Abstract Middle Miocene and middle Pliocene deep-water sandstone reservoirs of the western Gulf Coast Basin are associated with failed shelf margins and subregional unconformities referred to as submarine pediments. The submarine pediments formed broad, convex-landward arcs along nondeltaic slopes and on the southwestern flanks of subjacent delta systems. They were created by retrogressive failure and were later enlarged by erosion during periods of lowered sea level. At times of lowered sea level, the deep embayments that carved into the continental platform funneled nearshore sands downslope to basin-floor fans. The pediments were first backfilled by deep-water mudstones deposited by mass-transport processes. These slump blocks and high-energy turbidites exhibit mounded to chaotic seismic reflections that dip landward. Later, sand-rich channel-levee complexes were deposited above the basal mudstones and near the seismic-facies transition from chaotic reflections to overlying horizontal or wavy reflections. The pre-entrenchment morphology of the shelf margin was finally restored by coalescence of small, prograding deltas that are recorded as clinoform reflections. Unconfined lower-slope and basin-floor fans associated with the submarine pediments are generally sand poor. The sand-rich lowstand-fan deposits are restricted to highly elongate, dip-oriented leveed channels that mark the principal pathways of sediment transport. Sandstones confined to leveed channels of the upper-fan and pediment fill are the most prolific hydrocarbon reservoirs within each stratigraphic sequence. These channel sandstones exhibit high vertical continuity but low lateral continuity because interbedded turbidite mudstones increase away from the channel axes. Thin sandstones of the lower fan may exhibit high lateral continuity, but they typically have poor-reservoir properties because of high concentrations of original muddy matrix.

Abstract The Gulf of Mexico basin is best known for its vast and widespread oil and gas resources. They have been described in the preceding chapter of this volume. The basin, however, also contains important deposits of phosphate, lignite, and sulfur and small deposits of uranium. In addition, salt from several salt domes is produced by underground and solution mining and is used principally as a chemical feedstock for the manufacture of many industrial products. Large volumes of geopressured-geothermal water are also known from the Tertiary sediments of the Gulf of Mexico basin, particularly around its northern margin. It often contains natural gas in solution. This overpressured, gas-bearing hot water may someday be an important source of thermal and kinetic energy; it is now just a gleam in the eye of imaginative energy tacticians. The phosphate deposits of Florida and southeastern Georgia, the Florida Phosphogenic Province, represent about 75 percent of the total domestic phosphate production, and ranged between 34 and 28 percent of the total world production between 1983 and 1987. Important lignite deposits, for the most part of Eocene age, are known from the Gulf of Mexico basin. Two-thirds of the lignite is found in Texas, but it occurs also in parts of northeastern Mexico, Louisiana, Arkansas, Tennessee, Mississippi, and Alabama. Modest resources of Upper Cretaceous bituminous coal are found in northeastern Mexico. Once an important industry, the production of sulfur from the caprocks of some of the many salt domes in the U.S. Gulf Coastal Plain and shallow

