Sand Shoals Research Articles

New eocene oysters and the final regression at the southern Rim of the central High Atlas (Morocco)

The oysters Ostrea (Ostrea) todraensis n. sp. and Ostrea (Ostrea) sidialiensis n. sp. are described from the uppermost marine beds south of the central High Atlas in two sections west of Tinerhir. The oysters and the codiacean alga Ovulites margaritula ( Lamarck, 1801) Lamarck, 1816 indicate a final regression at the end of Middle Eocene times, i.e. earliest at the end of the Lutetian and latest at the end of the Bartonian. The oysters lived in muddy lagoonal environments sealed off from the sea by oolitic-bioclastic carbonate sand shoals and bordered in landward direction by sebkha-like redbed deposits. Taxonomy and systematic affinities of the oysters are discussed. Their shell structures and depositional history are described in respect to sedimentary infillings. Some selected aspects of biometry explain differences in shell morphometry both between different valves of the same species and equivalent valves from the different populations, illustrating the importance of size and location of the adductor muscle scar.

Review of Carbonate Sand-Belt Deposition of Ooid Grainstones and Application to Mississippian Reservoir, Damme Field, Southwestern Kansas

Ooid shoals are present (1) in the high-energy zones of carbonate ramps, (2) across broad shelves, and (3) along rimmed carbonate shelf edges. Shoals generally form as depositional strike-oriented or dip-oriented sand bodies in these settings. Marine sand belts, locally present along the shelf edge of the Great Bahama Little Bahama Banks, are depositional strike-oriented sand shoals made of flood ramps, shields, and channels that terminate in spillover lobes. Their geometry results from an interplay between storm-generated and tide-generated currents with the sea floor. Mississippian oolitic limestones (St. Louis Limestone B-zone), whose geometries resemble modern Bahamian ooid shoals, produce oil and are current exploration targets in southwestern Kansas (Hugoton embayment). Thus, a sedimentological and stratigraphic study of a producing field (Damme field, Finney County, Kansas) and a comparison to modern ooid sand bodies were justified. The St. Louis Limestone is overlain by the Ste. Genevieve Limestone (nonproductive) in Damme field. Both formations consist of five lithofacies: (1) skeletal wackestones in an open marine shelf, (2) porous, skeletal, ooid grainstone in a marine sand belt, (3) sandy, peloid-ooid, skeletal grainstone in a mobile grain shoal, (4) quartz-arenite in a tidal inlet, and (5) fenestral lime mudstone in an intertidal mud flat. Porous grainstones of lithofacies 2 form an elongate (northwest-southeast), lenticular body 10 mi (16 km) long, 2 mi (3.2 km) wide, and up to 18 ft (5.5 m) thick. Its shape and geometry are nearly the same as the marine sand belts of the Bahamas. Lithofacies arrangements indicate two styles of shallowing-upward depositional cycles; the lower St. Louis cycle was deposited during a relative sea level rise, and the overlying Ste. Genevieve cycles were deposited after a relative drop of sea level, an event signalled by the influx of siliciclastic sediments into the carbonate shelf from the Central Kansas uplift and/or Transcontinental arch.

Carbonate Facies and Reservoir Heterogeneity--The Value of Modern Analogs: ABSTRACT

Secondary and enhanced processing of hydrocarbon fields requires a critical understanding of reservoir heterogeneity by both geologists and engineers. Carbonates have more varied facies and diagenetic patterns than their siliciclastic counterparts, thus offering a greater challenge to reservoir evaluation. This challenge is illustrated by American Petroleum Institute data showing average primary plus secondary recovery efficiencies of carbonate reservoirs of only 32% original oil in place. Studies of modern analogs are valuable because they constrain interpretations and lend predictability to unraveling facies patterns in reservoirs. These patterns help to understand the lateral continuity of End_Page 1440------------------------------ stratification, variation within layers, heterogeneity, and performance of reservoir examples. An appreciation of facies variability and depositional processes for carbonates can come from examination of modern environments of deposition. Common patterns in structural, textural, and diagenetic trends can be summarized from several modern settings for reefs and mounds, sand shoals, and lagoons and tidal flats. The lessons learned from detailed studies of modern examples center on several important points: (1) the trend and continuity of facies belts vary, but the patterns are orderly when the setting is understood; (2) typically, carbonate deposits form in localized ovoid or elongate thicks, not in widespread sheets; (3) the depositional systems contain complex, highly variable facies patterns in map view; (4) a predictable sequence of sediments, although not fully developed thr ughout the depositional environment, typifies the setting; (5) the stratigraphy as revealed by sediment coring is highly variable, recording a short-lived, but exceedingly complex geologic history; and (6) early diagenesis related to evolving depositional environments can significantly alter the porosity and permeability of the sediments. Carbonate depositional systems, as shown by modern examples, are complex from the scale of a producing field right down to that of a pore throat. This fact, coupled with frequent control by facies over subsequent diagenesis, imparts the great heterogeneity to carbonate reservoirs. Log response and reservoir quality are directly related to facies and diagenesis, with varying grain size a major control over permeability amounts in porous intervals. Permeability affects recovery efficiency and thereby links the depositional facies through sediment texture to reservoir performance. End_of_Article - Last_Page 1441------------

