Subaerial Delta Plain Research Articles

Structure and formation conditions of the Yarakta productive horizon of the Late Vendian in the southern part of the Nepsko-Botuobinskaya Anticlise

The relevance. Eastern Siberia is a region with large raw material resources that plays a crucial role in development of Russian oil and gas complex. Since the 1970s, active exploration for hydrocarbons has been underway in the Nepsko-Botuobinskaya Anteclise. One of the main targets for development are terrigenous Vendian deposits, in which up to half of the region hydrocarbon reserves are concentrated. Therefore, for optimal development of the fields confined to them, a detailed study of lithological factors of formation of terrigenous reservoirs is required. The main aim. Lithological and facies analysis is used to reconstruct and trace the evolution of sedimentation conditions of the Yarakta productive horizon of the Nepa Formation. Objects. Terrigenous deposits of the Yarakta productive horizon of the Upper Nepa subformation of the Upper Vendian Formation, uncovered by deep drilling in the area of the Yarakta field. Methods. Mineralogical-petrographic study of core material, facial analysis of lithological rock types, correlation. Results. The Yarakta productive horizon was formed during Late Nepa time. At that time, there was a partly subaerial and to a greater extent subaerial delta plain (lower delta plain) within the tidal plain. It is assumed to be a delta of advancement on a coast dominated by tidal activity and to a lesser extent by wave activity (mixed type of coast). In the subsequent transgression, in the late Nepean time (upper part of the strata), a bar system began to form in place of the delta plain with the bars advancing toward the coastline in the northwestern direction. The bars had an elongated lentil-shaped shape along the conditional coastline. During the formation of the bars, due to the active wave and tidal activity, the sediments carried out by the delta were redeposited and re-sorted. At the Late Nepa and Early Tertiary boundary, the entire area was a shallow intertidal and terrigenous sebha (lagoon) with clay-siltstone sediments and sulfate-carbonate inclusions in them; sometimes, sandy sediments of small intertidal channels or sandbanks are noted in them. Improved filtration-capacitative properties are associated with the facies of distributary subaquatic channels and large (active) tidal channels.

Sharp‐based shoreface successions reconsidered in three‐dimensions: A forward stratigraphic modelling perspective

AbstractSea‐level fall is commonly inferred to generate a sharp‐based shoreface succession that displays an abrupt vertical transition from heterolithic, lower shoreface to sandy, upper shoreface deposits across a marine erosion surface. Three‐dimensional, process physics‐based, coupled hydrodynamic‐morphodynamic models are constructed to compare bedding architecture and facies patterns of wave‐dominated delta deposits preserved during normal (static sea level) and forced (falling sea level) regression and then transgression during subsequent sea‐level rise. The models suggest that wave‐dominated deltas will develop a sandy shoreface inner clinoform dipping from the subaerial delta plain to a relatively flat wave‐scoured subaqueous delta top, which is laterally separated from a delta front outer clinoform that dips from the subaqueous delta top edge to the shelf floor. As these systems prograde, deposits of these dual‐clinoforms will become vertically stacked and will be separated by a regressive surface of marine erosion. Significant grain‐size contrasts between these vertically stacked clinoform deposits reflect differences in sediment‐transport directions and sorting under river and wave‐driven littoral currents along the coast, and cannot be related uniquely to sea‐level changes. The marine erosion surface under a sharp‐based shoreface deposit records abrupt facies shift across a kilometres‐wide, wave‐eroded surface and defines a discontinuity in the preserved vertical succession. The continuity of a regressive surface of marine erosion mapped over many tens to hundreds of kilometres across mid‐shelf regions of some stratigraphic sequences reflects a gradual lateral shift in the position of littoral current erosion on a subaqueous delta top. Timelines cross such vertical lithic discontinuities throughout the extent of a prograding deposit, and the regressive surface of marine erosion thus has little chronostratigraphic significance. The results of these models suggest caution in inferring sea‐level changes from the character of vertical facies changes observed in individual well logs and isolated outcrop exposures.

