Shelf Sand Research Articles

Early diagenesis and its relationship to depositional environment and relative sea‐level fluctuations (Upper Cretaceous Marshybank Formation, Alberta and British Columbia)

ABSTRACTEarly diagenesis of the Upper Cretaceous (late Coniacian to early Santonian) Marshybank Formation was controlled by depositional environment (composition of depositional water, Fe and organic content of the sediment, sedimentation rate, proximity to the shoreline) and influx of meteoric water related to relative sea‐level fall.Five depositional environments, each characterized by a distinct early diagenetic mineral assemblage, have been recognized. Offshore shelf sediments that were deposited in a dysaerobic environment are characterized by abundant framboidal pyrite and rare septarian concretions, composed of ‘early’ calcite and siderite. Intense sulphate reduction, promoted by the dysaerobic depositional water, was the primary influence on early diagenesis. Offshore shelf sediments deposited under aerobic conditions are characterized by abundant concretions, composed of two generations of siderite (S1 and S2). In this environment, methanogenesis, rather than sulphate reduction, was more important. Early diagenesis of the inner shelf sands was generally limited. However, in sands deposited proximal to the shoreline, mixing of marine and meteoric waters promoted crystallization of Fe‐rich chlorite and siderite. The shoreface was characterized by dissolution of detrital minerals in the upper portion, and precipitation of kaolinite or illite/smectite in the lower portion. In the coastal plain environment, brackish water and early reducing conditions resulted in formation of abundant euhedral pyrite. Ankerite, rather than siderite, is the typical early diagenetic carbonate.The δ18O values of the earliest cements (i.e. ‘early’ calcite, siderite S1, inner shelf siderite) indicate crystallization from a low‐18O, marine‐derived porewater. Assuming crystallization at 25°C, a δ18O value of about −7‰ (SMOW) can be estimated for the seaway during Marshybank Formation time. Similar calculations for the overlying Dowling Member (Puskwaskau Formation) suggest that the δ18O value of the seaway increased to about −4% (SMOW), consistent with its transgressive nature. Very low δ18O values are exhibited by siderite S2. These results indicate crystallization during intermediate diagenesis (≥60°C) from meteoric water (≥− 15‰ SMOW) that entered the Marshybank Formation during sea‐level lowstand.

Shelf Sand Body Distribution and Transgressive Systems Tract Development on a High Energy Shelf: An Example from the East China Sea: ABSTRACT

Shelf Sand Body Distribution and Transgressive Systems Tract Development on a High Energy Shelf: An Example from the East China Sea: ABSTRACT

A Sequence Stratigraphic Model for Shelf Sand Ridge Genesis and Evolution: ABSTRACT

A Sequence Stratigraphic Model for Shelf Sand Ridge Genesis and Evolution: ABSTRACT

Depositional setting and sequence stratigraphic implications of the Upper Sinemurian (Lower Jurassic) sandstone interval, North Celtic Sea/St George’s Channel Basins, offshore Ireland