Abstract What additional insights to shelf sedimentation would significantly improve our ability to make new oil and gas discoveries, particularly in frontier basins, or in relatively unexplored stratigraphic intervals of otherwise mature basins? Lithofacies models are important for characterizing reservoirs and are most useful on the scale of a prospect or single sandbody. Depositional models are more appropriate for larger scale work, because knowledge of environmental setting places a sandbody in its regional context. Neither of these model types, however, adequately address the explorationist’s main concern which is to predict, in areas of limited or no well control, whether any sandstone will be present in the wellbore at the target interval. The economic significance of this prediction before drilling highlights the importance of models that provide more than just recognition criteria, modern analogs of depositional environments, or lithofacies classifications. Seismostratigraphic exploration for hydrocarbons in shelf deposits has been hampered by the lack of a basin-scale, time- and rock-stratigraphic framework which incorporates these shelf sequences. In a chronostratigraphic sequence, where are shelf-sand reservoirs most often preserved with respect to shoreline and shelf break? Where do they most often occur within third and fourth order sea-level cycles, and how are these occurrences expressed seismically? What are the tectonic controls on shelf sand location? Facies distributions on modern shelves have thus far provided insufficient data for answering these questions. This is because facies patterns on modern shelves reflect deposition under a wide range of sea levels, a fact which has often hindered consensus among sedimentologists about the depositional settings of modern shelf sandbodies. The best exploration approach to this difficulty is to first recognize the control that sea level exerts on positions and geometries not only of sandbodies originating on the shelf, but of other units likely to occupy similar stratigraphic positions, such as reworked delta-front sands or beheaded barrier island sequences. This range in diversity forms the primary physical constraint for understanding the geometries and acoustic properties of shelf sandbodies, and thus for constructing effective seismic reflection models, because in the absence of well control many other sandbodies could be mistaken for shelf deposits. Constraints on the temporal distribution of shelf sandbodies are much less clear, as very little published work relates shelf sandbody type to relative chronostratigraphic position. However, seismic sequence analyses from several foreland basin settings, as well as published outcrop studies, suggest that shelf sands of reservoir quality are best developed at chronostratigraphic sequence boundaries, with different types forming below and above regional transgressive surfaces in response to progradational and retrogradational episodes, respectively. Greatest storm sand amalgamation on the middle and outer shelf may occur between highstands and subsequent sequence boundaries, even in cases where evidence suggests that subsequent relative lowstands are of low magnitude, because lower rates of relative sea level rise favor a progradational rather than an aggradational mode. Although reflection seismic data provide the best time-stratigraphic control necessary to establish these relationships, sequence stratigraphic techniques allow the effective testing of these concepts with only outcrop and/or wireline log data.

ABSTRACT Shelf facies are defined by stratification and grain size characteristics. Most shelves are storm-dominated, and their strata are commonly storm beds (event strata). Strata that form on the less extensive tide-dominated shelf sectors are technically periodites rather than event strata. However, they are structurally similar, and can be viewed as being formed by very regular events. The variables controlling shelf storm bed formation are the time sequence of events and the time-averaged value of fluid power expended by waves and bottom currents, grain size of the available sediment, sedimentation rate, and the rate of biological mixing of the sediment. Within the last decade, our understanding of shelf sediment dynamics has reached a point where it is possible to present a simple numerical model for building storm strata sequences. The model uses extreme event statistics to bridge the gap between the dynamical time scale of strata formation and the stratigraphic time scale of strata accumulation. Our application of the model produces shelf storm beds with scales similar to observed deposits of the same type. Because such agreement is unlikely to be simply fortuitous, it supports, but does not prove, the appropriateness of the underlying marine sediment dynamic theories and the overall analytical methods. Numerical experiments conducted with the model show that storm beds may form across the continental shelf, with most of the activity in water depth of 50 m and less. Within this region, shelf facies comprise a continuum whose main trend extends from a fragmentary record of coarse, thick-bedded, extreme event deposits on the upper shoreface to a nearly continuous record of fine, thin-bedded deposits on the central or outer shelf. This trend may be a response to decrease in the available fluid power (directly, or as a consequence increasing depth), to increasing grain size or sedimentation rate, or to some combination of these factors. Deposits from relatively common storms ( e.g. , the annual extreme event) are preserved when there is a moderate to high net accumulation rate. The water depth at the time of deposition has a major influence on the thickness and statistical pattern of storm beds. This effect shows promise of providing a technique for mapping water depths for ancient shelf deposits.