Paleocurrent and Petrographic Analysis of Imbricate Intraclasts in Shallow-Marine Carbonates, Upper Cambrian, Southwestern Virginia

ABSTRACT The Conococheague Limestone and Copper Ridge Dolomite are coeval Upper Cambrian shallow-marine carbonates exposed in the Valley and Ridge Province of southwestern Virginia. Sheetlike conglomerates of probable storm origin showing well-developed intraclast imbrication are common in these units. Directional and petrographic data obtained from 48 such conglomerates and adjacent strata reflect the landward (Copper Ridge) versus seaward (Conococheague) depositional frameworks in which the two formations originated. Analysis of imbrication azimuths suggests that the Conococheague conglomerates were transported by multidirectional paleocurrents associated with middle- to outer-shelf storm activity. In contrast, the Copper Ridge imbricate intraclasts show much more consistent shoreward movement toward the northwest, probably the result of currents generated by storm waves refracted by shallower, middle-shelf to intertidal environments. At individual sample localities, apparent long-axis alignments of elongated clasts in the conglomerates show inconsistent relationships to transport directions deduced from imbrication, evidently because of high clast concentration and general lack of prolate grains. An overall shore-parallel trend of long axes, more evident in the Copper Ridge, may have developed fr m poststorm-tidal-current and wave activity and could serve as another shallow- versus deep-water discriminant. Paleoflow patterns derived from cross stratification in sandstones above and below the imbricate conglomerates reflect normal, day-to-day, tidal, longshore, and rip-current sedimentation. Petrographically, the intraclast conglomerates show a tripartite grouping strongly related to source sediments and their environments. Group I, found mainly in the Conococheague, formed in platform-margin ooid sand shoals; Group II, common in both formations, originated in subtidal middle-shelf to lower-intertidal inner shelf, peloid sand silt and mud flats; and Group III, present almost exclusively in the Copper Ridge, accumulated in inner-shelf intertidal environments where algal mats and carbonate mud were abundant. Paleocurrent directions determined from imbrication in the petrographic groups also reflect these differences in depositional environments. The importance of paleocurrent data from imbrication in shallow-marine carbonate conglomerates, which are abundant in the geologic record, has not been realized. This information can be synthesized with petrographic data to aid considerably in reconstructing ancient environments.

Facies-Specific, Calcitic and Bimineralic Ooids from Middle and Upper Cambrian Platform Carbonates, Western Newfoundland, Canada

ABSTRACT Oolites in the Port au Port Group consist of intercalated gray oolite, deposited on the shallow-subtidal mobile fringe of ooid sand shoals, and brown oolite, which accumulated on intertidal sand flats along the shoal crests. Gray oolite is composed of finely and coarsely preserved radial ooids that were precipitated and deposited in situ and whose fabric was controlled in large part by suspension versus bedload transport. Brown oolite is composed of concentric ooids or superficial radial ooids that were either formed and deposited in situ or swept onto the sand flats from the mobile fringe. The present fabric of the ooids is the result of protracted burial diagenesis. Cortices of finely and coarsely preserved radial ooids, excluding radial-concentric ones, and superficial radial ooids were Mg calcite originally; they changed to calcite, still retaining fine fabric, and subsequently to a variety of cortical fabrics during shallow burial, and altered to blocky fabrics during deep burial. Concentric ooids and radial-concentric ooids were bimineralic originally, composed of aragonite (now micrite and blocky calcite) and Mg calcite (now radial-fibrous calcite). Primary aragonitic laminae were partially or completely dissolved and crushed during shallow burial; resulting voids were filled with blocky calcite during deep burial. Correlation of original ooid mineralogy, fabric, and environment of formation indicates that local physical conditions such as turbulence influenced ooid formation in Cambrian time. This correlation is absent in modern deposits, suggesting that more widespread chemical and organic controls now prevail.

