Offshore Facies Research Articles

Depositional Patterns of a Silurian Shelf Sand in Central Appalachians: ABSTRACT

The Keefer Sandstone in the subsurface of West Virginia and Kentucky, where it produces small volumes of natural gas, was deposited on the western shelf of the Appalachian basin. The formation is a dolomitic quartzarenite and quartzwacke and ranges from 13 to 50 ft (4 to 15 m) thick. As interpreted from four cores and isopach and lithofacies maps, the sand was transported by longshore currents from a southeastern source area and laid down in a variety of environments. In Kentucky, Keefer deposition was on a wave-dominated coast. The rocks are characterized by physical sedimentary structures and textures which developed under high-energy conditions, and the formation is divided into shoreface and foreshore facies. In adjacent West Virginia, however, Keefer deposition occur ed offshore in water near the depth of wave base, swept out onto the shelf by storm-generated currents. Sedimentary structures and textures indicate a lower energy environment, bioturbation is more common, and the formation contains a greater amount of shale. The isopach map shows two linear tracts of thick sand in this offshore facies that may represent a coalescence of subtidal bars on the shelf. Between major sand bodies, the Keefer becomes appreciably thinner and is interbedded with fossiliferous dolomite. In the basin center of West Virginia, sandstone is replaced by shale. Thus, the geographical distribution of lithofacies shows a transition of shelf environments within a blanket sandstone. In all cores, regardless of depositional facies, the sandstone displays evidence of aggradin sedimentation; sedimentation exceeded subsidence, and the sand body built upward into shallower water. End_of_Article - Last_Page 1175------------

The Devonian System in China

SummaryThe Devonian System of China occurs in the Tianshan–Hingan* Geosyncline of North China, the Yangtze Paraplatform of South China and in the geosynclinal areas of West China. Devonian formations are entirely absent from the Sino-Korean Paraplatform and the Tarim Platform, but along the border of the Tarim Basin and the slopes of Qilianshan (Chilienshan) are continental sequences rich in vertebrate and plant fossils. On the basis of differences in biostratigraphic characteristics the Devonian is divisible into 8 regions: 1. Junggar–Hingan (Dzungar–Khingan), 2. South Tianshan (South Tienshan), 3. Qilianshan (Chilienshan), 4. Longmenshan (Lungmenshan), 5. South China, 6. South-eastern China, 7. West Sichuan–North Xizang (West Szechuan-North Tibet), 8. Himalaya–West Yunnan.Using both lithology and fossil content, five principal facies (types) may be further differentiated: 1. Hingan type (eugeosynclinal facies), 2. Baoxin type (miogeosynclinal facies), 3. Xiangzhou type (platform near-shore facies), 4. Nandan type (platformal off-shore facies), 5. Cuifengshan (Chuifeng shan) type (continental facies).The Devonian of South China occurs in various facies, but has a well-defined principal stratotype. It has been divided by Chinese geologists into eight stages, two in the Upper Devonian, two in the Middle Devonian and four in the Lower Devonian.The Devonian sequences of the geosynclinal regions of West China are characterized by facies which contain graptolites and tentaculites, and have largely been affected by regional metamorphism. The Devonian biota of the Junggar–Hingan Region is essentially an endemic one, but it is associated with a few European and North American elements. Marine pyroclastics are the principal rocks. General stratigraphical columns of those regions are given with a correlation with other parts of China. The continental Devonian of China is unique especially in the light of its vertebrate content. Red sandstones are widespread in the Lower Devonian, and intercalated with argillaceous limestone and mudstone. This sequence is rich in endemic East Asiatic faunal elements including agnatha (Galeaspida, Polybranchiaspida), Antiarchi, etc. Middle and Upper Devonian lithostratigraphic units consist principally of light coloured quartzose sandstone formations in Southeastern China, with some red beds in South and North China, while their fauna and flora are cosmopolitan in character. Their exact geological ages vary, however, as clearly shown in the South China region, especially towards the top of the Upper Devonian (with Asterolepis) and in the early Middle Devonian (with Bothriolepis).

