Distributary Mouth Research Articles

Petrology and Depositional Environments of Lower Tuscaloosa Formation (Upper Cretaceous) Sandstones in North Hustler and Thompson Field Areas, Southwestern Mississippi: ABSTRACT

ABSTRACT The lithology, diagenesis, and depositional environments of the Dykes, Denkman, and McComb sands of the Stringer Sand Member of the Lower Tuscaloosa Formation in the North Hustler and Thompson field areas of Amite County, Mississippi, have been determined by detailed description of cores from nine wells and examination of 61 thin-sections. The cores were recovered from driller depths of about 11,100 ft in the updip Tuscaloosa trend, which is being actively explored for stratigraphically trapped oil. Most of the sandstone samples (65%) are quartz arenites, and the remainder are sublitharenites: both types consist dominantly of fine- to very fine-grained sand. The average sandstone consists of 67.3% framework grains, 14.8% matrix, 9.2% cement, and 8.7% pore space. Monocrystalline quartz is by far the most abundant framework grain (88.7%), with polycrystalline quartz (6.6%), potassium feldspar (0.3%), plagioclase (0.1%), chert (1.9%), igneous (mostly volcanic) rock fragments (0.5%), metamorphic rock fragments (0.2%), and sedimentary rock fragments (1.5%) also present. The matrix for the most part consists of pore-filling clays, and includes organic matter and secondary pyrite. Quartz and ferroan dolomite (ankerite) are the most common cements; high concentrations of siderite and calcite occur as cement within some thin layers, and siderite is also found commonly as small concretions. Porosity is mostly secondary, resulting from partial dissolution of framework grains and cement. Quartz overgrowths probably formed during an early stage of diagenesis, followed by deposition of calcite cement which corroded and embayed quartz and other framework grains. Apparently the calcite was later replaced by ankerite. Late-stage pore-filling clays include vermicular kaolinite and chlorite, the latter occurring as rims and in dissolution-enlarged pore space. The depositional environment of the Dykes and Denkman sands in all wells is interpreted to be fluvial; stacked point bars are obvious in one well. The McComb sand, best developed in a wildcat well between Thompson and McComb fields, is marine, based on the occurrence of glauconite and shell fragments. The presence of a basal scour surface on shale indicates that this sandstone may be the product of marine reworking of a transgressed distributary mouth bar.

Palaeochannels in mine workings in the High Hazles Coal (Westphalian B), Nottinghamshire Coalfield, England

Palaeochannels have numerous deleterious effects on coal mining including the formation of ‘washouts’, and their detection and delineation in the subsurface is of paramount importance. A palaeochannel system in the roof of the High Hazles Coal (Westphalian B) at Gedling Colliery, Nottinghamshire, is described. The roof strata of this seam are interpreted as the deposits of a lacustrine delta complex formed on an upper delta plain, with the palaeochannels constituting minor deltaic distributaries. The lacustrine delta front deposits consist of a coarsening-upwards sequence of siltstones and sandstones, which form a sheet-like body of coalescing distributary mouth bars that were laid down by friction-dominated effluents. The distributary channels were filled by a variety of processes, giving rise to complex channel deposits. Bedload deposition of sand on bars, collapse and sloughing of channel banks, and deposition of silts and muds from suspension were important mechanisms. The nature of the deposits reflects the mode of channel abandonment, rather than equilibrium channel processes. Overbank flood deposits and prodelta/lacustrine mudstones are also described from the High Hazles roof. Based on this study, criteria are suggested that may enable the successful detection and delineation of palaeochannels in mine workings elsewhere.

