High Sediment Input Research Articles

Structure of the Makran subduction zone from wide-angle and reflection seismic data

Makran is one of the largest accretionary wedges on the globe, formed by the convergence between the Eurasian and the Arabian Plates. It is characterised by an extremely high sediment input of 7 km and a shallow subduction angle. We present seismic velocity models from four wide-angle seismic lines that image the wedge sediments and the subducted oceanic crust. Three strike-lines show complex but rather one-dimensional structures, where the observed seismic phases have been studied with the reflectivity method. A 160 km long dip line was surveyed coincident with a MCS line collected by Cambridge University in 1986. Prestack depth migration of this data with the new wide-angle velocities shows improved images compared to earlier results. Interpretation of the wide-angle and the MCS data indicates that a décollement is developed along a bright reflector within the turbiditic sequence. More than 3 km of sediment bypasses the first accretionary ridges (underthrusting) and is transported to greater depth. This might help to explain the sparse earthquake activity associated with subduction here. In the strike lines, low-velocity layers occur landward of the deformation front, in zones where active thrusting or underplating takes place. The oceanic crust is characterised by a strong velocity contrast between layer II and layer III.

Categorizing the morphologic variability of siliciclastic passive continental margins

Research Article| March 01, 2000 Categorizing the morphologic variability of siliciclastic passive continental margins Damian B. O'Grady; Damian B. O'Grady 1Institute of Arctic and Alpine Research, Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA Search for other works by this author on: GSW Google Scholar James P. M. Syvitski; James P. M. Syvitski 1Institute of Arctic and Alpine Research, Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA Search for other works by this author on: GSW Google Scholar Lincoln F. Pratson; Lincoln F. Pratson 2Department of Earth and Ocean Sciences, Duke University, Durham, North Carolina 27708, USA Search for other works by this author on: GSW Google Scholar J. F. Sarg J. F. Sarg 3Mobil Exploration and Producing Technical Center, Dallas, Texas 75247, USA Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (3): 207–210. https://doi.org/10.1130/0091-7613(2000)28<207:CTMVOS>2.0.CO;2 Article history received: 07 Sep 1999 rev-recd: 04 Nov 1999 accepted: 15 Nov 1999 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Damian B. O'Grady, James P. M. Syvitski, Lincoln F. Pratson, J. F. Sarg; Categorizing the morphologic variability of siliciclastic passive continental margins. Geology 2000;; 28 (3): 207–210. doi: https://doi.org/10.1130/0091-7613(2000)28<207:CTMVOS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The basic morphology of the continental shelf, slope, and rise on passive continental margins has many variations in the modern ocean environment. Through using a satellite-derived bathymetric data set of the global ocean, we have developed a classification for the shape of modern, siliciclastic-dominated, passive margins. The classification subdivides 50 margins into five distinct shape categories based on each margin's pattern of sea-floor slopes as a function of depth. By comparing these shape categories with the underlying stratal architecture, sediment input, and present degree of canyon incision of the margins, we find apparent correlations between these factors and basic margin shape. In general, gently sloped margins tend to occur in regions with high sediment input, few modern canyons, and unstable substrates. Higher gradient margins tend to have lower sediment input, more modern canyons, and a variety of subsurface architectures. These results suggest two important conclusions: (1) morphology differences among modern siliciclastic passive margins can be objectively and systematically categorized, and (2) these shape differences seem to be related to aspects of the modern sedimentary environment despite the fact that the margins have evolved over geologic time. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The late Mesozoic and Cenozoic evolution of the Sørvestsnaget Basin: A tectonostratigraphic mirror for regional events along the Southwestern Barents Sea margin?

