Download Depositional and structural architecture of prograding clastic

Depositional and structural architecture of prograding
clastic continental margins: tectonic inftuence on
patterns of basin filling
WILLIAM E. GALLOWAY
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,
Texas 78713 U.S.A.
Department of Geo/ogical Sciences, The University of Texas at Austin, Austin,
Divergent continental margins are generally con­
sidered to be settings of tectonic quiescence, little
affected by a prominent interplay between struc­
tural deformation and sedimentation. lndeed, the
stratigraphic patterns developed in 'passive' mar­
gin basins have been interpreted primarily in
terms of systematic, thermally driven subsidence
overprinted by eustatic sea-leve! changes (Har­
denbol et al. 1981; Watts 1982) The objective of
this paper is to examine the more subtle interplay
between depositional patterns and tectonic pro­
cesses 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 Mex­
ico 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 con­
tinental interior of western North America pro­
vided 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 Cre­
taceous 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 struc­
tural architecture as well as the evolution of con­
tinental margin sequences.
Four generalizations can be distilled from the
northwest Gulf Coast data base. These gen­
eralizations appear to have broad applicability
to sirnilar prograded divergent, clastic-dominated
margins such as those of the Niger and Mackenzie
basins (Perrodon 1983).
Continental margin progradation is concen­
trated 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 clastic­
margin wedges. Crustal loading and resultant
flexural deformation create areas of maximum
subsidence and peripheral uplift, which in turn
determine the geometry of depositional
l.
238
W. E.
Galloway
NORSK GEOLOGISK TIDSSKRIFf 67 (1987)
sequences. Within the sedimentary wedge,
gravity tectonics produces a variety of exten­
In the following sections, each of these gen­
eralizations will be discussed and illustrated.
sional, compressional, and diapiric structures.
3. Depositional outbuilding of the continental
margin is typically episodic.
Major depo­
sitional episodes were punctuated by periods
of regional basin-margin fiooding; resultant
genetic depositional sequences refiect this
alternation
of
progradation
and
Depositional elements of
prograding margins
Progradation of a continental-margin prism
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 (controlied largely by regional tectonics),
zone/shelf ). Three bathymetric and depositional
eustatic sea-level changes,
and subsidence
regimes - the continental slope, shelf edge, and
4. The depositional episodes, and concomitant
point as margin progradation proceeds beyond
rates.
shelf platform - successively pass a reference
geographic shifts of deltaic depocenters, are
that point. The shelf edge, as defined by 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
terranes) and directly (margin subsidence/
particular time or stratigraphic level ( for further
continental
margin
posmon
at
any
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
l
/ Shell edge
p Principal
positions
drainage axis
_... Facus of offlop
EJ] Sand- rich
continental-margin sequence
o
o
IOOmi
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).
TSGS Symposium
NORSK GEOLOGISK TIDSSKRIFr 67 (1987)
paleo-shelf edges do not connote a specific water
1986
239
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 defines the position of abrupt steep­
margin. As shown on the map, maximum out­
ening
and associated
building within any one stratigraphic interval is
change in dominant depositional process suite.
associated with a sand-rich depocenter. Regional
of
bathymetric
profile
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
\
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
NORSK GEOLOGISK TIDSSKRIFT 67 (1987)
Brazos/Colorado, and Mississippi) into the Gulf
of Mexico (Galloway 1981).
Paleogeography of the lower Miocene depo­
sitional 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 head­
lands 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 shelf­
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 sedi­
mentary 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 & Beau­
mont 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 depo­
center migrates basinward with ongoing offlap.
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
continental margin. lnterdeltaic shelf-edge seg­
ments prograded more slowly by Iongshore trans­
port 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, gravity
deformation within the sedimentary wedge pro­
duces a predictable but commonly complex family
of extensional and compressional structures.
