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

WILLIAM E. GALLOW A 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 ro le 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 con­sidered to be settings of tectonic quiescence, little affected by a prominent interplay between struc­tural deformation and sedimentation. lndeed, the stratigraphic patterns developed in 'passive' mar­gin basins have been interpreted primarily in terms of systematic, thermally driven subsidence overprinted by eustatic sea-leve! changes (Har­denbol et al. 1981; Watts 1982) The objective of this paper is to examine the more subtle interplay between depositional patterns and tectonic pro­cesses that occurs during an extended history of divergent continental margin progradation (offlap). The example used will be the Cenozoic sedimentary wedge of the northwest Gulf of Mex­ico margin. The Gulf basin originated with a brief period of rifting in Jurassic time, followed by an extended period of thermally induced subsidence during the remainder of the Mesozoic (Buffler & Sawyer 1985). Slow rates of sediment influx resulted in deposition being largely restricted to bounding shelf-platforms, and culminated in the creation of a reef-rimmed, sediment-starved oceanic basin (Jackson & Galloway 1984). Regional uplift and tectonism within the con­tinental interior of western North America pro­vided an abrupt surge of terrigenous clastic sediment into the northwest Gulf of Mexico in Paleocene time, and this continuing sediment

influx has prograded the continental margin as much as 350 km seaward of the inherited Cre­taceous shelf edge. The Cenozoic sedimentary wedge, which contains an estimated 40 billion barrels of producible oil (Dolton et al. 1981), provides a natural laboratory for the examination of the three-dimensional stratigraphic and struc­tural architecture as well as the evolution of con­tinental margin sequences.

Four generalizations can be distilled from the northwest Gulf Coast data base. These gen­eralizations appear to have broad applicability to sirnilar prograded divergent, clastic-dominated margins such as those of the Niger and Mackenzie basins (Perrodon 1983).

l. Continental margin progradation is concen­trated at one or more depocenters where large, oceanic delta systems build out to the shelf edge and deposit sediment directly onto the upper continental slope. Interdeltaic margin segments receive sediment by longshore transport.

2. A distinct syndepositional structural style is associated with the progradation of clastic­margin wedges. Crustal loading and resultant flexural deformation create areas of maximum subsidence and peripheral uplift, which in turn determine the geometry of depositional

238 W. E. Galloway

sequences. Within the sedimentary wedge, gravity tectonics produces a variety of exten­sional, compressional, and diapiric structures.

3. Depositional outbuilding of the continental margin is typically episodic. Major depo­sitional episodes were punctuated by periods of regional basin-margin fiooding; resultant genetic depositional sequences refiect this alternation of progradation and retrogra­dation/transgression. The episodes reveal the changing interplay between sediment input rates (con tro lied largely by regional tectonics), eustatic sea-level changes, and subsidence rates.

4. The depositional episodes, and concomitant geographic shifts of deltaic depocenters, are best explained by intraplate tectonics some­times overprinted by eustatic sea-level change. Plate interactions at convergent and transform margins affect thermal and stress regimes far into the interior of plates, indirectly (source terranes) and directly (margin subsidence/ uplift) infiuencing interregional depositional pattems.

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l

NORSK GEOLOGISK TIDSSKRIFf 67 (1987)

In the following sections, each of these gen­eralizations will be discussed and illustrated.

Depositional elements of prograding margins

Progradation of a continental-margin prism requires the accumulation of sediment in depo­sitional systems ranging from the base of the continental slope of paralic ( deltaic or shore­zone/shelf ). Three bathymetric and depositional regimes - the continental slope, shelf edge, and shelf platform - successively pass a reference point as margin progradation proceeds beyond that point. The shelf edge, as defined by the break in depositional slope separating shelf-platform sediments deposited by traction currents from slope sediments deposited by gravitational resedi­mentation, is the most useful guide for deter­mining continental margin posmon at any particular time or stratigraphic leve l ( for further discussion, see Winker 1982 and Jackson & Gal­loway 1984). It is important to emphasize that

/ Shell edge positions

p Principal drainage axis

_... Facus of offlop

l/ MISSISSIPPI EMBAYMENT

11 Mississippi ---V---�

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)

paleo-shelf edges do not connote a specific water depth, as is commonly assumed today. Rather, the shelf edge defines the position of abrupt steep­ening of bathymetric profile and associated change in dominant depositional process suite. In its position at the crest of a progradational sedimentary wedge many kilometers thick, the shelf edge provides a more significant and stable record of maximum basin-margin outbuilding than does the shoreline, which commonly shifts landward and seaward many tens of kilometers in response to minor base-level changes.

