Small-scale Sedimentary Structures Research Articles

Quaternary sedimentary processes on the Bahamas: From platform to abyss

Understanding the interaction between sediment production on and export from shallow-water areas of platforms and slopes is primordial when assessing sedimentary processes on a carbonate-platform scale. In this manuscript we explore variations in facies, sediment export, sediment deposition and reorganisation, hydroacoustic- and small-scale sedimentary structures, but also assess the variability in current systems as observed during the Quaternary for the north-facing margin of Little Bahama Bank (LBB) and compare those characteristics with features observed on other slopes surrounding LBB and Great Bahama Bank (GBB).Over the past decade, the northern margin of LBB was explored during a series of oceanographic cruises of the CARAMBAR project, which included the collection of 24.270 km2 of bathymetry data, 6.398 km of very high-resolution seismic profiles, and 42 cores covering water depths ranging from 177 m to 4873 m. This study evaluates the results obtained from the analysis of sediment cores retrieved in the Great Abaco Canyon area (GAC), located between the lower northern LBB slope, which is connected to the abyssal plain. The analysis of the shallower parts of the LBB slope relies on earlier studies and are complementary to our data, and allow for a detailed analysis of the sedimentary processes acting along the entire LBB slope.The data reveal that Quaternary sediment distribution differs when moving from the north-eastern to the north-western LBB slope. The entire LBB slope is dissected by numerous canyons. Gravity processes enriched in coarse platform components occur infrequently and are concentrated within lobes in the east. Only coarse-grained rich bank facies can concentrate coarse grains on this margin. The western LBB slope is mostly influenced by fine-grained platform export and current circulation. The deeper GAC area is dominated by pelagic sediments that are supplied from the canyon sides and through tributaries. The sediment composition confirms that pelagic sediment production and current movements determine the sediment-deposition and redistribution processes at this site.The comparison with other Bahamian slopes demonstrates that a leeward position agrees with high sedimentation rates on the slopes resulting in specific morphologic structures, such as gullies and sediment waves, related to fine-grained sediment export, whereas platform-derived coarse-grained facies are deposited downstream in larger structures, such as canyons, that are not affected by the main wind direction. Slope angle could also have an impact on grain-size export, as it appears that coarse-grained deposits are frequent on steep slopes like those bordering the Exuma Sound basin.

A method to generate small-scale, high-resolution sedimentary bedform architecture models representing realistic geologic facies

Small-scale (mm to m) sedimentary structures (e.g. ripple lamination, cross-bedding) have received a great deal of attention in sedimentary geology. The influence of depositional heterogeneity on subsurface fluid flow is now widely recognized, but incorporating these features in physically-rational bedform models at various scales remains problematic. The current investigation expands the capability of an existing set of open-source codes, allowing generation of high-resolution 3D bedform architecture models. The implemented modifications enable the generation of 3D digital models consisting of laminae and matrix (binary field) with characteristic depositional architecture. The binary model is then populated with petrophysical properties using a textural approach for additional analysis such as statistical characterization, property upscaling, and single and multiphase fluid flow simulation. One example binary model with corresponding threshold capillary pressure field and the scripts used to generate them are provided, but the approach can be used to generate dozens of previously documented common facies models and a variety of property assignments. An application using the example model is presented simulating buoyant fluid (CO2) migration and resulting saturation distribution.

The response of deltaic systems to climatic and hydrological changes in Daihai Lake rift basin, Inner Mongolia, northern China

Delta systems are ubiquitous around lacustrine rift basins. Its peripheral geometry, progradation structures and sedimentary successions were controlled by both tectonic settings and climatic changes. Peripheral geometry of a delta was strongly influenced by depositional gradients which formed the fan-shape delta on the steep slopes and developed the lobe-shape delta on the gentle slopes. Due to the discharge feed rivers can change rapidly driven by climatic variations, and the nearshore area of deltas display considerable facies variability. The rise of annual rainfall, which suggests the rivers feeding deltas are continuous, and result in distributary mouth bars that are prevalent in the front of deltas since the down-slope flows are greater than the along-slope currents. On the contrary, when the annual rainfall decreases and evaporation increases, the rivers only can feed deltas ephemerally. If the along-slope currents were in a dominant position, the distal bars were deposited. Progradation structure and sedimentary successions of deltas were controlled by the gradients of slopes. On gentle depositional slopes, shingle foreset beds predominate with fine sediments and small-scale sedimentary structures or vice versa.