Wave-, Tide-, and Current-Controlled Sandbodies of Miocene Molasse, Western Switzerland

The Miocene (Burdigalian) clastic sediments of western Switzerland were deposited in a wide variety of environments ranging from fan deltas to offshore coquina banks. Conglomerates and sandstones of the proximal fan-delta facies were deposited in distributary channels that were influenced by tides to varying extents. Tidal sand waves exhibiting bipolar paleocurrent patterns and intertidal sand flats are the most conspicuous features of the coastal facies belt. The nearshore facies is composed of thick, sandy subtidal shoals built of current-generated megaripples, intershoal swales containing horizontally stratified fine sandstones and mudstones, and swash bars or flood ramps with low angle accretionary bedding. The offshore facies belt contains very large coquina banks ashioned by vigorous (? tidal) currents and reworked by fair-weather waves. Wave power, calculated from preserved ripple marks, indicates a moderate-energy sea of limited fetch, perhaps 100 km. The ratio of wave power to reasonable estimates of discharge effectiveness index suggests wave-dominated rather than fluvial-dominated deltaic systems. Additionally, comparison of wave power with interpreted tidal range (microtidal to mesotidal) indicates strong influence from both tides and waves but little influence from river outflow. The predicted association of sand-filled distributaries and laterally persistent nearshore features is in agreement with the facies described from the Burdigalian Molasse of the Fribourg area, western Switzerland.

Sedimentary Facies Relations and Inferred Dynamics of a Single-Barred Nearshore Environment, Atlantic Coast of Eastern Long Island, New York: ABSTRACT

Facies relations in a conglomeratic single-barred nearshore environment on the glaciated Atlantic Coast of eastern Long Island, New York, were determined by examination of 42 can cores, bed-form distributions, and sediment textures. Two principal subtidal zones are recognized. The shoreface zone (0 to 10 m water depth) is composed of clean fine to coarse sand and consists of the planar parallel to megaripple-laminated upper shoreface facies, the massively bedded longshore trough facies, and the megaripple to parallel-laminated longshore bar facies. The inner shelf zone (> 10 m water depth) is composed predominantly of organic-rich fine sand and consists of the parallel-laminated transitional facies and bioturbated offshore facies. Scattered throughout this zone are coarse sand outcrops of megaripple-laminated inner shelf lag facies. By deployment of dye tracer and by inspection of bed forms in the longshore trough (during low wave-energy, fair-weather conditions) little evidence of longshore or seaward flow could be found. Landward migration of bar deposits over trough deposits was observed over a 6-week period during fair-weather conditions. A core in the trough facies adjacent to the distinct bar-trough contact revealed an underlying bar facies. An equilibrium model of near-continuous fair-weather landward migration of the bar facies interrupted by high wave-energy pulses of seaward movement of trough sediment is believed to account for the stable position of the longshore bar. End_of_Article - Last_Page 783------------

Morphological variability and temporal change in a Paleocene lucinid bivalve mollusk

Analysis of morphological variability, using six characters, in four stratigraphically successive popula­tions of the lucinid bivalve mollusk Myrtea (Gonimyrtea) uhleri shows a unidirectional trend toward decreasing mean size. Differences in mean length for stratigraphically adjacent populations are significant at the 5% level of probability. Chronoclinal trends toward decreased magnitude of apical angle meas- urements and of increased magnitude of two form ratios associated with apical angularity can be ex­plained by a tendency toward increase with growth in the quadrateness of the shell. Thus, populations of smaller mean size are also on average more trigonal. The observed decrease in mean size is associated with marine regression in the study area (Paleocene Brightseat and Aquia Formations of Maryland and Virginia, U.S.A.). Earlier representatives of the species are more common in an offshore facies, and later ones are more common in a nearer-shore but still subtidal one. Comparing specimens from nearshore and offshore facies without regard to stratigraphic position shows a statistically significant difference in size; the nearshore specimens are nearly 20% smaller, on average, than the offshore ones. Possible contributors to the size decrease may have been a slowdown of growth rate in the physically more· rigorous nearshore facies; an increased probability of mortality at earlier ages in episodic sedimentation events, expected to occur more frequently nearer shore; or selection for accelerated maturation in more rigorous and unpredictable environments.

