Small-scale Sedimentary Features Research Articles

Direct sediment transfer from land to deep-sea: Insights into shallow multibeam bathymetry at La Réunion Island

Abstract Submarine canyon heads are key areas for understanding the triggering factors of gravity currents responsible for the transfer of detrital sediment to the deep basins. This contribution offers a detailed picture of canyon heads off La Reunion Island, with high-resolution multibeam bathymetry in the water depth range of 4–220 m. The present feeding of the Cilaos turbidite system, one of the largest modern volcaniclastic systems in the world, is deduced from morphological and sedimentological interpretations of newly acquired data. The study highlights small-scale sedimentary features indicating hydrodynamic and sedimentary processes. A direct connexion between the Saint-Etienne river mouth and submarine canyons is evidenced by the complete incision of the shelf and the presence of canyon heads connected to the modern deltaic bar. This direct connection, supplied by river torrential floods (cyclonic floods every two or three years), suggests the continuity of high-density fluvial flows to submarine gravity flows, forming hyperpycnal flows in the canyon. The initiation of secondary submarine gravity flows by storm waves (large austral waves and cyclonic waves) is also proposed for submarine canyons with large canyon heads developed in the surf zone from a sandy coastal bar. Bedforms in active canyon axis are considered as an indicator of the frequent activity of high-density turbidity currents. Moreover, a morphological record of last glacial and deglacial sea level variations is preserved, and particularly the Last Glacial Maximum sea level with the presence of small vertical cliffs, observed in this bathymetric data, which likely corresponds to a paleo-shoreline or paleo-reefs.

Assessment and geological characterisation of the CO2CRC Otway Project CO2 storage demonstration site: From prefeasibility to injection

The CO2CRC Otway Project, located in south-eastern Australia, has demonstrated geological storage of CO2 in a depleted natural gas field. Prior to injection, a comprehensive site characterisation study established that the site should meet the requirements of safe and effective storage. In contrast to the conventional methods applied to characterising oil and gas reservoirs for production purposes, CO2 storage site assessments place greater emphasis on injectivity, capacity, and long-term containment. The site location was assessed in the context of accessibility for monitoring activities and impact on local communities and natural resources. Additional well-log data and cores were acquired from the gas field, in conjunction with the drilling of the CRC-1 injector well, in order to reduce the uncertainty surrounding the geological heterogeneity of the reservoir, capacity of the seal to retain CO2, fault seal geomechanics, and regional hydrodynamics. Specialised core analysis revealed that small scale sedimentary features, related to depositional environment impact reservoir quality, CO2 trapping and plume migration behaviour. Based on these effects, a depositional model was established to better understand storage potential away from well control. Finally, a nearby gas storage facility provided a valuable analogue for the project and added confidence that the CO2CRC Otway Project site would be suitable to inject, store and contain CO2 within the technological and economical limits of the project. Following the injection period, long term monitoring of the reservoir, as well as the overlying aquifers, soil, groundwater and the atmosphere above the site, have confirmed the storage concept is effective and that the CO2 is safely contained. As a result, the site characterisation methodology serves as an example for others contemplating CO2 storage into depleted gas fields.

The significance of mudstone fabric combined with palaeoecological evidence in determining sedimentary processes – an example from the Middle Jurassic of southern Poland