Small-scale sedimentary structures and their implications in recognizing large-scale ancient tidal bedforms. Example from Dur At Talah outcrop, Late Eocene, Sirt Basin, Libya

The Dur At Talah escarpment (150m thick and 150km long) is exposed at the southern side of the Sirt Basin, central Libya. This outcrop exposes an Upper Eocene succession, composed by highly bioturbated fine grained sandstones to claystones at the base (New Idam Unit; 80–100m thick), overlain by medium grained to microconglomeratic sandstones at the top (Sarir Unit; 60m thick). The latter is split into two subunits of nearly equal thickness: the lower Sarir subunit, composed of medium to coarse cross-bedded sandstones; and the upper Sarir subunit, composed of very coarse to microconglomeratic sandstones.The whole succession evolves from shallow marine estuarine (the New Idam Unit) to fluvial deposits (the upper Sarir subunit). The sandstone of the lower Sarir subunit, which is the focus of this article, is previously misinterpreted as being deposited in a purely fluvial environment. However, close observations revealed that the depositional environment is largely tide-influenced. It is notably marked by conspicuous subaqueous dune cross-stratifications that bear a variety of discrete, multi-scale, sedimentary structures evidencing their deposition in tidal rather than fluvial setting. Mud drapes, tidal bundles, and perpendicularly draining and oppositely climbing ripples are largely developed. Among these structures, the most diagnostic are of millimetric to centimetric scale.As a prime aim of this article, all these sedimentary structures are described, interpreted, and discussed for the first time from this outcrop. Their style of association and the quality of their preservation provide an outstanding ancient example of tide-dominated siliciclastic systems. Such structures are rarely found together in one outcrop as they are in Dur At Talah, and they provide a significant indicators in identifying ancient bedforms of tidal origin. Evidences of subtidal and intertidal depositional environments are afforded by these structures. Criteria indicative of semidiurnal regime of the tide are also presented. These criteria are especially well-preserved in the bundled foresets of the spring tides, the neap tide record is also distinguishable but it provides no discernible structures. Moreover, sedimentary features that can be used to infer a macrotidal range during the depositional time are also afforded by a combination of these structures. Above all, this study also concludes that one of the most reliable sedimentary structures for recognizing the tidal bedforms are the ripple-scale (centimetric) sedimentary structures. These are preserved inside and at the base of the cross-sets.

Stratigraphy of a Cretaceous Coastal-Plain Fluvial Succession: The Campanian Masuk Formation, Henry Mountains Syncline, Utah, U.S.A.

