Delta Plain Deposits Research Articles

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.

Three-Dimensional Study of a Modern Flood-Tidal Delta in South Carolina: ABSTRACT

A three-dimensional study of a modern flood-tidal delta in South Carolina was conducted using historical charts and photographs, geomorphic and sedimentologic process measurements, and shallow seismic profiling. Results suggest that the flood-tidal delta developed in response to shoreline erosion of the Santee River delta since the river was dammed and diverted in the first half of this century. The Santee flood-tidal delta complex consists of a transgressive sequence topped with tidal delta deposits 1 to 1.5 m thick. The clean, coarse to medium-grained sand of the flood-tidal delta is underlain by tidal flat, tidal channel fill, and bay-fill sediments. Beneath that lies estuarine-lower delta plain deposits. The relationship of peak ebb and flood currents to the depth of flow and the degree of shielding by topographic highs exerts control over bed-form orientation and distribution. Ebb-oriented bed forms dominate the tidal delta surface, but preferential preservation favors flood-oriented bed forms owing to higher hydraulic energy on the ebb shield and flood ramp, and by protection from ebb currents provided in the lee of the ebb shield. Textural and mineralogic evidence indicates that littoral-derived sediments are being deposited in the North Santee channel. Seismic and stratigraphic data indicate the maximum volume of clean sand in the flood-tidal delta is between 387,000 and 500,000 m3. Surface mapping suggests that the North Santee flood-tidal delta will continue to transgress the adjacent tidal flat, weld onto nearby Cane Island, and be capped by salt marsh if undisturbed by rediversion of the Santee. The flood-tidal delta would apper in End_Page 582------------------------------ the stratigraphic column as a thin sheet sand with local variation in internal physical and biogenic sedimentary structures. Overall bed-form orientation would be slightly flood dominant, and the flood-tidal delta sands would be sealed on top and bottom with fine-grained, organic-rich sediments. Modern flood-tidal deltas are excellent sources for beach nourishment projects and, given sufficient burial, ancient flood-tidal deltas could make good petroleum reservoirs. End_of_Article - Last_Page 583------------

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.

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

In the Devonian Bokkeveld basin on the southwestern periphery of Gondwanaland, over 3,000 m of mudstones and sandstones were deposited as a southerly prograding clastic wedge. Five thick regressive sequences record processes on arcuate deltas along a mixed wave and tidal energy coastline. Variations among the upward-coarsening sequences are the result of differential preservation and transgressive reworking caused by uneven rates of relative sea level rise. The base of each sequence is represented by dark shelf-prodelta shales which grade upward through mudstones to graywackes and lithic arenites of river-mouth bar facies. This upward-coarsening deltaic package is topped by tidal flat, shallow subtidal, and interdistributary bay deposits of the delta plain. These delta plain deposits are extensively bioturbated and contain brackish invertebrate taxa of the Malvinocaffric province. Reworked sands are present largely as tidal channel and tidal inlet quartzarenites that unconformably overlie the distributary-mouth bar facies and in places the delta plain facies. Plane-bedded sheet sands locally occur below the quartzarenites and are attributed to storm washover processes. It is suggested that deposition of the 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 near stillstand. The duration of these stable periods increased through time, culminating in stable shelf conditions in Witteberg time. End_of_Article - Last_Page 1000------------

Gallup Sandstone: Complex of Wave-Dominated Deltaic and Low-Sinuosity Braided Fluvial Deposits, Gallup Sag, New Mexico: ABSTRACT