Sedimentary environments of the Simla Group (Upper Precambrian), Lesser Himalaya, and their palaeotectonic significance

The Simla Group of the western Lesser Himalaya comprises a prograding muddy delta sequence that was built by the collapse of a carbonate shelf during Late Precambrian time. The Simla sequence consists of a thick, about 1500 m, coarsening-upwards clastic succession which is divisible into the Basantpur, Chhaosa and Sanjauli Formations. Carbonate appears only in the basal formation (Basantpur Formation). Three subfacies for the basal Simla have been discerned: a carbonate platform to the northeast (Basantpur subfacies), shallow water with more clastic influence to the south (Kunihar subfacies) and relatively deep water adjacent to a fault scarp to the northwest (Kashlog subfacies). The first signs of deltaic sediments appear in the middle of the Basantpur Formation and are well developed in the overlying Chhaosa Formation. The overall characteristics of the Chhaosa facies are those of a delta-front and distributary mouth bar where floods from distributaries and shelf storms lead to very rapid deposition, mostly above wave base. The uppermost Sanjauli Formation is relatively thick bedded, coarse grained and more frequently cross-bedded representing delta platform and distributary channel deposits. A significant change in environment of deposition is observed in the overlying beds, previously known as the Upper Sanjauli Member, and now being separated as the Jagas Formation. The Jagas facies suggests major uplifts and a climatic change to semi-arid conditions. The detritus of the Simla delta came from the Indian Shield situated to the south of the delta. Most constituents indicate mature sediments with occasional lithoclasts of sedimentary and intraformational characters. Faulting presumably continued during the deposition of the Simla Group as is characteristic for rifted continental margins.

Origin, Characteristics, and Provenance of Plant Macrodetritus in a Holocene Crevasse Splay, Mobile Delta, Alabama

Lower delta plains are primary sites for the deposition, accumulation, and preservation of macrodetrital plant parts. Chacaloochee Bay, an interdistributary estuary in the Mobile Delta, Alabama, is being infilled actively by crevasse-splay sedimentation. Within this system are a variety of sites in which plant macrodetritus has accumulated and has the potential of being preserved. Aerial plant parts constitute laminar leaf-litter beds within various sediments, in addition to extensive accumulations of relatively pure organics (peats). The leaf-litter beds in the crevasse-channel distributary mouth bar and a detrital peat shoal have been examined with respect to the distribution of accumulated plant parts and their size fractionation. Whole leaves recovered from the sediment-water interface of the crevasse channel provide an insight as to the provenance of the accumulated plant litter. The vast majority of leaves being incorporated into the interdistributary bay sediments have not originated from the surrounding vegetational communities. Rather, most recovered leaves originated from communities existing at least 13 km north of the depositional site. The taphonomic processes responsible for the transport, deposition, and alteration of the accumulated macrodetritus include heterochronous leaf fall, fluctuations in bedload and suspension-load transport, rates of periodic flooding, redistribution by tidal and wind-generated surface currents, biodegradation, and bioturbation.

Holocene evolution of the northeastern corner of the Nile Delta

The constructive phase of the modern Nile Delta, as manifested in a 48-m section drilled east of the Suez Canal, commenced in very early Holocene times. Sands rich in marine fauna were deposited in the littoral zone and the shoreline was more than 20 km landward of its present-day position. Subsequently, clays and silts were dumped from the Nile distributaries and the marine faunal spectrum became very limited and brackish. Later in early and middle Holocene times the sediments deposited were rich in freshwater, delta-plain diatoms and pollen and in allochthonous fern spores from the tropics, indicating proximity of a distributary mouth. The middle part of the section (22.5-17.5 m) is very poor in faunal and floral remains; pollen grains from sabkha vegetation are abundant. The environment, which seems lagoonal and slightly hypersaline, is related to the sea regression in middle Holocene times. Euryhaline pelecypods, dating from about 3000 yr B.P., are abundant around the 8-m depth. Upward, there is an increase in pollen grains from sabkhas; the section is poor in diatoms and those present are mostly euryhaline and lagoonal. Allochthonous spores derived from the nearby Pelusiac Branch are abundant. Between 3000 and 2000 yr B.P. the constructive phase of the modern delta terminated and winnowed sands began accreting in front of the delta plain.