Mudstones were deposited below the storm wave base in the shallow epicontinental sea, however, structures reflecting quiet settling from the suspension are not common. Most of recognized lamina types resulted from activity of bottom suspension currents generated by storms, suggesting that accumulation of mudstones proceeded under prevailing energetic conditions. Composition of benthic association indicates that bottom environment was depleted in oxygen but reoxygenetion events, linked with storms, were frequent. Short oxygenetion episodes resulted in development of microbioturbation, which was the most effective in destruction of subtle lamination. L onger lasting oxygenation led to development of more diversified association, consisting of ichnofossils Chondrites , Trichichnus , pyritized burrows, Palaeophycus and Protovirgularia and bivalves Bositra , nuculids and nuculanids. Analysis of fabric of laminated black mudstones of the Ore-Bearing Częstochowa Clay Formation (Bathonian, southern Poland) was used for reconstruction of sedimentary processes and environmental energy. Small-scale sedimentary features indicate that ore-bearing clays were deposited below the storm-wave base in the shallow epicontinental sea but structures reflecting quiet settling from the suspension are not common. Most of recognized lamina types record activity of bottom suspension currents generated by storms, suggesting that mudstones accumulated under prevailing energetic conditions. Composition of benthic faunal association indicates that on the seafloor suboxic conditions prevailed, restricting infaunal activity to cryptobioturbation. Recurrent short-lived reoxygenetion events, linked probably with storms, resulted in development of dysoxic conditions, recorded by more diversified trace fossil association (consisting of Chondrites , Trichichnus , pyritized burrows, Palaeophycus and Protovirgularia ) and colonization of the seafloor by epifaunal bivalves Bositra . Truly anoxic conditions might establish only for a short time, leading to mass mortality of Bositra . Analysis of the fabric of laminated black mudstones of the Ore-Bearing Częstochowa Clay Formation (Bathonian, southern Poland) is used for reconstruction of sedimentary processes and conditions. Small-scale sedimentary features indicate that ore-bearing clays were deposited below storm-wave base in a shallow epicontinental sea, but structures reflecting quiet settling from suspension are not common. Most of recognized lamina types record the activity of bottom currents generated by storms. The benthic faunal association indicates that suboxic conditions prevailed on the seafloor, restricting infaunal activity to cryptobioturbation. Recurrent short-lived reoxygenetion events, linked probably to storms, resulted in the development of dysoxic conditions, recorded by a more diverse trace fossil association (consisting of Chondrites , Trichichnus , pyritized burrows, Palaeophycus and Protovirgularia ) and colonization of the seafloor by epifaunal bivalves ( Bositra ). Truly anoxic conditions might have been briefly established, leading to mass mortality of Bositra .

Ground-penetrating radar facies characterization of deepwater turbidite outcrops

High-resolution outcrop studies are becoming an increasingly important tool in building reservoir architectural models (Slatt, 2000). By providing attributes of strata in both the lateral and vertical directions, outcrop studies can help constrain reservoir characterization models (e.g., identify seals or barriers to fluid flow). Outcrop exposures, at best, are limited in their extent. Boreholes, even when they are spaced tightly together, may yield an inaccurate picture of sedimentary features. Because the type and scale of information important to hydrocarbon production can be beneath the resolution of 3D seismic data, there is a need for a geophysical method that yields a high-resolution, continuous image of the subsurface. Ground-penetrating radar (GPR) is one of the more recent geophysical technologies used in examining outcrops (Slatt, 2000;Young et al., 2001). GPR is an electromagnetic imaging procedure that generates reflections from boundaries between layers and can use modified seismic processing to create images (Young et al., 1995). The short wavelengths of GPR signal provide data, which is subseismic scale in resolution. The resulting profiles are good for interpreting small-scale sedimentary features of bed continuity, stratification patterns, and facies boundaries. This paper presents the results from a GPR study of the Lewis Shale turbidite outcrops in Wyoming. The objective of the project is to collect subseismic scale resolution data to aid in determining small-scale sedimentary features. The interpretations, in turn, could be used to provide a better understanding of fluid flow and reservoir production modeling. The Lewis Shale, a Cretaceous age turbidite deposit, is an active gas exploration target and analog for the lower Cretaceous sequences in South Africa and tertiary deposits in the Gulf of Mexico. The formation crops out along the western edge of the Rawlins-Sierra Madre uplift within southeast Wyoming. The geology of three areas along the Lewis Shale outcrop has been studied. …

Immediate and longer-term impacts of hydraulic clam dredging on an offshore sandy seabed: effects on physical habitat and processes of recovery