Stepping out across the Karoo retro-foreland basin: Improved constraints on the Ecca-Beaufort shoreline along the northern margin

The Karoo Supergroup of South Africa, internationally renowned for its almost continuous Carboniferous to Jurassic record of deposition in a foreland basin setting, hosts an unparalleled record of fossil tetrapods that provides unique insight into terrestrial biodiversity change over this extended period. Understanding of the litho-, bio- and chronostratigraphy of the Karoo Basin has greatly improved in recent years but most work has focused on the thicker and better-exposed rocks in the basinal foredeep. A core issue that has remained ambiguous is the stratigraphic placement of the diachronous Ecca-Beaufort contact, for which different criteria have been used during mapping in different parts of the main Karoo Basin. Comparison of biostratigraphy and contrasting lithological facies changes with those in the better-studied foredeep enabled us to revise the lithostratigraphy of the Ecca and Beaufort groups in the distal part of the main Karoo Basin. The presence of proximal marine facies associations in the distal sector conform with the definition of the Waterford Formation of the Ecca Group, so far recognised only in the southern foredeep. The upper contact of the Waterford Formation represents a change from a subaqueous to a subaerial delta plain depositional environment and marks the boundary between the Ecca and Beaufort groups, as in the south. This has consequences for Beaufort Group stratigraphic subdivision in the distal sector of the Karoo foreland basin.

Biostratigraphy of the Eodicynodon Assemblage Zone (Beaufort Group, Karoo Supergroup), South Africa

Abstract The middle Permian Eodicynodon Assemblage Zone is the lowermost biozone of the Beaufort Group (Adelaide Subgroup, Karoo Supergroup) and occurs in the southwestern part of the main Karoo Basin. It is host to a diverse assemblage of basal therapsid genera of which Eodicynodon is the most abundant. The biozone reaches a maximum thickness of 1 100 m in the Prince Albert Road area and thins to the east and west. The biozone corresponds to the Combrinkskraal and Grootfontein members of the Abrahamskraal Formation, directly overlies the Waterford Formation of the Ecca Group, and records the earliest middle Permian terrestrial environments of Gondwana. Rocks of the biozone were deposited along the southern shoreline of the Karoo Basin in a subaerial delta plain environment as part of large-scale fan systems draining to the north and northeast within a second-order highstand systems tract.

Dynamic controls on shallow clinoform geometry: Mekong Delta, Vietnam

Compound deltas, composed of a subaerial delta plain and subaqueous clinoform, are common termini of large rivers. The transition between clinoform topset and foreset, or subaqueous rollover point, is located at 25–40-m water depth for many large tide-dominated deltas; this depth is controlled by removal of sediment from the topset by waves, currents, and gravity flows. However, the Mekong Delta, which has been classified as a mixed-energy system, has a relatively shallow subaqueous rollover at 4–6-m depth. This study evaluates dynamical measurements and seabed cores collected in Sep 2014 and Mar 2015 to understand processes of sediment transfer across the subaqueous delta, and evaluate possible linkages to geometry. During the southwest rainy monsoon (Sep 2014), high river discharge, landward return flow under the river plume, and regional circulation patterns facilitated limited sediment flux to the topset and foreset, and promoted alongshore flux to the northeast. Net observed sediment fluxes in Sep 2014 were landward, however, consistent with hypotheses about seasonal storage on the topset. During the northeast rainy monsoon, low river discharge and wind-driven currents facilitated intense landward and southwestward fluxes of sediment. In both seasons, bed shear velocities frequently exceeded the 0.01–0.02m/s threshold of motion for sand, even in the absence of strong wave energy. Most sediment transport occurred at water depths <14m, as expected from observed cross-shelf gradients of sedimentation. Sediment accumulation rates were highest on the upper and lower foreset beds (>4cm/yr at <10m depth, and 3–8cm/yr at ~10–20m depth) and lowest on the bottomset beds. Physically laminated sediments transitioned into mottled sediments between the upper foreset and bottomset regions. Application of a simple wave-stress model to the Mekong and several other clinoforms illustrates that shallow systems are not necessarily energy-limited, and thus rollover depths cannot be predicted solely by bed-stress distributions. In systems like the subaqueous Mekong Delta, direction of transport may have a key impact on morphology.

