngest rocks in order to preserve the proper STRATIGRAPHIC DISCUSSION geochronologic perspective. Valanginian(?) to Aptian Rocks. The As shown above, much of Cenozoic and Late Valanginian(?) to Aptian rocks are largely red Cretaceous time is well represented in the shales and sandstones of terrestrial origin, as Southeast Georgia Embayment by fossiliferous indicated by the sparsity of marine fossils and marine sedimentary rocks that show evidence of the presence of terrestrial palynomorphs in some thickening from surrounding core holes into the samples. To date, the only marine fossils ob­ GE-1 well. In contrast, rocks of Early Cre­ served in this interval are an assemblage of taceous age are largely sparsely fossiliferous dinoflagellates at 8,690-8,891 ft (2,649-2,710 or barren. Seven regional hiatuses interrupt m) Additional shallow-marine intervals are the depositional record. They occur between the indicated by the presence of anhydrite and Albian and Turonian, upper Maestrichtian and dolomite, but no marine fossils have yet been upper Paleocene, upper Eocene and lower Oligo- observed in them. cene, middle Oligocene and middle Miocene, Albian Rocks.--Albian sediments (cuttings middle Miocene and upper Pliocene, and upper samples) below 7,713 ft (2,351 m) contain Pliocene and lower Pleistocene (figs. 26, 27). scattered, sparse assemblages of shallow-water, A comparison of the Cenozoic rock record in inner shelf, benthic foraminifers, such as nearby core holes (J-l, J-2, and AMCOR 6002; miliolids, along with bioclastic limestones, fig. 27) reveals that the biostratigraphic zones aggregates of anhydrite or gypsum crystals, limy present there are nearly identical to those in casts of microgastropods and bivalves, and GE-1. The hiatuses at the top of the Eocene, numerous ostracodes. These assemblages indicate top of the Oligocene, and top of the Miocene are restricted, high-salinity, carbonate environ­ present in each core hole and can be traced ments. elsewhere throughout the embayment using high- resolution seismic-reflection profiles (C. A distinctly different assemblage is Paull, USGS, oral commun., 1978). They cor­ present between 6,105 and 6,466 ft (1,861-1,971 m) It is characterized by large, coarse­ respond to periods of low global sea level, as described by Vail and others (1977b; fig. 28). grained, agglutinant foraminifers of the genus The hiatuses at the tops of the Paleocene, Ammobaculites(?). Small specimens of Cyclammina Maestrichtian, and the Albian are also region­ sp. and fragile-walled Trochammina sp. are less ally recognizable on Common Depth Point (CDP) abundant. The ammobaculitid tests are composed seismic-reflection profiles (Dillon, oral of quartz grains held together by calcareous commun., 1978) but are not yet corroborated by cement, but nonagglutinated calcareous species additional deep wells. They too correspond to were not recovered in this interval. The pre­ times of low global sea level (Vail and others, sence of oolitic limestone and free ooids 1977b). attests to vigorous water agitation within this carbonate province. The water depthsd of quartz By comparing the paleobathymetric trends grains held together by calcareous cement, but that are discussed in the following section with nonagglutinated calcareous species were not estimates of sea-level drops given by Vail and recovered in this interval. The presence of others (1977b), we conclude that the surfaces oolitic limestone and free ooids attests to marking the following hiatuses in the GE-1 well vigorous water agitation within this carbonate

54 province. The water depths were probably Maestrichtian Rocks. Upper-slope faunas equivalent to those of today's middle shelf persist into the lower half of the Maestrichtian (fig. 26). beds (3,710-3,595 ft (1,131-1,096 m)). Benthic The upper part of the Albian section, species include Bulimina kickapooens is, 6,105-6,466 ft (1,861-1,971 m), contains a Brizalina incrassata. Globorotalites sparse assemblage of benthic species michelinianus, Bulimina trihedra. Bolivinita (Gavelinella, Lingulogavelinella), some of which eleyi. Planulina taylorensis. and others. A are characteristically pyritized. This shallowing of water to outer-shelf depths is assemblage indicates an inner-shelf environment indicated in the upper Maestrichtian interval (fig. 26). from 3,595 to 3,513 ft (1,096-1,071 m). The Turonian Rocks. The lowest Turonian rocks faunas there remain rich in specimens and in the GE-1 well contain sparse faunas similar species, but the deeper-water species to those of the upper Albian rocks; and they disappear. Typical benthic forms are represent, likewise, inner-shelf conditions. Arenobulimina americana. Gavelinella danica. Between 5,482 ft and 5,515 ft (1,671 m-1,651 m), Marssonella sp. Stensioina pommerana. however, the increased abundance of Bolivinoides__Reophax draco texanus. Ammodiscus Lingulogavelinella turonica, accompanied by a glabratus. Gaudryina austinana. Clavulinoides moderately well developed planktic assemblage, aspera, _C_. trilatera, several dentalinids (fig. suggests a deepening to middle-shelf water 26K depths (fig. 26). Paleocene Rocks. The oldest Paleocene Coniacian Rocks. The lower two-thirds of sediments, 3,513-3,448 ft (1,071-1,051 m), the Coniacian section, 5,416-4,990 ft (1,651- contain sparse faunas of both benthic and 1,521 m), contains a sparse assemblage of small planktic species. Gavelinella danica is one of benthic specimens belonging to Neobulimina the few identifiable forms. Several reworked canadensis, Gavelinella sp. , and Lenticulina Cretaceous forms also are present. This sp. This assemblage and the accompanying assemblage suggests middle-shelf conditions ostracode fauna suggest an inner-shelf (figs. 26, 27). A richer and deeper-water environment. The upper Coniacian beds, 4,951- assemblage occurs above this between 3,448 and 4,760 ft (1,511-1,451 m), contain a better 3,284 ft (1,051-1,001 m). Here, prominent developed planktic assemblage, plus a moderately benthic species include Gavelinella danica, large assemblage of several species of Eponides bollii, Spiroplectammina trinitatensis. Gavelinella (cf. berthelini. cf. thalmanni, cf. Dorothia bulleta. Marssonella dentata, sculptilis, cf. clementiana) that suggests Textularia sp., Lenticulina spp., Globobulimina middle-shelf environments (fig. 26). sp., Cibicides spp., and Bulimina sp. This Santonian Rocks. The lower part of the assemblage, accompanied by an increase in Santonian interval, 4,760-4,530 ft (1,451-1,381 planktic species, indicates outer-shelf m), contains assemblages of small benthic conditions (figs. 26, 27). species (Lingulogavelinella sp., Gavelinella cf. Eocene Rocks. The interval assigned to the berthelini, Bolivinita cos tata, and Eouvigerina lower and lower middle Eocene, 3,284-2,463 ft, aculeata) and a moderately rich planktic (1,001-751 m)) contains very few benthic assemblage, which suggest continued middle-shelf specimens. A few scattered tiny specimens of conditions. Between 4,530 and 4,169 ft (1,381- unidentified calcareous species appear to 1,271 m), the benthic fauna is much richer and indicate middle-shelf conditions (figs. 26, more diverse than any of the older 27). This assemblage contrasts significantly assemblages. Large specimens of Gavelinella with the contents of two sidewall cores at 3,750 clementiana are abundant, but the most prominent and 3,643 ft (1,129.4 and 1,110.5 m), which forms are large agglutinants such as Lituola contain abundant segments of large Bathvsiphon. taylorensis, Plectina watersi, Gaudryina rare Melonis cf. pompilioides, and abundant ellisorae. Mart inot t i ella sp., and Liebusella(?) large spherical radiolarians. This association sp. Planktic specimens are abundant and suggests that depths may have reached the Globorotalites multiseptus has its highest equivalent of today's upper to middle contin­ occurrence here. This assemblage indicates that ental slope. Extensive silicification of all the embayment had deepened to outer-shelf depths the calcareous elements in this interval, and (fig. 26). perhaps dissolution as well, complicates the Campanian Rocks. The Campanian assemblages interpretation even further. Obviously, the represent the deepest water conditions that nature of the paleoenvironment and the precise prevailed in this area during the Cretaceous. age of this interval must be determined by Both planktic and benthic assemblages are rich further analysis; but, for the present, we in specimens and species. Benthic forms include accept a middle-shelf interpretation, which is Neoflabellina rugosa. Osangularia sp., supported by the nannofossil analysis Clavulinoides trilatera forma concava, (Valentine, Calcareous nannofossil biostra- Pleurostomella sp., Heterostomella sp., tigraphy, this volume). Kyphopyxa christneri, Bolivinoides decoratus. The upper part of the middle Eocene, 2,463- and many others, which suggest conditions 2,103 ft (751-641 m), and the lower part of the equivalent to those of the modern upper upper Eocene, 2,103-1,627 ft (641-496 m), continental slope (fig. 26). contain a rich benthic assemblage including

55 frequent Uvigerina and Bulimina that indicate Oligocene at these sites, as indicated by the outer-shelf deposition (figs. 26, 27). abundance of Bulimina sculptilis and J5_. Nummulitids and operculinids, which are normally alazanensis (fig. 27). At J-l, lower Oligocene associated with shallow carbonate banks and rocks are missing, and the thin middle Oligocene reefs (for example, Frost, 1977), are also section accumulated in an outer-shelf present here, but appear to have been displaced environment; Brizalina, Florilus, Pararotalia, from shallower water along the rims of the and small planktic specimens are predominant. embayment. These larger foraminifers are Miocene Rocks. Following a period of present in greater abundance nearby in the J-l, erosion or nondeposition in the late Oligocene J-2, and AMCOR 6002 core holes, where they and and early Miocene (fig. 27), deposition of upper lepidocyclinids are the predominant benthic continental slope sediments continued at the GE- foraminifers in shallower-water carbonate-bank 1 well site during the middle Miocene. Rich and assemblages (fig. 27). The shallow-carbonate- diverse planktic assemblages accompany diverse bank environment, typified by abundant benthis assemblages typified by Bolivina lepidocyclinids, nummulitids, and a hash of floridana, Siphogenerina lamellata, Uvigerina cheilostome bryozoans, extended to the GE-1 well rustica. U.. peregrina, Laticarinina pauperata. site during the late Eocene, as seen in the Stilostomella sp., and large Vaginulinopsis. interval from 1,627 to 1,332 ft (496-406 m) At AMCOR 6002 and J-l, the middle Miocene (figs. 26, 27). The assemblages at this environments were somewhat shallower, and location and in the surrounding core holes foraminifers and other calcareous fossils are suggest that a broad carbonate platform rare (fig. 27). Instead, the predominant micro- developed at this time in waters of inner- and fossils are diatoms, which are accompanied by middle-shelf depths. silicoflagellates and radiolarians. Glauconite Near the end of the Eocene, outer-shelf and quartz sand are abundant; and fish conditions returned to the site of the GE-1 well vertebrae, teeth, and scales, and polished (1,332-1,217 ft (406-371 m)), and they prevailed phosphoritic grains are common. The diatom at J-2 and AMCOR 6002 as well (figs. 26, 27). assemblages contain shallow-water benthic forms Characteristic benthic species include and also species indicative of cool-water masses Textularia adalta. T\ recta. .T. dibollensis. (W. H. Abbott, South Carolina Geological Survey, large Lenticulina spp., Sigmoidella plummerae, written commun., 1978). Abbott suggests that Brizalina caelata, j^. huneri, B_. jacksonensis. water depths within the photic zone of about 300 Siphonina advena. Nodosaria spp., Gyroidina ft (100 m) or less were necessary to allow the octocamerata, Eponides jacksonensis, Cibicides benthic diatoms to photosynthesize. The few pippeni, Dentalina cocoaensis group, Uvigerina calcareous benthic foraminifers within these cocoaensis, Anomalina bilateralis, and siliceous assemblages are mainly Bulimina Cibicidina blanpeidi. Lepidocyclinids and elongata. Florilus mediocostatus. _F. nummulitids are still present, but appear to pizzarensis. and Epistominella sp., forms whose have originated from shallower biotopes such as modern counterparts are characteristic of that concurrently present at J-l (fig. 27). shallow waters having a high nutrient content Oligocene Rocks. The sediments bounding (Seiglie, 1968). A similar assemblage has also the hiatus separating Oligocene from Eocene been reported beneath the delta-front water mass rocks in the GE-1 well bear evidence of weather­ of the Mississippi River (Lankford, 1959). ing. Polished, abraded, and oxidized specimens Lower Miocene sediments are missing or are of foraminifers are common and are associated very thin in the GE-1 well, but are present in with polished glauconitic grains and abraded J-2 and AMCOR 6002 and perhaps at J-l (fig. mo Husk fragments. This characteristic mineral 27). The Bulimina/Florilus/Epistominella and fossil assemblage is also present at the assemblage is present in the lower Miocene same stratigraphic position in the J-2 and AMCOR section at AMCOR 6002, indicating middle-shelf 6002 core holes. deposition. At J-2 the lower half of the lower The indigenous faunas of the Oligocene in Miocene section also contains this high- the GE-1 well originated in deeper water than nutrient, middle-shelf assemblage, but the upper any of the older Cenozoic assemblages, except half of this unit is characterized by abundant perhaps those of the early Eocene (figs. 26, planktic species and predominantly Brizalina 27). Forms such as Planulina cooperensis, among the benthic assemblage. This is an outer- Bulimina sculptilis, Globobulimina sp., shelf assemblage. Chilostomella sp., Cancris sp., large The only upper Miocene strata recognized in Saracenaria sp., Sigmomorphina sp., the area were penetrated in the AMCOR 6002 core polymorphinids, and abundant diverse planktic hole (fig. 27). Diatomaceous strata are still faunas indicate deposition in depths equivalent common there; but, in addition, the strata to today's upper continental slope. On the contain a foraminiferal fauna of low diversity other hand, the lower Oligocene sediments at J-2 including primarily Bulimina elongata, Florilus and AMCOR 6002 accumulated in outer-shelf con­ grateloupi, and Lenticulina spinosa, a middle- ditions, as indicated by the predominance of shelf assemblage. uvigerinids and reduced planktic assemblages; Pliocene Rocks. Upper Pliocene sediments but slope faunas are present in the middle of outer-shelf origin lie above the early

56 Pliocene-late Miocene hiatus in the GE-1 well The Quaternary supercycle Q began with (fig. 27). Characteristic benthic species erosion and nondeposition during the early include Lenticulina americana, _L. spinosa, Pleistocene and culminated with middle-shelf Bulimina marginata, Ji.. elongata, Uvigerina sp., conditions that currently prevail at the GE-1 Fursenkoina sp., Fleetofrondicularia sp., and J-2 sites. Brizalina spp., Nodosaria sp., and Siphogenerina Corroboration of the third-order sea level lamellata. Planktic species are also abundant cycles of Vail and others (1977a, b) requires and diverse. Slightly deeper water (upper- further analysis; but evidence for some of them, continental-slope depths) prevailed during the such as T01 (early to middle Oligocene) and late Pliocene at J-2 (fig. 27), where a rich TM2.2 (middle Miocene), is readily apparent planktic assemblage is accompanied by a diverse (figs. 28A., _B). benthic assemblage of predominantly Uvigerina spp., Bulimina sp., and Cassidulina spp. At J- SEDIMENT ACCUMULATION RATES 1, the lower part of the upper Pliocene beds contains abundant specimens of Florilus By plotting the thickness of sedimentary grateloupi, small Uvigerina sp., and Cassidulina units against a geochronologic time scale for carinata. accompanied by a lesser number of the Cretaceous (van Hinte, 1976) and the Lenticulina spinosa, Bulimina elongata, and Cenozoic (Berggren and van Couvering, 1974), we Brizalina spp. These indicate outer-shelf have estimated the rate of sediment accumulation deposition. The upper part of the upper at the site of the GE-1 well (fig. 29). Using Pliocene at J-l was deposited in middle-shelf van Hinte's (1978) notation, the formula for environments, as indicated by the predominance this calculation is uR = T , where uR = among the benthic foraminifers of Trifarina sp., Cibicidoides sp., and Hanzawaia sp. The 10A Pliocene is missing at AMCOR 6002 (fig. 27). uncorrected accumulation rate in cm/1,000 years Pleistocene Rocks. Pleistocene paleo- (not corrected for compaction and thus a minimum environments are estimated to have ranged from rate; see van Hinte, 1978), Tp = present thick­ inner- to middle-shelf depths based on the sandy ness of a given unit in meters, and A = duration nature of the sediments and a general knowledge of deposition in millions of years. These rates of Pleistocene glacioeustatic events. We have must be taken only as estimates, especially for no good sample evidence from the boreholes, the Cretaceous, when absolute dates are less however (fig. 27). reliable and less abundant, and for which two different time tscales have recently been pub­ PALEOECOLOGIC DISCUSSION lished (Obradovich and Cobban, 1975; van Hinte, 1976). In spite of the generalized nature of The sequence of paleoenvironments repre­ the results, the relative changes are useful in sented in the GE-1 well and surrounding core interpreting the geologic history of the region, holes can be correlated with the second-order and the rate curves can be used for extrapo­ cycles (supercycles) of global sea level change lating the age of unfossiliferous sections, outlined by Vail and others (1977a, b; figs. detecting hiatuses, and estimating the age, 28A., jj). In the GE-1 well, supercycle Ka of thickness, or stratigraphic depth of units lying Vail and others, began with nonmarine and below the bottom of a given core hole. marginal-marine deposition in the Early Creta­ The accumulation-rate curves for the GE-1 ceous (Valanginian? to Aptian) and culminated well (fig. 29) show that the highest rates of with middle-continental-shelf conditions in the accumulation (2.0-2.5 in./I,000 yrs (5.0-6.4 late Albian. Supercycle Kb began with erosion cm/1,000 yrs)) took place during the Albian and nondeposition during the Cenomanian and through Santonian interval ("middle Creta­ early Turonian and culminated with upper-contin­ ceous"), middle and late Eocene, and middle ental-slope conditions during the Campanian and Miocene. Lowest rates (0.51-0.71 in./I,000 yrs early Maestrichtian. (1.3-1.8 cm/1,000 yrs)) prevailed during the In the Tertiary, supercycle Ta began with Late Cretaceous (Campanian and Maestrichtian), erosion and nondeposition during the early in early through middle Oligocene time (1.0 Paleocene (and latest Maestrichtian) and cul­ in./I,000 yrs (2.5 cm/1,000 yrs)), and during minated with perhaps middle-continental-slope the Pleistocene (1.0 in./I,000 yrs 2.5 cm/1,000 deposition in the early Eocene. Supercycle Tb yrs)). began with erosion and nondeposition during the Comparison of the Cenozoic rates at GE-1 early Eocene and culminated with upper- with those at nearby core holes (fig. 30) continental-slope conditions in the middle reveals that accumulation at GE-1 was generally Oligocene. Supercycle Tc began with erosion and more rapid than at the other sites. The central nondeposition in the late Oligocene and early part of the embayment appears to have remained Miocene and culminated with upper-continental- low and thus to have accumulated a thicker slope conditions in the middle Miocene. section of sediments throughout each Cenozoic Supercycle Td began with erosion and epoch, except during part of the middle Eocene nondeposition in the late Miocene and early when the rate at J-l was similar to that at GE- Pliocene and culminated in outer-shelf 1, and during the Pleistocene when the rates at conditions in the late Pliocene. J-2 and GE-1 were equivalent. The stratigraphic

57 RELATIVE CHANGE OF PALEOBATHYMETRY SEDIMENT ACCUMULATION RATE SUBSIDENCE OR UPLIFT RATE TIME GLOBAL SEA LEVEL (cm/1000 yrs) (cm/1000 yrs) -Increasing Depth (MX) -Rising Falling- -Rising Subsiding' UPPER OUTER MIDDLE INNER NON- | SLOPE | SHELF | SHELF | SHELF [MARINE 5-4-3-2-10 I 2345678S

Tc

Interval uRs Average uRs ~~~ based on based on subsidence of selected horizons. short sections subsidence measured measured over over total time elapsed duration of from deposition to present depostion

A c D

Figure 28. Comparison of paleobathymetry, sediment-accumulation rate, and subsidence rate of rocks penetrated in the COST No. GE-1 well with relative changes in global sea level postulated by Vail, Mitchum, and Thompson (1977a, b). uRs is the subsidence rate (uncorrected for compaction). Dashed lines in interval uRs curves represent inferred rates during hiatal gaps. changes in accumulation rate at these nearby 0.9 cm/1,000 yrs)) at J-l and J-2, but on the boreholes also generally coincide with the northern margin of the embayment (AMCOR 6002), changes at GE-1. The early and middle Oligocene the lowest rates occurred during the early rates were lowest (0.08-0.35 in./I,000 yrs (0.2- Miocene. Highest rates were obtained during the

'AGE(M.Y) SERIES AND STAGE DEPTH BELOW SEA FLOOR, IN METERS

500 1000 1500 2000 2500 3000 3500 4000 ^J-2.51 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 II MINI 1 1 1 1 1 II 1 1 1 1 1 1 1 1 1 1 iVF PLIO TO' 5 3.1 (3966m)-

10 UJ z Ul ^5.0 15 [^ 3279m bsf _ 8 Metamorphic 2 Basement 20

25 _ z O 30 8 \25

35 8 X

40 X. _ ^S^D.O UJ ^^^^ z 45 ^^^ O o ^w Ul ^V^ O Q 50 \ 55 Ul Z \3.3 Ul 60 8 IS

65 sfe 70 z 75 co _ s5 o2 80 "z 1- j LU DC 85 O °s ~ _ DC Ul Theoretical thickness Q_ oz 90 D_ +fr3 of Cenomanian rocks z ''^ N" 4;^.. 95 2Z o< '"*; > '"^- 100 § z Theoretical thickness ^^b.4 e 105 1 of Cenomanian rocks ^^^^

110 CO z ^ -^ p ^^ *^* ^ta^ 2 s.. . *^» ^^ L Minimum extrapolated age o £ ^*»* . 115 __ ' . /of sediment at basement 2 Ul %^. _ '"'-.( (middle Barremian) DC O 120 5 >v %. _ DC ^Intermediate extrapolated D 4 .' _ yj age of sediment at 125 i I ' '.' . basement (late Z V Valangiman) 130 | Maximum extrapolated age 1 of sediment at basement (middle Valangiman) 135 0 DC I 3 E 140- I I I I I I i i i i I 1 I i 1 1 I 1 1 I i 1 1 i i i 1 I i i i 1 i i i i i 1 i I i I I i i i i il i i i i i I i I I I I i I I 1 I i i I i i i I

Figure 29. Curve of sediment-accumulation rate for the COST No. GE-1 well. Gaps in the curve are hiatuses, whose time span can be measured on the time scale at the left. Number above segments of curve are sediment- accumulation rates measured in cm/1,000 yrs.