The Cenozoic sedimentary wedge of the north­west Gulf of Mexico illustrates the evolution of a prograding margin (Fig. 1). Successively younger paleo-shelf edges lie progressively basinward of the inherited Cretaceous reefal shelf edge. How­ever, although each successive depositional epi-

Extrobasinol streom

TSGS Symposium 1986 239

sode further prograded the shelf edge, the amount of outbuilding varies greatly along the basin margin. As shown on the map, maximum out­building within any one stratigraphic interval is associated with a sand-rich depocenter. Regional analysis of the Cenozoic depositional framework shows that these sand-rich depocenters cor­respond to major deltaic systems. It is further notable that nearly all of the sandy depocenters are localized at one of three preferred positions along the basin margin (Fig. 1). These foci for sediment input are broad, extremely subtle struc­tural sags, called 'embayments' by most Gulf Coast geologists. The three depocenters are, from southwest to northeast, the Rio Grande, Houston, and Mississippi embayments. lnter­estingly, they still define the entry points of the four largest extrabasinal rivers (the Rio Grande,

(a) Intrabosinol ond bosin fringe streoms

l

I nterdeltoic Bight Reworking

Deltoic Heodlond

Geomorphic or structurol focus edge of older depositionol sequence

\

Mud transport� � --:;!'

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

Brazos/Colorado, and Mississippi) into the Gulf of Mexico (Galloway 1981).

Paleogeography of the lower Miocene depo­sitional sequence (Galloway et al. 1986) illustrates the typical relationship between the principal depositional elements and contemporary shelf edge (Fig. 2). Depositional elements include one or more deltaic headlands that were constructed where major extrabasinal fluvial systems were funneled into the basin along topographically or structurally controlled axes, and interdeltaic shoreline reentrants or bights. The deltaic head­lands rapidly prograded across the foundered depositional platform of the earlier Oligocene delta systems to the shelf edge and directly onto the upper continental slope. These shelf-edge delta headlands thus became the sites for the most direct and rapid progradation of the continental margin (Winker & Edwards 1983). Headlands are subject to greatest marine reworking, whereas the concave bight becomes a sediment sink for sand and mud of the minor intrabasinal streams and for sediment reworked longshore. Longshore drift provided sediment for construction of the sandy barrier islands and strandplain. Suspended sediment was redistributed along the shelf edge and slope by longshore currents, providing the material for slower progradation of a muddy, interdeltaic shelf edge (Fig. 2).

In summary, direct feeding of sediment through structurally focused fluvial systems to major shelf­edge delta systems provided material for the most rapid and direct progradation of the Gulf Coast continental margin. lnterdeltaic shelf-edge seg­ments prograded more slowly by Iongshore trans­port and deposition of dominantly suspended sediment.

Structural style Two scales of structural deformation affect to varying degrees the sediments of a prograding continental margin. First, large-scale sedimentary loading of the crust induces regional subsidence and associated peripheral uplift. Secondly, gra vi ty deformation within the sedimentary wedge pro­duces a predictable but commonly complex family of extensional and compressional structures.

Crustal loading and isostatic subsidence

Replacement of several kilometers of water with

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

denser sediment induces isostatic adjustment of the underlying crust (Bott 1980). Subsidence of oceanic crust will result in a sedimentary section about three times thicker than the depth of water actually replaced. Transitional crust found along divergent plate margins will subside less, but the sedimentary section is expanded at least twofold. In the northwest Gulf of Mexico basin a sedi­mentary wedge 8 to 14 km thick resulted from progradational filling of the starved, deep-water basin created by thermal subsidence of underlying transitional to oceanic crust.

