U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY

SUBSURFACE CORRELATIONS AND SEQUENCE STRATIGRAPHIC INTERPRETATIONS OF LOWER SILURIAN STRATA IN THE APPALACHIAN BASIN OF NORTHEAST OHIO, SOUTHWEST NEW YORK, AND NORTHWEST PENNSYLVANIA

By

R.D. Hettinger

2001

Pamphlet to accompany GEOLOGIC INVESTIGATIONS SERIES I–2741 Printed on recycled paper CONTENTS Introduction ...... 1 Acknowledgments ...... 1 Stratigraphic nomenclature and previous interpretations ...... 1 Medina Group ...... 2 Clinton Group ...... 3 Lockport Group ...... 4 Tuscarora Formation ...... 4 Rose Hill Formation ...... 5 Stratigraphic correlations along cross sections B–B' and C–C' ...... 5 Sequence stratigraphy of the Medina and Clinton (lower part) Groups ...... 7 Sequence 1 ...... 8 Sequence 2 ...... 8 Sequence 3 ...... 9 Correlations from the Medina Group to the Tuscarora Formation ...... 9 Oil and gas ...... 10 Summary and conclusions ...... 11 References cited ...... 11

CROSS SECTIONS

B–B'. Correlations and sequence stratigraphic interpretations of Lower and Upper (part) Silurian strata along cross section B–B' ...... Map sheet C–C'. Correlations and sequence stratigraphic interpretations of Lower and Upper (part) Silurian strata along cross section C–C' ...... Map sheet

i FIGURES

1. Map showing locations of cross sections constructed by the U.S. Geological Survey to assess continuous gas accumulations in Lower Silurian strata of the Appalachian Basin ...... 14 2. Diagrams showing locations of cross sections B–B' and C–C' ...... 15 3. Chart showing chronostratigraphic correlations of Silurian strata in Ohio, southern Ontario and western New York (Niagara region), western-central New York, central New York, and central Pennsylvania ...... 16 4. Chart showing summary of the Niagaran Provincial Series in the Niagara region ...... 17 5. Diagrams showing previous depositional interpretations made from exposures of the Medina Group and Tuscarora Formation ...... 18 6. Diagram showing criteria used for lithologic interpretations in drill holes located along cross sections B–B' and C–C' ...... 19 7. Diagrams showing depositional units in the Medina Group and lower part of the Clinton Group along cross sections B–B' and C–C' ...... Map sheet 8. Diagrams showing summary of sequence stratigraphic interpretations for the Medina Group and Tuscarora Formation along cross sections B–B' and C–C' ...... Map sheet

TABLE

1. Identification of drill holes investigated along cross sections B–B' and C–C' ...... Map sheet

ii INTRODUCTION Varnes (USGS) and C.D. Laughrey (Pennsylvania Geological Survey) for their re- Significant quantities of recoverable views of the manuscript. natural gas resources are estimated to be in the regionally extensive Lower Silurian Medina STRATIGRAPHIC NOMENCLATURE AND Group and “Clinton” sandstone of the Ap- PREVIOUS INTERPRETATIONS palachian Basin (Gautier and others, 1995; Ry- der and others, 1996; Ryder, 1998). In order During the Silurian Period (439–408.5 to assess these accumulations of natural gas Ma, time scale of Harland and others, 1990), more accurately, the U.S. Geological Survey the northern Appalachian Basin was at a lati- (USGS) has investigated the Silurian stratigra- tude of 20°–25° S., and it was largely covered phy along six lines of cross section located in by an epeiric sea (Van der Voo, 1988; Brett and New York, Pennsylvania, Ohio, and West Vir- others, 1990). Early Silurian sedimentation in ginia (fig. 1). Cross section A–A' extends about the northern Appalachian Basin was influenced 375 mi from northeast to southwest and is ori- by tectonism as well as glacially induced sea- ented approximately parallel to the nearby pale- level fluctuations (Cotter, 1982, 1983; John- oshoreline of the Silurian epeiric sea. The re- son and McKerrow, 1991; Middleton, 1987; maining cross sections extend about 100–200 Ross and Ross, 1996). At least seven sea-level mi from northwest to southeast and are ori- fluctuations on the order of 150 ft occurred ented nearly perpendicular to the paleoshore- during the Silurian, several of which were dur- line. Stratigraphic investigations along A–A' ing the Llandoverian (Johnson and McKerrow, were reported by Ryder (2000), correlations 1991; Ross and Ross, 1996; and references along D–D' were reported by Keighin (1998), included within). Siliciclastic sediment was and investigations along E–E' and F–F' were in sourced from the Taconic Highlands and accu- progress at the time of this publication. This mulated in the adjoining foreland basin and on a report provides correlations for Silurian strata passive carbonate-dominated ramp located im- along cross sections B–B' and C–C', which are mediately westward of the foreland basin (Brett located in the northern part of the Appalachian and others, 1990) (fig. 2A). Basin (figs. 1 and 2). Cross section B–B' ex- Silurian rocks are exposed in the Niagara tends about 140 mi from Chautauqua County, region of New York and along the folded and N.Y., to Clinton County, Pa., and C–C' ex- thrust-faulted eastern margin of the Ap- tends about 200 mi from Lake County, Ohio, palachian Basin in New York and Pennsylvania to Clinton County, Pa. Correlations are made (fig. 2B). Cross sections B–B' and C–C' are lo- along B–B' and C–C' for the Lower Silurian cated 50–200 mi south of the Niagara region, Medina and Clinton Groups, Lower Silurian and they terminate in the Silurian exposures of Tuscarora and Rose Hill Formations, and central Pennsylvania (fig. 2B). Silurian strata Lower and Upper Silurian Lockport Group and were drilled at depths ranging from 2,000 to Mifflintown Formation (lower part). Emphasis 11,500 ft along the cross sections. In the Ni- is placed on the Medina Group because it is the agara region, Silurian strata are assigned to the principal natural gas reservoir in the Silurian Niagaran Provincial Series and overlying Ver- deposit. non Shale. The Niagaran Provincial Series is about 400 ft thick, and it includes (in ascending ACKNOWLEDGMENTS order) the Medina, Clinton, and Lockport Groups (figs. 3, 4). Coeval strata are about I thank personnel at the Ohio Division of 2,400 ft thick near the town of Williamsport, Geological Survey, the New York State De- Pa. (fig. 2B), where they are assigned (in as- partment of Environmental Conservation, and cending order) to the Tuscarora, Rose Hill, the Pennsylvania Topographic and Geological Mifflintown, and Bloomsburg Formations (fig. Survey, as well as W.J. Perry, Jr., R.T. Ryder, 3) (Cotter, 1988). and C.J. Wandrey (USGS) and P.M. Imbrogno Numerous outcrop and subsurface inves- and M.B. Walen (Cabot Oil), for providing geo- tigations have been conducted in Silurian strata physical and well-production data used in this throughout the northern part of the Ap- investigation. I thank M.A. Kirschbaum, P.J. palachian Basin. Significant to this study are McCabe, and R.T. Ryder (USGS) for their in- outcrop investigations and stratigraphic sum- sight and thoughtful discussions regarding maries by Brett and others (1990, 1991, stratigraphic correlations along the cross sec- 1995), Cotter (1982, 1983), and Duke and tions. My appreciation is also extended to A.J. others (1991), and subsurface investigations by Donatich, T.W. Judkins, R.T. Ryder, and K.L. Castle (1998) and Laughrey (1984). An