Abstract The Upper Sinemurian sandstone is a primary reservoir target in the North Celtic Sea/St George’s Channel Basins. Based on core examination, analysis of gamma ray logs, and interpretation of possible sandbody geometry from well log correlations and seismic character, a marine shelf depositional setting has been interpreted for the Upper Sinemurian sandstone on the northern margin of the North Celtic Sea/St George’s Channel Basins. Burrows observed in the cored interval are typical for a marine shelf depositional setting. Wavy, low-angle cross-bedding, reactivation surfaces and truncated cross-bedding suggest deposition by storm-driven, surging geostrophic currents in this setting. Examination of gamma ray logs through the Upper Sinemurian sandstone indicates that it coarsens and cleans upwards and can be divided into at least three units based on changes in curve patterns. Similar log patterns and sedimentary structure sequences in core and outcrop have been observed in elongate single and coalescing shelf ridges in the Ordovician of the eastern Williston Basin in Canada and the USA, in the Cretaceous of Wyoming and Colorado, and the Middle and Upper Jurassic of the North Sea. As will be discussed, the shelf ridge interpretation for some of these examples is controversial, with some workers suggesting an alternative incised shoreface interpretation. These examples range in thickness from 10 to 130m (33 to 425 ft). The 60+m (200+ ft) thickness of the Upper Sinemurian sandstone is well within this range. Such thicknesses suggest that ridge development (sand supply) kept up with basin subsidence. Thus, based on core data, gamma ray log curve examination, and comparisons to other sub-surface and surface examples, the Upper Sinemurian sandstone is interpreted as a series of complex elongate single and coalescing shelf sand ridges. Interpretation of recently acquired seismic data and sequence stratigraphic analysis of the Lower and Middle Jurassic section indicates that the Upper Sinemurian sandstone is part of a series of stacked sequences. These sequences result from increases in sediment accommodation space due to episodes of basin subsidence and their corresponding relative sea-level rises. Six sequences or cycles can be recognized, with each consisting of a basal onlapping, open marine phase which shoals upwards to shallow marine limestone or sandstone. Onlap occurs at and above a marine flooding surface (or its basinward sediment starvation equivalent) and proceeds towards the northern flexural margin of the basin. The next marine flooding surface is cut into or is in conformable contact with the top of the underlying sequence. The upper part of the sequence has a progradational to slightly aggradational seismic character, which results in an overall parallel seismic signature. The best potential Lower/Middle Jurassic clastic reservoir development in the North Celtic Sea/St George’s Channel Basins, such as the Upper Sinemurian sandstone, corresponds to the last phase of progradation at the top of a sequence. Six such sequences, with their upper boundaries at the tops of the Lower Sinemurian sandstone, Upper Sinemurian sandstone, Pliensbachian limestone, Toarcian marker, Bajocian sandstone/limestone and Bathonian limestone, respectively, can be delineated in the Lower and Middle Jurassic section in the North Celtic Sea/St George’s Channel Basins. The Lower and Middle Jurassic was largely a time of thermal subsidence in the North Celtic Sea and St George’s Channel Basins. Early thermal subsidence was interrupted by events of post-rift footwall readjustment along the northern margin of both basins. This resulted in the input of clastic sediments into both basins and deposition of Lower and Upper Sinemurian sandstones in rapidly coarsening-upward cycles and sequences. In contrast, Pliensbachian to Bathonian sequences represent deposition in a period dominated by thermal subsidence and thus coarsen upwards more gradually. With the exception of basin margin uplift and basin centre subsidence influenced by intraplate stresses, extrabasinal effects, such as eustasy, are not thought to be a factor in Upper Sinemurian sandstone deposition and Lower and Middle Jurassic sequence development in the North Celtic Sea and St. George’s Channel Basins.

Marine geology of the Sodwana Bay shelf, southeast Africa

The geostrophic current-controlled northern Zululand shelf displays a unique assemblage of interesting physical, sedimentological and biological phenomena. The shelf in this area is extremely narrow (3 km) and is characterised by submarine canyons, coral reefs, and steep gradients on the continental slope. Three submarine canyons occur in the study area and are classified as mature- or youthful-phase canyons depending on the degree to which they breach the shelf. These canyons originated as mass-wasting features which were exploited by palaeo-drainage during sea-level regressions. Shelf lithology is dominated by a series of coast-parallel patch coral reefs which have colonised beachrock and aeolianite sequences that extend semi-continuously from −5 to −95 m, and delineate late Pleistocene palaeocoastline events. The unconsolidated sediment on the shelf is either shelf sand (mainly terrigenous quartz grains) or bioclastic sediment. Large-scale subaqueous dunes commonly form in the unconsolidated sediment on the outer-shelf due to the Agulhas Current flow. These dunes occur as two distinct fields at depths of −35 to −70 m; the major sediment transport direction is towards the south, but occasional bedload parting zones exist where the bedform migration direction changes from south to north.