The Upper Cretaceous Gallup Sandstone in northwestern New Mexico is interpreted as coastal-barrier or delta-front deposits. A study of 160 closely spaced sections along nearly continuous exposures in the Gallup Sag, New Mexico, suggests that these deposits are related to wave-dominated delta and low-sinuosity braided fluvial systems. The Gallup Sandstone in this area is divided into three prograded depositional packages. The lowermost package consists of shoreface sheetlike sandstones, siltstones, and shales, which coarsen upward into coalesced distributary-mouth-bar and beach sandstones that are locally dissected by fluvially and tidally influenced channel sandstones. This package is overlain by constructional delta-plain deposits and coastal back-barrier deposits, the latter being associated with the destructional phase of the delta system. The constructional delta-plain deposits consist of meandering distributary channels interspersed with interdistributary crevasse-splay sandstones and backswamp coals, carbonaceous shales, siltstones, and shales. The destructional coastal back-barrier deposits consist of heavi y bioturbated lagoonal or bay sequences of sandstones, siltstones, shales, and carbonaceous shales. Coal beds associated with this destructional facies are as much as 4 ft (1.2 m) thick and extend laterally as much as 4.2 mi (6.7 km). These coal beds are blanketlike in contrast to the constructional delta-plain coal beds, which are thin, discontinuous, and lenticular. This overall package is overlain by an alluvial depositional package consisting of numerous stacked, overlapped, lenticular, varicolored, pebbly sandstones. These sandstones, which are blanketlike in extent and contain a few poorly developed coals, probably represent low-sinuosity braided-stream deposits. End_of_Article - Last_Page 561------------

Wave-Dominated Deltas--Important Economic Depositional Setting in Upper Cretaceous of Western Interior: ABSTRACT

In the western interior basin, wave-dominated, deltaic deposits provide an important economic model for exploration of oil and gas and thick, laterally extensive economic coals. Recent studies in the Upper Cretaceous Mesaverde Group (Rock Springs Formation of southwestern Wyoming and Blackhawk Formation of eastern Utah) provide insight to the characteristics of wave-dominated, deltaic environments. Widespread, delta-front, sheet sandstones provide significant reservoirs for oil and gas and a platform upon which thick laterally continuous coals can develop. Wave-dominated deltas are characterized by thick, prodelta deposits composed of graded, bedded siltstones and mudstones. Capping the prodelta deposits are distributary-mouth, bar sandstones at the mouth of distributary channels. In the interchannel areas, delta-front, sheet sandstones accumulate owing to high wave energy. They are laterally continuous along depositional strike as well as dip. Coeval, distributary channels tend End_Page 739------------------------------ to be straight or have a low sinuosity with sediment fills ranging from fine mudstones to coarse-grained sandstones. Delta-plain deposits consist of lagoonal, bayfill mudstones and small-splay or bayhead-delta sandstones formed in areas behind and marginal to the delta front. Numerous coarsening-upward sequences are capped by localized coals. Fluvial and upper delta-plain areas consist of channel, levee, and backswamp materials that are laterally discontinuous. Features such as those observed in these wave-dominated delta deposits are easily recognizable on seismic lines. The seismic lines can be used to target favorable areas of hydrocarbon and coal accumulation. End_of_Article - Last_Page 740------------

Sedimentology of Fluvial Upper Devonian Kanayut Conglomerate, Brooks Range, Alaska: ABSTRACT

The Kanayut Conglomerate, which extends across most of the Brooks Range in northern Alaska, is a widespread nonmarine clastic sequence as thick as 2,000 m. It records southwestward growth of a major fluvial-dominated coarse-grained delta. The Kanayut is underlain and overlain by fossiliferous marine strata of the Upper Devonian Hunt Fork Shale and Lower Mississippian Kayak Shale, respectively. It has been subdivided into four members, in ascending order: (1) a lower marine member, 560 m thick, consisting chiefly of sandstone; (2) a lower nonmarine member, 550 m thick, consisting chiefly of fining-upward cycles of sandstone to shale; (3) a middle nonmarine member, 450 m thick, consisting of massive interbedded conglomerate and sandstone; and (4) an upper nonmarine member, he Stuver Member, 400 m thick, consisting chiefly of fining-upward cycles of sandstone to shale. The distribution of maximum size of conglomerate clasts suggests a source area to the northeast, and paleocurrent measurements indicate sediment transport dominantly toward the southwest. The conglomerates are texturally and compositionally mature, containing primarily clasts of chert with lesser amounts of quartz, quartzite, and argillite. The sandstones are also compositionally and texturally mature, composed chiefly of subrounded grains of quartz, chert, and argillite, with negligible amounts of feldspar. The Hunt Fork Shale, Kanayut Conglomerate, and Kayak Shale record a major progradational-retrogradational deltaic cycle. The lower marine member of the Kanayut and underlying and overlying marine units represent prodelta, delta-margin, and delta-front deposits, the lower nonmarine member and the Stuver Member of the Kanayut represents meandering fluvial delta-plain deposits, and the middle nonmarine member of the Kanayut represents braided fluvial delta-plain deposits. End_of_Article - Last_Page 758------------