1 overview of their publications is provided in or- of central Pennsylvania. The Thorold, Cam- der to better understand the stratigraphy of bria, and Kodak of New York are considered to Silurian strata along cross sections B–B' and be correlative with the Castanea Member of the C–C'. The reader is referred to their publica- Tuscarora in Pennsylvania. tions for a comprehensive bibliography. Brett The Medina Group was considered by and others (1990) summarized depositional in- Brett and others (1990) to represent an uncon- terpretations for Silurian strata in the northern formity-bound sequence with lowstand fluvial Appalachian Basin, and they divided the rocks and transgressive marine deposits in its lower into six unconformity-bounded sequences. The part, and progradational marine and coastal sequence stratigraphic and chronostratigraphic plain deposits in its upper part. Sediment was correlations of Brett and others (1990) are derived from an eastern source (Yeakel, 1962) shown in figure 3. The stratigraphic nomencla- and possibly a northeastern source (Brett and ture in the Niagara region was subsequently others, 1991), and it was distributed along a revised by Brett and others (1995); their re- relatively straight to slightly arcuate shoreline vised nomenclature is shown in figure 3, and is that extended from north-northeast to south- used in this report. Dual references to Brett southwest across New York and Pennsylvania and others (1990, 1995) indicate that the (Cotter, 1982, 1983; Duke and others, 1991). stratigraphic nomenclature used in depositional Duke and others (1991) cited the German Bay interpretations by Brett and others (1990) was of the North Sea as a modern analog for the changed to match revisions by Brett and oth- Medina shorelines. ers (1995). The Whirlpool Sandstone overlies the regional Cherokee discontinuity (fig. 5A) that MEDINA GROUP truncates the Upper Ordovician Queenston Shale. Although the Queenston has commonly The Medina Group is about 80–155 ft been considered to be nonmarine, Middleton thick in the Niagara region of New York where (1987, p. 2) cited evidence that it is a supratidal it consists of siliciclastic sediment deposited in deposit with interbeds of marine mud. The fluvial, tidal, and marine environments (fig. 4). lower part of the Whirlpool consists of trough In the Niagara region, the Medina includes (in cross-stratified sandstone that has been inter- ascending order) the Whirlpool Sandstone, preted as a braided stream deposit (Middleton Power Glen Shale, Devils Hole Sandstone, and others, 1987) on the shelf margin (Brett Grimsby Formation, Thorold Sandstone, Cam- and others, 1990). Hummocky cross-bedded bria Shale, and Kodak Sandstone (figs. 3, 4). sandstone in the upper part of the Whirlpool, Brett and others (1995) showed the distribu- fossiliferous mudrock in the Power Glen, and tion of these formations along a dip-oriented horizontally laminated to hummocky cross- profile (fig. 5A). The Whirlpool, Power Glen, bedded sandstone in the Devils Hole have been and Devils Hole are lower Llandoverian described as shallow shelf sands and deeper (Rhuddanian), the Grimsby is lower Llandove- shelf muds associated with an overall marine rian (uppermost Rhuddanian to lowermost transgression (Brett and others, 1990, 1991, Aeronian), and the Thorold and Cambria are 1995). probably Aeronian (Brett and others, 1995). Burrowed shale and hummocky cross- The Kodak was shown to be Aeronian, B2 –B3 bedded sandstone in the lower part of the by Brett and others (1990) (fig. 3) but was re- Grimsby as well as planar and trough cross- vised as being probably B3 by Brett and others bedded and bioturbated sandstone in the upper (1995). The entire Power Glen to Thorold part of the Grimsby have been described within succession passes laterally into the Cabot Head coarsening-upward stratal successions (Brett Shale in central Ohio (Knight, 1969). In the and others, 1990, 1995; Duke and others, subsurface of eastern Ohio, the Whirlpool 1991; Martini, 1971). Each succession has Sandstone is informally referred to as the been interpreted to represent the progradation Medina sandstone; the Power Glen Shale is of shoreface and marine tidal flat sand over ma- called the Cabot Head Shale (lower); and the rine mud (Brett and others, 1990; Duke and remaining part of the Medina Group is infor- others, 1991) within shallow shelf and tidal flat mally referred to as the “Clinton” sandstone environments (Brett and others, 1991). Duke (fig. 3). In western Pennsylvania, the Medina and others (1991) described channel-form de- Group is divided into (in ascending order) the posits in the Grimsby and interpreted that they Whirlpool Sandstone, Cabot Head Shale, and accumulated in tidally influenced creeks and es- Grimsby Sandstone. The entire Medina Group tuaries. Coarsening-upward and progradational passes laterally into the Tuscarora Formation deposition are also described for the overlying

2 Thorold-Kodak succession (Brett and others, CLINTON GROUP 1990). The Thorold is a massive to locally cross-bedded channel sandstone that contains The Clinton Group is divided into lower, brachiopod fragments; the Cambria is an middle, and upper parts that are dominated by interbedded shale, siltstone, and sandstone marine mudrock and carbonates (figs. 3, 4). characterized by ostracodes, caliche horizons, Brett and others (1990) interpreted that the and desiccation cracks; and the Kodak is a lower and middle parts each represented uncon- rhythmically interbedded sandstone and shale formity-bound sequences, and the upper part that contains abundant trace fossils (Brett and represented two additional unconformity-bound others, 1995). sequences. The lower part of the Clinton on- Subsurface investigations in Pennsylvania lapped a regional unconformity that beveled the by Piotrowski (1981) and Laughrey (1984) re- underlying Medina Group (Brett and others, sulted in interpretations for the Medina that 1990, 1995) (fig. 3). The lower and middle were generally consistent with those made for parts of the Clinton were also beveled in a the Niagara region. Piotrowski (1981) consid- westward direction by the unconformity at the ered the Medina Group to have been deposited base of the upper Clinton (Brett and others, in a large delta complex that was dominated by 1990) (fig. 3); therefore, the middle Clinton is channels near the shoreline and dominated by not found in rocks exposed in the Niagara re- bars farther seaward. Laughrey (1984) inter- gion of New York. preted the Whirlpool and lower part of the In the Niagara region, the lower Clinton Cabot Head as a sublittoral and offshore de- is less than 10 ft thick, and it is represented by posit associated with a marine transgression inner shelf mudrock of the Neahga Shale and that stabilized along a line that extended from offshore carbonate of the overlying Reynales Warren to Beaver Counties (fig. 1). He consid- Limestone. The Neahga was shown as Aero- ered the Grimsby to have been deposited during nian, B3 to C1 by Brett and others (1990) (fig. a subsequent marine regression; the lower part 3), and it was revised to Aeronian, B2 to C1 by of the Grimsby contained shoreface deposits, Brett and others (1995). In west-central New and the upper part of the Grimsby contained York, the lower Clinton is about 90 ft thick and braided river and tidal deposits within a coastal consists of (in ascending order) the Maplewood sand/mud complex. More recently, Castle Shale, Furnaceville Member, Bear Creek Shale, (1998) concluded that the Whirlpool, lower Sodus Shale, and Wolcott Limestone. The Power Glen/Cabot Head, and lowermost estu- Maplewood is laterally equivalent to the Neahga arine interval in the Tuscarora were deposited Shale, and the Furnaceville and Bear Creek are within a transgressive systems tract over a laterally equivalent to the Reynales Limestone; regional sequence boundary unconformity that the overlying Sodus Shale (Aeronian, C3 to correlated to the Cherokee discontinuity. He Telychian, C4 ) and Wolcott Limestone further interpreted that the Grimsby was de- (Telychian, C4 ) are offshore deposits. The posited within a highstand systems tract along Reynales-Wolcott succession is equivalent to a wave-dominated shelf, a wave- and tide-influ- the basal 100 ft of the lower Rose Hill Forma- enced inner shelf, and a tide-dominated shore- tion in central Pennsylvania (Brett and others, line. Castle also recognized two regionally ex- 1990). In Ohio, the lower Clinton appears to tensive flooding surfaces that allowed for pre- pass into (in ascending order) the upper Cabot cise correlations between the Medina and Tus- Head (Plum Creek) Shale, Oldham Limestone, carora; the lower flooding surface is at the base and Lulbegrud Shale. Although Brett and oth- of Cambria Shale-equivalent strata, and the up- ers (1990, p. 209) considered the Plum Creek– per is at the base of Neahga Shale-equivalent Lulbegrud succession to be approximately strata in the overlying Clinton Group. Strata equivalent to the Neahga-Sodus succession, located between the flooding surfaces were in- correlations by Ryder (2000) suggested that terpreted to represent a landward shift in facies the Plum Creek–Lulbegrud is slightly older than that was also recognized in the Tuscarora. As the Neahga-Sodus. a result, Castle postulated that shoreline de- The middle part of the Clinton Group is posits in the Grimsby were laterally equivalent represented in central New York by the 45- to with fluvial and estuarine strata in the Tus- 120-ft-thick Sauquoit Formation (upper Llan- carora, and intertidal and subtidal facies in the doverian, Telychian, C ) and laterally equivalent upper part of the Medina were correlative with 5 Otsquago Sandstone (Brett and others, 1990). the coastal plain strata of the Tuscarora’s Cas- tanea Member.

3 The Sauquoit comprises primarily shale with in- Island is about 26–56 ft thick and comprises terbeds of siltstone and sandstone, and the Ot- thick- to massive-bedded dolomite overlain by squago consists of interbedded mudrock, sand- argillaceous dolomite with shale partings. The stone, and conglomerate. The middle Clinton Eramosa is Ludlovian; it is about 38–50 ft thick is equivalent to at least 300–360 ft of the mid- and consists of medium- to massive-bedded, dle shaly member of the Rose Hill Formation in biostromal dolomite. The Guelph is upper central Pennsylvania (Brett and others, 1990). Ludlovian; it is about 36 ft thick and consists of The upper Clinton was divided into two laminated, fine-grained, oolitic dolomite that unconformity-bound sequences by Brett and grades upward to shale. The Gasport and others (1990). In the Niagara region, the lower part of the Goat Island were interpreted lower sequence contains relatively deep-water as shallow shelf deposits. The upper part of the marine shale and carbonates in the Merritton Goat Island was interpreted as a deep-water de- Limestone and Williamson Shale (uppermost posit, and the Eramosa and Guelph were inter- Llandoverian, upper Telychian, C6 ), and overly- preted as shoaling-upward successions of ma- ing Rockway Dolomite (lowermost Wenlockian) rine strata (Brett and others, 1990, 1995). (Brett and others, 1990, 1995). The lower se- quence also includes the Dayton Limestone and TUSCARORA FORMATION overlying Estill Shale in Ohio (Telychian, C6 ), as well as the Center Sandstone Member and The Tuscarora Formation is considered superposed upper shaly member of the Rose Hill to be Lower Silurian, extending from the base Formation in central Pennsylvania (Brett and of the Llandoverian to the Llandoverian, C2 –C3 others, 1990). In the Niagara region, the up- stage (Berry and Boucot, 1970; Cotter, 1983, per sequence contains shallow-water crinoidal 1988). However, the Tuscarora does not con- grainstones in the Irondequoit Limestone tain age-diagnostic fossils, and its age is in- (lowermost Wenlockian, lower Sheinwoodian), ferred from its stratigraphic position between marine mudrock in the Rochester Shale (lower the Upper Ordovician Juniata Formation and to middle Wenlockian), and argillaceous to Lower Silurian Rose Hill Formation (Cotter, sandy carbonate in the DeCew Dolomite 1982). Brett and others (1990) indicated that (middle Wenlockian) (Brett and others, 1990, the Tuscarora is no younger than Llandove- 1995). Shallow-water deposits of the Ironde- rian, C1 (fig. 3) based on its inferred correlation quoit represent a basinward facies shift over to the Medina Group. The Tuscarora overlies deeper-water mudrock. The Rochester and strata of terrestrial origin in the Juniata For- DeCew were interpreted to have stratal suc- mation (Cotter, 1983). Facies variations in the cessions arranged in retrogradational, aggra- Tuscarora are shown in a dip-oriented diagram dational, and progradational stacking patterns by Cotter (1982) (fig. 5B). According to Cot- (Brett and others, 1990). ter, the lower part of the Tuscarora was de- posited along beaches and estuaries during a LOCKPORT GROUP major marine transgression that stabilized in central Pennsylvania, near the city of Harris- The Lockport Group consists of argilla- burg (fig. 2B). The main body of the Tuscarora ceous dolomite and minor amounts of dolomitic was deposited during a series of minor shoreline limestone and shale (Brett and others, 1995). fluctuations; sediment accumulated on the ma- Coeval rocks in central Pennsylvania are within rine shelf northwest of the shoreline, and in the McKenzie Member of the Mifflintown For- braided river systems southeast of the shore- mation. In the Niagara region of New York, line. Cotter contended that the upper part of the Lockport is 160–175 ft thick and includes the Tuscarora accumulated along a coastal sand (in ascending order) the Gasport Dolomite, and mud flat as the shoreline advanced to the Goat Island Dolomite, Eramosa Dolomite, and northwest. Guelph Dolomite. The Gasport is uppermost The Tuscarora is about 600 ft thick Wenlockian; it is about 20–37 ft thick and con- where it is exposed near the town of Mill Hall, tains biohermal and biostromal grainstone, Pa. (fig. 2B). Tuscarora successions at the Mill dolomite, and shaly dolomite. The Goat Island Hall exposure were described and interpreted by was shown to be Ludlovian by Brett and others Cotter (1982), and they are shown at the (1990, 1995, figs. 14 and 15, respectively); southeast ends of cross sections B–B' and C– however Brett and others (1995, p. 53) stated C'. According to Cotter, the Tuscarora at Mill that the Goat Island is probably uppermost Hall consists (in ascending order) of the follow- Wenlockian to lowermost Ludlovian. The Goat ing facies: (1) channeled coast, (2) lagoonal, (3)