Gulf of Mexico Oil and Gas Atlas Series: Chronostratigraphically Bound Reservoir Plays in Texas and Federal Offshore Waters: ABSTRACT

The search for additional hydrocarbons in the Gulf of Mexico is directing exploration toward both deep-water frontier trends and historically productive areas on the shelf. The University of Texas at Austin, Bureau of Economic Geology, in cooperation with the Minerals Management Service, the Gas Research Institute, and the U.S. Department of Energy, is responding to this need through a coordinated research effort to develop an oil and gas atlas series for the offshore northern Gulf of Mexico. The atlas series will group regional trends of oil and gas reservoirs into subregional plays and will display graphical location and reservoir data on a computerized information system. Play methodology includes constructing composite type logs with producing zones for all fields, identifying progradational, aggradational, and retrogradational depositional styles, and displaying geologic data for type fields. Deep-water sand-rich depositional systems are identified separately on the basis of faunal ecozones, chronostratigraphic facies position, and log patterns. To date, 4 Oligocene, 19 Lower Miocene, and 5 Upper Miocene plays have been identified in Texas state offshore waters. Texas state offshore plays are gas prone and are preferentially trapped in rollover anticlines. Lower Miocene plays include deep-water sandstones of Lenticulina hanseni and jeffersonensis; progradational sandstones of Marginulina,more » Discorbis b, Siphonia davisi, and Lenticulina; transgressive sandstones associated with a barrier-bar system in the Matagorda area; and transgressive sandstones below Amphistegina B shale. Particularly productive gas-prone plays are progradational Siphonia davisi, shelf-margin deltas in the High Island area, and progradational Marginulina shelf and deltaic sands in association with large rollover anticlines in the Matagorda Island and Brazos areas.« less

Lithofacies Evolution from Transgressive to Highstand Systems Tracts, Holocene of the Alabama Coastal Zone: ABSTRACT

ABSTRACT The distribution of Holocene marginal marine lithofacies from Alabama State waters and the inner shelf was determined from vibracores, surface sediment samples, and shallow seismic surveys. Facies range from silty clay (bay muds) to orthoquartzitic sand (shelf sand ridges) and shell gravel (transgressive lag). A type 1 sequence boundary separates the Holocene transgressive tract from the eroded pre-Holocene; the surface locally exhibits oxidized paleosols, change in seismic velocity, or underlying stiff clay. The irregular surface preserves Wisconsinan paleotopography, including dip-trending paleochannels up to tens of meters deep (e.g., the Mobile River system) and shore-parallel lows. Early transgressive systems tract deposits include marsh deposits in low-energy lagoons in the paleolows; muddy channel fills; onlapping muddy shallow shelf deposits; and coarse-graded shelly transgressive lags in some open-shelf settings. Carbon-14 dates from oyster and wood samples from Mobile Bay and Mississippi Sound indicate that the present shelf and paleochannels were inundated ca. 7,000 years before present (YBP). By 4,000 YBP the present bays and lagoon areas were largely inundated. The early Holocene lithofacies configuration of more open circulation in the bays evolved to that of the present latest transgressive to highstand systems tract as sea-level rise decelerated. Pleistocene paleohighs acted as nuclei for the development of the present barrier complexes, which restricted water energy and salinity in the bays. These bays acted as sinks for aggrading bay/lagoonal muds and muddy sands from Mobile River, with a bayhead delta prograding rapidly in the incised Mobile River valley. The coastal Escatawpa River exhibited significant delta destruction. Development of extensive wetlands and downbay migration of oyster reefs reflected this slowing of sea-level rise. On the inner shelf, quartz sand ridges and hardbottoms developed updrift from Mobile Bay outfall; downdrift the shelf continued to receive fine-grained sediments. This lithofacies pattern represents a classic sequence evolution due to climatic-induced transgression.

