Depositional and structural architecture of prograding clastic continental margins: tectonic inftuence on patterns of basin filling

WILLIAM E. GALLOWA Y

Galloway W. E.: Depositional and structural architecture of prograding clastic continental margins: tectonic inftuence on patterns of basin filling. Norsk Geologisk Tidsskrift, Vol. 67, pp. 237-251. Oslo 1987. ISSN 0029-196X.

Progradation of a sediment wedge into an oceanic basin induces regional isostatic subsidence and a predictable structural and depositional architecture. The combination of large scale, slope morphology; and undercompaction makes continental margin wedges prime sites fur syndepositional gravity tectonics. Extension dominates the shelf margin; shortening characterizes the lower slope. Outbuilding clastic wedges, such as the Cenozoic Gulf Coast margin, contain one or more deltaic depocenters that episodically prograde directly onto the continental slope. Eustatic sea-leve! changes may inftuence depositional patterns, but regional intraplate tectonic events and extrabasinal controls on rate and location of sediment supply play an equally important role in determining genetic depositional sequence stratigraphy.

William E. Galloway, Department of Geo/ogical Sciences, The University of Texas at Austin, Austin, Texas 78713 U.S.A.

Divergent continental margins are generally con­ influx has prograded the continental margin as sidered to be settings of tectonic quiescence, little much as 350 km seaward of the inherited Cre­ affected by a prominent interplay between struc­ taceous shelf edge. The Cenozoic sedimentary tural deformation and sedimentation. lndeed, the wedge, which contains an estimated 40 billion stratigraphic patterns developed in 'passive' mar­ barrels of producible oil (Dolton et al. 1981), gin basins have been interpreted primarily in provides a natural laboratory for the examination terms of systematic, thermally driven subsidence of the three-dimensional stratigraphic and struc­ overprinted by eustatic sea-leve! changes (Har­ tural architecture as well as the evolution of con­ denbol et al. 1981; Watts 1982) The objective of tinental margin sequences. this paper is to examine the more subtle interplay Four generalizations can be distilled from the between depositional patterns and tectonic pro­ northwest Gulf Coast data base. These gen­ cesses that occurs during an extended history eralizations appear to have broad applicability of divergent continental margin progradation to sirnilarprograded divergent, clastic-dominated (offlap). The example used will be the Cenozoic margins such as those of the Niger and Mackenzie sedimentary wedge of the northwest Gulf of Mex­ basins (Perrodon 1983). ico margin. The Gulf basin originated with a brief period of rifting in Jurassic time, followed by an l. Continental margin progradation is concen­ extended period of thermally induced subsidence trated at one or more depocenters where large, during the remainder of the Mesozoic (Buffler oceanic delta systems build out to the shelf & Sawyer 1985). Slow rates of sediment influx edge and deposit sediment directly onto the resulted in deposition being largely restricted to upper continental slope. Interdeltaic margin bounding shelf-platforms, and culminated in the segments receive sediment by longshore creation of a reef-rimmed, sediment-starved transport. oceanic basin (Jackson & Galloway 1984). 2. A distinct syndepositional structural style is Regional uplift and tectonism within the con­ associated with the progradation of clastic­ tinental interior of western North America pro­ margin wedges. Crustal loading and resultant vided an abrupt surge of terrigenous clastic flexuraldeformation create areas of maximum sediment into the northwest Gulf of Mexico in subsidence and peripheral uplift, which in turn Paleocene time, and this continuing sediment determine the geometry of depositional 238 W. E. Galloway NORSK GEOLOGISK TIDSSKRIFf 67 (1987)

sequences. Within the sedimentary wedge, In the following sections, each of these gen­ gravity tectonics produces a variety of exten­ eralizations will be discussed and illustrated. sional, compressional, and diapiric structures. 3. Depositional outbuilding of the continental margin is typically episodic. Major depo­ Depositional elements of sitional episodes were punctuated by periods prograding margins of regional basin-margin fiooding; resultant genetic depositional sequences refiect this Progradation of a continental-margin prism alternation of progradation and retrogra­ requires the accumulation of sediment in depo­ dation/transgression. The episodes reveal the sitional systems ranging from the base of the changing interplay between sediment input continental slope of paralic ( deltaic or shore­ rates (contro lied largely by regional tectonics), zone/shelf). Three bathymetric and depositional eustatic sea-level changes, and subsidence regimes - the continental slope, shelf edge, and rates. shelf platform - successively pass a reference 4. The depositional episodes, and concomitant point as margin progradation proceeds beyond geographic shifts of deltaic depocenters, are that point. The shelf edge, as definedby the break best explained by intraplate tectonics some­ in depositional slope separating shelf-platform times overprinted by eustatic sea-level change. sediments deposited by traction currents from Plate interactions at convergent and transform slope sediments deposited by gravitational resedi­ margins affect thermal and stress regimes far mentation, is the most useful guide for deter­ into the interior of plates, indirectly (source mining continental margin posmon at any terranes) and directly (margin subsidence/ particular time or stratigraphic leve l (for further uplift) infiuencing interregional depositional discussion, see Winker 1982 and Jackson & Gal­ pattems. loway 1984). It is important to emphasize that