Devonian lacustrine limestones at the margin of the Orcadian Basin, Scotland

A complex overlap of middle Old Red Sandstone occus in N. Caithness and Sutherland. The relief of the underlying surface of Moine metamorphics and intrusions is irregular. The facies variation of the sediments reflects lacustrine transgression and regression within the Orcadian Basin. When the lake level was high and the lake margin coincided with the basin margin, thick ( c. 4m) carbonates were deposited on the unconformity. Carbonate deposition was controlled by photosynthesizing algae. At such sites the depositional environment was oxygen-rich and the carbonates do not contain biotite, organic carbon or pyrite. The offshore facies was deposited in deeper stratified lakes, anoxic at depth, and comprises organically-enriched sediments (with pyrite and biotite). When the lake margin was not coincident with the basin margin alluvial sediments mantled the unconformity as rivers cut to a lower base level. At marginal sites compaction of the lacustrine sediments over the irregular basement during periods of low lake level, when no deposition occurred, produced local unconformities.

Calcareous Nannoplankton of Salt Mountain Limestone (Jackson, Alabama): ABSTRACT

A sample of the Salt Mountain Limestone (Paleocene) from Jackson, Alabama, has yielded a diverse nannoplankton flora. Preservation differs greatly between species. Some forms are well preserved whereas others have been subjected to extensive dissolution and/or recrystallization. Placoliths generally are well preserved, but the centers of many specimens are obscured by pelatoid overgrowths. Many specimens also bear sparry extensions of isolated shield elements. The presence of Discoaster gemmeus, and the absence of recognizable specimens of Discoaster nobilis and Heliolithus riedeli, places the Salt Mountain Limestone within the Discoaster gemmeus Zone. This suggests that the Salt Mountain Limestone is older than the Nanafalia Formation which has been placed in the Heliolithus riedeli Zone. Toulmin suggested that the Salt Mountain Limestone was an offshore facies equivalent of the Ostrea thirsae beds of the Nanafalia Formation. End_of_Article - Last_Page 2214------------

Depositional Models from a High-Energy Coast: ABSTRACT

The 2 most common depositional systems along the coast of southern Oregon are the nonbarred nearshore and the longshore bar-rip channel systems. Detailed observation of these systems, largely by scuba diving, has led to delineation of facies of sedimentary structures and recognition of their genetic relation to physical processes operating in the high-energy coastal environment. A 3-dimensional analysis of the geometric interrelation of the facies within each system permits construction of progradational depositional models that can be used to identify deposits of similar origin in the stratigraphic record. In the most simple depositional system, the nonbarred nearshore, the bottom profile extends smoothly seaward into deeper water. Progradation of the facies in this system will produce a distinctive vertical sequence. At the base lies fine sand of the offshore facies, which shows landward-dipping ripple lamination and scour-and-fill structure. The upper part of this facies is likely to contain lenses of crossbedded coarser sand in which foresets dip landward. The facies grades upward into predominantly cross-stratified beds formed by different facies of the surf zone. The upper crossbeds are likely to be gravelly and inclined seaward. They are overlain by planar-bedded swash facies. Progradation of the longshore bar-rip channel system produces a different sequence. An erosion surface separates the offshore facies from overlying rip channel facies in which crossbedding dips seaward. Above the rip channel facies lies a gradational sequence of longshore trough facies, wave-current complex facies, and, at the top, swash facies. In an actual situation, the bar facies, in which cross-stratification dips onshore, may be preserved locally within the deposit. End_of_Article - Last_Page 609------------

The Tully Clastic Correlatives (Upper Devonian) of New York State: A Model For Recognition of Alluvial, Dune (?), Tidal, Nearshore (Bar and Lagoon), and Offshore Sedimentary Environments in a Tectonic Delta Complex