Seismic interpretation and mapping of the Sørvestsnaget Basin at the southwestern Barents Sea Margin has revealed a complex basin history, related to large scale plate tectonic movements in the Norwegian-Greenland Sea area. The pre-Cenozoic evolution of the Sørvestsnaget Basin is poorly understood, due to lack of well data and poor seismic data quality. Regional pre-Cenozoic geologic models, that includes the Sørvestsnaget Basin, should await more and better data. Prior to break-up and sea-floor spreading in the Norwegian-Greenland Sea, Late Cretaceous and Palaeocene shear movements along the Senja Fracture Zone led to structuring and possibly elevation of the southern part of the basin. As sea-floor spreading took place and propagated northwards along the Senja Fracture Zone, the western areas of the basin were uplifted and eroded. Two large and two smaller intrabasinal features in the southern Sørvestsnaget Basin are characterised by: (a) low values on the gravimetric anomaly maps; (b) surrounding synforms; and (c) little response on magnetic anomaly maps. The features have been interpreted as salt structures. The Tertiary evolution of intrabasinal salt-structures seems to have been governed by extensional faulting and subsidence of the margin during the Late Palaeocene and Eocene periods. The Oligocene and Miocene periods were characterised by local and regional uplift of the Sørvestsnaget Basin, respectively. During these periods, the area in particular the western part of the basin, was subjected to transpressional/compressional forces, which can be tentatively correlated to contemporaneous, large scale plate reorganisations and variations of sea-floor spreading rates in the Norwegian-Greenland Sea. The Oligocene and Miocene transpressional/compressional events influenced the salt structures, and is here suggested to be partially responsible for the complex expression of the largest salt structures. The Late Neogene and Pleistocene periods are represented by a thick, westward prograding wedge, indicating subsiden3e with coinciding high sediment input to, and minor adjustment of the salt structures in, the Sørvestsnaget Basin.

Oceanography and sedimentation in the semi-enclosed, deep-water Gulf of Corinth (Greece)

Sedimentological and oceanographic characteristics in the Gulf of Corinth are examined, to define modern sedimentation processes over its central part; this is a restricted deep water body (depths > 900 m), with a sill depth of 65 m. CTD data collected in May 1983 indicate the presence of a thermocline layer (0–60 m) associated with a rapid temperature decrease. This layer is separated from an almost uniform bottom water mass (> 100 m) by a transitional zone (60–100 m), in which temperature decreases gradually with depth. The salinity remains almost constant (38.5 ± 0.1 psu) throughout the whole of the water column. Dissolved oxygen levels vary from 1.9 to 2.6 ml/l; in water depths > 600 m, these indicate the absence of anoxic conditions. Near-bed currents are < 8 cm/s, which are not capable of resuspending bottom sediments. These currents relate to atmospheric pressure changes. The modern sedimentary cover of the Gulf is mainly terrigenous in origin. Generally, coarser-grained (gravely muddy sand) material is present along the narrow and steep southern continental shelf and slope. Mud deposits dominate the deeper parts of the Gulf, whilst a bauxitic red-mud slurry (discharged by an aluminium processing factory) is accumulating over the gently-sloping northern shelf (Antikyra Bay). Some of this artificially-induced fine-grained material reaches the abyssal plain, up to 18 km from its depocentre. Depositional processes are dominated by gravity-driven mass flows (i.e. turbidites, debris flows, and slides), for which the main triggering mechanism appears to be earthquake activity. In some cases, slope steeping through structural control and flood events associated with high sediment inputs (from ephemeral rivers and streams) contribute to the process.

The glacier-fed fan at the mouth of Storfjorden trough, western Barents Sea: a comparative study

The Middle and Late Pleistocene succession on the glacier-fed fan at the mouth of Storfjorden trough was studied using high-resolution seismic data. Seven glacial advances to the shelf break during Middle and Late Pleistocene resulted in episodic high sediment input to the fan with real sedimentation rates of up to 172 cm/1000 years, separated by sediment-starved interstadials and interglacials. On the upper fan the high sediment input resulted in frequent slides and slumps, generating debris flows which dominate the mid-fan strata. Compared with the larger neighbouring Bear Island trough mouth fan, the Storfjorden trough mouth fan has a steeper fan gradient, narrower, thinner and shorter debris flow deposits and lower frequency of large scale sliding. Glacier-fed submarine fans receive their main sediment input from a glacier margin at the shelf break, as opposed to river-fed fans where sediment input occurs through a channel-levee complex. As a result, the depocentre of a river-fed fan is found on the mid-fan and the upper slope is mainly an area of sediment bypass, whereas the glacier-fed fan has an elongated depocentre across the uppermost fan. The river-fed fans are dominated by deposition from turbidity currents, whereas glacier-fed fans are dominated by debris flow deposits.