Distribution and Significance of Coarse Biogenic and Clastic Deposits on Texas Inner Shelf: ABSTRACT

Sediments of the Texas inner shelf are fine grained; coarse clasts (> 0.5 mm) are uncommon (< 1%) over much of the area. Higher concentrations of coarse material, however, occur in discrete areas which apparently represent positions of former deltas. Coarsest constituents are predominantly whole shells and shell fragments with subordinate amounts of lithic clasts. The calcareous skeletal debris represents a mixture of extant shelf fauna and relict brackish-water mollusks including Ragina sp. and Crassostrea virginica. Rounded sandstone and mudstone clasts up to 7 cm long and caliche nodules are common in some areas. Maps showing (1) coarse-fraction percent, (2) distribution of brackish-water mollusks, and (3) rock fragments show similar trends outlining ancestral Ri Grande, Brazos-Colorado, and Trinity deltas. A patchy, arcuate trend between Pass Cavallo and Aransas Pass is enigmatic. Criteria used to determine relative ages of shell debris for each of the four trends are degree of abrasion, fragmentation, etching, boring, and discoloration. Possible explanations for concentration of coarse material include high productivity, low rates of terrigenous clastic sedimentation, selective deposition by modern shelf processes, and reworking of locally shelly relict deposits exposed on the seafloor during the Holocene transgression. However, no single explanation adequately accounts for areal variations in coarse material. Reworking of delta-plain and estuarine deposits during and following sea-level rise is common to all areas which at present are also sites of insignificant coarse-sediment influx. Sabine-Bolivar trends are interpreted as transgressive lags derived from erosion of a late Pleistocene Trinity delta previously dissected by the Sabine River during Wisconsin glaciation. In contrast, Brazos-Colorado and Rio Grande tre ds are interpreted as possibly compound strandline features associated with subsidence, erosion, and retreat of Holocene deltas. Upwelling of nutrient-rich shelf waters and freshwater inflow also may have increased productivity of shelf benthos near the Rio Grande delta. End_of_Article - Last_Page 1608------------

Tidally influenced barrier island and estuarine sedimentation in the upper carboniferous of southern West Virginia

Upper Carboniferous orthoquartzites exposed in the Guyandotte Valley of southern West Virginia are interpreted as barrier island and backbarrier deposits. They overlie fossiliferous marine limestones and shales, and are overlain in turn by fluvio-deltaic subgraywacke sandstones, siltstones and coals. Only the upper one-third of the 180 m thick orthoquartzitic interval is exposed, but these rocks outcrop over a distance of 16 km, and reveal landward transition of the barrier orthoquartzites into backbarrier and alluvial or delta-plain deposits. Within the exposed orthoquartzitic interval are three major orthoquartzite bodies, separated by subordinate siltstones containing smaller sandstone bodies. The major orthoquartzites are 10–25 m thick and are lenticular in transverse section, with a north-south elongation approximately perpendicular to the palaeoslope. These sandstone units are subdivisible into three facies. Facies 1 is most widespread and comprises mainly upward-thinning sets of cross-stratification, with a silty interval occasionally preserved towards the top. Palaeocurrent indicators reflect increasing dispersal from the base of the sequence upward. These deposits are interpreted as originating within laterally migrating tidal inlets, in which ebb currents were dominant. Facies 2 contains low-angle bedding suggestive of wave swash processes, with patterns of minor sedimentary structures similar to those observed in modern ebb tidal deltas. Facies 3 consists of sandstone with a preponderance of landward-directed cross-stratification, and is attributed to a flood tidal delta, flood-channel and washover berm origin. The orthoquartzites merge eastward and southeastward (landward) into inferred estuarine or lagoonal siltstones, but in places they are in lateral contact with fluvio-deltaic subgraywackes. Thus the barrier sediments are thought to have been derived at least in part by the reworking of abandoned delta lobes. Longshore drift created spit extension towards the south and southwest, thereby enclosing lagoons within which accumulated siltstones and sandstones with brackish or marine fossils and a variety of structures suggestive of tidal processes and local subaerial exposure. Washover sands were deposited to landward of the barrier. Both barrier and backbarrier deposits are incised by small tidal channels. Occurrence of washover features in association with small but well-developed tidal deltas suggests moderate to strong wave activity and a mesotidal (2–4 m) range.