ABSTRACT A spectrum of sedimentary environments, including alluvial, tidal, dune (?), nearshore (bar and lagoon) and offshore, existed side by side in central New York State during early Late Devonian time. These environments, part of the great Catskill deltaic complex, are represented in the stratigraphic record by the Tully Limestone and its eastern clastic correlatives. This very limited interval of the rock record was studied in order to develop associations of criteria for recognition of sedimentary environments in ancient tectonic delta complexes. Recognition criteria include sedimentary structures. lithology, geometric relationships of rock units, and character of biologic content. The Tully Limestone and associated strata are subdivided into four lithosomes. Lithosome A is the thinned eastward extension into the study area of the Tully Limestone. The unit consists of argillaceous calcilutite and sandy biocalcarenite. Lithosome B, which lies directly above Lithosome A and is co-extensive with the tongue-shaped eastern extension of the Genesee Shale, consists of very dark fissile shale grading eastward into shaly siltstone. Lithosome C forms a massive clastic wedge which in cross-sectional view envelops Lithosomes A and B. It is comprised of interlensing gray siltstone and slightly lighter gray, very fine-grained sandstone which contains flow-rolls, trace fossils, coquinite lenses and, at the eastern margin of the lithosome, fossil seed-ferns. Lithosome D consist of red and green siltstone and mudstone with interbeds of gray, fine to medium grained, texturally very immature sandstone. The spectrum of Tully interval sedimentary environments includes sandstone bodies of alluvial channel origin which truncate underlying beds, contain basal shale-pebble lag-concentrates, are well trough cross-bedded, texturally immature, and display a fining-upwards. The alluvial strata of overbank origin are horizontally laminated, red and green siltstones. Strata of the marsh facies consist of black organic lenses containing abundant plant remains some of which are coalified. At the distal margin of the alluvial plain, just below the Tully interval, a swamp environment is represented by three levels of abundant fossil seed-ferns. Sedimentation that resulted in strata of tidal origin within the Tully interval was of the Wadden-type. The tidal flat facies consists of gray, very finely cross-laminated muddy siltstone and very fine-grained sandstone, which contain allochthonous brachiopods and locally well developed mud-cracks. Sedimentary structures of the tidal channel facies are essentially identical to those of the alluvial channel facies, but can be distinguished by the unique character of the basal lag-concentrate, which is coquinite or coquinoid pebble conglomerate consisting largely of allochthonous brachiopod shells. Strata of the nearshore facies consist of thick bedded bar sandstone bodies interbedded with very thinly bedded lagoonal siltstone which is rich in wood fragments. The offshore facies consists of very thinly bedded and laminated, fossiliferous siltstone that becomes increasingly calcareous westward and grades into the very argillaceous eastern extension of the Tully Limestone. Well developed trends of change in texture, general biologic character, and type and scale of sedimentary and biologic structures are present in both the nearshore (bar and lagoon) facies and the offshore facies. Synthesis of the environmental pattern of the Tully interval indicates that (1) Tully sedimentation occurred during the transgressive phase of a transgressive-regressive cycle, (2) terrigenous material was trapped east of a submarine topographic high, thus permitting deposition of carbonate material in a basin that was for the most part being overwhelmed by clastic influx, and (3) landward migration of the strandline during the transgressive phase caused river mouth drowning and resulted in more widespread estuarine (tidal) conditions than were present immediately prior to and following Tully time. In addition, the transgression raised the base level of streams that were flowing westward across the deltaic plain, thus causing alluviation of fine-grained sediment in quantity greater th n that deposited directly preceding and following Tully time.

A faunal analysis of the Lias (Lower Jurassic) of South Wales (Great Britain)

Detailed analysis of faunal and petrological variations in the Lias, South Wales, proved useful for interpretation of regional paleoecology. The most significant factor that influenced most Lias genera was the texture of the substratum which became increasingly stabilized with increasing quantities of intermixed bioclastic debris. Bottom turbidity was of secondary importance to faunal distribution and was indirectly related to the substratal stability; bottom stability was in turn influenced by the abundance of burrowing organisms and the rate at which bottom sediment was loosened by biological reworking. Abundant collophane and pyrite resulted in a decline of fossil populations and indicate that the Lias sea floor had a low oxygen content; a dearth of collophane suggests aerating and winnowing currents so strong that phosphates were not preserved. Current-transported lithoclasts in the Lias offshore facies reflect currents sufficiently strong to transport fossil debris and to intermix level-bottom marine communities. The widespread distribution of fossil species throughout rocks of varying lithology in both the Lias nearshore and offshore facies reveal a high degree of faunal tolerance to environmental change.

Ecological distribution of Devonian Atrypid brachiopods

In the shallow Devonian shelf seas of northwestern Europe, individual groups of atrypid brachiopods belonging to the families Atrypidae and Palaferellidae are distributed along narrow, separate parallel bands which probably roughly followed ancient shore-lines. Each brachiopod group is associated with particular bottom sediments and frequently also with certain types of corals. Seven different associations or biotopes of atrypid brachiopods, ranging from a littoral (Rhenish) to an off-shore (Hercynian) facies, are outlined. For select Middle Devonian time intervals, the areal distribution of such biotopes in the Eifel region of Germany is plotted.