Morphosedimentary behaviour of the deltaic fringe in comparison to the relative sea-level rise on the Rhone delta

For the first time, mareographic data obtained by the ‘Compagnie du Salin du Midi’ have been used to estimate the relative sea-level rise (RSLR) in the Rhone delta. The results were compared with the data obtained in the Marseille region which is known to be a tectonically stable zone, at least, on the secular time scale. The magnitude of relative sea-level rise is equal to 2.1 mm/year, from which 1 mm/year could be attributed to subsidence. The morphosedimentary behaviour of the deltaic fringe has been analyzed in order to see if the amount of sediment input is sufficient to allow the wetlands to continue to exist with RSLR. The horizontal variations (erosion and progradation) have been quantified from land surface changes obtained by shoreline variation analysis. The vertical accretion is measured using isotopic dating ( 137Cs). It appears that the eastern part of the mouth of the Rhone is characterized by high sediment input, sufficient enough to offset the recent rise of sea level. The western part remains vulnerable due to the lack of sediment in this area.

Impacts of Sea-Level Rise on Deltas in the Gulf of Mexico and the Mediterranean: The Importance of Pulsing Events to Sustainability

In deltas, subsidence leads to a relative sea-level rise (RSLR) that is often much greater than eustatic rise alone. Because of high RSLR, deltaic wetlands will be affected early by an acceleration of eustatic sea-level rise. If there is sufficient vertical accretion, wetlands can continue to exist with RSLR; however, lack of sediment input eventually leads to excessive water logging and plant death. Areas with low tidal range, such as the Mediterranean and Gulf of Mexico, are especially vulnerable to rising water levels because the elevational growth range of coastal vegetation is related to tide range. Reduction of suspended sediments in rivers and prevention of wetland flooding by river dikes and impoundments have reduced sediment input to Mediterranean and Gulf of Mexico deltaic wetlands. This sediment deficit will become more important with an acceleration in sea-level rise from global warming. Most sediment input occurs during strong pulsing events such as river floods and storms, and management policies and decisions are especially designed to protect against such events. Management approaches must be reoriented to take advantage of pulsing events to nourish marsh surfaces with sediments. We hypothesize that deltas can be managed to withstand significant rates of sea-level rise by taking advantage of pulsing events leading to high sediment input, and that this type of management approach will enhance ecosystem functioning.

Evolution of the intra-coastal tidal range in the Rhine-Meuse delta and Flevo Lagoon, 5700-3000 yrs cal B.C.

The evolution of the coastal and intra-coastal tidal range in the western Netherlands during the period 5700-3000 yrs cal B.C. (6875-4400 yrs B.P.) is considered by comparing one coastal mean high water curve and two local mean high water curves (MHW) with respect to three alternative relative mean sea level (MSL) curves: (1) the existing 1982-MSL curve for the western and northern Netherlands, (2) a MSL curve based on the oldest part of the 1982-MSL curve and the youngest part of a recently published MSL curve based on data from the east-central coastal plain, and (3) a hypothetical MSL curve for the east-central coastal area assuming crustal subsidence of 0.10 m/1000 yr relative to the 1982-MSL curve. With regard to coastal tidal range, the first and third MSL curve yield a similar result: a minor increase in coastal tidal range between 5200 yrs cal B.C. (6200 yrs B.P.) and 4200 yrs cal B.C. (5350 yrs B.P.). The second MSL curve gives an increase in coastal tidal range from an assumed value of 2 m by 5200 yrs cal B.C. (6200 yrs B.P.) to ca. 3 m by 4100 yrs cal B.C. (5300 yrs B.P.), conflicting with existing model results. Intra-coastal tidal damping was a widespread phenomenon and played an important role in the evolution of the coastal plain. For the first and second MSL curves, tidal attenuation in the Rhine-Meuse delta and the east-central coastal sector begins around 5250 yrs cal B.C. (6275 yrs B.P.); with the third MSL curve, the reduction of the tidal range starts is 500 years later. The first and third MSL curves result in complete tidal damping in the Rhine-Meuse delta and the east-central coastal area; with the second MSL curve, the tidal range in the Rhine-Meuse delta remains ca. 1.3 m, whereas in the east-central area tidal reduction is complete. In spite of these different results, it is clear that the period 5300-4300 yrs cal B.C. (ca. 6375-5400 yrs B.P.) was very important for the subsequent evolution of the West Netherlands coastal plain given that (a) the relative MSL rise slowed down from about 0.8 m/100 yr to 0.23 m/100 yr, (b) the coastal tidal range increased, possibly by as much as 1 m, resulting in higher sediment input into the intra-coastal area, while (c) further landward transportation of sediment was hampered due to intra-coastal tidal damping, which reduced rates of local MHW rise, tidal current velocities and accommodation space.