TERRESTRIAL VERTEBRATE COLONISATION AND THE ECCA-BEAUFORT BOUNDARY IN THE SOUTHEASTERN MAIN KAROO BASIN, SOUTH AFRICA: IMPLICATIONS FOR PERMIAN BASIN EVOLUTION

Rocks of the Late Carboniferous to Early Jurassic aged Karoo Supergroup of South Africa preserve a sedimentary succession, deposited in a retro-arc foreland setting. This succession documents environmental change from glacial-marine, through full marine to continental fluvial and aeolian environments, culminating in rift associated continental flood basalt extrusions. The Karoo Basin is internationally renowned for its wealth of fossil tetrapods, enabling the establishment of a reliable and useful biostratigraphic framework which has international applicability for correlation of Permian-Triassic tetrapod-bearing continental deposits. The transition from marine to continental deposition in the Karoo has been the subject of much recent research, particularly in regard to the position of the Ecca-Beaufort contact. Our study indicates for the first time that in the south-eastern part of the basin, as for the rest of the basin, this transition comprises three separate facies associations deposited respectively in the prodelta, subaqueous delta plain and subaerial delta plain environments. The Tapinocephalus Assemblage Zone is the lowermost vertebrate biozone in the Koonap Formation indicating that the Ecca-Beaufort boundary is diachronous in the southern part of the basin, younging towards the east. This supports the easterly to northeasterly prograding shoreline model previously proposed for the Ecca-Beaufort transition and provides new insight on the distribution of the earliest land-living vertebrates in the south-eastern Karoo Basin.

The Middle Pleistocene tunnel valley at Schöningen as a Paleolithic archive

Schöningen represents one of the key sites for Lower Paleolithic archaeology in central Europe, where a Middle to Late Pleistocene sedimentary succession, locally up to 45 m thick, has been preserved in an Elsterian tunnel valley. After deglaciation, the tunnel valley remained underfilled and provided the accommodation space for Holsteinian interglacial deposition and also kept the artifact-bearing strata below base level for subsequent erosion. The Holsteinian (MIS 9) succession consists of laterally and vertically stacked lacustrine delta systems, which were controlled by repeated lake-level changes. In the face of changing climatic and environmental conditions the long-lived interglacial lake provided an attractive site for animals and early humans. Artifacts were deposited on the subaerial delta plain and became embedded during lake-level rise. Although the area was considerably affected by erosion and glacitectonic deformation during the subsequent Saalian glaciation, the artifact-bearing Holsteinian strata were preserved in the deeper part of the tunnel valley.Tunnel valleys should be regarded as potential archives for interglacial deposits, which may contain important Paleolithic sites. Tunnel valleys may provide accommodation space and also have a high preservation potential. Interglacial lakes situated within underfilled tunnel valleys represented attractive sites for animals and early human hunter-gatherers.

Waterford Formation in the south-eastern Karoo: Implications for basin development

Extensive research on the rocks of the Karoo Supergroup has shown that this sequence, which contains an unsurpassed record of Permian–Jurassic tetrapods, records a largely unbroken stratigraphic succession from 300 Ma to 180 Ma. This Gondwanan succession was deposited in a changing environmental setting reflecting glacial marine through deltaic to fluvial and aeolian desert conditions. The contact between the Ecca and Beaufort Groups (at the top of the Waterford Formation of the Ecca Group) in the southern and western Karoo represents a change in depositional environment from a subaqueous to a subaerial delta plain. By contrast, the Waterford Formation has not yet been recognised in the south-eastern Karoo Basin, which might imply that a major unconformity is present between the Fort Brown Formation of the Ecca Group, deposited in a prodelta environment, and the overlying fluvially deposited Koonap Formation of the Beaufort Group. From careful documentation of lithofacies and sedimentological data, it can be demonstrated that the Waterford Formation is indeed present in the south-eastern part of the basin and that no major unconformity is present – a fact that has implications for the mapping of Karoo rocks in the Eastern Cape Province of South Africa, for understanding the depositional environment of ’reptilian‘ fossils from the lowermost Beaufort in this part of the Karoo basin, and for basin development models.

