Delta Clinoforms Research Articles

Spatial analysis of Holocene delta compound clinoforms

Recognition of compound delta clinoforms has led to a new understanding of delta progradation and architecture. Limited study has been dedicated to spatially delineate three dimensional morphologies of subaqueous deltas and their migration away from active sediment sources. Qualitative and quantitative analyses of 38 Holocene deltas lead to the observation of two main gradient breaks at tens-of-meters water depth and variable subaqueous geometries. Geometric variability was captured relative to riverine-sediment sources and basinal energies and presents steeper shoreline and subaqueous delta clinoforms, reaching 1°, separated by a gently-dipping 0.11° platform with widths from few kilometers (Ebro Delta) to 200 kilometers (Amazon Delta). In wave-dominated systems, the platform is absent ahead of the sediment source but widens laterally away as a smooth arcuate surface, whereas in tide-dominated systems the platform widens ahead of active sources and narrows ahead of starved ones while hosting erosional features and tidal channel/bar couplets.

Sedimentation of the Oligocene-Lower Miocene clinoforms of the Maikop formation in the Eastern and Central Pre-Caucasus region as a key criteria for reservoir exploration

Oligocene-Lower Miocene clinoforms of the Maikop formation are productive in the Eastern and Central Pre-Caucasus region. In spite of commercial discoveries and long exploration history, Maikop formation is poor characterized by borehole data in the deep basins such as Terek-Caspian trough. Furthermore, there are challenges of the detail stratigraphic subdivision in the deep basins, correlation, sedimentary conditions and criteria of reservoirs exploration. Based on seismic and borehole data analysis, sequent-stratigraphic framework, distribution area, progradation direction, seismic unconformities and thicknesses of the 17 Maikop sequences are established. Estimation of the clinoforms height allowed reconstructing paleobathymetry and paleogeography of the Pre-Caucasus region in the Oligocene – Early Miocene. Clinoforms M1-M7 prograded from the northeast, sea depth increased from 300 to 450 m. During clinoforms M8 sedimentation, new provenance area appeared on the west, sea depth increases up to 800 m. Clinoforms M12-M16 prograded from the north, sea depth decreases from 480 to 270 m. To the end of the M17 sequence deposition, starved deep basin was totally compensated. Three morphologic types of the clinoforms are identified. Type I represents by tangential clinoforms up to 75 m of height, that interpreted as subaqueous delta. Type II are the sigmoid shelf-edge delta clinoforms up to 800 m of height. Clinoforms of type III are low-angle wedges, confined to the slope and floor of the deep basin. Height of the wedges does not exceed 270 m. In the topset of the shelf-edge deltas, shallow-marine sandstones and structural traps predicted, while basin floor fans and stratigraphic traps expected in the bottomset. Subaqueous deltas are regarded to stratigraphic traps, while the wedges are predominantly mud-prone.