Processes on the continental slope off North Carolina with special reference to the Cape Hatteras region

Historical data from the slope off Cape Hatteras show this environment to be atypical of the rest of the Atlantic slope in terms of high rates of sedimentation and high benthic/demersal standing stocks. This paper focuses on identifying potential sources for sediments and nutrients on the Hatteras slope and on likely transport mechanisms. Sediments consist mainly of subequal mixtures of sand, silt, and clay and contain an average of 1% carbon. This pool of carbon represents weathered organic matter containing polyunsaturated fatty acids (PUFA) and sterols typical of relatively refractory shelf/estuarine sediments. The labile organic fraction is derived from phytoplankton and zooplankton as reflected in short chain fatty acids (<C22), planktic sterols, and chlorophylla that are found in higher concentrations than observed elsewhere on the eastern U.S. continental slope. The inventory of organic nitrogen is much higher on the mid-slope (800–1 000 m) than shallower or deeper bottom areas as predicted from plots of organic nitrogen versus grain-size for the U.S. Atlantic continental margin. The midslope region is the major focusing area for sedimentation. Bioturbation is an important diagenetic process that has profound influence on sediment profiles of sulphate, methane, fatty acids, sterols, chlorophylla, viable diatoms, and metals. The low inventory of relatively refractory carbon (1%) stands in contrast to high measured rates of organic carbon sedimentation (28–121 g organic C m−2 year−1). This paradox is probably related to high remineralization rates in the water column and on the bottom. The refractory nature of the small residual pool of deposited organic matter may define the trophic niche filled by those benthos found on this slope that are more typically encountered on the shelf (e.g. oligochaetes and opportunistic polychaetes). A likely mechanism for high input rates of both organic and inorganic particulates to the Hatteras slope may be attributed to the position of Cape Hatteras relative to the adjacent narrow shelf. The Cape extends outward almost to the shelf edge topographically diverting sediment and primary production seaward that is moving southward along the outer shelf (the ‘funnel’ hypothesis). Transport of outer shelf sands to the slope may take place by the impingement of Gulf Stream eddies on the outer shelf and upper slope as well as by storm-generated waves. Once shelf sediment is deposited on the upper slope, it is apparently redistributed, as no strong on-shore to off-shore or depth-related gradients are observed in any of the measured sedimentary parameters. The Hatteras slope apparently represents an estuarine type of sedimentary/nutrient regime that is displaced to an otherwise oceanic deep-sea environment by outwelling and funnelling of nutrients and sediments from the shelf to the slope.

Carbon budget for the mid-slope depocenter of the Middle Atlantic Bight

A mass budget was constructed for organic carbon on the upper slope of the Middle Atlantic Bight, a region thought to serve as a depocenter for fine-grained material exported from the adjacent shelf. Various components of the budget are internally consistent, and observed differences can be attributed to natural spatial variability or to the different time scales over which measurements were made. The flux of organic carbon to the sediments in the core of the depocenter zone, at a water depth of ∼1000 m, was measured with sediment traps to be ∼65 mg C m−2 day−1, of which 6–24 mg C m−2 day−1 is buried. Oxygen fluxes into the sediments, measured with incubation chambers attached to a free vehicle lander, correspond to total carbon remineralization rates of 49–70 mg C m−2 day−1. Carbon remineralization rates estimated from gradients of Corg within the mixed layer, and from gradients of dissolved ammonia and phosphate in pore waters, sum to only ∼4–6 mg C m−2 day−1. Most of the Corg remineralization in slope sediments is mediated by bacteria and takes place within a few mm of the sediment-water interface. Most of the Corg deposited on the upper slope sediments is supplied by lateral transport from other regions, but even if all of this material were derived from the adjacent shelf, it represents <2% of the mean annual shelf productivity. This value is further lowered by recognizing that as much as half of the Corg deposited on the slope is refractory, having originated by reworking from older deposits. Refractory Corg arrives at the sea bed with an average 14C age 600–900 years older than the pre-bomb 14C age of DIC in seawater, and has a mean life in the sediments with respect to biological remineralization of at least 1000 years. Labile carbon supplied to the slope, on the other hand, is rapidly and (virtually) completely remineralized, with a mean life of < ∼ 1 year. Carbon-14 ages of fine-grained carbonate and organic carbon present within the interstices of shelf sands are consistent with this material acting as a source for the old carbon supplied to the slope. Winnowing and export of reworked carbon may contribute to the often-described relationship between organic carbon preservation and accumulation rate of marine sediments.

