Major Distributary Channels Research Articles

Sedimentary characteristics and model of seasonal deltaic sandstone: a case study on the continental red bed from the Cretaceous Baxigai Formation, Tarim Basin, NW China

Seasonal delta is a sandy fan-shaped sedimentary system formed by seasonal river flooding in continental basins. Modern sedimentary finds that seasonal deltas mainly develop in an arid-semi-arid climate environment with gentle terrain. However, the study of seasonal deltas in ancient rock records is still insufficient, leading to misinterpretations as other sedimentary systems, which poses challenges to the correct understanding of the origin and spatial distribution of sand body deposits. In this study, the red bed deposits of the Cretaceous Baxigai Formation in the west of the Tabei Uplift were selected as the research object. Based on core observation and description, a comprehensive analysis of the sedimentary environment, sedimentary characteristics and sedimentary model of the Baxigai Formation was carried out using data such as well-logging and analytical testing, combined with modern depositional characteristics. Research shows that the sedimentary sequence of the Baxigai Formation red bed developed a seasonal delta sedimentary system in the arid climate. The seasonal delta deposits provide an opportunity to understand the influencing of river flow and total sedimentary loading vary greatly on the sedimentary model, sedimentary mechanism, and distribution of sand bodies in arid climates. The sedimentary process of the seasonal delta in the Baxigai Formation was controlled by the dual-stage sedimentation process in an arid paleoclimate. During the flood period, the supercritical flow sedimentary structures are widely developed in the major distributary channels, and the distributary channels migrate and bifurcate frequently. On the delta plain, crevasse channels are formed through the avulsion of major distributary channels. During the dry period, due to the consumption of water seepage and evaporation transfer, terminal distributary channels are abandoned, crevasse channels are not developed, and the sediment is exposed. The density of the distributary channels is reduced, and the total sedimentary loading is low. The distributary channels sand body is the skeleton sand body of the seasonal delta. This study is of great significance for accurately explaining the formation process of seasonal deltas and provides a useful reference for further exploration and deployment of oil and gas reservoirs and paleogeographic reconstruction in the study area.

MESOZOIC DELTAIC SYSTEM AND SEQUENCE STRATIGRAPHIC MODEL OF THE LOWER JURASSIC DATTA FORMATION ALONG THE WESTERN SALT RANGE, PAKISTAN

In this research, the Early Jurassic Datta Formation from three outcrop sections of the western Salt Range has been extensively studied to elucidate facies architecture, sedimentary features, diagenetic features, sequence stratigraphy, and depositional model. Datta Formation is well exposed with massive thickness in the western Salt Range and the Trans Indus Ranges, while tapering in the central Salt Range further eastward, it dies out. Its mainly dominated by variegated sandstone, siltstone, shale, fire clays, laterites, and carbonates. Seven lithofacies has been identified and interpreted in the Datta Formation, i.e., (i). Fluvial Meandering Lithofacies (JDL-01), (ii). Braided Fluvial Lithofacies (JDL-02), (iii). Lateritic Lithofacies (JDL-03), (iv). Flood Plain Lithofacies (JDL-04), (v). Siliciclastic Lagoon Lithofacies (JDL-05), (vi). Carbonates dominant Lagoon Lithofacies (JDL-06), (vii). Swamps related Deltaic Lithofacies (JDL-07). Sequence stratigraphic analysis is primarily based on recognizing sequence surfaces, finning-coarsening upward trends, and types of facies. Two transgressive-regressive cycles are identified in the south-eastern section (Kassan Nala and Kaowali sections), while four sequences are identified in the Nammal Gorge section. These sequences are started with the underlying sequence boundary of LST and ended with the sequence bounding surface after HST. Datta Formation is possibly deposited in fluvial to deltaic environments in the study area, revealing a prograding delta with a shoreline oriented in the SW-NE direction and the siliciclastic detritus fed the fluvial system running over the Indian cratonic basement in the direction of N-NW. The lower part of the formation is deposited in fluvial settings, probably in the incised valley. The point bar sequences result from meandering channel facies and show association with overlying flood plain/overbank facies. Moreover, the lagoonal facies mainly covered the upper part of the formation and displayed mixed lithologies of delta plain to delta front setting. However, the repetition of these lithofacies in the study area at several points indicates the recurrence of depositional phases. The carbonaceous shales and coal streaks are evident by the presence of swamp or marshy lands. These conditions meet the criteria of deltaic settings. Furthermore, several laterites and fire clay horizons are also argue periodic sub-aerial exposure of the depositional area. Thus it is concluded that the depositional area lies in a fluvial to deltaic setting with major distributary channels and their flood plain in the adjoining delta plain to delta front part.