Outer-shelf conduit within growth-fault compartment Pliocene Orinoco Delta

Prominent outcrops of the Pliocene Orinoco Delta along the southeast coast of Trinidad Island display a well-known, very thick Icehouse succession of storm-wave dominated delta lobes, as part of a 10–12 km thick Neogene continental-margin prism. There is a previously discovered, steep incision or conduit (400 m wide) with south and north-bounding erosion surfaces, cutting deeply into the Mayaro Formation shelf-edge delta deposits. The ca. 100 m-thick infill of the conduit shows an initial muddy succession passing upwards to very coarse-grained debrites, rotated and slumped blocks, and structureless sandstones that pass upward to a stacked series of sandy to muddy channel fills. The channels show spectacular tidal rhythmites, probably developed on the point bars of subaqueous tidal channels. As one piece of the topset of the shelf-edge delta clinoform, this conduit displays features of the linkage hub between the landward on-shelf delta systems and the basinward slope deposits.New fieldwork and a re-examination of the incision-infill reveal new details and suggest three possible origins of the conduit: ① the first is a no river feeder, shelf-edge collapse scar, extending onto the upper slope and infilled by deeper to shallower deposits. This shelf-edge cutting canyon interpretation is supported by the presence of hardground ichnology on the steep walls of the incision and the abundance of chaotic blocks and debrites in the lower two-thirds of the incision infill. ② The second is a shelf channel or valley linking fluvial channels to on-shelf and outer shelf deltas. This river-connected, subaqueous shelf valley origin, is supported by the upward-coarsening nature of the infill succession and especially the occurrence of shallow-water tidal channels in the upper third of the infill. ③ The third is a canyon developed mainly during transgression or highstand with a collapsed canyon-head infill and an overlying muddy tidal-channel system that had only limited sediment bypass to the deepwater slope and basin floor. This case represents both the shallow-water and the deeper water features of the infill succession, and is supported by similar large shelf-edge collapse features seen elsewhere in the Cruse and Moruga Formations of the paleo-Orinoco margin. This case, unrelated to river supply, also benefits from slight rotational shelf-edge uplift to trigger collapse.The known Cedar Grove growth fault was active and located some 4 km to the west from the present coast on the southeast corner of Trinidad Island. The growth-fault trapped thickened outer shelf to shelf-edge delta-front and delta-plain sediments within the hanging-wall compartment as the fault stretched about 20 km along strike. All the shelf-edge aggradation and its partial collapse were likely triggered by growth-faulting. After some muddy backfilling and coarse-grained block collapse into the invaginated shelf-edge, this scalloped area attracted a belt of subaqueous tidal channels, thus healing the eroded topography.

Late Pleistocene shelf‐edge delta clinoforms along the rift margin of the northern Okinawa Trough

Sedimentological study into the architecture of continental margins reveals its importance in predicting the partition of sediment budget between the shelf and the deep‐water settings. The sediment delivery towards the deep‐water basin during sea‐level lowstands can be hindered along rift margins. Here we use a 2D seismic survey from the northern Okinawa Trough to identify a set of clinoform seismic units elongating along the western slope, where the rifting activity occurred later than the late Pleistocene. The clinoform units are interpreted as a shelf‐margin delta complex based on the following observations: (a) the sediments clearly prograde across the eroded paleo‐shelf on the dip‐oriented sections; (b) there is some negative relief along two extensively traceable reflections, indicating the probable existence of paleo‐channels working as the potential pathways of sediment delivery for the shelf‐margin deposits; and (c) an eroded surface is easily recognized in the seismic data, possibly marking the sea‐level lowstand during which the shelf‐margin delta formed. The oxygen isotopes of the core QZ01 are taken as a proxy for the variation of the relative sea level and indicate its fast fluctuation in the past 180 kyr. We interpret how the shelf‐margin delta evolved during the late Pleistocene and propose that the subsidence and the rift faulting lead to the starvation of sediments near the shelf edge. As a result, there could be more sediments depositing along the active rift margin, while few of them would be transported to the deep‐water basin and re‐deposit as turbidites.

Characteristics of a Campanian delta deposit controlled by alternating river floods and tides: the Loyd Sandstone, Rangely Anticline, Colorado, U.S.A.

ABSTRACTThe Campanian Loyd Sandstone Member of the Mancos Shale (Loyd) along the Rangely Anticline, Piceance Basin, Colorado, contains a series of multi-meter-thick, heterolithic, coarsening-upward successions (CUSs) with internal clinoforms comprising low-angle-dipping (< 5°), parallel-laminated sandstone beds interbedded with bioturbated flaser-wavy-lenticular bedded sandstone and siltstone. Loyd clinoforms are delta foresets composed of prodelta and delta-front deposits, including mouthbars. Many CUSs exhibit scours filled with bioturbated sandstone, or interbedded bioturbated sandstone and siltstone or mudstone representing aggradational fill of subaqueous terminal distributary channels or the migration of mouthbars into channel scours. Mud drapes on sedimentary structures and mud rip-up clasts are extremely common. A high-abundance, high-diversity, trace-fossil assemblage includes vertical, 1–4-m-deep Ophiomorpha that may penetrate multiple bedsets of parallel-laminated sandstones and highly bioturbated finer-grained interbeds. Bioturbation increases sandstone content in finer-grained interbeds, and provides sandy conduits that increase connectivity between beds.Although parallel-laminated sandstones volumetrically dominate Loyd delta clinoforms, they likely represent relatively short-term freshwater and sediment input during river flooding that produced delta-front turbidity currents. Interbedded finer-grained beds, mud-draped sedimentary structures, and the high-abundance, high-diversity trace-fossil assemblages record longer amounts of time during reduced fluvial discharge, tidal reworking of sediments, and intense bioturbation under marine salinities. Controls on the internal characteristics of the Loyd delta deposits include: 1) a high sediment influx into a relatively shallow marine basin protected from wave action during normal regression; 2) relatively short-duration, episodic freshwater and sediment discharge from distributaries that reduced salinities, deposited sand as turbidity currents, and promoted delta-front channelization; and 3) longer-duration periods of reduced discharge with deposition of finer-grained sediment, tidal reworking, and bioturbation of sediments under higher salinities. These controls combined to produce the heterolithic, highly bioturbated, river-flood delivered, and tidally modified clinoforms of the Loyd that differ from deposits typically considered to be classic examples of fluvial-flood-dominated or extensively tidally modified deltas.