Abstract The internal architecture and genetic-stratigraphic analysis of coastal-plain fluvial successions has applications to both academic understanding of continental-margin stratigraphy and to the economic evaluation of subsurface hydrocarbon and water reservoirs. Despite the abundance of exposed ancient examples of such coastal-plain to shallow marine transitions, however, the lateral facies changes and stratigraphic stacking patterns of such transitions remain largely unresolved in detail. This study presents a sedimentological and stratigraphic analysis of the Campanian Masuk Formation in the Henry Mountains syncline of Utah, western USA, and proposes a sequence-stratigraphic model to explain the geometries of fluvial-channel lithosomes found in the unit. At its type section, the Masuk Formation is an ~ 220-m-thick succession of interbedded sandstone, mudstone, and coal with minor intraformational breccias. Six facies are defined: Facies 1, coal and carbonaceous shale, records accumulation of organic sediments in coastal-plain mires; Facies 2, mudrock, records coastal-plain floodbasin settings; Facies 3, thinly interbedded sandstone and siltstone, represents estuarine-basin deposits with one interval of marine flooding; Facies 4, admixed siltstone-sandstone with intraformational breccia, represents chaotically filled coastal plain channels; Facies 5, interbedded sandstone-siltstone with inclined heterolithic stratification (IHS), records deposits of coastal-plain channels with variable sediment supply; and Facies 6, trough-cross-bedded sandstone, records sand-dominated coastal-plain channel deposits. Facies 4, 5, and 6 all show evidence of coastal and tidal influence on sediment accumulation (IHS, low-diversity trace-fossil assemblages, bimodal to bipolar paleocurrent distributions, mud drapes on small-scale sedimentary structures) to varying degrees. Facies analysis and tracing of key surfaces such as regionally extensive erosion surfaces (sequence boundaries) and flooding surfaces suggest that the entire Masuk Formation (and the uppermost part of the underlying Muley Canyon Sandstone) is a single depositional sequence. The typically multilateral channel bodies are traceable in a depositional strike direction over distances much greater than empirically calculated estimates of channel-belt width. This pattern suggests an allogenic control on stacking patterns. Multistory and multilateral channel bodies typically have a Facies 4 unit at the base, overlain by one or more stories of Facies 5, and in turn capped by Facies 6. This stacking pattern could reflect cycles of incision, initial lowstand channel filling, and subsequent backfilling of estuarine channels during transgressions. This pattern further hints at the possibility of preservation of high-frequency sequences nested within the longer-term sequence. This study provides new insights into the stacking patterns of coastal-plain fluvial successions and their interpretation.

Relating small-scale sedimentary structures and permeability in a cross-bedded aquifer

The objective of this study is to investigate the relation between small-scale sedimentary structures and permeability in the Brussels Sands formation, an early Middle-Eocene shallow marine sand deposit in Central Belgium that constitutes a major groundwater source in the region. A field campaign was carried out consisting of field observations of the sedimentary structures and in situ measurements of air permeability. The sedimentary structures were interpreted, sketched, digitally photographed and measured in a representative outcrop. Additionally, a total of 2750 cm-scale air permeability measurements were carried out in situ. Analysis of the spatial distribution of sedimentary structures and permeability shows that clay-rich sedimentary features such as bottomsets and distinct mud drapes exhibit a different statistical and geostatistical permeability distribution compared to the other lithofacies in the cross-bedded sands. Spatial analysis of the air permeability data shows that permeability anisotropy in the cross-bedded lithofacies is dominated by the foreset lamination orientation. These results show that small-scale sedimentary heterogeneity strongly influences the local spatial distribution of the hydraulic properties and results in permeability heterogeneity and stratification that would produce anisotropy in upscaled permeability values.

Rapid lake infill following major rockfall (bergsturz) events revealed by ground-penetrating radar (GPR) measurements, Reintal, German Alps

The capability of GPR (ground-penetrating radar) measurements to decipher the sedimentary history of young alluvial plains is investigated. The sediment basins result from the infill of two lakes, which were dammed by high-magnitude rockfalls 400—600 and 200 years ago in an Alpine trough valley. The thickness and structure of the dammed sediments was derived from a series of longitudinal and cross profiles. In order to take advantage of both high penetration and high resolution measurements, we applied 25, 50, 100 and 200 MHz antennas. The radargrams allow for a detailed analysis of the history of the alluvial fill. The pre-existing valley bottom is clearly visible under one of the basins. The profiles display the former clayey lake bottom and illustrate several stages of lake infill. Small-scale sedimentary structures, such as topset beds or foreset beds, are discernible and provide proxy information on water level, sediment supply and phases of infill activity. At the second basin, discordant structures in the adjacent sediment cones indicate the abrupt rise of the base level resulting from the rockfall event. Radar-derived information and historical records of lake retreat were used to reconstruct the sediment budget of the 200 year lifespan of the younger alluvial plane. Therefore, it can be concluded that the sedimentation rates of the twentieth century (489 ± 87 m3/yr) are several times larger than those of the nineteenth century (103 ± 7 m3/yr). A 130-yr rainstorm record suggests that the overall increase in sediment supply in the twentieth century corresponds to increased rainstorm frequencies during warm periods.