59 middle and late Eocene and Pleistocene (0.59- rates for the Campanian are next to lowest in 0.67 in./I,000 yrs (1.5-1.7 cm/1,000 yrs)) at J- the entire drilled section in GE-1 (0.71 2, and during the middle Eocene and early and in./lOOO yrs (1.8 cm/1000 yrs)). Thus, the middle Miocene at J-l and AMCOR 6002. rapid Campanian rates noted by Whitten (1977) Whitten (1976a, b; 1977) calculated sedi­ seem to have been the result of localized ment-accumulation rates in a similar manner phenomena. (based on present thickness of units) for As noted above, the sediment-accumulation selected coastal wells located between Long rates can be extrapolated in order to detect the Island and Florida. He found that when using presence of hiatuses and to estimate the age of either the time scale of Obradovich and Cobban unfossiliferous strata. We have used it, for (1975) or of van Hinte (1976), the most rapid example (fig. 29), to support the paleontologic rates (>2.0 to >6.0 in./1,000 yrs (>5 to >15 and paleoenvironmental evidence that Cenomanian cm/1,000 yrs)) were established in the Aptian- rocks are missing in the GE-1 well. By extra­ Albian interval and in the Campanian. He polating the Turonian rate downward to the top speculated that the high Campanian rates might of the Albian (fig. 29), we can determine that have extended through Florida, but he had no Cenomanian rocks should theoretically be 1,541 data for that region. Our calculations for the ft (470 m) thick. Extrapolation of Albian rates GE-1 well show that here, too, Aptian and Albian upward to the Turonian would yield a slightly rates were higher than those of later Cretaceous thicker theoretical Cenomanian unit 1,679 ft time (3.0 in./lOOO yrs (6.4 cm/1000 yrs), com­ (512 m). On the other hand, if we assume that parable to Whitten's data), although apparently the entire Cenomanian is represented by the 82- they were not significantly greater than those ft (25-m) interval between the lowest observed of earlier Cretaceous time (fig. 29). In Turonian and highest Albian samples, the cal­ contrast to Whitten's findings, however, the culated rate of deposition would be 0.12 in./I,000 yrs (0.3 cm/1,000 yrs). This is 4 times lower than the lowest rate calculated for rocks actually penetrated at GE-1 (0.51 J) SEDIMENT ACCUMULATION RATE, UJ UJ in./1,000 yrs (1.3 cm/1,000 yrs) in the o 2 0 cm/IOOOyrs. Maestrichtian) and nearly 20 times lower than VJ CO 0.5 1 1C the Turonian rate. In view of these results, it i i i i i ii 1 i i i i i i i i PLE1ST 22/23 _ o '° seems reasonable to postulate that Cenomanian 21 i° rocks were eroded from, or not deposited at, the PL 10 9/20 5 - GE-1 well during the long (4 to 6 m.y.)

17 Cenomanian low sea level stand described by Vail NEOGENE 16 10- and others (1977b). MIOCENE |M|uL We have also extrapolated the Albian t i accumulation rate downward in order to estimate I5 I g i ' the age of the unfossiliferous sedimentary rocks 7 o 0 6 0 resting on Devonian metamorphic basement (fig. o _5_ 20- ° 29). A straight-line extrapolation dates the 4 ° oldest post-Devonian rocks as approximately late 22 25 : Valanginian. Two alternative estimates can be

2 made by using the maximum (Barremian) and LU _ 21 minimum (Aptian) possible ages of the deepest 0 5 30 ; i ° O 20 ^ o fossiliferous sample in the GE-1 well, at 9,524 0 O 19 ft (2,903 m). If this sample is assumed to be OO of early Aptian age, the downwardly extrapolated 18 rate would yield an age of middle Barremian at 16 ± 0 40 ; ± o basement. If the sample at 9,524 ft (2,903 m) OGENE 15 i : ° is assumed to be of early Barremian age (which UJ H-E I § is also suggested by extrapolating the Albian z 13 45 - LU 2 12 i rate downward), then the extrapolated rate 0 2 10 yields a middle Valanginian age at basement. LU __ 50 - I 8 Thus we have estimated the age of the sediments -« 7 EXPLANATION R at basement to be Valanginian(T). 55 - GE-1 J-l J-2 6002 UJ z. 5 i ° * UJ 4 SUBSIDENCE AND UPLIFT OF THE EMBAYMENT o 3 60 - Approximate subsidence and uplift rates for & 1 given horizons in the GE-1 well have been calculated and plotted against a time scale to Figure 30. Comparison of sediment accumulation yield a subsidence curve (fig* 28JJ). We have rates for Cenozoic rocks at the COST No. used two different approaches to calculate GE-1 well and nearby core holes J-l, J-2, subsidence rates. The first method determines and AMCOR 6002. the subsidence of a given horizon as an average

60 rate measured over the total time elapsed since deposition. The basis for the calculation is illustrated in the following example, using a modified version of van Hinte's (1978) notation. Let us calculate the uncorrected rate of subsidence, uRs, for the top of the Mae- strichtian section in the GE-1 well (rate un­ corrected for compaction). The present depth (D ) of this horizon is 1,000 m below the present sea floor (SF , fig. 31A) and 1,040 m below present sea level (SL ). (Present water depth (W ) is 40 m.) BtPt Vail and others (1977b, rig. 6) estimated that sea level was about 325 m higher than at present during the late Maestrichtian (SL ). We have estimated that the late Maestrichtian sea floor (SFffl ) was 200 m below the late Maestrichtian sea level (Maestrichtian water depth Wm = 200 m), or 125 m above the present sea level. Thus, the Maestrichtian sea floor (SFffl) was 165 m higher than the present sea floor, and the uncorrected subsidence for the Maestrichtian top is 1,000 + 165 or 1,165 m. This is divided by the time elapsed for this subsidence (66 m.y.) yielding an uncorrected subsidence rate of 1.8 cm/1,000 yrs. Thus, the equation for the calculation may Figure 31. -A, Diagrammatic examples (not to be expressed as: scale) of calculations used to derive averaged and interval subsidence rates. uRs = Dp +(SLm - Wm) +Wp Qr Averaged subsidence rate. 10A Formula: Dp + (SLm - Wffl) + Wp ) uRs = 1,000 + (325 - 200) + 40 = uRs 660 10A where uRs = uncorrected subsidence rate 1.8 cm/1,000 yrs (0.71 in./I,000 yrs). Negative (uncorrected for compaction), D = present answers indicate uplift rather than subsidence. depth to Maestrichtian top, %L = sea level at end of Maestrichtian relative to The averaged subsidence rates in figure 28D^ today, Wffl = water depth at end of show that Lower Cretaceous rocks (Valanginian? Maestrichtian (note this refers to the top to Albian) have subsided at an average rate of of Maestrichtian deposits as seen in this 0.79-1.0 in./I,000 yrs (2-2.6 cm/1,000 yrs) well, not the true end of Maestrichtian since their deposition. Because the well time), W = water depth at present over reached basement just below Valaginian(?) rocks, COST No. GE-1 well site, SF = sea floor the average subsidence rate of the oldest at present at COST No. GE-1 well site, and Valanginian(?) rocks (1.0 in./I,000 yrs (2.6 SL = sea level at present at GE-1 well cm/1,000 yrs)) is also the average subsidence of site. _B_, Interval subsidence rate. the basement during the past 128 m.y. Similar subsidence rates continued into the early Formula: T - (W + E) Eocene, although significant changes occurred in uRs = P______; W = W, - paleobathymetry and sediment-accumulation E = Ev - E t* rates. Thus, according to this curve, the 10A significant deepening of the embayment during where uRs = uncorrected subsidence rate the Campanian seems to have been largely a (uncorrected for compaction), T = present result of rising global sea level and reduction thickness of unit (forP example of accumulation rate to 0.51 in./I,000 yrs (1.3 Maestrichtian beds), W = change in water cm/1,000 yrs), perhaps due to increased distance depth during deposition of unit, W, = to clastic sources on a uniformly subsiding water depth during deposition of base of shelf. unit, Wfc = water depth during deposition Rocks deposited during the middle and late of top of unit, E = change in eustatic sea Eocene have subsided more slowly. This was a level during deposition of unit, E, = period of thick carbonate buildup when accu­ eustatic sea level (relative to present) mulation rates rose to more than 2.36 in./I,000 during deposition of base of unit, E = yrs (6 cm/1,000 yrs), the highest rate of the eustatic sea level (relative to present) Cenozoic. Subsidence stopped during the middle during deposition of top of unit, SF, = Oligocene, but a major global rise in sea level sea floor during deposition of base of increased the water depths and the accumulation unit, SFfc = sea floor during deposition of rates were lowered. top of unit, and SL = present sea level. 61 Middle Miocene and upper Pliocene rocks account for the significant thickness of have been uplifted at a rate of 0.79-1.2 shallow-water carbonates; it probably was in. /I, 000 yrs (2-3 cm/1,000 yrs), although high triggered by accelerated sediment accumula­ sea level stands maintained outer-shelf and tion. The interval subsidence rates for the upper-slope environments during their deposi­ Oligocene and Pliocene slowly decreased, tion. converging toward the averaged curve, and the The second method of calculating the Pleistocene rates became nearly identical with uncorrected subsidence rate is taken directly the long-term average. During the Miocene, from van Hinte (1978). The equation is however, subsidence increased substantially, in unison with rising global sea level, probably p - E > uRs because sediment accumulation accelerated 10A significantly. Following this, near equilibrium (fig. 31B_), where T is present thickness of a was reached in the late Pliocene, and the given unit (for example, the Maestrichtian embayment was uplifted during the Pleistocene. interval), W is the change in water depth during This resulted in the filling of the embayment to deposition of the unit (taken from our its present topographic level, even though paleobathymetric interpretation), and E is the sediment accumulation decreased. The paleo- eustatic sea level change during deposition of ecologic analysis supports this interpretation the unit (taken from Vail and others, 1977b, of Pliocene-Pleistocene uplift and filling by fig. 6). Deepening from 0 to 164 ft (0-50 m) showing a significant change in the late would yield W = 50; a eustatic rise of 164 ft Pliocene paleobathymetry. During most of the (50 m) would yield E = +50. A is the duration earlier Cenozoic, water depths were deeper at of deposition in millions of years. This method GE-1 than at the other borehole sites (implying yields a short-term or "interval" rate of that GE-1 occupied a topographic depression). subsidence by measuring average subsidence of However, during the late Pliocene, site GE-1 the base of a given unit only during the time of became shallower than site J-2; the two sites deposition of the unit. occupied similar depths during the Figure 28D_ shows that the curve constructed Pleistocene. by this method produces significant departures It is clear that the averaged subsidence from the averaged curve at several points. The curve yields only generalized results that Lower Cretaceous rates are rather uniform (2.2- illustrate long-term relative changes. The 2.6 in. /I, 000 yrs (5.5-6.5 cm/1,000 yrs)), but calculations that produce the averaged curve higher than those shown by averaging, and are encompass the time span (A) represented by approximately equivalent to the Lower Cretaceous several major hiatuses, whereas the interval- accumulation rates. The interval curve cor­ curve calculations deal only with time spans (A) roborates the previous interpretation from the during which the existing rocks were averaged curve that the Campanian deepening was deposited. Therefore, the averaged curve due mainly to rising global sea level. A major displays rates that are generally lower than discrepancy is present in the Maestrichtian, those of the interval curve. As a result, the where the slight decrease shown on the averaged interval subsidence curve allows more detailed curve is a pronounced uplift on the interval comparisons with the sediment-accumulation curve. The interval method produces this curve, because they are calculated over departure through the necessity to account for identical time spans. The interval curve is the 1,000-ft (300-m) shoaling of waters that more accurate in relative terms than the occurred near the end of the Maestrichtian. The averaged curve, but its accuracy in absolute 33-ft (10-m) sea-level drop proposed for this terms has a wide margin of error, because time interval by Vail and others (197 7b, fig. 6) calculations are made from parameters that are would account for only an insignificant amount gross estimates. of the shoaling. However, if we assume that the Because of the smoothing effect inherent in 660-ft (200-m) drop in sea level, estimated by the averaged curve, it yields information Vail and others (197 7b) to have occurred at the concerning subsidence during the hiatal gaps in end of the Maestrichtian, actually occurred the interval curve. For example, from the somewhat earlier in the late Maestrichtian, the averaged curve (fig. 28DJ, we see that the top required uplift would be reduced to 0.5 of the middle Oligocene section has been in. /I, 000 yrs (1.2 cm/1,000 yrs). Furthermore, uplifted at the rate of 0.16 in./I,000 yrs (0.4 if the estimated paleo-bathymetric change were cm/1,000 yrs) over the past 30 m.y. But the 660 ft rather than 1,000 ft (200 m rather than interval curve shows that the embayment actually 300 m) , and it could well have been, then the subsided during deposition of the Miocene and calculation would yield a uRs of 0 (equili­ Pliocene rocks penetrated in the well. There­ brium). This demonstrates the dependence of the fore, we conclude that during the hiatuses, interval method on the accuracy of the estimated significant uplift must have occurred. Accord­ parameters, which unfortunately are only gross ingly, we have schematically inserted dashed estimates. lines in the hiatal parts of the interval curve The next major departure of the interval to show uplift during the hiatuses. This hiatal curve is an acceleration of subsidence in the uplift apparently was a response to sediment middle and late Eocene that is necessary to unloading (erosion) or the cessation of loading

62 (nondeposltion). It seems sufficient to con- the major paleobathymetric changes were caused elude from these curves that (1) Cretaceous by eustatic sea level fluctuations; and (4) subsidence was more rapid than that of most of isostatic balance of the embayment was sensitive the Cenozoic in the Southeast Georgia Embayment; to sediment loading; subsidence correspondingly (2) Cenozoic rates have steadily decreased since increased during periods of rapid sediment the middle or late Eocene, and uplift has been accumulation and decreased or reversed during dominant since the middle Oligocene; (3) most of periods of prolonged erosion or nondeposition.

63 CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY AND PALEOENVIRONMENTAL INTERPRETATION

Page C. Valentine

INTRODUCTION NEOCENE (390-720 ft (119-220 m))

The COST No. GE-1 well, located on the The first sample was collected from the outer part of the Continental Shelf off Georgia, COST No. GE-1 well at 390 ft (119 m). The was drilled into a deep part of the Southeast highly calcareous strata from this level down to Georgia Embayment. The well penetrated sedi­ the top of the Oligocene at 720 ft (220 m) are mentary strata of Early Cretaceous through almost barren of nannofossils, no doubt a result Quaternary age that overlie metamorphic rocks of postdepositional dissolution. Although other believed to be of Devonian age. Paleoenviron- groups of calcareous marine fossils are present ments include both nonmarine and marine in the in this interval, nannofossils were found in Lower Cretaceous, whereas the Upper Cretaceous only one sample (600-630 ft (183-192 m)), a and Cenozoic are marine throughout. Calcareous moderately well preserved assemblage indicative nannofossils are rare in the Lower Cretaceous of a late early to early middle Miocene age. but are common and usually well preserved in The following species are present: Coccolithus younger strata. eopelagicus, £. pelagicus, Cyclicargolithus abisectus, £. floridanus, Discoaster deflandrei, NANNOFOSSIL BIOSTRATIGRAPHY D_. exilis, Discolithina sp., Helicosphaera carteri, II. recta?, Reticulofenestra sp and Calcareous nannofossil biostratigraphy of Sphenolithus heteromorphus. The presence of the COST No. GE-1 well is based on the study of Sphenolithus heteromorphus places this flora in samples of rotary drill cuttings collected over the Helicosphaera ampliaperta and Sphenolithus 10- to 30-ft (3- to 9-m) intervals (table 6). heteromorphus Zones of Bukry (1973, 1975). With few exceptions, cutting samples were an­ alyzed at intervals of 90 ft (27 m) or less from OLIGOCENE (722-1,230 ft (220-375 m)) 390 to 8,860 ft (119-2,702 m) in the well. A complete sequence of samples was studied from Oligocene calcareous sedimentary beds are 570 to 780 ft (174-238 m) and from 3,360 to present in a 510-ft (155-m) interval from 720 to 4,020 ft (1,025-1,226 m). Discrete rock fra­ 1,230 ft (220-375 m). The occurrence of calca­ gments representing each of the major litho- reous nannofossil species varies from rare to logic units present in a single cuttings sample abundant, and preservation is poor to moder­ were processed for calcareous nannofossils. The ate. On the basis of assemblages of rather low oldest nannofossil assemblage identified in a diversity, the sequence down to 1,020 ft (311 m) sample was considered to indicate the age of the represents undifferentiated Oligocene strata, strata of that level. Species listed as repre­ and characteristic species include Coccolithus sentative of a stratigraphic interval do not pelagicus, Cyclicargolithus abisectus, C_. necessarily occur in every sample studied from floridanus, Dictyococcites bisectus, J). that interval. In all, 260 subsamples from 110 scrippsae, Discolithina spp., Helicosphaera levels in the well were studied. Sample depths bramlettei, JI. compact a, jl. euphratis, are relative to the KB. The stratigraphic Reticulofenestra hillae. Reticulofenestra cf. ranges for the Mesozoic calcareous nannofossils scissurus of Bramlette and Wilcoxon, _R. sp., and treated here are based, for the most part, on Transversopontis sp. determinations made by Thierstein (1971, 1973, Nannofossil assemblages are more diverse 1976) and Smith (in press). The reports on COST from 1,020 to 1,230 ft (311-375 m). The highest No. GE-1 well biostratigraphy and paleoenviron- (youngest) occurrence of Cyclococcolithina ments by International Biostratigraphers, Inc. formosa and Reticulofenestra umbilica is at the (1977) and Amato and Bebout (1978) were con­ top of this interval, which is dated as early sulted; and selected information, as cited, has Oligocene and contains the following assem­ been incorporated from these reports into the blage: Blackites sp., Braarudosphaera present study. bigelowii, _B_. discula, Chiasmolithus sp. ,

64 Table 6. Calcareous nannofossil biostratigraphy of the COST No. GE-1 well

[Depths are relative to the KB, 98 ft (30 m) above sea level. In sample column, blacked-in areas indicate a complete sequence of samples studied; B indicates cuttings barren of nannofossils. Ages are based on findings of this study but incorporate some biostratigraphic information from International Biostratigraphers, Inc. (1977). Ranges of selected Late Cretaceous calcareous nannofossil species are based on cuttings samples of this study]

UJ DEPTH _J RANGES OF SELECTED Q. AGE NANNOFOSSIL SPECIES IN THE METERS FEET $ UPPER CRETACEOUS OF COST No. GE-1

SEA LEVEL 98 -100 - SEA FLOOR234 _ _ - >£o. MIOCENE - 5_720'______1 *^ OLIGOCENE £a ,. -300- *~^«. - 1000- - \ ca < 0 _g I s _ _ _ 1230, EARLY OLIG. ( M o uT ^ ~ 1290' >* ""^ _ o * *«. £ en

- en j3 1 z> V) en EOCENE en a: h _J 3 LU a: J C c en r (j X 13 .O

^ " - - - SANTONIAN -1500- -5000- - _ _ 5170' - = 5^i CONIACIAN - 5400' -1700- - TURONIAN __ 1 - -6000-: y Mixed Turanian and barren cuttings ~

-1900- ~" _ Cuttings barren to 8860 feet :