Abstract Sediments delivered to the seas surrounding Antarctica are not subjected to the fluvial and coastal processes which would otherwise sort them before reaching the continental shelf. For this reason, the Antarctic continental shelf is an ideal environment in which to study the role of marine currents in sediment transport and deposition. The results of theoretical studies and laboratory flume experiments on sediment transport indicate that moderately strong marine currents are capable of transporting fine-to-very fine sand in suspension. These currents are therefore capable of transporting these sands great distances on the shelf. Modern sand-rich facies, derived from the unsorted glacial debris of the Antarctic continental shelf, provide striking evidence for this. Four areas of the Antarctic continental shelf were examined in detail, the Ross Sea shelf, George V shelf, Pennell Coast shelf, and the continental shelf of the northwestern corner of the Weddell Sea. All of these areas have extensive sand-rich facies on their outer shelves. These facies are hundreds of kilometers long and tens of kilometers wide. The thickness of these deposits is limited by the availability of sand to the outer shelf. Physical oceanographic data from the Ross Sea and George V continental shelves indicate that sand-rich facies of these areas occur where the modified derivatives of Circumpolar Deep Water extend onto the shelf. Near-bottom current meter records from the Ross Sea shelf show frequent and sustained currents with speeds capable of suspending very-fine to fine sand. Wind-driven currents appear to be active sediment transporting agents to a depth of nearly 300 m, but most of the Antarctic continental shelf is deeper than this. The sea floor of the inner shelf is mainly blanketed by muds, which implies weak thermohaline circulation at depths below 300 m.

ABSTRACT The inner continental shelf off southwest Florida is a shallow, low-energy environment that displays many of the characteristics of ancient epicontinental and pericontinental seas. The gradient is about 1:3,000, mean annual wave height is less than 0.5 m, and tides are in the lower microtidal range. This inner shelf area contains two distinct provinces. The northern one near Cape Romano is characterized by tide-dominated quartz sand ridges, whereas the southern province contains a transition from siliciclastic to carbonate sand. The area of WNW-trending linear sand ridges extends for about 30 km south of Cape Romano. These ridges range from a few to 10 km in length and have a relief of up to 4 m with an asymmetric profile. Sediments on the ridges are well-sorted, fine quartz sand and are covered with various types of bedforms including ripples, megaripples, and sand waves. Swales are less well-sorted with a relatively high shell content. Tidal currents in this area typically peak at 50-60 cm/sec during spring tidal cycles and about half that during neap tides. They show bidirectional patterns with little time-velocity asymmetry. South of the linear ridge complex and north of Florida Bay is a broad, gently sloping shelf that is subjected to very low wave and tidal energies. The 10 m isobath is approximately 35 km from the coast. The transition zone from siliciclastic sediment in the north to carbonate sediment in the south is several kilometers wide and trends generally WNW.

Abstract Using available wind, tide, and wave data, it has been possible to reconstruct the kinematics of sand transport during Hurricane Carla, which made landfall at Pass Cavallo, Texas in September, 1961. The analysis indicates that the peak bed shear stress occurred just prior to hurricane landfall. The quasi-steady shelf flow was driven by the wind-induced sea-level slope and consequent hydrostatic pressure gradient forces. The downwelling bottom current probably was oriented obliquely offshore and along-shelf to the southwest. Integration of hindcast wave data with the proposed bottom flow patterns allows development of a descriptive model for the combined-flow transport pathway for sand and silt. This model suggests a regional transport pattern characterized by offshore transport from shoreface sources near the point of hurricane landfall and weak onshore flow on the southern part of the Texas shelf. This kinematic reconstruction is compatible with the observed strike-trending morphology of a thin (< 10 cm), regional sand bed deposited during the storm. The shelf sand transport model developed here differs considerably from an earlier explanation for the Carla sand bed which invokes turbidity currents generated during the ebb phase of the storm surge following hurricane landfall. The combined-flow reconstruction indicates that it is unnecessary to call upon post-storm processes to explain the occurrence or areal distribution of the sand bed deposited during this storm.