Shallow‐water deltaic clinoforms and process regime

AbstractUtilizing two outcrop data sets with dip direction exposures of shallow‐water (tens of meters) deltaic clinoforms, this paper quantifies sedimentary facies proportions and clinoform lengths and gradients, and links process regimes to delta clinoform dimensions. Both data sets are from foreland basins, the Cretaceous Chimney Rock Sandstone of the Rock Springs Formation from the US Western Interior, and the Eocene Brogniartfjellet Clinoform Complex 8 of the Battfjellet Formation from the Central Basin of Spitsbergen. Sedimentary facies indicate presence of both river‐ and wave‐dominated clinothems in each data set. Facies characteristics and distribution implies that river‐dominated clinothem progradation was primarily driven by deposition from weak hyperpycnal flow turbidity currents across the clinoforms, and minor slumps. Wave‐dominated clinothems were constructed by wave processes rather than alongshore currents, and are also progradational subaerial clinoforms, with one exception, where the formation of a compound subaqueous clinoform set indicates erosion and sediment bypass above the wave base. Sediment distribution and lithological heterogeneity in the river‐dominated clinothems is controlled by individual hyperpycnal flow events or mouth‐bar collapse events, and thus by self‐organization and minimal reworking that results in a heterogeneity that is difficult to predict (high entropy). The efficient reworking of river‐derived sediments in wave‐dominated clinothems results in predictable lithological sediment partitioning (low entropy). Clinoform dimension analyses show that although of similar sediment caliber, river‐dominated clinoforms in both data sets are on average 3–4 times steeper and 3–4 times shorter than the wave‐dominated clinoforms, with mean gradients of ca 4 degrees and ca 1 degree, respectively, and mean lengths of 150–230 m and 640–760 m. These results require corroboration from additional data sets, but do suggest that river‐ and wave‐dominated delta clinoforms are likely to have distinct downdip extents (lengths) and gradients for given clinoform heights. Clinoform shape can thus be a method for differentiating ancient river‐ vs. wave‐dominated deltaic clinoforms, in addition to their sedimentary facies, biogenic features and sandstone maturity, and helpful when incorporated into reservoir models.

Benthic foraminifera distribution in a deltaic clinoform (São Francisco Delta, eastern Brazil): A reference study

Abstract Foraminifera distribution in surface sediment of the continental shelf is controlled by several environmental factors. Expansion and spreading of fluvial plumes significantly change environmental conditions at shelf areas facing river mouths. The present work contributes with an in situ reference study on the spatial distribution of foraminiferal assemblages in the surface sediments of the Sao Francisco delta clinoform and adjacent areas (outer shelf and homonymous submarine canyon). Four benthic foraminiferal biofacies were identified based on quantitative and qualitative analyses of foraminiferal microfauna from 116 sediment samples. Biofacies I and II, both dominated by Ammonia beccarii, perfectly delimit respectively the topset and upper foreset portions of the delta clinoform. Biofacies III characterizes the area around the Sao Francisco canyon head, which is partially buried by the clinoform. This biofacies is dominated by Globocassidulina subglobosa and Hanzawaia concentrica and typifies an important stratal surface, known as the Maximum Flooding Surface, over which the clinoform progrades. Biofacies IV, dominated by Textularia spp., characterizes the clinoform bottomset and outer shelf sediments. The outer shelf is typically covered by bioclastic sand and gravel. In this region, besides Textularia spp., biofacies IV also comprises macro-foraminifera species, such as Amphistegina spp., Archaias angulatus and Peneroplis spp., typical of reef bottoms.