Nearshore carbonate deposits in Lake Tanganyika

An exceptionally wide variety of carbonate fades, dominated by high-magnesian calcite, occurs along the littoral and shallow sublittoral zones (<50 m) of Lake Tanganyika in central Africa. These facies include exposed and submerged, calcite-cemented ridges of nearshore terrigenous sand, ooid sand shoals, and lithified oolite ridges, Chara meadows of bioturbated calcareous silts, gastropod shell blankets and related coquinas, and extensive thrombolitic microbial reefs. Though texturally and compositionally similar to many modern and ancient shallow-water marine facies, these deposits record carbonate deposition and cementation in a large, tropical, deep-water lake of tectonic origin. Lithofacies along Lake Tanganyika represent the broadest spectrum of carbonate deposits yet reported from any modern lake and serve as important analogues for lacustrine carbonate sequences in the stratigraphic record.

Banktop responses to Quaternary fluctuations in sea level recorded in periplatform sediments

Periplatform sediment from a 12-m core recovered from Northwest Providence Channel, Bahamas, contains a geochemical and paleontological record of sea-surface temperatures, ice volumes, and the response of banktops to highstands of sea level. A comparison of these data suggests that fluctuations of carbonate mineralogy in periplatform sediments result from fluctuations of sea level and from patterns of banktop sedimentation. Highstands of sea level that flood carbonate platforms are recorded in periplatfonn sediments as abrupt increases of exported Sr-rich aragonite derived from banktop orgiuiisms superposed on a background of Sr-poor aragonite (pteropods) and calcite (foraminifera and coccoliths) derived from planktonic sources. The pulses of banktop sediment coincide with increases of water temperature determined from foraminiferal assemblages and with decreases in ratios of oxygen isotopes, indicating decreased ice volume and rising sea level. Following these abrupt changes is a more gradual decline in bank-derived sediment, although paleotemperatures from foraminiferal assemblages and oxygen isotopic data clearly show that warm conditions and a highstand of sea level persist. We suggest that this decrease of offbank transport is part of autocyclic sedimentation patterns of shallow-water carbonate environments. Offbank transport is restricted as reefs, sand shoals, and islands reach sea level. Also, green algal production may decline because progradation of tidal flats decreases lagoon area, and the hydrologic and ecologic conditions change as lagoons are filled.

Upper Jurassic Ramp Carbonate and Associated Evaporite, Neuquen Province, Argentina: ABSTRACT

The Oxfordian La Manga Limestone (10-65 m) and overlying Auquilco Gypsum (315 m maximum thickness) crop out along the west flank of the Neuquen basin, Neuquen Province, Argentina (36/sup 0/40/sup 0/S lat.). The contact with the underlying Lotena Sandstone is gradational, and both formations are cut by the Late Jurassic Araucanian angular unconformity. Seven lithofacies have been identified within sections measured through the entire interval along the northeast to southwest trending, 30-km long Sierra de la Vaca Muerta ridge (38/sup 0/30'-39/sup 0/S). The La Manga Limestone is interpreted as a temperate ramp carbonate that developed over the Lotena Formation siliciclastic shelf. Interpretations of lithofacies from southwest to northeast are: behind-barrier subtidal lagoon with washovers; coral and red algae biostromes; ooid and peloid sand shoals; downslope wackestone and packstone mud mounds; and deep-water carbonate turbidites. A minor regression separates La Manga and Auquilco Formations. Lithofacies of the Auquilco Formation indicate a shallowing-up sequence comprised of initially deep (hundreds of meters) subaqueous evaporite deposition followed by shallow, subtidal carbonate peloidal and shell fragment grainstones and evaporites. Thickness of the subaqueous evaporite gives an order of magnitude estimate of Auquilco basin depths of a few hundred meters at most. The Neuquen basin hasmore » an intermediate proportion of carbonate in comparison to relatively carbonate-poor basins to the south and carbonate-rich basins to the north.« less

Deposition of a Jurassic Ooid Sand Shoal--Smackover A"Zone of Buckner Formation, Corney Bayou Field, Union Parish, Louisiana: ABSTRACT"