Sedimentological and ichnological signatures of changes in wave, river and tidal influence along a Neogene tropical deltaic shoreline

AbstractAnalysis of Neogene cores from the Eastern Venezuela Basin along 65 km of a west–east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty‐three facies are defined and grouped into four categories (wave‐influenced, river‐influenced, tide‐influenced and basinal). Wave‐dominated deltaic deposits occur mostly in the Tácata Field. The delta plain was characterized by tide‐influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm‐wave reworking and suspended sediment fallout. Delta‐front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid‐generated ichnogenus Scolicia. River‐dominated deltaic deposits are present in the Santa Bárbara, Mulata, Carito and El Furrial Fields. Low‐sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher‐sinuosity rivers. The subaqueous delta plain includes distributary channels and tide‐influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary‐channel and distributary mouth‐bar deposits, as well as by delta‐front and prodelta deposits showing evidence of sediment gravity‐flow and fluid‐mud emplacement. Delta‐front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river‐dominated deltas, tide‐influenced delta plains, wave‐dominated deltas and wave‐dominated strandplain–offshore complexes.

3D modelling of grain size distribution in Quaternary deltaic deposits (Llobregat Delta, NE Spain)

The Llobregat delta constitutes a Pliocene-Quaternary sequence recording progradation. An 8 meter-thick, delta-plain succession temporarily accessible in the subaerial delta plain was sub-divided into four units (A to D), which constitute a shallowing upwards sequence recording sedimentation from shoreface at the bottom (Unit A) to lacustrine settings at the top (Unit D). Unit B basically comprises gravelly, coarse-grained sandy lenticular bodies (up to 20 m wide, up to a few decimetres thick), within a homogenous, medium-grained sandy background, interpreted as prograding foreshore bars. 3D facies models, built by using the sequential conditional indicator simulation method, reproduced the sedimentary heterogeneities identified in Unit B, using as input data the sedimentary body geometry and its correlation with sandstone grain size values, with the grain size modes taken as a continuous, quantitative proxy for facies distribution. The simulations not only reproduced satisfactorily the significant sedimentary heterogeneities generated by the facies arrangement, but also predicted and described the spatial relation and the three-dimensional shape of sedimentary bodies accurately. Quality of the models should be ensured by a consistent prior analysis of the spatial structure of the grain size mode based on semivariogram models.

Bay-head deltas across the northern Gulf of Mexico back step in response to the 8.2 ka cooling event

Estuaries along the northern Gulf of Mexico show rapid landward shifts in estuarine environments (back-stepping) around 8200 cal yr BP. Paleogeographic maps of the estuaries based on seismic data and cores, and constrained chronologically by radiocarbon dates, depict bay-head delta retreat in excess of 25 m/yr. During these events the sub-aerial delta plain flooded to create sub-tidal upper- and middle-bay environments. Abrupt sea-level rise associated with the 8200 cal yr BP climate-change event caused these estuaries and others around the world to reorganize abruptly. In the northern Gulf of Mexico, the sea-level rise associated with the event was only ∼0.1 m over a sub-decadal time period. This rise was superimposed on a background decelerating rate of sea-level rise of ∼1.0–0.5 m/century. The widespread bay-head delta back-stepping, documented here, may be an appropriate analog for possible abrupt future climate change in response to global warming. It also shows that estuaries fringed by low-gradient shorelines are highly sensitive to low-amplitude and sudden sea-level rise.