A hydraulic clam dredging experiment was conducted on a deep (70–80 m) offshore sandy bank on the Scotian Shelf in order to examine the immediate impacts of hydraulic dredging on physical habitat and to follow processes of recovery over a 3-year period. Seabed structural complexity in this low-relief habitat consists of small-scale sedimentary features including pits and bivalve burrow openings as well as polychaete tubes and empty mollusc shells. The most obvious effect of dredging was a dramatic change in seabed topography due to the numerous deep (20 cm), wide (4 m) curvilinear furrows that were cut by the dredges. The loss of burrows, tubes, and shells through destruction or burial, and local sedimentation created a smooth surface. Both spatial scale and sampling resolution were critical in identifying longer-term impacts. Dredge furrows were no longer visible in video 1 year after dredging due to their low relief; however, they persisted, while undergoing changes, as evidenced in sidescan sonograms. The margins of furrows were gradually degraded, likely through the combined actions of slumping, sediment transport and bioturbation. Over time, dredge furrows act as traps for empty shells. Differences in patterns of acoustic reflectance between dredge furrows and the surrounding seabed indicate long-lasting effects on sediment structure. Densities of large burrows were reduced by up to 90% after dredging with no signs of recovery after 3 years due to the high mortalities of their architect, the propellerclam, Cyrtodaria siliqua. Dredging effects were detectable against a background of natural temporal (annual) and large-scale spatial variability in physical habitat.

Distribution and deposition of mudstone facies in the Upper Devonian Sonyea Group of New York

ABSTRACT Petrographic and small-scale sedimentary features of the mudstone-dominated Upper Devonian Sonyea Group of New York were examined in order to (1) improve our understanding of mudstone facies, (2) examine coeval mudstone facies within a broader depositional context, and (3) promote a simple methodology for the study of mudstones. Six mudstone facies are distinguished, each characterized by several component mudstone types or subfacies. Sedimentary conditions and environments are reconstructed from primary sedimentary structures and bioturbation characteristics. Mudstone facies are characterized by soils and flood deposits in the coastal-plain region, by rapid sediment deposition and frequent reworking in nearshore areas, by storm-dominated offshore transport on a wide, basin-margin platform, by turbidite slope deposits below storm wave base, and by bottom currents and slow settling in the distal, deep-basin black mudstones. The latter, although commonly thought of as deposits of a stratified anoxic basin, contain indications of benthic life, such as burrows, disrupted laminae, and clay/silt fecal pellets. These observations are incompatible with a stratified-basin model, and indicate the need to search for alternative models of black-shale accumulation.

Sand-Rich Fairways in Deep-Water Clastic Reservoirs: Genetic Units, Capturing Uncertainty, and a New Approach to Reservoir Modeling

Correlation in sand-rich deep-water clastic reservoirs using sedimentary characteristics is examined using well-exposed outcrop data. Individual bed thickness, or the sedimentary characteristics of single beds, and packages of beds when viewed in one dimension are found to give an inadequate basis for interwell correlation over distances greater than 100 m. Offset stacking of beds is the main control over bed thickness variations with scouring playing a minor role. Composite units are identified that can be traced over kilometer-scale distances. These units are potentially identifiable on seismic data and are valuable for interwell-scale reservoir modeling. Pinch-out of sand units is characterized in two end-member styles, onlap and infill. Onlap and infill indicate the influence of paleotopography on deposition, reflecting deposition onto low and high relief, respectively. Onlap is characterized by gradual deterioration of reservoir quality toward the pinch-out, whereas infill maintains good reservoir quality without significant deterioration close to the pinch-out. Prediction of the proximity of 1-D (one-dimensional) sections to termination of reservoir units at a pinch-out is problematic for both styles. Although 1-D sections provide invaluable data for reservoir characterization prediction of 3-D (three-dimensional) sand body geometry from borehole sections is problematic. Commonly cited channelized and lobate sandstone body geometry does not have diagnostic sedimentary features and recognition of alternative sand body geometry is needed. The difficulty in selecting key, small-scale sedimentary features that, in turn, are diagnostic of larger scale geometric features limits their value in the up-scaling of reservoir characteristics. A new approach to reservoir modeling of sand-rich deep-water clastic reservoirs is proposed that involves down-scaling from mapping of seismically detectable, interwell-scale packages of beds and the paleotopography on which they lie. Once these features are constrained smaller scale characteristics are modeled.