Sedimentology of an Upper Paleozoic River, Wave and Tide Influenced Delta System in Southern Morocco

ABSTRACT Upper Devonian and lower Carboniferous deposits exposed along the north flank of the Tindouf Basin in southern Morocco are assigned to 12 sedimentary facies belonging to four interacting depositional environments: progradational deltaic, marine shelf, beach and tidal flat. These environments together form an ancient delta system which records regressive-transgressive episodes due to phases of constructive and destructive delta building. The constructive phase of delta building occurs when an actively depositing fluvial system progrades a platform of deltaic plain deposits onto a marine shelf, without significant contemporaneous reworking of sediment by marine processes. The first destructive phase of delta building occurs when a less actively depositing fluvial system enables marine p ocesses of a microtidal and moderate wave energy sea, to build a beach on the margin of the deltaic plain. The final destructive phase of delta building occurs when detrital influx is almost absent and the deltaic plain converts to a back-barrier tidal flat which is eventually transgressed by the barrier beach and marine shelf.

Upper permian coal measures of the Central Sydney Basin, N.S.W. – seismic data analysis and drilling results

The results of a preliminary eight hole coal drilling program in the Dural-Spencer-St. Albans region are presented. The data from the South Colah, Frederick and St. Albans holes are discussed with respect to lithologies, stratigraphy, depositional environments, coal intersections and qualities. The holes were drilled beyond 800m. The north western bores intersected coking coal seams, the thicknesses of which varied between 0.69m and 4.85m. A study was made of the regional seismic reflection data available from petroleum prospecting operations. The data was very useful in the drilling programme in that it enabled the recognition of, and estimation of depths to, the coal measures as a gross unit. However it was not possible to determine seam configurations owing to fundamental limitations in the resolving power of the reflection method. The coal measure sediments intersected in the drill holes comprise: (a) A lower unit in which sandstones are mostly thin; shales, dirty coals and thin tuff beds are common. The beds appear to be lenticular. Evidence of salinity (indicated by burrowing) is common; (b) An upper unit in which sandstone and conglomerates are the dominant rock types. The sandstones appear to be sheet-like bodies. The coals are cleaner and burrowing in the finer sediments is rare. Coking quality coals are developed in this unit, they are thickest in the northwest. Assuming a thick tuff bed to be a time plane, the facies change is diachronous becoming younger from windeyer's Hawkesbury Bore southwards. The relationships in the South Colah drill hole may be more complex. Environmentally the lower is almost entirely deltaic ranging from bay to upper delta plain deposits becoming more distal westwards and downwards. One possible point bar sequence is present in the South Colah hole. The upper unit is mainly fluvial, probably deposited by braided streams but some point bar and deltaic deposits are present in South Colah. The sediments of the upper sequence could have been derived from the northeast or the north to northwest. The simplest model is that the upper and lower units are the downstream deposits of the streams that laid down the coal measures in the Newcastle-Lake Macquarie area.

Short-Headed Stream Delta: Model for Pennsylvania Haymond Formation, West Texas

The most commonly applied deltaic model in lithogenetic analysis of ancient deposits has been that constructed by major river systems. Perhaps an equally important model is the short-headed stream delta model exemplified by the Pennsylvanian Haymond Formation in the Marathon basin. This model consists of clay, silt-sand couplets in the lower part, and bouldery conglomeratic sand in the upper part. The lower fine-grained deposits represent prodelta and delta front, the upper coarse-grained deposits are the delta plain. The difference between short-headed stream-delta deposits and their large river-delta equivalents arises in the nature of the delta front--delta plain deposits. In the Haymond Formation, the flysch-like delta-front interval probably indicates proximity to the source terrane in the southeast and sediment influxes in amounts greater than the organisms can rework. The gravelly sand delta-plain deposits indicate that most of the clay and silt common on other delta plains was flushed farther offshore. The small proportion of current-bedded shale and siltstone suggests a largely subaqueous delta plain; however, some subaerial sedimentation is indicated by bleached, root-marked sandstone. In addition, the delta plain was traversed by braided distributaries as evidenced by multiple-channel sa dstones. That the channels were choked by a vast assortment of large particles probably transported during periods of greatest runoff is indicated by the basal conglomerates and boulder beds. However, some large boulders may have been reworked from nearby coastal scree detritus and steep cliffs during intense storms.