The Smackover A zone of the Buckner Formation in northern Louisiana and southern Arkansas is lithologically similar to the more thoroughly studied upper Smackover Formation. However, in contrast to the regionally occurring thick, blanket-like ooid sands of the upper Smackover, the Smackover A zone carbonates occur as strike-parallel, individual carbonate sand bodies isolated within the clays, muds, and evaporites of the upper Buckner. This isolation and its potential effects on fluid movement, diagenetic processes, and ultimate porosity evolution in similar sequences across northern Louisiana and southern Arkansas emphasize the importance of a well-constrained core-based field study.

Depositional environments along a carbonate shelf to basin transect in the Silurian of Nevada, U.S.A.

During the Llandovery (Silurian) in central Nevada foundering of the shelf margin and drowning of a homoclinal carbonate ramp produced a rimmed carbonate shelf. Westward the shelf was bordered by a slope which passed into a deep-water basin. At the shelf margin, carbonate sand shoals (Lone Mountain Dolomite) aggraded and prograded westwards over the slope. The spatial and temporal facies variation in a transect across the slope (Roberts Mountains Formation) records the history of a prograding shelf margin.Initial slope sedimentation adjacent to the carbonate shelf margin was lime mudstone hemipelagically derived from the shelf. Westwards and downslope dilute carbonate turbidites and hemipelagic lime mudstone (laminated facies) were deposited. Contemporaneous slope-rise sediments (Fourmile Canyon Formation) consist primarily of siliciclastic mudstone with minor dilute carbonate turbidites and turbiditic siliciclastic siltstone, the latter essentially being part of the deep-water basin. Subsequent shelf margin progradation and increasing slope differentiation is reflected in the downslope introduction of calcarenite and conglomerate derived from the shelf margin. Slope sedimentation records an overall coarsening- and thickening-upwards cycle. Superimposed on this, in some areas, are small-scale coarsening- and thickening-upwards cycles reflecting deposition on carbonate debris aprons. Abandonment and subsequent build-out of the aprons reflects the pulsatory and uneven progradation of the outer shelf margin. Simultaneous with these depositional patterns, the immediate shelf margin periphery (top-of-slope) received hemipelagic lime mudstone sediments. Many rotational slides, some generating debris flows, indicate upper-slope instability. Most allochthonous carbonate debris from the shelf margin by-passed the top of slope and was deposited further downslope. With the westward progradation of the shelf margin the fringing facies migrated basinward.

Caractères sédimentologiques, morphologiques et paléogéographiques de la Plate-forme Carbonatée de l'avant-pays Rifain Oriental (Maroc) au Jurassique supérieur et à l'Eocrétacé

On the northwestern African passive margin, from Upper Kimmeridgian to Lower Berriasian, the Rift platform is characterized by a very thick muddy carbonate sedimentation. This platform grew up between the northward Rift basin and the southward emerged Atlatic domain. The sequential arrangement shows a general regressive evolution but without a true progradation of the sedimentary bodies because sedimentation and subsidence rates are approximately equivalent. Three sedimentary systems are described; each system is defined as a functional unit with elementary patterns, the evolution of which is controlled by a number of factors which interact with each other. The factors include basement tectonics, hydrodynamics, biologic activity and climate. In the first system, the general pattern is a homoclinal ramp that may have exceptionally skeletal sand shoal complexes (northern edge of Beni Snassen mountains). The second system is characterized by instability of the basement: at the beginning the platform appears as a mosaic of low and shallow zones (lagoons) and pellet sand shoals, then between the Rift basin and inner part of the platform the mobility of the basement induces tilting of blocks as those of passive (extensional) continental margins with high or moderated energy sandy deposits or reef-buildups on the block crests; at last the system finds again a situation of relative stability. In the third system, sheltered behind a series of bioclastic or oolitic sand shoals restricted to the platform edge, a very shallow zone develops with confined and supersalted or unsalted lagoons and algmats on intertidal or supratidal shoals and on the margino-littoral area. A number of common features allow us to define a most general pattern of neritic carbonate platform with: (1) a circalittoral muddy outer part with sponges in quiet deep water, (2) a transition zone with a sand bioclastic barrier or not, (3) a sheltered shallow inner part with muddy sedimentation where a more or less important benthic biologic activity (specially algal activity) develops and (4) a margino-littoral area with algmats and evaporitic deposits.