Stratigraphic Framework of Inner Shelf Storm-Dominated Sand Ridges, Alabama EEZ: Implications for Sequence Stratigraphy, Global Climate Change, and Petroleum Exploration

ABSTRACT The Alabama Exclusive Economic Zone contains an abundance of orthoquartzitic shelf sand ridges that are elongate in a northwest-southeast orientation diagonally from the shoreline. Local topographic relief can be greater than 4 meters. They are found most commonly in water depths of <15 m, with many being shoreface attached, although they are found in all water depths of the inner Alabama shelf. Ridges are abundant on the Alabama Eastern Shelf, but rare on the West Shelf due to high sediment flux from Mobile Bay. They average 6 km in length and range from 1.7 to 11 km long with an average width of 1.6 km (the range in width is 1 to 3 km). Soft-sediment peels from 59 vibracores from the Alabama inner shelf permit detailed description of sand ridge sedimentary structures, fabrics, and eight seafloor sediment types. These overlie the pre-Holocene sequence boundary and early Holocene transgressive sediments. Cross-sections utilizing the peels detail internal facies relationships. The ridges and inter-ridge troughs are embedded in a blanket shelf sand sheet representing widespread deposition of reworked palimpset orthoquartzites following Holocene transgression. In general, the ridges are capped by coarse stacked, graded shelly sands, echinoid sands, and clean sands deposited well above storm wave base. The graded shelly sand microfacies, the most common sediment type, is inferred to represent shelf storm deposits; its graded nature, sharp base, and variable thickness (0.1 to 4 m) is typical of tempesites. Often, graded shelly sands are found as stacked storm deposits on upper ridge flanks, with only the basal, graded shelly portions preserved. The orthoquartzite facies is also found on the ridge crests and upper flanks, as well as on the sand sheet proper. The echinoid sand microfacies is a background, agitated water deposit that forms on sand ridge crests as well as low relief parts of the sand sheets. It is compostionally very similar to the orthoquartzite microfacies, but it contains very recently dead parautochthonous echinoid hash. Thus, it is restricted to surficial deposits in areas of high concentrations of echinoid colonies. The inter-ridge troughs receive quieter water, fine-grained sediment between storms; thin stacked shelly washovers may be deposited during storms. Where clean units are correlative from ridge to swale, they often thin into the swales. Therefore, the ridges contain thicker sequences of coarse, reworked Holocene sediments than do the surrounding swales; they represent positive build-ups of Holocene sediment above the pre-Holocene surface. There is considerable patchiness of facies on a single sand ridge, even with close (c. 1 km) core spacing. Often, muddy vs. sandy sediment can be correlated seaward (i.e., parallel to ridge axis) in a general manner; however, specific muddy or sandy microfacies grade laterally. The internal facies pattern does not indicate lateral migration of the ridges; there are no obvious differences in facies patterns of Gulf-facing versus shoreline-facing ridge flanks. The facies patchiness may result from the interplay between relict sediment distribution, present hydrodynamics (especially helical storm flow parallel to the ridge axes), the local differences in preserved shell content. Sana ridge distribution is not controlled by the paleotopography of the pre-Holocene unconformity; ridges are apparently Holocene in origin, with their location, morphology and internal facies geometry controlled by the present hydrodynamic regime. Due to the microtidal regime of the Alabama EEZ and the prevalence of the graded sands on the ridge crests, the ridges are interpreted to be dominantly storm-wave in origin. This type of coarse, clean deposit is a poorly studies yet important possible model for many shelf sand petroleum reservoirs. End_of_Record - Last_Page 511-------

Sedimentology and Stratigraphy of Tidal Sand Ridges Southwest Florida Inner Shelf