DIVERSITY, TAPHONOMY and PALAEOECOLOGY OF AN ANGIOSPERM FLORA FROM THE CRETACEOUS (CENOMANIAN–CONIACIAN) IN SOUTHERN PATAGONIA, ARGENTINA

Abstract: This paper describes the diversity, taphonomy and palaeoecology of angiosperm leaves that dominate a palaeoflora of Cretaceous (Cenomanian–Coniacian) age from the Mata Amarilla Formation in the Austral Basin, south‐west Patagonia, Argentina. Twelve morphotypes of angiosperm leaves are recognized based on foliar morphotype analysis of more than 500 specimens. These were divided into six morphological groups based on major architectural patterns. The relative dominance of these morphotypes, mode of preservation and relationship with sedimentary facies were evaluated from two levels within the formation. This analysis identified two different plant palaeocommunities. The lower, María Elena, level (MEL) was deposited in a marine coastal area on a subaerial delta plain; the dominant angiosperm morphotypes preserved in it are group 1 (MA100) and group 2 (MA101, 102); morphotypes MA109 and 110 are scarce but exclusive to this level. The upper, Mata Amarilla, level (MAL), accumulated inland in flood‐plain environments; the most abundant angiosperm morphotypes are groups 3 (MA103–105), 4 (MA106) and 1 (MA100); morphotypes MA103–105 and 108 are exclusive to this level. Comparisons with other floras of similar age from Antarctica, Australia and New Zealand indicate that the Mata Amarilla flora has a slightly higher morphological diversity of angiosperm leaves, providing the first evidence for an angiosperm‐dominated early Late Cretaceous macroflora in south‐west Gondwana.

The Mackenzie Delta: sedimentary processes and facies of a high‐latitude, fine‐grained delta

The Mackenzie Delta is a large fine‐grained delta deposited in a cold arctic setting. The delta has been constructed upon a flooding surface developed on a previous shelf‐phase delta. There are three principal depositional zones: the subaerial delta plain, the distributary channel mouth region and the subaqeous delta. The subaerial delta plain is characterized by an anastomosing system of high‐sinuosity channels and extensive thermokarst lake development. This region is greatly influenced by the annual cycle of seasonal processes including winter freezing of sediments and channels, ice‐jamming and flooding in the early spring and declining river stage during the summer and autumn. Deposition occurs on channel levees and in thermokarst lakes during flood events and is commonly rhythmic in nature with discrete annual beds being distinguishable. In the channel mouth environment, deposition is dominated by landward accretion and aggradation of mouth bars during river‐ and storm surge‐induced flood events. The subaqeous delta is characterized by a shallow water platform and a gentle offshore slope. Sediment bypassing of the shallow‐water platform is efficient as a result of the presence of incised submarine channels and the predominance of suspension transport of fine‐grained sediments. Facies of the shallow platform include silty sand with climbing ripple lamination. Offshore facies are dominated by seaward‐fining fine sand to silt tempestites. Sea‐ice scouring and sediment deformation are common beyond 10 m water depth where bioturbated muds are the predominant facies. The low angle profile of the shallow‐water platform is interpreted to be the combined response of a fine‐grained delta to (1) storm sediment dispersal; (2) autoretreat as a result of the increasing subaerial and subaqeous area of deposition as the delta progrades out of its glacial valley; (3) limited water depth above the underlying flooding surface; and (4) efficient nearshore bypassing of sediment through subice channels at the peak of spring discharge. Several indicators of the cold climate can be used as criteria for the interpretation of ancient successions, including thermokarst lake development, submarine channel scours, freeze–thaw deformation and ice‐scour deformation structures. Permafrost inhibits compaction subsidence and, together with the shallow‐water setting, also limits autocyclic lobe switching. The cold climate can thus influence stratal architecture by favouring the development of regional‐scale clinoform sets rather than multiple, smaller scale lobes separated by autocyclic flooding surfaces.