Crustal depression occurs by flexural loading, forming a broad subsidence bowl (Fig. 3, middle section) that extends on the order of 150 km around the locus of loading (Bott 1980). Thus, a broad lens of sediment, including not only the deposits of the continental margin depocenter but also extensive shore-zone and coastal-plain facies, is accommodated. Concomitant with subsidence, a halo of uplift - the peripheral bulge - forms around the subsidence bowl (Quinland & Beau­mont 1984; Cloetingh et al 1985). The effect is to produce a hinge line with subsidence and sedi- . ment storage occurring on the basinward side while, at the same time, gentle uplift, sediment bypass, valley incision, and erosion occur on the landward fringe. The peripheral bulge migrates basinward just as the continental margin depo­center migrates basinward with ongoing offlap. Modeling indicates that the peripheral bulge will also migrate toward the focus of loading following a discrete loa ding event ( Quinlan & Beaumont 1984). Furthermore, the rates and magnitudes of both basin subsidence and peripheral uplift will

DEPRESSION OF CRUST

GR411TY SLIOtNG � � COMPRESSIONAL TRANSLATION THtCKENING

EXTENSIONAL THINNING

Fig. 3. Deformation and subsidence domains of a prograding clastic continental margin. A broad subsidence bowl and per· ipheral bulge result from sediment loading. Gravitational insta­bility within the sediment prism results in extension (normal faults and enhanced subsidence) at the shelf edge and com­pression ( folds and reverse faults and uplift) at the slope toe. Modified from Winker (1982).

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

be modified by changes in the horizontal stress field within crustal plates (Cloetingh et al. 1985). In summary, the inherent pattern of flexural sub­sidence of the crust ensures the presence of numerous intraformational disconformities and even low-angle unconformities within the updip margin of the sedimentary prism.

Gravity tectonics The thick sedimentary prism of a prograding con­tinental margin is an ideal habitat for gravity­driven deformation, and the Gulf of Mexico Cenozoic section exhibits a spectacular assem­blage of growth faults, diapirs, gravity-glide folds, and allochthonous salt wedges (see Martin 1978, Jackson & Galloway 1984, and Galloway 1986, for reviews).

Gravity tectonic structures, by definition, must exhibit a geometry that refiects a descrease in gravitational potential when compared to the undeformed state (Ramberg 1981). Further, the decrease in gravitational potential must be suf­ficient to overcome the fundamental strength of

TSGS Symposium 1986 241

the sedimentary section and to account for energy dissipated during deformation. Because gravi­tational force increases in proportion to the cube of the length scale, it is very effective within the thick sedimentary prisms deposited at continental margins.

Three potential styles of gravity tectonics are shown diagramatically in Fig. 4: gravity gliding, gravity spreading, and diapirism. Gravity gliding and gravity spreading both create regional stress fields that focus tension at the shelf edge and compression at the base of the slope. Though similar, these two styles have different boundary conditions. Gravity gliding requires a sloping lower boundary surface down which the glide sheet moves. In contrast, gravity spreading occurs in a mounded sediment pile with a free surface, toward which spreading is directed. The extruded sediments can mo ve up the slope of the underlying boundary layer. Lateral movement of the Green­land ice cap is an excellent example of gravity spreading. Two further points should also be emphasized. First, although vertical displacement is usually more obvious, horizontal displacement

GRAVITY TECTONICS-CONTINENTAL MARGINS

z

�··· ·············��·········· --t ­�j [ �j�]\\l\11\[[j[�nun::::::::�::.::�:�

·: : : : : : : : ... ................ . ............. . ............................ . . . .. ..................... . ............. . ........................... ........... ......................................... ............................................

:::::::::: • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ::-: • • "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); modified from Bally (1983).

of sediment volume commonly dominates. Sec­ondly, density inversion is neither necessary nor sufficient to initiate gravity gliding or spreading. However, density inversions, such as produced by deposition of normally compacted sandy sedi­ment on a foundation of undercompacted, over­pressured continental slope mud, will augment the inherent instability of the sloping, free boundary surface and enhance the potential for diapirism.