Visualisation and analysis of shear-deformation bands in unconsolidated Pleistocene sand using ground-penetrating radar: Implications for paleoseismological studies

Deformation bands in unconsolidated sediments are of great value for paleoseismological studies in sedimentary archives. Using ground-penetrating radar (GPR), we investigated an array of shear-deformation bands that developed in unconsolidated Pleistocene glacifluvial Gilbert-type delta sediments. A dense grid (spacing 0.6 m) of GPR profiles was measured on top of a 20 m-long outcrop that exposes shear-deformation bands. Features in the radargrams could be directly tied to the exposure. The shear-deformation bands are partly represented by inclined reflectors and partly by the offset of reflections at delta clinoforms. 3-D interpretation of the 2-D radar sections shows that the bands have near-planar geometries that can be traced throughout the entire sediment volume. Thin sections of sediment samples show that the analysed shear-deformation bands have a denser grain packing than the host sediment. Thus they have a lower porosity and smaller pore sizes and therefore, in the vadose zone, the deformation bands have a higher water content due to enhanced capillary forces. This, together with the partially-developed weak calcite cementation and the distinct offset along the bands, are likely the main reasons for the clear and unambiguous expression of the shear-deformation bands in the radar survey. The study shows that deformation-band arrays can clearly be detected using GPR and quickly mapped over larger sediment volumes. With the 3-D analysis, it is further possible to derive the orientation and geometry of the bands. This allows correlation of the bands with the regional fault trend. Studying deformation bands in unconsolidated sediments with GPR is therefore a powerful approach in paleoseismological studies. Based on our data, we postulate that the outcrop is part of a dextral strike-slip zone that was reactivated by glacial isostatic adjustment.

Chronological evolution of some morphological, tectonic and volcanic features in Lake Van, based on correlation of seismic and core data

The Lake Van is the largest Lake in Turkey and the fourth largest soda lake in the World. High resolution (HR) seismic data acquired during 2012 reveals sub-lacustrine terraces, delta clinoforms, channel-networks, active faults and volcanic edifices. The seismic data were correlated with the cores recovered during the 2008 coring and 2010 ICDP PaleoVan drilling campaigns, using synthetic seismograms produced from the MSCL data of the age-modelled cores, thus allowing to date seismic reflector surfaces and to discuss the temporal evolution of the some of the morphological features and structures. Sub-lacustrine channel-network systems are well developed on the shelf areas connect with the river drainage systems on land and extend to water depths of more than 100 m. These drainage systems developed during the major low-stand periods of Lake Van, the last two of which occurred during 15 ka BP and the Younger Dryas with water levels at −200 m and −70 m, respectively. High lake levels similar to today during the early Holocene were followed by lake level oscillations with some low-stands during the Late Holocene. The low stands are characterized by terraces and berms located at 15 m below lake surface (mbls), 25 mbls, 35 mbls and 60 mbls, 70 mbls and 105 mbls. They were most likely related to the rapid climate change (RCC) events such as the 4.2 ka aridity event, Dark Age Cold Period, Medieval Warm Period and Little Ice. The volcanic features are located along the Southern Boundary Fault (SBF). A significant NW-SE trending active fault in the eastern shelf left-laterally offsets a channel by about 500 m and has a small reverse component in the HR seismic profiles. This fault is considered to be the source of Mw = 5.7 Edremit Earthquake of 9 November 2011. NW-SE trending normal faults with a right-lateral strike slip component north of the Northern Basin provides extension in the area. Normal faults are also dominant in the Erciş Strait. Using ages of reflector surfaces we estimate a vertical slip rate of 0.40 mm/yr to ∼0.5 mm/yr on the normal faults in the shelf areas over the last 2–3 ka.