4 barrier beach inlets, (4) marine shelf, and (5) ROSE HILL FORMATION coastal sand and mud flat. The following de- scriptions are based on Cotter (1982): The Rose Hill Formation occupies the Channeled coastal facies are about 90 ft thick stratigraphic interval between the Tuscarora and contain fine- to medium-grained, cross- Formation and the Keefer Sandstone Member bedded sandstone and thin interbeds of mu- of the Mifflintown Formation. The Rose Hill drock. Trace fossils of Monocraterion and consists of about 500 ft of marine mudrock and Rusophycus indicate a marine influence, minor amounts of marine limestone where it and landward-directed foresets suggest es- has been drilled near the eastern ends of cross tuarine flow. sections B–B' and C–C'. The Rose Hill is di- Lagoonal facies are about 18 ft thick and con- vided (in ascending order) into lower shaly tain dark fissile shale and thin interbeds of member, Cabin Hill Sandstone Member, middle fine-grained, ripple-laminated sandstone shaly member, Center Sandstone Member, and with mud drapes. Trace fossils of upper shaly member. Only the lower, middle, Monocraterion and Rusophycus are and upper shaly members are present near the common. eastern ends of cross sections B–B' and C–C'. Barrier beach inlet facies are about 28 ft thick. Brett and others (1990) correlated the basal They overlie an erosional surface and con- 180–300 ft of Rose Hill to the lower Clinton tain white, medium-grained, moderately Group of west-central New York, and they cor- sorted to well-sorted sandstone with flood- related the 300- to 360-ft-thick middle shaly and ebb-oriented cross-beds. member to the middle part of the Clinton Group Marine shelf facies (western cross-laminated in west-central New York (fig. 3). The upper facies of Cotter, 1982, 1983). The marine shaly member was correlated to the Merritton shelf facies are about 300 ft thick. Subfa- Limestone and Williamson Shale of New York, cies include (in ascending order): (1) a and the Dayton Limestone and Estill Shale of beach-attached shelf sand wave complex, Ohio. (2) a slightly deeper shelf complex, and (3) an inner shelf sand wave complex. Each STRATIGRAPHIC CORRELATIONS subfacies is heterolithic; sandstone domi- ALONG CROSS SECTIONS B–B' AND C–C' nates the lower and upper subfacies, but the deeper shelf complex has nearly equal Correlations of the Medina, Clinton, and amounts of sandstone and shale. Sand- Lockport Groups and the Tuscarora and Rose stone is medium to coarse grained in the Hill Formations are shown along cross sec- basal sand wave complex, and grain size tions B–B' and C–C'. Stratigraphic correla- generally decreases in the overlying subfa- tions were made using geophysical logs from cies. Sandstone bodies have flat bases and 113 drill holes; the drill holes are identified in wavy tops that may be capped by granules. table 1 and their locations are shown in figure Beds are trough and planar cross-stratified 2C. The holes are spaced about 1 mi apart in and transport directions are predominantly Ohio and New York, and about 2–52 mi apart in seaward, although shoreline parallel and un- Pennsylvania. Lithologies are interpreted from common landward directions are also re- a combination of natural gamma and density ported. Arthrophycus trace fossils are logs responses as described in figure 6. Se- common throughout, and Rusophycus is lected natural gamma logs are shown on each also found in the slightly deeper shelf de- cross section; the base of the Reynales Lime- posits. Sedimentary features are inter- stone was used as a datum for display purposes. preted by Cotter to have been derived by The southeast ends of both cross sections are storm-driven currents although he stated at the exposure of the Tuscarora Formation at that tidal currents might also be possible. Mill Hall, which was described by Cotter Coastal sand and mud flat facies are about 140 (1982). ft thick and form the Castanea Member of Group and formational boundaries shown the Tuscarora. They consist of very fine on the cross sections are based, in part, on grained to fine-grained, bioturbated sand- comparisons to geophysical logs shown in stone with Skolithos, Arthrophycus, and Brett and others (1990, 1995), Castle (1998), Chondrites trace fossils. Foresets in two and Laughrey (1984). Most importantly, cross channel-form sandstone bodies indicate a section A–A' (Ryder, 2000) provided correla- seaward transport direction. Marine in- tions from the Niagara region to cross sec- terbeds are more common in the upper part tions B–B' and C–C'. Cross section A–A' in- of the member. tersects B–B' and C–C' at drill holes 18 and 50,

5 respectively (table 1). The Medina Group was Reynales Limestone, or the unnamed limestone divided into the Whirlpool Sandstone, Power and unnamed shale. The Reynales splits into Glen Shale, and Grimsby Formation along the two parts that diverge to the southeast. In New York part of cross section B–B', and co- this report, the upper split of the Reynales is eval units along the Pennsylvania part of B–B' tentatively correlated to the Wolcott Lime- include the Whirlpool Sandstone, Cabot Head stone, and strata between the upper and lower Shale, and Grimsby Sandstone, respectively. splits are tentatively correlated to the Sodus The Clinton Group was divided into the Neahga Shale based on diagrams and descriptions by Shale, Reynales Limestone, Williamson Shale, Brett and others (1990, p. 206, 207, 235– Irondequoit Limestone, and Rochester Shale 237). The interval between the base of the along the New York part of cross section B–B', lower Reynales and top of the upper Reynales and the Clinton also contains the Dayton (Wolcott-equivalent) is correlated to the lower Limestone, an unnamed shale, and an unnamed shaly member of the Rose Hill based on de- limestone along parts of cross section C–C'. scriptions by Brett and others (1990, p. 207) The unnamed shale and unnamed limestone that equate the lower Clinton to the lower were tentatively correlated to the Lulbegrud 180–300 ft of the Rose Hill. The middle part of Shale and underlying Oldham Limestone, re- the Clinton Group is represented by a 0- to spectively, based on correlations made by 250-ft-thick wedge of Sauquoit-equivalent strata Ryder (2000). Ryder’s correlations also sug- located above the Wolcott-equivalent strata. gest that the unnamed shale is equivalent to Brett and others (1990, p. 210, 211) consid- the Neahga Shale. In the subsurface of Penn- ered the Sauquoit to be equivalent with the sylvania, the Irondequoit, Dayton, and Rey- 300- to 360-ft-thick middle shaly member of the nales are composed of hard dense dolomite and Rose Hill Formation. The upper part of the are therefore labeled as the Irondequoit, Day- Clinton Group is represented by the interval ton, and Reynales Dolomites, following the containing the Dayton Limestone/Dayton nomenclature used by the Pennsylvania Dolomite, Williamson Shale, Irondequoit Lime- Geological Survey. Other formations that stone/Irondequoit Dolomite, and Rochester might also be present on the cross sections are Shale. Southeastern transitions shown on the not shown because they are difficult to identify cross sections for the Irondequoit and underly- on geophysical logs and their presence is not ing Williamson are based on diagrams by Brett certain. For example, the Grimsby might and others (1990, their fig. 26) that correlate contain strata equivalent to the Devils Hole, the Irondequoit to the Keefer Sandstone Mem- Thorold, Cambria, Kodak, and Neahga. ber, and the underlying 40- to 60-ft-thick Additionally, the lower part of the Irondequoit Williamson Shale to the upper part of the Rose might be laterally equivalent to the Rockway Hill Formation. The Keefer-Irondequoit interval Dolomite, and the DeCew Dolomite might be was identified between the depths of 9,460 ft within the undivided strata of the Lockport and 9,518 ft in drill hole 51 (cross section B–B') Group. by Heyman (1977, his drill hole 34). Both cross sections show southeastern Several unconformable relations are indi- transitions of the Medina Group to the Tus- cated by the correlations shown for the Clin- carora Formation, and southeastern transitions ton Group on cross sections B–B' and C–C'. of the Clinton Group to the Rose Hill and Mif- The most notable unconformity is at the base flintown (lower part) Formations. The Medina– of the upper part of the Clinton Group; it un- Tuscarora and Clinton–Rose Hill intervals derlies the Williamson Shale on cross section thicken dramatically southeast of Elk and Potter B–B', and the Dayton Limestone/Dayton Counties, Pa. The thicker stratigraphic inter- Dolomite and Williamson Shale-equivalent on vals are attributed to increased subsidence and cross section C–C'. The unconformity trun- preservation of strata within the main deposi- cates Sauquoit Formation-equivalent strata tional trough of the Appalachian foreland basin. (middle Clinton) on both cross sections, and Correlations of the Medina-Tuscarora transi- appears to truncate the Reynales between drill tion are described in the section of this report holes 14 and 16 on cross section C–C'. Fur- regarding sequence stratigraphy. Correlations ther evidence for the unconformity is provided showing Clinton–Rose Hill–Mifflintown transi- along the northwest end of cross section B–B' tions are based on descriptions and diagrams by where the Williamson Shale (Telychian, C6 ) Brett and others (1990) and Heyman (1977). overlies the Reynales Limestone (Aeronian, At the northwest end of the cross sections, C2 ). Other apparent stratal truncations sug- the lower part of the Clinton Group is repre- gest additional unconformities; these include: sented either by the Neahga Shale and (1) an apparent truncation of Sodus Shale-