Sediment transport in the shelf canyon “Swatch of No Ground” (Bay of Bengal)

The Swatch of No Ground is a shelf canyon that deeply incises the Bengal shelf near the Ganges–Brahmaputra river mouth, cuts the foreset beds of the subaqueous river delta and acts as temporary depocenter between river mouth and Bengal fan. Sedimentation rates in the Swatch of No Ground are highest near the canyon head at ∼50 cm a−1, decreasing to ∼15 cm a−1 in 600 m water depth. The canyon deposits consist of intercalated fine (silt–clay) and coarse (silt–sand) grained deposits. In seismic profiles, small-scale sedimentary structures and parallel-bedded layers reveal that sediment in the canyon is mainly deposited from suspension. During fair weather conditions tidal currents are the dominant mechanism that transports plumes of suspended river load towards the canyon, forming fine-grained silt–clay layers. During storm conditions, sediment is resuspended on the inner shelf and subaqueous delta east of the canyon. Storm-generated bottom currents transport the resuspended sediment alongshelf to the canyon where the particles are trapped and form graded coarse silt–sand layers. Channels and gullies in the Swatch of No Ground indicate active gravity-driven currents with an erosional character in the upper canyon and non-depositional character below ∼450 m, suggesting that persistent or ephemeral currents presently export sediment to the Bengal fan. Numerous slumps and slides observed in the canyon show small-scale acoustically transparent layers that indicate sediment liquefaction presumably initiated along the steep canyon margin. These mudflows move downslope and halt where the canyon gradient decreases. One widespread acoustically transparent layer with an age of approximately 140–160 years BP can be traced throughout much of the canyon and is probably the result of large-scale sediment remobilization triggered by a catastrophic event like an earthquake. This may represent decennial-to-centennial scale events that remove large quantities of sediment and prevent the canyon from rapid infilling.

A GPR study of sedimentary structures within a transgressive coastal barrier along the Danish North Sea coast

Abstract The Danish North Sea coast is characterized by the presence of coastal barrier systems. One of these systems, the Holmsland Barrier, is a transgressive wave-dominated barrier. For the purpose of studying large-scale architecture in a transgressive barrier, as well as small-scale sedimentary structures, a ground penetrating radar (GPR) field experiment has been carried out. The study focuses on the identification of high-amplitude reflections of the large-scale architecture of the barrier and the recognition of small-scale structures for interpretation of coastal processes. The observed radar facies fall into two groups, both interpreted as storm washover deposits. One group, dominated by parallel to subparallel reflection, is related to the seaward horizontal stratification in the washover fans. The other group, dominated by sigmoid and oblique clinoforms, is related to delta foreset stratification, indicating that the washover fans are terminated in standing water. The observations derived from the GPR study of the Holmsland Barrier suggest that this transgressive barrier is composed almost entirely of washover deposits with local small amounts of aeolian deposits. This study has shown that the GPR method is outstanding in mapping both large-scale architecture and small-scale internal structures in a coastal barrier.

An assessment of steady-state scale-up for small-scale geological models

The calculation of pseudo-relative permeabilities can be speeded up considerably by using steady-state methods. The capillary equilibrium limit may be assumed at small scales (30 cm or less), when the flood rate is low. At high flow rates and larger distance scales, we may use a viscous-dominated steady-state method which assumes constant fractional flow. Steady-state pseudos may also be calculated at intermediate flow rates using fine-scale simulations, and allowing the flood to come into equilibrium at different fractional flow levels. The aim of this paper is to assess the accuracy of steady-state scale-up for small-scale sedimentary structures. We have tested steady-state scale-up methods using a variety of small-scale geological models. The success of steady-state scale-up depends not only on the flow rate, but also on the nature of the heterogeneity. If high permeability zones are surrounded by low permeability ones (e.g. low permeability laminae or bed boundaries), oil trapping may occur in a water-wet system. In this case pseudo-oil-relative permeabilities are very sensitive to flow rate, and care must be taken to upscale using the correct viscous/capillary ratio. However, in permeability models, where phase trapping may not occur (unconnected low permeability regions), the pseudos are similar, whatever the viscous/capillary ratio. The disadvantage of steady-state scale-up is that it cannot take account of numerical dispersion, in the manner in which dynamic methods can. However, we show examples of coarse-scale simulations with viscous-dominated steady-state pseudos which agree favourably with fine-scale simulations. Provided there are sufficient grid blocks in the coarse-scale model, the smearing of the flood front due to numerical effects is not serious.