Diagenetic alterations related to sedimentary architecture of deltaic distributary channels in red beds of the Cretaceous Yaojia Formation, Songliao Basin

Fine-scale diagenetic alterations linked to architectural patterns of the sandstones were studied in meter-scale deltaic distributary channels of the Upper Cretaceous Yaojia Formation, Songliao Basin. The deltaic reservoirs of the Yaojia Formation are mostly developed in three types of distributary channels: major distributary channels, crevasse channels, and terminal distributary channels. The three types of distributary channels have different sedimentary architecture and lithofacies. The distribution of reservoir intervals varies greatly in the upward fining distributary channel depositional cycles (meter-scale). From the diagenetic alterations in the distributary channels successions, compaction, calcareous cementation, grain-coating clays with hematite in the red bed sandstones, and dissolution of feldspar or rock fragments are main control factors of high-quality reservoirs. More porosity loss is generated by strong compaction near the top and bottom part of the distributary channel succession. This could be caused by the high clay content and mud clasts which contribute much to the porosity loss in the sandstones. Calcareous cementation is mainly developed near the clay-rich intercalations. Dissolution pores in the sandstones without hematite cement and grain-coating clays are well developed in the middle position of the successions. The cementation is restricted in sandstones with unique hematite cement and grain-coating clays in the reddish crevasse distributary channels. The distribution pattern of high-quality reservoirs within the three different types of distributary channels in red beds is proposed by linking the diagenetic processes to depositional environments and architecture. High-quality hydrocarbon reservoirs are mostly preserved in the massive and cross-bedded sandstones from the middle part of the major distributary channel successions.

Sedimentary characteristics and pattern of distributary channels in shallow water deltaic red bed succession: A case from the Late Cretaceous Yaojia formation, southern Songliao Basin, NE China

Shallow water delta developed during the Late Cretaceous greenhouse climate depositing in the red bed succession of the Yaojia Formation in the Southern Songliao Basin, NE China. The shallow water delta deposits provide an opportunity to understand the influence of high discharge variability in a semi-arid climate on the fluvial patterns, internal sedimentary details, and reservoir quality of distributary channels. The sedimentary process of the shallow water delta in the Yaojia Formation was controlled by a dual-stage process, including a high-flow stage and a low-flow stage within a semi-arid paleoclimate. During the high-flow stage, crevasse channels formed through avulsion of major distributary channels on the delta plain. Subaqueous terminal distributary channels and mouth bars formed at the delta front during progradation of the delta. During the low-flow stage after flood events, crevasse channels were abandoned and frequently exposed becoming reddish in color. This process can be reason for the classification for two main types of distributary channels: major distributary channels and crevasse channels in the delta plain. Sand bodies in distributary channels are distinguished by sedimentary structures indicate critical and supercritical flow conditions. Based on well logs and 3D seismic data, the sand distribution of was analyzed from the sandstone isopach map of the 2nd sand bed in the first member of the Yaojia Formation. The framework sand belts develop coincides with distribution of the major distributary channels. Based on detailed observational and reservoir test data from cores, the distribution of major distributary channels revealed a belt of high-quality reservoirs in the shallow water delta. This study provides a new perspective on the sedimentary pattern of distributary channels in shallow water deltas with high discharge variability, which has important implications for reservoir exploration in lacustrine basins.

The Table Rocks Sandstone: A fluvial, friction-dominated lobate mouth bar sandbody in the Westphalian B Coal Measures, NE England