6 equivalent strata along the base of the upper in the Power Glen Shale/Cabot split of the Reynales in both cross sections; Head/Cabot Head (lower) Shales. the truncation appears to extend through the Unit B contains progradational deposits of shelf lower Reynales between drill holes 42 and 47 mudrock and sandstone in the upper part of on cross section C–C', and (2) a possible trun- the Power Glen Shale/Cabot Head/Cabot cation of the Dayton at the base of the Ironde- Head (lower) Shales, as well as prograda- quoit between drill holes 4 and 6 on cross sec- tional deposits of shelf/shoreface sand- tion C–C'. Although these unconformities are stone in the lower part of the Grimsby also supported by the chronostratigraphic cor- Formation/Grimsby Sandstone. Unit B relations by Brett and others (1990) (fig. 3) conformably overlies unit A. they were not shown on the cross sections Unit C contains retrogradational deposits of in- because the apparent truncations were local in terbedded sandstones and mudrock. The extent, and they could simply represent areas strata are interpreted as tidal channels, and where the underlying strata thin and pinch out. the upper part of the unit might contain a thin deposit of marine mudrock. Unit C is SEQUENCE STRATIGRAPHY OF THE encased in shelf mudrock of the Cabot MEDINA AND CLINTON (LOWER PART) Head Shale (lower) along the northwest GROUPS part of cross section C–C'. The sharp un- dulating base of unit C scours unit B; it has Recognition of sequence boundary un- as much as 50 ft of relief and is interpreted conformities and systems tracts might be criti- to be unconformable. cal for understanding the variability of reservoir Unit D contains progradational shelf and performance for the Medina Group. Sequence shoreface mudrock and sandstone in the stratigraphic interpretations of the Medina upper part of the Cabot Head Shale (lower), Group and lower part of the Clinton Group lower part of the “Clinton” sandstone, and along cross sections B–B' and C–C' are based lower part of the Grimsby Forma- on the superposition and interpretations of in- tion/Grimsby Sandstone. The unit is lo- formally designated units A through E that are cated along the northwest part of cross shown in figure 7. References to coarsening- section C–C', and it is also found locally and fining-upward stratal successions are based along B–B' in drill holes 21 (4,140–4,178 ft) on criteria described in figure 6. Units A, C, and 32 (4,483–4,496 ft). Unit D con- and E are dominated by fining-upward succes- formably overlies unit C in cross section C– sions and are interpreted to be retrogradational C', but its contact over unit B in cross sec- based on their deepening-upward succession of tion B–B' is interpreted to be facies. Units B and D are dominated by coars- disconformable. ening-upward successions and are interpreted Unit E contains retrogradational deposits in the to be progradational based on their shallowing- upper part of the Medina Group, unnamed upward and forward-stepping stratal succes- limestone, Neahga Shale (and its laterally sions. Unconformities at the bases of units A, equivalent unnamed shale), and Reynales C, and E are interpreted from regionally exten- Limestone/Reynales Dolomite. The sand- sive surfaces that scour underlying strata. stone-dominated lower part of unit E is Maximum marine flooding surfaces are placed probably equivalent to tidal- and estuarine- along regionally extensive boundaries that di- channel deposits in the Grimsby. The mu- vide retrogradational strata from superposed drock-dominated upper part of the unit is progradational strata. Brief descriptions of probably equivalent to tidal-flat deposits in each unit are provided below. References to the upper Grimsby, Thorold, Cambria, and fluvial, tidal, estuarine, shoreface, or offshore Kodak Formations, and it might also include facies in the units are based on published de- inner shelf deposits of the Neahga Shale. scriptions and interpretations of coeval rocks The top of unit E is represented by offshore described in previous sections of this report. carbonates of the Reynales Lime- Unit A contains retrogradational successions stone/Reynales Dolomite or the unnamed of fluvial and nearshore marine sandstone in shale where the Reynales is not present. the Whirlpool Sandstone/Medina sand- The sharp undulating base of unit E has as stone, and shelf mudrock in the lower part much as 60 ft of relief; it scours units B, C, of the Power Glen Shale/Cabot and D and is interpreted to be uncon- Head/Cabot Head (lower) Shales. Unit A formable. overlies the Cherokee discontinuity and its Stratal successions within units A top is at a maximum marine flooding surface through E indicate that the Medina Group and

7 lower part of the Clinton Group can be divided braided fluvial deposits in the basal part of the into three unconformity-bound sequences along Lower Silurian Whirlpool Sandstone/Medina cross sections B–B' and C–C'. The sequences sandstone. The juxtaposition of braided fluvial and their respective basal sequence boundary strata over supratidal mudrock is interpreted to unconformities are referred to by the numbers represent a significant basinward shift of facies. 1, 2, and 3 on the cross sections. Following Castle (1998) attributed thickness variations of the sequence stratigraphic terminology and the lower Whirlpool to irregular topography on concepts of Van Wagoner and others (1988, the basal Silurian unconformity. The trans- 1990), each sequence boundary unconformity gressive systems tract is represented by unit A. is recognized by truncation of underlying strata Its deepening-upward successions have been in- and a basinward shift in overlying strata. Inci- terpreted as retrogradational, whereby shallow sion is attributed to a loss of accommodation incised valleys were backfilled with fluvial strata space associated with a fall in relative base level. during the initial rise in sea level, and overlying Each overlying sequence was deposited during a nearshore and offshore strata were deposited subsequent rise in relative base level. The se- during the subsequent marine transgression. quences are divided into transgressive and The highstand systems tract of sequence 1 is highstand systems tracts; the transgressive represented by progradational deposits of systems tract developed when accommodation shoreface and shelf parasequences in unit B. exceeded sediment supply, and the highstand systems tract developed when accommodation Sequence 2 was equal to, or less than, sediment supply. Each transgressive systems tract overlies the Sequence 2 includes the upper part of the basal sequence boundary unconformity and Cabot Head Shale (lower) and lower part of the contains deepening-upward successions of “Clinton” sandstone along the western part of strata that are capped by a maximum marine cross section C–C'. Sequence 2 is also found flooding surface. Ravinement surfaces are in the Grimsby Formation at drill holes 21 and transgressive surfaces of erosion within trans- 32 along cross section B–B'. The sequence gressive systems tracts (Dalrymple and others, comprises units C and D, and the sharp 1992; Walker, 1992); they are situated be- undulating contact at the base of unit C is the tween shoreface deposits and underlying fluvial, basal sequence boundary (sequence boundary tidal, or estuarine strata. The ravinement sur- 2). The sequence boundary extends 60 mi faces within units A and C are not shown on along C–C' and scours as much as 50 ft into the cross sections because contacts between unit B. Variations in the thickness of unit C the various facies could not be differentiated on are attributed to irregular topography along the the geophysical logs. Each highstand systems sequence boundary. The sequence boundary tract overlies a maximum flooding surface and separates shelf and shoreface deposits in unit B contains aggradational or progradational suc- from overlying tidal channel deposits in unit C, cessions of strata that are truncated by an un- and these juxtapositions are interpreted to rep- conformity of disconformity. resent a minor basinward shift in facies. Sequences 1, 2, and 3 within the Medina The transgressive and highstand sys- Group and lower part of the Clinton Group are tems tracts in sequence 2 are represented by described below. Tentative correlations of each units C and D, respectively, and they comprise sequence southeast to the Tuscarora and Rose tidal channel deposits that are overlain by ma- Hill Formations are provided in the following rine mudrock. In this interpretation, incised section of this report. valleys were backfilled with tidally influenced strata during the initial rise in sea level, and the Sequence 1 overlying offshore-marine strata were deposited during the subsequent marine transgression. Sequence 1 includes the Whirlpool Sand- The highstand systems tract is represented by stone/Medina sandstone, Power Glen/Cabot progradational parasequences in unit D. Head/Cabot Head (lower) Shales, and lower Unit D conformably overlies unit C along part of the Grimsby Formation/Grimsby Sand- the western part of C–C', and it disconformably stone. It comprises retrogradational and overlies unit B in drill hole 49 on C–C' and drill progradational strata in units A and B. The un- holes 21 and 32 on B–B'. The disconformable derlying sequence boundary (sequence bound- contact between units B and D is labeled R/2 ary 1) is equivalent to the regional Cherokee on the cross section, and it is interpreted as a discontinuity, and it separates supratidal mu- transgressive surface of erosion (ravinement drock in the Ordovician Queenston Shale from surface) that was cut as the shoreline retreated