l/ MISSISSIPPI EMBAYMENT 11 ---V-Mississippi--�

N / Shell edge positions p Principal drainage axis

_... Facus of offlop l EJ]Sand- rich continental-margin sequence

o IOOmi

o 200km

Fig. l. Progressive Cenozoic shelf-edge positions and location of sand-rich depocenters of the Northwest Gulf of Mexico continental margin. Modified from Winker (1982). NORSK GEOLOGISK TIDSSKRIFr 67 (1987) TSGS Symposium 1986 239 paleo-shelf edges do not connote a specific water sode further prograded the shelf edge, the amount depth, as is commonly assumed today. Rather, of outbuilding varies greatly along the basin the shelf edge definesthe position of abrupt steep­ margin. As shown on the map, maximum out­ ening of bathymetric profile and associated building within any one stratigraphic interval is change in dominant depositional process suite. associated with a sand-rich depocenter. Regional In its position at the crest of a progradational analysis of the Cenozoic depositional framework sedimentary wedge many kilometers thick, the shows that these sand-rich depocenters cor­ shelf edge provides a more significant and stable respond to major deltaic systems. It is further record of maximum basin-margin outbuilding notable that nearly all of the sandy depocenters than does the shoreline, which commonly shifts are localized at one of three preferred positions landward and seaward many tens of kilometers in along the basin margin (Fig. 1). These foci for response to minor base-level changes. sediment input are broad, extremely subtle struc­ The Cenozoic sedimentary wedge of the north­ tural sags, called 'embayments' by most Gulf west Gulf of Mexico illustrates the evolution of a Coast geologists. The three depocenters are, prograding margin (Fig. 1). Successively younger from southwest to northeast, the Rio Grande, paleo-shelf edges lie progressively basinward of Houston, and Mississippi embayments. lnter­ the inherited Cretaceous reefal shelf edge. How­ estingly, they still define the entry points of the ever, although each successive depositional epi- four largest extrabasinal rivers (the Rio Grande,

(a) Extrobasinol streom Intrabosinol ond bosin fringe streoms l

I nterdeltoic Bight Reworking Deltoic Heodlond

Geomorphic or structurol focus edge of older depositionol sequence \

ort Mud transp � � --:;!'

Fig. 2. Depositional elements of a prograding clastic continental platform and shelf edge. One or more principal deltaic headlands prograde to the shelf edge and onto the upper continental slope. The interdeltaic margin is fed by longshore transport of sand and mud that is reworked from the deltaic headlands. a. Schematic depositional elements include alluvial/deltaic aprons of the large, extrabasinal as well as the smaller, basin-margin streams and the interheadland coastal bight. b. lnterpreted paleogeography of the lower Miocene depositional episode showing the mosaic of environments that equate to the schematic elements. From Galloway et al. (1986). 240 W. E. Galloway NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

Brazos/Colorado, and Mississippi) into the Gulf denser sediment induces isostatic adjustment of of Mexico (Galloway 1981). the underlying crust (Bott 1980). Subsidence of Paleogeography of the lower Miocene depo­ oceanic crust will result in a sedimentary section sitional sequence (Galloway et al. 1986) illustrates about three times thicker than the depth of water the typical relationship between the principal actually replaced. Transitional crust found along depositional elements and contemporary shelf divergent plate margins will subside less, but the edge (Fig. 2). Depositional elements include one sedimentary section is expanded at least twofold. or more deltaic headlands that were constructed In the northwest Gulf of Mexico basin a sedi­ where major extrabasinal fluvial systems were mentary wedge 8 to 14 km thick resulted from funneled into the basin along topographically or progradational filling of the starved, deep-water structurally controlled axes, and interdeltaic basin created by thermal subsidence of underlying shoreline reentrants or bights. The deltaic head­ transitional to oceanic crust. lands rapidly prograded across the foundered Crustal depression occurs by flexural loading, depositional platform of the earlier Oligocene forming a broad subsidence bowl (Fig. 3, middle delta systems to the shelf edge and directly onto section) that extends on the order of 150 km the upper continental slope. These shelf-edge around the locus of loading (Bott 1980). Thus, a delta headlands thus became the sites for the most broad lens of sediment, including not only the direct and rapid progradation of the continental deposits of the continental margin depocenter but margin (Winker & Edwards 1983). Headlands are also extensive shore-zone and coastal-plain facies, subject to greatest marine reworking, whereas is accommodated. Concomitant with subsidence, the concave bight becomes a sediment sink for a halo of uplift - the peripheral bulge - forms sand and mud of the minor intrabasinal streams around the subsidence bowl (Quinland & Beau­ and for sediment reworked longshore. Longshore mont 1984; Cloetingh et al 1985). The effect is to drift provided sediment for construction of the produce a hinge line with subsidence and sedi- . sandy barrier islands and strandplain. Suspended ment storage occurring on the basinward side sediment was redistributed along the shelf edge while, at the same time, gentle uplift, sediment and slope by longshore currents, providing the bypass, valley incision, and erosion occur on the material for slower progradation of a muddy, landward fringe. The peripheral bulge migrates interdeltaic shelf edge (Fig. 2). basinward just as the continental margin depo­ In summary, direct feeding of sediment through center migrates basinward with ongoing offlap. structurally focused fluvialsystems to major shelf­ Modeling indicates that the peripheral bulge will edge delta systems provided material for the most also migrate toward the focus of loading following rapid and direct progradation of the Gulf Coast a discrete loa ding event (Quinlan & Beaumont continental margin. lnterdeltaic shelf-edge seg­ 1984). Furthermore, the rates and magnitudes of ments prograded more slowly by Iongshore trans­ both basin subsidence and peripheral uplift will port and deposition of dominantly suspended sediment.