Crustal loading and isostatic subsidence
Replacement of several kilometers of water with
a discrete loading event (Quinlan & Beaumont
1984). Furthermore, the rates and magnitudes of
both basin subsidence and peripheral uplift will
DEPRESSION OF CRUST
GR411TY SLIOtNG
�
EXTENSIONAL
THINNING
�
COMPRESSIONAL
TRANSLATION THtCKENING
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 insta­
bility within the sediment prism results in extension (normal
faults and enhanced subsidence) at the shelf edge and com­
pression ( folds and reverse faults and uplift) at the slope toe.
Modified from Winker (1982).
TSGS Symposium
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 sub­
sidence 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 con­
tinental margin is an ideal habitat for gravity­
driven deformation, and the Gulf of Mexico
Cenozoic section exhibits a spectacular assem­
blage 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 suf­
ficient to overcome the fundamental strength of
1986
241
the sedimentary section and to account for energy
dissipated during deformation. Because gravi­
tational 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 move up the slope of the underlying
boundary layer. Lateral movement of the Green­
land 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
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GRAVITY TECTONICS-CONTINENTAL
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MARGINS
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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.
Fig.
242
W. E.
Galloway
NORSK GEOLOGISK TIDSSKRIFT 67 (1987)
E
w
5. Gravity·glide sheet in the Upper Cretaceous section of the Southwest Africa continental margin. From Galloway (1986);
modified from Bally (1983).
Fig.
of sediment volume commonly dominates. Sec­
ondly, 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 sedi­
ment on a foundation of undercompacted, over­
pressured 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 con­
tinental margin (Bally 1983). Upper Cretaceous
sediments have been displaced down the con­
tinental 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 pattems produced domi­
nantly by gravity spreading ( Fig. 6). A typical
regi onal transect shows that the toe of the con­
tinental slope is characterized by numerous imbri­
cate thrust faults and appears to have been
extruded from beneath the prograding deltaic
COMPRESSION
Fig.
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 thick­
ening of depositional units against the down­
thrown side of the fault planes.
Both gravity gliding and spreading produce a
comparable suite of compressional and exten­
sional 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 char­
acterizes the upwardly convex shelf-to-slope tran­
sition. 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 pat­
tern 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 con­
siderable impact on sediment deposition and
storage. Extension enhances regional, load­
induced 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 pro­
grading shelf-edge deltaic headland, rapid shelf-
EXTENSION
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)
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TSGS Symposium
1986
243
EXPLA NAT ION
Terrestriol Aogrodotion
Coostol Progrodotion
Slope Progradotion
Marine Aggradotion
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).
Fig.
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, gravity is a ubiquitous source of
energy that affects both the structural and depo­
sitional architecture of prograding continental
margins. Specific structural patterns and the
extent of gravity-tectonic modification of the sedi­
mentary 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 strati­
graphic 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 lithostra­
tigraphy of the upper 3 to 5 km of the sedimen­
tary sequence. Although the prominence and
nomenclature of the individual sandy tongues
vary from depocenter to depocenter, several prin­
cipal 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 sig­
nificance of such repetitive depositional packages
and introduced the extremely useful concept of
depositional episodes and resulting genetic depo­
sitional sequences. A depositional sequence was
defined as a complex of facies derived from com­
mon sources around the basin margin and
deposited in a period of relative base-level or
tectonic stability. Depositional episodes are typi­
cally 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)
RIO GRANDE EMBAYMENT
NW
SE
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
o
30mi
30km
V.E.•AOx
20
CONTINENTA L
CRUST
ATTENUATED CONTL.
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.
Fig.
(Frazier 1974). The Cenozoic wedge of the North­
west Gulf of Mexico contains multiple depo­
sitional 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 Sacra­
mento 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 history.
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 strati­
graphic architecture of the genetic depositional
sequence. For simplicity, effects of gravity tec­
tonics are not shown. The episode and genetic
sequence consist of three families of elements offlap components, onlap or transgressive com­
ponents, and bounding intervals refiected in the
stratigraphic record as disconformities (hiatal sur­
faces of Frazier (1974)).