Late Holocene distal mud deposits off the Nakdong delta, SE Korea: evidence for shore-parallel sediment transport in a current-dominated setting

The distal mud deposits (DMDs) off the Nakdong delta represent a subaqueous delta on the inner continental shelf aligned parallel to the southeast coast of Korea and displaying a clinoform geometry. Hydrographically, the coast is characterized by a micro-tidal regime, the strong Korean Coastal Current (KCC) and the East Korean Warm Current (EKWC). Age models and sedimentary facies related to the clinoform geometries are based on high-resolution chirp subbottom profile data and have provided information on shore-parallel sediment transport and accumulation during the late Holocene sea-level highstand. The highest sedimentation rates (6.19–9.17 cm/year) produced steep foresets in the central DMDs at water depths of 35–50 m. Here, vertical burrows are repeatedly truncated by laminated mud packages displaying erosional surfaces. This region represents the main depocenter of the Nakdong subaqueous delta. The topset sediments of the southern DMD at ~40 m water depth closer to the river mouth show relatively low sedimentation rates (0.01–0.12 cm/year). Here, the muds have a predominantly mottled character. Similarly, the foreset sediments of the northern DMD at ~71–80 m water depth with sedimentation rates of 0.10–2.03 cm/year are also predominantly characterized by mottled muds. The spatial dispersal pattern of the DMDs is consistent with the coast-parallel front between the KCC and EKWC along the southeast Korean coast. In addition, the depocenter of the Nakdong subaqueous delta clinoform is affected by the near-bed turbulence generated by episodic storm events.

Organic matter compositions and loadings in soils and sediments along the Fly River, Papua New Guinea

The compositions and loadings of organic matter in soils and sediments from a diverse range of environments along the Fly River system were determined to investigate carbon transport and sequestration in this region. Soil horizons from highland sites representative of upland sources have organic carbon contents (%OC) that range from 0.3 to 25wt%, carbon:nitrogen ratios (OC/N) that range from 7 to 25mol/mol, highly negative stable carbon isotopic compositions (δ13Corg<−26‰) and variable concentrations of lignin phenols (1<LP<5mg/100mgOC). These compositions reflect inputs from local vegetation, with contributions from bedrock carbon in the deeper mineral horizons. Soils developed on the levees of active floodplains receive inputs of allochthonous materials by overbank deposition as well as autochthonous inputs from local vegetation. In the forested upper floodplain reaches, %OC contents are lower than upland soils (0.8–1.5wt%) as are OC/N ratios (9–15mol/mol) while δ13Corg (−25 to −28‰) and LP (2–6mg/100mgOC) values are comparable to upland soils. These results indicate that organic matter present in these active floodplain soils reflect local (primarily C3) vegetation inputs mixed with allochthonous organic matter derived from eroded bedrock. In the lower reaches of the floodplain, which are dominated by swamp grass vegetation, isotopic compositions were less negative (δ13Corg>−25‰) and non-woody vegetation biomarkers (cinnamyl phenols and cutin acids) more abundant relative to upper floodplain sites. Soils developed on relict Pleistocene floodplain terraces, which are typically not flooded and receive little sediment from the river, were characterized by low %OC contents (<0.6wt%), low OC/N ratios (<9mol/mol), more positive δ13Corg signatures (>−21‰) and low LP concentrations (∼3mg/100mgOC). These relict floodplain soils contain modern carbon that reflects primarily local (C3 or C4) vegetation sources. Total suspended solids collected along the river varied widely in overall concentrations (1<TSS<9000mg/L), %OC contents (0.1–60wt%), OC/N ratios (7–17mol/mol) and δ13Corg signatures (−26 to −32‰). These compositions reflect a mixture of C3 vascular plants and freshwater algae. However, little of this algal production appears to be preserved in floodplain soils. A comparison of organic carbon loadings of active floodplain soils (0.2 and 0.5mgC/m2) with previous studies of actively depositing sediments in the adjacent delta–clinoform system (0.4–0.7mgC/m2) indicates that Fly River floodplain sediments are less effective at sequestering organic carbon than deltaic sediments. Furthermore, relict Pleistocene floodplain sites with low or negligible modern sediment accumulation rates display significantly lower loadings (0.1–0.2mgC/m2). This deficit in organic carbon likely reflects mineralization of sedimentary organic carbon during long term oxidative weathering, further reducing floodplain carbon storage.