Structural style DEPRESSION OF CRUST Two scales of structural deformation affect to varying degrees the sediments of a prograding continental margin. First, large-scale sedimentary GR411TY SLIOtNG loading of the crust induces regional subsidence � � COMPRESSIONAL and associated peripheral uplift. Secondly, gra vity TRANSLATION THtCKENING EXTENSIONAL deformation within the sedimentary wedge pro­ THINNING duces a predictable but commonly complex family Fig. 3. Deformation and subsidence domains of a prograding of extensional and compressional structures. clastic continental margin. A broad subsidence bowl and per· ipheral bulge result from sediment loading. Gravitational insta­ bility within the sediment prism results in extension (normal Crustal loading and isostatic subsidence faults and enhanced subsidence) at the shelf edge and com­ pression ( folds and reverse faults and uplift) at the slope toe. Replacement of several kilometers of water with Modified from Winker (1982). NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 241 be modified by changes in the horizontal stress the sedimentary section and to account for energy field within crustal plates (Cloetingh et al. 1985). dissipated during deformation. Because gravi­ In summary, the inherent pattern of flexuralsub­ tational force increases in proportion to the cube sidence of the crust ensures the presence of of the length scale, it is very effective within the numerous intraformational disconformities and thick sedimentary prisms deposited at continental even low-angle unconformities within the updip margins. margin of the sedimentary prism. Three potential styles of gravity tectonics are shown diagramatically in Fig. 4: gravity gliding, gravity spreading, and diapirism. Gravity gliding Gravity tectonics and gravity spreading both create regional stress The thick sedimentary prism of a prograding con­ fields that focus tension at the shelf edge and tinental margin is an ideal habitat for gravity­ compression at the base of the slope. Though driven deformation, and the Gulf of Mexico similar, these two styles have different boundary Cenozoic section exhibits a spectacular assem­ conditions. Gravity gliding requires a sloping blage of growth faults, diapirs, gravity-glide folds, lower boundary surface down which the glide and allochthonous salt wedges (see Martin 1978, sheet moves. In contrast, gravity spreading occurs Jackson & Galloway 1984, and Galloway 1986, in a mounded sediment pile with a free surface, for reviews). toward which spreading is directed. The extruded Gravity tectonic structures, by definition, must sediments can move up the slope of the underlying exhibit a geometry that refiects a descrease in boundary layer. Lateral movement of the Green­ gravitational potential when compared to the land ice cap is an excellent example of gravity undeformed state (Ramberg 1981). Further, the spreading. Two further points should also be decrease in gravitational potential must be suf­ emphasized. First, although vertical displacement ficient to overcome the fundamental strength of is usually more obvious, horizontal displacement �GRAVITY TECTONICS-CONTINENTAL MARGINS �··· ·············�·········· · --t ­ �j[�j�]\\l\11\[[j[�nun::::::::�::.::�:� :: : : : : :: ...... z :::::::::: •••••••••••••••••••••••••••••••••••••••••::-: ••"7". -,..,••,..,- -:-••�."7".:-: •• ......

Fig. 4. Schematic geometries of the three manifestations of gravity tectonics. Modified from Ramberg (1981). A prograding sedimentary wedge provides a free surface toward which sediment can flow and may also create a density inversion (rho vs. z) as sandy, normally compacted sediment is deposited on a foundation of undercompacted muds. 242 W. E. Galloway NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

w E

Fig. 5. Gravity·glide sheet in the Upper Cretaceous section of the Southwest Africa continental margin. From Galloway (1986); modifiedfrom Bally (1983). of sediment volume commonly dominates. Sec­ sedimentary prism along a seawardly rising thrust ondly, density inversion is neither necessary nor plane. Sediment layers thin over or lap out against sufficient to initiate gravity gliding or spreading. folds. The shelf edge is again a zone of extensional However, density inversions, such as produced faults (growth faults) that show landward thick­ by deposition of normally compacted sandy sedi­ ening of depositional units against the down­ ment on a foundation of undercompacted, over­ thrown side of the fault planes. pressured continental slope mud, will augment Both gravity gliding and spreading produce a the inherent instability of the sloping, free comparable suite of compressional and exten­ boundary surface and enhance the potential for sional structures. This observed and theoretical diapirism. distribution of stress/strain regimes characterizes An excellent example of a gravity glide sheet prograding continental margins (Dailly 1976; is shown in Fig. 5, which is based on a published Winker 1982). The structural pattern reveals the seismic line traversing the West African con­ presence of three strain regimes within the shelf tinental margin (Bally 1983). Upper Cretaceous edge and continental slope. Extension char­ sediments have been displaced down the con­ acterizes the upwardly convex shelf-to-slope tran­ tinental slope along a glide zone within the older sition. At the toe of the slope, compression Cretaceous section. Prominent structural features dominates. Beneath the middle continental slope, include a series of syndepositional listric normal between the zones of extension and compensatory faults within the Upper Cretaceous section near compression, Iies a domain of lateral translation. the crest of the slope and an imbricate thrust sheet Together, these zones overprint the simple pat­ at the toe of the glide sheet near the base of tern of crustal subsidence with localized uplift at the slope. The overall structural pattern closely the toe of the slope and enhanced subsidence resembles that predicted by Crans et al. (1980). across the outer shelf to upper slope (Fig. 3). The outer shelf and continental slope of the Thus, the well-defined strain domains have con­ Niger delta sedimentary prism provide an siderable impact on sediment deposition and example of the structural patte ms produced domi­ storage. Extension enhances regional, load­ nantly by gravity spreading (Fig. 6). A typical induced subsidence at the shelf edge, creating a regional transect shows that the toe of the con­ localized depocenter that retains much of the tinental slope is characterized by numerous imbri­ sediment transported to the shelf margin and cate thrust faults and appears to have been upper slope. In the typical facies tract of a pro­ extruded from beneath the prograding deltaic grading shelf-edge deltaic headland, rapid shelf-

COMPRESSION EXTENSION

Fig. 6. Niger delta continental slope and shelf edge showing compressional and extensional structural domains resulting primarily from gravity spreading of the thick delta/continental slope sediment pile. Length of the section is approximately 250 km. From Galloway (1986). NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 243

EXPLA NAT ION fc;;>.�,:i:l Terrestriol Aogrodotion IH!> l Coostol Progrodotion PS•;'� Slope Progradotion O Marine Aggradotion

Fig. 7. Comparative depositional styles of a stable (upper profile)and unstable (lower profile)prograding basin margin sedimentary wedge. Enhanced deposition and preservation of delta facies sequences at the expense of lower-slope facies characterizes the gravitationally deforrned continental margin wedge. From Galloway (1986).