An excellent example of a gravity glide sheet is shown in Fig. 5, which is based on a published seismic line traversing the West African con­tinental margin (Bally 1983). Upper Cretaceous sediments have been displaced down the con­tinental slope along a glide zone within the older Cretaceous section. Prominent structural features include a series of syndepositional listric normal faults within the Upper Cretaceous section near the crest of the slope and an imbricate thrust sheet at the toe of the glide sheet near the base of the slope. The overall structural pattern closely resembles that predicted by Crans et al. (1980).

The outer shelf and continental slope of the Niger delta sedimentary prism provide an example of the structural patte ms produced domi­nantly by gravity spreading (Fig. 6). A typical regional transect shows that the toe of the con­tinental slope is characterized by numerous imbri­cate thrust faults and appears to have been extruded from beneath the prograding deltaic

COMPRESSION

sedimentary prism along a seawardly rising thrust plane. Sediment layers thin over or lap out against folds. The shelf edge is again a zone of extensional faults (growth faults) that show landward thick­ening of depositional units against the down­thrown side of the fault planes.

Both gravity gliding and spreading produce a comparable suite of compressional and exten­sional structures. This observed and theoretical distribution of stress/strain regimes characterizes prograding continental margins (Dailly 1976; Winker 1982). The structural pattern reveals the presence of three strain regimes within the shelf edge and continental slope. Extension char­acterizes the upwardly convex shelf-to-slope tran­sition. At the toe of the slope, compression dominates. Beneath the middle continental slope, between the zones of extension and compensatory compression, Iies a domain of lateral translation. Together, these zones overprint the simple pat­tern of crustal subsidence with localized uplift at the toe of the slope and enhanced subsidence across the outer shelf to upper slope (Fig. 3). Thus, the well-defined strain domains have con­siderable impact on sediment deposition and storage. Extension enhances regional, load­induced subsidence at the shelf edge, creating a localized depocenter that retains much of the sediment transported to the shelf margin and upper slope. In the typical facies tract of a pro­grading shelf-edge deltaic headland, rapid shelf-

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)

EXPLA NAT ION

fc;;>.�,:i:l Terrestriol Aogrodotion

IH!> l Coostol Progrodotion

PS•;'� Slope Progradotion

O Marine Aggradotion

TSGS Symposium 1986 243

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 delta-margin progradational sand and prodelta mud facies (Fig. 7). In contrast, compression of the lower slope results in abbreviated, locally truncated, and diapirically intruded submarine fan and lower slope facies. Much of the sediment that does escape the shelf-margin extensional sediment trap also bypasses the lower slope and is deposited on the abyssal plain (Galloway 1986).

In summary, gra vi ty is a ubiquitous source of energy that affects both the structural and depo­sitional architecture of prograding continental margins. Specific structural patterns and the extent of gravity-tectonic modification of the sedi­mentary prism are determined by variations in depositional rate, degree of density inversion, and inhomogeneities within and between depositional sequences (Galloway 1986).

Episodic depositional history The episodic nature of Cenozoic deposition in the Northwest Gulf of Mexico Basin has been long recognized as a prominent aspect of the strati­graphic architecture (Deussen & Owen 1939; Fisher 1964). A succession of sandy tongues that consist of coastal plain and paralic deposits

extends progressively basinward, where they overlie and grade into thick marine mudstones (Fig. 8). The sand-rich tongues are separated by thinner, updup-extending tongues of marine mudstone, effectively punctuating the lithostra­tigraphy of the upper 3 to 5 km of the sedimen­tary sequence. Although the prominence and nomenclature of the individual sandy tongues vary from depocenter to depocenter, several prin­cipal progradational units are regionally delimited. They include (Fig. 8) the Lower and Upper Wilcox (Paleocene-early Eocene), Queen City, Yegua, and Jackson (Eocene), Vicksburg and Frio (Oligocene), lower Miocene, upper Miocene, and Plio-Pleistocene sequences.