8 to areas located southeast of the valley-fill de- unconformity between the Reynales Limestone posits of unit C. The ravinement surface is in- (Aeronian, C1–2) and Williamson Shale terpreted to merge northwest with the basal (Telychian, C6 ). An alternative interpretation sequence boundary and maximum flooding sur- by Ryder (2000) places the top of the trans- face of sequence 2. gressive systems tract at a maximum flooding surface that he described in the unnamed shale Sequence 3 in the basal Clinton Group of Ohio. According to Ryder, the highstand systems tract of se- The lower part of sequence 3 is repre- quence 3 would include strata in the Neahga sented by the upper part of the Medina Group Shale and unnamed shale, as well as the and the lower part of the Clinton Group. The Reynales Limestone/Reynales Dolomite. basal sequence boundary unconformity (sequence boundary 3) truncates sequences 1 CORRELATIONS FROM THE MEDINA and 2 along much of both cross sections. The GROUP TO THE TUSCARORA FORMATION sequence boundary is interpreted to represent broad and gentle topography of a regionally ex- The Tuscarora Formation and Medina tensive valley system that was incised during a Group have been described as a siliciclastic fall in relative base level. wedge whereby proximal facies of the Tus- The transgressive systems tract in se- carora grade northwest to distal facies of the quence 3 is represented by unit E. Tidally influ- Medina. Deposition of the wedge has been at- enced strata in unit E overlie over shelf and tributed to a single, overall transgressive-re- shoreface deposits in units B and D, and these gressive marine cycle. Transgressive deposi- juxtapositions are interpreted to represent a tion has been interpreted for the Whirlpool basinward shift in facies associated with the fall Sandstone, Power Glen Shale, and Devils Hole in relative base level. During the initial phase of Sandstone (Brett and others, 1990; Duke and the subsequent rise in base level, the topogra- others, 1991; Middleton and others, 1987); phy is interpreted to have been backfilled by Whirlpool and lower part of the Cabot Head tidal- and estuarine-channel deposits that form Shale (Laughrey, 1984; Piotrowski, 1981); and the lower part of unit E. As base level contin- lower and main part of the Tuscarora Forma- ued to rise, the topography was covered by tion (Cotter, 1983). Regressive deposition has finer grained tidal-flat deposits described in the been interpreted for the Grimsby-Kodak suc- upper part of unit E. The tidal flat was then cession (Brett and others, 1990; Duke and scoured by a transgressive surface of erosion others, 1991), upper part of the Cabot Head (ravinement surface) as the shoreline moved and Grimsby (Laughrey, 1984; Piotrowski, southeast across the area. The ravinement 1981), and Castanea Member (Cotter, 1983). surface is interpreted to be the same uncon- However, facies transitions between the Med- formable surface that truncated the Thorold, ina and Tuscarora are not straightforward be- Cambria, and Kodak Formations in the Niagara cause accommodation has been affected by region (fig. 5A). A conglomerate located at the variations in sea level and tectonic subsidence in base of the Neahga Shale in the Niagara region the foreland basin. Descriptions of erosional- (identified as the Densmore Creek Phosphate based estuarine channel-fill deposits in the Bed in figure 4) might represent a lag deposit Grimsby-Thorold succession (Duke and Brusse, associated with the ravinement. Inner shelf 1987; Duke and others, 1991), and descrip- mudrock of the Neahga Shale and offshore car- tions of marine beds in the upper part of the bonate of the Reynales Limestone/Reynales Castanea (Cotter, 1983), provide evidence that Dolomite were subsequently deposited after the these formations were partially transgressive. shoreline had retreated southeast of the cross Castle (1998) contended that the Cambria sections, and the top of the Reynales Shale (New York), upper Grimsby (eastern Ohio Limestone/Reynales Dolomite is tentatively in- and western Pennsylvania), and Castanea terpreted as the top of the transgressive sys- Member were transgressive deposits rather tems tract. than regressive deposits as previously thought. The highstand systems tract in sequence Sequence stratigraphic interpretations 3 might be represented along the southeast made in this report indicate the Medina Group parts of each cross section by the Sodus (?) was deposited during several transgressive and Shale equivalent and lower shaly member of the regressive marine cycles, and an analysis of fa- Rose Hill Formation. However to the north- cies at Mill Hall, as described by Cotter (1982), west, these presumed highstand deposits have suggests that Tuscarora deposition was influ- been removed by erosion along the enced by the same transgressive-regressive

9 cycles. In this report, the Tuscarora Forma- by the contact between the shelf sand-wave tion is divided into three sequences at Mill Hall. complex and overlying strata deposited on a The sequences are described below, and they slightly deeper shelf. The contact is interpreted are tentatively correlated northwest to the as a ravinement surface that converges in a Medina Group in cross sections B–B' and C–C'. seaward direction (northwest) toward the basal Sequence 1 is represented by the basal sequence boundary unconformity and maximum 240 ft of the Tuscarora at the Mill Hall section. flooding surface described for unit C within se- Its inferred basal sequence boundary divides flu- quence 2. The ravinement surface is labeled vial deposits of the Juniata Formation from su- R/2 on the cross sections and it has been cor- perposed fluvial and estuarine deposits of the related northwest through drill holes 53, 51, Tuscarora. The inference of the sequence 32, and 21 on B–B', and 60 and 49 on C–C'. boundary is based on the recognition of: (1) a At Mill Hall, the ravinement surface is overlain sea-level lowstand at the end of the Ordovician by about 50 ft of slightly deeper shelf deposits (Middleton, 1987), (2) an Ordovician-Silurian that form the upper part of the transgressive age sequence boundary unconformity through- systems tract; the lower part of the trans- out much of the northern Appalachian Basin gressive systems tract is represented by unit C (Castle, 1998), and (3) channeled coastal de- along the northwest part of cross section C– posits in the basal Tuscarora (Cotter, 1982, C'. The maximum flooding surface and top of 1983). The transgressive systems tract over- the transgressive systems tract is inferred by lies the sequence boundary and includes about an increase in mudrock about 295 ft above the 175 ft of rock that passes upward through de- base of the Tuscarora. The overlying high- posits of a channeled coast, lagoon, barrier stand systems tract is about 150 ft thick and beach inlet, and basal part of a shelf sand-wave consists of shallowing-upward successions that complex. The maximum flooding surface was pass upward through slightly deeper shelf, and placed at a horizon in the shelf sand-wave inner shelf facies. In this report, the highstand complex that was overlain by an increased deposits are interpreted to prograde westward amount of marine mudrock. These deepening- and correlate with shoreface deposits described upward stratal successions are interpreted to in unit D of the Medina Group. correlate with unit A, which contains the Sequence 3 is represented by the 140-ft- Whirlpool Sandstone/Medina sandstone and thick Castanea Member at Mill Hall and it might lower part of the Power Glen/Cabot Head also include strata in the overlying Rose Hill Shale. The highstand systems tract is repre- Formation. The Castanea comprises tidally in- sented by about 65 ft of the shelf sand-wave fluenced coastal strata that contain channel- complex that overlies the maximum flooding sur- form deposits associated with through-flowing face. In this report, the upward increase of fluvial systems; marine interbeds are more sandstone within the complex is attributed to common in the upper part of the member progradation of the shoreline. The shelf sand- (Cotter, 1983). These deepening-upward suc- wave complex is interpreted to grade northwest cessions are interpreted to represent retrogra- into shelf mudrock and hummocky cross-bedded dational deposition associated with a rise in rel- sandstone of unit B. Similar facies transitions ative base level. The deepening-upward succes- have been described by Driese and others sions are correlated to the finer grained upper (1991) for deposits in the Clinch Sandstone part of the Grimsby and “Clinton” sandstone (Tuscarora-equivalent strata) and Rockwood that are within the transgressive systems tract Formation of east Tennessee. The Clinch is of sequence 3 along the northwest part of the dominated by a cross-stratified sandstone facies cross sections. Castle (1998) also correlated that is similar to the sand-wave complex of the the Castanea to the finer grained upper part of Tuscarora (Driese and others, 1991). Driese the Grimsby Sandstone in western Pennsylva- and others interpreted the cross-stratified nia and interpreted the strata as retrograda- sandstone facies as megaripples and shelf sand tional. The highstand systems tract of se- waves that were deposited in a progradational, quence 3 might be represented by marine de- storm-dominated, shoreface setting. The posits in the overlying lower shaly member of megaripples and sand waves were interpreted to the Rose Hill Formation. grade into hummocky cross-stratified beds of the Rockwood Formation that were situated on OIL AND GAS the inner shelf. Sequence 2 occupies the stratigraphic in- Initial production data from the Medina terval located 240–450 ft above the base of the Group and Tuscarora Formation are provided, Tuscarora at Mill Hall. Its base is represented where available, for drill holes displayed on