edge subsidence enhances storage of thickened extends progressively basinward, where they delta-margin progradational sand and prodelta overlie and grade into thick marine mudstones mud facies (Fig. 7). In contrast, compression of (Fig. 8). The sand-rich tongues are separated the lower slope results in abbreviated, locally by thinner, updup-extending tongues of marine truncated, and diapirically intruded submarine mudstone, effectively punctuating the lithostra­ fan and lower slope facies. Much of the sediment tigraphy of the upper 3 to 5 km of the sedimen­ that does escape the shelf-margin extensional tary sequence. Although the prominence and sediment trap also bypasses the lower slope and nomenclature of the individual sandy tongues is deposited on the abyssal plain (Galloway 1986). vary from depocenter to depocenter, several prin­ In summary, gravi ty is a ubiquitous source of cipal progradational units are regionally energy that affects both the structural and depo­ delimited. They include (Fig. 8) the Lower and sitional architecture of prograding continental Upper Wilcox (Paleocene-early Eocene), Queen margins. Specific structural patterns and the City, Yegua, and Jackson (Eocene), Vicksburg extent of gravity-tectonic modificationof the sedi­ and Frio (Oligocene), lower Miocene, upper mentary prism are determined by variations in Miocene, and Plio-Pleistocene sequences. depositional rate, degree of density inversion, and Frazier (1974) discussed the stratigraphic sig­ inhomogeneities within and between depositional nificanceof such repetitive depositional packages sequences (Galloway 1986). and introduced the extremely useful concept of depositional episodes and resulting genetic depo­ sitional sequences. A depositional sequence was definedas a complex of facies derived fromcom­ Episodic depositional history mon sources around the basin margin and The episodic nature of Cenozoic deposition in the deposited in a period of relative base-level or Northwest Gulf of Mexico Basin has been long tectonic stability. Depositional episodes are typi­ recognized as a prominent aspect of the strati­ cally temtinated by regional transgressive events; graphic architecture (Deussen & Owen 1939; thus their physical stratigraphy provides a record Fisher 1964). A succession of sandy tongues that of coastal progradation bounded by transgressive consist of coastal plain and paralic deposits deposits and/or nondepositional (hiatal) surfaces 244 W. E. Galloway NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

NW RIO GRANDE EMBAYMENT SE

FACIES ASSEMSLAGE c::::JCoostal plain fluvial E:'l Bayl lagoon l:2:lPoroiic' shore·zone/deltaic D Shell and slope c:;:Jlntraslope basin

O 30mi o 30km V.E.•AOx C RUST

20 CONTINENTAL ATTENUATED CONTL.

Fig. 8. Generalized dip-oriented stratigraphic cross section through the Rio Grande depocenter, Northwest Gulf Coast sedimentary wedge. Principal Cenozoic depositional sequences are labeled. Note the expansion of sequences across major shelf-margin growth faults.

(Frazier 1974). The Cenozoic wedge of the North­ The time-space diagram at the top of the figme west Gulf of Mexico contains multiple depo­ illustrates the tempora! and spatia! relationships sitional sequences that refiect the ongoing of principal environmental assemblages. The interplay between coastal outbuilding and retreat. lower cross section illustrates the basic strati­ Such repetitive genetic stratigraphy is typical of graphic architecture of the genetic depositional terrigenous clastic deposits in many basins of all sequence. For simplicity, effects of gravity tec­ ages and plate settings (for example, the Sacra­ tonics are not shown. The episode and genetic mento Basin, Rocky Mountain Cretaceous sequence consist of three families of elements - Seaway, Anadarko Basin, Beaufort Sea, and offlap components, onlap or transgressive com­ North Sea). Thus the conceptual framework ponents, and bounding intervals refiected in the established by Frazier, with modifications to stratigraphic record as disconformities (hiatal sur­ incorporate improved knowledge of deep water faces of Frazier (1974)). depositional processes, remains a powerful Offiap components (Fig. 9) include (l) facies approach for basin analysis. Further, such (commonly sandy) refiecting terrestrial aggra­ sequences are in part the stratigraphic record of dation of the coastal plain, (2) coastal pro­ regional tectonic activity of the basin margin and/ gradational deposits (also sandy) that initially or source terranes. Systematic documentation of overly the transgressed depositional platform of the processes responsible for their formation and the preceding depositional sequence and then of their stratigraphic, paleogeographic, and build out onto contemporaneous facies of the chronologic framework improves our ability to offiapping continentalslope, and (3) mixed aggra­ interpret this indirect record of tectonic his tory. dational lower slope and progradational upper slope facies. It is worth noting that although slope offiap and progradation are commonly used syn­ Elements of depositional episodes and onymously, the in terna! facies architecture of sequences many slope deposits is dominated by aggra­ The depositional architectme of a continental dational stacking of sediment at the toe and on margin episode is shown schematically in Fig. 9. the adjacent basin floor.Thus, offiap slope sys- NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 245