Depositional History of the Coal-Bearing Upper Jurassic-Lower Cretaceous Kootenay Formation, Southern Rocky Mountains, Canada

The coal-bearing Kootenay Formation of Late Jurassic-Early Cretaceous age in Alberta and British Columbia is a classic example of sedimentary sequences built by delta progradation. The formation is composed of 3,500 ft of cyclicly alternating sandstone, shale, and coal, with a few conglomerate beds. The sandstone beds are poorly to moderately sorted, fine- to medium-grained lithic graywacke and proto-quarzite. Shale is of illitic composition with minor amounts of kaolinite. These Upper Jurassic-Lower Cretaceous sediments were derived mainly from the sedimentary-rock terrains located in the Purcell Mountains and the Shuswap Complex. Direction of transportation was to the northeast. Deposition of the Kootenay Formation was initiated by epeirogenic uplift of the source area in Late Jurassic time. Terrigenous sediments were initially deposited on a low-lying coastal plain, which advanced seaward by accretion processes into the shallow sea of the eastern Canadian Cordillera miogeosyncline. In the vertical section, the siltstone and mudstone beds of the open marine environment grade upward into transitional prodelta facies of the Fernie Formation composed of sandstones and siltstones containing the trace fossil Thalassinoides. The prodelta deposits in turn grade upward into delta front sheet sand containing the trace fossil Rhizocorallium. The overlying well-sorted and highly burrowed sandstone of a barrier bar and “misch” tidal flats contains the trace fossil Arenicolites, and grades upward into lower delta plain deposits. These are composed of sandstone beds ranging up to 50 ft in thickness and represent channel fill and point bars of meandering distributaries, laterally interfingered into interdistributary sequences. Coal, as much as 30 ft thick formed as peat in coastal marshes and in fresh-water swamps of interdistributary basins of the lower delta plain. Within the upper delta plain, thick sandstone beds are more common, and coal formed in fresh-water swamps of the flood basins. The delta-plain sediments grade upward into alluvial plain deposits characterized by the occurrence of sheet form beds of conglomerate, lithic sandstone, siltstone and silty shale, and decreased thickness of individual cyclothems. Increased frequency in alternation of rock units is explained by the increase of channel instability and tendency of river channels to form a braided channel system. This system provided unfavorable conditions for coal development. Uplifting of the depositional area in Neo-comian time terminated sedimentation and initiated denudation of the previously deposited sediments. During additional strong uplifting of the inland area, the rejuvenated piedmont-plain deposits represented by conglomerate beds of the Cadomin Formation were spread over the eroded surface of the Kootenay Formation. This study demonstrates the use of sedimentological studies as a tool for exploration and evaluation of coal basins.

Deltaic Origin of Difunta Group (Late Cretaceous to Paleocene), Parras Basin, Coahuila and Nuevo Leon, Mexico: ABSTRACT

The Difunta Group is predominantly gray calcareous mudstone, siltstone, and sandstone with 3 wedge-shaped redbed units. The redbeds provide a basis for subdividing the group into 7 formations with a composite thickness of 12,000 ft. Simple structure, continuous exposures, and lithologic variability provide an excellent opportunity for paleoenvironmental interpretations. The Difunta Group is largely the product of deltaic sedimentation in the Parras basin, a shallow embayment off the ancestral Gulf of Mexico. Major delta progradation was from southwest to northeast as shown by facies changes, thickness trends, and paleocurrent data. The redbed units are interpreted as delta-plain deposits that accumulated as lake and bay muds and silts and fluvial and distributary-channel sands. Redbeds are bounded by resistant gray blanket sandstones 20-60 ft thick that have sharp bases, scour-and-fill bedding, Ophiomorpha, and local mudstone, wood, and oyster clasts. These sandstones are interpreted as delta-front and delta-destructional deposits formed during progradation and retrogradation of the deltaic complex. Prodelta facies are gray sequences of either (1) burrowed mudstone or muddy sandstone, many beds of which have ball-and-pillow structure and a sparse molluscan fauna, or (2) interbedded graded sandstone and burrowed mudstone. The graded sandstone beds of the later facies were deposited by turbidity currents that were generated at the delta front probably by hyperpycnal flow. Patchlike carbonate banks up to 1,000 ft thick developed in the north during episodes of low terrigenous influx in this area. Bank deposits are chiefly micritic bioclastic rocks; boundstone is rare. End_of_Article - Last_Page 352------------