Westphalian B (Duckmantian) alluvial Coal Measures along the Northumberland coast, NE England, comprise coal-capped coarsening-upward crevasse-splay sequences of shale, siltstone and sandstone, interbedded with a number of major distributary channel sandbodies, including the Table Rocks Sandstone. Lithofacies, architectural analysis and outcrop geometries divide the Table Rocks Sandstone into flaggy sandstone, massive sandstone, heterolithic, and mudstone facies associations, each comprising up to 7 lithofacies types. The three sandy facies associations are characterised by lenticular bed geometries on different scales producing a hierarchy of lensoid packages and associated bounding surfaces, all showing typical offset stacking patterns: (1) lenses, represent individual lenticular cross-bed sets, bounded by 1st order surfaces; (2) packages of lenses, called lens clusters are bounded by 2nd order surfaces, and are the basic architectural building block of the sandy facies associations; and (3) vertically stacked lens clusters called amalgamated lens clusters, bounded by 3rd order surfaces. The Table Rocks sandbody has a laterally extensive, irregular, lobate subsurface plan geometry, it displays a radial palaeocurrent pattern with 180° dispersion, and it forms part of a 14-m thick coarsening-upward regressive sequence. It is interpreted as a composite, lobate crevasse-splay delta system that prograded into a shallow interdistributary fresh to brackish water lake up to 14 m deep. The shallow lake water, fluvial input, and extensive development of traction structures such as cross-bedding and ripple cross-lamination suggests a friction-dominated delta, in which the four facies associations can be interpreted in terms of discrete elements of the mouth bar environment. The flaggy sandstone facies association represents the main, axial part of the mouth bar system, the erosively based massive sandstone facies association major subaqeous distributary channels, the lithologically more variable heterolithic facies association the medial mouth bar, and the mudstone facies association the distal mouth bar fringe and prodelta. Within this environmental setting amalgamated lens clusters are interpreted as small, discrete mouth bar sand lobes, whose offset, imbricate stacking pattern reflects channel spacing and bifurcation, the rate of channel shifting, or shallow depths and lack of accommodation space. Thus, lens clusters are interpreted as discrete growth elements of the mouth bar sand lobes, and lenses as individual bedforms making up these growth elements. Because of the high rate of channel shifting, lack of extensive erosion of the mouth bar lobes, and deposition of low discharge fines, the lobes retained much of their original depositional geometry, thereby providing advantageous gradients for offset deposition and stacking of adjacent sand lobes. Although the delta complex was maintained by frequent crevassing from the feeder channel, and by subsidence due to contemporaneous compaction and/or local tectonism, it was deeply incised on two occasions by subaqeous channels in response to high magnitude floods or falling lake level.

Geomorphology of the Boao coastal system and potential effects of human activities - Hainan Island, South China

The Boao coastal system along the eastern coast of Hainan Island is a dynamic delta-tidal inlet-barrier formed during the late Holocene. The delta developed inside a shallow lagoon barred by a sandy barrier with a narrow, shallow tidal inlet opening. Two major distributary channels separated by small islands characterize the delta. The lagoon is silting up receiving and trapping sediments from both the river and, in minor measure during storms, through the tidal inlet opening and barrier washovers. The barrier at the tidal inlet is highly dynamic and changes its form, accreting (migrating spit) against the inlet during fair-weather conditions and being eroded during storms and river floods. The delta has almost completely filled the lagoon and major concerns exist on the effect that ongoing large development plans may have on the environment. These concerns include the effect on floods and rate of siltation once banks of the islands have been stabilized and floodwater and sediment load are impeded from spreading over the lowlands, and the effect of increasing pollutant loads from the new facilities on the ecosystems of the increasingly restricting lagoon water and on the seashores.

Vegetation, hydrology, and sedimentation patterns on the major distributary system of the Okavango fan, Botswana

This study investigated the distribution and determinants of marsh vegetation along the major distributary channel system of southern Africa’s largest wetland, the Okavango Delta, using a large, inter-disciplinary data set. Eight communities were recognized, dominated respectively by Pennisetum glaucocladum, Phragmites mauritianus, Cyperus papyrus, Cyperus papyrus/Miscanthus junceus, Miscanthus junceus, Imperata cylindrica, Pycreus nitidus and a mixed bog community. The Pennisetum glaucocladum community is situated in the Panhandle (a narrow valley reach at the head of the fan) on elevated scroll bars that are flooded seasonally for short periods (days to weeks). The Phragmites mauritianus community occurs largely in the Panhandle where channel margins have high clay contents and where soils are seasonally flooded for moderate periods (months). Fires are widespread in the Panhandle, but both Pennisetum glaucocladum and Phragmites mauritianus are stoloniferous, and meristems occur below the soil surface and are protected from fire. In contrast, the rhizomatous sedge C. papyrus dominates in situations where meristems are permanently submerged, and therefore protected from fire, such as areas of open channel water where current velocities are sufficiently low to enable the extension of C. papyrus into the channel. This situation exists where channel avulsion has recently taken place or where discharge is reduced by water loss from the channel by overspill. The semi-floating habit of C. papyrus in the channel fringe results in high hydraulic conductivities, which promotes water loss from channels and leads to sediment deposition within channels. Miscanthus junceus occurs in areas where the nutrient status of water is low, seasonal changes in water level are small, and the average water level is approximately constant over decadal time scales. It occurs some distance from the channel on the upper reaches of the Delta and progressively closer to the channel downstream such that it is the dominant channel fringe species in the distal reaches. This pattern, where a community occurs progressively closer to the channel downstream is similar for communities dominated by Imperata cylindrica and Pycreus nitidus suggesting that environmental gradients (probably nutrient supply) perpendicular to the channel axis are steep and that they are mirrored by long-range environmental gradients downstream. An analysis of hydraulic characteristics of this distributary river system illustrates that channel width varies most with variation in discharge, while channel depth and current velocity are relatively constant over the range of discharges in the study area. Since channel width is primarily a consequence of vegetation processes in the channel margin, especially the growth of the giant sedge C. papyrus it is clear that channel hydraulics are affected largely by vegetation.