Frazier (1974) discussed the stratigraphic sig­nificance of such repetitive depositional packages and introduced the extremely useful concept of depositional episodes and resulting genetic depo­sitional sequences. A depositional sequence was defined as a complex of facies derived from com­mon sources around the basin margin and deposited in a period of relative base-level or tectonic stability. Depositional episodes are typi­cally temtinated by regional transgressive events; thus their physical stratigraphy provides a record of coastal progradation bounded by transgressive deposits and/or nondepositional (hiatal) surfaces

244 W. E. Galloway NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

NW RIO GRANDE EMBAYMENT SE

20

FACIES ASSEMSLAGE c::::J Coostal plain fluvial

E:'l Bay l lagoon

l:2:l Poroiic' shore·zone/deltaic

D Shell and slope

c:;:J lntraslope basin

O 30mi

o 30km V.E.•AOx

CONTINENTA L C RUST

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 North­west Gulf of Mexico contains multiple depo­sitional sequences that refiect the ongoing interplay between coastal outbuilding and retreat. Such repetitive genetic stratigraphy is typical of terrigenous clastic deposits in many basins of all ages and plate settings ( for example, the Sacra­mento Basin, Rocky Mountain Cretaceous Seaway, Anadarko Basin, Beaufort Sea, and North Sea). Thus the conceptual framework established by Frazier, with modifications to incorporate improved knowledge of deep water depositional processes, remains a powerful approach for basin analysis. Further, such sequences are in part the stratigraphic record of regional tectonic activity of the basin margin and/ or source terranes. Systematic documentation of the processes responsible for their formation and of their stratigraphic, paleogeographic, and chronologic framework improves our ability to interpret this indirect record of tectonic his tory.

Elements of depositional episodes and sequences The depositional architectme of a continental margin episode is shown schematically in Fig. 9.

The time-space diagram at the top of the figme illustrates the tempora! and spatia! relationships of principal environmental assemblages. The lower cross section illustrates the basic strati­graphic architecture of the genetic depositional sequence. For simplicity, effects of gravity tec­tonics are not shown. The episode and genetic sequence consist of three families of elements -offlap components, onlap or transgressive com­ponents, and bounding intervals refiected in the stratigraphic record as disconformities (hiatal sur­faces of Frazier (1974)).

Offiap components (Fig. 9) include (l) facies (commonly sandy) refiecting terrestrial aggra­dation of the coastal plain, (2) coastal pro­gradational deposits (also sandy) that initially overly the transgressed depositional platform of the preceding depositional sequence and then build out onto contemporaneous facies of the offiapping continental slope, and (3) mixed aggra­dational lower slope and progradational upper slope facies. It is worth noting that although slope offiap and progradation are commonly used syn­onymously, the in terna! facies architecture of many slope deposits is dominated by aggra­dational stacking of sediment at the toe and on the adjacent basin floor. Thus, offiap slope sys-

NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 245

l TIME

DEPOSITIONAL PLATFORM SHELF EDGE SLOPE

l DEPTH

l

............. "\, '•

'•• ••• ISOCHRON

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 pro­gradational and aggradational facies sequences.

Onlap components consist of (1) reworked shore-zone and shelf facies deposited during and soon after shoreline retreat, and (2) an apron of gravitationally resedimented upper slope and shelf-margin deposits at the toe of the slope. The transgressive period, following upon active outbuilding of the continental margin, is an opti­mum time for slump initiation and headward ero­sion of submarine canyons (Coleman et al. 1983; Galloway 1985). Again for simplicity, Fig. 9 illus­trates a genetic depositional sequence in which little sediment is added during the transgressive part of the cycle. In reality, sediment commonly continues to be supplied by major fluvial systems during onlap, resulting in thick deposits and recording an extended period of gradual relative base-leve! rise. The term retrogradation is useful to describe such long-term periods of shoreline and shelf-edge retreat.