Permo-Triassic Sedimentation Patterns, Sydney Basin, Australia

Fluvial, deltaic, and marine-shelf sediments were deposited in a 100- by 200-mi, north-south oriented trough centered around Sydney, where up to 12,000 ft accumulated during the Permo-Triassic. A basin analysis was made using depositional models based on rock outcrops and closely spaced boreholes. These depositional models were used to form a best fit-of-environment correlation along three lines of section through the basin using 21 deep boreholes as control. Patterns of sedimentation can be delineated from the correlation charts at several intervals through the sequence. Lower Permian sediments were derived mainly from the north and, except for minor delta-plain facies in the nearshore area, were principally deposits of a marine shelf and delta front with an abundant fau a of thick-shelled pelecypods, brachiopods, and bryozoans. Most of these marine sandstones and siltstones contain predominantly lithic fragments but some, apparently representing subaqueous bars, are distinctly quartzose. From the Middle Permian into the Early Triassic, the sediments were deposited in a southward prograding delta and were derived mainly from the north. Two fluvial wedges formed which merged southward into a deltaic plain facing a shallow sea. Close borehole control within the fluvial system establishes channel-bar sandstones and conglomerates 50-200 ft thick grading laterally into levee and lacustrine siltstones and back-swamp coal beds. Alluvial deposits grade seaward into delta-plain sediments consisting of distributary-mouth bar sandstones 30-50 ft thick which interfi ger laterally with interdistributary bay sequences. These bay sequences commonly grade from fine to coarse upward and commonly are capped by intensely burrowed sandstones or root claystones and coal beds. Delta-front sandstones flank the delta-plain deposits and merge with mottled gray siltstones of the open shelf. Several episodes of delta outbuilding separated by periods of marine transgression can be delineated with the last most widespread progradation followed in the Triassic by an equally extensive transgression of shallow marine water. Fluviodeltaic deposits in this latest episode lack coal, and the sedimentary sequence is dominated by an orthoquartzitic barrier bar-tidal delta system locally 1,000 ft thick which grades laterally into marine red claystones and gray tidal-flat silt tones and fine sandstones.

AN HISTORIC GEOLOGY OF THE GIPPSLAND BASIN

The Mesozoic and Tertiary rocks of the Gippsland Basin form a simple natural sequence as the deposits of a main regressive - transgressive cycle, followed by a regressive phase of lesser magnitude.After a period of early Mesozoic erosion in Gippsland, a river flowing from the north-west built extensive deposits which drove the sea back well beyond the present coast line. Strzelecki Formation is the name applied to the flood-plain sediments of this period. The channel belt deposits have been given various names and of these the Childers Formation appears to be the most valid. However, with the deltaic plain deposits, it can be regarded as forming a part of the Latrobe Valley Complex.About the end of Mesozoic time the sea began to transgress, also at this time there was some vulcanism in the western part of the basin. As transgression proceeded, the delta became digitate in form, similar to today's Mississippi delta. Silty and muddy sediments accumulated beyond the distributaries and between them, whilst the distributaries were areas of dominantly sand desposition. Marine agencies reworked some of the sand to form beaches and occasional barrier islands.As the sea transgressed the prodelta silty muds (Lakes Entrance Formation), covered the submerged distributaries and these in turn became covered by the cleaner water calcareous deposits of the Lower Gippsland Limestone. Meanwhile onshore, the flood-plain and earlier channel-belt deposits became overlain by later channel-belt deposits, and then by deltaic plain deposits with abundant coal. Transgression reached its zenith, probably during Miocene time, and a final regressive phase culminated in the situation as we see it today.The formation water in the main body of the Latrobe Valley complex is very fresh, and calculations show a static situation for both the Latrobe Valley and the contiguous glauconitic sandstone member, and thus the existence of down-dip escape is most unlikely. The oil at Lakes Entrance has not been flushed down-dip, rather it has moved up-dip.It follows from a consideration of the depositional history in Gippsland that producing structures at the top Latrobe Valley level are basically stratigraphic traps. They lie along ancient distributary channel trends and are flanked by contemporaneous muddy facies. Separate closures along the trends have been brought about by the development of saddles due to differential compaction and probably normal faulting at depth.The oil and gas pools today are more or less at their greatest depth of burial. An association is obvious between depth and hydrocarbon content for the top Latrobe Valley pools. A progression is noted from dry gas at Golden Beach (2,000') to a thick oil i Halibut (8,000'). Coal rank increases with depth. It is concluded that coal is the main hydrocarbon source in Gippsland. Thus all the Latrobe Valley section is prospective. The top Latrobe Valley horizon is probably the most productive to date because it has very effective cappingBarracouta 1, intersected a barrier island sand. There is likely to be more of these parallel to ancient shorelines. They could prove productive.