Depositional Systems and Reservoir Quality of Clinton Sandstone, Guernsey County, Ohio: ABSTRACT

The Lower Silurian Clinton sandstone is the best producer of gas and oil in eastern Ohio. Successful exploration for its subtle stratigraphic traps requires detailed knowledge of its depositional systems, which were defined by sandstone thickness maps, gamma-ray log cross sections covering a 25-mi area, log signatures, and four slice thickness maps. Two cores and some thin sections helped substantiate environmental interpretations based on logs. These deltaic deposits produce from multistory and laterally discontinuous distributary mouth bars, distributary channels, and delta-plain point bars that are predictably interrelated, each having a unique isopach pattern, log signature, and production characteristics. Distributary mouth-bar reservoirs occur in more than 70% of the wells. They have coarsening-upward log signatures and elongate isopach patterns, and are 0-22 ft thick. They also exhibit poor production, with median payout times exceeding 5 years. Distributary channel-fill reservoirs are superimposed on underlying mouth-bar deposits. They have blocky log signatures and form linear, narrow shoestrings. These reservoirs are found in 18% of the wells, and more than 70% of the wells reach payout in less than 2 years. Meander point-bar reservoirs are the best economically but are encountered in only 8% of the wells. These fining-upward, ovoid to kidney-shaped reservoirs aremore » prolific gas producers and range up to 25 ft thick. Isopach maps define and project the extent of distributary channels, but point bars, because they are thin, are not apparent and must be identified by cross sections, log signatures, and stratigraphic position.« less

Stratigraphy and Depositional Environments of Part of Pennsylvanian Pottsville Formation in Black Warrior Basin, Alabama and Mississippi: ABSTRACT

The upper part of the Pennsylvanian Pottsville Formation of the Black Warrior basin of Alabama and Mississippi is composed of cyclic sequences of conglomerates, sandstones, siltstones, mudstones, and coals, deposited in response to alternate phases of constructive and destructive deltaic sedimentation. The coal-bearing interval has been divided into seven coal groups (Black Creek, Mary Lee, Pratt, Cobb, Gwin, Utley, and Brookwood), each of which contains several coal beds. A detailed study of the three oldest coal groups, the Black Creek, Mary Lee, and Pratt, identified a variety of deltaic facies. These include distributary channel, interdistributary bay, crevasse splay, distributary mouth bar, lagoon/prodelta, and barrier island. The small, elongate deltas of the Black Creek, Mary Lee, and Pratt coal groups prograded northeastward from an orogenic source southwest of the Black Warrior foreland basin. A possible southeastern source terrain is suggested during deposition of Pratt strata. Distributary channels of the upper Black Creek and Mary Lee coal groups shifted extensively across the lower delta plain. Two general centers of deposition, one in Mississippi and one in Alabama, were maintained. Associated coals, trending northeast, accumulated in bays adjacent to the channels. In contrast, the northwest-trending coals of the lower Black Creek and Pratt coal groups were deposited in back-barrier settings during destructive phases of deltaic sedimentation. A single Pratt depocenter, located in Mississippi, is co ncident with the upper Black Creek and Mary Lee depocenter. Determination of depositional environments is based on the interpretation of geophysical log signatures and the distribution of sandstones and coals. Most of these data have been obtained from dual induction and density logs of wells drilled to underlying Mississippian reservoirs. The Pottsville sediments have not been a major petroleum objective; however, the numerous coal beds are a potential source of methane. End_of_Article - Last_Page 1447------------

SEDIMENTOLOGICAL AND PALEONTOLOGICAL ANALYSIS OF JURASSIC NORTH SEA DEPOSITS FROM DELTAIC ENVIRONMENTS

Core samples from Well 33/9‐3, Norwegian North Sea, have been analysed sedimentologically, palynologically and micro‐paleontologically. The cores cover the Toarcian Drake Member of the Dunlin Formation, and nearly the whole thickness of the Middle Jurassic Brent Formation.The Drake Member is characterized by pro‐deltaic depositional conditions. In the Brent Formation, five members are recognized (from below): the Broom, Rannoch and Etive Members represent distributary mouth bar facies; while the Ness and Tarbert Members are recognized as interdistributary bay fill sequences.The composition of the kerogen assemblages is controlled by the changing energy regimes of the different depositional facies. Terrestrial plant débris are dominant throughout the studied interval. Marine palynomorphs are present in the Drake Member and in the lower part of the Broom Member. The development of rich, arenaceous foraminiferal assemblages in the Drake Member are interpreted as a response to shallow, pro‐deltaic conditions. Calcareous foraminifera constitute only a small, accessorial part of these assemblages.