Calculating lunar retreat rates using tidal rhythmites

ABSTRACT Tidal rhythmites are small-scale sedimentary structures that can preserve a hierarchy of astronomically induced tidal periods. They can also preserve a record of periodic nontidal sedimentation. If properly interpreted and understood, tidal rhythmites can be an important component of paleoastronomy and can be used to extract information on ancient lunar orbital dynamics including changes in Earth-Moon distance through geologic time. Herein we present techniques that can be used to calculate ancient Earth-Moon distances. Each of these techniques, when used on a modern high-tide data set, results in calculated estimates of lunar orbital periods and an Earth-Moon distance that fall well within 1 percent of the actual values. Comparisons to results from modern tidal data indicate that ancient tidal rhythmite data as short as 4 months can provide suitable estimates of lunar orbital periods if these tidal records are complete. An understanding of basic tidal theory allows for the evaluation of completeness of the ancient tidal record as derived from an analysis of tidal rhythmites. Utilizing the techniques presented herein, it appears from the rock record that lunar orbital retreat slowed sometime during the mid-Paleozoic.

Organic geochemistry in Pennsylvanian tidally influenced sediments from SW Indiana

Tidal rhythmites are vertically stacked small-scale sedimentary structures that record daily variations in tidal current energy and are known to overlie some low-sulfur coals in the Illinois Basin. Tidal rhythmites from the Pennsylvanian Brazil Formation in Indiana have been analyzed sedimentologically, petrographically, and geochemically in order to understand the character and distribution of organic matter (OM) preserved in an environment of daily interactions between marine and fresh waters. The concentration of organic matter (TOC) ranges from traces to 6.9% and sulfur rarely exceeds 0.1% in individual laminae. Angular vitrinite is the major organic matter type, accounting for 50–90% of total OM. The C/S ratio decreases as the vertical distance from the underlying coal increases. A decreasing C/S ratio coupled with decreases in Pr/Ph, Pr/ n-C17, Ph/ n-C18 ratios and a shift of carbon isotopic composition towards less negative values suggest an increase in salinity from freshwater in the mudflat tidal rhythmite facies close to the coal to brackish/marine in the sandflat tidal rhythmite facies further above from the coal. Within an interval spanning one year of deposition, TOC and S values show monthly variability. On a daily scale, TOC and S oscillations are still detectable but they are of lower magnitude than on a monthly scale. These small-scale variations are believed to reflect oscillations in water salinity related to tidal cycles.

The submarine delta of the Ganges–Brahmaputra: cyclone-dominated sedimentation patterns

The Ganges–Brahmaputra belongs to the world's largest rivers in terms of sediment discharge, but little is known yet about the processes controlling sediment distribution on the shelf and the documentation of these processes in large- and small-scale sedimentary structures. For this study the submarine delta of the river system was investigated by high-resolution seismic reflection profiles, and piston and gravity coring. Logged physical properties of the cores were combined with visual core description, grain-size analysis, and radiography. Sedimentation rates were estimated by 210Pb and 137Cs gamma spectrometry and cross-checked with evaluation of seismic profiles. The data reveal that the broad delta topset (<20 m waterdepth) is formed by a thick layer of sand + silt. Highest sedimentation rates (about 10 cm a −1) occur at the centre of clinoform delta foreset beds where about 20% of the total riverine sediment load is deposited and thus cause an annual progradation of the subaqueous delta front by about 15 m. The foreset beds consist of graded sand + silt layers which are thought to be deposited by sediment-laden flows generated during the surge of tropical cyclones (tempestites) interbedded with silty clay layers. The head of a canyon, the Swatch of No Ground, that deeply incises into the shelf, forms a sediment trap along the westward cyclonic transport path where sedimentation rate increases to about 50 cm a −1. Episodically earthquakes or storms destabilize especially the rapidly accumulated foreset beds in the eastern delta creating up to 8 m thick mass flows. On the outer shelf in water depth beyond 80 m no Holocene sediment is deposited.