65 Coccolithus eopelagicus, £. pelagicus, J2. Campylosphaera dela, Cepekiella lumina, sarsiae of Bybell, Cyclicargolithus floridanus, Chiasmolithus bidens/solitus. _C. gigas, _C. Cyclococcolithina formosa. ^. kingii, eopelagicus, C_. jugatus, £. pelagicus. £. Dictyococcites bisectus. IK scrippsae. sarsiae of Bybell, Cyclicargolithus floridanus, Discolithina spp., Helicosphaera compacta, JI. Cyclococcolithina f ormosa, C_* gammation, intermedia, _H_. wilcoxonii, Lanternithus minutus, Daktylethra punctulata, Dictyococcites Reticulofenestra hillae, _R. cf. scissurus of scrippsae, Discoaster barbadiensis, _D_. Bramlette and Wilcoxon, _R. umbilica, deflandrei, D.. delicatus, J). lodoensis, J). Transversopontis duocavus?. JT. obliquipons, _T. nodif er?, J). spp., Discolithina bicaveata, D.. zigzag, and Zygrhablithus bijugatus. spp., Helicosphaera compacta, .H- lophota, _H- seminulum, Lophodolithus nascens, Neococcolithes EOCENE (1,290-3,510 ft (393-1,071 m)) dubius. Reticulofenestra coenura, _R. sp., Sphenolithus radians, £. spiniger, Below the Oligocene lies a sequence of Transversopontis obliquipons, JF. pulcher, and Eocene limestones that is 2,220 ft (678 m) Z yg rhab1i thus bijugatus. thick. Nannofossils vary in occurrence from The early Eocene is represented by a rare to abundant, and preservation is poor to relatively thin limestone interval, 150 ft (46 moderate, reflecting differential solution m) thick, from 3,360 to 3,510 ft (1,025-1,071 within the beds of this interval. The top of m). These strata are darker gray than the the Eocene is correlated with the highest occur­ overlying units. Nannofossil occurrences are rence of Reticulofenestra reticulata at 1,290 ft rare to common, and preservation is poor to (393 m). Upper Eocene beds are believed to be moderate. Several samples at the top of this about 930 ft (284 m) thick based on the interval contain diverse assemblages. The occurrence of the following species: highest occurrences of typical early Eocene Braarudosphaera bigelowii, J3. discula, species, such as Discoaster binodosus. _D_. Chiasmolithus titus. Coccolithus eopelagicus. £. diastypus. Discoasteroides kuepperi, and sarsiae of Bybell, Cyclicargolithus floridanus. Tribrachiatus orthostylus, were observed at Cyclococcolithina formosa, £. kingii, 3,360 ft (1,025 m). The following were identi­ Dictyococcites bisectus, J). scrippsae. fied: Braarudosphaera bigelowii, Campylosphaera Discoaster saipanensis. Discolithina spp., dela, Chiasmolithus californicus, _C_. consuetus, Helicosphaera compacta. _H. euphratis, jl. Coccolithus eopelagicus, C^. pelagicus. intermedia, Lanternithus minutus, Cyclicargolithus floridanus, Cyclococcolithina Reticulofenestra reticulata, _R. umbilica. f ormosa. C_* gamma t ion, Dictyococcites scrippsae. Sphenolithus predistentus. Transversopont is Discoaster barbadiensis, J). binodosus, J). obliquipons. JT. zigzag, and Zygrhablithus diastypus. IK lodoensis, JD^. s a li sbur gens i s. bijugatus. Discoasteroides kuepperi. Ellipsolithus Between 2,220 and 2,790 ft (677-851 m) the distichus. JE. macellus, Helicosphaera lophota. cuttings are characterized by rare, poorly _H. seminulum. Lophodolithus nascens, preserved nannofossil species constituting Micrantholithus flos. Neochiastozygus distentus. assemblages of low diversity. Reticulofenestra Neococcoli thes dubius, Prinsius bisulcus, reticulata occurs in several samples, whereas _R. Sphenolithus anarrhopus. £. radians. £. umbilica is absent. This interval is ten­ spiniger. Transversopontis duocavus, JT. tatively dated as late middle Eocene to early obliquipons, JT. pulcher. Tribrachiatus late Eocene based on the occurrence of orthostylus, and Zygrhablithus bijugatus. Braarudosphaera bigelowii, Ji. discula, Coccolithus eopelagicus. £. sarsiae of Bybell, PALEOCENE (3,510-3,660 ft (1,071-1,116 m)) Cyclicargolithus floridanus. Cyclococcolithina formosa, £. kingii, Dictyococcites scrlppsae. The Paleocene, like the early Eocene, is Discoaster saipanesis. Reticulofenestra represented by a thin interval of dark-gray reticulata. and Sphenolithus predistentus. limestones not more than 150 ft (45 m) thick. Middle Eocene strata that are 570 ft (174 Although, in this study, Maestrichtian nanno­ m) thick occur from 2,790 ft to 3,360 ft (851.m- fossil assemblages first occur at 3,660 ft 1,025 m). The samples from this interval con­ (1,116 m), the Cretaceous-Tertiary boundary is tain common, moderately well preserved nanno- placed somewhat higher by International Bio- fossils in diverse assemblages. The highest stratigraphers, Inc. (1977). The uppermost occurrences of Cyclococcolithina gammation and sample believed to be in the Paleocene interval, Sphenolithus radians are at 2,790 ft (851 m); at 3,510-3,540 ft (1,071-1080 m), contains a and Campylosphaera dela. Helicosphaera lophota. small assemblage of poorly preserved nannofossil _H. seminulum, and Neococcolithes dubius are not species of Paleocene-Eocene aspect and includes found higher than 3,000 ft (915 m). Discoaster multiradiatus, Fasciculithus Chiasmolithus gigas occurs at 3,000 ft (915 m) involutus. Prinsius bisulcus, and Toweius and 3,090 ft (942 ra); and Discoaster lodoensis, eminens. The other samples studied exhibit at 3,270 ft (997 m). The nannofossil assemblage richer, more diverse, and better preserved from this interval includes Blackites creber. _B_. assemblages. The following species indicative sp., Braarudosphaera bigelowii. JB. discula. of the middle and late Paleocene were identified

66 in this interval: Braarudosphaera bigelowii, JU Broinsonia parca. Tetralithus aculeus. JC. discula, Chiasmolithus bidens. _C. californicus, gothicus. and J_> trif idus are present here; but C_. consuetus, Coccolithus eopelagicus, C_. Eiffellithus eximius. a species observed in the pelagicus, Cyclococcolithina robusta. Discoaster next lower sample whose extinction marks the top multiradiatus, Discolithina sp., Fasciculithus of the Carapanian (table 6), is not. involutus, _F. tympaniforrais. Micrantholithus crenulatus, M. flos. II. sp, Neochiastozygus CAMPANIAN (3,750-4,290 ft (1,144-1,308 m)) distentus, Neococcolithes concinnus, JN. protenus, Prinsius bisulcus, Toweius eminens, Based on the highest occurrence of Zygolithus aff. plectopons, and _Z_. sigmoides. Eiffellithus eximius at 3,750 ft (1,144 m) and the lowest occurrence of Broinsonia parca at MAESTRICHTIAN (3,660-3,750 ft (1,116-1,144 ra)) 4,290 ft (1,308 ra), the Carapanian sequence is about 540 ft (164 m) thick. The top of the Maestrichtian is placed at 3,570 ft (1,078 ra) in the COST No. GE-1 well by SANTONIAN (4,340-5,170 ft (1,324-1.577 m)) International Biostratigraphers, Inc. (1977) based on foraminifers in cuttings samples; and a Santonian strata' occur in an 830-ft (253- study of calcareous nannofossils in the same m)-thick interval from 4,340 to 5,170 ft (1,324- report indicates the presence of the late 1,577 m). The Santonian extends from just below Maestrichtian Micula mura Zone in a sidewall the lowest occurrence of Broinsonia parca down core from 3,604 ft (1,099 ra). to the lowest occurrence of Tetralithus obscurus/ovalis (table 6). These beds also In the present study, the top of the exhibit the highest occurrences of Chiastozygus Maestrichtian, based on observed nannofossil cuneatus at 4,340 ft (1,324 m) and of occurrences, is placed at 3,660 ft (1,116 m) Lithastrinus floralis at 5,070 ft .(1,546 ra). (table 6). This depth coincides with a color change apparent in the cuttings samples. The CONIACIAN (5,200-5,350 ft (1,586-1,632 ra)) highest occurrence of Cretaceous ostracodes in cuttings is at 3,630 ft (1,107 m). Late The Coniacian interval is rather difficult Cretaceous (Maestrichtian and older) beds in to delineate in this section. It is represented this well are chiefly fine-grained, dark-gray by several samples in a 150-ft (46-m) sequence calcilutites that contain terrigenous components occurring between the base of the Santonian such as quartz and organic matter in somewhat recognized at the lowest occurrence of greater amounts than are found in the overlying Tetralithus obscurus/ovalis (5,170 ft (1,577 ra)) Tertiary limestones. In contrast to the Tertiary, the nannofossil assemblages of the and as the top of the Turonian, at the highest occurrence of Corollithion achylosura (5,400 ft Upper Cretaceous are more consistently diverse, (1,647 ra)). contain species that are common to abundant, and exhibit moderate to good preservation. The TURONIAN (5,400-5,800 ft (1,647-1,769 ra)) Upper Cretaceous nannoflora is characterized by many species that range throughout most of the Turonian strata at least 400 ft (122 ra) interval: Ahmuellerella octoradiata. thick occur from approximately 5,400 to 5,800 ft Arkhangelskiella cymbiformis, Biscutum blackii, (1,647-1,769 ra). The top of the Turonian is Braarudosphaera bigelowii, Chiastozygus recognized at the highest occurrence of araphipons, £. plicatus, Corollithion signum, Corollithion achylosura (table 6). International Cretarhabdus conicus, £. coronadventis, jC_. Biostratigraphers, Inc. (1977) reported this crenulatus, Cribrosphaera ehrenbergii, species to be present in a sidewall core at coronatus, Eiffellithus Cylindralithus 5,342 ft (1,629 m), and although it was not turriseiffeli. Gartnerago costatum, Q. observed in the cuttings sample from 5,340-5,350 Kamptnerius magnificus, K. segmentatura, ft (1,629-1,632 m) of the present study, the Lithraphidites carniolensis, punctatus, species does occur in the next lower sample cayeuxii, Manivitella Lucianorhabdus studied (5,400-5,410 ft (1,647-1,650 m)). The pemmatoidea, Microrhabdulus belgicus, _M. lowest occurrences of both Eiffellithus eximius decoratus, M. stradneri, Micula staurophora, (5,710 ft (1,742 ra)) and Corollithion exiguura Parhabdolithus angustus, £. embergeri, £. (5,800 ft (1,769 m)) are within this interval. regularis, f_. splendens, Prediscosphaera cretacea, Tranolithus orionatus, Vagalapilla The base of the Turonian is placed at 5,800 elliptica, V^. matalosa, Watznaueria barnesae, JW. ft (1,769 m), a level at which a significant biporta, Zygodiscus acanthus, j^. diplogrammus, lithologic break occurs in the cuttings. The and Z. fibuliformis. The ranges of strati- sample from 5,790-5,800 ft (1,766-1,769 ra) graphic marker species are discussed below and contains Turonian dark-gray argillaceous chalk are illustrated in table 6. of marine origin, but the chalk is associated While no nannofossils of late Maestrichtian with fragments of wood and mollusk shells and age occur in the cuttings, early Maestrichtian with nonfossiliferous calcareous sandstone assemblages are found in a 90-ft (28-ra) interval containing abundant quartz, pyrite, and organic from 3,660 to 3,750 ft (1,116-1,144 ra). matter, all of which are indicative of a

67 shallow-nearshore environment of deposition. gray shales contain a discontinuous sequence of The age of this lithologic change at 5,800 ft sporomorph and dinoflagellate assemblages (1,769 m) is in doubt. International Biostra- (International Biostratigraphers, Inc., 1977) tigraphers, Inc. (1977) placed the base of the that are indicative of fluctuating nonmarine and Turonian at 5,830 ft (1,778 m) and reported the shallow-marine conditions. highest occurrence of Albian spores and pollen Overlying the red beds, from 7,500 to 5,950 at 5,950 ft (1,815 m); however, no definitively ft (2,288-1,815 m) is a varied sequence of Cenomanian nannofossils, foraminifers, or paly- sedimentary strata 1,550 ft (473 m) thick that nomorphs were identified in sidewall cores or includes limestones, calcareous sands, and minor cuttings from the intervening beds (5,830-5,950 amounts of dolomite, oolite, and anhydrite. ft (1,778 m-1,815 m)). Moreover, no nannofossil These beds represent a sedimentary regime and assemblages of Cenomanian Age are present in regional paleoclimate that differed markedly cuttings samples of the present study. Thus, from that of the underlying red-bed facies. the contact at approximately 5,800 ft (1,769 m), Marine depositional environments prevailed, and where Turonian marine limestone overlies shallow-marine calcareous sands, limestones, and shallow-marine calcareous sandstone, may mark even some evaporites were deposited. Sporo­ the top of a short sequence of Cenomanian beds morphs and dinoflagellates are present, and or could represent a hiatus separating Albian several samples near the top of the interval and Turonian rocks. exhibit poor assemblages of calcareous nanno­ Calcareous nannofossils older than Turonian fossils and predominantly arenaceous were not observed in samples from 5,800 to 8,860 foraminifers (International Biostratigraphers, ft (1,769-2,702 m), the deepest sample studied Inc., 1977; Poag and Hall, this volume). here. International Biostratigraphers, Inc. Sporomorphs studied by Ray Christopher (oral (1977) reported the occurrence of late Albian commun., 1978) from three levels in this nannofossils in a sidewall core from 6,495 ft interval suggest a probable Aptian Age for a (1,981 m), but all other samples studied by them sample from 7,520 ft (2,294 m), an Albian Age (13,196 ft (4,025 m)) are barren of nannofos­ for a sample from 7,370 ft (2,248 m); and an sils. Albian-Cenomanian Age for an assemblage at 6,110 PALEOENVIRONMENT ft (1,864 m). Palynomorph species that are not known to range above the Albian occur at the top The paleoenvironmental interpretaton of the of this sequence (5,850 ft (1,815 m)); however, COST No. GE-1 well presented here is based on the base of the Albian has not been identified, the composition and diversity of calcareous and most of the section from about 8,700 to nannofossil and ostracode assemblages, general 6,500 ft (2,654-1,982 m) represents undiffer- lithology, and occurrences of glauconite, wood, entiated Aptian-Albian strata (International and molluskan shell fragments. In addition, Biostratigraphers, Inc., 1977). paleoenvironmental interpretations based on the A very short interval containing argil­ occurrences of benthic and planktic forami­ laceous limestones and calcareous sandstones of nifers, sporomorphs, and dinoflagellates are equivocal age extends from 5,850 to 5,800 ft considered (International Biostratigraphers, (1,769-1,815 m). Sporomorphs, dinoflagellates, Inc., 1977). and poor assemblages of arenaceus foraminifers The lower part of the sedimentary sequence are present and, although they are not age- at GE-1, from about 11,000 ft to total depth at diagnostic, they are collectively indicative of 13,254 ft (3,355-4,042 m), is radiometrically a shallow-marine environment (International dated as Paleozoic (Devonian). The sequence Biostratigraphers, Inc., 1977). These poorly includes nonfossiliferous quartzite, shale, and dated beds may mark an unconformity at the base slate, underlain by schist and metavolcanic of the Turonian, dark marine transgressive rocks. Overlying these strata are approximately chalks. 3,500 ft (1,067 m) of red sandstones, silt- The Upper Cretaceous in the COST No. GE-1 stones, and shales, including some gray shales, well consists of approximately 2,200 ft (671 m) anhydrite, and fragments of lignite or wood. of predominantly dark calcilutites and calcar- The lower 2,300 ft (702 m) of this nonmarine enites from 5,800 to 3,600 ft (1,769-1,098 m). red-bed sequence is almost barren of fossils. Turonian through Maestrichtian strata contain However, a dark-gray siltstone from 9,670 ft rich assemblages of calcareous nannofossils, (2,934 m) yielded a sporomorph assemblage of foraminifers, and palynomorphs, especially Barremian to Aptian Age (R. Christopher, USGS, dinoflagellates. Ostracodes are also present, oral commun., 1978) A 180-ft (55-m) interval of but not in abundance, and assemblages are not dark siltstones and shales from 8,720 to 8,900 diverse. The entire Upper Cretaceous sequence ft (2,660-2,715 m) contains spores, pollen, and has been interpreted, on the basis of forami­ Aptian dinof lagellates, marking the occurrence nifers, to indicate a marine environment of of a shallow-marine interval within the deposition at shelf and upper-slope water depths nonmarine red-beds (International Biostra­ (International Biostratigraphers, Inc., 1977; tigraphers, Inc., 1977). From approximately Poag and Hall, this volume). 8,700 ft upward to 7,500 ft (2,654-2,288 m), red The environmental preferences of calcareous sandstones, siltstones, and shales and minor nannofossils are not as well known as those of

68 foraminifers, but among nannofossil species that interval. Poag and Hall (this volume), on the are typically found in continental-margin other hand, interpret most of this interval to environments (Thierstein, 1976; Gartner, 1977), be indicative of an outer-shelf paleoenviron­ Braarudosphaera bigelowii. Lithastrinus ment. Calcareous nannofossil assemblages from floralis, L uci ano rh ab du s cayeuxii. and the middle and upper Eocene vary in diversity Tetralithus obscurus/ovalis are well represented and preservation, but numerous species and within their appropriate ranges in the Upper genera are present that occur predominantly in Cretaceous of GE-1. continental shelf sediments (Braarudsphaera, Ostracodes, on the other hand, are good Discolithina, Daktylethra, Helicosphaera environmental indicators, and the modest Upper lophota, _H. seminulum, Lanternithus, Cretaceous assemblages display an association of Reticulofenestra reticulata, and genera indicative of shelf and upper-slope Transversopontis). Other forms that occur in paleoenvironments that include the following: both shelf and deeper-water deposits include Krithe, a reliable outer shelf and slope genus; Cyclococcolithina gammation, Helicosphaera Phacorhabdotus, an indicator of open-ocean compacta, and Sphenolithus predistentus. conditions, most commonly associated with chalks Ostracodes, though never common, are also (J. E. Hazel, USGS, written commun., 1978); present in the middle and upper Eocene part of Bairdia and Cytherella, two genera that occur at this section. In the upper Eocene, ostracodes all depths; Haplocytheridea, a genus that are associated with larger foraminifers, bryo- generally occurs on the inner and middle shelf; zoans, and bivalves; they are very rare and and other species whose environmental poorly preserved below the base of the upper preferences are not well known (Alatacythere. Eocene (2,220 ft (677 m)). The Upper Eocene Brachycythere, Cythereis, and Veenia). assemblage is characteristic of the middle shelf Paleocene gray limestones contain and contains several genera that are chiefly nannofossil assemblages that include several found in inner- and middle-shelf deposits species of Braarudosphaera and Micrantholithus, (Actinocythereis, Cytheretta, and Hermanites); two genera that characteristically occur in one genus that is most commonly found in outer- continental shelf deposits. The presence of shelf and slope environments (Echinocythereis); these nannofossils thereby corroborates the and genera that inhabit a wider range of depths interpretation based on foraminifers that these (Bairdia. Cytherella. and Cytherelloidea). strata were deposited in an outer-shelf en­ Several genera are also represented whose depth vironment (International Biostratigraphers, preference is not known (Digmocythere. Inc., 1977; Poag and Hall, this volume). The Jugosocythereis, and Oertliella). environmental preferences attributed to Tertiary Above the Eocene, a thin sequence (210 ft nannofossil species are based on reports by (64 m)) of fine-grained limestone and dolomite Bukry and others (1971), Bybell (1975), Bybell of early Oligocene age contains a relatively and Gartner (1972), Gartner (1977), Martini (1965, 1970), and Sullivan (1964, 1965). diverse assemblage of nannofossils. Many of the constituent species belong to genera that Lower Eocene limestones exhibit a diverse nannofossil assemblage that contains many exhibit preference for a neritic environment (Braarudosphaera, Discolithina. Transverso­ species and genera indicative of a marine-shelf pontis, and Zygrhablithus). Ostracodes are very environment of deposition (Braarudosphaera, Discoaster blnodosus, Helicosphaera lophota. II. rare, and only two species are present: a seminulum, Micrantholithus, three species of species of Cytherella (shelf and slope Transversopontis, and Zygrhablithus environments), and a single occurrence of a bi.-jugatus). Other species that occur in shelf species of Cytheretta (usually found on the inner shelf). Although the ostracode occur­ deposits as well as in sediments from deeper rences are equivocal, the nannofossils indicate environments are Cyclococcolithina gammation, the presence of a marine-shelf environment in Ellipsolithus macellus, and Tribrachiatus orthostylus. This interpretation that the lower the early Oligocene rather than the upper-slope Eocene strata were deposited in a marine-shelf environment based on foraminifers from cuttings environment agrees with the preliminary con­ samples (International Biostratigraphers, Inc., clusions of Poag and Hall (this volume). 1977; Poag and Hall, this volume). The upper 300 ft (92 m) of the Oligocene is However, it disagrees with the International composed of limestone and shelly carbonate Biostratigraphers, Inc. (1977) report that suggests a paleoenvironment of middle- to lower- sands. The nannofossil assemblage is not very slope depths based on the presence of radio- diverse, and most of the neritic species found larians and a species of arenaceous foraminifer. in the lower Oligocene are absent. An ostracode assemblage indicative of outer-shelf and upper- The very thick sequence (2,070 ft (631 m)) slope deposits is consistently present though, of middle and upper Eocene strata is interpreted and contains Buntonia. Cytherella. by International Biostratigraphers, Inc. (1977) Echinocythereis. Henryhowella» Krithe, and to be a marine-middle-shelf deposit based on the Paracypris. Other ostracode genera present are occurrence of larger benthic foraminifers in the Acanthocythereis, Actinocythereis. Cytherel­ upper part of the Eocene (above 1,750 ft (534 loidea, Digmocythere, and Pterygocythereis. m)) and lithologic uniformity throughout the Ostracode and calcareous nannofossil assemblages

69 therefore indicate that these beds were nonmarine sandstone and shale red beds, inter­ deposited at upper-slope depths and support the rupted at least once by a shallow-marine trans­ interpretation based on forami-nifers gression in the Aptian and overlain by alternat­ (International Biostratigraphers, Inc., 1977; ing nonmarine to shallow-marine sandstones and Poag and Hall, this volume). shales of Aptian-Albian Age. These clastic The overlying lower to middle Miocene rocks are succeeded by shallow-marine carbonate carbonates are similar lithologically to those strata associated with evaporites. A relatively of the upper part of the Oligocene. Only one thin, poorly dated interval composed of shallow- nannofossil sample is fossiliferous, and the marine limestone and calcareous sandstone, which assemblage has a pelagic aspect, as almost all occurs above Albian carbonate rocks and below the commonly occurring neritic species are Turonian chalks, may mark an unconformity of absent. Ostracodes in two samples from the probable Cenomanian Age. The Upper Cretaceous Miocene include species of Cytheropteron. argillaceous chalks represent a marine carbonate Ech inocythereis, Henryhowella. Marocypris. and environment indicative of shelf and upper-slope Muellerina. an association indicative of outer- depths that prevailed through the Maestrich- shelf and upper-slope environments. Several tian. A marine carbonate-shelf environment other ostracode genera that are generally found persisted throughout the Paleocene, Eocene, and at outer-shelf and shallower depths are also lower part of the Oligocene interval. The present (Actinocythereis. Acanthocythereis. limestones and shelly carbonate sands of the "Cytheretta," and Pterygocythereis). Benthic upper part of the Oligocene sequence and the foraminifers occurring in the Miocene section lower to middle Miocene were, however, deposited point to deposition in upper- to middle-slope at upper-slope depths and represent a deepening water depths (International Biostratigraphers, that probably resulted from a eustatic rise in Inc., 1977; Poag and Hall, this volume), whereas sea level. Previous investigations of cores the ostracodes and calcareous nannofossils imply from this area have also documented a deepening upper-slope depths. of the marine environment during the Miocene In summary, the post-Paleozoic rocks in the (Charm and others, 1969). Post-Miocene bio- COST No. GE-1 well represent, in a broad sense, clastic calcareous sands are characteristic of a transgressive sequence of Lower Cretaceous deposition in a marine-shelf environment.