Progradation of the Changjiang River delta since the mid-Holocene

Subaqueous deltaic deposits with approximately 30 radiocarbon ages show that the Changjiang River delta was strongly affected by tides and that the delta progradation rate after 2 kaBP was almost double the rate before 2 kaBP. This change in the progradation rate correlates well with the active extension of the subaerial delta plain shown by previous work. Widespread human activities, such as farming, deforestation, and dike construction, probably resulted in an increase in sediment discharge to the river-mouth area.

The significance of large sediment supply, active tectonism, and eustasy on margin sequence development: Late Quaternary stratigraphy and evolution of the Ganges–Brahmaputra delta

Abstract Borehole data from the Ganges–Brahmaputra delta system unveil an intriguing Late Quaternary history controlled by immense sediment discharge, tectonics, and eustasy. Sea level first intersected a major portion of the lowstand surface ∼10,000–11,000 cal yr BP, marking the onset of sediment trapping and delta growth. Despite rapid sea-level rise (>1 cm/yr), sediment load was sufficient to maintain relative shoreline stability during this time. By ∼7000 cal yr BP, accommodation was quickly infilled with slowed sea-level rise, leading to upstream channel migration and widespread dispersal of sands. This forced coastal progradation along the western delta front where the Ganges was situated, and much of the river's fine-grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf. Concurrently, Brahmaputra sediments were sequestered to an inland tectonic basin, thus starving the adjacent shoreline and leading to transgression along the eastern delta. By ∼5000 cal yr BP, the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river-mouth estuary. Present delta configuration appears to have developed by ∼3000 cal yr BP. Evolution of the tectonically active Ganges–Brahmaputra (G–B) system reveals important similarities and differences with other deltas. Overall facies succession follows that of basic models, progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain. However, the timing, thickness, and controls of these deposits differ. Immense sediment discharge from the Ganges and Brahmaputra rivers allowed initial delta growth 2000–3000 yr earlier than the global average, despite debouching onto a canyon-incised, high-energy margin. Subsequently, a thick (∼50 m) transgressive systems tract was formed during the early Holocene phase of delta aggradation, contrasting with extensive shoreline retreat documented along most margins. Highstand progradation of coastal and shelf sequences ensued by the middle Holocene, but subsiding inland basins also favored accumulation of a thick (to 40 m) highstand sequence in the lower floodplains. Unlike many other delta systems, subsidence in the subaerial G–B delta is not dominated by compaction, but rather by plate-driven tectonic processes that generate rates up to 4 mm/year. Overall, the huge sediment load, tectonic subsidence, major seismic events, and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta, shelf, and deep-sea Bengal Fan throughout the Late Quaternary.

Lake sedimentation in the Mackenzie Delta, NWT

Spatial variation in lake sedimentation within the Mackenzie Delta was documented, and total lake sedimentation over the past 30 years was estimated, by classifying (via aerial photography) the flooding characteristics of a complete transect of lakes (about 2600 lakes) across the upper subaerial delta plain, then selecting representative samples of lakes (81 lakes) from the transect, and a random sample of lakes from a second transect (7 lakes) across the lower subaerial plain, for direct measurement of sedimentation rates. The depth corresponding to the 1963 sediment layer was obtained from a sediment core from each of these lakes by sampling the core at frequent depth intervals to identify the peak in 137 Cs (Caesium-137) content. Lake sediment bulk densities, as obtained from the cores, averaged 931 kg/m 3 , a value lower than reported in previous studies. Using mean bulk density above each 137 Cs peak, and the depth of the 137 Cs peak, mean lake sedimentation for different lake types varied between 1.19 kg/m 2 /yr and 5.34 kg/m 2 /yr, with a decrease in sedimentation as lake sill elevation increased. Values for the lower delta transect averaged 1.36 kg/m 2 /yr By combining relationships between sedimentation and lake type with a classification of lake types, total sedimentation across the upper-delta transect is estimated as 8.2 Mt/yr, and 0.8 Mt/yr for the lower delta. These values are lower than previously reported primarily because they are derived from lower bulk density values. Of the total upper-delta sedimentation, 17% occurs in high-closure lakes, 53% in low-closure lakes, and 30% in no-closure lakes. Lake sedimentation is higher in the Middle Channel segment of the upper-delta transect (44% of total sedimentation across the transect) than in the East Channel (25%) or Peel Channel (32%) segments.