The application of high‐resolution sequence stratigraphy to fluvial systems: a case study from the Upper Carboniferous Breathitt Group, eastern Kentucky, USA

ABSTRACTThe Pennsylvanian Pikeville, Hyden and Four Corners formations of the Breathitt Group in eastern Kentucky, USA, contain six major facies associations along with a number of subassociations. These facies associations are offshore siltstone, rhythmically bedded mouthbar heteroliths, predominantly fine‐grained floodplain deposits, minor channel fills, major distributary channels and major, stacked fluvial bodies. The stacked fluvial bodies are incised into a variety of open marine and delta plain deposits, have widths of several kilometres and exhibit a range of sandy fill types. These fluvial complexes are interpreted as incised valley fills.Parasequences and parasequence sets are not identifiable. Nonetheless, it is possible to identify systems tracts on the basis of sequential position, facies associations and systematic changes in architectural style and sediment body geometries. The studied portion of the Breathitt Group comprises stacked 4th‐order sequences, which occur in lowstand, transgressive and highstand sequence sets related to the development of a lower frequency base level cycle.In the lowstand sequence set, incision associated with successive 4th‐order sequence boundaries has commonly removed all the HST and TST of the underlying sequences, such that succeeding 4th‐order incised valley fills are amalgamated. Within the transgressive sequence set, incision is at a minimum and incised valley fills tend to stack discretely with the maximum amount of fine‐grained TST and HST between them. The highstand sequence set is transitional between the lowstand and transgressive sequence sets in terms of the amount of transgressive and highstand deposits preserved. Incised valley fills tend to stack discretely.

Quaternary sandy deposits and canyons on the Venezuelan margin and south Barbados accretionary prism

The geomorphological and tectonic setting, distribution and origin of Late Quaternary sandy deposits on the southern termination of the Barbados accretionary prism and the adjacent abyssal plain have been investigated using Seabeam and 3.5 kHz echosounding data, and lithological and mineralogical analyses of 28 Kullenberg cores. Quaternary sands on this prism are deposited in piggyback basins. They are transported along three major canyons, the morphology and trend of which are highly variable and which are tectonically controlled. The narrow and relatively shallow canyon sections are in some ways like the major distributary channels of deep sea fans, but they are true canyons in terms of the sedimentary processes taking place, namely active erosion associated with the continuous uplift of tectonic structures and deposition of only slightly elevated sediment levees. The sand is partly trapped on basin floors as flat sediment sheets composed of thinly and thickly bedded fine grained sandy turbidites. High deposition rates occurred during the last period of low sea levels. The sands originated from the Orinoco-Trinidad shelf, as inferred from geographical considerations and clay and heavy mineral provenance data (predominently kaolinite, illite, hornblende and epidote). Sands of the abyssal plain settled on a deep sea fan system that trends north-south, parallel to the toe of the prism. Most of the sediments are introduced at the southern end of the system where they are delivered by the three major canyons which merge, on the fan, in a northward braided channel system. This material consists of sandy turbidites that originated from the Orinoco-Trinidad margin. The grain size of the sands is similar to or coarser than the grain size of the prism sands. Sand components are identical and preclude supply from the Amazon river. The sedimentation rate, during periods of low sea level, seems to be lower. A significant percentage of smectite in the clay fraction suggests that a minor part of the material may be derived from the Demerara margin, located closer to the main smectite source, i.e. the Amazon delta. These results emphasize the difference between the sand distribution on active and passive margins.

Sedimentology and palynostratigraphy of Permian rocks exposed at Fairbairn Dam, central Queensland

Sedimentological and palynological studies have been undertaken on a sequence of Permian rocks exposed in the vicinity of Fairbairn Dam, near the town of Emerald in central Queensland. The rocks represent part of the fill of the northern Denison Trough, a north‐south elongate feature on the western margin of the Bowen Basin. The present investigation sheds new light on the depositional environment and stratigraphic context of the exposed sequence. The section is interpreted as the product of coastal environments of deposition. Five lithofacies are recognized and interpreted as: (1) major distributary channel fill: thick, erosively based, cross‐bedded sandstone; (2) interdistributary bay deposits: thinly interbedded sandstone, siltstone and claystone with carbonaceous shale; (3) tidal inlet and tidal delta deposits: erosively based, bimodally cross‐bedded sandstones; (4) beach ridge deposits: massive, quartzose sandstone beds; and (5) middle‐upper shoreface (i.e. inner marine shelf) facies: amalgamated units of hummocky cross‐stratified sandstones. Evidence for tidal flux and the importance of wave‐generated structures suggests that the coastal complex was of mixed‐influence, affected by a balance between fluvial outflow, waves and tides. Regional correlations suggest that the exposed section represents an eastward‐prograding coastal plain without any pronounced deltaic depocentres. Palynological assemblages from four samples support an interdistributary bay interpretation for the thin‐bedded facies, and indicate a Late Permian age for the exposed sequence. Palynofloras, which are dominated by pterophyte spores and disaccate gymnosperm pollen in three samples, and algae and acritarchs in the fourth, are characteristic of the upper stage 5 palynozone of Price (1983). The age and lithostratigraphic correlation suggest that the rocks exposed in the vicinity of the dam spillway are part of the Freitag Formation.