70 RADIOMETRIC AGE DETEKMINATIONS I/'

E. K. Simonis

Radiometric dating of selected samples The Mesozoic (159 ± 6 m.y.) K-Ar age of the between 11,250 and 13,254 ft (4,329-4,040 m) felsic rock from 13,247 ft (4,038 m) is made appears to indicate a Late Devonian (approxi­ questionable by Rb-Sr age determinations, Paul mately 355 m.y.) age for the sampled section. D. Fullagar of the University of North Carolina The radiometric ages may, however, reflect the (written commun., 1977) obtained Rb-Sr model effects of a Late Devonian thermal event and at ages on seven whole-rock samples (table 7).The least some of the rocks may be older. isochron age of all seven samples is 363 ± 7 The dominant lithology from an unconformity m.y. However, Fullagar pointed out that, with at 11,050 ft to 12,750 ft (3,368-3,886 m) the exception of the sample from 13,128 ft consists of green and green-gray, very fine (4,038 m) (table 7), the remaining six points grained, highly indurated or weakly meta­ are highly colinear on the isochron plot and morphosed, pelitic, possibly bentonitic give an age of 355 ± 3 m.y. He considered this sedimentary rocks. Meta-igneous rocks contain­ to be the most reliable age for the sampled ing albite, epidote, chlorite, and numerous section. quartz veins increase in abundance below 12,750 ft (3,886 m). A brief examination of a thin Table 7. Rb-Sr whole-rock analyses from the section from core No. 15 (13,252-13,254 ft (4039 COST No. GE-1 well to 4040 m)) reveals a relict igneous texture consisting of fine-grained felted plagioclase and epidote with phenocrysts and (or) xenocrysts Sample ___ Depth Radiometric of altered feldspar and less abundant olivine No. Feet Meters age (m.y.) and (or) pyroxene. It is not clear if the igneous rocks represent flows or shallow intrusives. Presence of epidote, albite, and 12,900-13,025 3,932-3,970 353 ± 4 chlorite, and the absence of minerals associated 12,900-13,025 3,932-3,970 360 ± 4 with higher grades of metamorphism indicate a 13,128 4,001 383 ± 4 relatively low grade of metamorphism. 13,128 4,001 350 ± 10 Whole-rock K-Ar ages of three samples were .13,207 4,025 353 ± 10 determined by Geochron Laboratories (written 13,247 4,038 331 + 85 commun., 1977) with the following results: 13,252-13,254 4,039-4,040 372 i 30 11,250 ft (3,429 m) meta-bentonite(?) 374 ± 14 m.y. 13,128 ft (4,001 m) quartz sericite schist(?) 346 ± 12 m.y. 13,247 ft (4,038 m) felsic rock 159 ± 6 m.y.

' This section reprinted, with minor changes, from Amato and Bebout (1978).

71 GEOTHERMAL GRADIENTS

E. I. Robbins

Present-day subsurface temperatures can be The liquid-petroleum window is based used to approximate the depth range of maximum largely on empirical observations and generally hydrocarbon generation (liquid-petroleum window) is considered to have a lower limit of about in a basin. This can be particularly useful 150°F (66°C) and an upper limit of 270°-300°F when other studies are performed to determine (132°-149°C) (Pusey, 1973; Harrison, 1976). The whether the modern temperatures are the highest upper and lower temperature limits may be to which the section has been subjected. significantly higher if heating times are For the COST No. GE-1 well, 58 geothermal short. Using the present geothermal gradient in values have been plotted (fig. 32) using the GE-1 well, the liquid-petroleum window would approximate equilibrium temperatures calculated be expected to occur between 4,700 and 20,000 ft from the three available temperature logs. Data (1,425-6,100 m), provided these temperatures -points were chosen at approximately 500-ft (150- have been maintained over a significant period m) intervals. The eight other points, of time. Organic geochemical analyses, coupled represented by crosses in figure 32, are with vitrinite reflectance and visual analysis temperature values taken from headers of other of color alteration of organic matter geophysical logs run in the well. Although (summarized by Miller and others, this volume) these latter points are considered less reliable suggest that similar geothermal gradients have than the temperature log values, they do provide prevailed over the past 120 m.y. However, most some check on the overall accuracy of the reconstructions of the geologic history of the temperature-logs. A linear regression was Atlantic Continental Margin (Bott, 1971; Falvey, computed using data points from the two deepest 1974; Sleep, 1971) would predict considerably temperature logs; it shows a very systematic higher geothermal gradients in the past, increase in temperature with depth (correlation particularly during Triassic and Jurassic coefficient = 0.99). The geothermal gradient is rifting. Thus, the older sedimentary rocks calculated to be 0.89°F/100 ft (!6.2°C/km), with within the Southeast Georgia Embayment may have a surface intercept of 108°F (42°C). This been subjected to higher temperatures than those gradient is somewhat lower than typical values calculated from present-day geothermal gra­ for basins around the North Atlantic Ocean dients. (table 8).

Table 8. Circum-Atlantic offshore geothermal gradients

Basin Gradients References (°F/100 ft) ( °C/km)

Scotian Shelf basin 1.2 21.9 Robbins and Rhodehamel, 1976. Southern North Sea 1.7 29.7 Harper, 1971. Baltimore Canyon basin 1.3 23.7 Scholle, 1977a. Niger Delta shelf 1.4 25.5 N wa chukwu, 1976. South Pass, La. 1.2 21.9 Pusey, 1973. Offshore Louisiana 1.3 23.7 Jam and others, 1969. Southeast Georgia Embayment 0.9 16.2 This paper.

72 TEMPERATURE IN DEGREES FAHRENHEIT 5O 1OO 15O 2OO 25O 3OO

500

10OO

15OO

2OOO CO DC UJ UJ UJ 8 2500 X Q. UJ D 10 3OOO

11

35OO 12

13 4OOO

14

45OO 15

16 -18 25 5O 75 1OO 125 15O TEMPERATURE IN DEGREES CENTIGRADE

Figure 32. Present-day temperature data for the COST No. GE-1 well. Dots are resting temperatures taken from three available temperature logs; crosses are temperatures taken from headers of other geophysical logs. Geothermal gradient is shown as a least-squares fit to the data from the two deepest temperature logs.

73 ORGANIC GEOCHEMISTRY

R. E. Miller, D. M. Schultz, G. E. Claypool, M. A. Smith, H. E. Lerch, D. Ligon, C. Gary, and D. K Owings

Purpose and Scope and maturity discussion are based, consisted only of rock chips larger than Tyler 10 to 20 The COST No. GE-1 well was drilled largely mesh from which all the diesel-stained walnut for the purpose of evaluating the petroleum husks and mica were physically removed. potential of the Southeast Georgia Embayment. Although the solid contaminants were removed, To aid in this evaluation, the following some of the rock chip samples were very likely objectives were established: (1) characterize still contaminated from the liquid "free pipe" any rocks that might be considered prospective and possibly diesel added to the mud circulation of petroleum and natural gas; and (2) determine system. Organic-carbon and thermal-pyrolytic and identify, where possible, the apparent analyses were performed by the USGS on 27 influence of drilling-mud additives on the true unwashed and unpicked well cuttings ranging in hydrocarbon-source character. depth from 450 to 13,220 ft (137-4,029 m). Organic geochemical studies of GE-1 well These analyses were used to supplement the cuttings were carried out by USGS offices in organic-richness and thermal-maturation inter­ Reston, Va., and Denver, Colo. The geochemical pretation derived from the composition and analyses employed in these studies of sedimen­ amount of extractable organic matter. tary rocks measure (1) total organic carbon content by wet oxidation and combustion, (2) Terms and Definitions extractable hydrocarbons by solvent soxhlet extraction and chromatographic analyses, and (3) The terminology used in this report to hydrocarbon-generating capacity of solid organic describe the petroleum source-rock potential of matter by pyrolysis. The interpretation of such the COST No. GE-1 area follows, in general, the measurements normally assumes the organic concepts of organic richness and maturity substances detected to be indigenous to the expressed in the COST No. B-2 well studies by sediment composing the stratigraphic unit Claypool and others (1977). The organic sampled. However, in the COST No. GE-1 well, richness of a source bed and its petroleum loss of mud circulation at approximately 2,800 potential may be measured by (1) a minimum ft (850 m) resulted in the need for mud amount of total organic carbon on a weight- additives such as IMCO "free pipe," walnut percent basis (0.7-1.0 percent for argillaceous husks, mica, and possibly diesel fuel to be rocks); (2) pyrolytic oil yields in excess of added to the mud system in order to restore 0.2-0.3 percent; and (3) solvent-extractable circulation and free the drill string. The hydrocarbon concentrations in excess of 100-500 effect of these mud additives seriously ppm. The maturity of a source bed is a term complicates a true interpretation of these that expresses the degree to which the con­ source-rock parameters by masking, either vertibility of organic matter to petroleum totally or partially, the indigenous hydrocarbon hydrocarbons has progressed through temperature- characteristics. and time-dependent kinetic reactions. An important point to be considered is that some In this present study, the USGS analyzed source beds must contain more organic carbon two sets of well cuttings samples from 570 to than normal to be considered possible source 10,370 ft (174-3,161 m) for their C 15+ rocks. This arises from the much poorer con­ hydrocarbon type, amount, and composition. The version capability of some types of organic first set of samples included the coarse- and matter such as humic and lignin compounds. The fine-sediment-size fractions (Tyler No. 10 to availability of hydrogen and the molecular 100 mesh size), and also contained varying structure of the organic constituents will amounts of "free pipe" and diesel-stained walnut influence the degree of convertibility of husks, mica flakes, and some traces of iron organic matter to petroleum; therefore, the type filings. The second set of samples analyzed, of organic matter is important to source-bed upon which the quantitative source-rock richness performance and evaluation.

74 Organic-rich sedimentary rocks are defined of volatile organic compounds produced by as immature when the petroleum-generating pyrolysis of the solid organic matter is at a reactions have not altered the organic matter to maximum. the point at which a minimum amount of petroleum In the extractable-heavy-hydrocarbon (Cjc+) has been thermochemically generated and portion of this study, unwashed well cuttings physically expelled. Furthermore, those from approximately 570 to 10,370 ft (175-3,160 sedimentary rocks that contain predominantly m) were removed from their plastic storage bags; biochemically derived hydrocarbons in low and the drilling mud was carefully removed, as concentrations but yet are shown to be capable well as possible, from each rock chip by of generating petroleum hydrocarbons by thoroughly washing the sample under running pyrolysis are described as immature. water through a Tyler number 100 mesh screen. The term mature may be used to describe The sediment retained on the screen was air-and those source beds in which the thermochemical oven-dried at 95°-104°F (35°-40°C). Each washed conversion of the organic matter disseminated in and dried sample was then carefully examined argillaceous sediments has occurred to the under a binocular microscope and lithologically extent that oil expulsion could have taken described, and all "foreign material" including place. The principal criteria used to define string, rubber, and plastic was removed. The the degree of thermochemical conversion of the lithologic descriptions of the samples analyzed organic matter to oil are the molecular are shown in table 9. composition and relative concentration of In addition to the "foreign material," the indigenous extractable hydrocarbons. When the binocular examination of the samples showed the extractable hydrocarbons are present in amounts presence of variable amounts of mud additives of about 1-2 percent of the total organic carbon such as walnut husks and mica flakes. The and the hydrocarbons are chemically identical organic composition of the husks was confirmed with petroleum, then the source beds are by burning the husk to form a soft, gray-white, described as being mature. fluffy ash. These walnut husks have a deceiving The terms potential source and possible crystalline appearance similar in some regards source rock are used in the same context as that to that of rock fragments; they were darkly defined by Claypool and others (1977, p. 46): stained around their edges, probably by diesel potential source rock "A sedimentary rock that or IMCO "free pipe." In an effort to identify is rich in organic matter, and that is the possible effects of the mud additives on the immature." Possible source rock "A sedimentary gas-chromatographic character of the saturated rock that is rich in organic matter, and that is hydrocarbons, separate sets of samples were mature." analyzed. One 'set was composed only of those rock chips larger than 10 to 20 Tyler mesh and Analytical Procedure from which all the walnut husks and mica flakes had been physically removed by sieving and In the thermal-analysis studies, unwashed visual hand picking of the samples. The other and unpicked samples of wet drill cuttings and set contained all the sediment-size fractions mud were freeze-dried and analyzed for organic larger than 100 Tyler mesh and included the carbon by wet oxidation and for hydrocarbon walnut husks, mica flakes, and some traces of yield by thermal-evolution analysis. Organic iron filings. Both sample sets were then ground carbon was determined by Rinehart Laboratories, using mortar and pestle to pass through a 60 Arvada, Colo. using a chemical (chromic acid) Tyler mesh screen and weighed. oxidation technique modified from Bush (1970). The solvent-extraction technique and li­ Thermal analysis was done in the manner quid-column- and gas-chromatography methods were described by Claypool and Reed (1976), except adopted and modified from the procedures that the response of the flame ionization described by Miller and others (1976). The detector was calibrated by analysis of a ground sediment samples were soxhlet-extracted synthetic standard (nrC20H42 at 4.24 percent on for 24 hours using redistilled chloroform. If ^2^3) * Total pyrolytic hydrocarbon yield elemental sulfur was found to be present in the (weight percent) is the integrated response of extract, a column of activated copper was the flame ionization detector converted to an employed to remove the sulfur. Each extract was equivalent weight of hydrocarbon as Jl-^O*^ separated into paraffin-naphthene, aromatic, and during the time that the rock sample is heated NSO fractions by elution-column chromatography in a flowing stream of helium from about 120° to using Bio-Sil HA, 100 to 200-mesh, silica gel as 1,330°F (50°-720°C) at 72°F (40°C) per minute. the solid support. The liquid-solid chroma­ Volatile hydrocarbon content is the component tography columns employed were approximately 8 (ppm) of the total pyrolytic hydrocarbon yield in. (20 cm) long by 0.4 in. (1 cm) in interior that is obtained from the rock at lower diameter. temperatures (<600°F (<320°C)). The ratio of The saturated (paraffin-naphthene) frac­ pyrolytic hydrocarbon yield to organic carbon, tions were analyzed on Perkin Elmer 3920 gas in percent, is a measure of the convertibility chromatographs. Each saturated hydrocarbon of the organic matter to hydrocarbons. Also fraction was analyzed for both qualitative and reported is the temperature at which the yield quantitative information. The qualitative

75 Sample Depth interval Sample Depth interval No. Feet Meters Llthology No. Feet Meters Lithology

7 570-600 174 - 183 Shell fragments (micro 194-196 4,740-4,770 1,445-1,454 Very argillaceous biomicrite, and macro), whitish-gray light gray to yellow, with limonite staining (100 percent) . sparite and sand, clear and Abundant drilling mud; light- brown angular walnut husks; ooze (55 percent).

22 soft, finely crystalline, biomicrite; fossiliferous shale; slightly fossiliferous (05 glassy spheres; traces of percent: sand, clear to aragonite. Abundant drilling frosted, subangular to subrounded mud; light-brown angular walnut (5 percent) . husks; mica.

37 1,470-1,500 448 - 457 Soft limestone, fragments of 254-256 5,340-5,370 1,628-1,637 Light gray shale, very calcarenite plus ooids, chalk calcareous and silty (100 (biomicrite) (70 percent); percent); slight amount of

subrounded; Foraminifera (30 of pyrite. Abundant drilling percent) . mud; light-brown walnut husks: mica. 48 1,800-1,830 549-558 Limestone (biomicrite), argillaceous, traces of dense, 350-353 6,300-6,330 1,920-1,929 Light- to pale-pinkish-red crypto-crystalline dolomite limestone (oomicrite) , some (100 percent) . sparite in packstone and grapestone (60 percent); shale, 76 2,640-2,670 805 - 814 Limestone (chalk-biomicrite) , hard to medium, gray (40 percent). slightly argillaceous, moderately Light-brown walnut husks, mica.

brownish-gray (40 percent) . of pink clay (5 percent); small 98 3,300-3,330 1,006-1,015 White, soft, argillaceous amounts of carbonaceous, coaly shale; calcareous, dense sand­ stone (30 percent); calcareous chert (100 percent). Mud shale (20 percent) . additives: mica, light-brown, angular to subangular walnut 578-580 8,580-8,610 2,615-2,624 Frosted quartz sandstone with husks: iron fillings. Dipsel- carbonate cement (65 percent). fuel odor in unwashed samples. Shale, slightly calcareous, small amount of light-red to medium- 101 3,390-3420 1,033-1,042 White, soft, argillaceous gray silt. (35 percent): Trace limestone (70 percent): gravish- coaly organic matter. Light-brown walnut husks. possible glauconite (30 percent). Drilling mud, walnut husks, 656-658 9,360-9,390 2,853-2,862 Coarse- to medium-grained, iron fillings, mica: diesel- fuel odor in unwashed samples. frosted; slightly calcareous 110 3,660-3,690 1,116-1,125 Calcareous shale grading shale, silty shales, light- gray to red (25 percent); trace chalk. linonitic staining, traces of quartz (30 percent). Abundant 755-757 10,350-10,380 3,155-3,164 Reddish-brown to pale-gray drilling mud, nica, light- shale with traces of

(90 percent); approximately 1 148-149 4,280-4,300 1,305-1311 Very poor sample recovery. percent dark-brown dolomite, Light-grav calcareous shale, fossiliferous; white micrite quartz sandstone. pellets; abundant drilling mud. 805-807 10,850-10,880 3,307-3,316 Sandstone, silica-cemented, 156-158 gray to light-red, with some biomicrite (100 percent) , with conglomerate fragments (75 percent) ; gray shale with siltsone (25 percent); gray shale with black crypto- crystalline dolomite (6 percent): coaly, carbonaceous organic matter (4 percent). analyses were performed on a 12.5 ft x 0.09 in. temperature programed at 7°F/tnin (4°C/tnin) to a (3.8 m x 0.236 cm) stainless steel column, final temperature of 473°F (245°C) and held at packed with 5 percent OV-1, 80- to 100-mesh CW- this temperature until n-C^ eluted. H.P. The helium flow rate was 1.8 in. /tnin (30 The determination of total organic carbon ml/min). The instrument temperature was pro­ content of the rock samples involved first gramed from 176°F (80°C) at injection at drying, then grinding the sample to less than 60 29°F/min (16°C/tnin) for 8 minutes; it was ten mesh. A weighed aliquot of approximately 0.2 g temperature programmed at 14°F/min (8°C/min) to was then digested in hot, 6-Normal hydrochloric a final temperature of 572°F (300°C). A OV-1, acid to remove the carbonates and dried. The 66-ft (20-m) SCOT column was employed for the dried samples were then reweighed and combusted quantitative determinations. The column con­ in a F and M Model 185 Carbon, Hydrogen, Nitro­ ditions were 212°F (100°C) at injection and were gen Analyzer. Duplicate splits of the sample held isothermally for 1 minute; they were then were analyzed for total organic carbon content.

76 The following expression, modified from Hunt (1974), was used to calculate the Carbon Preference Index (CPI):

CPI (%nC25 %nC2?