Abstract A computer generated three-dimensional plot of 220 sq km of the Virginia inner continental shelf revealed three distinct bathymetric zones, not detectable on navigation charts. The survey area is characterized by broad low-relief plains, “ridge and swale” regions, and “dome and swale” regions. Low-relief plains are featureless areas of the sea floor that gently dip seaward at 0.03 to 0.06 degrees. Ridge and swale regions have linear trends in bathymetry with up to 6 m of local relief and 2 to 3 degree slopes. The dome and swale regions are differentiated from the ridge and swale areas by the lack of well-defined parallel ridges. Bathymetrically high areas stand out as individual mounds or domes. Detailed studies were made at a ridge and swale area located 30 km east of the Chesapeake Bay entrance. Three ridges in a 3.4 sq km survey area are spaced approximately 0.75 km apart and have a maximum relief of 5.5 m. Each of the ridges is transected by shallow submarine depressions that are subnormal to the trends of the ridges. As a result, the ridge crests are undulating surfaces formed by linear trending domes and the shallow depressions. Spatial distribution of five lithosomes in the study area corresponds with bathymetrically distinct portions of the ridge and swale system. The five areas are described as dome crests (along the ridge axis), cross-ridge depressions and upper-ridge margins, lower-ridge margins, swale floors, and scour troughs. The characteristics of these lithosomes suggest that the surface deposits are not relict, but are due to active marine processes. Grain size increases between the swale floor and the ridge crest. The only exceptions to this trend are coarse deposits at scour troughs on the swale floors. Surface sediments on domes along the ridge crests contain primary sedimentary structures in a coarse sand lag. Cross-ridge depressions contain primary sedimentary structures in fine sand, indicative of both bedform migration and storm-related sand drapes. The presence of bioturbated fine sand along the adjacent ridge-face slope, between the cross-ridge depressions, suggests that these areas are relatively stable.

ABSTRACT Thick, aggradational sequences of shelf and distal shoreface sandstones are prolific hydrocarbon reservoirs in the deep, downdip part of the Frio Formation fronting the Greta/Carancahua shorezone system. Near Corpus Christi, Texas, geopressured shelf reservoirs have produced more than 190 bcf of gas just in the Corpus Channel and Encinal Channel fields; within these fields, two thinly bedded shelf-sandstone units (K2 and K8 reservoirs) have produced 26 and 38 bcf of gas respectively. Cross sections and maps show that shelf sandstones extend basinward from the distal shoreface toes of barrier-island and beach-ridge sandstone bodies. Shelf sequences are typically upward-coarsening, although upward-fining and heterogeneous sequences also occur. Conventional cores reveal that shelf sequences consist of bioturbated muddy sandstone and sandy mudstone thinly interbedded with planar laminated, sparsely burrowed, and occasionally low-angle cross-laminated or ripple-laminated fine to very fine sandstone. Associated burrow-homogenized siltstone to very fine sandstone sequences are from 1.5 to 6 m (5 to 20 ft) thick. Scattered thin zones contain locally derived mudstone clasts, macerated plant fragments, or shell debris. Individual shelf sandstone bodies commonly exceed 30 m (100 ft) in thickness, particularly where expanded on the downthrown side of major growth faults. In plan view, shelf sandstones are irregular sheets covering areas of several hundreds of square kilometers. Sandstone percent maps reveal subparallel, discontinuous, strike-oriented buildups lying seaward of the contemporary shoreface sandstone unit. These shore-parallel belts are typically interconnected and attached to the shoreface sand body by one or more dip-oriented channel-like axes. The geometry and internal features of the shelf sandstones document their deposition and longshore reworking by storm processes. Focused bottom return flow swept sand from the shoreface onto the inner shelf where it was distributed alongshore by wind-forced geostrophic currents. Between the infrequent, high-energy storm events, a diverse shelf infauna reworked the sediments. Although a turbidity current origin has commonly been suggested for similar shelf sequences in the Frio and other stratigraphic units, the geometry, trend, setting, and internal sedimentary structures are incompatible with this depositional process. Reservoir characteristics of Frio shelf sandstones are controlled by original pore properties inherited from the depositional environment and by subsequent diagenetic processes. The reservoirs are composed of moderate to well sorted, fine to very fine quartzose lithic arkoses that contain abundant glauconite pellets. Volcanic rock fragments dominate the lithic component, although some mudstone fragments are present in all samples. Cements are mainly quartz and feldspar overgrowths with minor amounts of sparry calcite, kaolinite, and clay coats; porosity also is reduced by deformed rock fragments. Optimum reservoir porosities range from 16 to 30 percent and average about 23 percent. Permeabilities for the same intervals range from 0 to 1200 md; however, such extremes are rare and values of tens to hundreds of millidarcies are most common. Reservoir quality improves at depth by dissolution of feldspar grains. This secondary porosity accounts for nearly half of the total porosity. Subregional megascopic heterogeneities occur as alternating zones of higher and lower permeability oriented parallel to depositional strike and spaced a few kilometers apart. Within a field, lateral and vertical variations in pore properties primarily depend on facies sequences and degree of burrowing. High and intermediate permeabilities coincide with ripple cross laminations and plane parallel laminations respectively, whereas low permeabilities coincide with bioturbated strata.