Sedimentary Basin Analysis of Middle Ordovician Limestones in Central Appalachians: ABSTRACT

Seven of 28 wells penetrating the Trenton Limestone in West Virginia have reported shows of natural gas, enough to continue industry's interest in this potential reservoir. The formation consists of thin limestones interbedded with shales and bentonites. Sediments were deposited on a ramp that sloped eastward from a shallow platform in northwest Ohio into the foreland basin of Virginia, and the unit forms a wedge-shaped mass that thickens into the basin. Limestones of the upper ramp were deposited on sand shoals (skeletal grainstones) and restricted flats (lime mudstones), which passed downslope into skeletal patches of a deep, muddy environment (packstones and wackestones). Rapid downwarping of the carbonate ramp produced a major transgression, and deeper limestone facie migrated westward during time. Bentonites spread across the region from distant volcanic islands. In West Virginia, a lower bentonite package is present in the Black River Limestone to the southwest, whereas an upper package occurs in the Trenton to the east and north. This distribution End_Page 1444------------------------------ indicates that the site of volcanic activity shifted during time. A growing orogenic source area shed terrigenous sediment into the basin and onto the Trenton ramp. Initially, these muddy influxes came from the north and from the south, but terrigenous mud eventually swamped the entire ramp and carbonate sedimentation then came to an end. Trenton Limestones have only minor intergranular porosity because of abundant mud matrix and cementation. The only significant contribution to porosity is in fractures. Thus, wells with Trenton shows are characterized by high initial potential that decreases rapidly. End_of_Article - Last_Page 1445------------

Trinity Shoal: a Reworked Deltaic Barrier on Louisiana Continental Shelf: ABSTRACT

Abandonment and reworking of deltaic complexes of the Holocene Mississippi River have produced a series of sandy shoals on the muddy Louisiana continental shelf. Trinity shoal, one of these transgressive deposits, is located 30 km offshore of Atchafalaya Bay and the Point AuFer-March End_Page 309------------------------------ Island shell reefs. Approximately 1,000 km of high-resolution uniboom and 3.5 kHz subbottom-profile seismic data, taken in this area in 1983 and 1984, provide the data base for this study. Trinity shoal, associated with the abandoned Maringouin delta complex, is a lunate shore-parallel feature approximately 36 km long and 5-10 km wide. Relief on the shoal ranges from 2 to 3 m, and minimum water depths over the shoal vary from -5 to -2 m. The shoal sand body is from 5 to 7 m thick and is composed largely of parallel to low-angle clinoform reflectors. Several levels of buried fluvial channels, ranging in age from early Wisconsinian to Holocene, are associated with the shoal deposit. The occurrence of channel features within the shoal sand itself suggests the presence of tidal inlets, indicating a possible barrier-island origin for the shoal. The underlying deltaic sediments reach approximately 15 m in thickness and are made up of low-angle clinoform reflectors dipping to the southwest. Distributary, bay-fill, estuarine, and buried oyster-reef deposits can be recognized, making these similar to modern Atchafalaya delta deposits. Continued progradation of the Atchafalaya delta will probably result in burial of the Trinity shoal and Maringouin delta deposits by fine-grained sediments, giving these shoal deposits a high-preservation potential and creating an excellent stratigraphic trap. End_of_Article - Last_Page 310------------

Silurian carbonate shelf and slope evolution in Nevada: A history of faulting, drowning, and progradation

In the Early Silurian a broad subtidal to peritidal shallow carbonate ramp was situated in central and northern Nevada. Along both the northern and western margins the shallow ramp merged into a deep-water basin. The northern shallow-ramp margin, coincident with the Tooele Arch, had a narrow grainstone shoal facies belt that passed downslope into deep-ramp environments. The grainstone shoal facies at the western shallow-ramp margin formed a wide belt to the north and a narrow belt to the south. Shallow-ramp sediments thinned northward toward the shallow-ramp margin, indicating slight differential down-to-south subsidence of the shallow ramp. Later in the Silurian (C 4 —Llandovery) the ramp was abruptly drowned. Simultaneous down-to-north subsidence along the northern shallow-ramp margin possibly led to surface faulting and the formation of an escarpment-bounded shelf. Subsequently, the drowned ramp evolved into a rimmed shelf, the rim consisting of high-energy skeletal sand shoals. Eastward progradation of peritidal sediments from the shelf rim filled in the shelf during the Llandovery and Wenlock. The rim facies also prograded oceanward over the western slope during the later Silurian. Along the northern shelf margin the rim facies was less well developed, and oceanward progradation over the northern slope was slow.