(%nC24 %nC28 %nC3Q ) 26 %nC28

Results and Discussion this interval is 0.16 percent, considerably less than the worldwide average of 0.27 percent for Tertiary. The Tertiary section in the COST carbonate rocks (Hunt, 1961; Gehman, 1962). The No. GE-1 well, 390-3,570 ft (118 to 1,088 m), extractable C^^+ hydrocarbon content averages 32 consists largely of unconsolidated, moderate- to ppm for the Tertiary, which is below the deep-water, chalky, cherty, and shaly limestones limiting range (50 ppm) needed for a poor or grading into calcareous shales at about 3,090 ft adequate petroleum source bed (Philippi, 1957; (942 m) (Amato and Bebout, 1978). Baker, 1972). The data for total pyrolytic hydrocarbon A seemingly mature character of the C^+ yield, total extractable hydrocarbon, and total hydrocarbons in the interval from 600 to 2,800 organic carbon (weight percent), shown in tables ft (183-853 m) in the Tertiary is suggested by 10 and 11, suggest a very organic-lean Tertiary the comparatively high values of the saturated section. The average total organic carbon for paraffin-napthene-to-aromatic ratio, which

Table 10. Organic-carbon and thennal-evolution analyses. COST No. GE-1 well

[Leaders ( ), no data available; (*) indicates oil staining or contamination present]

Pyrolytic Total hydrocarbon Sample organic Pyrolytic Volatile Temperature total organic depth carbon hydrocarbon hydrocarbon of maximum carbon Bag interval (weight yield content pyrolysis yield No. Feet Meters percent) (weight percent) <°C) ratio (jjercent) (ppm)

3 450- 480 137-146 0.13 0.007 9 508 5.4 21 990-1,020 302-311 0.39 0.081 105 472 20. b 36 1,440-1,470 439-448 0.16 0.028 31 484 17.5 51 1,890-1,920 576-585 0.17 0.035 47 500 20.0 66 2,340-2,370 713-722 0.13 0.038 30 463 jy.2

81 2,780-2,820 847-860 12.1 4.03 16,000 * _ 33.3 96 3,240-3,270 988-997 0.41 0.14 290 * 4o4 33.1 111 3,690-3,720 1,125-1,134 1.19 0.72 1,230 * bO.5 138 4,180-4,190 1,274-1,277 1.15 0.38 720 * 45S J3.U 189 4,690-4,700 1,430-1,433 0.25 0.037 bO 44b 14.6

240 5,200-5,210 1,585-1,588 14.5 5.35 17,000 * _ 3b.9 291 5,710-5,720 1,740-1,743 0.63 0.14 185 464 2 L *! 342 6,220-6,230 1,896-1,899 0.18 0.017 19 b2U 9.4 393 6,730-6,740 2,051-2,054 0.20 0.017 20 312 8.3 444 7,240-7,250 2,207-2,210 0.34 0.015 IS 472 4 . -4 495 7,750-7,760 2,362-2,365 0.50 0.029 44 47b b.b

546 8,260-8,270 2,518-2,521 0.47 0.052 85 * _ 11.1 597 8,770-8,780 2,673-2,676 0.31 0.027 50 470 8.7 648 9,280-9,290 2,829-2,832 0.14 0.010 92 476 7.1 699 9,780-9,800 2,981-2,987 0.10 0.019 36 44o 19.0 750 10,300-10,310 3,139-3,142 0.08 0.006 27 7.5

800 10,800-10,810 3,292-3,295 0.17 0.010 10 470 5.y 850 11,300-11,310 3,444-3,447 0.08 0.010 12 480 12.5 900 11,800-11,810 3,597-3,600 0.13 0.004 7 492 3.1 950 12,300-12,316 3,749-3,754 0.08 0.003 11 490 3.8 1,000 12,800-12,810 3,901-3,904 0.12 0.002 3 495 1.7 1,042 13,220-13,230 4,029-4,032 0.12 0.002 5 492 1.7

77 Table 11. Organic carbon and extractable organic matter, COST Mo. GE-1 well [Leaders ( ) indicate no data available]

Well Organic Total Hydrocarbon Hydrocarbon Saturated CPT Interval carbon hydrocarbons Organic carbon Extractable Aromatic ratio Organic matter ratio ratio Feet Meters (percent) (ppm) (percent)

570- 600 174- 183 0.20 25 1.26 0.43 2.63 1.08 1,470-1,500 448- 457 0.04 6 1.60 0.13 1.23 1.17 1,800-1,830 549- 558 0.15 15 1.00 0.24 l.«2 1.00 2,040-2,070 622- 631 0.10 26 2.60 0.2S 1.76 l.?4 2,640-2,670 805- 814 0.07 46 6.63 0.28 2.16 0.90

3,300-3,330 1,006-1,015 0.09 61 6.79 0.39 1.30 1.93 3,390-3,420 1,033-1,042 0.32 43 1.35 0.17 1.22 4,280-4,300 1,305-1,311 0.15 360 23.90 0.37 0.29 1.09 4,360-4,390 1,329-1,338 0.46 410 8.91 0.62 1.89 0.85 4,670-4,700 1,423-1,433 0.09 40 4.39 0.15 0.40 0.97

4,960-4,990 1,512-1,521 4.30 95 0.22 0.20 0.33 0.92 5,340-5,370 1,628-1,637 1.20 175 1.46 0.25 0.29 1.23 6,300-6,330 1,920-1,929 0.28 23 0.80 0.35 0.55 1.51 7,470-7,490 2,277-2,280 0.35 32 0.92 0.14 0.22 1.47 8,580-8,610 2,615-2,624 0.18 9 0.49 0.05 2.00 1.55 9,360-9,390 2,853-2,862 0.01 6 6.24 0.06 1.75 1.14 10,350-10,370 3,155-3,161 0.02 3 1.59 0.04 6.95 1.26

ranges from 2.63 to 1.22. The relative con­ With the exception of one sample interval, centration of saturated paraffin-naphthene-to- 2,780-2,820 ft (847 to 860 m), the total organic aromatic hydrocarbons is sensitive to the carbon (weight percent) and total pyrolytic thermal history, type and percent of organic hydrocarbon yield for these Tertiary rocks matter of a sample, and preferential destruction correlate well with the interpretation of a very of aromatic hydrocarbons (Baker, 1972; Baker and lean to poor source-rock character. The pyro­ Claypool, 1970; Louis and Tissot, 1967). More lytic hydrocarbon-to-total-organic-carbon ratio recently it has been suggested that thermally averages 23.3 for these Tertiary sed-iments, and immature sediments may have paraffin-naphthene- may reflect the convertibility of the more to-aromatic ratios of less than 0.5 and a lipoidal, hydrogen-rich, marine amorphous hydrocarbon-to-organic carbon ratio less than kerogens present in these rocks (tables 9, 10). 0.8 percent (Claypool and others, 1978). In Figures 33 and 34 show a series of gas addition, it has been shown that hydrocarbon-to- chromatograms of the saturated paraffin-naph- total-organic-carbon ratios of more than 1 thene hydrocarbons representing the Tertiary percent may indicate that hydrocarbon generation interval. The unresolved complex mixture is by thermal alteration has occurred (Philippi, equal to or predominates, in most cases, over 1965; Claypool and others, 1978). If the the resolved aliphatic peaks, a characteristic hydrocarbons in these Tertiary rocks (600-2,800 that is usually associated with thermally ft (183-853 m)) are truly indigenous, then the immature hydrocarbon extract compositions. In high proportion of total hydrocarbon to total contrast, however, the CPI ratios for the extract (bitumen), a saturated paraffin- resolved alkanes are in the range from 0.9 to naphthene-to-aromatic ratio of greater than 1.0, 1.24. These CPI values may not be reliable and hydrocarbon-to-organic-carbon values that because of the limited range of the Cj^ to C«9 range from 1.0 to 6.8 (table 10) would seem to hydrocarbons and their low concentrations (table imply that the hydrocarbons are mature and that 11). The isoprenoids (pristane and phytane) hydrocarbon generation had occurred. However, are, however, dominated slightly by the resolved the high proportion of the extractable normal alkanes, a characteristic that is usually hydrocarbons (ppm) relative to the very low associated with mature saturated hydrocarbons. percentage of total organic carbon would The limited range of the resolved alkanes (C,g probably imply a nonindigenous origin for the to C99 ) is inconsistent with an immature greater part of the C^+ hydrocarbons present in hydrocafrbon assemblage, but does closely these rocks. Because of the effect that small resemble both refined diesel fuel and the absolute concentration values, either in the commercial mud additive IMCO "free pipe." denominator or numerator, may have on specific The probability that nonindigenous hydro­ geochemical ratios, this factor must be taken carbons in this Tertiary interval are a result into consideration in the interpretation of the of up-dip migration seems remote considering the geochemical significance of these ratios low geothermal gradient and regional (Swetland and others, 1978). structure. This Tertiary interval from 600 to

78 CARBON NUMBER CARBON NUMBER______1 rf i i r^i 1 ' i' i' n ' i' i' i' i' H~~Ii 10 12 14 16 18 20 22 24 26 28 30 32 34 36 10 12 1416 18 20 22 24 26 28 30 32 34 36

570-600 FT 174-183 M

2040-2070 FT 622-631 M

2640-2670 FT 805-814 M

Figure 33. Gas-chromatographic analyses of saturated paraffin-naphthene hydrocarbons of Tertiary rocks above 3,300 ft (1,000 m), COST No. GE-1 well.

79 CARBON NUMBER CARBON NUMBER

10 12 14 16 18 20 22 24 26 28 30 32 34 36 10 12 14 16 18 20 22 24 26 28 30 32 34 36

4670-4700 FT 1423-1433 AA

3390-3420 FT 1033-1042 AA

4280-4300 FT 4960-4990 FT 1305-1311 AA 1512-1521 AA

Figure 34. Gas-chromatographic analyses of saturated paraffin-naphthene hydrocarbons of Tertiary and Upper Cretaceous rocks below 3,300 ft (1,000 m), COST No. GE-1 well.

80 2,800 ft (183-853 m) is believed to contain low seemingly inconsistent behavior in the matura­ amounts of indigenous hydrocarbons of marine tion character of the saturated-hydrocarbon biogenic origin that are not related, for the chromatograms tends to support the notion that main part, to a thermal-chemical history. In the assemblage of extractable, resolved addition, this low-level biogenic background may saturated hydrocarbons in these samples probably have been strongly influenced by nonindigneous, reflects the effects of drilling-mud additives; possibly anthropogenic, hydrocarbons that impart therefore, an interpretation of thermal maturity a more mature signature and more restricted and organic richness based only on the range to the extractable C + hydrocarbons than distribution and composition of extractable would be expected normally. hydrocarbons may be overly optimistic. Upper Cretaceous. The Upper Cretaceous The Upper Cretaceous interval shows a interval from 3,570 to 5,950 ft (1,088-1,814 m), comparatively reasonable and consistent a section of gray, calcareous deepwater shales, agreement in degree of increase between the contains the zone of highest organic carbon total organic carbon (weight percent), total (average 1.24 weight percent) and the highest extractables (ppm), total hydrocarbons C^

81 CARBON NUMBER CARBON NUMBER TTH I ' I ' I ' I ' I I I 10 12 U 16 18 20 22 24 26 28 30 32 34 36 26 28 30 32 34 36

Figure 35.~Gas-chromatographic analyses of saturated paraffin-naphthene hydrocarbons of Upper Cretaceous and Lower Cretaceous rocks, COST No. GE-1 well.

82 content are in the range from 0.015 to 0.052 decreases from 1.5 to 1.1, the saturated-to- weight percent and from 19 to 85 ppm, aromatic-hydrocarbon ratio increases from less respectively. Each of these organic-richness than one to greater than one, and the proportion parameters indicates a poor source-rock quality of volatile hydrocarbons in the total pyrolytic for these Lower Cretaceous rocks (tables 10, hydrocarbon yield increases from 0.1 to 0.2, all 11). over the same depth interval (tables 10, 11). The saturated paraffin-naphthene-to-aro- These measurements all suggest that if rocks matic ratio varies from 0.22 to 2.0, and the containing a sufficient amount of hydrogen-rich hydrocarbon-to-organic-carbon ratio ranges from organic matter had been present in the section 0.92 percent to 0.49 percent, respectively, for below 9,000 ft (2,743 m), they might have the Albian (7,470-7,490 ft (2,276-2,283 m)) and generated petroleum hydrocarbons. The organic part of the Aptian (8,580 to 8,610 ft; (2,615 to carbon content (weight percent) for the rocks 2,624 m)). The gas chromatograms of the deeper than 9,000 ft (2,743 m) is very low, resolved alkanes for this interval show an generally less than 0.1 percent by weight, with increase in the 11-624 to n~^32 hydrocarbons, total hydrocarbons averaging about 5.0 ppm. The which may be indicative of increasing amounts of interval from 9,600 tq 10,800 ft (2,926-3,292 m) terrestrial organic matter. The resolved normal also shows high relative contents of amorphous alkanes are predominant over the isoprenoids kerogen; but, except for one sample, this pristane and phytane. With the exception of the increased relative percentage of apparently 7,470- to 7,490-ft (2,276- to 2,28-3-m) interval hydrogen-rich kerogen types is not reflected by in which the resolved alkanes are dominant, the increased pyrolytic-hydrocarbon-to-organic-car- unresolved complex mixture of branched and bon ratios. cyclic paraffins continues to be comparable with Because of the very organic-lean nature of the resolved normal alkanes (fig. 35). Thermal- these sediments, very small differences in pyrolysis analyses indicate that the nonvolatile concentrations can cause significant changes in organic matter decomposed thermally at temper­ the value of a given ratio; therefore, any atures ranging from 968°F (520°C) at 6,220-6,230 interpretation based on such ratios must take ft (1,896-1,899 m) to 878°C (470°C) at 8,770- this factor into consideration. These probable 8,780 ft (2,673-2,676 m) (table 10). The reason Lower Cretaceous rocks are believed to have very for the seemingly inverse gradient is not known; little or no potential as liquid-petroleum or however, the organically lean nature of these gas source rocks because of inadequate amounts rocks may be a limiting factor in the pyrolysis of organic matter. method for evaluating maturity. Comparison of>Geochemical Measurements and These Lower Cretaceous sedimentary units the Possible Effects of Mud Additives on Source- have organic-richness characteristics that are Rock Interpretation. Figure 36 shows a associated with very poor source beds for comparison between the COST No. GE-1 well liquid-petroleum hydrocarbons and, in addition, analyses performed by the contract-service probably have an immature to moderately immature research company, Geochem Laboratories, Inc., time-temperature history. The oil-generating and similar analyses carried out by the USGS. potential, or convertibility, of the kerogen in The parameters that were compared are the total these Lower Cretaceous rocks has decreased extractables (ppm), paraffin-naphthene hydro­ primarily because of the apparent lower hydrogen carbons (ppm), aromatic hydrocarbons (ppm), content due to the more terrestrial nature of eluted NSO's (ppm), total organic carbon (weight the kerogen. The probability is good that the percent), and total-hydrocarbon-to-total-extrac- geothermal history of this Lower Cretaceous tables, paraffin-naphthene-to-total-extract, interval has been insufficient to have generated total-hydrocarbon-to-total-organic-carbon, and and expulsed significant quantities of liquid- paraffin-to-aromatic ratios. petroleum hydrocarbons or natural gas from the organic matter present in these sediments. Good agreement exists between the ana­ The 8,900- to 11,000-ft (2,712- to 3,353-m) lytical values reported by the USGS and those section of the well has not been definitively reported by Geochem Laboratories, Inc., in terms dated. This interval has, however, been tenta­ of "numerical tracking;" that is, at a given tively described as Lower Cretaceous (probably sample depth, when one value is high, both Valanginian to Aptian) by Poag and Hall (this analyses are high, and, conversely, when a value volume). For the greater part, these rocks are is low, both analyses are low. In terms of fluvial and intertidal, grading into continental absolute quantitative recoveries, the USGS red beds. Occasional marine intercalations values are consistently lower than those occur, however, indicated by carbonate beds. reported by Geochem Laboratories, Inc., but are relatively close in numerical value, as indic­ At a depth of about 9,000 ft (2,743 m), the ated by the paraffin-naphthene, aromatic hydrocarbon-to-organic-carbon ratios increase hydrocarbons, eluted NSO's, and total hydro­ from values of less than one in samples at carbon-to-total-organic-carbon, total-hydrocar­ depths shallower than 9,000 ft (2,743 m) to bon-to-total-extract, and paraffin-naphthene-to- values of greater than one (6.2, 1.6) in samples total-extract ratios. A similar relationship below that level. In similar fashion, the CPI exists for the total-organic-carbon measure-

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S4 merits: the USGS values are slightly less than, CARBON NUMBER but of the same order of magnitude as, those reported by Geochem Laboratories, Inc. The 10 12 14 16 18 20 22 24 26 30 32 34 36 largest spread in the analytical determinations occurs in the interval from 3,600 to 5,400 ft (1,097-1,646 m), where the total extractable yields differ by a factor of three. The saturated paraffin-naphthene-to-aromatic ratios obtained by the USGS are consistently higher than those from Geochem Laboratories, Inc., as are the NSO values for the interval from 4,500 to 10,200 ft (1,372-3,109 m). The total extractable hydrocarbon values reported by the USGS are generally less than those of Geochem WALNUT SHELL Laboratories, Inc., and may be the result of the difference between sample splits and method of storage (canned for Geochem Laboratories, Inc., and bagged for USGS). In general, the quanti­ tative analytical differences are relatively small, with the possible exception of source- U.S.G.S. rock richness for the interval from 3,600 to 5,400 ft (1,097-1,646 m). The possible effect that mud additives may have on the extractable hydrocarbon analyses and the subsequent source-rock interpretation must be given ample consideration, especially for the middle Tertiary to Lower Cretaceous interval from approximately 2,000 to 6,300 ft (600-1,900 m). Gas chromatograms of the saturated paraffin-naphthene hydrocarbons for such £ DIESEL FUEL FREE PIPE additives as IMCO "free pipe," stained and crushed walnut husks, and diesel fuel (IMCO Services) are shown in figure 37. Seven gas chromatograms were made (fig. 38) of the saturated paraffin-naphthene hydrocarbon fractions of well-cutting samples from 2,040 to 6,300 ft (622-1,929 m), in which no attempt was M_____ made to separate the "rock chips" (Tyler 10 to NAPHTHENES NAPHTHENES 20 mesh)' from the mica, fine-clay fractions, and Courtesy Geochem, Inc. Courtesy Geochem, Inc. crushed walnut husks. The gas chromatograms of the saturated paraffin-naphthene hydrocarbons in Figure 37. Gas-chromatographic analyses of these unfractionated sediment samples show additives to the mud drilling system, COST significant differences when compared to the No. GE-1 well. Analyses performed by saturated-hydrocarbon gas chromatograms of the Geochem Laboratories, Inc. fractionated samples that are composed only of "rock chips." The gas chromatograms of the continued comparability of the unresolved branch saturated hydrocarbons of the fractionated and cyclic paraffins to the resolved aliphatic samples are shown in figures 33, 34, and 35. compounds may be, in part, the result of the The most apparent difference in the gas liquid-hydrocarbon mud additives (free pipe and chromatograms is the presence of higher-boiling- diesel fuel) adsorbed on the "rock-chip," point resolved normal alkanes (Zjk to ^32^ in cutting samples. Because of the similarity in the unfractionated samples as compared with the molecular composition and restricted range of predominance of a restricted range of the major the dominant resolved aliphatics (Cjc to C~/) of components of resolved normal alkanes (Cj^ to the mud additives to both the unfractionated and ^2^ in the fractionated "rock-chip" samples. fractionated well-cutting samples, it can be Because these sediment samples are predominantly inferred that the mud additives IMCO "free pipe" marine calcareous shales deposited in an outer- and possibly diesel fuel may act to mask, at shelf environment with no nearby source of least partially, the indigenous saturated terrigenous-plant-wax hydrocarbons, the longer- paraffin-naphthene source-rock character of the chain normal aliphatic hydrocarbons (£>2k to ^32^ middle Tertiary to Lower Cretaceous sediments. present in this interval may be, in part, The preliminary thermal-pyrolysis analyses derived from the lipids of the walnut husks, as of unwashed, unpicked drill cuttings from the well as from a petroleum-derived mud additive. COST No. GE-1 well indicate the presence of In the fractionated samples containing only sedimentary rocks which are generally low in the "rock chips," the restricted range and the organic matter content, except in the interval

85 CARBON NUMBER CARBON NUMBER r rrr r r rIT i ' i ' i ' i r i ri' i i i' i' i' i ' r 10 12 M 16 18 20 22 24 26 28 30 32 34 10 12 14 16 18 20 22 24 26 28 30

Figure 38. Gas-chromatographic analyses of unfractionated sediment samples of Tertiary to Lower Cretaceous rocks, COST No. GE-1 well. from 2,780 to 5,210 ft (847-1,588 m), which is samples appear to be less pronounced because of believed to be contaminated by organic sub­ the more consistent agreement between source- stances added to the drilling mud. rock parameters; therefore, a clearer assessment Determinations of organic carbon content may be made of the source-rock potential of the and pyrolytic hydrocarbon yield for the unwashed interval from 6,000 to 13,200 ft (1,800-4,000 and unpicked samples from the COST No. GE-1 well m). are reported in table 10. These preliminary Integrated discussion of USGS geochemical analyses suggest that at least some of the analyses and measurements of the stable carbon samples contain large amount of nonindigenous isotopes, vitrinite, T.A.I., kerogen types, and organic matter. In particular, about half of light-hydrocarbon analyses. In addition to the each sample from the intervals 2,780-2,820 ft USGS studies, contract organic-geochemical data, (847-860 m) and 5,200-5,210 ft (1,585-1,588 m) performed for the COST Group and provided by the consisted of seemingly tar-stained, brown, Conservation Division, Atlantic Resource angular fragments which were later determined to Evaluation Office, USGS, Washington, D.C., are be walnut husks. Excluding those samples in the reported. The contractual organic-geochemical interval from 2,780 to 5,210 ft (847-1,588 m) analyses reported here were carried out by that were believed to be contaminated, only one Geochem Laboratories, Inc. (1977); vitrinite and sample, at 5,710-5,720 ft (1,740-1,743 m), T.A.I. studies, by Core Laboratories, Inc. contained sufficient organic matter (0.63 (1977); stable-carbon-isotope analyses, by J. G. percent organic carbon, 0.14 percent total Erdman, Phillips Petroleum Company (written pyrolytic hydrocarbon yield) to be considered commun., 1978); and light-hydrocarbon analyses even marginally adequate as a potential or by K. F. Thompson, Atlantic Richfield Company, possible petroleum source rock. (written commun., 1977). Although the above- Within the 4,180- to 4,190-ft (1,274- to listed geochemical analyses were not performed 1,277-m) interval, rocks with marginally in laboratories of the USGS, the interpretations adequate petroleum source-rock capability may be of the data that are expressed here are those of present. If the organic carbon contents and the the USGS. pyrolysis characteristics of this interval are Properties sensitive to the type of organic due primarily to the indigenous solid organic matter visual kerogen abundance, pyrolytic- matter, then some rocks within this interval may hydrocarbon-to-organic-carbon ratio, and stable- contain a sufficient (1.15 percent organic carbon-isotope ratio are summarized in figure carbon) quantity of hydrogen-rich (pyrolytic- 39. In addition, those properties sensitive to hydrocarbon-to-organic-carbon ratio of 33 the maturity of the organic matter include percent) organic matter to be considered as vitrinite-reflectance data, volatile-pyrolytic- potential source rocks of petroleum (table 10). hydrocarbon ratios, kerogen-maturation data, and The apparent degree of thermal maturity CPI indices, which are summarized in figure 40. based on the results given in table 10 is higher The C^ to Cy hydrocarbons occur in very than would be expected considering the small amounts, usually less than 200 ppm, in the stratigraphic ages and burial depths of the 600- to 2,800 ft (180- to 850-m) interval of the samples taken from less than 800-ft (1,768-m) Tertiary, indicating poor source quality depth. This maturity is indicated by the (richness) and thermally immature, non- relatively high proportion of volatile generative conditions. The normal C> (butane) hydrocarbons in the total pyrolytic hydrocarbon -to-normal-Cy (heptane) ratio (fig. 41) is less yield and by temperatures of maximum pyrolysis than unity and may suggest diesel contamination yield in the range from 896° to 932°F (480°- (K. F. Thompson, written commun., 1977) 500°C). Rather than indicating only maturity, throughout this interval, whereas the kerogen- it is very possible that the relatively high color maturation rank suggests the organic contents of volatile hydrocarbons represent low matter to be thermally immature and capable of levels of drilling-mud contamination, especially producing only biogenic methane. Vitrinite- for shallow depths less than 5,800 ft (1,768 m) reflectance values average about 0.2 and are in the COST No. GE-1 well. The temperature of consistent with an interpretation of thermal maximum pyrolysis yield probably is unreliable immaturity. An inconsistency exists, however, as an indicator of thermal maturity for these between the apparent thermal immaturity of the samples because they have low organic carbon organic matter inferred from the visual-kerogen- content and have been subjected to drilling-mud maturity rank, vitrinite-reflectance values, and contamination. low amounts of C-, to C-, hydrocarbons, in Those stratigraphic units from approxi­ comparison with the apparently mature character mately 6,000 to 13,200 ft (1,800-4,000 m), which of the gas chromatograms of the saturated contain much less than 0.6 percent organic paraffin-napthene hydrocarbon mixture (figs. 33- carbon and less than 0.14 percent total pyro­ 35, 40, 41). lytic hydrocarbon yield, are believed to be The environment in which organic matter is inadequate for potential or possible petroleum generated by photosynthesis and the nature of source rocks on the basis of insufficient the source of this organic matter can be organic-matter content. The effects of drill­ characterized by stable-carbon-isotope ratios. ing-mud additives on the deeper strat-igraphic The stable carbon-isotope ratio, C/ C, for

87 PYROLYTIC HYDROCARBON CARBON ISOTOPIC COMPOSITION RELATIVE KEROGEN TYPE AND TO ORGANIC CARBON (6C'3PDB) ABUNDANCE (PERCENT) RATIO (WEIGHT PERCENT) -30 -29 -28 -27 -26 -25 -24 -23 -22 -21 -20 0 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 35 40 45 55 60 65 I I I I I I l I I T 1 I I I I I I I I I I I 1 I \ II I I I 1 I

JC ..