Effects of tides on deltaic deposition: Causes and responses

This paper explores the response of deltas to tidal reworking and discusses the conditions at which deltas may become strongly tide influenced or tide dominated. Three case studies are presented, from the Cretaceous Western Interior, the Devonian Baltic Basin and the Eocene Central Basin of Spitsbergen. All datasets contain extensive outcrop data, the Baltic Basin dataset also extensive core data.The paper aims to show that beyond the commonly recognized morphological features and the recognizable tidal facies, the main effects of tidal reworking of deltas are associated with delta clinoform morphology, delta lobe switching rates, delta progradation rates, and the nature of the delta plain. Strong tidal influence is here documented to promote subaqueous, rapid progradation of deltas, by efficiently removing sediment from river mouth and thus reducing mouth bar aggradation and fluvial delta plain construction rates. Such subaqueous progradation of the delta front is decoupled from shoreline progradation. The delta plain of such tide-dominated deltas consists of a few distributary channels and tidal flats. The delta front clinoforms become gentler and longer, as tidal currents efficiently transport sediment to the basin. Tide-dominated deltas tend to maintain a funnel shape and show low lobe switching rates, compared to fluvial-dominated and tide-influenced deltas.This paper emphasizes topographic restrictions or invaginations, caused by incision, delta-lobe deposition or tectonic uplift and subsidence, as significant controls on the occurrence of strongly tide-influenced deltas. The role of such topographic restrictions is twofold, by reducing wave energy and amplifying tidal energy. Significantly, such invaginations are not restricted to inner-shelf reaches as commonly assumed, but may also occur at the shelf edge, promoting tidal reworking of shelf-edge deltas. Moreover, in very shallow epicontinental seas, significant tidal reworking may occur throughout the basin evolution, independent on sea-level cycles or sediment supply.

Tidal and seasonal dynamics of a muddy inner shelf environment, Gulf of Papua

Subaqueous delta clinoforms are accretionary features created on the continental shelf where there is a large supply of sediment. Sediment is transported across the shallow topset region and accumulates rapidly in the deeper foreset region. In the Gulf of Papua, Papua New Guinea, the absence of cyclonic storms makes tidal currents and seasonal variation of wind direction primary controls on the timing and magnitude of across‐shelf sediment transport. To investigate processes that disperse sediment across the topset of the Fly River clinoform, anchor stations were occupied during spring and neap tidal cycles of the trade wind season, the monsoon season, and the transition period. During the energetic trade wind season, surface waves coupled to strong spring tide currents generated peaks in suspended‐sediment concentration (&gt;100 g L−1 at some locations). However, seabed erosion and deposition over tidal timescales could not be discerned, indicating that sediment was being advected across the outer topset. Although seaward gravity flows occur with significant spatial heterogeneity, they are important for carrying sediment to the foreset region because the near‐bed circulation generally has a landward net flow. Seaward transport is restricted to the energetic trade wind season. During the quiescent monsoon season, 4–11 cm of erosion occurs on parts of the outer topset as fluvial sediment is temporarily deposited closer to shore. Seasonal patterns in sediment dispersal are likely impacted by El Niño–Southern Oscillation, and consideration of El Niño conditions of 2002–2003 are reflected in the interpretation of observations.

Facies Architecture of Flexural Margin Lowstand Delta Deposits in Lake Edward, East African Rift: Constraints from Seismic Reflection Imaging