Surface area control of organic carbon accumulation in continental shelf sediments

The relationship between organic carbon (OC) and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments. Cores from many North American shelf environments show downcore decreases in OC to similar refractory background concentrations if expressed relative to the surface area of the sediments. This consistent concentration is 0.86 mg-OC m −2, which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces. A more global collection of sediment-water interface samples show that this relationship is even more extensive, with exceptions occurring in areas of very high riverine sediment input, organic pollution, or low-oxygen water columns. Density separations indicate that organic matter is largely adsorbed to mineral grains. The microtopography of surfaces was examined with N 2 sorption and most surface area was found to be inside pores of <10 nm width. These data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay. Such protection would likely work in concert with other protection mechanisms such as humification. This consistent surface area correlation with OC concentration may explain control of spatial and temporal variations in OC burial rates by sedimentation rates; the pore protection hypothesis provides a causal mechanism for this observed control.

Tectonism: the dominant factor in mid-Cretaceous deposition in the Jeanne d'Arc Basin, Grand Banks

The development of stratigraphic sequences has been demonstrated to be controlled by a set of factors including variations in subsidence, sediment input, eustatic sea level and physiography. Well and seismic data from the Jeanne d'Arc Basin, Grand Banks indicate that mid-Cretaceous tectonism controls at least three of these factors, namely subsidence, sediment input and physiography. North Atlantic rift tectonism was therefore the dominant factor in controlling the migration of coastal to shallow marine environments and the development of sequence stratigraphy in this basin during the mid-Cretaceous. The Avalon Formation respresents a mainly Barremian to Early Aptian regressive phase of clastic, marine to marginal marine sedimentation. This followed the deposition of a thick sequence of mainly marine limestones and shales of the Whiterose Formation above a mid-Valanginian sequence-bounding unconformity. The increased clastic input and northward progradation of coastal environments represented by the Avalon Formation occurred during uplift of a basement arch to the south with subsidence of the basin increasing to the north, accompanied by only relatively minor faulting. These features indicate that a period of epeirogenesis was initiated during the Barremian. Continuing uplift over an expanding area at the southern end of the basin is interpreted to have resulted in the development of an angular unconformity with incised valleys. This mid-Aptian unconformity defines the top of the Whiterose/Avalon sequence. Initiation of brittle fracturing of the sedimentary package and underlying basement (i.e. rifting) in mid-Aptian times resulted in rapid fault-controlled subsidence and fragmentation of the Jeanne d'Arc Basin. This great increase in subsidence rate caused retrogradation of coastal environments across the previously developed sequence-bounding unconformity, despite continuing high rates of sediment input from the uplifted basin margins. The transgressive, siliciclastic Ben Nevis Formation comprises two separate but related facies associations. A locally preserved basal association represents interfingering back-barrier environments and is herein defined as the Gambo Member. An upper, ubiquitous facies association comprises tidal-inlet channel, shoreface and lower shoreface/offshore transition sandstones. This upper facies association onlapped marine ravinement diastems above the laterally equivalent back-barrier facies. The rapid fault-controlled subsidence and high sediment input rate of this mid-Aptian to late Albian rift period resulted in the accumulation and preservation of very thick shoreface sandstones. The transgressive sandstones were buried by laterally equivalent offshore shales of the Nautilus Formation. Flooding of the basin margins induced by the onset of thermal subsidence in latest Albian or early Cenomanian times marks the top of the Ben Nevis/Nautilus syn-rift sequence.