Depositional Systems of Clinton Sandstone and Petroleum Exploration, Guernsey County, Ohio: ABSTRACT

The Lower Silurian Clinton Sandstone is the most commonly drilled formation in eastern Ohio. Successful exploration for subtle stratigraphic traps requires detailed knowledge of Clinton depositional systems. Two highly constructive cratonic delta systems (Claysville and Salt Fork deltas) are present in Guernsey County, Ohio. These deltas are typical of the small deltaic complexes present along the eastern margin of the Clinton-Medina production trend. Production from these deltaic deposits occurs in multistory and laterally discontinuous sandstone bodies deposited as distributary mouth bars, distributary channel fill, and delta-plain point bars. Criteria used to define depositional environments and patterns include: (1) sandstone isopach maps, (2) gamma-ray log cross sect ons, (3) log signature, and (4) slice isopach maps. Environmental interpretations are augmented by examination of two cores and thin sections. The three types of sandstone deposits are interrelated in a predictable manner and each has a unique isopach pattern, log signature, and production characteristics. Distributary mouth bar deposits are the most common reservoirs, and are characterized by coarsening-upward log signatures and elongate isopach patterns. Distributary channel-fill deposits are the most prolific reservoirs, and have eroded into underlying mouth-bar deposits. They are characterized by blocky log signatures and linear, narrow isopach patterns. Meander point-bar deposits have fining-upward log signatures and an ovoid to kidney-shaped isopach pattern. These methods and resul s provide a visualization of paleogeography and sedimentologic processes that should be used as a guide for development of and exploration for the Clinton Sandstone. End_of_Article - Last_Page 1922------------

Progradational Sequences in Springer Formation, Ardmore Basin, Oklahoma: ABSTRACT

The transitional Mississippian-Pennsylvanian Springer Formation, exposed in the Ardmore basin of southern Oklahoma, consists of coarsening-upward progradational sequences that were deposited in the southern Oklahoma aulacogen. The unit is divided into three sandy members: the Rod Club, Overbrook and Lake Ardmore, which are separated by shale intervals. Each of the sandy members consists of one or more of the coarsening-upward sequences. A typical sequence includes from the base upward, dark gray shale with abundant siderite concretions, rhythmically interbedded siltstones and shales, interbedded burrowed sandstones and shales, and abundantly burrowed sandstones. The latter contain wood impressions, ripple cross-laminations, and occasional festoon cross-stratification. In ddition, one of the sequences contains a thin, discontinuous marine limestone. These sequences represent the transition from an offshore/prodelta setting to a distributary mouth bar/lower shoreface setting. One of the sequences in the Rod Club contains an additional lithofacies at its base which consists of interbedded shales and green-gray sandstones. Sedimentary features of the sandstones include: massive nongraded bedding, large lutite casts, ripple and dish laminations, flute casts, and numerous soft sediment microfaults. The general characteristics of the sandstones suggest deposition by sediment gravity flows. This lithofacies represents deposition in a slope setting, with the sandstones derived from the proximal delta/shoreface. Offsets on microfaults in the lower Rod Club occur on two different scales. Small scale microfaults have displacements of a few millimeters. Offset on the larger microfaults (up to 5 cm, 2 in.) is expressed on both the upper and lower surfaces of a sandstone bed. There are no fault zones within the beds which suggest the faults are synsedimentary and represent deposition on an unstable slope. The microfaults are consistently oriented approximately 90° to the flute casts, and most are downthrown in the direction of transport. Paleoslope data from the flute casts and microfaults indicate the sandstones were transported southeastward along the axis of the Ardmore-Anadarko basin during deposition of the Springer. End_of_Article - Last_Page 512------------