Impacts géomorphologiques côtiers des aménagements du Var et de son delta, Côte d'Azur, France

Coastal geomorphic impacts of human-induced transformations of the Var River and Delta, French Riviera The high-relief coast of the Baie des Anges, on the French Riviera, is bounded by two gravel barriers linked to the central Var Delta. This deltaic protuberance forms the terminus of a 135 km-long, steep-gradient river. Over the past century, the lower Var valley and the delta have been massively transformed in various ways. These transformations have included reclamation of large sectors of the river bed for agricultural and transport networks, embankments against flooding of the reclaimed land and construction of several low rock dams across the river to enable gravel accumulation and enhancement of the height of the the alluvial aquifer of the Var, which, by the mid-sixties, had dropped considerably as a result of massive human withdrawal. Two important consequences of these changes were a drastic reduction in the supply of gravel to the coast and destabilization of the morphodynamics of the delta front. The drop in gravel supply to the barrier beaches fringing the Baie des Anges resulted in accelerated barrier breakdown. Following a long period of stability during which permanent loss of gravel to the steep nearshore shelf was compensated in the past by fresh supplies from the Var, the implantation of various installations such as seafront roads, rail links, yachting harbours and reclamation fill structures on the barriers had, by the 1950's, already led to the onset of this phase of erosion. The reclamation of much of the lower valley and the entire delta plain resulted in the irreversible transformation of these morphodynamic units into an artificial system bounded seaward by revetments that have replaced the former gravel beaches. Reclamation of the lower valley and of the subaerial delta plain led to enhanced seaward transport of suspended sediments that could no longer be stored within the fluvial system, and to greater fine-grained sedimentation on the steep and unstable delta front slopes. One of the consequences of these transformations, notably the modification of the upper delta front slope, was a catastrophic submarine landslide, in October, 1979. This landslide caused several casualties, destroyed part of the reclamation works on the upper delta front, including a port under construction at the time, and generated a small tidal wave in the Baie des Anges. Whether gradual, as in the case of beach erosion, or associated with threshold exceedance, as in the case of this landslide, prediction of the effects of human destabilization of coastal systems is hampered by a poor grasp of the functional mechanisms and processes underlying such systems, whose stability depends as much on what goes on upland as in the sea.

Carbonate platform, lagoon and delta “high-frequency” cycles from the Carnian of Lombardy (Southern Alps, Italy)

The Early and Middle Carnian succession of the Bergamasc Alps (Southern Alps, Italy) was deposited in three different environments: a deltaic depositional system passing seaward to a lagoon (or bay) and then to a carbonate platform. The corresponding lithofacies (sandstones and siltstones; shales and limestones) are organized in “high-frequency” cycles. Fining-upward trends are typical of the subaerial delta plain cycles, and shallowing-upward trends are widespread in the lagoon and platform cycles. The genesis of the lagoonal and carbonate platform cycles is possibly accounted for by sea-level changes, mainly controlled by small-scale eustatic oscillations. Sea-level changes are interpreted as exerting a major control also in the development of the deltaic cycles.