Heightened North Pacific Storminess during Synchronous Late Holocene Erosion of Northwest Alaska Beach Ridges

A progradational regime of falling sea level and/or high sediment input has produced extensive beach ridge plains in northwest Alaska during the last 4000 yr. Eleven Chukchi Sea beach ridge complexes, oriented at various angles to wind fetch, provide a cumulative history of longshore transport and erosion. Archaeological and geological upper limiting radiocarbon ages ( n = 59) allow correlations between depositional units on seven beach ridge complexes. Progradation started 4000 yr B.P. at nearly all complexes, as eustatic sea level stabilized. Two disconformities or truncations are found on most of the complexes, providing time-parallel storm horizons, dated at 3300-1700 and 1200-900 14C yr B.P. Between 1700 and 1200 14C yr B.P. most of the complexes prograded, indicating the predominance of less-stormy conditions. Modern synoptic patterns that produce Chukchi beach ridge erosion are linked to northerly shifts in North Pacific storm tracks. The regionwide beach ridge erosional truncations correlate with records of glacier expansion, heightened precipitation evident in tree-rings, stream flooding, and shelf deposits reworked by storm surges.

Modern and Ancient Continental Shelf Anoxia

Drs. R. V. Tyson and T. H. Pearson convened a symposium on modern and ancient shelf anoxia in May, 1989 under the aegis of the Gologica] Society Marine Studies Group. This volume is the product of that meeting. The meeting was multidisciplinary including geologists, biologists and chemists. The examination and interpretation of anoxic sedimentary environments is of significance in our understanding of the interplay of organic production, water circulation and sediment accumulation. When the flux of degradable organic material exceeds the oxygen supply, then anoxia and the exclusion of macrobenthos removes the more common bioturbation effects preserving primary depositional structures. It has also been postulated that the anoxic environment permits preservation of a larger proportion of the incoming organic matter. Studies in a variety of modern environments has tended to argue against that hypothesis although certainly the role of sulfate reducing bacteria versus aerobic bacterial processes is a major factor in the ultimate sedimentary product. Within the broad spectrum of anoxic environments, shelf anoxia is a special case. One complicating factor for the stratigrapher is that the anoxic condition in several shelf environments discussed in these papers is temporary and ephemeral. Thus a primary question is what artifacts of these periods of low or zero oxygen will remain in the sedimentary record? As the convenors point out in their opening review, the modern effects impact fisheries and with the overprinting of anthropogenic effects produce some complex questions for the marine biologist. They also aver that much of the hydrocarbon-rich sedimentary resources were laid down and developed in organic-rich and oxygen-depleted depositional environments. As noted earlier, the sometimes higher organic content of such sediments reflects the common association of high productivity regions (upwelling regions) with oxygen-deficient depositional environments. Even if degradation and removal rates are similar for all depositional environments, certainly the net input under high production areas will be larger. Major factors influencing oxygendeficiency in shelf regions are the higher biological production in margins as compared to open ocean, the short fall path in margins compared to open oceans and resulting higher sediment input rates, higher sediment accumulation rates and thus thicker mass accumulation and higher burial rates for carbon. Bioturbation is also operating at higher rates, and fluctuations in depositional regime are larger and faster. The editors also introduce a suggested standard terminology for such environments in which the environmental facies are termed oxic, dysoxic (with subdivisions), suboxic and anoxic with respective bottom water oxygen contents of > 2.0 ml/L, 0.2-2.0 ml/L, 0-0.2 ml/L and <0.0 ml/L (increasing dissolved H2S). Biofacies terms for these are respectively aerobic, dysaerobic, quasi-anaerobic and anaerobic at the same oxygen boundaries. Physiological regimes would be Normoxic, Hypoxic (no matching term for Suboxic) and anoxic respectively. The volume is subdivided into modern shelf anoxia studies, and ancient shelf anoxia studies. The papers in the two sections number 11 and 16 respectively and space does not permit review of each paper. The modern studies examine the northern Gulf Of Mexico shelf associated with or adjacent to the Mississippi River discharge (papers by Boesch and Rabalais; Rabalais, Turner, Wiseman and Boesch; Harper, McKinney, Nance and Salzer)) which discuss effects on benthos, recovery of communities and general oceanographic characteristics of the environment. This area may well be one in which the anoxia has an anthropogenic source as noted in the paper by Rabalais and others. Contrasts with other areas are noted. The effects are of the order of 4-5 months in duration in the delta front area. A paper by Van der Zwaan and Jorissen compares the effects in the northern Adriatic, Orinoco-Paria shelf and the Mississippi locale. All are influenced by high river discharges and pollution effects. The authors utilize benthic foraminifera as indi-