125

IS"

EXPLANATION EXPLANATION

« ~ TOTAL EXTRACT o 30 FT INTERVAL AMORPHOUS Y//A POINT SATURATES o 30 FT INTERVAL HERBACEOUS I I POINT AROMATICS « 30 FT INTERVAL WOODY t^S POINT KEROGEN - 30 FT INTERVAL COALY Hffttl POINT Courtesy of Phillips Petroleum Company __ Courtesy of Geochem, Inc.

Figure 39. Summary profiles of types of organic matter present in the COST No. GE-1 well. Relative-kerogen-type and abundance data from Geochem Laboratories, Inc., (1977); carbon isotopic composition provided by G. Erdman, Phillips Petroleum Corp. (written commun., 1978); pyrolytic hydrocarbon to organic carbon (weight percent) from G. C. Claypool, USGS (written commun., 1978). the Tertiary interval, 600-2,800 ft, (183-853 Laboratories, Inc., and Phillips Petroleum m)), is shown in figure 39 in parts per thousand Company data can be made. (permil) deviations from the C/ 1 C ratio of A genetic relationship can be inferred from the PDB carbonate standard. The stable-carbon- the spread (permil) between the kerogen isotope analyses reported here were conducted by (solvent-insoluble organic matter), total - Phillips Petroleum Company (J. G. Erdman, extractable- (bitumen), saturated-paraffin-naph­ written commun., 1977) on total hydrocarbon thene, and aromatic-hydrocarbon fractions. extracts, paraffin-naphthene, aromatic, and Theoretical considerations suggest that as the kerogen sample splits prepared by Geochem spread (permil) between fractions decreases, the Laboratories, Inc. The well-cutting sample potential increases for the extractable intervals analyzed are nearly identical with hydrocarbons to be genetically derived from the those reported by the USGS; and, therefore, a kerogen by thermal-chemical diagenesis. Con­ reasonably good comparison between USGS, Geochem versely, when the spread (permil) increases, the

88 VOLATILE HYDROCARBON KEROGEN MATURATION VITRINITE REFLECTANCE (R°) TOTAL PYROLITIC HYDROCARBON (%) CPI 01 1+2-2 2+3-3 3+ .1 0.2 0.4 0.6 0.8 1.0 2.0 .1 .2 .3 .4 .5 .6 .7 0.3 0.7 1.1 1.5 1.9 1 I I I I I 1 I I I I I T I T I 1

1000

4000

2000 z $

10,000 11,000 H 12,000 2 * S

13,000 USGS Courtesy of Qeochem, Inc Courtesy of Core Labs, Dallas. Texas I I I I I I I

Figure 40. Maturity profiles of organic matter in samples from the COST No. GE-1 well. Vitrinite-reflectance data (Core Laboratories, 1977); volatile-pyrolytic-hydrocarbon ratios (USGS); kerogen-maturation data (Geochem Laboratories, Inc., 1977); CPI values (Geochem Laboratories, Inc., 1977; and USGS). Where comparisons are shown, crosses identify USGS data and dots signify Geochem Laboratories, Inc., data. potential decreases for the saturated paraffin- for the greater part, genetically unrelated to naphthene, aromatic and total-extractable hydro­ the kerogen and, therefore, are not the product carbons to be derived from the solid, solvent- of in situ thermal-chemical diagenesis but most insoluble organic matter. The stable-carbon- likely have a low-temperature biogenic, and isotope values shown in figure 39 indicate a possible anthropogenic, origin. wide spread of 3-7 permil between the kerogen, the total extractables, the paraffin-naphthene, The Tertiary sequence from 2,800 to 3,570 and the aromatic hydrocarbons for the interval ft (853-1,088 m) follows a geochemical pattern from 600 to 2,800 ft (180-850 m). This would similar to that described for the overlying part indicate that the saturated paraffin-naphthene, of the Tertiary, with the noticeable exceptions aromatic fractions, and total extractables are, of the increase in the Cc to C 7 gasoline-range

89 ,- 4 HYDROCARBONS TOTAL C5-C7 PPM) 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 800 900 0 0.5 1.0 1.5 2.0 2.5 3.0 I I I I I I I I I I I I I I I I I I I I I I I I 1 I I I I I I I I I_____I_____|_____I_____I_____I

-2000

0 7000-

=(X7.30|

-3000

10,000-

11,000-

12,000

13,000 Courtesy of Atlantic Richfield Co.

Figure 41. Summary of light-hydrocarbon analyses of samples from the COST No. GE-1 well. Data from Geochem Laboratories, Inc. (1977), and Atlantic Richfield Co. (written commun., 1977). hydrocarbons and a smaller spread between the to the hydrogen-rich, amorphous-algal-marine 5 C value of the kerogen and the extract able kerogen (figs. 34, 35, 39). organic matter. These exceptions generally The small amounts of Cj to C^ and C5 to Cy correlate with the depth at which the drilling hydrocarbons present in the upper part of the operations encountered a loss in mud-fluid Lower Cretaceous interval, 5,950-8,900 ft, circulation. These parameters may reflect both (1,814-2,713 m) may suggest a relatively low the influence of drilling-mud additives and the time-temperature diagenetic history for these indigenous nature of the extractable hydro­ rocks, although the small increase in the ^ to carbons . C, content (ppm) and a normal butane-to-normal- The vitrinite-reflectance (R ) values for pentane ratio of about 7.0 at about 8,000 ft the 3,570- to 4,800-ft (1,090- to 1,450-m) (2,450 m) may indicate that generation of traces interval are in the immature range from 0.2 to of natural gas may have started. The R values 0.3 and are consistent with the kerogen- *are in the 0.40 to 0.45 range, while the maturation rank of 1- to 2-, indicating that the kerogen-alteration rank occurs in the 2- to 2 solid organic matter in these Upper Cretaceous range (figs. 40, 41). rocks probably has not experienced sufficient The stable-carbon-isotope ratios for the thermal alteration to form and expulse mature part of the Lower Cretaceous interval from 5,950 liquid-petroleum hydrocarbons (fig. 40). The to 8,900 ft (1,814-2,713 m) show a relatively spread of C/ C ratios between the kerogen- wide spread in the 5 C values between the saturated-, aromatic-, and total-hydrocarbon kerogen-, saturated-, aromatic-, and total- extracts is about 2 permil, which tends to hydrocarbon extracts that would suggest, first, suggest that the kerogen and extractable that a thermal genetic relationship probably hydrocarbons are probably, for the major part, does not exist between the soluble hydrocarbons genetically unrelated. To further amplify this and the kerogen and, second, that a change is point, the C,- to Cy gasoline-range hydrocarbons occurring in the type of kerogen. The type of show a significant increase while the C, to C, kerogen present in these Lower Cretaceous rocks light hydrocarbons are very low and the normal- shows a significant trend towards smaller butane-to-normal-heptane ratio is less than amounts of amorphous-algal-marine material and unity (fig. 41). This apparent inconsistency, an increase in the relative abundances of in the proportions of the C, to C, and Cc to Cy herbaceous and woody forms (fig. 39). hydrocarbon amounts, relative to the spread in The various fractions of the organic matter the stable-isotope values between the kerogen in sedimentary rocks of probable Early and extractable hydrocarbons, suggests that Cretaceous age below 8,900 ft (2,700 m) have a these hydrocarbons were not generated totally in somewhat narrower range of 6 C values. In situ by geothermal-maturation processes but some of these samples, carbon-isotope ratios rather may reflect the signature of hydrocarbons might support the interpretation of a general possibly related to mud additives. relationship between the hydrocarbons and the solid organic matter. Each of the maturity In contrast to the much wider spread of indicators shown in figure 40 indicates similar about 2 permil for the 13C/ 12C ratio in the trends that are generally consistent with a 3,570- to 4,800-ft (1,090- to 1,450-m) interval, transition from immature to marginally mature a more narrow range (1 permil) exists for the with respect to generation of petroleum. 4,800- to 5,700-ft (1,463- to 1,737-m) interval However, because the organic matter content is between the kerogen, total-extractable, so low, these rocks are inadequate as potential saturated, and aromatic hydrocarbons, which may or possible source rocks. indicate that the presence of liquid hydrocarbons is related, at least in part, to Summary and Conclusions the kerogen. In addition, a comparatively reasonable and consistent agreement exists in The organic geochemical parameters of the degree of increase between the total organic source-rock richness and maturity suggest that carbon, total extractables, total C 15+ the Tertiary interval from 390 to 3,570 ft (119- hydrocarbons, total eluted NSO's, C, to 1,088 m) probably contains very low concen­ content, and total Cc to Cy. The kerogens of trations of indigenous hydrocarbons of biogenic this Upper Cretaceous interval are composed of origin that are not related to a thermal- approximately 40 percent of the amorphous-algal- chemical history. In the lower part of this marine type and have a kerogen maturation rank interval, the influence of the drilling-mud of 2-. The R values are in the 0.35 range and additives is believed to have significantly imply immaturity. The saturated-hydrocarbon influenced the C^+ extractable-hydrocarbon chromatograms continue to show maturation signatures of maturity and richness to the point inconsistencies. However, because of the that an interpretation of source-rock potential comparatively consistent agreement between the based on the extractable-hydrocarbon character richness and type characteristics, a high alone is misleading. probability exists that the extractable liquid hydrocarbons in this interval are, at least in The Upper Cretaceous interval from 3,570 to part, indigenous and may be genetically related 5,950 ft (1,088-1,814 m) contains the zone of

91 highest organic carbon (average 1.24 weight hydrocarbon-to-organic-carbon ratios generally percent), total extractables (981 ppm), greater than 20 percent in this same interval. paraffin-naphthenes (268 ppm), aromatics (278 The Lower Cretaceous sediments from 5,950 ppm), and NSO's (416 ppm) present in COST No. to 8,900 ft (1,814-2,713 m) show an increase in GE-1. Although the influence of mud additives hydrogen-deficient types of kerogens. This on the extractable hydrocarbons is still present increase correlates with pyrolytic-hydrocarbon- and significant in the zone from 3,570 to 4,800 to-organic-carbon ratios of 10 percent or less ft (1,090-1,460 m), Upper Cretaceous sedimentary and, in this same interval, with geochemical units from 4,800 to 5,700 ft (1,469-1,740 m) characteristics that are associated with very show relatively consistent agreement between the poor source beds for liquid-petroleum source-rock parameters, lending support to the hydrocarbons. These rocks appear to have an view that the extractable C, c+ hydrocarbons in immature to moderately immature time-temperature this interval are probably not all the products history that probably has been inadequate to of contamination. They may be, at least in have generated expulsed-liquid-or gaseous- part, indigenous and may possibly be genetically petroleum hydrocarbons. related to the hydrogen-rich amorphous-marine kerogens. The kerogen-maturation rank of 2- and Vitrinite-reflectance and kerogen-color- R values (0.35) still, however, imply thermal alteration values apparently were not affected immaturity. Interpretation of maturity based on severely by the contamination problem. Both of relative concentrations of extractable or these maturity indicators exhibit fairly gradual volatile hydrocarbons is susceptible to increase with increasing depth of burial in the interference because of the presence of mud well. The onset of petroleum-hydrocarbon additives or other sources of nonindigenous generation at geologically significant rates is hydrocarbons. Such interference appears to be a believed to coincide with R values of 0.6 significant factor that affects analyses of percent and kerogen-color-alteration values of sediments from most of the upper half of the 2+. These values are reached or approached in COST No. GE-1 well. Thus, high ratios of samples of the COST No. GE-1 well at depths of volatile to pyrolytic hydrocarbons, and about 9,000 ft (2,750 m). extractable hydrocarbons to organic carbon at The interval from 8,900 to 10,370 ft depths of 5,300 ft (1,615 m) and shallower may (2,710-3,160 m) (probable Lower Cretaceous) indicate contamination. contains sediments which for the greater part Visual-kerogen analyses show high relative are fluvial to intertidal grading to continental contents of hydrogen-rich, "oil-prone" kerogen red beds; because of their very low organic- types in the upper 5,800 ft (1,770 m) of the matter content, they possess little or no section. This is also indicated by pyrolytic- potential as petroleum source rocks.

92 GEOPHYSICAL STUDIES

R. C. Anderson and D. J. Taylor

Velocity information obtained from the assumed to best match the typical frequency interval transit-time log of the COST No. GE-1 range of normally processed marine seismic data well was used to generate synthetic seismograms (fig. 42). The 8-70 Hz synthetic seismogram and plots of vertical depth, average velocity, (fig. 42) shows the additional resolution that and root-mean-square (RMS) velocity versus two- might be obtained with further processing of the way travel time. Manual digitizing of the well- seismic data. log data was performed by visually averaging The interval-velocity curve in figure 42 zones of constant-interval transit times, shows the velocity distribution as a function of producing an irregularly sampled digital time plotted linearly, and can be used to curve. Values from this curve were entered in a correlate stratigraphic changes with reflec­ computerized synthetic-seismogram package, tions . producing the results shown in figure 42. The Velocity informaton from the interval program was restricted to velocity information transit-time log is also useful for converting only. Density data from the well were not seismic travel times to depth. The relationship incorporated into the construction of the between two-way travel time and vertical depth synthetic seismogram because, in the majority of is shown plotted in figure 43. From this rock sequences, the reflection coefficient information, three types of velocity functions depends primarily on changes in velocity rather can be generated: interval velocity (already than in density. shown in fig. 42), average velocity, and RMS The synthetic-seismogram program assumes a velocity. These velocities have been tabulated normal-incident, plane-wave model and computes in table 12 for every 164 ft (50 m) of vertical the reflection coefficient as the ratio of the depth. The interval velocity is shown in figure amplitude of the reflected wave to the amplitude 42 as a function of two-way travel time plotted of the normal-incident wave. The reflection linearly. The average velocity curve is shown coefficient is computed according to the by the heavy line in figure 44. The average formula: velocity can be used in seismic data processing for time-to-depth conversion and for migration. RC = Finally, the RMS velocity, V^g, is a good approximation to stacking velocity in areas of where V represents wave velocity. The gentle regional dip. The stacking velocity is subscripts "i" and "i + 1" denote the rock layer used for correcting the hyperbolic moveout of above and below the reflecting interface common-depth-point (CDP) seismic-data traces respectively. To complete the synthetic seis­ prior to stacking. This velocity function is mogram, an approximation of the Earth's impulse given by response is then applied to the reflection N coefficients. The resulting seismogram can be V. used to correlate geologic intervals to their RMS corresponding expression on a seismic trace. Z Figure 42 shows three synthetic seismograms 1-1 Vi obtained by applying three bandpass wavelets with different frequency ranges. The bandpass and is shown in figure 44. The Vj^g velocity is wavelet represents an approximation of the useful for checking the accuracy of stacking Earth's impulse response to a seismic wave. velocities that have been derived strictly from Bandpass wavelets of 8-30 and 8-40 Hz were the seismic data.

93 SYNTHETIC SEISMOGRAMS REFLECTION VELOCITY, m/sec £ = COEFFICIENTS ° = 0 £2 , + ro *. o> 2 o '* ooono .. ooo^zs 0 > « *» ^ " z Ki -( 1 1 f " i^ 1 p- 1 PJ 1 ' ! i 1

9 i ' , . ' r , - ^=- » >> -f- "C?p p _ _ > s 3 5 >. o i i i i . . -l-_ 01 s| '-* - rrrrfr -*- ' »»- , t \< : Si r r r r r r - t |: =1 (_ ? i J^ j "° +1 -t 2- S »>»>»> *>»»»> _ »»>»»>»>»> _ kkkkkk _ o -I P5H- -| 1 ~" h fe 9 CO _ _ 1. a 5 mm 0 £l /- 1 b» H- II unit - nuii i -1 1 ^ S J _ ? : o ? j- S. o* -L- 01 S 0 >>>>>> -ei i ^ »>»>»» -< h r s S ^-_ ^ i i § kvkvkvkvkvkv J-L tttttt | -f- ^U>f=T_ J . ^ "^" - kkkkkk - LLLLLL & ar 37 ro ^ si- r^^^^^r^ » » » » »»» crcrrt o £ -*~ -~ T^- ~ ^a §3 " -- fc-- g S _ ,- __J- ^F - . >>>>> >>>>>> V LLLLLL - - _-^_ . t P ro I "^ » » »» » ^ CJ1 O co - ^ S f[ c m 2 >>>>.. r r r r r r -*= s § -T M =5 a i ~ 1 f- c_ " :::::: -' C CO ro CO g> fl> >>>>>> 1 1 * * 1 1 o S " __- rv "_ -4E - n , , m m ;;;;:: ^ t- >>>>>> ! u \. t -I-M, i i i > i i ,,,,,, ,.,,,, r CO -r CJ1 TJ coc^ £ S-i. " - _ 4- 5'S 8 3 IT i i i i i i l N "» i i i i i i J- _ --v- «i 1 ^ 1 II Table 12. Velocity data from the COST No. GE-1

Depth Two-way Interval Average RMS velocity (seconds) (m/s) (m/s) (m/s)

951 290 0.359 1,954 1,616 1,616 984 300 0.369 1,792 1,625 1,625 1,148 350 0-416 2,309 1,684 1,693 1,312 400 0.456 2,796 1,756 1,781 1,476 450 0.494 2,746 1,822 1,858

1,640 500 0.532 2,540 1,879 1,923 1,804 550 0.569 3,208 1,933 1,984 1,968 6UO 0.604 2,697 1,987 2,046 2,133 650 0.638 3,048 2,037 2,103 2,297 700 0.670 2,903 2,088 2,162

2,461 750 0.703 3,243 2,134 2,214 2,625 800 0.734 3,110 2,180 2,266 2,789 850 0.768 2,822 2,213 2,299 2,953 900 0.802 3,110 2,242 2,328 3,116 950 0.839 2,605 2,263 2,346

3,281 1 ,000 0.877 2,771 2,279 2,359 3,445 1 ,050 0.909 3,243 2,309 2,392 3,609 1 ,100 0.940 3,672 2,340 2,425 3,773 1 ,150 0.977 2,849 2,354 2,437 3,937 1 ,200 1.014 2,876 2,367 2,448

4,101 1 ,250 1.046 3,208 2,389 2,470 4,265 1 ,300 1.077 3,503 2,414 2,496 4,429 1 ,350 1.106 3,175 2,439 2,523 4,593 1 ,400 1.139 3,079 2,458 2,542 4,757 1 ,450 1.169 3,349 2,481 2,565

4,921 1 ,500 1.198 3,464 2,503 2,589 5,085 1 ,550 1.227 3,387 2,526 2,613 5,249 1 ,600 1.254 3,717 2,550 2,640 5,413 1 ,650 1.284 3,503 2,570 2,660 5,577 I ,700 1.317 3,810 2,581 2,670