10 cross sections B–B' and C–C'. Forty-four Subsurface sequence stratigraphic inves- holes have initial production values that range tigations in this report provide alternate from 20 to 7,000 thousand ft3 of gas (MCFG) interpretations whereby the Medina and per day, and the mean value is about 650 Tuscarora were deposited during two MCFG per day. Additionally, most of those transgressive-regressive marine cycles and the holes located along the Ohio part of cross sec- transgressive phase of a third marine cycle. tion C–C' also yielded an initial production of Strata in the lower part of the Clinton Group less than 20 barrels of oil. Fourteen drill holes might have been deposited in the transgressive shown on the cross sections are either dry and phase of the third cycle as well. The deposi- abandoned or lack initial production data; most tional cycles spanned a 6- to 7-million year pe- of these holes are located in central Pennsyl- riod that extended from the earliest Rhuddanian vania where reservoir porosity and permeability to the latest Aeronian, C1 or C2 Stages of the of the Medina Group and Tuscarora Formation Llandoverian Series. Three sea level lowstands are reduced due to depth of burial. However, existed during Rhuddanian Stage, and each the Amoco/UGI Texas-Gulf #1 well (drill hole lowstand was followed by a sea level rise that 60, cross section C–C'), located in the Devils culminated in a sea level highstand. Uncon- Elbow field, Centre County, Pa., had an initial formities associated with the sea level lowstand production of 7,000 MCFG per day from are represented by sequence boundaries 1, 2, depths between 10,759 and 10,974 ft in the and 3, as defined in this report. The develop- Tuscarora Formation. Harper and others ment of transgressive and highstand systems (1999) related the large yield in the tracts in each unconformity-bounded sequence Amoco/UGI Texas-Gulf #1 well to fracture are summarized in figure 8. porosity and bedding-plane partings. Initial production data indicate that nearly Perforated zones in the Medina Group all formations in the Medina Group have been and Tuscarora Formation are also shown for explored for natural gas along the northwest holes displayed on cross sections B–B' and C– parts of cross sections B–B' and C–C'. The C'. Although all of the sequences and associ- mean initial gas production is about 650 MCFG ated systems tracts defined in this report have per day based on values reported for 44 produc- been perforated in various holes, the initial pro- ing drill holes located along the cross sections. duction data do not specify which strata are One well (drill hole 60, cross section C–C') had productive. However, a tally of the perforated an initial production of 7,000 MCFG per day. zones suggest which intervals the operators Fourteen additional drill holes were either dry considered most productive on a regional basis. and abandoned or lacked initial production data. The transgressive systems tract of sequence 3 Perforated zones in the producing drill holes in- is the most frequently perforated interval, hav- dicate that transgressive deposits of sequence ing been perforated in about 90 percent of the 3 and highstand deposits of sequences 1 and 2 holes tested for initial production. The high- were the most frequently targeted strata for stand systems tracts of sequences 1 and 2 initial production tests. were targeted by operators in about 50 percent of the holes tested. The least targeted inter- REFERENCES CITED vals were the transgressive systems tracts of sequences 1 and 2, which were perforated in Berry, W.B.N., and Boucot, A.J., 1970, Corre- less than 25 percent of the holes tested. lation of North American Silurian rocks: Geological Society of America Special Pa- SUMMARY AND CONCLUSIONS per 102, 289 p. Brett, C.E., Goodman, W.M., and LoDuca, Previous investigators have interpreted S.T., 1990, Sequences, cycles, and basin that Lower Silurian rocks in the northern Ap- dynamics in the Silurian of the Appalachian palachian Basin accumulated in marine shelf, Foreland Basin: Sedimentary Geology, v. nearshore marine, and terrestrial environments 69, p. 191–244. within a foreland basin. Sedimentation was af- ——1991, Silurian sequences in the Niagara fected by various rates of subsidence, accom- Peninsula, in Cheel, R.J., ed., Sedimentol- modation space, and sediment supply owing to ogy and depositional environments of Sil- sea-level fluctuations and tectonic activity. urian strata of the Niagara Escarpment, Most investigators have considered deposition Ontario and New York: Sudbury, Ontario, of the Lower Silurian Medina Group and Tus- Geological Association of Canada, Miner- carora Formation to have been associated with alogical Association of Canada, Society of a single transgressive-regressive marine cycle.

11 Economic Geologists, Joint Annual Meet- Lower Silurian Medina Group, Ontario and ing, Field Trip B4 Guidebook, p. 3–26. New York—Storm- and tide-influenced Brett, C.E., Tepper, D.H., Goodman, W.M., sedimentation in a shallow epicontinental LoDuca, S.T., and Eckert, Bea-Yeh, 1995, sea, and the origin of enigmatic shore- Revised stratigraphy and correlations of normal channels encapsulated by open the Niagaran Provincial Series (Medina, shallow-marine deposits, in Swift, D.J.P., Clinton, and Lockport Groups) in the Type Oertel, G.F., Tillman, R.W., and Thorne, Area of western New York: U.S. Geolog- J.A., eds., Shelf sand and sandstone bod- ical Survey Bulletin 2086, 66 p. ies—Geometry, facies, and sequence Castle, J.W., 1998, Regional sedimentology stratigraphy: International Association of and stratal surfaces of a Lower Silurian Sedimentologists, Special Publication clastic wedge in the Appalachian foreland Number 14, p. 339–375. basin: Journal of Sedimentary Research, Gautier, D.L., Dolton, G.L., Takahashi, K.I., v. 68, no. 6, p. 1201–1211. and Varnes, K.L., eds., 1995, 1995 na- Cotter, Edward, 1982, Tuscarora Formation of tional assessment of United States oil and Pennsylvania: Society of Economic Pale- gas resources; results, methodology, and ontologists and Mineralogists, Eastern supporting data: U.S. Geological Survey Section, 1982 Field Trip, Guidebook, 105 Digital Data Series 30 [1 CD-ROM]. p. Harland, W.B., Armstrong R.L., Cox, A.V., ——1983, Shelf, paralic, and fluvial environ- Craig, L.E., Smith, A.G., and Smith, D.G., ments and eustatic sea-level fluctuations in 1990, A geologic time scale, 1989: Cam- the origin of the Tuscarora Formation bridge, Cambridge University Press, 263 p. (Lower Silurian) of central Pennsylvania: Harper, J.A., Kelley, D.R., and Linn, E.H., Journal of Sedimentary Petrology, v. 53, 1999, Petroleum—Deep oil and natural p. 25–49. gas, in Shultz, C.H., ed., The geology of ——1988, Hierarchy of sea-level cycles in the Pennsylvania: Pennsylvania Geological medial Silurian siliciclastic succession of Survey and Pittsburgh Geological Society Pennsylvania: Geology, v. 16, p. 242– Special Publication 1, p. 507–530. 245. Heyman, Louis, 1977, Tully (Middle Devonian) Dalrymple, R.W., Zaitlin, B.A., and Boyd, Ron, to Queenston (Upper Ordovician) correla- 1992, Estuarine facies models—Concep- tions in the subsurface of western Penn- tual basics and stratigraphic implications: sylvania: Pennsylvania Geological Survey, Journal of Sedimentary Petrology, v. 62, 4th Series, Mineral Resource Report 73, p. 1130–1146. 16 p. Driese, S.G., Fischer, M.W., Easthouse, K.A., Johnson, M.E., and McKerrow, W.S., 1991, Marks, G.T., Gogola, A.R., and Schoner, Sea level and faunal changes during the A.E., 1991, Model for genesis of shoreface latest Llandovery and earliest Ludlow and shelf sandstone sequences, southern (Silurian): Historical Biology, v. 5, p. 153– Appalachians—Paleoenvironmental recon- 169. struction of an Early Silurian shelf system, Keighin, C.W., 1998, Depositional dip-oriented in Swift, D.J.P., Oertel, G.F., Tillman, cross section through the Lower Silurian R.W., and Thorne, J.A., eds., Shelf sand “Clinton” Sands and Medina Group in and sandstone bodies—Geometry, facies, northeastern Ohio and western Pennsyl- and sequence stratigraphy: International vania: U.S. Geological Survey Open-File Association of Sedimentologists, Special Report 98–500, 1 sheet. Publication Number 14, p. 309–338. Knight, W.V., 1969, Historical and economic Duke, W.L., and Brusse, W.C., 1987, Cyclicity geology of Lower Silurian Clinton sand- and channels in the upper members of the stone of northeastern Ohio: American Medina Formation in the Niagara Gorge, Association of Petroleum Geologists Bul- in Duke, W.L., ed., Sedimentology, letin, v. 53, p. 1421–1452. stratigraphy, and ichnology of the Lower Laughrey, C.D., 1984, Petrology and reservoir Silurian Medina Formation in New York characteristics of the Lower Silurian Med- and Ontario: Society of Economic Paleon- ina Group sandstones, Athens and Geneva tologists and Mineralogists, Eastern Sec- Fields, Crawford County, Pennsylvania: tion, 1987 Annual Field Trip Guidebook, p. Commonwealth of Pennsylvania Depart- 46–65. ment of Environmental Resources Office of Duke, W.L., Fawcett, P.J., and Brusse, W.C., Resource Management, Bureau of 1991, Prograding shoreline deposits in the