Sequence Stratigraphy of the Jurassic-Lowermost Cretaceous of East Greenland (1)

The Jurassic-lowermost Cretaceous succession of East Greenland was deposited in a seaway formed over a series of old extensional basins between Greenland and Norway. The succession is interpreted within a sequence stratigraphic framework with the main emphasis on the Middle and Upper Jurassic. The vertical and lateral dimensions of the stratigraphic units are measured in kilometers and hundreds of kilometers, respectively. The interpretation is comparable to seismic-scale sequence stratigraphy, and the results can be compared directly to those derived from conventional reflection-seismic studies of subsurface successions. Sequence boundaries, and thus sequences, are defined differently by various research groups. In contrast, systems tracts representing linkages of deposi ional systems are considered the basic building blocks in both genetic and sequence stratigraphy. In the present study, systems tracts are recognized as the unit of highest rank within the concept of sequence stratigraphy. Ten major systems tracts are recognized. A Pliensbachian-Toarcian transgressive systems tract consisting of a slightly retrogradational parasequence set is overlain by a thin uppermost Toarcian-Aalenian(?) highstand systems tract (290-420 m in total thickness). The 140-700 m thick upper Bajocian-middle Callovian interval is represented by a basal aggradational to slightly retrogradational parasequence set interpreted as a shelf margin systems tract transitional to a transgressive systems tract. This interval is overlain by a strongly onlapping retrogradational set form d under rapid sea level rise and high sediment input, and interpreted as a transgressive systems tract. The upper Callovian-middle Oxfordian forms a composite progradational parasequence set that downlaps onto the top of the transgressive systems tract and represents a highstand systems tract. The transitional strata between the two systems tracts are highly condensed distally and contain the maximum flooding surface. Major regional deepening began in the late Oxfordian, and a thick succession of shales and turbiditic gully sandstones was deposited across the whole region. This succession represents another transgressive systems tract formed during a rapid sea level rise reaching highstand in the early Volgian when a sandy highstand systems tract prograded into the basin. The middle Volg an-Valanginian interval was characterized by rotational block faulting in northern East Greenland, in contrast to southern East Greenland, which continued its regular subsidence. A succession of a lowstand or shelf margin systems tract, a transgressive systems tract, and a highstand systems tract is recognized in both regions. This similarity may allow separation of sea level and tectonic signals in the two contemporaneous successions. The sequence stratigraphic analysis forms the basis for a coherent genetic model for the Jurassic-lowermost Cretaceous succession. The model may prove to be of value in interpreting deeply buried correlative hydrocarbon reservoirs in the northern North Sea and the Norwegian shelf.

Modification of Coral Reef Zonation by Terrigenous Sediment Stress

Four coral reefs nearPonce, Puerto Rico were examined for the effects of terrigenous influx into a reef environment. The four coral reefs are successively farther from a point source of frequently occurring, westward-moving terrigenous sediment plumes which are generated by resuspension of fine-grained sediments. The coral cover was measured from linear photographic transects parallel to each 5 meter depth. Living coral was marked on each photograph, the species identified, and area of cover was measured with a jandel digitizer pad and SigmaScan program. Statistics were compiled for percent cover by species, total cover, and number of colonies. Total coral cover was reduced near the source of terrigenous sediment influx. Coral cover and diversity increased with distance from the source and amounts of sediment trapped on the reefs decreased, suggesting that the plume influx was an important factor contributing to the deterioration of these reefs. Sediment stress has drastically reduced the coral cover and number of species. The reefs with high sediment inputs showed decreased coral species diversity and percent cover. Sediment-resistant coral species tolerated this adverse environment and their percent of cover remained relatively constant. The effects from sediment influx include partial or total burial of coral colonies, bleaching, and colonization of the coral surface by filamentous bluegreen algae and sponges. The reduced light levels resulted in domination of the community by deeper fore-reef coral. INTRODUCTION

Freshwater and marine coupling in estuaries of the Mississippi River deltaic plain1