Offiap components (Fig. 9) include (l) facies
(commonly sandy) refiecting terrestrial aggra­
dation of the coastal plain, (2) coastal pro­
gradational 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 aggra­
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-
TSGS Symposium
NORSK GEOLOGISK TIDSSKRIFT 67 (1987)
1986
245
l
TIME
DEPOSITIONAL
PLATFORM
SHELF EDGE
l
DEPTH
l
SLOPE
.............
"\,
'•
'•• •••
ISOCHRON
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.
Fig.
tems more commonly contain a mix of pro­
gradational 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 opti­
mum time for slump initiation and headward ero­
sion of submarine canyons (Coleman et al. 1983;
Galloway 1985). Again for simplicity, Fig. 9 illus­
trates 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 strati­
graphic '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 mani­
festations, 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 discor­
dant reflector patterns such as downlap or toplap
(Vail et al. 1984). Significantly, however, the
depositional episode model also predicts the pres­
ence 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). Subsidence­
induced uplift and resultant peripheral uncon­
formities will be temporally and spatially related
246
W. E.
Galloway
NORSK GEOLOGISK TIDSSKRIFT 67 (1987)
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 char­
acterized 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 mul­
tiple, local, and most likely, autocyclic facies
sequences that record local variations in sediment
supply. Each episode is defined 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 trans­
gression and subsequent flooding of the basin
margin. Thus, the sedimentary record in the Gulf
documents two aspects of episopdicity. The over­
all facies tract (most readily defined by the posi-
tion of the coastal facies sequences) has oscillated
widely, reflecting progradation followed by retro­
gradation (Fig. 10). The continental margin
(defined 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 paleon­
tologically well-dated Frio (Oligocene) and upper
and lower Miocene depositional sequences retro­
gradation 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 encom­
passes several million years. The concept of geo­
logically 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 inter­
esting. Certainly the interpretation that eustatic
sea level controls sedimentary cycles (Vail et al
1984) currently enjoys broad popularity. How­
ever, many authors have pointed out the fact that
----- CONTINENTAL
MARGIN ------
�======sHORELINE ========::::;.
----
BASINWARD-------
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.
Fig.
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 vol­
urne (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, load­
induced isostatic adjustment, and local and
regional plate-margin and intraplate stress
regimes. Both absolute rate and changes in
rate determine stratigraphic evolution of con­
tinental margins (Pitman 1978).
As shown in Fig. 11, a full range of depositional
architectures reflecting continental margin pro­
gradation, aggradation, retrogradation, and
transgression can result from variations of sedi­
ment 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 com­
binations of progradational components followed
by retrogradational or transgressive components.
Aggradational intervals rnay also be incorporated
at various levels in the sequence. The conclusion
is that all three factors - sediment supply, sub­
sidence rate, and eustatic sea level - must be
Depositional Architecture
-
Transgressive
�etrogradational
Il. Schematic stratigraphic architectures and the three
variables that control their formation.
Fig.
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 northwest Gulf of Mexico are summarized in