Finally, the genetic depositional sequence is bounded on its basinward periphery by two strati­graphic 'surfaces' (Fig. 9) that record the relative sediment starvation of the outer shelf and slope during and following transgression. These hiatal intervals have many potential stratigraphic mani­festations, including beds of autochthonous chemical sediments (i.e. marine limestone), paleontologic condensed intervals, thin drapes of

pelagic sediment, and submarine unconformities. Unconformities may have compound origins including nondeposition, localized or regional erosion by submarine currents or mass wasting, or transgressive ravinement. Seismically, such sequence boundaries are recognized by discor­dant reflector patterns such as downlap or toplap (Vail et al. 1984). Significantly, however, the depositional episode model also predicts the pres­ence of an internal hiatus along and within the landward margin (Fig. 9). As the shoreline, which is the base leve! for deposition, shifts basinward, the inner coastal plain increasingly becomes a graded fluvial surface that primarily acts as a sediment bypass zone. The inherent peripheral uplift that occurs as deposition loads the crust will readily lead to nondeposition, valley incision, or even low-angle truncation of older parts of the depositional sequence. Such surfaces and their seaward projections have been proposed as sequence boundaries (Vail et al. 1984). However, their regional extent and importance in separating genetically distinct depositional systems are dependent upon the assumption that stratigraphic architecture of continental margins is uniquely dominated by eustatic falls of sea leve! to or below the shelf edge, an assumption worthy of considerable skepticism (Miall 1986). Subsidence­induced uplift and resultant peripheral uncon­formities will be temporally and spatially related

246 W. E. Galloway

to the major depositional episodes and their depocenters.

Genetic depositional episodes of the northwest Gulf basin The major depositional episodes of the Cenozoic wedge of the northwest Gulf basin are char­acterized by active, ongoing supply of abundant . terrigenous sediment. As would be expected, their stratigraphic architecture, though reflecting the simple sequence model outlined above in broad aspect, presents a more complex pattem of sequence development (Fig. 10). The regional depositional sequences are composites of mul­tiple, local, and most likely, autocyclic facies sequences that record local variations in sediment supply. Each episode is de fin ed by progradation and retrogradation of the shoreline by several tens of kilometers, although maximum transgression usually does not extend as far upslope as previous transgressions. In contrast, the shelf edge is built outward during offlap, but commonly remains a more permanent marker of successive steps in basin filling. It is merely submerged more deeply and prone to modest regrading during trans­gression and subsequent flooding of the basin margin. Thus, the sedimentary record in the Gulf documents two aspects of episopdicity. The over­all facies tract (most readily defined by the posi-

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

ti on of the coastal facies sequences) has oscillated widely, reflecting progradation followed by retro­gradation (Fig. 10). The continental margin (de fin ed most specifically by the shelf edge) has alternated between periods of active outbuilding and relative stability or local retrogradation.

Finally, it is of interest to compare the relative time intervals of progradation and retrogradation within Gulf Coast episodes. For the paleon­tologically well-dated Fri o ( Oligocene) and upper and lower Miocene depositional sequences retro­gradation occupied from about 50% to as little as 15% of the total duration of the episode. Time span of the three episodes varies, but each encom­passes several million years. The concept of geo­logically instantaneous eustatic transgressions is certainly not borne out by the depositional record of slow coastal retreat in this sediment-rich basin.

Variables controlling depositional episodicity Given the prominence of episodic deposition in many continental margin sedimentary wedges, the question of cause becomes particularly inter­esting. Certainly the interpretation that eustatic sea level controls sedimentary cycles (Vail et al 1984) currently enjoys broad popularity. How­ever, 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)

depositional patterns of prograding basin margins must in reality reflect the dynamic interplay of three principal factors (Curray 1964; Hardenbol et al. 1981).

l. Eustatic changes in sea level directly influence the location of the shoreline and thus the locus of sediment input into the basin. The eustatic component includes changes in the geoidal surface, ocean basin volume, and water vol­urne (Morner 1980).