Hydrocarbon transport in the Mackenzie River, Canada

This study examines the sources and transport of hydrocarbons to the lower Mackenzie River in response to concern over the environmental impact of human activities, particularly those related to hydrocarbon development, within the Mackenzie River basin. We carried out two time-of-travel studies by boat, commencing at the confluence of the Liard and Mackenzie Rivers and extending 1250km downstream to a major distributary channel in the Mackenzie delta near Inuvik. The first study, carried out in 1985, characterized summer flow conditions, and the second study reflected snowmelt inputs during spring breakup in 1986. Samples of suspended sediment and water were obtained at a total of eight sites and analyzed for n-alkanes and PAHs. n-Alkanes were present in suspended sediments at all sites in both sampling periods. Based on their distributions and isomer patterns, alkanes in the river appear to originate from three main sources: Great Slave Lake is a source of algal alkanes; a source of undegraded petroleum-related alkanes, possibly from erosion of bituminous shale bedrock; and a source of high molecular weight alkanes, possibly related to combustion. PAHs also enter the river from several sources. Under low flow conditions, low levels of PAHs from petroleum-related sources, possibly seeps, were detected, while under high flow conditions, PAH patterns were dominated by atmospherically-deposited combustion-related PAHs that had been carried into the river by snowmelt.

Model for Unstable Shelf Deltas--A Sequence Conducive to Accumulation of Giant Gas Fields; Example from South Lake Arthur Field: ABSTRACT

ABSTRACT In the south Louisiana onshore, exploration emphasis has now shifted to deeper plays in interdomal areas as the majority of the domes have been discovered. Unstable shelf deltas develop in such areas. Several discoveries have been made during the past few years and a giant one is South Lake Arthur Field. This field ideally fits the unstable shelf delta model. Unstable shelf deltas are deeply buried structures of very large magnitude in which thick reservoir sandstones have been deposited on extensive organic rich prodelta shales (excellent source rocks). Deposition of reservoir sandstones generates the trapping structures -- growth fault and rollover anticline. The sequence is sealed on top by large thicknesses of shales rich in marine organisms. In the unstable shelf delta sequences, rapid deposition of sands on large prodelta shale platforms triggers a series of chain reactions that are self sustaining and supportive -- initiation of growth fault, development of rollover anticline, geopressuring of sediments, shale movement and diapiric activity around the sand depocenter and finally the abandonment and bypassing of the area by major distributary channels initiating marine transgression, reworking of the older deltaic sands and deposition of thick marine shales on top of the sequence. Unstable shelf deltas build in localized basins on the part of the shelf that GIVES actively, through combined effects of subsidence, shale compaction and shale flowage. They develop in stratigraphic units where a strong regressive episode of deposition is immediately followed by a large marine transgression. South Lake Arthur Field is a giant gas field discovery with potential reserves in excess of a trillion cubic feet of gas. More than 25 wells have been drilled in the field and both the log and the seismic data display the typical unstable shelf delta sequence -- 400 to 800 thick reservoir sandstones of Miogyp (Oligocene) age -- sandwiched between greater than 2000 of prodelta shales at the bottom and 4000 or more of transgressive marine shales at the top. Production is between the depths of 16,000 to 18,000. The field is bounded by an arcuate down to the basin fault on the north and is surrounded by shale and salt diapirs all around it. Productive Miogyp sands were deposited in three separate depositional episodes. Lower and Middle Miogyp members are thick in the middle part of the field and are dominantly of distributary mouth bar origin -- rapidly deposited, poorly sorted, and very shaly. The Upper Miogyp sands were developed by the reworking of older Miogyp delta sands. They are clean, better sorted, and are elongated along depositional strike. Lack of knowledge of the unstable shelf delta model combined with structurally emphasized interpretation hindered the recognition, proper development, and accurate mapping of this field. Maps made without the depositional model erroneously used faults to explain variations of interval thicknesses, shale out of sands and anomalies of gas/water levels. In excess of 200 faults were mapped in the field with a map looking like a bowl of fettucini. Utilizing the unstable shelf delta model and the understanding of the depositional sequences, all variations of data could be mapped as a simple east-west trending anticlinal structure with a bounding arcuate fault on the north and no other internal faulting. Other similar features exist, not only on the south Louisiana shelf but in deltaic basins all around the world. End_of_Record - Last_Page 755-------