TIMEl

DEPOSITIONAL PLATFORM SHELF EDGE SLOPE

...... "\, DEPTHl '• '•• ••• ISOCHRON l Fig. 9. Idealized facies architecture of a depositional episode and resultant genetic sequence. The upper diagram (episode) has time as the vertical scale. The lower diagram (sequence) shows the resultant stratigraphic architecture with depth as the vertical scale. Similar stipple pattems show the tempora! and stratigraphic relationships of corresponding facies sequences. For further explanation, refer to Fig. 7. tems more commonly contain a mix of pro­ pelagic sediment, and submarine unconformities. gradational and aggradational facies sequences. Unconformities may have compound origins Onlap components consist of (1) reworked including nondeposition, localized or regional shore-zone and shelf facies deposited during and erosion by submarine currents or mass wasting, soon after shoreline retreat, and (2) an apron or transgressive ravinement. Seismically, such of gravitationally resedimented upper slope and sequence boundaries are recognized by discor­ shelf-margin deposits at the toe of the slope. dant reflectorpatterns such as downlap or toplap The transgressive period, following upon active (Vail et al. 1984). Significantly, however, the outbuilding of the continental margin, is an opti­ depositional episode model also predicts the pres­ mum time for slump initiation and headward ero­ ence of an internal hiatus along and within the sion of submarine canyons (Coleman et al. 1983; landward margin (Fig. 9). As the shoreline, which Galloway 1985). Again for simplicity, Fig. 9 illus­ is the base leve! for deposition, shifts basinward, trates a genetic depositional sequence in which the inner coastal plain increasingly becomes a little sediment is added during the transgressive graded fluvial surface that primarily acts as a part of the cycle. In reality, sediment commonly sediment bypass zone. The inherent peripheral continues to be supplied by major fluvialsystems uplift that occurs as deposition loads the crust will during onlap, resulting in thick deposits and readily lead to nondeposition, valley incision, or recording an extended period of gradual relative even low-angle truncation of older parts of the base-leve! rise. The term retrogradation is useful depositional sequence. Such surfaces and their to describe such long-term periods of shoreline seaward projections have been proposed as and shelf-edge retreat. sequence boundaries (Vail et al. 1984). However, Finally, the genetic depositional sequence is their regional extent and importance in separating bounded on its basinward periphery by two strati­ genetically distinct depositional systems are graphic 'surfaces' (Fig. 9) that record the relative dependent upon the assumption that stratigraphic sediment starvation of the outer shelf and slope architecture of continental margins is uniquely during and following transgression. These hiatal dominated by eustatic falls of sea leve! to or intervals have many potential stratigraphic mani­ below the shelf edge, an assumption worthy of festations, including beds of autochthonous considerable skepticism (Miall 1986). Subsidence­ chemical sediments (i.e. marine limestone), induced uplift and resultant peripheral uncon­ paleontologic condensed intervals, thin drapes of formities will be temporally and spatially related 246 W. E. Galloway NORSK GEOLOGISK TIDSSKRIFT 67 (1987) to the major depositional episodes and their ti on of the coastal facies sequences) has oscillated depocenters. widely, reflectingprogradation followed by retro­ gradation (Fig. 10). The continental margin (defin ed most specifically by the shelf edge) has Genetic depositional episodes of the alternated between periods of active outbuilding northwest Gulf basin and relative stability or local retrogradation. The major depositional episodes of the Cenozoic Finally, it is of interest to compare the relative wedge of the northwest Gulf basin are char­ time intervals of progradation and retrogradation acterized by active, ongoing supply of abundant . within Gulf Coast episodes. For the paleon­ terrigenous sediment. As would be expected, tologically well-dated Frio ( Oligocene) and upper their stratigraphic architecture, though reflecting and lower Miocene depositional sequences retro­ the simple sequence model outlined above in gradation occupied from about 50% to as little as broad aspect, presents a more complex pattem of 15% of the total duration of the episode. Time sequence development (Fig. 10). The regional span of the three episodes varies, but each encom­ depositional sequences are composites of mul­ passes several million years. The concept of geo­ tiple, local, and most likely, autocyclic facies logically instantaneous eustatic transgressions is sequences that record local variations in sediment certainly not borneout by the depositional record supply. Each episode is defin ed by progradation of slow coastal retreat in this sediment-rich basin. and retrogradation of the shoreline by several tens of kilometers, although maximum transgression usually does not extend as far upslope as previous Variables controlling depositional transgressions. In contrast, the shelf edge is built outward during offlap, but commonly remains a episodicity more permanent marker of successive steps in Given the prominence of episodic deposition in basin filling.It is merely submerged more deeply many continental margin sedimentary wedges, and prone to modest regrading during trans­ the question of cause becomes particularly inter­ gression and subsequent flooding of the basin esting. Certainly the interpretation that eustatic margin. Thus, the sedimentary record in the Gulf sea level controls sedimentary cycles (Vail et al documents two aspects of episopdicity. The over­ 1984) currently enjoys broad popularity. How­ all facies tract (most readily defined by the posi- ever, many authors have pointed out the fact that

----- CONTINENTAL MARGIN------�======sHORELINE ======::::;.

---- BASINWARD------

Fig. 10. Schematic representation of the complex history of outbuilding of a typical Gulf Coast depositional episode. Smaller cycles of progradation and retreat punctuate the long-term episode of regional offlap followed by shoreline retrogradation. Patterns used are the same as in Fig. 9. NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 247 depositional patterns of prograding basin margins considered in any critical examination of basin must in reality reflect the dynamic interplay of margin stratigraphic evolution (Miall 1986). three principal factors (Curray 1964; Hardenbol The principal Cenozoic depositional episodes et al. 1981). that resulted in continental margin outbuilding of the north west Gulf of Mexico are summarized in l. Eustatic changes in sea level directly influence Fig. 12. Comparison with a proposed eustatic sea­ the location of the shoreline and thus the locus leve! curve and regional tectonic events of the of sediment input into the basin. The eustatic North American plate reveals the importance of component includes changes in the geoidal this interplay of multiple factors. Although sub­ surface, ocean basin volume, and water vol­ sidence is primarily induced by sedimentary load­ urne (Morner 1980). ing, large-scale variation of sediment supply is 2. Sediment supply is determined by tectonics required to explain the distinct pulses of shelf­ of the source terranes as well as by regional edge progradation. By the late Neogene, increas­ climate (Schumm 1977). ing ice vol urnesalso significantlylowered eustatic 3. Basin subsidence rate is primarily the product sea level, and changes in ice volume provided a of the interplay among thermal history, load­ mechanism for rapid eustatic fluctuations. induced isostatic adjustment, and local and Simple comparison of the episode history and regional plate-margin and intraplate stress proposed eustatic curves shows that correlation is regimes. Both absolute rate and changes in indeed good during the Pliocene and Pleistocene. rate determine stratigraphic evolution of con­ By Miocene time, the cycles still show good cor­ tinental margins (Pitman 1978). relation, but the magnitudes of progradational (low stand) and bounding transgressive (high As shown in Fig. 11, a fullrange of depositional stand) events and proposed sea-level fluctuation architectures reflecting continental margin pro­ are disparate. Correlation of the Oligocene Vicks­ gradation, aggradation, retrogradation, and burg and Frio episodes with the single major mid­ transgression can result from variations of sedi­ Oligocene sea-level drop poses a problem, and ment influx, subsidence rate, or eustatic sea level. is further complicated by uncertainties in the In gross aspect, the stratigraphic product looks paleontologic dating of these two units. Within the same regardless of which variable is changed. the Paleogene, correspondence between the Genetic depositional sequences are simply com­ eustatic and episode curves is fair to poor at binations of progradational components followed best. Again, age determinations using standard by retrogradational or transgressive components. planktonic zonation poses some uncertainties, but Aggradational intervals rnay also be incorporated published dates may be interpreted to mean that at various levels in the sequence. The conclusion offlap events are not perfectly synchronous is that all three factors - sediment supply, sub­ around the basin margin. sidence rate, and eustatic sea level - must be Comparison of episode history with the onset or duration of major tectonic events of the central and western North American plate reveals some compelling associations. The terminal Laramide Depositional Architecture deformation phase spread progressively from the