Abstract A detailed palynologic, sedimentologic, and mineralogic investigation of the Robulus L No. 2 and the Robulus L No. 5 sands in cores from Texaco well No. 6 of Vermilion Block 31, offshore Louisiana, confirms that the units were deposited in the middle to outer part of the continental shelf during the early Miocene Epoch. The co-occurrence of the dinocysts, Hystrichosphaeropsis obscura and Lejeunecysta hyalina , further restricts the cored intervals to the Burdigalian Age. Variations in the abundances of Polysphaeridium zoharyi , Lingulodinium machareophorum, Tuberculodinium vancampoae , and terrestrial organic matter indicate the sediments are mixtures of marine and estuarine debris accumulating in the neritic zone.Five lithofacies and seven palynofacies are recognized. Current-structured, medium-grained sandstone containing abundant terrestrial organic matter and shell fragments (lithofacies B) forms the best potential reservoirs. Original high clay content and bioturbation restrict the permeability of other lithofacies and, hence, their producing potential. The Robulus L No. 2 is a better producer because lithofacies B is more abundant. Sandstones of this lithofacies often exhibit porosities between 15-28 percent and permeabilities as high as 2000 md because of their coarser grain size and the dissolution of shell fragments. In the Texaco No. 6 well, there is a clear relationship between original characteristics of the sediments, diagenesis, and hydrocarbon production.

Abstract Moderate volumes of dry gas are produced in the South Padre Island Area of offshore south Texas from a 120-m-thick aggregation of mixed shoreface and shelf sandstones that are interbedded with thin neritic shales. These shallow marine sediments aggraded on a broad stable shelf platform during the middle Miocene depositional episode when block faulting and basin filling within the Rio Grande Rift disrupted regional drainage patterns and caused an abrupt decrease in sediment supply to the Rio Grande Embayment. Because sediment influx to south Texas was significantly reduced, middle Miocene sandstones were deposited less than 10 km landward of the previous shelf edge, which was constructed during the early Miocene (pre-Amphistegina B) regressive episode. Thick, widespread marine shales, containing the Cibicides opima and Textularia stapperi faunal assemblages, occur beneath and above the sequence of nearshore sediments. The Textularia stapperi transgressive shale serves as the primary seal for hydrocarbon accumulation that is preferentially trapped on the upthrown side of minor down-to-the-west faults. These reactivated early Miocene faults did not significantly influence middle Miocene sediment distribution or interval thickness except near the contemporaneous slope. Individual sandstone reservoirs of the delta fringe and mid-shelf facies are 3 to 12 m thick and exhibit thin interbedded to upward-coarsening log responses. They form laterally-persistent but irregular sand sheets that pass landward into either thick coastal-plain sandstones or embayment mudstones, they merge updrift (northeastward) into fluvial-deltaic sandstones, and they grade seaward into slope mudstones. Progressive basinward and southwestward decreases in net sandstone thickness and sandstone percent indicate predominant sediment transport by shelf currents in the same directions. Repetitive cyclic depositional events are also indicated by the nearly uniform vertical spacing of sandstone beds near their southwestern pinchout. Shallow reservoir depths (~ 1500 m, 5000 ft) and relatively young sediment age suggest that the hydrocarbons trapped in middle Miocene sediments originated in older Tertiary shales and migrated into the shelf sandstone reservoirs when early Miocene faults were reactivated. The presence of humic organic matter in Miocene sediments and high thermal gradients within the Rio Grande Embayment probably are responsible for the predominance of methane which results from cracking of previously generated hydrocarbons.