ABSTRACT Detailed investigation of linear shelf sand ridges located off the southwest coast of Florida shows them to be tide-dominated sand bodies. These ridges are remarkably similar to the large sand ridges of the North Sea, and they have abundant apparent analogs in the stratigraphic record, many of which are important petroleum producers. The Florida ridges are asymmetric in profile, about 10 km long, 1 km wide, with relief of 3-4 m with the adjacent sea bed. Extensive tidal current monitoring, sediment distribution patterns and side scan sonar surveys permit characterizing their morphodynamics. Tidal currents show distinct bidirectional patterns with speeds up to 70 cm/s. There is slight flood-dominance, and currents show much higher velocities in the troughs as compared to the crests of he ridges. Megaripples and sand waves are widespread and migrate obliquely across the ridges at opposite directions on the gentle and steep sides of the ridge. Shallow, high-resolution seismic data and 39 vibracores in the area of the ridges show a consistent sequence characterized by three ascending Holocene lithofacies: 1) muddy quartz sand with limestone clasts; 2) bioturbated muddy shelly quartz sand; and 3) well-sorted, cross-stratified quartz sand that characterizes the sand ridges themselves. Each of the tidal sand ridges displays a coarsening-upward sequence of fine, well-sorted sand. Small-scale, multidirectional, cross stratification dominates the stratigraphy of the cores in this facies, but megaripple(?) cross stratification is also present. All data indicate that these tidal ridges are good modern analogs for many of the shelf sand bodies in the ancient record, especially the Mesozoic of the mid-continent area.

Ichnofabrics containing Ophiomorpha: significance in shallow-water facies interpretation

In an attempt to interpret Ophiomorpha ichnofabrics observed in core, three ichnofabrics are described from outcrops where O. nodosa is a conspicuous element. These ichnofabrics enable sandy shoreline sedimentary environments to be characterized and differentiated: (1) shoreface with mottled- Ophiomorpha—Planolites ichnofabric generally without primary lamination; (2) offshore tidal shelf sand wave facies with Macaronichnus-Ophiomorpha ichnofabric associated with primary, mainly cross-laminated or cross-bedded sands; (3) estuarine facies with Ophiomorpha ichnofabric associated with primary lamination and, commonly, heterolithic sands and mudstones. Distinctions between the ichnofabrics are attributed to differences in primary stratification, the total ichnocoenoses, morphological features (such as burrow attitude, shaft restriction, pellet wall lining), to the nature of the substrate and, particularly, to the time available for colonization (larval settlement or relocation) and burrow construction, referred to here as the colonization window The analysis is applied to an interval of core (Upper Jurassic, Central Graben, North Sea) and a sequence in Eocene sediments in southern England.

Quaternary evolution of the Abu Quir bay, Egypt

The Pleistocene/Holocene history of Abu Quir bay and the adjacent shoreline has been studied using textural, petrological and geotechnical information obtained from 33 boreholes. The sedimentary vertical sequence is as follows reading from bottom to top: Late Pleistocene shelf sand and stiff mud, Late Pleistocene/Holocene transgressive sand, Holocene calcareous shelf mud, Holocene nearshore sand, prodelta mud, delta plain lagoonal and marsh mud, delta front mud and sand and coastal sand of beach and dunes. These units are produced as a response to shoreline fluctuation, resulting from a wide variety of deltaic and shelf environments. The study identifies delta lobes of the former Canopic branch which was located in the western part of the bay.

A note on the factors controlling the sedimentation rate along the western continental shelf of India

To study variations in sediment accumulation along the western continental shelf of India, sedimentation rates have been measured in two cores from the shelf off Mangalore. The rates obtained are 0.72 and 0.56 mm/yr for water depths of 35 and 45 m respectively. These are significantly lower than those of the northern continental shelf (north of 18°N) where sedimentation rates range between 1.8 and 19 mm/yr. The low rates in the southern area are due to the low concentrations of suspended particulate matter (SPM) coming from rivers such as the Netravati and Gurpur that drain into the study area. These rivers contribute only about 8% of the total discharge of all the west-flowing rivers of the Indian peninsula, whereas rivers flowing onto the northern continental shelf have high SPM concentrations and contribute about 45% of the total discharge of the west-flowing rivers. The occurrence of outer shelf sands ranging in age from 9000 to 11,000 yrs B.P. at various depths below mean sea level in the northern and southern parts of the western continental shelf may be indicative of neotectonic activity, and the presence of such activity is supported by the occurrence of recent earthquakes and hot springs in Western India, the northward compression of the Western Indian continental margin, coastal geomorphic features and epeirogenic movement of the West Indian landmass. However, it could also be the result of differing influxes of terrigenous sediment and of riverine particulates travelling over different distances into the Arabian Sea.