Fig. 12. Comparison with a proposed eustatic sea­
leve! curve and regional tectonic events of the
North American plate reveals the importance of
this interplay of multiple factors. Although sub­
sidence is primarily induced by sedimentary load­
ing, large-scale variation of sediment supply is
required to explain the distinct pulses of shelf­
edge progradation. By the late Neogene, increas­
ing ice volurnes 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 cor­
relation, but the magnitudes of progradational
(low stand) and bounding transgressive (high
stand) events and proposed sea-level fluctuation
are disparate. Correlation of the Oligocene Vicks­
burg and Frio episodes with the single major mid­
Oligocene 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 out­
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
the southern Rocky Mountains were beveled,
forming a regional erosional surface (Epis &
248
W. E.
Galloway
NORSK GEOLOGISK TIDSSKRIFT 67 (1987)
LITHOCH RONOSTRATIGRAPHY
STRATIGRAPHY
OUTCROP
BASIN
DEPOSITIONAL
EPISODES and
DEPOCEN TERS
TECTONIC
EVENTS
l
5
Epeirogenic uplift of
Rocky Mins. ond
OCEANOGRAPHY/
CLIMATE
t Pleistocene
*
Greot Ploins
cycles
EUSTATIC
CU RVE
climotic
t
lO
15
Stable Antorctic
ice sheet
w
z
w
C!>
o
w
z
Basin ond Range
eJttension
*
Acceleroted
i
a
����� �? �� te-
t
l1
MaJor reoroamzat1on
of E. Pocif1c
spreod1ng
system
t .l11
Regio
stress li eld
b
reorganization
·S. Rocky mtn. t
Active groben
for motion -Rio
Grande Rift
Polar bottom
water formotion
ond Antorctic
montone gtaciotion
Regional rhyolitic
volconism:
o. Chihuahua
b. West Texas
c. New Mexico
*
o
Flottening of
subducted
Forollon plate
---RG
_
l
l
l
MaJOr N-S
Lorom1de pulse
Decoupling of
Colorado
Plateau
•
l
Major NE-SW
Laromld& pulse
l
4lJ1
w
Marine shale tongue
�lnner coastal plain unconformity
Deep-water wedge or
-RG�Comporotive mognitude ond depocenter
of continental margin progrodotion
submarine canyon
*
Abrupt
....._
d8 increose
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 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
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 content of volcanic rock fragments,
entered the Gulf basin. Beginning in late Oligo­
cene, the initial subsidence along the Rio Grande
Rift occurred (Chapin 1979), culminating in large-
TSGS Symposium
NORSK GEOLOGISK TIDSSKRIFr 67 (1987)
scale graben formation 27-20 mya. Rapid sub­
sidence 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 inner margin of the northwest Gulf of Mexico
coastal plain, where the Balcones fault was reac­
tivated. Regional epeirogenic uplift of the Rocky
Mountains and adjacent western midcontinent
ORAINAGE
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 con­
tinental-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-
BASINS
--- MIOCENE- HOLOCENE
- - OLIGOCENE
- ·
- L. PALEOCENE- E. EOCENE
, ..' �.:,_..._.,
O
O
400mi
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
under Grant EAR - 8416138. Initial drafts were reviewed by
responsible for continental margin progradation 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 Bally, A. W. 1983: Seismic expression of structural styles.
American Association of Petroleum Geologists Studies in Geo­
the Neogene when polar ice caps began to affect
logy 15-2. 327 pp.
ocean circulation and volume (3) Low-angle, Bott, M. H. P. 1980: Mechanisms of subsidence at passive
continental margins. InBally, A. W. , Bender, P. L. , McGet­
basin-fringing unconformities are a repetitive
chin, T. R. & Walcott, R. l. (eds.): Dynamics of Plate
consequence of episodic, load-induced subsid­
Interiors. American Geophysical Union, Washington, D. C. ,
ence and peripheral uplift. (4) Finally, as pointed
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 shifting position of the major deltaic
early history, Gulf of Mexico Basin. Gulf Coast Association
of Geological Societies Transactions 35, 333-344.
depocenters along the Gulf margin noted earlier
(Fig. 13). In the early Paleogene, sediment was Chapin, C. E. 1979: Evolution of the Rio Grande rift - a
summary. InRiecker, R. E. (ed.): Rio Grande Rift: Tectonics
derived primarily from the southern Rocky
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
sedimentation in the Colorado Plateau - Rocky Mountains
canism and regional uplift of Trans-Pecos Texas
area.InDickinson, W. R. & Payne, W. D. (eds.): Relations of
and the Sierra Madre Occidental in northern Mex­
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
mechanism for regional sealevel variations. Earth & Planetary
axis was partly beheaded in early Miocene time
Science Letters 75, 157-166.