ABSTRACT A high-resolution single-channel seismic survey of the Mississippi-Alabama continental shelf and upper slope revealed relatively thin upper Pleistocene and Holocene deposits. The stratigraphy was defined by identifying depositional and erosional episodes recognized on seismic profiles by geomorphic patterns, high-angle clinoform progradational deposits, buried stream entrenchments, planar conformities, and erosional unconformities. The regional bathymetry shows a gently sloping shelf with a relatively steeper continental slope. Local tectonics have had minimal effect on the shelf, with surface and subsurface faults exhibiting a random pattern. Diapirs on the shelf break and upper slope, in contrast, have created several topographic highs with associated faults. The upper Pleistocene shelf and slope deposits are examples of two depositional environments, including four differentiated areas: inner shelf, middle shelf, outer shelf, and upper slope. The presence of sandy sediments in the shelf deposits is inferred to be the result of reworking by marine processes. A five-stage model of sedimentation and shelf evolution is presented. The earliest stage (stage 1) is the early Wisconsinan transgression across a seaward-sloping erosional horizon. During the transgression, this paleosurface was overlain by a thin (< 10 m) layer of sediments (stage 2). The third stage was the late Wisconsinan regression, during which fluvial channels eroded the shelf, forming a thick (> 90 m) shelf margin delta. The Holocene transgression deposited a thin layer of sediment over parts of the shelf (stage 4). The last depositional episode occurred during the Holocene highstand and was the progradation of the St. Bernard delta (stage 5).

ABSTRACT Detailed measured sections, subsurface data and regional observations along the southern and western margins of New Mexico’s San Juan Basin provide a precise data base for testing proposed sequence stratigraphic models which describe the architecture of depositional sequences. It is concluded that Coniacian strata in this basin represent lithosomes deposited during relative sea level lowstand and subsequent transgressions. The base of the Torrivio Member of the Gallup Sandstone records the landward, unconformable Coniacian sequence boundary. The uppermost tongues (C-A) of the Gallup Sandstone and the associated upper member of the Lower Mancos Shale represent the lowstand wedge. The base of this wedge is the seaward, mostly conformable, continuation of the basal Torrivio sequence boundary. In distal shelf settings, beyond the influence of the Gallup highstand deposits (highstand systems tract), the sequence boundary merges with the condensed section of the next underlying sequence—the Juana Lopez Member of the Mancos Shale. The Tocito Sandstone represents a set of linear shelf sand ridges which formed during transgressions associated with parasequence boundaries within this regressive lowstand wedge. Ridge-building also continued into the overlying transgressive systems tract. Dominant ridge-building currents were along-shelf tidal flows, partly modified by geostrophic storm flows. As articulated in sequence stratigraphy, analysis of the genetic linkage between contemporaneous depositional systems and the associated relative sea level stage is a powerful tool in stratigraphic and sedimentological analysis. The many conflicting interpretations already published on the Gallup-Tocito stratigraphy partly owe their origin to a lack of appreciation for the direct connection between styles of sedimentation and patterns of relative change in sea level.

Abstract More than 1,750 m of late Cenozoic deposits are exposed in a structurally continuous section in sea cliffs south of the city of San Francisco. Well-defined facies in this succession reflect deposition in the eolian dune, alluvial, freshwater pond/swamp/marsh, embayment, backshore, foreshore, nearshore and inner-, mid- and outer-shelf environments. The facies are arranged throughout the succession in alternating regressive and transgressive sequences. The most complete regressive-transgressive cycles show the following upward progression: outer-shelf, mid-shelf, inner-shelf, nearshore, foreshore, backshore/eolian dune, embayment, inner-shelf, mid-shelf, outer-shelf. In this progression, an erosional surface separates the embayment facies from the overlying shelf facies. Although many of the cycles are incomplete, the alternating upward-shallowing and upward-deepening trends are present throughout. The upward-shallowing sequences typically are continuous, whereas upward-deepening sequences contain a distinct hiatus. Nearshore and foreshore deposits do not occur in transgresssive sequences; instead, an erosional surface marks the interval at which they would be expected. The transgressions and regressions are interpreted to reflect Pleistocene glacio-eustatic sea level oscillations. The facies sequences in this deposit differ from some of the transgressive-regressive sequences described elsewhere. Widespread erosion seems not to coincide with relative lowstands of the sea, but rather occurs with the landward translation of the shoreface during a relative sea-level rise. This situation may be due to the rapid subsidence required to develop such a thick section in a short period of time. It may also be that, because of the high frequency of sea-level fluctuation, insufficient time existed during the lowstands to allow for widespread planation. Vertical textural trends in the regressive and transgressive sequences are more complicated than the simple upward-coarsening and upward-fining trends commonly ascribed to such sequences. These complications probably result from some combination of the effects of the high-gradient and high-energy character of this coast and the rapid rate of subsidence in this tectonically active area.