12 Topography and Geological Survey Min- nia, to Orleans County, New York: U.S. eral Resource Report 85, 125 p. Geological Survey Miscellaneous Investi- Martini, I.P., 1971, Regional analysis of sedi- gations Series Map I–2726, 2 sheets. mentology of the Medina Formation Ryder, R.T., Aggen, K.L., Hettinger, R.D., (Silurian), Ontario and New York: Ameri- Law, B.E., Miller, J.J., Nuccio, V.F., Perry, can Association of Petroleum Geologists W.J., Prensky, S.E., SanFilipo, J.R., and Bulletin, v. 55, p. 1249–1261. Wandrey, C.J., 1996, Possible continuous- Middleton, G.V., 1987, Geologic setting of the type (unconventional) gas accumulation in Northern Appalachian Basin during the the Lower Silurian “Clinton” sands, Med- Early Silurian, in Duke, W.L., ed., Sedi- ina Group, and Tuscarora Sandstone in mentology, stratigraphy, and ichnology of the Appalachian basin—A progress report the Lower Silurian Medina Formation in of 1995 project activities: U.S. Geological New York and Ontario: Society of Eco- Survey Open-File Report 96–42, 82 p. nomic Paleontologists and Mineralogists, Van der Voo, R., 1988, Paleozoic paleography Eastern Section, 1987 Annual Field Trip of North America, Gondwana and inter- Guidebook, p. 1–15. vening displaced terranes—Comparisons Middleton, G.V., Rutka, M., and Salas, C.J., of paleomagnetism with paleoclimatology 1987, Depositional environments in the and biogeographical patterns: Geological Whirlpool Sandstone Member of the Med- Society of America Bulletin, v. 100, p. ina Formation, in Duke, W.L., ed., Sedi- 311–324. mentology, stratigraphy, and ichnology of Van Wagoner, J.C., Mitchum, R.M., Jr., Cam- the Lower Silurian Medina Formation in pion, K.M., and Rahmanian, V.D., 1990, New York and Ontario: Society of Eco- Siliciclastic sequence stratigraphy in well nomic Paleontologists and Mineralogists, logs, cores, and outcrops—Concepts for Eastern Section, 1987 Annual Field Trip high-resolution correlation of time facies: Guidebook, p. 31–45. American Association of Petroleum Geol- Piotrowski, R.G., 1981, Geology and natural ogists Methods in Exploration Series 7, 55 gas production of the Lower Silurian p. Medina Group and equivalent rock units in Van Wagoner, J.C., Posamentier, H.W., Pennsylvania: Pennsylvania Topographic Mitchum, R.M., Vail, P.R., Sarg, J.F., and Geologic Survey Mineral Resource Loutit, T.S., and Hardenbol, J., 1988, An Report 82, 21 p. overview of sequence stratigraphy and key Ross, C.A., and Ross, J.R., 1996, Silurian sea- definitions, in Wilgus, C.W., and others, level fluctuations: Geological Society of eds., Sea-level changes—An integrated ap- America Special Paper 306, p. 187–192. proach: Society of Economic Paleontolo- Ryder, R.T., 1998, Characteristics of discrete gists and Mineralogists Special Publication and basin-centered parts of the Lower Sil- 42, p. 39–45. urian regional oil and gas accumulation, Walker, R.G., 1992, Facies, facies models and Appalachian basin—Preliminary results modern stratigraphic concepts, in Walker, from a data set of 25 oil and gas fields: R.G., and James, N.P., eds., Facies models U.S. Geological Survey Open-File Report response to sea level change: Geological 98–216, 71 p. Association of Canada, p. 1–14. ——2000, Stratigraphic framework and de- Yeakel, L.S., 1962, Tuscarora, Juniata, and positional sequences in the Lower Silurian Bald Eagle paleocurrents and paleogeogra- regional oil and gas accumulation, Ap- phy in the central Appalachians: Geologi- palachian Basin; from Jackson County, cal Society of America Bulletin, v. 73, no. Ohio, through northwestern Pennsylva- 12, p. 1515–1540.

13 82° 80° 78° Lake Ontario A Orleans CANADA Genesse

Erie Wyoming B Cattaraugus Lake Erie Chau- tauqua NEW YORK

42° C Erie Warren Potter D Lake Crawford McKean Ashtabula Forest Elk Lycoming Geauga Cameron Venango Mercer Clinton Jefferson OHIO Trumbull B' Clarion Mahoning D' Clearfield E Wayne C' Ashland Lawrence Centre Stark Butler Columbiana Beaver Carroll

Tuscara- Jefferson was PENNSYLVANIA F Harrison 0 50 mi Guern- 40° Licking Muskingum sey Fayette 0 50 km Noble MARYLAND

Athens WEST Vinton Washington VIRGINIA Preston Wood E' Wirt Calhoun A' EXPLANATION Roane B B' Cross section described in this report Clay

Nicholas A A' Other cross sections associated with ° Fayette 38 this study F'

Figure 1. Locations of cross sections constructed by the U.S. Geological Survey to assess continuous gas accumulations in Lower Silurian strata of the Appalachian Basin. A–A' trends approximately parallel to the paleoshoreline of an epicontinental sea that covered much of North America during the Silurian Period. Correlations along cross-sections B–B' and C–C' are shown in this report. Correlations along A–A' were reported by Ryder (2000), and correlations along D–D' were reported by Keighin (1998). Investigations by R.T. Ryder along cross sections E–E' and F–F' were in progress at the time of this publication and were expected to be published at a later date.

14 Michigan Basin EXPLANATION B NY Appalachian Foreland Basin area C Main depositional trough

B' and Highlands Intrabasinal platform Passive carbonate- PA C' dominated western ramp A 0 100 mi Source Cross sections B–B' and C–C' Taconic 0 100 km

EXPLANATION

Niagara region NY Silurian outcrops CANADA Cross sections Buffalo B–B' and C–C' B Lines of panels C shown in figure 5 B' and C' Williamsport OH 0 100 mi Mill Hall 0 100 km PA Harrisburg Folded and thrust-faulted eastern B margin of the Appalachian Basin

B 10 13 NEW YORK 18 3 17 6 32 21 27 0 50 mi 37 28 Lake Erie 45 Chautauqua 0 50 km Erie 44 46 48 19 35 37 38 32 Warren C 10 39 41 Potter 13 53 1 54 McKean 6 46 52 4 14 30 42 48 55 8 16 21 Lake 26 44 51 Ashtabula 45 Crawford 47 49 Forest Elk 52 56 Cameron 50 51 Lycoming Clinton 53 Venango 57 59 OHIO 58 Clarion B' Jefferson Clearfield 60 PENNSYLVANIA Centre C' EXPLANATION B B' Outcrop of Tuscarora Fm. near Mill Hall, Pa. Line of cross-section C Oil Gas Drill hole

Figure 2. Locations of cross sections B–B' and C–C'. A, Cross sections located with respect to paleogeography during the medial Silurian; modified from Brett and others (1990). B, Cross sections located with respect to Silurian outcrops in the northern Appalachian Basin. The Niagaran Provincial Series is exposed in the Niagara region, and coeval strata near the town of Williamsport, Pa., are assigned to the Tuscarora, Rose Hill, Mifflintown, and Bloomsburg Formations. C, Location of drill holes and measured section along B–B' and C–C', and areas with oil and gas accumulations in the Lower Silurian Medina Group and Tuscarora Formation. Areas with oil and gas accumulations are based on Ryder (1998). The southeastern end of both cross sections is at an outcrop of the Tuscarora Formation near the town of Mill Hall, Pa. (fig. 2B); the outcrop was described by Cotter (1982, 1983). Drill holes are numbered consecutively from left to right on each cross section, and selected drill hole numbers are shown. All drill holes are identified in table 1. 15 Series Southern Ontario and West-central Central Central Ohio western New York New York New York Pennsylvania (Niagara region) and others (1990) European Stage System European Eastern North American interpretations of Brett Sequence stratigraphic

Lud. Vernon Guelph Dol. Bloomsburg ? Guelph Dol. Shale Formation

Eramosa Eramosa Peebles Dol. Dolostone Dolostone Vinemount

Gorstian Mbr. Ludlovian Upper (part) Ancaster VI Mbr. McKenzie Penfield Lilley-Peebles Niagara Falls

Goat Island Dol. Formation Mbr. transition Ilion Shale Lockport Group Lockport Group Lockport Group Lockport Group Mbr. Sconondoa Dolomite Pekin Mbr. Lilley Dolostone Gothic Hill Mbr. Gasport Dol. Homerian

Glenmark DeCew Dol. DeCew Dol. DeCew Dol. Sh. Mbr. -equiv.

Gates Mbr. Mifflintown Formation Burleigh Hill Burleigh Mbr. Rochester Hill Mbr. V Mbr.

Mbr. Shale Mbr. Wenlockian

Lewiston Lewiston Joslin Hill ?? Herkimer Ss. Jordanville Ss. Rochester Shale Mbr. Rochester Shale Mbr. Bisher Sheinwoodian Dolostone Irondequoit Ls. Irondequoit Ls. Kirkland Ls. Keefer Ss. Mbr. Clinton Group (upper) Clinton Group (upper ) Rockway Dol. Clinton Group (upper ) Rockway Dol. Dawes Formation Dawes -equiv.

Silurian Estill Sh. Williamson Williamson Shale Willowvale Shale upper shaly C Sh. 6 member IV Clinton Group (upper) Dayton-Waco Ls. Merritton Ls. Westmoreland Westmoreland Hem.

C Center Ss. Mbr. 5 Otsquago Ss. Clinton Sauquoit middle shaly mbr. Gp. (m.) Telychian Fm. Sauquoit Fm. III Lower Cabin Hill Ss. Mbr. Clinton (m.)

C4 Wolcott Ls. W. (upper) Sodus Sh. Cacapon Ss. (upper) Sodus Sh.