- southwestern United States (late Paleocene-early Eocene) into northern Mexico (Middle Eocene) (Chapin & Cather 1981; Dickinson 1981; Eaton Transgressive 1986). Deformation and uplift in the southern Rocky Mountains corresponds directly to out­ �etrogradational building of the Lower and Upper Wilcox con­ tinental margins (Fig. 12). As the locus of uplift moved south into Mexico, supply of sediment to the northwest Gulf basin decreased, and the continental margin was generally flooded.By late Eocene time, tectonic quiescence dominated and

Fig. Il. Schematic stratigraphic architectures and the three the southern Rocky Mountains were beveled, variables that control their formation. forming a regional erosional surface (Epis & 248 W. E. Galloway NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

LITHO- DEPOSITIONAL CH RONO- TECTONIC OCEANOGRAPHY/ EUSTATIC STRATIGRAPHY EPISODES and STRATIGRAPHY EVENTS CLIMATE CU RVE OUTCROP BASIN DEPOCENTERS Epeirogenic uplift of t Pleistocene climotic cycles Rocky Mins. ond * Greot Ploins

5 lt Stable Antorctic w ice sheet lO z w Basin ond Range C!> eJttension o w * 15 z

Acceleroted �i�? �� te- a

Active groben MaJor reoroamzat1on for motion -Rio of E. Pocif1c Grande Rift spreod1ng t system l Polar bottom 1 water formotion ond Antorctic montone gtaciotion Regio Regional rhyolitic stress li eld b volconism: reorganization o. Chihuahua ·S. Rockyt mtn. .lt b. West Texas 11 c. New Mexico * o

Flottening of l subducted Forollon plate MaJOr N-S Lorom1de pulse ---RG _ l Decoupling of Colorado Plateau l • Major NE-SW l Laromld& pulse l

* 4lJ1 Marine shale tongue �lnner coastal plain unconformity Abrupt d8 increose

Deep-water wedge or -RG�Comporotive mognitude ond depocenter ....._ Proposed major condensed w submarine canyon of continental margin progrodotion sections

Fig. 12. Comparative tempora! history of Gulf Coast Cenozoic depositional episodes, proposed eustatic sea-leve! changes, oceanographic evolution in response to Cenozoic climatic cooling, and tectonic events of western North America. Major Continental-margin outbuilding episodes and their depocenters are shown by excursions to the right on the episode curve. The principal lithostratigraphic elements (including basin-margin unconformities and submarine erosion features) are tabulated according to most recently published and unpublished planktonic dates. The chart indicates progressive evolution from input­ dominated sequences in the Paleogene to increasingly eustatic-dominated sequences in the Neogene. Principal references for tectonic, oceanographic/climatic, and eustatic events include: Chapin (1979), Chapin & Cather (1981), Davis (1982), Dickinson (1981), Gries (1983), McDowell & Clabaugh (1979), Price & Henry (1984), Leggett (1982), and Haq et al. (1987).

Chapin 1975). Meanwhile, volcanism began in and Oligocene time (McDowell & Clabaugh the southwestem U.S. and northem Mexico. 1979). A contemporaneous surge of sediment, Concomitantly, the modest Yegua and Jackson notable for its content of volcanic rock fragments, sequences were deposited. A tremendous episode entered the Gulf basin. Beginning in late Oligo­ of explosive rhyolitic volcanism spread from West cene, the initial subsidence along the Rio Grande Texas into northem Mexico during late Eocene Rift occurred (Chapin 1979), culminating in large- NORSK GEOLOGISK TIDSSKRIFr 67 (1987) TSGS Symposium 1986 249 scale graben forma ti on 27-20 mya. Rapid sub­ by more than 1000 m occurred in· the Pliocene sidence of this north-trending feature (Fig. 13) (Chapin 1979; Dickinson 1981). At the same time created an immense sediment trap, beheading major pulses of continental margin outbuilding the southwestern U. S. drainage system that had occurred, primarily in the northern Gulf of played a prominent role in Oligocene continental Mexico (Fig. 12). margin outbuilding. By the end of early Miocene Also shown in Fig. 12 are the tempora! and time, a major reorganization of the intraplate stratigraphic positions of several subregional stress regime initiated regional extension across inner-coastal plain unconformities. Note that western North America. This Basin and Range each is closely associated with a major con­ episode of normal faulting extended as far east as tinental-margin depositional episode. They are the inn er margin of the northwest Gulf of Mexico tentative! y interpreted to be a result of peripheral coastal plain, where the Balcones fault was reac­ upliftcaused by active crustal loading and crustal tivated. Regional epeirogenic upliftof the Rocky subsidence. Mountains and adjacent western midcontinent This cursory examination of comparative tec-