Petroleum Geology of Bangladesh: ABSTRACT

The easternmost part of the Bengal foredeep or Surma basin is the most prospective area for finding additional gas because the degree of folding diminishes markedly in a westward direction. The foothills of the Tripura-Chittagong area and the Bengal basin (sometimes called Bengal foredeep or Surma basin) are locations of the gas fields in Bangladesh. These areas have sometimes been called the Outer Molasse basin. Folding occurred in four phases. Gas discoveries are in the Chittagong foothills. Similar structural features to those of the Chittagong foothills appear to be present in the extreme eastern part of the Bay of Bengal. Compressional folding did not affect the central and western part of the Bay of Bengal. However, by comparison with other areas of deltaic depositi n, rollover structures associated with growth faults may be significant. The Oligocene to Holocene rock sequences were deposited in environments that range from abyssal marine prodelta to subaerial delta plain. In productive areas onshore and offshore, hydrocarbon traps include asymmetric, elongate, faulted anticlines. Strategic traps and sedimentary growth structures are found in the Bengal basin. Miocene sandstones constitute the gas reservoirs; Eocene, Paleocene, and Oligocene carbonaceous shales and Miocene shales are the source rocks. In the central and western part of the Bay of Bengal area, the major uncertainties are the development and thickness of sandstones and the possible size of structures. Thirteen gas fields have been discovered: (1) Kutubdia, (2) Chhatak (8 mmcf/day), (3) Kailashtilla, (4) Habiganj (14 mmcf/day), (5) Bakhrabad, (6) Murdi, (7) Begumgoni, (8) Beanibazar, (9) Sylhet, (10) Rashidpur, (11) Titas, (12) Semutang, and (13) Jaldi. Chhatak, Habiganj, Sylhet, and Titas fields were on production during 1979. The gas in these fields occurs in multi-sandstone reservoirs in anticlines that probably developed in late Miocene to Pliocene time. The gas reservoirs are the lower Miocene Bhuban Formation and lower to lower-middle Miocene Boka Bil Formation. Both formations are included in the Surma Group. Total recoverable gas reserves are 7 to 7.8 tcf. Total estimated gas reserves in place are 9.33 to 10.39 tcf and possibly 10 to 20 tcf of gas resources yet to be discovered. End_of_Article - Last_Page 572------------

Fan-Delta Sedimentology and Tectonic Setting--Yallahs Fan Delta, Southeast Jamaica

Fan deltas have been defined as alluvial fans that prograde into a standing body of water from an adjacent highland. Most large Holocene fan deltas are located at the edge of active continental margins and island-arc systems where high-gradient braided streams deposit their sediment load of coarse detritus. Such coastlines are usually wave dominated and receive between 100 and 300 cm of annual precipitation. The Yallahs fan delta along the southeast coast of Jamaica is a typical example of this type of fan-delta complex. The Yallahs River debouches from the Blue Mountains and has built a 10.5-sq km lobate fan delta composed of sand to boulder-size detritus. The morphology of the Yallahs delta is controlled in part by the foothills of the Blue Mountains which bound the delta on three sides and by a steep offshore profile which causes waves to break and expend most of their energy directly against the delta front. Environments comprising the subaerial delta plain include erosional and depositional beaches, abandoned and braided channels, flood plains, and salt-water ponds. The subaqueous delta environment is characterized by steep slopes and numerous submarine channels dominated by mass-gra ity processes resulting in patchy sediment-size distribution. Active coral growth occurs along the western delta margin. Sedimentologic characteristics of the Yallahs fan delta and published information on the southeast coast of Alaska provide data for the construction of two stratigraphic models that can be used for the recognition of ancient fan-delta deposits. The model based on the characteristics of the Yallahs fan delta is applicable to ancient fan-delta deposits that grade directly onto continental and/or island-arc slopes. These deposits are characterized by proximal, gravelly, braided-stream deposits that grade seaward into interbedded gravels and sands at the shoreline and to muddy gravels and muds of the slope. The second model, based on the characteristics of the southeast Alaska coastline, is applicable to fan-delta deposits that prograde onto continental and/or island shelves. These deposi s are characterized by gravelly, proximal, braided-stream deposits that grade seaward into sandy, distal braided-stream deposits, well-laminated sands of the beach and nearshore zone, and finally into burrowed shoreface muds. The two models represent end members of a spectrum of fan-delta types common in ancient sedimentary sequences ranging in age from Precambrian to Pleistocene.