by faulting and subsidence of the Rio Grande
Coleman, J. M. Prior, D. B. & Lindsay, J. F. 1983: Deltaic
Rift. Late Neogene epeirogenic uplift of the west­
influences on 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
of growth faulting: a geomechanical delta model based on
ported southward into the Mississippi embayment
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
Conclusions
The stratigraphic architecture of divergent con­
tinental margins records the influences of gravity
tectonics, flexural subsidence and related per­
ipheral uplift, and evolving stress regimes within
the bounding continental plate. Thus the term
'passive' margin is perhaps something of a mis­
nomer. 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 stra­
tigraphy and depositional history of any pro­
gradational continental margin.
memorative Volume, 175-203. Macmillan, New York.
Dailly, G. C. 1976: A possible mechanism relating progradation,
growth faulting, clay diapirism and overthrusting in a regress­
ive sequence of sediments. Bulletin of Canadian Petroleum
Geology 24, 92-116.
Davis, G. A. 1980: Problems of intraplate extensional tectonics,
western United States In National Research Council. Studies
in Geophysics- Continental Tectonics, 84-95.
Deussen, A. & Owen, K. D. 1939: Correlation of surface and
subsurface formations in two typical sections of the Gulf
Coast of Texas. American Association of Petroleum Geol­
ogists Bulletin 23, 1603-1634.
Dickinson, W. R. 1981: Plate tectonic evolution of the southern
Cordillera. Arizona Geological Society Digest 14, 113-135.
Dolton, G. L. , Carlson, K. H., Charpentier, R. R. , Coury, A.
B. , Crovelli, R. A. , Frezon, S. E. , Khan, K. S. , Lister, J.
H. , McMullin, R. H. , Pike, R. S. , Powers, R. B. , Scott,
E. W. & Varnes, K. L. 1981: Estimates of Undiscovered
Recoverable Conventional Resources of Oil and Gas in the
United States. 87 pp. United States Geological Survey Circular
860.
Epis, R. C. & Chapin, C. E. 1975: Geomorphic and tectonic
implications of the post-Laramide, late Eocene erosion sur-
NORSK GEOLOGISK TIDSSKRIFT
TSGS Symposium
67 (1987)
face in the southero Rocky Mountains. Geological Society of
144, 45-74.
Fisher, W. L. 1964: Sedimentary patteros in Eocene cyclic
deposits, northero Gulf Coast region. Kansas Geological Sur­
vey Bulletin 169, 151-170.
Frazier, D.E. 1974: Depositional Episotks: Their Relationship
America Memoir
to the Quaternary Stratigraphic Framework in the North­
western Portion of the Gulf Basin, 28
pp. The University of
Texas at Austin, Bureau of Economic Geology Geological
Circular 74-1.
Galloway, W. E. 1981: Depositional architecture of Cenozoic
Gulf Coastal Plain fluvial systems. Society of Economic
Paleontologists and Mineralogists Special Publication 31, 127155.
Galloway, W. E. 1985: Submarine canyon system in the Frio
Formation of South Texas. Gulf Coast Association of Geo­
logical Societies Transactions 35, 75-84.
Galloway, W. E. 1986: Growth faults and fault-related struc­
tures of prograding terrigenous clastic continental margins.
Gulf Coast Association of Geological Societies Transactions
36, 121-128.
Galloway, W. E., Jirik, L. A., Morton, R. A. & DuBar, J. R.
1986: Lower Miocene (Fleming) Depositional Episode of the
Texas Coastal Plain and Continental Shelf' Structural Frame­
work, Facies, and Hydrocarbon Resources. 50
pp. The Uni­
versity of Texas at Austin, Bureau of Economic Geology
Report of Investigations 150.
Gries, R. 1983: North-south compression of Rocky Mountain
foreland structures. American Association of Petroleum Geol­
ogists Bulletin 66, 574.