Rose Hill Formation lower shaly C (lower) II 3 (lower) Sodus Sh. Sodus Oneida member Lulbegrud l. Sodus Sh. Congl. C2 Sh. Sh. Bear Creek Sh. Clinton Gp. (l.) ? Reynales Ls. C Oldham Ls. Furnaceville Mbr.

1 Clinton Gp. (l.) Clinton Gp. (lower) Maplewood Sh. ? Plum Cr., u. Cabot Head Sh. Clinton Gp. (l.) Neahga Sh. B 3 Aeronian Llandoverian B 2 Kodak Ss. Kodak Ss. Castanea Cambria Sh. Cambria Sh. Mbr. B1 "stray Clinton" Thorold Ss. Thorold Ss. -equiv. Medina Gp.

A4 "red Clinton" Grimsby Grimsby Formation Formation I

Medina Group ? A "Clinton" ss. "white Clinton" Devils Hole Ss. 3 Medina Group Devils Hole Ss. Cabot Head Sh. (l.) Power Glen Sh. Rhuddanian Medina Group

Brassfield Ls. Whirlpool Tuscarora Formation A2 Ss. Whirlpool Ss. (Medina ss.) A1

Figure 3. Chronostratigraphic correlations of Silurian strata in Ohio, southern Ontario and western New York (Niagara region), west-central New York, central New York, and central Pennsylvania. Diagram is compiled and modified from Brett and others (1990, their figs. 7B and 14). Many units in the Medina Group and Tuscarora Formation are not precisely dated. Vertical ruling represents unconformities. Gray areas represent unconformity-bound sequences, labeled I through VI, of Brett and others (1990). Nomenclature in southern Ontario and New York has been modified to match revisions by Brett and others (1995, their figs. 9 and 15). The lower part of the Clinton Group was extended into Ohio based on descriptions by Brett and others (1990, p. 209). Abbreviations: Group (Gp.), lower (l.), middle (m.), upper (u.), Formation (Fm.), Member (Mbr., mbr.), Conglomerate (Congl.), Sandstone (Ss., ss.), Shale (Sh.), Dolomite (Dol.), Hematite (Hem.), Limestone (Ls.), Ludfordian (Lud.), Creek (Cr.), Wolcott Limestone (W.), equivalent (equiv.). 16 Thick- Lithologic description Depositional Formation ness summary

Group (ft)

Guelph Dol. < 40 ? Fine-grained oolitic dolomite, shaly in upper part. Contains sparse fossils and stromatolites. Progradational marine Eramosa Dol. 38–50 Thick- to thin-bedded, bituminous, fossiliferous dolomite with chert horizons. Some units lack fossils. Deeper marine Vinemount Member is a thin- to medium-bedded, fossiliferous, shaly dolomite; Goat Island (Vinemount 26–56 Ancaster Member is a thin-bedded, cherty, fossiliferous dolomite; Niagara Falls

Lockport Mbr.) Dol. Member is a biohermal grainstone.

Pekin Member grades laterally from argillaceous dolomicrite to bioherms and Shallow Gasport Dol. 20–37 dolorudites; Gothic Hill Member is a fossiliferous dolograinstone to marine shelf dolopackstone.

Argillaceous to sandy dolomite with thin shale partings. Unit has sparse fossils DeCew Dol. 5–12 and is characterized by contorted bedding. Deep water The Burleigh Hill Member is calcareous to dolomitic mudstone; its upper part has offshore marine Rochester Sh. 2–120 abundant laminated pelletal grainstones and is increasingly calcareous upwards, grading to limestone. The underlying Lewiston Member is calcareous mudstone that is fossiliferous in its lower and upper parts.

Thick- to massive-bedded, fossiliferous, dolomitic packstone and grainstone with Shallow water upper Irondequoit Ls. 12–22 medium- to massive-bedded, fossiliferous limestone and thin shale interbeds in high energy upper part.

Thin- to medium-bedded, burrowed, dolomitic wackestones with thin interbeds of 7–12 dolomitic shale. Base contains Salmon Creek Phosphate Bed, which comprises

Clinton Rockway Dol. bioturbated dolomicrite with quartz and phosphate pebbles.

Fissile shale containing graptolite fossils. Base contains Second Creek Phosphate Deep shelf Williamson Sh. < 1–79 Bed, which comprises quartz and phosphate pebbles.

< 3.5 Dolomitic to argillaceous limestone with shale partings. Limestone contains Merritton Ls. brachiopod fossils and glauconite. Lower beds are bioturbated. Basal part has phosphate nodules and chert pebbles.

Represented by Hickory Corners Member, which consists of thin- to medium- Offshore

Niagaran Provincial Series Reynales Ls. 0–12 bedded limestone with shale partings, fossils, and some chert. carbonates

Fissile shale with sparse fossils. Basal Densmore Creek Phosphate Bed contains lower Neahga Sh. < 2–6 Inner shelf nodules, pebbles, and cobbles of quartzite and phosphate in a sandstone muds matrix.

Rhythmically interbedded sandstone and shale. Contains abundant trace fossils, Kodak Ss. < 1–11 locally. Shoreface , shallow Interbedded fine-grained sandstone, siltstone, and shale with sparse fossils. Cambria Sh. < 1–14 Caliche horizons and desiccation cracks are in some western sections. shelf, tidal flat, and Thorold Ss. 5–10 Mottled, cross-bedded, channel sandstone that grades laterally to a massive tidal pelletal sandstone. channels Interbedded sandstone, conglomerate, and shale. Sandstone is fine to medium grained. Sandstone beds in upper part of Grimsby are trough cross-stratified; Condensed Grimsby Fm. 29–72 sandstone beds in lower part of Grimsby are planar laminated and hummocky interval (Artpark cross-stratified. Basal 10–15 ft of Grimsby is fossiliferous and bioturbated, and Phosphate Bed) includes a phosphatic dolomite and pebble lag in the Artpark Phosphate Bed. Medina Sandstone with shale interbeds; sandstone is fine to medium grained, horizontally Devils Hole Ss. 13–15 Shallow shelf to hummocky cross laminated, and well sorted.

Power Glen Sh. 11–28 Shale containing hummocky cross-stratified and bioturbated sandstone, dolomite, Deeper shelf siltstone, limestone, and sparse fossils. Nearshore marine, Upper part is finer grained and hummocky cross-stratified sandstone. Lower part shallow shelf Whirlpool Ss. 18–28 is fine- to medium-grained trough cross-stratified sandstone and contains large superposed over channel fills. braided fluvial

Figure 4. Summary of the Niagaran Provincial Series in the Niagara region. Nomenclature, descriptions, depositional summaries, and unconformities are from Brett and others (1990, 1991, 1995, and references included therein). Their interpretations of major un- conformities are shown by thick dashed lines, and their interpretations of minor unconformities are shown by thin dashed lines. Ab- breviations: Dolomite (Dol.), Formation (Fm.), Limestone (Ls.), Member (Mbr.), Sandstone (Ss.), Shale (Sh.).

17 West East (distal, seaward) Basal Clinton unconformity (proximal, landward) EXPLANATION Thorold Ss. Kodak Ss. Cambria Sh. Tidal

Grimsby Fm. Very shallow shelf, shoreface, tidal flat, Grimsby Fm. tidal channel, and estuarine

about 100 ft Manitoulin Medina Group Devils Hole Ss. Shallow shelf Dol. Power Glen Shale Whirlpool Ss. Deeper shelf Cherokee discontinuity Shallow shelf in upper part, and braided about 100 mi fluvial in lower part A of Whirlpool Ss.

Northwest (distal, seaward) Southeast (proximal, landward) near Harrisburg, Pa. near Mill Hall, Pa. Castanea Mbr. Coastal flats

Braided Shelf fluvial sand waves Fm. Tuscarora about 660 ft Braided fluvial and estuary Beach about 60 mi B (Cotter (1982) reported the palinspastic distance to be about 155 mi)

Figure 5. Previous depositional interpretations made from exposures of the Medina Group and Tuscarora Formation. Both panels show facies distributions along depositional dip; panel locations are shown in figure 2B. A, Depositional interpretations for Medina Group in the Niagara region; modified from Brett and others (1995). Interpretations are based on Brett and others (1990, 1991, 1995, and references included therein) and Duke and others (1991); tidal interpretations in the upper part of the Medina Group are based on Castle (1998) and Laughrey (1984). B, Depositional interpretations for the Tuscarora Formation along the folded and thrust-faulted eastern margin of the Appalachian Basin; modified from Cotter (1982). Abbreviations: Formation (Fm.), Member (Mbr.), Sandstone (Ss.), Shale (Sh.), Dolomite (Dol.).

18 7 LITHOLOGIC INTERPRETATIONS 31-031-10298 GR DEN Sandstone Mudrock

Sandy shale Carbonate; silty or siltstone or sandy (gray)

11 31-031-11605 GR 3000 3100 3100 3200 3200

STRATAL SUCCESSIONS Fining-upward successions 3300 Coarsening-upward successions

Figure 6. Criteria used for lithologic interpretations in drill holes located along cross sections B–B' and C–C'. Examples from drill holes 7 and 11 (table 1, cross section B–B'). Down-hole depths are labeled at 100- ft intervals. Lithologies are interpreted from combined responses of natural gamma (GR) and density (DEN) logs; values increase to the right as indicated by arrows. Sandstone has low to moderate GR and DEN values; siltstone has moderate GR and DEN values; mudrock has high GR and moderate DEN values; carbonates have low GR and moderate to high DEN values. Coarsening-upward siliciclastic successions have upwardly decreasing GR values, and fining-upward siliciclastic succes- sions have upwardly increasing GR values.

19