2. Sediment supply is determined by tectonics of the source terranes as well as by regional climate ( Schumm 1977).

3. Basin subsidence rate is primarily the product of the interplay among thermal history, load­induced isostatic adjustment, and local and regional plate-margin and intraplate stress regimes. Both absolute rate and changes in rate determine stratigraphic evolution of con­tinental margins (Pitman 1978).

As shown in Fig. 11, a full range of depositional architectures reflecting continental margin pro­gradation, aggradation, retrogradation, and transgression can result from variations of sedi­ment influx, subsidence rate, or eustatic sea level. In gross aspect, the stratigraphic product looks the same regardless of which variable is changed. Genetic depositional sequences are simply com­binations of progradational components followed by retrogradational or transgressive components. Aggradational intervals rna y also be incorporated at various levels in the sequence. The conclusion is that all three factors - sediment supply, sub­sidence rate, and eustatic sea level - must be

Depositional Architecture

-Transgressive

�etrogradational

Fig. Il. Schematic stratigraphic architectures and the three variables that control their formation.

TSGS Symposium 1986 247

considered in any critical examination of basin margin stratigraphic evolution (Miall 1986).

The principal Cenozoic depositional episodes that resulted in continental margin outbuilding of the north west Gulf of Mexico are summarized in Fig. 12. Comparison with a proposed eustatic sea­leve! curve and regional tectonic events of the North American plate reveals the importance of this interplay of multiple factors. Although sub­sidence is primarily induced by sedimentary load­ing, large-scale variation of sediment supply is required to explain the distinct pulses of shelf­edge progradation. By the late Neogene, increas­ing ice vol urnes also significantly lowered eustatic sea level, and changes in ice volume provided a mechanism for rapid eustatic fluctuations.

Simple comparison of the episode history and proposed eustatic curves shows that correlation is indeed good during the Pliocene and Pleistocene. By Miocene time, the cycles still show good cor­relation, but the magnitudes of progradational (low stand) and bounding transgressive (high stand) events and proposed sea-level fluctuation are dis parate. Correlation of the Oligocene Vicks­burg and Fri o episodes with the single major mid­Oligocene sea-level drop poses a problem, and is further complicated by uncertainties in the paleontologic dating of these two units. Within the Paleogene, correspondence between the eustatic and episode curves is fair to poor at best. Again, age determinations using standard planktonic zonation poses some uncertainties, but published dates may be interpreted to mean that offlap events are not perfectly synchronous around the basin margin.

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 deformation phase spread progressively from the southwestern United States (late Paleocene-early Eocene) into northern Mexico (Middle Eocene) (Chapin & Cather 1981; Dickinson 1981; Eaton 1986). Deformation and uplift in the southern Rocky Mountains corresponds directly to out­building of the Lower and Upper Wilcox con­tinental 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 the southern Rocky Mountains were beveled, forming a regional erosional surface (Epis &

248 W. E. Galloway

5

lO

CH RONO-LITHO-

STRATIGRAPHY STRATIGRAPHY

OUTCROP BASIN

15

w z w C!> o w z

4lJ1

w

Marine shale tongue

Deep-water wedge or submarine canyon

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

DEPOSITIONAL EPISODES and DEPOCENTERS

TECTONIC EVENTS

Epeirogenic uplift of

l Rock y Mins. ond Greot Ploins

t

Acceleroted

Basin ond Range eJttension

����� i�?a��te-

MaJor reoroamzat1on l of E. Pocif1c t spreod1ng

system 1 Active groben for motion -Rio Grande Rift

Regiot .l stress li eld b reorganization 11 ·S. Rocky mtn.t

Flottening of subducted

o

l

Regional rhyolitic volconism:

o. Chihuahua b. West Texas c. New Mexico

Forollon plate l MaJOr N-S

---RG _ l Lorom1de pulse

Decoupling of Colorado Plate au

• Major NE-SW l Laromld& pulse

l

�lnner coastal plain unconformity

-RG�Comporotive mognitude ond depocenter of continental margin progrodotion

OCEANOGRAPHY/ CLIMATE

t Pleistocene climotic

* cycles

Stable Antorctic ice sheet

*

Polar bottom water formotion ond Antorctic montone gtaciotion

*

*Abrupt d8 increose

EUSTATIC CU RVE

....._ Proposed major condensed 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 input­dominated 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 the southwestem U.S. and northem Mexico. Concomitantly, the modest Yegua and Jackson sequences were deposited. A tremendous episode of explosive rhyolitic volcanism spread from West Texas into northem Mexico during late Eocene