Wave-Dominated Deltaic Sedimentation in the Devonian Bokkeveld Basin of South Africa

ABSTRACT In the Devonian Bokkeveld Basin on the southwestern periphery of Gondwana, over 3,000 m of shales and sandstones were deposited as a southward-prograding clastic wedge. These rocks are organized into five thick regressive sequences that record processes on arcuate deltas along a mixed wave and tidal energy coastline. Variations between each of the upward-coarsening sequences are the result of differential preservation and transgressire marine reworking caused by uneven rates of relative sea-level rise. Each npward-coarsening sequence consists typically of shelf-prodelta, distributary mouth bar, tidal flat, and interdistributary bay deposits and is usually capped by tidal- and wave-reworked delta-front sands. The base of each sequence is represented by dark shelf-prodelta shales which grade upward through mudstones to graywackes and lithic arenites deposited as river mouth bars. This upward-coarsening deltaic package is topped by two types of intertidal and shallow subtidal sequences deposited on the delta plain: tidal-flat facies consist of sandstones, mudstones, and shales deposited in upward-fining units up to 10 m thick; bayfill deposits comprise small upward-coarsening subdelta sequences. The delta-plain deposits are extensively bioturbated and contain brackish invertebrate taxa of the Malvinokaffric Province. Reworked delta-front sands are present largely as tidal-channel and tidal-inlet deposits of abandoned distributaries. These consist of upward-fining, cross-bedded quartz arenites that occur as lenticular bodies up to 8 m thick, and that erosively overlie both the distributary-mouth-bar facies and in places the delta-plain facies. Thin, flat-bedded sheetsands which occur locally at the base of the quartz arenite units are attributed to storm washover processes. It is suggested that deposition of the entire Bokkeveld Group occurred on arcuate deltas similar to the modern Brazos and Niger deltas. Rates of relative sea-level rise were high and were punctuated by periods of decreased rates or near stillstand. The duration of these stable periods increased through time, culminating in stable shelf conditions that characterized Witteberg deposition.

Elements of High-Constructive Deltaic Sedimentation, Lower Frio Formation, Brazoria County, Texas: ABSTRACT

ABSTRACT The lower Frio Formation in eastern Brazoria County, upper Texas Gulf Coast, was deposited in a high constructive deltaic environment in the Houston Delta System. Constructive elements of the stacked, elongate to lobate deltas, intersected in three cores through the lower Frio Formation, are storm-induced delta-front splays, delta-front slump deposits, distributary mouth bar, distributary channel, and delta-plain assemblages. Reworked and winnowed abandonment facies are volumetrically insignificant relative to constructive elements, are subdivided into a crossbedded shoreface-foreshore subfacies and a cyclic sequence of fine-grained storm deposits on the distal delta front. Micropaleontological evidence indicates that deposition of constructive and abandonment facies took place in water depths of less than 120 feet.

Subsurface Paleoenvironmental Analysis of Gas-Producing Medina Group (Lower Silurian), Chautauqua County, New York: ABSTRACT

A paleoenvironmental interpretation of the gas-producing Medina Group from the subsurface of Chautauqua County, New York, was made by making a lithologic study of a core from Panama, New York, and analyzing over 140 gamma-ray well logs. The oldest formation, the Whirlpool, is a light gray sandstone interbedded with thin lenses of siltstone. Isopach patterns reveal that the Whirlpool Formation was deposited in elongate parallel thick areas trending NE-SW which are similar to the patterns produced by modern tidal current ridges. The Whirlpool Formation is interpreted to be deposited as a tidally influenced sublittoral sheet sandstone. The Power Glen Formation is a medium dark gray shale interbedded with a light gray siltstone. It is interpreted as being deposited in: (1) a arine shelf and prodelta environment; and (2) as distal bar deposits. The Grimsby Formation has a light gray basal sandstone followed by medium red sandstone and interbedded with blackish red shales. The isopach patterns indicate that it was deposited in elongated dendritic areas that trend nearly north-south. The Grimsby is interpreted as a tidal-dominated delta, analogous to the modern Ord River delta of western Australia. There are four sub-environments: (1) channel and distributary mouth bars, (2) overbank splay deposits, (3) tidal channel, and (4) tidal flat. A regional correlation was made with the northeastern Ohio White Clinton sands to the basal light gray section of the subsurface Grimsby of northwestern Pennsylvania and Chautauqua County, New York. The Red Clinton of Ohio is c rrelated with the upper sections of the subsurface Grimsby. End_of_Article - Last_Page 606------------