Three-Dimensional Facies Architecture of River-Dominated Deltas, Upper Cretaceous Dunvegan Formation, Alberta Subsurface: ABSTRACT

The Dunvegan Formation comprises a thick clastic wedge deposited into the West Alberta Foreland basin. This wedge consists of a complex series of interbedded shales and sandstones and has been divided into seven allomembers (A to G), each separated by widespread flooding surfaces. Member E consists of four shingled sandstone bodies that progressively step out into the basin. Detailed mapping of each of these sandstones reveals they consist of several architectural components. These include well-developed, birdfoot-shaped lobate sand bodies that narrow landward and correlate with northwest-southeast-oriented shoestring-shaped linear sand bodies. Core through the lobes show well-developed coarsening-upward sandy facies successions, typical of prograding river-dominated delta fronts. Cores and cross sections indicate that the shoestring sandstones are fluvial channels. They are erosively based, and the facies successions fine upward and are typical of sandy channel fills. Facies successions through interlobe areas shallow upward but do not contain thick sandstones. The association of lobes and channels and the facies associations within these components suggest deposition as river-dominated deltaic lobes fed by major distributary channels. Superposition of sand bodies within each shingle reveals the fashion in which the sedimentary basin was filled. The areal pattern and scale of overlapping depositional delta lobes ismore » similar to the Mississippi delta during the last 5000 years.« less

Fluvial channel and overbank deposits from the Westphalian of the Durham coalfield, NE England

ABSTRACTThe Westphalian (Upper Carboniferous) Coal Measures of the Durham coalfield in NE England were deposited in lower and upper delta plain environments. Distributary channels crossed the plain and were separated by shallow, interdistributary lakes and bays. Detailed observation of three‐dimensional (3‐D) opencast (surface) mine exposures, in collaboration with subsurface borehole analysis, has revealed the existence of five varieties of channel deposits and two associated overbank facies within the Durham Coal Measures.Major distributary channels were the major avenues of sediment transport across the Coal Measures plain, were variably sinuous, mostly 1–2 km wide and deposited elongate belts of sand mostly up to 5 km wide. Proximal, major crevasse splay channels formed by the breaching of major channel banks during flood events, were straight, sand‐filled and up to 400 m wide. Minor distributary channels formed by the sustained operation of such crevasses, varied from straight to highly sinuous, and deposited ‘shoestring’ sand/mud belts up to a few hundred metres wide. Minor crevasse channels, generally straight and up to 50 m wide, were formed through bank breaching of minor, and in a few cases major, distributaries. Distal feeder channels formed down‐palaeocurrent extensions of minor distributaries which supplied interdistributary minor delta subsystems, were generally straight and up to 200 m wide.Of the two types of channel overbank (levee) deposits recognized, one, comprising thinly interbedded fine‐grained sandstone and siltstone/claystone, is mostly, though not exclusively, associated with major distributary channels. The other, consisting of ‘massive’ siltstones with regularly spaced, thin claystone bands, is uniquely developed at the margins of minor distributary channels.The lower part of the Westphalian A succession in the northern Pennines records a change in the depositional environment upwards from a lower to upper delta plain. Through this transition, major channel deposits show evidence of having evolved from being of dominantly low sinuosity to being more variable in morphology. Channel sedimentation was profoundly influenced by regular, possibly seasonal, variations in flow stage and sediment load.

Styles of Interdistributary Basin Sedimentation: Mississippi Delta Plain, Louisiana: ABSTRACT