Haq, B. U., Hardenbol, J. & Vail, P. R. 1987: Chronology of
fluctuating sea levels since the Triassic. Science 235, 11561166.
Hardenbol, J., Vail, P. R. & Ferrer, J. 1981: Interpreting
paleoenvironments, subsidence history and sea lcvel changes
of passive margins from seismic and biostratigraphy. 1n Blan­
chert, R. & Montadert, I. (eds.): Geology of Continental
Margins. International Geological Congress, 26th Proceed­
ings. Oceanologica Acta, 3>-44.
Jackson, M. P. A. & Galloway, W. E. 1984: Structural and
Depositional Styles
of Gulf Coast
Tertiary
Continental
Margins: Applications to Hydrocarbon Exploration. 225
pp.
American Association of Petroleum Geologists Continuing
Education Course Note Series 25.
Leggett, J. K. 1985: Deep-sea pelagic sediments and palaeo­
oceanography: a review of recent progress. In Brenchley,
P. J. & Williams, B. P. J. (eds.): Sedimentology: Recent
Developments and Applied Aspects, 95-121. Blackwell Scien­
tific Publications, London.
1986
251
Martin. R. G. 1978: Northero and eastern Gulf of Mexico
continental margin: stratigraphic and structural framework.
American Association ofPetroleum Geologists Studies in Geo­
logy 7, 21-42.
McDowell, F. W. & Clabaugh, S. E. 1979: Ignimbrites of the
Sierra Madre Occidental and their relation to the tectonic
history of western Mexico. Geological Society of America
Special Paper 180, 113-124.
Miall, A. D. 1986: Eustatic sea level changes interpreted from
seismic stratigraphy: a critique of the methodology with par­
ticular reference to the North Sea Jurassic record. American
Association of Petroleum Geologists Bulletin 70, 131-137.
Moroer, N. A. 1980: Eustasy and geoid changes as a function
of core/mantle changes. In Moroer, N. A. (ed.): Earth Rhe­
ology, lsostasy and Eustasy, 535-553. Wiley & Sons, New
York.
Perrodon, A. 1983: Dynamicsof Oil and Gas Accumu/ations,
368 pp. Elf Aquitaine, Pau.
Pitman, W. C. 1978: Relationship between eustasy and strati­
graphic sequences on passive margins. Geological Society of
America Bulletin 89, 1389-1403.
Price, J. G. & Henry, C. D. 1984: Stress orientations during
Oligocene volcanism in Trans-Pecos Texas: timing the tran­
sition from Laramide compression to basin and range tension.
Geology 12, 238-241.
Quinlan, G. M. & Beaumont, C. 1984: Appalachian thrusting,
lithospheric flexure, and the Paleozoic stratigraphy of the
Eastero Interior of America. Canadian Journal of Earth
Science 21, 973-996.
Ramberg, H. 1981: Gravity Deformation and the Earth's Crust
in Theory, Experiments and Geological Application, Second
Edition. 452
pp. Academic Press, London.
Schumm, S. A. 1977: The Fluvial System, 338 pp. Wiley &
Sons, New York.
Vail, P. R. & Hardenbol, J. 1979: Sea-level changes during the
Tertiary. Oceanus 22, 71-79.
Vail, P. R. , Hardenbol, J. & Todd, R. G. 1984: Jurassic uncon­
formities, chronostratigraphy, and sea-level changes from
seismic stratigraphy and biostratigraphy. American Associ­
ation of Petroleum Geologists Memoir 36, 129-144.
Watts, A. B. 1982: Tectonic subsidence, flexure and global
changes of sea level. Nature 297, 469-474.
Winker, C. D. 1982: Cenozoic shelf margins, northwestero
Gulf of Mexico basin. Gulf Coast Association of Geological
Societies Transactions 32, 427-448.
Winker, C. D. & Edwards, M. B. 1983: Unstable progradational
clastic shelf margins. Society of Economic Paleontologists and
Mineralogists Special Publication 33, 139-157.