The Upper Cretaceous Beartooth Sandstone of Southwestern New Mexico: A Transgressive Deltaic Complex on Silicified Paleokarst

The Beartooth Sandstone in the Silver City Range, southwestern New Mexico, is part of a long, linear belt of transgressive, paralic deposits of Late Cretaceous age. The Beartooth rests unconformably on an extensive paleokarst surface developed on Paleozoic-age strata. Paleo-caves, pipes, and other karst features are filled with silicified detritus on the order of 30 m thick and, in places, overlain by an in situ terra rossa soil. Silica for the chertification of the karst-fill was provided by the thorough weathering of the terra rossa soil. The Beartooth Sandstone itself represents a general transgressive deltaic deposit complicated by a minor regressive phase and several shifts in the sites of deltation. The initial transgressive deposits (12 m thick) represent a low energy reworking of the soils and karst-fill in a strandline to shallow nearshore zone laterally associated with deltaic deposits. These strata are muddy, very fine sandstones to siltstones with an abundance of wood chip and tree limb impressions, no skeletal material, and a paucity of burrows. These lowermost Beartooth Sandstone strata are vertically succeeded by a massive, unipolar, thoroughly trough cross-stratified medium sandstone (4 m thick) that was deposited as part of a distributary mouth bar. Overlying the lower distributary mouth bar deposit is a ripple cross-laminated and burrowed siltstone in which occur thin cross-stratified and parallel laminated beds of sandstone displaying ball and pillow structures. These strata represent deposition in a prodelta environment. The 5-m-thick prodelta sequence is succeeded by another unipolar, thoroughly trough cross-stratified sandstone containing conglomerate lenses. This upper distributary mouth bar accumulation represents either a regression and/or progradation as a result of deposition concomitant with a stillstand of sea level. Capping the sequence is the thick accumulation of predominantly shales and siltstones of the Colorado Shale. The lower part of these strata accumulated as part of the prodelta and were succeeded vertically by normal open marine shelf deposits.

Deposition and Diagenesis of Glauconite Sandstone, Berrymore-Lobstick-Bigoray Area, South-Central Alberta: ABSTRACT

The depositional environments of the Glauconite sandstone in the Berrymore-Lobstick-Bigoray area are distributary channels, delta platform, distributary mouth bars, and interdistributary bays. The diagenetic mineralogy is consistent with the formation water chemistry . A simplified model for evolution of Glauconite sandstone water compositions includes (1) original derivation in a deltaic setting giving composition of early pore waters as brackish to normal marine, (2) alteration due to inorganic and organic chemical diagenesis, and (3) dilution through time due to meteoric water recharge. The formation waters now evolved have such a composition as to be (1) oversaturated with respect to hematite, kaolinite, and illite (late-stage cements); and (2) near equilibrium to undersaturated with respect to quartz, calcite, siderite, and dolomite. The early diagenetic mineralogy is a function of early pore waters and thus the subenvironment. For example, ankerite cement forms early at the base of distributary channels. The intermediate to late diagenetic cementation is a function of early diagenetic mineralogy. For example, hematite is formed by the oxidation of siderite due to meteoric water recharge. Oxidation of pyrite is quantitatively unimportant. Occurrence of late stage hematite is associated with structural highs which are most affected by meteoric water recharge. Delta platform deposits contain gas, and distributary channel and distributary mouth bar deposits contain water. Delta platform deposits are isolated from distributary channel and distributary mouth bar deposits by a vertical permeability barrier of ankerite cement and a lateral permeability barrier of siderite and kaolinite cement, respectively. End_of_Article - Last_Page 606------------