ABSTRACT Deposits in interdistributary basins are largely controlled by the relative rates of subsidence and the amount of sediment transported into the basin. Marine delta progradation and subsequent abandonment on approximately a 1000 to 1500 year cycle isolate extensive backswamp and marshy areas between major distributary channels. The resulting interdistributary basins areally occupy a large portion of the lower alluvial valley and upper delta plain. Extensive backswamps occur at the apex of these basins and grade seaward into fresh, brackish, and saline marshes. Following delta-lobe abandonment, high subsidence rates combined with low sediment input lead to flooding of the basin, both by marine incursion and the enlargement of lakes. Analyses of sedimentary processes in the Barataria and Atchafalaya Basins of the Louisiana delta plain reveal two contrasting styles of sediment accumulation. Thick sequences of peat, interfingering with fine-grained lacustrine, marsh, and backswamp deposits are characteristic of a basin starved of terrigenous clastic sediment (Barataria Basin). Formation of a distributary channel in the basin (Atchafalaya Basin) results in the introduction of large quantities of coarser-grained detrital clastics to the lowland. Coarsening-upward lacustrine delta sequences, which average several meters in thickness, prograde tens of kilometers into the interdistributary basin, ultimately coalescing with adjacent deltaic, alluvial and backswamp deposits. In the Atchafalaya Basin, deposition of backswamp deposits with coarser-grained lacustrine deltaic and alluvial sediments aggrades the interdistributary basin and delta plain in the process of prograding a distributary channel and marine delta. Once this marine delta complex is abandoned, the site of active deposition shifts to another area. The lack of detrital influx increases the relative subsidence rate, and peat accumulation becomes the dominant process in the basin. Continual subsidence leads to complete inundation of the backswamp and termination of distributary channel in the basin. An upstream channel avulsion, or distributary formation, may again initiate siliciclastic deposition in the basin.

Basin and Range-Age Reactivation of the Ancestral Rocky Mountains in Texas Panhandle: Evidence from Ogallala Formation: ABSTRACT

The Ogallala Formation (Neogene) is a widespread syntectonic alluvial apron that was shed eastward from the Rio Grande rift and related uplifts in Colorado and New Mexico during Basin and Range extension. In the Texas Panhandle, the Ogallala completely buried Ancestral Rocky Mountain (Pennsylvanian) structures. Renewed movement on these older structures during the Neogene influenced the thickness and facies distribution of the Ogallala. The Ogallala thickens into the Palo Duro, Dalhart, and Anadarko basins. Major distributary channels on Ogallala alluvial fans coincide with the axes of these basins, whereas major interchannel areas overlie intervening uplifts. Second-order structures subtly influenced the unit as well. For example, in the Carson basin, a Pennsylvanian rhomb graben along the Amarillo uplift, the Ogallala is over 250 m (820 ft) thick compared with 90 m (275 ft) in adjacent areas. Within the Palo Duro basin, local highs controlled the distribution of thin, interchannel fl od-basin and lacustrine deposits. Thicker, braided-stream channel deposits follow local lows. Later movement on the Amarillo uplift broadly folded the Ogallala. The southern high plains surface subtly reflects basement structure, with topographic highs overlying basement highs, suggesting post-Ogallala deformation within the Palo Duro basin. The Amarillo uplift is approximately perpendicular to the Rio Grande rift and parallel to the direction of Basin and Range extension. Thus, the stress field that produced the rift may have caused strike-slip movement and reactivation of the Carson basin along the Amarillo uplift. End_of_Article - Last_Page 459------------

Paleogeographic controls of coal accumulation, Cretaceous Blackhawk Formation and Star Point Sandstone, Wasatch Plateau, Utah

Analysis of sequences, associations, and relationships of facies in the Upper Cretaceous Blackhawk Formation and Star Point Sandstone in the Wasatch Plateau, Utah, leads to inferences regarding the paleogeographic controls affecting the accumulation of coals in delta–barrier-island complexes. Progradation, lateral shifting, and abandonment of these complexes created four major landward-thinning tongues of the upper sandstone bed of the Star Point Sandstone that extend into the lowermost Blackhawk Formation. Each tongue of the Star Point Sandstone trends northwest to southeast, subparallel to the general orientation of the Late Cretaceous paleoshoreline, and represents accretion-ridge, distributary-channel, and mouth-bar sandstones. The accretion-ridge or barrier-island sandstones grade landward into coastwise lagoonal and back-barrier swamp deposits, which, in turn, merge with bayhead deposits of short-headed streams. These deposits grade laterally into delta-plain deposits that consist of major distributary channels which crosscut the coastwise lagoons. Two coal populations were formed in this paleogeographic setting: delta-plain and back-barrier coals. Delta-plain coals formed as thin to thick, discontinuous bodies. Back-barrier coals are thicker and more laterally continuous. They parallel depositional strike and accumulated in swamps on surfaces of infilled lagoons. Here, the swamps were sites of accumulation of thick coals and were influenced by abandonment of associated deltaic barrier-island complexes. Successive offlap events resulted in accumulation of the Hiawatha coal, not as a single continuous bed, but as a series of discontinuous, related coal bodies.