Great storms originating in both tropical and extratropical regions represent upper limits in the continuum of physical forces that affect sedimentation in shallow marine basins. The energy expended and sediment transported during a few storm hours may equal many years of non-storm work, or they may surpass thresholds unequaled by less energetic processes; consequently, the volume of storm sediment preserved in the basin fill is disproportionate to the frequency of these convulsive events. Storms can produce large geomorphic features and exaggerated bedforms, especially on low-energy coasts and adjacent shelves where they commonly exert their greatest influence. During peak intensity, combined wind-driven geostrophic currents and wave-induced oscillatory currents flow alongshore and slightly offshore at up to 2 m/s in relatively shallow water (<50 m). The coarsest nearshore sediments (gravel and sand) are transported and deposited by these strong currents, which have a principal unidirectional component. As the unidirectional current component diminishes, wave orbital velocities assume greater importance, and the subequal oscillatory and unidirectional motion molds fine sand that settles from suspension; in the final phase of deposition, mud accumulates below fair-weather wave base in slack water. The resulting graded beds constitute a single but common class of shelf storm deposit. Thicknesses of graded storm beds typically range from a few decimeters to a few meters, depending directly on wave and current energy and flow duration, and inversely on distance from the shoreline (water depth). Volumetric estimates suggest that each graded bed is not necessarily a product of sand eroded from the beach and shoreface during a single storm. Instead, they may also result from sand being transported offshore by a series of moderately intense events and later selectively sorted during a single extreme storm lasting several days. Most amalgamated storm beds are preserved in regressive sequences deposited on broad, moderately stable shelves of passive continental margins; however, some accumulated in thick transgressive sequences deposited on narrow, rapidly subsiding and tectonically active shelf margins associated with converging plates. Episodic storm deposition also accounts for coastal and shelf aggradation under static sea-level conditions. Similarities between ancient storm deposits and their modern analogs suggest that extant marine processes adequately explain the observed sedimentologic properties, textural patterns, and vertical sequences of sedimentary structures. Super storms, which are predicted by analysis of extreme values (e.g., wave heights or wind speeds) may be statistical artifacts or, if they exist, their influence may be unrecognized or unimportant in the geological record.

Abstract Modem depositional environments prominently displayed along the northwestern Gulf of Mexico served as the first widely accepted sedimentary facies models that have successfully guided exploration and production of a variety of energy and nonfuel resources (petroleum, lignite, uranium, geothermal energy, construction aggregate, ground water). Because these industrial activities both required and provided a tremendous data base, the depositional environments of the gulf coast are among the best known anywhere. The following sections of the guidebook describe selected geological features that typify late Pleistocene and Holocene sediments of the Texas coastal plain and illustrate the diversity of associated depositional environments. The described sites represent a spectrum of transgressive and regressive sequences coexisting along a microtidal storm-dominated coast undergoing a significant reduction in sediment supply and a slight sea-level rise. Late Quarternary history. The Texas coastal plain is a broad, flat, depositional surface created by several rivers that eroded large volumes of sediment from remote areas of the state and adjacent mountainous regions and deposited the sediment as coalescing deltas in the Gulf of Mexico (Barton, 1930, Doering, 1935; Winker, 1979). It extends eastward and merges with the coastal plain of Louisiana, which has a similar morphology but a slightly different origin. Rather than being formed by several rivers and deltas of moderate size as was the Texas coast, the Louisiana coastal plain was mostly deposited by two large alluvial systems (Mississippi and Red rivers) that drained much of the continental interior. Like present-day rivers, the load and discharge of these late