Depositional Environment of Carter Sandstone (Chesterian) of Black Warrior Basin in Northwestern Alabama and Northeastern Mississippi: ABSTRACT

The Late Mississippian (Chesterian) Carter sandstone, which is present in the subsurface of the Black Warrior basin in northwestern Alabama and northeastern Mississippi, was deposited as lower to subaqueous delta-plain facies of a high-constructive delta. Specific deltaic environments identified include bar finger, which is a combination of distributary mouth bar and channel facies, delta front, and prodelta or interdistributary bay. These paleoenvironmental interpretations are based on primary sedimentary rock properties and characteristic spontaneous potential curves. The Carter delta prograded from the northwest toward the southeast in the basin. The morphology of the delta is elongate through most of the basin; however, in the area of the southeasternmost extent of Carter deposition the morphology becomes lobate. The change in morphology is a result of reworking of the delta-front sands by marine proceses. The overall compositional maturity of the sandstone suggests that the constituents had a long distance of transport, with the source area being most likely a sedimentary source terrane. The direction of transport was from the northwest to the southeast, as indicated by sandstone morphology, grain size and thickness trends, paleontology, and facies distribution. End_of_Article - Last_Page 1437------------

Sedimentary Facies of an Arid Zone Delta: Gascoyne Delta, Western Australia

The Gascoyne delta is forming on a semi-arid, wave-dominated coast at the northern end of Shark Bay, Western Australia. The Gascoyne River flows intermittently. The delta has a flattened lobate form with a peripheral strandplain of elongate lagoons and tidal flats separated by beach spits and low beach ridges. Three major stratigraphic components form the Gascoyne delta: basal sheet, deltaic sequence, and plain sequence. The basal sheet is a thin transgressive unit deposited during the Holocene sea-level rise. Offshore it is shelly quartz sand with a relict, shallow-water sandflat fauna. Inshore it contains sediment admixed from contemporary seagrass communities or overlying units. The deltaic sequence is a regressive terrigenous wedge overlying the basal sheet. It consists of the bar unit, the delta bay unit, and the strandplain sheet. The bar unit is an upward-coarsening sand body being deposited on the distributary mouth bars and the shoreface; it grades laterally into the delta bay unit, a muddy sand body accumulating in the protected embayment to the south of the delta. The strandplain sheet overlies both units and is composed of sand and muddy sand with minor mud. It is being deposited in beach, beach ridge, tidal flat, and supratidal flat environments. The plain sequence consists of three units: the channel and levee units and the plain sheet. The channel unit is coarse sand and gravel; the levee unit is fine sand, silt, and mud. The channel and levee units form a narrow belt of bedded fluvial deposits normal to the strandline. The plain sheet is homogeneous, red-brown mud and sandy mud deposited by sheetwashing. The marine deltaic sequence is a laterally extensive, upward-coarsening sand body typical of other wave-dominated deltas (e.g., Sao Francisco and Rhone). However, the subaerial sequence, a narrow, axial belt of channel sands and levee silts flanked by extensive red-brown muds, forms a different cap. In contrast both the Sao Francisco and Rhone deltas have sandy deltaic plain sequences. Other arid zone deltas (Colorado and New River) differ markedly in their process style. Neither are wave-dominated. Arid climate in the Gascoyne does not markedly influence the marine sequence but is reflected in subaerial units by extensive red-brown, desiccated muds and confinement of marsh deposits to the narrow coastal, tidal zone. The Gascoyne is a potential redbed delta, particularly the subaerial sequences. Reddening of marine sequences is theorized as being dependent on the incorporation of organic matter, and possibly on inshore productivity.