ORAINAGE BASINS --- MIOCENE- HOLOCENE -- OLIGOCENE

-· - L. PALEOCENE- E. EOCENE

, .. ' �.:,_... _.,

O 400mi O 400km

Fig. 13. Limits of the three principal age-defined source terranes of the North American craton and their depocenters in the Gulf of Mexico basin. Source areas and drainage axes were controlled by intraplate tectonic evolution. Modified from Winker (1982). 250 W. E. Galloway NORSK GEOLOGISK TIDSSKRIFT 67 (1987) tonic and depositional histories suggests several Acknowledgements. - Development of the concepts presented generalizations: (l) Major depositional episodes here was supported in part by the National Science Foundation responsible for continental margin progradation under Grant EAR - 8416138. Initial drafts were reviewed by W. R. Muehlberger and A. D. Miall. These contributions are correspond to principal tectonic events within the gratefully acknowledged. North American plate. These events are in turn related to the evolution of the active plate margin along the western rim of the continent. (2) Details References of sequence development may also reflect a eustatic sea-leve! overprint, particularly during Ball y, A. W. 1983: Seismic expression of structural styles. the Neogene when polar ice caps began to affect American Association of Petroleum GeologistsStudies in Geo­ logy 15-2. 327 pp. ocean circulation and volume (3) Low-angle, Bott, M. H. P. 1980: Mechanisms of subsidence at passive basin-fringing unconformities are a repetitive continental margins. In Bally, A. W., Bender, P. L., McGet­ consequence of episodic, load-induced subsid­ chin, T. R. & Walcott, R. l. (eds.): Dynamicsof Plate ence and peripheral uplift.(4) Finally, as pointed Interiors. American Geophysical Union, Washington, D.C., Geodynamics Series Volume l, 27-35. out by Winker (1982), tectonic history readily Buffter, R. T. & Sawyer, D. S. 1985: Distribution of crust and explains the shiftingposition of the major deltaic earl y history, Gulf of Mexico Basin. Gulf Coast Association depocenters along the Gulf margin noted earlier of Geological Societies Transactions 35, 333-344. (Fig. 13). In the early Paleogene, sediment was Chapin, C. E. 1979: Evolution of the Rio Grande rift - a derived primarily from the southern Rocky summary. InRiecker, R. E. (ed.): Rio Grande Rift: Tectonics and Magmatism. l-5. American Geophysical Union, Wash­ Mountain terrane and directed into the closest ington, D.C. depocenter - the Houston embayment. Vol­ Chapin, C. E. & Cather, S. M. 1981: Eocene tectonics and canism and regional uplift of Trans-Pecos Texas sedimentation in the Colorado Plateau - Rocky Mountains and the Sierra Madre Occidental in northern Mex­ area.InDickinson, W. R. &Payne, W. D. (eds.): Relationsof Tectonicsto Ore Deposits in the Southem Cordillera. Arizona ico sent a tremendous surge of sediment into the Geological Society Digest 14, 199-213. adjacent Rio Grande embayment. This drainage Cloetingh, S. , McQueen, H. &Lambeck, K. 1985: On a tectonic axis was partly beheaded in early Miocene time mechanism for regional sealevel variations. Earth & Planetary by faulting and subsidence of the Rio Grande Science Letters 75, 157-166. Rift.Late Neogene epeirogenic upliftof the west­ Coleman, J. M. Prior, D. B. & Lindsay, J. F. 1983: Deltaic influenceson shelfedge instability processes. Society of Econ­ ern interior of the continent diverted drainage far omic Paleontologists and Mineralogists Special Publication to the east into the midcontinent. Here it was 33, 121-137. collected by the paleo-Mississippi River and trans­ Crans, W. , Mandl, G. & Haremboure, J. 1980: On the theory ported southward into the Mississippi embayment of growth faulting: a geomechanical delta model based on gravity sliding. Journal of Petroleum Geology 2, 265-307. on the north-central rim of the Gulf. This pattern Curray, J. R. 1964: Transgressions and regressions. In Miller, continues today. R. L. (ed.): Papers in Marine-Geology, Shepard Com­ memorative Volume, 175-203. Macmillan, New York. Dailly, G. C. 1976: A possible mechanism re lating progradation, growth faulting, clay diapirism and overthrusting in a regress­ Conclusions ive sequence of sediments. Bulletin of Canadian Petroleum Geology 24, 92-116. The stratigraphic architecture of divergent con­ Davis, G. A. 1980: Problems of intraplate extensional tectonics, tinental margins records the influences of gravity western United States In National Research Council. Studies tectonics, flexural subsidence and related per­ in Geophysics- Continental Tectonics, 84-95. ipheral uplift, and evolving stress regimes within Deussen, A. & Owen, K. D. 1939: Correlation of surface and subsurface formations in two typical sections of the Gulf the bounding continental plate. Thus the term Coast of Texas. American Association of Petroleum Geol­ 'passive' margin is perhaps something of a mis­ ogists Bulletin 23, 1603-1634. nomer. Observed strain rates and magnitudes of Dickinson, W. R. 1981: Plate tectonic evolution of the southern resultant structures rival those of active plate Cordillera. Arizona Geological Society Digest 14, 113-135. Dolton, G. L., Carlson, K. H., Charpentier, R. R. , Coury, A. margin settings. By examining the Gulf of Mexico B. , Crovelli, R. A., Frezon, S. E. , Khan, K. S. , Lister, J. Cenozoic basin fill, I hope to have made the H. , McMullin, R. H., Pike, R. S., Powers, R. B. , Scott, point that tectonic influences, though commonly E. W. & Varnes, K. L. 1981: Estimates of Undiscovered manifested in subtle ways, must be rigorously Recoverable Conventional Resources of Oil and Gas in the United States. 