Multiple Submarine-Cemented Grainstone Sequences Along Leeward Carbonate Margins: Examples from Late Quaternary of Little and Great Bahama Banks: ABSTRACT

Coarse-grained, leeward-margin sand shoals, developed during the late Quaternary along the western edges of Little and Great Bahama Bank, have been deposited and preserved in response to regional sediment-transport processes and local physicochemical conditions. These sand bodies are fundamental depositional sequences, chronostratigraphically bounded by subaerial exposure crusts, and thus are of major importance in determining rates of bank-margin growth and in understanding the dynamics of carbonate margin buildup. In the Holocene sediments, these sand bodies are 1-2 km wide, up to 35 m thick, and are present along 400 End_Page 316------------------------------ km of the western Bahamas. Surficial sediment is composed of coarse sand and granules dominated by composite-ooid grains. Submarine cementation is active and has led to the development of coarser grain sizes during the depositional interval. Fine to medium-sized grains are cemented with fibrous aragonite and surficially coated at the tidally active margin. The resultant composite-ooid sediment is more hydrodynamically stable and is rapidly cemented into hardgrounds characterized by a smooth (tidally abraded) upper surface. Lithification is gradational through a thickness of 50-100 mm to an irregular lower boundary transitional with uncemented material. These hardgrounds are submarine discontinuity surfaces developed during inter-storm conditions of winnowing and bypass sedimentation a ong a depositional profile of equilibrium. Rock cores into the Pleistocene section have recovered sediments and submarine discontinuity surfaces identical to those in the Holocene. These are present in the two latest Pleistocene sequences representing the last major interglacial intervals. Both the Holocene and Pleistocene sequences have one or two cemented zones per meter in the upper section. The presence of these deposits throughout the preserved stratigraphic package indicates the persistence of characteristic leeward depositional processes during the late Quaternary and their importance in bank-margin growth. A combination of characteristics including typical microfacies, cements, and discontinuity surfaces (if seen in the ancient) should be good indicators of leeward-margin setting . End_of_Article - Last_Page 317------------

Cay Sal Bank, Bahamas — A partially drowned carbonate platform

Recent high-resolution seismic reflection profiling, sediment sampling, SCUBA observations and Landsat imagery show that Cay Sal Bank (CSB) has very limited reef development, no active sand shoals, few islands, a thin to non-existent sedimentary cover and a relatively deep margin (20–30 m) and shelf lagoon system (10–20 m), Windward and leeward margins can be discerned, but their general development is poor when compared to the shallower, more active margins of Little Bahama Bank (LBB) and Great Bahama Bank (GBB). Windward margins (facing north and east) along CSB are generally deep, rocky, sediment-barren terraces supporting limited, low-relief, relict(?) reefs. Leeward margins have small sand bodies (maximum thickness 10 m) covering reef structures at the bank edge indicating that offbank transport of sands has occurred. However, these marginal sand bodies are limited in areal extent, suggesting that this transport system was not ubiquitous along the south- and west-facing margins. Seismic and grab sample data from the deep (200–500 m) slopes seaward of the leeward margins show a thin, discontinuous unit of periplatform sand containing Halimeda, molluscs, and non-skeletal components derived from shallow water. The limited basinward extent (no deeper than 300 m) of this unit, recognized by its reflection-free seismic facies, also indicates that sand production and transport off the bank were never prolific. This is in stark contrast to new seismic data from the leeward margins of GBB which clearly show thick (20 m) sand bodies covering high reefs 12–15 m along the outer margin and multiple reflection-free units extending to great depths (600 m) all along the adjacent slope. The apparent immature development of normal bank-top processes and facies, and the absence of key modern depositional environments on CSB may be related to the rate at which this platform was submerged. Due to its comparatively low elevation, the initial Holocene flooding occurred at approximately 8–10 ka when sea level rise was rapid (6 m ka −1). By comparison, the higher LBB/GBB were flooded later at a much slower rate (1.5 m ka −1). The relatively rapid flooding of CSB provided little time for the shallow depositional environments to start up. The continued rapid rate of sea-level rise after drowning, and offbank transport of sediment and the export of chilled waters formed during winter months, prevented the resulting facies from catching up. Consequently, CSB appears to be partially drowned, particularly when compared to the other “healthier” rimmed Bahamian platforms. Drowned carbonate banks are very common in the ancient record and potentially provide excellent stratigraphic traps for hydrocarbons. CSB provides a modern example of a bank that may be in the very early stages of termination. Final drowning of carbonate platforms may occur as a result of frequent higher order sea-level fluctuations superimposed on initial stage of a lower order sea-level cycle.