Forced Regressions in a Sequence Stratigraphic Framework: Concepts, Examples, and Exploration Significance (1)

Sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level. On the shelf, lowstand deposits, which form during falls and subsequent stillstands of relative sea level, can be recognized by the presence of an unconformity at the base, the isolated and basinward position relative to the previous shoreline, and the abrupt seaward translation of shallow-water and shoreline facies into the basin across an unconformity surface. This seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a regression. A forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf (i.e., accommodation). Forced regressions commonly are associated with a zone of sedimentary bypass, subaerial exposure, and possible fluvial erosion between the newly formed and preceding shorelines. Certain shelf sands, previously interpreted as offshore or mid-shelf sand bodies, thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions. This alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields. Examples of forced regression can be observed at a variety of scales and ages. Several such examples include the modern East Coulee fan delta and the Lower Cretaceous Viking Formation in Alberta, Canada, the Quaternary Rhone Delta, and the Quaternary Hudson Valley system.

Typical Pleistocene Oil and Gas Plays-Offshore Louisiana

ABSTRACT In south and offshore Louisiana, regional surface and subsurface studies from the eastern part of the High Island Area through the western part of the south Timbalier area show that Pleistocene sediments from the Mississippi, Red, and Sabine River systems can be recognized and mapped for exploration purposes. The Pleistocene section has been divided into eleven stratigraphic units, the oldest of which can be traced to the outcrop of the High Terrace in Louisiana and Mississippi. The deposition of clastic sediments in each unit is related closely to a major cycle of sea-level change with the development of an erosional unconformity overlain by a coarse-grained substratum and a fine-grained topstratum. Each unit contains deltaic, shelf, and slope sand bodies mapped along depositional axes for the three river systems. The distribution of sand along these axes constitutes prospective trends for petroleum exploration from nearshore out to sea floor water depths of greater than 3000 feet on the modern continental slope.

Differentiating between Shelf and Shoreline Sand Bodies by the Use of Paleocurrent Information

Differentiating between Shelf and Shoreline Sand Bodies by the Use of Paleocurrent Information

Variation in Diapiric Structure Development and Productivity, Northern Gulf Coast Basin

ABSTRACT Paleo-geologic (palinspastic) reconstructions of salt dome development are used to compare growth patterns and processes characteristic of different salt dome classes, to understand diapiric processes, and for salt structure evaluation prior to drilling. Coastal Salt Basin diapiric structures (salt domes and clay [shale] domes) can be classified by the diapir top/objective section relationship (continental shelf sands and shales and sandy upper slope strata comprise the objective section) as: Penetrant (or shallow piercement) salt domes where salt pierces the entire objective section and the overlying non-objective alluvium; semi-penetrant (or intermediate piercement) salt domes where salt pierces part of the objective section and arches the overlying beds; and non-penetrant (or deep-seated) diapiric structures where the diapir (salt or clay) is buried in upper slope shales beneath the arched objective section base. Cross-sections from 44 published salt dome field studies were palinspastically restored to illustrate developmental patterns. They represent 15 penetrants, 8 semi-penetrants and 21 non-penetrants. All fields are in the Eocene, Oligocene, Miocene or Pliocene producing trends of southeast Texas and south Louisiana. Penetrant salt diapirs maintain their crests close to the depositional surface. Internal convergence (thinning of flanking strata toward the diapir) is very common, as are major growth faults and deep flank unconformities. Traps form early and are preserved. Semi-penetrant diapirs include more diapiric clay than penetrants. All were buried before alluvial deposition commenced. Major, downthrown-away, diapir-peripheral faults and unconformities are characteristic, and internal convergence is common at depth. Traps form early, but some lose critical dips later. Non-penetrant diapiric structures develop much differently. The diapirs were already buried and inactive in slope facies shales when objective section deposition commenced. Differential flank subsidence, modest internal convergence and growth faulting characterize development. Effective traps sometimes form late and critical dips may be reduced or lost, reducing productivity. Salt dome development in the Mesozoic-filled interior salt basins is very different. Penetrant diapirs are common. Most are slim salt cylinders, some with circumdomal overhangs. Many exhibit internal divergence (thickening of strata toward the diapir) in part of the section, and productivity is limited or absent. Evidence of semi-penetrants and non-penetrants is limited. Most salt-associated production in the interior basins occurs on non-diapiric salt pillows or on salt-flow-related turtle structures. Differences in salt dome development patterns between the coastal and interior basins relate to basin type and depth, depositional rate, the presence or absence of thick marine shales, abnormal pressures, and large growth faults.