and Oligocene time (McDowell & Clabaugh 1979). A contemporaneous surge of sediment, notable for its con tent of volcanic rock fragments, entered the Gulf basin. Beginning in late Oligo­cene, the initial subsidence along the Rio Grande Rift occurred (Chapin 1979), culminating in large-

NORSK GEOLOGISK TIDSSKRIFr 67 (1987)

scale graben forma ti on 27-20 mya. Rapid sub­sidence of this north-trending feature (Fig. 13) created an immense sediment trap, beheading the southwestern U. S. drainage system that had played a prominent role in Oligocene continental margin outbuilding. By the end of early Miocene time, a major reorganization of the intraplate stress regime initiated regional extension across western North America. This Basin and Range episode of normal faulting extended as far east as the inn er margin of the northwest Gulf of Mexico coastal plain, where the Balcones fault was reac­tivated. Regional epeirogenic uplift of the Rocky Mountains and adjacent western midcontinent

ORAINAGE BASINS

TSGS Symposium 1986 249

by more than 1000 m occurred in· the Pliocene (Chapin 1979; Dickinson 1981). At the same time major pulses of continental margin outbuilding occurred, primarily in the northern Gulf of Mexico (Fig. 12).

Also shown in Fig. 12 are the tempora! and stratigraphic positions of several subregional inner-coastal plain unconformities. Note that each is closely associated with a major con­tinental-margin depositional episode. They are tentative! y interpreted to be a result of peripheral uplift caused by active crustal loading and crustal subsidence.

This cursory examination of comparative tec-

--- 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

tonic and depositional histories suggests several generalizations: (l) Major depositional episodes responsible for continental margin progradation correspond to principal tectonic events within the 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 of sequence development may also reflect a eustatic sea-leve! overprint, particularly during the Neogene when polar ice caps began to affect ocean circulation and volume (3) Low-angle, basin-fringing unconformities are a repetitive consequence of episodic, load-induced subsid­ence and peripheral uplift. (4) Finally, as pointed out by Winker (1982), tectonic history readily explains the shifting position of the major deltaic depocenters along the Gulf margin noted earlier (Fig. 13). In the early Paleogene, sediment was derived primarily from the southern Rocky Mountain terrane and directed into the closest depocenter - the Houston embayment. Vol­canism and regional uplift of Trans-Pecos Texas and the Sierra Madre Occidental in northern Mex­ico sent a tremendous surge of sediment into the adjacent Rio Grande embayment. This drainage axis was partly beheaded in early Miocene time by faulting and subsidence of the Rio Grande Rift. Late Neogene epeirogenic uplift of the west­ern interior of the continent diverted drainage far to the east into the midcontinent. Here it was collected by the paleo-Mississippi River and trans­ported southward into the Mississippi embayment on the north-central rim of the Gulf. This pattern continues today.

Conclusions The stratigraphic architecture of divergent con­tinental margins records the influences of gravity tectonics, flexural subsidence and related per­ipheral uplift, and evolving stress regimes within the bounding continental plate. Thus the term 'passive' margin is perhaps something of a mis­nomer. Observed strain rates and magnitudes of resultant structures rival those of active plate margin settings. By examining the Gulf of Mexico Cenozoic basin fill, I hope to have made the point that tectonic influences, though commonly manifested in subtle ways, must be rigorously considered for a full understanding of the stra­tigraphy and depositional history of any pro­gradational continental margin.

NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

Acknowledgements. - Development of the concepts presented here was supported in part by the National Science Foundation under Grant EAR - 8416138. Initial drafts were reviewed by W. R. Muehlberger and A. D. Miall. These contributions are

gratefully acknowledged.

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