Age relationships of distributary channels on Amazon deep-sea fan: Implications for fan growth pattern

Seismic-reflection profiles and long-range side-scan (GLORIA) sonographs reveal the relative age relationships between the major distributary channels of the upper and middle Amazon deep-sea fan. These age relationships, in turn, suggest that the fan grows through formation of a succession of broad levee complexes, each of which consists of several individual leveed distributary channels whose levee systems overlap or coalesce with one another. Only one channel is apparently active at any given time on an actively growing levee complex. Eventually this channel is abandoned, probably through avulsion, and a new channel and associated levee system form nearby, thereby enlarging the levee complex. Occasionally, the formation of a new channel will cause the present levee complex to be abandoned and will initiate a new levee complex at a different location. In this manner, a succession of overlapping levee complexes is formed through time, and the fan grows upward as well as radially outward downslope.

Growth patterns of the acheloos and evinos deltas, Western Greece

The coastal plain complex on the north side of the Gulf of Patras has been built by the overlapping Acheloos and small Evinos deltas. Upstream river channels are braided; on the lower part of the delta they are meandering. Classical writings indicate that the present Acheloos channel has been occupied for at least 2300 years; air photographs show three former major distributary channels. The present annual sediment discharge (suspended: 3–4 million tons; bedload: 2 million tons) is sufficient to build all the modern delta plain by progradation of these four distributaries in the last 5000–7000 years of relatively stable sea level. Old maps and air photographs show that river mouths prograde by periodic plugging of sub-distributaries within 1 km of the mouth. Where the river progrades into deep water (> 20 m) a steep sandy pro-delta slope forms and a shoal mouth bar is developed by wave reworking. The freshwater muddy plume of the Acheloos extends up to 15 km offshore. Analysis of about 30 bottom samples shows that very silty sediment settles within a few kilometers of the river mouth, becoming progressively more clayey offshore. Marine reworking of abandoned distributaries takes place rapidly (0.5 km in the first ten years, and a further 0.5 km in the next 30 years). Sand is redistributed to form parallel accretionary spits and barrier beaches. A shallow pro-delta platform up to 2 km wide has formed by such reworking along the southern margin of both deltas. Gradual subsidence (less than 0.5 mm/year) leads to flooding of shallow lagoons behind barrier beach systems. Subaerial delta facies have been mapped in the field, and extensively sampled in artificial channel cuts. Normal tidal range is only 16 cm, but storm tides up to 1 m high flood low supratidal flats that locally extend several kilometres inland. Broad immediately subtidal silt flats are found on the inner part of the western Evinos pro-delta platform. Silt is drifted in suspension by strong winter easterly winds from the Evinos mouth, and the pro-delta platform is sufficiently wide and shallow to damp down most large waves that would subsequently remove the silt. Where sand supply is limited, the coastline is marked by a sandy low-tide terrace, and a berm or storm ridge built of variable proportions of sand and reeds. Only near delta mouths is sand supply sufficient for broad barrier beach-dune systems to form.

Facies, Paleoecology, and Depositional Environments of Energy Shale Member (Pennsylvanian) and Their Relation to Low-Sulfur Coal Deposits, Southern Illinois: ABSTRACT

Thick deposits of Energy Shale (Carbondale Formation, Desmoinesian) are associated with low-sulfur coal deposits in the underlying Herrin No. 6 coal in southern Illinois. The Energy Shale consists of wedges, up to 85 ft (25.9 m) thick, which thin away from the Walshville Channel (interpreted as a major distributary channel deposit in the study area). Facies recognition and interpretation of depositional environments are based upon lithology, sedimentary structures, vertical and lateral relations, geometry, and paleoecology as well as organic matter content and total sulfur content. Four facies are recognized in the surface mines studies. The thickest and coarsest facies is adjacent to the Walshville Channel and is characterized by numerous fining-upward channel-fill sequences of sandstone, siltstone, and silty shale. It is interpreted as a series of crevasse distributary channel-fills in the proximal parts of splays. This proximal splay facies grades laterally into shale with abundant plant remains, thin coal beds (splits from the Herrin No. 6 coal) and some siderite concretions. It is interpreted as the distal deposits of crevasse splays. The distal splay facies grades laterally into what is interpreted as an interdistributary bay-fill facies consisting of shale with laterally persistent siderite layers. Sixteen miles (25.8 km) from the Walshville Channel, a zo e at the top of this facies is extensively burrowed and contains pectinoid bivalves, indicating some marine influence. The bay-fill facies grades laterally and vertically into shale, containing a moderately diverse marine fauna composed mostly of stunted individuals. It is interpreted as a marginal marine facies. The Walshville Channel and the crevasse splay facies of the Energy were partly contemporaneous with Herrin peat deposition. The sulfur content of the Herrin No. 6 coal is highest beneath the marginal marine facies (the thinnest) and the proximal splay facies (the thickest) of the Energy Shale. End_of_Article - Last_Page 1283------------