5,741 1 ,750 1.345 3,048 2,600 2,692 5,905 1 ,800 1.377 2,650 2,612 2,703 6,070 1 ,850 1.408 4,618 2,629 2,721 6,234 1 ,900 1.432 4,549 2,654 2,751 6,398 1 ,950 1.460 4,763 2,672 2,772 6,562 2 ,000 1.481 5,080 2,701 2,808

6,726 2 ,050 1.503 5,347 2,727 2,841 6,890 2 ,100 1.526 4,689 2,752 2,871 7,054 2 ,150 1.551 3,277 2,772 2,895 7,218 2 ,200 1.580 4,762 2,784 2,905 7,382 2 ,250 1.608 3,386 2,799 2,920 7,546 2 ,300 1.635 3,349 2,813 2,934 7,710 2 ,350 1.662 3,763 2,827 2,948

7,874 2 ,400 1.689 3,674 2,842 2,962 8,038 2 ,450 1.717 3,208 2,854 2,973 8,202 2 ,500 1.747 3,672 2,863 2,982 8,366 2 ,550 1.773 3,858 2,875 2,994 8,530 2 ,600 1.799 4,064 2,890 3,009 8,694 2 ,650 1.824 4,996 2,906 3,025

8,858 2 ,700 1.848 4,064 2,922 3,043 9,022 2 ,750 1.872 4,118 2,937 3,059 9,186 2 ,800 1.898 3,242 2,942 3,072 9,350 2 ,850 1.924 4,118 2,962 3,084 9,514 2 ,900 1.948 4,175 2,976 3,099

9,678 2 ,950 1.972 4,064 2,991 3,115 9,843 3 ,000 1.997 4,064 3,003 3,127 10,006 3 ,050 2.022 3,424 3,015 3,139 10,170 3 ,100 2.048 4,064 3,026 3,149 10,335 3 ,150 2.079 4,064 3,031 3,154 10,499 3 ,200 2.104 4,417 3,042 3,164

10,663 3 ,250 2.127 4,354 3,055 3,178 10,827 3 ,300 2.152 4,482 3,068 3,192 10,991 3 ,350 2.173 4,064 3,084 3,209 11,155 3 ,400 2.193 5,347 3,101 3,230 11,319 3 ,450 2.212 5,347 3,120 3,253

11,483 3 ,500 2.230 5,644 3,139 3,278 11,647 3 ,550 2.248 5,644 3,159 3,304 11,811 3 ,600 2.265 5,751 3,170 3,303 11,975 3 ,650 2.282 5,751 3,199 3,355 12,139 3 ,700 2.299 5,751 3,218 3,379

12,303 3 ,750 2.317 5,751 3,237 3,403 12,467 3 ,800 2.334 5,861 3,256 3,428 12,631 3 ,850 2.351 5,862 3,276 3,452 12,795 3 ,900 2.367 6,096 3,294 3,477 12,959 3 ,950 2.385 5,080 3,313 3,499 13,123 4 ,000 2.402 5,977 3,331 3,521

95 0 50 1 00 1 50 2 00 TWO-WAY TRAVEL TIME, IN SECONDS TWO-WAY TRAVEL TIME, IN SECONDS Figure 43. Depth, measured from sea-level Figure 44. Interval velocity, V»; average datum, versus two-way vertical travel velocity, V*; and EMS velocity, VRMS» time. Geologic age boundaries are approx­ cross-plottea against vertical two-way imate. travel time. Light line is V. STRUCTURE OF THE CONTINENTAL MARGIN NEAR THE COST NO. GE-1 DRILL SITE FROM A COMMON DEPTH-POINT SEISMIC-REFLECTION PROFILE

William P. Dillon, Charles K. Paull, Alfred G. Dahl, and William C. Patterson

INTRODUCTION spacing was 330 ft (100 m). The streamer was towed 985 ft (300 m) behind the seismic source A deep-penetration, CDP seismic profile has at a depth of 30 ft (10 m). A pass band of 8- been recorded for the USGS along a track which 124 Hz was applied during field recording on a passes through the site of the COST No. GE-1 Texas Instruments DFS IV system. Shot-point well (fig. 2). The line, recorded under interval was 165 ft (50 m) for most of the contract by Teledyne Exploration Company, is line. However, the shoreward part of the part of a grid of seismic profiles totalling profile (shelf and slope at water depths less 5,200 mi (8,400 km) that have been recorded than about 1.0 s) was shot at 330-ft (100-m) south of Cape Hatteras in order to examine the intervals. This was necessary because of the structure, history, and resource potential of impossibility of steaming slow enough to fire at the Continental Margin. In addition to crossing 165-ft (50-m) intervals while maintaining a the COST No. GE-1 location, the line crosses the straight cable in the strong flow of the Gulf Atlantic Slope Project (ASP) 3 and Deep Sea Stream. The seaward end of the line, at water Drilling Project (DSDP) 390 drill sites on the depths of more than about 3.6 s, was also shot outer Blake Plateau (fig. 45). The seaward end at 330-ft (100-m) intervals in order to allow of the line, in the Blake-Bahama Basin, can be sufficient recording time between shots. tied to the DSDP 391 drill site through other The line was processed by Teledyne profiles of our grid. Thus, the profile affords Exploration Co. in their Houston processing one of the best opportunities on the United center. The data were demultiplexed, and States eastern Continental Margin to correlate parameter selection (such as filter tests, reflectors and sampled horizons along a profile deconvolution tests, scaling tests, and so from the shelf to the deep sea. forth) were completed. Then basic processing Older regional seismic-reflection profiles was applied, including binary-gain recovery, reported for the United States southeastern spherical-divergence correction, predictive Continental Margin have been single-channel deconvolution, velocity analysis at 2-mi (3-km) data, with limited penetration (Ewing and intervals, nortnal-moveout correction, 48-fold others, 1966; Emery and Zarudzki, 1967; Uchupi CDP stack, post-stack time-variant decon­ and Emery, 1967; Uchupi, 1970). CDP reflection volution, and time-variant filtering and data, which achieve deeper penetration, have scaling. The parts of the line with 330-ft been reported in more recent publications (100-m) shot-point intervals were stacked 24- (Dillon and others, 1976; Dillon and Paull, fold. 1978; Shipley and others, 1978; Buffler and others, 1978; Dillon and others, in press; Special processing was applied to a 6-mi Buffler and others, in press). (10-km) section of the profile landward of the COST No. GE-1 well. For this part of the profile, continuous velocity analysis was METHODS performed by Teledyne. The USGS seismic processing center then applied multi-pass The seismic source consisted of four 540- predictive deconvolution, a technique designed in. (8,850-cm ) airguns towed abreast, 13 ft (4 to attack individual pegleg or interbed mul­ m) apart and at a depth of 26 ft (8 m) ± 10 tiples that interfere with primary events. percent. Firing pressure was 2,000 psi (13,800 Subsequently, both F-K migration and wave- kPa) ± 10 percent. The multichannel streamer equation (finite-difference) migration were cable was 2.2 mi (3.6 km) long and contained 48 used. All special processing was aimed at hydrophone groups. The hydrophone-group spacing improving the signal-to-noise ratio in the nearest the ship was 165 ft (50 m), and the far region of basement reflections.

97 UPPER EOCENE OLIGOCENE MIDDLE EOCENE CON1AC1AN

APTIAN ^"FALEOCENE

MARINE -KIMMERIDGIAN KIMMERIDGIANC?) TRANSGRESSION TRANSGRESSION BERRIASIANC?) FIG 47

TRIASSIC (?) CONTINENTAL MARINE TRANSGRESSION (TOP AND BOTTOM)

IV - i INFERRED MARINE-CONTINENTAL FACES BOUNDARY 14H ___ (CONTINENTAL ON PATTERNED SIDE) INFERRED CARBONATE BANK OR BACK-REEF DEPOSITS 16 INFERRED REEF DEPOSITS F=| REFLECTORS SHOWN IN DEPTH SECTION P23 CONTINENTAL BASEMENT g^T^ TRANSITIONAL BASEMENT K^a OCEANIC BASEMENT

Figure 45. Line-drawing interpretation and depth section calculated from the interpretation of seismic profile (location of profile shown in fig. 2). Heavy lines indicate reflectors used brackets beneath the profile. _B, Depth section.

Stratigraphic horizons determined pa- the reflectors for which depths were calculated leontologically (Amato and Bebout, 1978; Poag and which therefore correspond to the lines in and Hall, this volume; Valentine, this volume) figure 45B^ and seismic reflectors were correlated using a con-version table produced by Schlumberger, DESCRIPTION OF THE SEISMIC PROFILE Inc. For the conversion table, velocities were AND ESTTJMATESOF AGES OF REFLECTORS determined both by conventional down-hole logging (by Schlumberger) and by measuring Continental Shelf, mi 0-25 (km 0-41) arrival times of waves originating at a seismic source at the water surface to a sensor in the Correlation of reflectors and stratigraphy hole (by Seismic Reference Service, Inc.). is most satisfactory beneath the Continental The profile is presented as a line drawing Shelf, where reflectors can be related to strata (fig. 45) intended to display selected re­ of known age penetrated in the COST No. GE-1 flectors as accurately as possible. Estimated well. This part of the profile and our inter­ ages corresponding to selected reflectors and pretation are shown in figure 46. Basement possible environments of deposition are also rocks penetrated in the GE-1 well have been included. Depths to some of the reflectors identified as quartzites and argillites (Halley, shown in figure 45A were calculated by the this volume) and have been radiometrically dated method of Taner and Koehler (1969) and are as Devonian (Simonis, this volume). This base­ presented in the form of a companion profile ment surface apparently does not produce a (fig. 46B). Heavy lines in figure 45A indicate strong reflection event in the seismic record, 550Km

i i J2(?) J1 ' BASEMENT PALEOCENE BERR'ASIAN ALBIAN 550Km

' $$?* ^ BASE OF ESCARPMENT REEF +v

_i____i____u _±l_ _j____i___i_____i_

USGS seismic profile across Southeast Georgia Embayment. A., Line-drawing interpretation of to construct the depth section. Locations of photographs shown in figures 46-49B by

presumably due to the small change in acoustic characteristic of such deposits (Sangree and impedance between the oldest Mesozoic sedi­ Widmier, 1977). The Albian marine limestone and mentary units and the underlying Paleozoic sandstone above the continental terrigenous rocks. Reflections from within the basement, section are characterized by strong and however, are discernible (fig* 46). The ir­ distinctly more continuous reflectors, as is the regular basement surface probably results from Aptian transgressive unit. This good faulting and erosion during Triassic rifting; correlation of reflector characteristics and and, therefore, we assume that at least some of facies at the drill site has encouraged us to the sediments filling the resultant grabens are estimate the marine-continental facies boundary of Triassic age. An unconformity which may in the profile as shown in figure 45A. represent the top of Triassic deposits is The Upper Cretaceous marine shaly chalks observed (figs. 45, 46), but was not penetrated (Halley, this volume) drilled at GE-1 were in the drilling. deposited in a deepening-outer-shelf to upper- Lower Cretaceous (pre-Albian) deposits at slope environment (Poag and Hall, this volume; the drill site are mostly continental, except Valentine, this volume). On the profile these for those corresponding to a brief marine Upper Cretaceous deposits, which were laid down transgression in the Aptian (Poag and Hall, this in an upper-slope environment, are characterized volume; Valentine, this volume). These by a set of continuous but very weak reflec­ continental facies appear in the seismic profile tors. The small amplitude of the reflections (figs. 46, 47) as zones of low reflector attests to the homogeneity of the section. The continuity and variable amplitude, a pattern entire Upper Cretaceous section assumes this

99 COST No. GE-1

CO ST 3E-1 SHOT POINT NUMBER FLORIDA-HATTERAS TURN-^n IK)1 1181 I 1901 2001 2101 SL/°PE 22 01 11.11''''! ' -! /' ' ' -o MIDDLE MIOCENE TWO-WAYTRAVEL MIDDLE AND LOWER OLIGOCENE-- UPPER EOCENE-17^- MIDDLE EOCENE LOWER EOCENE-?- : UPPER PALEOCENE MAESTRICHTIAN-SANTONIAN M CONIACIAN-pS" TURONIAN-rr-: ALBIAN (MARINEH r- CONTINENTAL DEPOSITSV -2 APTIAN MARINE TRANSGRESSION J . ^ ~~- ^^J^ TRIASSIC(') DEPOSITS >^%>vX ^> ^?7^>-r^I7>^ ^=^^=^= -3 Xv^--"---""" ^^"-"^^^ DEVONIAN BASEMENT -4

pTl CONTINENTAL-MARINE FACIES BOUNDARY " + * + "~~ ~ - t^i^ (PATTERN ON CONTINENTAL SIDE) X'^r^^T^^. -5 \\\\ BASEMENT-DEVONIAN METAMORPHIC ROCKS ++ %% +

Figure 46. Section of the profile through the COST No. GE-1 drill site crossing the Continental Shelf and upper Florida-Hatteras Slope and a line-drawing interpretation, both at the same scale. Correlations of reflectors to stratigraphy, as derived from the well, are shown to the left of the interpretation. Location shown in figure 45A. pattern farther seaward (fig. 47), and we con­ faulted, continental Paleozoic rocks to a clude that much of this section was deposited transitional basement that was formed during the across the Blake Plateau as a homogeneous shaly earliest part of the rift stage. The basement chalk at water depths greater than those pre­ rocks are inferred to underlie all of the Blake sently characteristic of continental shelves. Plateau (Klitgord and Behrendt, in press) and to The Paleocene strata produce strong, distinctly be composed of a mixture of mantle-derived mafic continuous reflections, even though their en­ igneous rock and continent-derived sediments, vironment of deposition is not considered to be perhaps including some fragments of old conti­ greatly different from that of the underlying nental basement. Maximum depth to basement, at Upper Cretaceous strata. about mi 137-143 (km 220-230) along the line, Reflections from the Eocene interval appears to exceed 8 mi (13 km). suggest a progradational filling by sediments of We have made tentative age estimates of a restricted area of subsidence in the center of several horizons beneath the Blake Plateau that the Southeast Georgia Embayment (Charles K. are stratigraphically lower than any beds Paull, unpublished data, 1978). The Eocene drilled at the COST No. GE-1 well. The deepest wedge is covered by a fairly uniform blanket of of these is the J2 horizon, which has been upper Tertiary and Quaternary deposits. identified beneath the Blake Basin to seaward; its origin and age, about 175 m.y. , will be Florida-Hatteras Slope and inner discussed in the section on the Blake Basin part Blake Plateau, mi 25-149 (km 41-240) of the line. The correlation of the J2 horizons beneath the Blake Plateau and Blake Basin is In the inner part of this zone beneath the based on structural character. Faults break the Florida-Hatteras Slope (near mi 37 (km 60), fig. strata below J2 in the deep sea but not above; 45), the basement probably changes from block- and a similar structural pattern, with faults

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M 0 ro c fD J~* 0 S w R- § CD * H- O 0 E3 QJ P Ql fD M I HI o CO istical i-i. rrelati 0 assumed o hales S 0 3 Ql * 01 01 O CD sg about CO n CO rt P O O rt XI H co cr TWO-WAY TRAVEL TIME, IN SECONDS (D Hi ha 3* (D o fD fD 3 h-> p ITJ rT' P 3 ft O O O (X ^ o XI rt t~* m fD 0 ET £ CO ![- &. CO o i-1 _^ m _.. H» HM fD h{ 03 ^ ft 01 O m H fD rt 3 w ft 3* *"^ Ql 11 i-ri O Hi 0 ° o 3* I-1 H 01 ft ft H- H- Q) <3> fD CO Ql h-1 CD X3 O o j-1 Q > n co Q> fD ft w 1- S B S ° ft 11 00 fD ^y* Jj I 1 H- a r o fD H 0. H- H- XI o 00 ^ s « O 0 ft fD H- B SI Ql rt rt Hi QI a (D Q) CD CD fD Ql fD 3 ft) fD 1 * fD H o co ^ 3 ft ^_i fD X^f CO ft 3 fD rt fD H- O CT* Ql CO O ' Ql Ql 6 o ?sr ft3" 0 fD OJ O rt 3*ft "83 o a S £ 2 3 03 H 01 fD 3 CD C VO 1 3" hh CXi co co CD CO ft ^ fD 1 P O. o n C^ &i fD fD &. the seismic record corresponding to this layer, fairly energetic current conditions, as broad small breaks appear in the weak reflectors at crossbedding and some cut-and-fill structures the foot of the Florida-Hatteras Slope (fig. 47) are displayed in nearby high-resolution pro­ and CDP-determined interval velocities increase files. from about 2.4 km/s beneath the inner Blake Shipley and others (1978) discussed a Plateau to about 3.2 km/s as the layer becomes multichannel seismic profile that intersects our thinner beneath the Continental Shelf. This line at mi 173 (km 279 or shot point 4630). We change in velocity probably is due to compaction estimate the tops of Upper and Lower Cretaceous by the weight of Cenozoic sediment forming the to occur slightly shallower at the intersection shelf, and the small breaks in reflectors may than they estimate by 650-980 ft (200-300 m). represent small faults resulting from variation Our Aptian top is deeper by 2,000-2,300 ft (600- in the amounts of compaction. 700 m), but we are in quite close agreement on The lower Florida-Hatteras Slope and inner the placement of the Barremian. Our inter­ Blake Plateau appear to be zones of scour, where pretation shows basement at considerably deeper the Paleocene top is truncated and Cretaceous depth than Shipley's by 2-3 mi (3-4 km). rocks are exposed. Dredge hauls of Cretaceous Beneath the outer Blake Plateau, the rocks from the Plateau confirm this (Uchupi, seismic profile contains several zones that we 1967, 1970). have interpreted as representing reflections from carbonate-bank and back-reef facies (fig. Outer Blake Plateau, mi 149-233 (km 240-375) 45A). We believe that we can distinguish these features from reefs which are considered to have Beneath the outer Blake Plateau, the been "bathymetrically positive structures formed basement rises irregularly toward the east from by sedentary, intergrowing organisms" (Bubb and a depth of about 9 mi (14 km) to about 7 mi (11 Hatlelid, 1977, p. 185). The inferred carbonate km) (fig. 45B) Reflectors below the Tertiary banks in the outer Blake Plateau appear on the show a consistent westward dip following the seismic profile as zones in which reflectors basement trend, indicating that maximum overall abruptly disappear or become very weak (fig. subsidence occurred beneath the western part of 48). The zones generally have a sigmoidal lense the Plateau. However, the locus of subsidence, shape and rise in the section toward land. The defined on the basis of maximum interval thick­ inferred back-reef deposits are distinguished ness (fig. 45B), has migrated considerably from the carbonate banks only by their location during the Cenozoic and late Mesozoic. The directly landward of reef structures. thickest interval between basement and the J2 horizon (probably of late Early Jurassic age) Blake Spur, mi 233-283 (km 375-455) occurs at mi 186-196 (km 300-315). Based on interval thickness between J2 and a probable The Blake Spur (fig. 2) is a seaward- middle Upper Jurassic reflector, the depocenter projecting rampart which forms the northward apparently was located at mi 137-143 (km 220- termination of the Blake Escarpment. Its 230). Between this reflector and one inferred surface dips rather gently between water depths to be near the Jurassic-Cretaceous boundary, the of 3,600 and 9,500 ft (1,100-2,900 m). At a thickest interval occurs at about mi 87-93 (km water depth of about 9,500 ft (2,900 m), the dip 140-150). Lower Cretaceous rocks are almost steepens abruptly, assuming an angle similar to uniform in thickness, with a poorly defined that of the Blake Escarpment to the south (fig. maximum at about mi 75-118 (km 120-190), whereas 45). the Upper Cretaceous maximum thickness occurs at Basement rises toward the east beneath the mi 62-75 (km 100-120). Maximum subsidence Blake Spur from 5 to 7 mi (8-11 km) (fig. 46), during most of the Cenozoic, as measured from continuing the general trend of basement re­ the top of the lower Eocene to the sea surface, flectors becoming more shallow seaward, noted occurred at mi 25-34 (km 40-55), but again, as beneath the outer Blake Plateau. Internal in the Cretaceous, the subsidence was very broad basement reflectors dip seaward beneath the and the depocenter poorly defined. We therefore outer Blake Spur (fig. 45), attesting to the see a pattern in which the locus of maximum layered nature, at least in part, of this subsidence shifted landward through time, transitional basement. Although we have not accompanied by a broadening of the area of applied migration, we believe that these are subsidence. truly intrabasement reflectors because dif­ The outer Blake Plateau is covered by a fractions, which are also observed originating thin, seaward-thickening wedge of Cenozoic from the basement surface, assumed a con­ deposits. At the ASP 3 drill site an apparently siderably steeper apparent dip. complete Tertiary section from Paleocene to the The Blake Spur is bounded and structurally Miocene, except for the Oligocene, was pene­ controlled by a pair of reefs (fig. 45A). The trated. The absence of Oligocene, however, reef situated at the seaward margin of the spur might have resulted from a sampling gap of 46 ft developed first. The base of a reef commonly is (14 m) between lower Miocene and upper Eocene difficult to observe in seismic profiles, (Page Valentine, oral commun., 1978). This whereas fore-reef and back-reef deposits are Tertiary section probably was deposited under much easier to penetrate. Therefore, we have