Abstract Lake Edward is single half-graben rift basin unique in the East African Rift System as a result of its connection to Lake George via the Kazinga Channel, a paleo–river valley which is incised into the flexural margin of the lake. Using recently acquired normal-incidence seismic reflection data and thirty-three lake-bottom sediment samples, we examine the consequences of this hydrologic connection for understanding depositional processes and facies development along the flexural margin of Lake Edward. Three depositional sequences were defined in the study area based on top and bottom discordances of seismic reflections. Climatically induced changes in lake level appear to have forced sequence development. Sequence I deposits indicate that highstand conditions prevailed earlier in the Pleistocene. Evidence for high-frequency lake-level change comes from lowstand delta clinoforms in two separate stratigraphic intervals on the flexural margin. An older lowstand event occurred during Sequence II time and reduced lake level by at least ~ 37 m. A younger lowstand event occurred during the deposition of Sequence III and reduced lake level by ~ 18 m. Clastic sedimentation on the flexural margin of Lake Edward is strongly modified by inflow from the Kazinga Channel and Lake George during periods of lowered lake level. We suggest that the presence of an incised valley at the margin of the rift controls the stratigraphic development of clinoforms diagnostic of lowstand deltaic sedimentation. The morphology and orientation of these clinoforms suggest that discharge from Lake George during arid periods utilized the Kazinga Channel to transport sediment toward the exposed flexural margin of Lake Edward. Dipping reflections in sigmoid-oblique clinoforms suggest that these flows were capable of transporting sediment loads that included coarse grain sizes. Deposition by this process occurred several times during the late-Quaternary history of Lake Edward, and represents an important modern analogue for linked incised valley-lacustrine delta sequences in the stratigraphic record.

Late Quaternary sequence stratigraphy of the Mackenzie Delta

The Late Wisconsinan and Holocene sequence stratigraphy of the Mackenzie Delta provides insights into the glacial history of the region. The base of the described succession is a hummocky regional reflector interpreted to be a flooding surface formed immediately after retreat of glacial ice from the Mackenzie Trough. Above this flooding surface, two progradational parasequences are present. The first, assigned to the transgressive systems tract, is correlated with the Tutsieta Lake readvance of the ice sheet at approximately 13 000 BP. A flooding surface forming the upper boundary of this parasequence extends inland to at least Inuvik, developing as a response to glacial retreat and early Holocene relative sea level rise. The second parasequence of Holocene deltaic deposits is assigned to the highstand systems tract and is characterized by progressive progradation of the delta into the Mackenzie Trough to a position seaward of the present delta coastline. A distinct reduction in gradient of the most recent delta clinoforms is consistent with other data suggesting regional transgression and is interpreted to represent the development of a healing-phase wedge. The reasons for this recent transgression are not clear, because relative sea level rise has decreased and sediment supply probably increased over the last 2000 years. Transgression may be related to decreased efficiency of channels, increased trapping of sediments by thermokarst lakes, overspill of the delta across the eastern margin of the valley, and (or) progressively greater exposure to wave action as the delta became less sheltered by the confines of the glacial valley.

Seismic Stratigraphy, Sedimentology, and Reservoir Potential of a Late Pleistocene Shelf-Edge Delta

Seismic Stratigraphy, Sedimentology, and Reservoir Potential of a Late Pleistocene Shelf-Edge Delta

Latest Quaternary Outer Shelf and Slope Deposits, Northern Gulf of Mexico, USA: Industry Research Consortium

A consortium of oil companies is undertaking a multiyear shallow coring and high-resolution seismic investigation of the outer shelf-slope deposits of the northern Gulf of Mexico. The program's objective is to reconstruct the depositional, seismic stratigraphic, biostratigraphic, isotopic, and organic geochemical record of the latest Quaternary of a portion of the Gulf margin. Results will serve as an analog to deeper hydrocarbon reservoirs and to help in understanding sedimentation with glacial-interglacial sea-level changes. Nearly continuous cores up to 850 ft long from Main Pass Blocks 303, 242, 288 and Viosca Knoll Block 774 are being described and analyzed. Main Pass 303 core samples Holocene to Oxygen Isotope Stage 6 deposits and as old or older strata are expected to have been penetrated in the other boreholes. Three or four sea level cycles are represented. Seismic acquisition is planned for late 1990. The outer shelf and slope of the northern Gulf is an area of active growth faulting, high sedimentation rate, diapirism, and slumping. Recent work, including this study, shows that near-surface stratigraphy consists of coarse clastics alternating with carbonate-rich mud. Sands and gravels are inferred to have been deposited mostly during lowstands and rising sea level in deltas and valley-fillmore » systems. Delta clinoforms on high-resolution seismic may represent shelf-margin wedges, prograding wedges, or highstand systems tracts. Highstand deposits are characterized largely by thin, continuous intervals of bioturbated muds containing abundant diagenetic carbonate and pyrite. Precise dating and correlation of outer shelf-upper slope facies to magnitude and direction of sea level change is intended during the project.« less