Sedimentology of an ordovician fan delta complex, Western Libya

The Ordovician Haouaz Formation in the western-central Gargaf Arch of western Libya provides a well-exposed example of an ancient fan delta system. The Haouaz sequence consists of delta front deposits overlain by subaqueous distributary channels with intervening distributary mouth shoals and swash bars. This delta front complex is abruptly and erosively overlain by braided fluvial and distributary channels of the lower deltaic plain. Lower deltaic plain deposits may show reworking by wave swash processes in the upper portion, indicating marine destructive phases of delta building. This process is attributed to diminished or absent detrital influx allowing wave reworking of the deltaic plain margin. These reworked deposits may in turn be overlain by thin transgressive marine shelf sequences, also formed during periods of minimal detrital influx. The lower deltaic plain may also pass upward into an upper deltaic plain and alluvial plain environment.

PETROLOGY, PROVENANCE, AND DEPOSITIONAL ENVIRONMENTS OF THE RESERVOIR SANDSTONES OF OSSU‐IZOMBE OILFIELD, IMO STATE, NIGERIA

Upper Eocene immature subarkoses derived from Cretaceous sedimentary rocks, older granites and metamorphic rocks, form the reservoir sandstones of the Ossu‐Izombe oilfield, Imo State, Nigeria. These sandstones accumulated as part of a deltaic deposit. The vertical section of deltaic facies consists, from base to top, of a progradational sequence (prodelta and delta front), an aggradational unit (delta plain, marsh and interdistributary bay), and an overlying transgressive shallow marine interval. This sequence was penetrated by nearly all of the 15 wells drilled in the area. Reservoir sandstones are present within the distributary mouth bars, point bars, braided stream deposits, and the shallow marine interval.Thin section studies, electric‐log analyses, and data from modern deltas have aided in establishing the environments of deposition of these sandstone bodies. The principal results of this study are: (1) the basal C sandstone represents distributary mouth bar sands deposited at water depths of about 10 m; (2) the intermediate B group consist of point bar and distributary channel sandstone deposits of the lower delta plain; (3) the A‐2 sandstone tops the deltaic cycle as a product of braided streams; and (4) a shallow marine deposit, the A‐1.1 and A‐1 sandstones, occurs at the top of the deltaic cycle.

Differentiation of Delta-Front and Barrier Lithofacies of the Upper Cretaceous Pictured Cliffs Sandstone, Southwest San Juan Basin, New Mexico

The Upper Cretaceous Pictured Cliffs Sandstone represents a regressive littoral marine unit deposited during the final retreat of the Cretaceous epeiric sea in the southwestern San Juan Basin, New Mexico. Differences in rock type, internal and penecontemporaneous deformation structures, textural sequences, mineral composition, and trace fossil content permit recognition of laterally contemporaneous delta-front and barrier lithofacies in the Pictured Cliffs Sandstone. The delta-front lithofacies consists of distal bar, distributary mouth bar, and distributary channel deposits. The barrier lithofacies consists of shoreface, beach, washover channel, tidal inlet, tidal channel, and ebb-tidal delta deposits; these lithofacies are coarsening-upward sequences of shale, siltstone, and sandstone, locally scoured in the upper part by fining-upward channel deposits. The delta-front lithofacies contain abundant penecontemporaneous deformation structures in the form of ball-and-pillow, diapiric, and slump structures as well as convolute bedding and microfaults. In contrast, the laterally equivalent shoreface deposits of the barrier lithofacies lack these deformation structures. The channel deposits of the delta-front lithofacies contain large-scale convolute lamination and a few burrows, in contrast to the channel deposits of the barrier lithofacies, which contain common herring­bone crossbeds separated by reactivation surfaces, bidirectional but ebb dominated festoon crossbeds, and more abundant burrows. Quartz content of the barrier lithofacies varies from 43 to 74 percent, whereas in the delta­ front lithofacies it varies from 29 to 60 percent. The difference in quartz content between these lithofacies suggests some cleaning-up process in the deposition of the barrier lithofacies, particularly in the preserved beach deposit. The delta-front and barrier lithofacies were probably deposited along mesotidal shorelines. The recognition of laterally contemporaneous delta-front and barrier lithofacies of the Pictured Cliffs Sandstone allows delineation of potentially economic coal-producing deposits in the delta-plain lithofacies flanked by back­ barrier lithofacies in the overlying prograded Fruitland Formation.