The estuaries of Louisiana’s Mississippi River deltaic plain (MRDP) exhibit sharp physical and biological contrasts due to their different successional stages in delta development. The Atchafalaya‐Fourleague Bay complex is a young deltaic system with high freshwater and sediment inputs. The area has been undergoing rapid land building since 1973. The Barataria Basin estuary occupies a deltaic land mass which formed over 2000 b.p. and has since been isolated from riverine inflow and sediment renewal. It is experiencing a high rate of land loss. A comparison of these estuaries offers an excellent opportunity to observe coupling of freshwater and marine environments at the extremes of their scale of interaction. In the Atchafalaya system, riverine input is highly seasonal, and the estuary receives most of its sediment input and high loadings of nutrients during spring. However, high turbidity and colder temperatures limit phytoplankton productivity during this time. During the period of low flow in summer and fall, GulfofMexico waters dominate Fourleague Bay and provide an important interlude of clearer water when riverborne and regenerated nutrients can be maximally exploited by phytoplankton. The Barataria Basin estuary does not exhibit the strong seasonality found in Fourleague Bay. Barataria receives no direct riverine input, and the main hydrologic inputs are precipitation and upland runoff. The Barataria‐estuary depends on organic production and resuspended lake bottom sediments for much of the marsh building that occurs. Therefore, in the Barataria system, tides, gulf water levels, and wind‐driven resuspension are much more important to maintenance of the wetland. The upper basin is heavily impacted by nutrient runoff from urban and agricultural areas and is eutrophic. Lower Barataria Basin remains in a more natural state. Despite being at opposite ends of the delta life cycle, both estuaries are highly productive and perform similar functions as important nursery grounds for juvenile marine and estuarine fishes.

Holocene sedimentation in glacial Tasikutaaq Lake, Baffin Island

Tasikutaaq Lake, on Cumberland Peninsula, Baffin Island, receives inflow and fine sediment from a 448 km2 drainage basin, 21% of which is glacier covered. During the summer of 1983 the lake remained essentially isothermal between about 4 and 6 °C. The suspended sediment concentration of inflow never exceeded 100 mg L−1 with overflow and homopycnal flow dominant.Surface sediments are clearly laminated, although varves are not apparent. The sediments are very fine, with less than 3% sand in all but the most proximal sites. Average sedimentation rates between 1976 and 1983 ranged from about 4 mm a−1 to 0.25 mm a−1 down lake from the point of inflow. The absence of varves is a function of the low rates of sediment accumulation and the long residence time of the fine sediments in the water column.Three sediment cores up to 135 cm in length reveal marked changes in sediment characteristics and diatom assemblages through the Holocene. During the late Foxe Glaciation it is likely that glacier ice contacted the lake, with retreat upvalley recorded by thinly varved (?) silts. By 7580 ± 140 BP ice had retreated to near its present margins. The earliest diatom assemblage in the cores is dominated by small Fragilaria spp., typical of late glacial, pioneering environments. Sedimentation rates during much of the Hypsithermal were about five times less than at present, with the resulting massive sediments having "nonglacial" characteristics despite the presence of glacial ice in the drainage basin. A planktonic diatom flora suggests that summer lake ice cover was minimal at this time. A climatic deterioration at about 4500 BP marks the onset of the Neoglacial, recorded by a shift in the diatom assemblage to species characteristic of more shallow water environments. Retreat from Neoglacial moraines is recorded by clearly laminated sediments and increasing accumulation rates. In general, laminated sediments relate to periods of high sediment input associated with glacial retreat, whereas massive sediments relate to low sediment input in association with glacial stabilization or advance.

Field evidence for hydraulic jumps in subaqueous sediment gravity flows

Despite the significance of subaerial and subaqueous sediment-laden gravity flows as geological and geomorphic agents1–5, our understanding of such transport and depositional processes is limited. In particular, there is little known about the mechanisms involved in the conversion of higher-density subaqueous debris flows into lower-density turbidity currents. To address this issue, a detailed field study of this conversion process was undertaken on flows and their deposits within a reservoir. The results presented here provide field evidence indicating the occurrence of hydraulic jumps as a conversion mechanism in such flows, and the deposition of large quantities of sediment in the immediate down-jump area. The findings are significant both in terms of basic geological processes as well as in applications to problems of reservoir management in areas with high sediment inputs.