Paleogeography Applied to Exploration: ABSTRACT

Paleogeography, the mapping of seas, shorelines, rivers, deltas etc., at some time in the past, is a valuable tool in exploring for reservoir rocks in the Cretaceous section of the Rocky Mountain region. Paleogeographic mapping is accomplished by means of stratigraphic studies to determine two essential factors--environment of deposition and correlation. Classification of the Rocky Mountain Cretaceous rocks into four broad facies (Inland, Shoreline, Sandy Offshore, and Shaly Offshore) is a useful device which aids in the study of the complex intertonguing of varying lithologies. An understanding of shoreline movements (transgression and regression) is the key to correlation in the Cretaceous section. The End_Page 112------------------------------ Cretaceous sequence of most of the Rocky Mountain region can be considered to be the result of superimposition of many paleogeographic settings. Analysis of a hypothetical time-stratigraphic unit shows the variation in location, shape, and size of potential reservoir rocks deposited in various paleogeographic locations. In one example, correlation of an inland facies and an offshore facies suggests an intervening shoreline facies. Paleogeographic considerations are fundamental in stratigraphic analysis and consequently paleogeography is the base on which stratigraphic exploration is built. End_of_Article - Last_Page 113------------

A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan

The rocks of the Blue Lias in Dorset and Glamorgan can be divided into off-shore and near-shore facies. The off-shore facies has a characteristic pattern of regular, small-scale alternations of argillaceous calcilutites, marls and sometimes bituminous shales, the main variable being CaCO3content. Two distinct types of limestone are recognized and termed laminated and normal limestone, respectively. The marls are generally comparable with the limestones in all respects except their much lower CaCO3content. The bituminous shales are rich in bituminous matter which is arranged in fine laminae parallel to the bedding. Determinations of percentages of insoluble residues reveal a consistently large difference between the limestones and marls. Fuller chemical analyses of major constituents indicate, among other things, that nearly all the carbonate is present as CaCO3. The clay mineral content consists almost entirely of illite, with subsidiary kaolinite. The strontium content of the carbonate fraction of the marls appears to be markedly higher than that of the limestones. Vertical variation in Dorset, Glamorgan and Somerset is studied and compared by a graphical method based on the limestone-shale ratio. It has been established that the Blue Lias rhythm is primary in origin, but that there has also been a limited amount of early diagenetic segregation of CaCO3to produce nodular structures. The limestone textures are accounted for by recrystallization from an original lime mud and the respective importance of several processes including drusy and grain growth and granular and rim cementation assessed. Pyrite is considered to have been formed early in diagenesis under anaerobic conditions within the sediments. Its association in some drusy cavities with calcite is explained as due to the local fall in pH of interstitial fluids. The difference between the normal and laminated limestones and marls is considered to be the result of aerobic and anaerobic bottom conditions, respectively. The microlaminae in the bituminous shales are interpreted as varves due to the annual fall of plankton into anaerobic bottom waters. Evidence is put forward that the Blue Lias rhythm may be the result of repeated epeirogenic oscillations. Rocks of the near-shore facies are confined to Glamorgan. They include (besides calcilutites and subsidiary marls) skeletal limestones, oolites, conglomerates and cherty beds; locally the rocks lie unconformably on Carboniferous Limestone. Silica is found in the form of bands of nodules and silicified limestone pebbles and shells. The facies relationships of the different rock types can be satisfactorily related to the approach of an old shoreline. The silica was almost certainly derived from detrital chert weathered from the Carboniferous Limestone. Although there is a broad similarity in the fauna between Dorset and Glamorgan, a number of important differences are recognizable. Differences between the off-shore and near-shore facies are also described; whereas the former has pelecypods and ammonites as its most conspicuous elements the latter is notable for the abundance of corals and gastropods and, locally, of ribbed pectinids. A relationship between the fauna and the sediments is recognized in three cases: (1) shell enrichment in condensed beds (with glauconite and/or collophane); (2) dwarfing and general faunal impoverishment in the laminated rocks, related to poor aeration of the sea bottom ; and (3) variations in sedimentary rate and depth of sea probably account for the faunal differences between Dorset and Glamorgan in the off-shore facies. On the other hand, no relationship can be perceived in three other cases: (1) the increase in size up the succession in a number of forms, which is evolutionary; (2) the succession of different organisms due to ecological replacement and extinction; and (3) certain shell enrichments which may be due to population fluctuations. In a summary of the Blue Lias environment deductions are made about temperature, salinity, rate of sedimentation, depth of sea and current strengths.