Rain-impact Microtopography (RIM): An Experimental Analogue for Fossil Examples from the Maroon Formation, Colorado

ABSTRACT Small-scale sedimentary features in medium- to coarse-grained siltstones (loessites) of the Maroon Formation (Middle Pennsylvanian to Early Permian) of northwestern Colorado are interpreted to be of rain-impact origin. These features, which have been experimentally produced by simulated oblique rain impact, are herein termed rain-impact microtopography, or RIM. Experiments indicate that RIM forms only in sediments ranging from fine silts to medium very fine sands (7 to 3.5 ). Probable fossil (ancient) examples of RIM superficially resemble numerous other sedimentary and biogenic structures, but a variety of observations and experimental procedures demonstrate their uniqueness. ecognition of RIM can be valuable as an indicator of subaerial exposure and paleowind direction.

Stratigraphic Dipmeter Applications in Clastic to Carbonate Shoreline Transition: Core-Dipmeter Comparison: ABSTRACT

The James (Lower Cretaceous) shoreline complex cored in a southern Arkansas well consists of cyclic carbonate, clastic, and mixed lithologies. Arm-to-arm and button-to-button computations from the stratigraphic high-resolution dipmeter and core observations have been combined to illustrate the usefulness as well as limitations of the dipmeter in identifying and interpreting sedimentary and diagenetic features. The button-to-button dipmeter correlations were found to be most effective in discerning small-scale sedimentary features (< 4 cm), such as laminations and fine-scale cross-bedding due to variations in grain size, lithology, and porosity. Where bioturbation, compaction, or irregular porosity distribution have been overprinted on original bedding features, thin localized variations can be overcome by using the arm-to-arm computations. A larger cross section across the borehole combined with a larger correlation window reduced the effects of these very fine-scale irregularities. An expanded scale display of the 8 microresistivity curves was instrumental in qualifying the computed results. Using this display, horizontal porosity variations from diagenesis, bioturbation, and even stylolites observed in the core could be recognized. Computer processing often force-fits a planar solution to nonplanar features. By calibrating the dipmeter with a core, useful sedimentary features and their orientations can be identified. These relationships can then be extendedmore » to wells where dipmeter data is available, aiding in the development of area-wide depositional models.« less

Local variability in the distribution of Windscale fission products in estuarine sediments

The local variations of fission product activites (144Ce, 106Ru, 137Cs and 134Cs) in surface sediments from a location in the Ravenglass estuary, north-west England, have been studied. The sediments are contaminated by the radioactive effluents discharged to the eastern Irish Sea by the Windscale nuclear fuel reprocessing facility. The results indicate that significant activity variations occur for surface samples taken from small-scale sedimentary features of the estuarine environment, and that sediment grain-size characteristics and radioactive decay exert controls on fission product distributions. The assessment of radiation exposure of the public by external gamma doses from contaminated sediments should take intt account the, variability of radionuclide distributions over small-scale features as well as the broader surveying of estuarine areas.

Les sediments superficiels des Approches occidentales de la Manche

The part of the continental terrace stretching along a 250 km axis from the Western English Channel to the shelf edge is covered by a blanket of sediments ranging from fine sands to pebbles. Three terrigenous fringing areas are of Pleistocene origin: the periarmorican pebbly strip and the quartzitic sands located off Devon—Cornwall and near the shelf edge. The remainder is composed preponderantly of bioclastic sands, deposited through the Holocene and up to the present. The tidal currents, occasionally augmented by storm-induced oscillatory water movements, exert the main control on the distribution of the sediments. An evolutionary model starting from the Würmian low sea-level environment is proposed. It takes into account the specificity of the great sand-bank area in the southwest which was submerged whilst, onshore, subaerial and periglacial conditions led to the deposition of pebbles and, to a minor extent, of detritic sands south of England. The progression of the Holocene transgression triggered the productivity of the epifauna on the submerged, hard floor zones and modified the hydrodynamic conditions which, in turn, reacted upon the distribution of the organogenic debris. The velocity of the tidal currents is maximum (over 2 knots) around Brittany, keeping the pebbly area generally free of sedimentation of smaller particles. The distribution pattern of the bioclastic fraction (concerning mainly Bryozoa) enables one to consider a general axial transportation trending towards the SW which is corroborated by previous studies dealing with small-scale sedimentary features.