Sedimentary Models of Pattern, Process, and Succession Derived from Bahamian Carbonates: ABSTRACT

Three decades of research on the Cenozoic carbonate platforms of the Bahamas have produced several models of deposition and diagenesis. The models are sufficiently varied and numerous that they can be categorized and their interactions evaluated. The models can be divided into three types: two-dimensional patterns of depositional facies; process models of sediment formation, deposition, and diagenesis; and concepts of facies succession. Patterns of depositional facies range in scale from a kilometer or less in the bar and channel morphology of ooid sand spreads, to a few kilometers in channeled tidal flats, to as much as 100 km (62 mi) in the platform to basin zonation of reefs and sediments. The spectrum of process models extends from grain formation (peloids and ooids), to major modes of accumulation (net shoreward movement of lime mud with attendant seaward progradation of shoaling cycles), to sea-floor cementation that yields aggregate grains and hardgrounds. Vertical facies successions on the scale of several meters are seen in the shoaling deposits of tidal flats and in the coarsening-upward sequences of sand shoals. Larger scale changes in facies, from skeletal to nonskeletal deposits that are tens of meters thick, occur in the subsurface Pliocene-Pleistocene carbonates. These three kinds of sedimentary models interact in various ways. An example of the influence of pattern on process is seen in the channels between tidal bars of ooid sand. The growth of tidal bars of ooid sand leads to accelerated tidal currents in the intervening channels; the result is a channel lag of coarser sediments that limits traction movement of lime sands and especially the nuclei for continued formation of ooids. Deceleration of ooid formation can in time lead to stabilization of the moving sands with attendant alterations of grains and sedimentary structures. An example of the effect of process on pattern is seen in the formation of extensive hardgrounds on lime sands of the platform interior by submarine cementation. These hardgrounds are rapidly colonized by attached or anisms, including reef-building corals; the end result is a major transformation of sediment type and the appearance of the preferred substrate for reefs. An example of the interaction between facies is seen in two aspects of reefal deposits: the preferential development of some reefs seaward of islands or growing sand shoals and the burial of other reefs that fringe leeward margins of the platforms by off-bank transport of lime sands from the interiors. End_of_Article - Last_Page 1836------------

Paleoenvironments of Late Albian Stage (Early Cretaceous) of Eastern Trans-Pecos, Texas: ABSTRACT

Upper Albian rocks in eastern Trans-Pecos Texas consist of a depositional sequence that records a major transgressive-regressive cycle, beginning with an initial transgression of the early late Albian seas over a regionally disconformable surface and ending with the onset of late Albian regression. The history of these events is recorded in rocks of the Fort Lancaster Formation and its western equivalent, the Boracho Limestone. These rocks were deposited in several depositional systems that developed on the Comanche shelf and an associated shelf basin called the Fort Stockton embayment. These depositional systems include the carbonate-shelf system, the shelf-basin system, and the carbonate-shoal system. Each system is recognized by unique suites of lithofacies, differentiated on the basis of petrographic and outcrop characteristics: skeletal wackestone, interbedded shale and wackestone, oyster packstone/shale, skeletal grainstone, and calcareous shale. Associated with these lithofacies are the following biofacies: Protocardia-Macraster, Globigerina, Nuculana-Syncyclonema, Texigr phaea, Lopha, and Miliolina. Within a biostratigraphic framework based on ammonite zones, the geographic and stratigraphic distribution of a depositional systems and their associated lithofacies and biofacies indicate that late Albian deposition primarily was influenced by periodic terrigenous influxes from the northwest over the Comanche shelf. This was coupled with tectonically controlled subsidence in the Fort Stockton embayment. The depositional sequence culminated in the development of widespread carbonate sand shoals prior to the onset of emergence. End_of_Article - Last_Page 461------------