102 CARBONATE BANK FACIES BACK REEF FACIES APTIAN-ALBIAN REEF BASEMENT

Figure 48. Photograph of part of the profile, showing the outer Blake Plateau and inner Blake Spur. Location shown in figure 45A. selected the subhorizontal reflector directly m)) above the top of the Barremian as traced on below the base of relatively steeply dipping seismic section from the DSDP 390 drill site. fore-reef or back-reef facies as a horizon The reef seems to have died by the end of Albian equivalent in age to onset of reef time, as the Albian reflector, traced both from development. By this criterion, it appears that the DSDP 390 and COST No. GE-1 wells, extends the outer reef (mi 278-281 (km 448-453)) did not across the top of the reef. During the life of become well established until more than 6,600 ft this landward reef, the Blake Spur was covered (2,000 m) of sediments had accumulated above by seaward-dipping fore-reef deposits. Pre­ basement, although unidentified patch reefs or viously, beds were deposited horizontally, carbonate banks may have existed which cannot be trapped by the older reef to seaward. After the observed at that depth. Onset of true reef de­ death of the landward reef, the spur was covered velopment occurred after the formation of the by pelagic oozes, deposited at least through the reflector that is presumed to be equivalent to middle Eocene (Benson and others, 1976; DSDP the J2 horizon of the deep sea (figs. 45, 49). 390). A fairly complete Cenozoic section, If the assumed age of the J2 horizon is approx­ except perhaps for the Oligocene, was penetrated imately correct (about 175 m.y.), then true reef at the ASP 3 well (Page Valentine, oral commun., construction probably began at the outer edge of 1978) (fig. 45), and rather thick remnants of the Blake Spur in Middle to early Late Jurassic Cenozoic deposits are present on the Blake Spur time. The death of this reef is well documented as compared to the inner Blake Plateau. This by studies of samples from DSDP holes 390 and indicates that the major erosion which is 392 (Benson and others, 1976) showing that reef indicated by the scoured topography of the spur or carbonate-bank limestones of Neocomian- (fig. 45) is the result of post-Eocene changes Barremian Age occur beneath an eroded surface in deep-sea current activity perhaps related to covered by Barremian-Albian deepwater oozes. major changes in sea level in Pliocene and Apparently, the reef died in Barremian time. Pleistocene time (Vail and others, 1977b). Our correlation of reflectors to the drilled stratigraphic horizons (DSDP 390) also suggests Blake Basin, mi 283-352 (km 455-567) this time for the cessation of reef growth. The deepest coherent reflector that crosses the reef Based on seismic interpretation, basement occurs just 0.08 s beneath the top of the just seaward of the Blake Escarpment is believed Barremian (fig. 49). to be relatively shallow (4.6-5 mi (7.5-8 km)) The reef that forms the landward boundary (figs. 45A, J3), but it drops abruptly into a of the Blake Spur (km 382-386) apparently began basin at least 6 mi (10 km) deep farther to to form just after the end of the Barremian. seaward. This basin, interrupted by a small Its base occurs about 0.10 s (about 650 ft (200 basement peak on the line of profile, becomes

103 Figure 49. Photograph of part of the profile showing the outer Blake Spur and western Blake Basin. Location shown in figure 45A. shallower to seaward and is bounded by a broad tured west of the Blake Spur anomaly. An east­ basement high-centered at about mi 320 (km ward jump of the oceanic spreading center to the 515). This broad high is associated with a Blake Spur anomaly location 175 m.y. ago has magnetic anomaly related to the opening of the been proposed (Vogt, 1973), and this assumption Atlantic, the Blake Spur anomaly (Vogt, 1973; seems to result in the best reconstruction of Klitgord and Behrendt, 1979). Seaward of the early spreading events. If such an eastward Blake Spur anomaly, basement remains at a depth jump did occur, then fracturing of the sedi­ of about 4.0-4.6 mi (6.5-7.5 km) and is believed mentary layers in the region between the old and to be of normal oceanic composition and thick­ new spreading centers might be anticipated. We ness. The change from transitional basement believe that this may account for the faults (modified oceanic) beneath the Blake Plateau to which extend upward to the J2 reflector. We oceanic crust is gradual, no doubt, but we con­ have previously mentioned that a deep reflector sidered it to be completed at the location of beneath the Blake Plateau overlies a similarly the Blake Spur anomaly because the anomaly is faulted sedimentary section, and using the continuous and associated with the ocean assumption that tectonic episodes probably opening. represent regional events, we have tentatively The J2 reflector (fig. 45) pinches out correlated that deep reflector with the horizon against the basement high believed to be the identified as J2. source of the Blake Spur anomaly. Because this A buried Jurassic reef formed on basement basement material is considered to be 175 m.y. has been identified at the foot of the Blake old (Klitgord and Behrendt, 1979), the J2 Escarpment (fig. 45). This reef may have died reflector is considered to be approximately the at J2 time, but subsequently another smaller same age. The sedimentary section directly reef appears to have been initiated slightly above basement and below the J2 horizon appears eastward of the older reef. The younger reef on the seismic section to be extensively frac­ grew atop the latter's fore-reef talus. This

104 interruption in reef development might also be 185 to 175 m.y. ago related to tectonic events that occurred about 175 m.y. ago. The younger reef subsequently In Early Jurassic time, perhaps 180-185 died and was covered by the time of formation of m.y. ago (Vogt, 1973), rifting of continental the strong reflector located between horizons Jl basement slowed, as thinning of this material and Beta. approached a limit, and intrusions of mafic Horizon Jl has been traced from a seismic mantle material began to fill the gap between profile presented by K. Klitgord and J. Grow the separating continents. The oceanward limit (written commun., 1978). They consider this of rifted continental crust probably occurs reflector to represent a 140-m.y.-old stratum beneath the Florida-Hatteras Slope. The mantle because it pinches out against basement that is material which welled up in the gap between estimated to be of this age. Their interpreta­ continental masses was, no doubt, extensively tion was based on magnetic-anomaly patterns. mixed with sediment shed from the nearby, still- The unit bounded by Jl and J2 is unfaulted in rugged continents and may have included sizable the Blake Basin west of the Blake Spur fragments of continental rocks. Therefore, we anomaly. Some small breaks in the unit to the refer to this as transitional or rift-stage east are attributed to compaction over a rough crust (Klitgord and Behrendt, 1979). Although basement. The Jl and J2 horizons probably were Sheridan (1974, 1976) originally referred to the not penetrated at DSDP site 391 in the Blake basement as oceanic, he pointed out (1974, p. Basin. 399; 1978) that it has intermediate seismic Horizons Beta and AU and a Miocene turbi- velocity and mantle depth and should be con­ dite were, however, penetrated at DSDP site 391 sidered a "rift crust" (1978). Tarr (1969) (Benson and others, 1976) and have been traced noted that the Rayleigh wave-dispersion curve to the profile along connecting seismic lines. for this region is intermediate between theo­ Horizon Beta is presumed to correlate with the retical curves for oceanic and continental crust transition from Aptian clays to Neocomian and that this might be accounted for by the limestone. At DSDP site 391, horizon AU presence of a variety of crustal types. represents an unconformity between Miocene However, it is also consistent with a tran- turbidites and unfossiliferous Cretaceous Ositional type of basement, as pointed out by claystones. The Miocene carbonate turbidites at Sheridan (1974). A basement beneath the that drill site are similar to those cored just southern Blake Plateau derived mostly of mantle east of the Blake Escarpment by Sheridan and material was proposed by Dietz and others (1970) others (1974). The Miocene turbidites in our on the basis of an overlap in a pre-rift fit of profile apparently are represented by dis­ the continents. Recent, improved fits are continuous, irregular, strong reflectors (figs. somewhat different (Le Pichon and others, 1977; 45, 49). Below these reflectors and above AU is Schouten and Klitgord, written commun., 1978); a section of very weakly reflective or acous­ but the overlap at continental closure, as well tically transparent material that may represent as the nature of basement estimated from mag­ sedimentary deposits equivalent to the Miocene netic anomaly patterns (Klitgord and Behrendt, hemipelagic deposits drilled on the Blake Ridge 1979) still precludes the presence of con­ at DSDP site 101 (Hollister and others, 1972a) tinental basement beneath the Blake Plateau. and at site 102 (Hollister and others, 1972b). Sheridan (1974, 1976) has proposed a narrow, Above the Miocene turbidites, two uncon­ high, continental basement beneath the Blake formities are present at depths of approximately Escarpment, which our profile does not show. 0.05 and 0.4 s sub-bottom, both of which roughly Early in the Jurassic (185-175 m.y. ago), parallel the sea floor. Either of these might in conjunction with the development of tran­ be the Pliocene-Holocene contact as drilled at sitional basement, the grabens became filled nearby locations. with continental deposits, extensional movement on the faults ceased, and a smooth subaerial HISTORY OF THE CONTINENTAL MARGIN surface developed. This is interpreted as the unconformity shown as the top of Triassic Pre-185 m.y. ago deposits in the profile. This post-rift unconformity surface probably did not develop Development of the Continental Margin at until Early Jurassic time. On the eastern the latitude of the COST No. GE-1 well began in (African) side of the rift basin, a reef, now in the Triassic, presumably with rifting and the Blake Basin at the foot of the Escarpment, thinning by fracture of the continental crust. began to develop on the transitional basement. The irregular, blocky nature of the Devonian (and older) basement seen in the profile (fig. 45) probably results from this faulting. The 175 to 140 m.y. ago crustal extension would have resulted in con­ siderable volcanic activity. The grabens would Approximately 175 m.y. ago, a reorgani­ then have filled rapidly with continental ter­ zation of plate movement occurred (Vogt, 1973; rigenous deposits, similar to those found in Klitgord and Behrendt, 1979) and the spreading Triassic basins all along the U.S. east coast. center jumped eastward by about 120 mi (200 km)

105 on this profile. The change in depth of base­ 130-115 m.y. ago ment just west of the Blake Spur anomaly is considered to be the contact between older crust The end of this period is characterized by to the west and younger (170-175 m.y. old) crust a major regression. The shoreline apparently to the east. The younger basement was hotter moved farther seaward than ever before, and the and therefore formed at a shallower level than related unconformity extended far seaward across the older, cooler, and more dense basement to the plateau. The escarpment reef died at this the west. Although both crusts are now old time, possibly as a result of the regression. enough, and therefore cool enough, to have reached nearly their maximum subsidence, the 115-100 m.y. ago step was frozen into the basement topography and so remains (Klitgord, oral commun., 1978). The In Aptian-Albian time, reef growth abruptly "jump" in spreading center referred to above recommenced 40 mi (70 km) to landward of the certainly was not an instantaneous leap, and it Blake Escarpment, resulting in development of a is probable that considerable faulting of the small reef just seaward of the ASP 3 drill site basement and sediments took place. We correlate (fig. 45). The present Blake Spur then became a the extensive fracturing of the deepest strata zone of deposition of fore-reef talus, and a in the western Blake Basin (fig. 45) to the time broad zone of back-reef deposits and carbonate of spreading-center jump. These deposits are banks appears to have extended about 25 mi (40 covered by a quite undisturbed reflector, J2. km) behind the reef, westward beneath the Blake The episode of readjustment of plate movement Plateau. Beneath the present shelf at the GE-1 may also be related to a period of Early site and beneath the inner Blake Plateau, a Jurassic basaltic intrusion and volcanism around period of continental terrigenous sedimentation the North Atlantic basin. Dikes and flows of was interrupted by a brief Aptian transgression this age have been distinguished paleomagneti- and regression, followed by a major transgres­ cally by deBoer (1967) in eastern North America sion in the Albian. The reef on the outer Blake and by their inferred stress pattern by May Plateau died by the end of Albian time, perhaps (1971) around the North Atlantic. Extensive drowned by this transgression. volcanic flows occur near this profile, directly In the deep waters of the Blake Bahama overlying the post-rift unconformity (Dillon and Basin and generally in the western North others, 1979). These have shown a range of Atlantic, anoxic conditions prevailed, resulting dates of 162-204 m.y. (Gohn and others, 1978) in the deposition of carbonaceous clays between but probably represent volcanic activity asso­ horizons Beta and AU (fig. 45) (Ewing and ciated with stresses involved in the spreading- Hollister, 1972; Benson and others, 1976). center jump. The oldest reef observed in this profile (at the foot of the Blake Escarpment) 100 to 55 m.y. apparently died at about this time, or slightly later. During Late Cretaceous to Paleocene time, submergence exceeded deposition and a general deepening of the Blake Plateau took place. The 140 to 130 m.y. plateau previously had been a shallow carbonate shelf. At the COST No. GE-1 site, the argil­ During the time represented by the J2-J1 laceous chalks and calcareous shales of this interval (about 175-140 m.y. ago), no reef interval are indicative of paleobathymetric building appears evident in the profile; but environments that increased from shelf to slope after Jl time, a small reef began to grow in the depths (Poag and Hall, this volume); however, Blake Basin, atop the fore-reef talus of the these strata remained relatively flat-lying, previous reef. At about the same depth (4 mi both beneath the present shelf and the Blake (6.5 km)) and somewhat to the west, we find the Plateau. Presumably a shelf was present to the probable base of the reef structure that forms west. The weak acoustic layering of the Upper the Blake Escarpment. The Blake Basin reef Cretaceous and Paleocene at the GE-1 well survived only briefly, but the Blake Escarpment changes to an almost acoustically transparent reef survived until perhaps as late as Barremian signature farther seaward, beneath the plateau, time, about 115 m.y. ago (Benson and others, showing that the unit becomes even more homo­ 1976). These two structures may have begun to geneous to the east. The deposition of such form simultaneously during an episode of renewed homogeneous, fine-grained deposits suggests that reef growth, or the escarpment reef may re­ quiet current conditions existed across the present a site of reef development initiated present Blake Plateau at the time. During the after the death of the basin reef. Based on Late Cretaceous and Paleocene, the area of character of the seismic reflections (fig. 45), maximum subsidence was beneath the inner Blake the most extensive development of carbonate Plateau, as shown by the location of maximum banks on the outer shelf occurred during this sediment thickness there as well as the presence period (Tithonian-Berriasian). of a sag at the base of the unit in that region.

106 55 to 0 m.y. others (1970). Above the Miocene turbidites, probable Pliocene-Quaternary hemipelagic de­ High-resolution airgun profiles in the posits are interrupted by at least two erosional study area show that extensive erosion took episodes; one of these unconformities is just place near the end of Paleocene time. Paleocene below the sediment surface and might be related and Eocene strata on both sides of the uncon­ to changes in ocean circulation associated with formity were deposited at middle- to outer-shelf Pleistocene glaciation. water depths at the well site (Poag and Hall, this volume), and no significant regression was SUMMARY proposed by Vail and others (1977b, fig. 2). Therefore, we believe that at the end of the An unprecedented opportunity to analyze the Paleocene, the Gulf Stream, which previously structure and development of the Continental probably flowed northward along the Blake Margin off the southeastern United States is Escarpment, changed its location and began to afforded by correlation of the COST No. GE-1 flow through the Straits of Florida and across stratigraphic information with data from other the western Blake Plateau. The post-Paleocene drill sites and with a high-quality, deep- depositional history of the area is charac­ penetration, multichannel seismic profile. terized by the seaward progradation of the The Continental Margin is underlain by a Continental Shelf against the flank of this broad, asymmetric basin and basement rocks fast-moving current and of scour beneath the probably consist of transitional (modified Gulf Stream on the inner Blake Plateau. oceanic) crust; the basin's center of subsidence The maximum development of the present is located beneath the inner Blake plateau. Continental Shelf occurred during the Eocene as Maximum subsidence occurred beneath the outer sediments prograded in a wedge from the west. plateau early in margin development, and the Within the resolution of this profile, the locus of subsidence moved landward and broadened reflectors seem to prograde smoothly with no with time. The seaward limit of shelf devel­ contemporaneous erosion and redeposition (fig. opment was set by two reefs which acted as 46). After deposition, the Eocene deposits were sediment dams. extensively eroded, and differential erosion The present Continental Margin began to features are evident on the slope and inner form in the Triassic, with rifting of con­ Blake Plateau (fig. 47). The Gulf Stream may tinental crust. Sediments eroded from the have migrated eastward during the Eocene, rough, faulted topography accumulated in the allowing the shelf to prograde seaward, and then grabens. In Early Jurassic time, from about 185 resumed its former course to scour the Eocene to 175 m.y. ago, intrusions and extrusions of deposits. Subsequently this current prevented mafic material, extensively mixed with con­ any further progradation of the shelf, eroded tinent-derived sediments, formed new transi­ the inner Blake Plateau, and swept away most of tional basement beneath the Blake Plateau. As the sediment that otherwise could have built up marine water flooded the subsiding zone, reefs the plateau. began to form on fault-block highs. About 175 On the outer Blake Plateau, seaward of the m.y. ago, reorganization of plate movements main zone of Gulf Stream influence, a condensed resulted in a jump of the spreading center about wedge of Cenozoic strata is present which under­ 120 mi (200 km) eastward, close to the African went extensive sediment migration during de­ continental block. About 140 m.y. ago, a reef position. The profile (fig. 45) shows that the began to develop at the location of the Blake Blake Spur has also been very extensively Escarpment and grew upward for almost 6,600 ft eroded. Based on ages of sediments drilled, (2,000 m) before dying in Berriasian time. A erosion on the spur occurred after Eocene time broad carbonate platform developed behind this (Benson and others, 1976). This erosion may reef; and when the initial reef died, another have been related to Gulf Stream flow, although reef was initiated 40 mi (70 km) to the west, the main flow axis probably was located on the which took over the function of a sediment inner Blake Plateau by that time. Deep-sea dam. Until the latter reef died at the end of deposits were extensively eroded between late the Early Cretaceous, the outer Blake Plateau Eocene and early Miocene time to form the was characterized by carbonate-bank development; unconformity known as AU. to the westward, sediments were more terrig­ At DSDP site 391, in the Blake Basin, enous. During Late Cretaceous and Paleocene horizon AU is formed at the contact of Miocene time, quiet-water deposition of chalk and carbonate turbidites and Cretaceous black clays terrigenous mud characteristic of slope water (Benson and others, 1976). The turbidites also depths covered the plateau and Outer Continental have been cored near the escarpment (Sheridan Shelf area. The top of the Paleocene sediments and others, 1974). However, on the profile was sharply eroded, presumably due to the onset (fig. 45), an acoustically transparent layer of Gulf Stream flow across the inner Blake which probably represents Oligocene or lower Plateau after the Paleocene. The present Miocene pelagic deposits occurs between the Continental Shelf has resulted from sedimentary irregularly bedded, channeled, discontinuous accumulation, mostly in Eocene, restricted on Miocene turbidite deposits and horizon AU. This the seaward side by the flank of the Gulf Stream is the horizon A reflector X unit of Markl and flow.

107 CONCLUSIONS

1. The COST No. G-l well penetrated 13,039 5. Petrographic analysis indicates that most ft (3,974 m) of sedimentary and of the sandstones in the COST No. GE-1 metasediraentary rocks in the Southeast well are arkoses or subarkoses. Georgia Embayment of the AOCS. The age Feldspars are generally fresh and of sedimentary units was determined unaltered; they neither show breakdown through the examination of Foraminifera, into pore-filling clays nor leaching to calcareous nannofossils, and palyno- form secondary porosity. morphs. The section is Cenozoic down to 6. The present geothermal gradient in the about 3,500 ft (1.060 m). From 3,500 to COST No. GE-1 well is 0.89°F/100 ft 5,900 ft (1,060-1,800 m), the sediments (16.2°C/km). are Late Cretaceous in age, whereas the 7. Organic geochemical studies, coupled with interval from 5,900 to about 11,000 ft data on vitrinite reflectance and color (1,800-3,350 m) consists of units of alteration of visible organic matter, probable Early Cretaceous (Kimmeridgian?) indicate poor source-rock potential for and younger age. The section from 11,000 the section down to 3,600 ft (1,100 m) ft (3,350 m) to the total depth of the because of low organic-carbon contents well (13,254 ft (4,040 m below KB)) and thermal immaturity. The section from contains metasedimentary and meta-igneous about 3,600 to 6,000 ft (1,100-1,800 m) rocks which have been radioraetrically has the highest organic carbon content of dated as Devonian or older. any interval in this well but still 2. The sediments down to about 3,300 ft appears to be thermally immature. This (1,000 m) are predominantly chalks; interpretation is complicated by limestone and calcareous shales are most contamination by drilling additives, abundant in the underlying interval down however. The section from about 6,000 to to about 7,200 ft (2,200 m). In the 8,900 ft (1,800-2,700 m) has low levels 7,200- to 11,000-ft (2,200- to 3,350-m) of organic carbon, and much of this interval, interbedded sandstones and material is of the terrestrial, "gas- shales (mostly red beds) are the dominant prone" type. The basal part of the lithology, and these rocks unconformably interval does appear to be thermally overlie basement units at about 11,000 ft mature, however. The section below 8,900 (3,350 m). ft (2,700 m) has very low organic-carbon 3. Oligocene and Miocene units as well as content and, in spite of some small gas some Upper Cretaceous strata were shows, is unlikely to have any deposited in outermost-shelf to middle- significant source-rock potential. slope water depths. Pliocene, Eocene, 8. The combination of the above factors and Paleocene beds, as well as some Upper indicates that the intervals with the and Lower Cretaceous units, were laid best reservoir rocks have generally low down in middle- to outer-shelf water associated source potential. Source beds depths. The bulk of the Lower Cretaceous capable of generating oil or gas are rocks were deposited in inner-shelf to present in the well, however, and where terrestrial environments. thermally mature they may have formed and 4. Porosities in excess of 25 percent are expelled hydrocarbons. Lateral migration present in sandstone and limestone units into stratigraphic traps or upward down to approximately 10,000 ft (3,000 migration into overlying low-permeability m). Sandstones with reservoir-quality but high-porosity chalks may provide petrophysical properties are especially potential for oil or gas production in common in the 6,000- to 10,000-ft (1,800- the upper parts of the section. to 3,000-m) interval. No systematic decrease of porosity or permeability appears to occur with depth. Chalks at the top of the section in the GE-1 well, although they have low permeabilities, could form adequate reservoirs, especially where fractured.

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* U.S. Government Printing Office: 1979-677-026/26