Transport of mud on continental shelves: Evidence from the Texas Shelf

Transmissivity and suspended-sediment data from three cruises on the East Texas continental shelf confirm that a bottom nepheloid layer (BNL) exists across the shelf; this layer is most turbid in the nearshore and outer shelf regions. In addition, surface nepheloid layers exist close to shore due to river and bay outflows, and on the outer shelf intermediate nepheloid layers are present as a result of quasi-horizontal advection from the BNL. The character of the BNL is not directly linked to local hydrography or substrate, being instead a function of the shelf dynamics. In the nearshore regime high-energy coastal processes and high sediment input from bays and rivers maintain the BNL. At the shelf edge, the BNL is formed by near-inertial internal waves and shelf waves which resuspend and transport fine sediment. Present-day sediment deposition on the East Texas Shelf can be directly related to the above processes. Mud is accumulating in the nearshore environment, due to river input, and on the inner and midshelf regions as a result of deposition from BNLs and SNLs. On the outer shelf, BNLs and INLs transport fine sediment further offshore, so that in effect suspended sediment bypasses the outer shelf.

Stratigraphic Reconstruction Using Digitized Well Logs: Lewis Shale, South-Central Wyoming: ABSTRACT

Advances in manipulating and displaying log data and improved methods of well-log digitizing have greatly enhanced explorationists' ability to incorporate large volumes of well data into basin-wide stratigraphic reconstructions. Computer manipulation of digital traces expedites construction of cross sections, generation of log-derived lithologic columns, normalization of log response, and updating of regional studies. The ease and speed with which cross sections can be changed and printed allow use of numerous datums to test correlations and permits construction of paleoslope configurations. Additionally, the ability to reduce a large cross section to a single field of view, without loss of definition, produces enhanced basin-side perspective and reveals stratigraphic rel tionships not apparent at larger scales. The approach proved critical in depositional reconstruction of the Maestrichtian-aged Lewis Shale in the Washakie and Red Desert basins, Wyoming. Deep-water sandstones within the Lewis are hydrocarbon reservoirs at Wamsutter and Hay Reservoir fields. Core data, cross section thickness patterns, and lithology computed from logs show the Lewis to consist of a thin transgressive shale overlain by progradational sequences. Progradation occurred as deltas entered the basin initially from the northeast and later from the south. Correlation of log response indicative of volcanically derived clay-rich layers results in stratigraphic patterns on log cross sections similar to patterns on seismic sections. The transgressive shale onlaps the Almond Sandstone; progradational sequences are depicted as irregular, sigmoidal clinoforms. Patterns indicate high sediment input and very rapid basin subsidence. End_of_Article - Last_Page 843------------

What is COMFAN?: ABSTRACT

COMFAN (Committee on Deep-Sea Fans) is an informal, international group of scientists that was hosted by Gulf Research & Development Co. in September 1982 to analyze the findings of deep-sea fan research accomplished during previous years. The group also reviewed the future plans for DSDP drilling on the Mississippi fan (Leg 96) and provided recommendations. It was realized that tectonic setting and sea level variations have major influences on volumes of sediment supply, type of sediment, rates of accumulation, nature of fan growth, facies distribution, and stratigraphic sequences. Deep-sea fans can be divided into three categories. Elongate fans develop in response to medium to high sediment input dominated by mud and fine sand. These fans have a major river as their primary source (e.g., Mississippi, Bengal, Indus, Amazon, and Rhone fans). Radial fans result from a lower sediment input with higher sand/clay ratios (e.g., La Jolla, Navy, San Lucas, Redondo fans). Slope aprons are a third category that, although not submarine fans, are closely related turbidite systems. Most fans are hybrids rather than true end members. Three major conclusions were generated: (1) comparing modern fans with ancient turbidite systems is almost impossible because of scale differences and different study techniques; (2) slope failure may be a more direct source of fan deposits rather than deltaic systems; and (3) major fan accumulations most likely occur during the end of a sea level lowering or during the initial period of sea level rise, and sedimentation rates may be very high. End_of_Article - Last_Page 456------------