Geologic Framework of Sand Shoals on Muddy Mississippi Delta Shelf: ABSTRACT

More than 1,000 km 620 mi) of high resolution ore-Boomer and 3.5 khz subbottom seismic profiles correlated to seventeen 10-12 m (33-56 ft) vibracores provide the data base for analyzing the sedimentologic and stratigraphic framework of transgressive sand shoals on the Louisiana inner continental shelf. Trinity and Ship Shoals are comprised of reworked sands of the abandoned Holocene Teche and Maringouin deltas and provide a possible modern analog for some Cretaceous shelf sandstones of the Western Interior. Ship Shoal transgressive sands lie disconformably over Maringouin deltaic muds. The sand body pinches out seaward on the erosional inner shelf and is terminated landward by a depositional surface. Maximum sand body thickness is 7 m (23 ft) in the western shoal region. Internally, the sand body is characterized by landward dipping subhorizontal reflectors. The underlying Maringouin deltaic sequence contains a series of low-angle seaward-dipping clinoforms and numerous small channels in the western shoal area. Core analysis reveals a 3-7 m (10-23 ft) thick upward-coarsening sequence of very fine to fine-grained (100-125µ) well-sorted, clean, quartzose sand. Grain size, percent sand and shell, and percent cross bedding increase upwards. The shoal sequence is capped by a 1-2 m (3-6 f ) thick deposit of horizontally laminated fine-grained (125-175µ) sand and shell. The lower two-thirds of the shoal sequence is massive in appearance with minor amounts of burrowing. The shoal sequence abruptly overlies a dark, organic rich, silty clay (prodelta?) with numerous wavy and lenticular interbeds of silt; burrowing is rare. The Trinity Shoal sand body is 5-7 m (16-56 ft) thick and lies disconformably over Teche deltaics. Internally, the sand body is composed of a set of westward-dipping clinoform reflectors. Three levels of channeling related to sea level stands in the early Wisconsin, late Wisconsin, and Holocene (Maringouin delta) underlie and occur seaward of Trinity Shoal. Continued Atchafalaya delta sedimentation will soon encase Trinity Shoal in mud. End_of_Article - Last_Page 533------------

Sand Shoal Development on Muddy Mississippi River Delta Shelf: ABSTRACT

Trinity and Ship Shoals are transgressive sand bodies on the Louisiana inner continental shelf, and they represent the reworked sands of the abandoned Holocene Teche and Maringouin deltas. The development of these shoals is initiated by an episode of delta abandonment followed by subsidence-enhanced sea level rise. Through the process of shoreface retreat, the abandoned delta lobe evolves from an erosional headland with flanking barrier islands to a barrier-island arc and finally into a submerged inner-shelf shoal system. Trinity and Ship Shoals represent the final stage in the Mississippi River delta barrier shoreline cycle and provide a possible modern analogue for some Cretaceous shelf sandstones of the Western Interior. More than 1,000 km (620 mi) of high-resolution s ismic profiles correlated with cores provide the data base for interpretation of the depositional history of sand-body development on the muddy Louisiana shelf. Ship Shoal is the oldest inner-shelf shoal associated with the abandonment and subsequent reworking of the Maringouin delta 6,150 y.B.P. Located 25 km (16 mi) south of the Isles Dernieres, the Ship Shoal transgressive sands lie disconformably over the Maringouin deltaic muds. The Ship Shoal sand body is shore parallel, 32 km (20 mi) long and 2-4 km (1-2 mi) wide. The inner-shelf relief ranges (east to west) from 2-6 m (7-20 ft) with a corresponding decrease (east to west) in the water depth over the shoal crest from -6 to -3 m (-20 to -10 ft). The shoal profile is asymmetric landward. The Ship Shoal sand body is composed of an upward-coarsening sequence of well-sorted fine-grained sand with a median size of 125µ. Trinity Shoal is associated with the Teche delta, abandoned 3,500 y.B.P., and it is located 20 km (12 mi) south of Marsh Island. The base of Trinity shoal lies unconformably over the Teche deltaic sediments. Trinity shoal is a shore-parallel lunate sand body, 36 km (22 mi) long and 5-10 km (3-6 mi) wide. The inner-shelf relief ranges (east to west) 2-3 m (7-10 ft), with a corresponding decrease (east to west) in the water depth over the shoal crest 5-2 m (16-7 ft). The Trinity Shoal sand body is 5-7 m (16-23 ft) thick. End_of_Article - Last_Page 515------------