A High-Resolution Local Sea level Curve From Detailed Mapping of the Wales Sand Interval (Wilcox Group, Paleogene), LaSalle and McMullen Counties, South Texas

ABSTRACT Eight sands, B1 through B8, from oldest to youngest, comprise the Wales sand interval in LaSalle and McMullen Counties, South Texas. B1 and B2, the lower and upper Wales of industry are the most widespread sands and make up much of the approximately 150 feet of the Wales interval. B8, which caps the sequence and is probably equivalent to the Bartosch sand along strike to the northeast, has the most limited distribution and is the thinnest of the eight sands (<10). The lower Wales (B1) strandplain extends basinward across the shelf almost to the growth-fault zone. The upper Wales (B2) also extends down paleoslope almost to the shelf edge. Following deposition of the upper and lower Wales, the shoreline stepped back some 15 to 20 miles to the northwest, and shelf muds accumulated across McMullen County. Stacked shoreline sands (B3-B5) in LaSalle County are higher on paleoslope than B1 and B2 sands. Sand carried from the northeast by shelf currents was deposited in McMullen County while the B5 shoreline stood in LaSalle County. The shoreline continued to retreat to the northwest and deposition of shelf muds took place over most of the two county area except in northern McMullen County where sands (B6-B8) preserve the advance of shelf sands from the northeast. Changes in local relative sea level in the Wales sand interval can be determined by mapping the individual sands, but not from the stacking patterns seen on individual well logs. The sea level curve shows an overall rise in relative sea level, with successively younger parasequence sets deposited farther landward in a retrogradational pattern, punctuated by periods of relative stillstand during which the parasequences within each set stack in an aggradational pattern.

Surface and near-surface sediments from the continental shelf off the Russian River, northern California

There are two basic types of detrital sediments in the study area: modern and relict. Modern sediment consists mostly of sand on the inner shelf, and silt and clay on the middle shelf. The silt and clay overlie an older transgressive sand and extend northwest-southeast with an average thickness of 10 m. This unit pinches out toward the outer shelf, where relict detrital sand dominates with some authigenic glauconite. The detrital sand is much cleaner and darker than the inner shelf sand. The important heavy minerals of the sand fraction for detecting sediment dispersal patterns are biotite, hornblende, glaucophane and glauconite. Biotite and hornblende are abundant in the southern parts of the study area close to Point Reyes Beach, and decrease in abundance northward and offshore. Glaucophane is abundant close to the Russian River mouth, and decreases in abundance southward. Glauconite is abundant on the outer shelf, and decreases in abundance southward. There are two sediment sources for the inner and middle shelf: Russian River discharge and coastal erosion, particularly of beaches, headlands and cliffs in the southern part of the area. The sand-sized sediment of the Russian River is being deposited on the inner shelf between Salt Point (to the north) and Bodega Head (to the south). The Russian River silt and clay are confined to the middle shelf. Some of this silt and clay is being transported into Bodega Submarine Canyon. The coastal material is eroded in the south and transported northward, seaward and also into Bodega Submarine Canyon.