A Recent Model for Pennsylvanian Deposition in the Marathon Basin, West Texas

ABSTRACT The Haymond Formation comprising the middle Pennsylvanian strata of the Marathon Basin of West Texas consists of a series of interbedded sandstones, siltstones, shales, and boulder beds which have been interpreted as deep water turbidites deposited in a trench-like area surrounded by tectonically active highlands. Detailed studies of this sequence in conjunction with investigations in the south shore of Lake Erie in western New York, suggest an alternative explanation in which the Haymond is interpreted as a prograding deltaic unit formed from short headed and steep gradient streams in a nearby but not necessarily high source area. Proximity of the source provides an explanation for the turbidite-like character of the delta front portion of the sequence as well as for the presence of the boulder beds in the delta plain deposits.

Patterns of Permo-Triassic Sedimentation, Southeastern Australia: ABSTRACT

Fluvial, deltaic, and marine-shelf sediments were deposited in a 100 × 200 mi, north-south-oriented trough centered around Sydney where as much as 18,000 ft of strata accumulated during the Permo-Triassic. The trough is between and was filled by debris from two major blocks of older, deformed Paleozoic rocks. The northern block consists mainly of radiolarian chert, volcanic graywacke, and mudstone, intermediate-composition volcanics, and Permian granite. The southern and western block consists mainly of quartzose sediments, silicic volcanic rocks, quartzite, granite, and Permian basaltic rocks. Short transport distances and relatively slight reworking of these diverse rock types within the basin of deposition yielded a petrographically complex sequence but one in whi h contributions from southern, western, and northern sources can be distinguished readily. Lower Permian sediments apparently were derived mainly from the southern and western blocks and, except for small deltas in the nearshore area, were principally marine-shelf deposits having an abundant fauna of thick-shelled pelecypods, brachiopods, and bryozoans. Most of the marine sandstone and siltstone deposits contain a very large proportion of lithic fragments but some, apparently representing subaqueous bars, are mainly quartzose. During the Middle Permian the sequence was covered by a rapidly prograding sedimentary sequence derived from the northern block, which until Middle Triassic time provided most of the sediments to the trough. The principal sediment complexes derived from this northern system were two fluviatile wedges. One wedge each was deposited on either side of a b sement high, and the two wedges coalesced southward into a deltaic plain facing a shallow sea. Close borehole control within the fluvial system establishes the presence of channel-bar sandstone beds 50-200 ft thick. These grade laterally into levee and lacustrine siltstone and back-swamp coal beds. The alluvial deposits grade seaward into delta-plain deposits consisting of distributary-mouth bar sandstone beds 30-50 ft thick which interfinger laterally with interdistributary bay fills. Bay fill sequences generally grade from fine- to coarse-grained upward and commonly are overlain by intensely burrowed sandstone or root-clay and coal beds. Delta-front sandstone flanks the delta-plain deposits and merges with mottled gray siltstone of the open shelf. Several episodes of delta outbuilding separated by periods of marine transgression can be delineated. The last, most widespread progradation was followed in early Middle Triassic time by an equally extensive marine transgression. Fluvio-deltaic deposits which formed during this latest episode lack coal, and the sedimentary sequence is dominated by an orthoquartzitic barrier bar-tidal delta system, locally 1,000 ft thick, which grades laterally into marine red claystone and gray tidal-flat siltstone and fine-grained sandstone. The mineral composition of the latter sediments shows an increasing quantity of basaltic detritus from the south and a concurrent reduction of sediment influx from the north. End_of_Article - Last_Page 713------------