87 pp. United States Geological Survey Circular considered for a full understanding of the stra­ 860. tigraphy and depositional history of any pro­ Epis, R. 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face in the southero Rocky Mountains. Geological Society of Martin. R. G. 1978: Northero and eastern Gulf of Mexico America Memoir 144, 45-74. continental margin: stratigraphic and structural framework. Fisher, W. L. 1964: Sedimentary patteros in Eocene cyclic American Association ofPetroleum GeologistsStudies in Geo­ deposits, northero Gulf Coast region.Kansas Geological Sur­ logy 7, 21-42. vey Bulletin 169, 151-170. McDowell, F. W. & Clabaugh, S. E. 1979: Ignimbrites of the Frazier, D.E. 1974: Depositional Episotks: Their Relationship Sierra Madre Occidental and their relation to the tectonic to the Quaternary Stratigraphic Framework in the North­ history of western Mexico. Geological Society of America western Portion of the Gulf Basin, 28 pp. The University of Special Paper 180, 113-124. Texas at Austin, Bureau of Economic Geology Geological Miall, A. D. 1986: Eustatic sea level changes interpreted from Circular 74-1. seismic stratigraphy: a critique of the methodology with par­ Galloway, W. E. 1981: Depositional architecture of Cenozoic ticular reference to the North Sea Jurassic record. American Gulf Coastal Plain fluvial systems. Society of Economic Association of Petroleum Geologists Bulletin 70, 131-137. Paleontologistsand MineralogistsSpecial Publication 31, 127- Moroer, N. A. 1980: Eustasy and geoid changes as a function 155. of core/mantle changes. In Moroer, N. A. (ed.): Earth Rhe­ Galloway, W. E. 1985: Submarine canyon system in the Frio ology, lsostasy and Eustasy, 535-553. Wiley & Sons, New Formation of South Texas. Gulf Coast Association of Geo­ York. logical Societies Transactions 35, 75-84. Perrodon, A. 1983: Dynamicsof Oil and Gas Accumu/ations, Galloway, W. E. 1986: Growth faults and fault-related struc­ 368 pp. Elf Aquitaine, Pau. tures of prograding terrigenous clastic continental margins. Pitman, W. C. 1978: Relationship between eustasy and strati­ Gulf Coast Association of Geological Societies Transactions graphic sequences on passive margins. Geological Society of 36, 121-128. America Bulletin 89, 1389-1403. Galloway, W. E., Jirik, L. A., Morton, R. A. & DuBar, J. R. Price, J. G. & Henry, C. D. 1984: Stress orientations during 1986: Lower Miocene (Fleming) Depositional Episode of the Oligocene volcanism in Trans-Pecos Texas: timing the tran­ Texas Coastal Plain and Continental Shelf' Structural Frame­ sition from Laramide compression to basin and range tension. work, Facies, and Hydrocarbon Resources. 50 pp. The Uni­ Geology 12, 238-241. versity of Texas at Austin, Bureau of Economic Geology Quinlan, G.M. & Beaumont, C. 1984: Appalachian thrusting, Report of Investigations 150. lithospheric flexure, and the Paleozoic stratigraphy of the Gries, R. 1983: North-south compression of Rocky Mountain Eastero Interior of America. Canadian Journal of Earth foreland structures. AmericanAssociation of Petroleum Geol­ Science 21, 973-996. ogists Bulletin 66, 574. Ramberg, H. 1981: Gravity Deformation and the Earth's Crust Haq, B. U., Hardenbol, J. & Vail, P. R. 1987: Chronology of in Theory, Experiments and Geological Application, Second fluctuating sea levels since the Triassic. Science 235, 1156- Edition. 452 pp. Academic Press, London. 1166. Schumm, S. A. 1977: The Fluvial System, 338 pp. Wiley & Hardenbol, J., Vail, P. R. & Ferrer, J. 1981: Interpreting Sons, New York. paleoenvironments, subsidence history and sea lcvel changes Vail, P. R. & Hardenbol, J. 1979: Sea-level changes during the of passive margins from seismic and biostratigraphy. 1n Blan­ Tertiary. Oceanus 22, 71-79. chert, R. & Montadert, I. (eds.): Geology of Continental Vail, P. R., Hardenbol, J. & Todd, R. G. 1984: Jurassic uncon­ Margins. International Geological Congress, 26th Proceed­ formities, chronostratigraphy, and sea-level changes from ings. Oceanologica Acta, 3>-44. seismic stratigraphy and biostratigraphy. American Associ­ Jackson, M. P. A. & Galloway, W. E. 1984: Structural and ation of Petroleum Geologists Memoir 36, 129-144. Depositional Styles of Gulf Coast Tertiary Continental Watts, A. B. 1982: Tectonic subsidence, flexure and global Margins: Applications to Hydrocarbon Exploration. 225 pp. changes of sea level. Nature 297, 469-474. American Association of Petroleum Geologists Continuing Winker, C. D. 1982: Cenozoic shelf margins, northwestero Education Course Note Series 25. Gulf of Mexico basin. Gulf Coast Association of Geological Leggett, J. K. 1985: Deep-sea pelagic sediments and palaeo­ Societies Transactions 32, 427-448. oceanography: a review of recent progress. In Brenchley, Winker, C. D. & Edwards, M. B. 1983: Unstable progradational P. J. & Williams, B. P. J. (eds.): Sedimentology: Recent clastic shelf margins.Society of EconomicPaleontologists and Developments and Applied Aspects, 95-121. Blackwell Scien­ Mineralogists Special Publication 33, 139-157. tific Publications, London.