Heterogeneity Modeling and Geopseudo Upscaling Applied to Waterflood Performance Prediction of an Incised Valley Reservoir: Countess YY Pool, Southern Alberta, Canada

The objective of this study is to analyze the effects of different modeling approaches and various scales of geological heterogeneity on waterflood recovery and volumetrics of an incised valley reservoir. Seismic, well-log, and core data are integrated with an incised valley facies model to create cross sections used to perform two-phase 2-D (two-dimensional) fluid-flow simulations. Core observations and probe-permeameter data are acquired to perform a geopseudo upscaling exercise, which simulates the effects of small-scale sedimentary structures on fluid flow. Applying this method and incorporating small-scale sedimentary structures in 2-D fluid flow simulations have proved to make a significant difference in individual-well oil recovery (up to 8%) depending on the facies types involved in a well's drainage area. Incorporating variations in sand-body dimensions and connectivities has proved to have a major impact on field oil recovery (30% difference between extreme 2-D cases), whereas variations in incised valley size have the greatest impact on original oil-in-place values (21% between extreme 2-D cases). A layercake model of an incised valley reservoir results in optimistic performance compared to a geopseudo upscaled model (11% higher oil recovery for a 2-D case). In a highly favorable scenario, an incised valley reservoir indeed may behave like a layercake, but it is more likely that it will not perform as well. Not taking into account small-scale sedimentary structures, uncertainty in reservoir architecture, and incised valley size in reservoir simulation studies can introduce substantial errors in reserves estimation and production forecasting. Lessons learned from this 2-D study will be used in a future full-field three-dimensional waterflood simulation of the Countess YY pool.

Facies and fluvial architecture of a high-energy braided river: the Upper Proterozoic Seglodden Member, Varanger Peninsula, northern Norway

The Seglodden Member (Late Proterozoic), as studied in two areas of the Varanger Peninsula, northern Norway, is dominated by sand-rich braided river deposits. A quantitative summary of described facies emphasises the dominance of structures deposited under high-energy conditions, and a general absence of fine-grained deposits and visible small-scale sedimentary structures. The sigmoidal and concave-up cross-stratified, planar horizontally stratified and massive sandstone facies represent 80–95% of the total vertical thicknesses of the measured sections. Architectural elements are defined as ‘sheet sandstone bodies’ and ‘channel forms and fills’ and comprise simple bars, transitional simple bar-dune complexes, dune complexes, downstream accretion complexes and various types of channel deposits. The Seglodden fluvial system was operating in a non-vegetated setting with well exposed sediment sources, probably of a sedimentary origin. This situation gave a very erratic discharge system with rapid runoff, high denudation rate and very high sediment discharge. Increased flow within channels was compensated for by a broadening of the channels, and it is inferred that the channels were very wide and shallow relative to their depths. Bounding surfaces of at least six different orders (0th–5th) are interpreted within the studied successions, with the third-order surface as the lowest-order surface that bound architectural elements as defined in this study. The highest-order surface (5th order) is probably the result of channel belt switching due to a combination of sedimentary processes and tectonic subsidence of the basin. The east and northeasterly palaeocurrent trend in the Seglodden area compared to the southeasterly trend in the Smellror area indicates that the axis of the depositional system changed significantly in direction basinwards over the 15–20 km distance between the two study areas. The controlling mechanisms for this may be autocyclic as well as allocyclic processes, although the tectonic influence on sedimentation was probably significant.