Basic Delta Systems in Eocene of Gulf Coast Basin: ABSTRACT

Studies of certain of the thick terrigenous clastic wedges that comprise most of the Eocene fill of the western Gulf Coast basin indicate that principal deposition of sediments occurred as parts of major delta and associated systems. Two basic types of delta systems are recognized in the Gulf Coast Eocene. One type developed at the mouths of a few, very large, distant-source streams which resulted in extensive shoreline progradation of numerous, elongate to lobate, terrigenous, delta lobes. Streams supporting this type of delta system entered the basin at only a few places and deposited local fluvial facies along the updip basin margin. The second type developed at the mouths of several, relatively small, local streams, and consists of a series of nearly uniformly prograding arcuate to cuspate delta lobes. Streams supporting this type of delta system entered the basin at many places and deposited a more or less continuous fluvial facies along the basin margin. In the first type of delta system sediment input was great and exceeded the energy of coastal processes so that, within the component facies of the system, the ratio of constructional or progradational facies to destructional facies is high. In the second type, sediment input was moderate and only slightly exceeded the energy of coastal processes so that ratio of constructional to destructional facies is relatively low. Extensive delta systems of the lower Wilcox in the Gulf Coast basin (Rockdale system in Texas, Holly Springs system in Louisiana and western Mississippi) are of the first type; large delta and associated systems of the Jackson (Fayette) in Texas and the Cotton Valley in the northern Gulf Coast basin also are primarily of this type. The recent counterpart is the Mississippi delta system. Main component facies of this delta system include: (1) extensive delta-plain deposits with abundant lignite or peat (distributary-channel and interdistributary deposits); (2) well-developed delta-front sand deposits made up chiefly of distributary mouth bars; (3) very thick, dark prodelta mud; and (4) interdelta mud. Lobes are elongate perpendicular to the coast where associated with sequences contain ng large amounts of mud, or are rounded where associated with sequences which are relatively low in mud content. Destructional components include thin, locally persistent, marine units across distal parts of abandoned delta lobes and widespread lignite or peat on the landward or updip parts of abandoned lobes. Constructional and destructional units are vertically distinct. Associated depositional systems include well-developed delta-flank mud facies along margins of the delta-fluvial systems, and prominent strandplain and barrier-bar (with complementary lagoon) systems along strike and down longshore drift. The large delta systems of the upper Wilcox in Texas and Louisiana, of parts of the Vicksburg, and probably of the Frio in Texas are chiefly of the second type. Recent analogues include the Appalachicola, Rhone, and certain of the U.S. Atlantic coast delta systems. Because of approximate balance of moderate sediment input and energy of coastal processes, much of this type of delta consists of locally redistributed marine sediments; construction and destruction were more or less contemporaneous and units of these phases are not vertically distinct. Principal facies consist of numerous coastal barrier-sand bodies. Constructional delta-plain facies (including distributary-channel and interdistributary deposits) are only partly preserved. Lignite or peat is not important. Frontal shoal-sa d facies commonly mark the original progradation of individual deltas. Lagoon, marsh, and lacustrine deposits are minor components. Prodelta mud is not thick and generally is similar to open-shelf deposits. No extensive marginal delta-flank deposits nor prominent strandplain and barrier-bar systems developed along strike. Significant, and commonly multipay, oil and gas reservoirs are present within both types of delta systems in the Gulf Coast basin. In the first type, main trends are chiefly coincident with marine delta-front sand; in the second type, marine coastal barrier- and frontal shoal-sand bodies are the principal reservoirs. Distribution of deltaic oil and gas trends depends on the type of delta system in which they are present. Reconstruction of the system from facies composition and three-dimensional facies geometry and facies relations provides a useful guide for exploration. End_of_Article - Last_Page 1826------------