Characteristics of velocity profiles along riffle-pool sequences and estimates of bed shear stress

Numerous velocity profiles were measured along a riffle-pool sequence for three different flow conditions. For each flow condition (ranging from baseflow to about 2 3 of bankfull depth), ten to fifteen velocity profiles were measured along the same transect. The objectives of the study were: (i) to differentiate between components of bed roughness and shear stress attributed to surface friction and large-scale bed undulations, and (ii) to investigate the effects of fluctuating discharges on the mean vertical flow structure in pools and riffles. No previous attempts have been made to investigate scales of bed roughness from velocity profiles along riffle-pool sequences and to determine how near-bed velocity gradients change with increasing flow discharge. At low flows (and as expected), riffles are characterized by higher near-bed velocity gradients than pools. Similarly, average bed shear stresses, as determined from measurements of flow depth and energy gradient, indicate a greater resistance and bed shear stress in riffle sections. Invariably, however, local shear stresses as determined from near-bed velocity gradients in pools and in riffles are much smaller than reach averages estimated from measurements of depth and slope. The shape of spatially averaged velocity profiles changes as discharges increases. The difference in terms of near-bed velocities between pools and riffles decreases as discharge increases which suggests that a velocity reversal may occur at or near bankfull. Further away from the bed surface, velocity differences between pools and riffles do remain significant. Finally, the curvature of the velocity profiles is more significant at high flows and surface roughness is maximized when flow conditions are intermediate, perhaps a result of increasing density of small-scale sedimentary structures.

Environmental overview of Eckernf�rde Bay, northern Germany

This environmental overview of Eckenforde Bay (northern Germany) summarizes the results of previous studies relevant to the Office of Naval Research's Coastal Benthic Boundary Layer (CBBL) Baltic field exercise conducted during 1993-1994. Significant environmental characteristics include the following: (1) surface sediment distribution is related to water depth, dictated primarily by hydrodynamic reworking of older glacial deposits: (2) the origin and characteristics of small-scale sedimentary structures depend on storm-generated waves and currents; (3) the proximity of the sea surface and sediment -water interface results in a pelagic-benthic coupling that drives biogeochemical processes and produces organic-rich, acoustically turbid sediments and (4) the bay floor is complicated topographically by pockmarks and manmade sedimentary structures.

Sequence model for coastal-plain depositional systems of the Upper Triassic (Betic Cordillera, southern Spain)

The deposits of the Upper Triassic of the Subbetic Zone (Betic Cordillera) offer an example of semi-arid fluvial deposits passing into shallow-water facies through a mud-flat environment. From the study of the vertical changes in facies, a sequence model is proposed for the Upper Triassic succession, relating, for each systems tract, the changes in sea level, the accommodation space and the evolution of the facies within a coastal-plain depositional system. The lowstand depositional system is made up of a package of thick, amalgamated sandstone strata limited below by an erosive surface with little incision. These are sheet-flood deposits and channel fills which form part of an extensive alluvial system in which wide but shallow water courses appear, with the development of extensive sand bars. These sandbodies from the lowstand phase often contain mudrock intraclasts, plant remains and erosive surfaces. The accommodation space being extremely limited, the preservation of the floodplain or mud-flat deposits and palaeosols is practically nil. The result is an amalgamation of extensive sandbodies with fine intercalated claystone levels. The transgressive depositional systems consist mainly of a red claystone series, although the first sediments of these systems are composed of sandstone with major claystone intercalations. These lower sandy levels of the transgressive phase do not exceed 50 cm in thickness and usually contain burrows and small-scale sedimentary structures. In the intermediate part of the red claystone series of this transgressive episode of saline mud-flat facies, there are sandstone levels of a little-developed fluvial system with channels which migrate laterally due to the rise of the base level. In addition, the predominance and the great development of the red claystone facies reflect the increase in the accommodation space, which permitted a greater accumulation of sediments during a transgressive phase than during the lowstand phase. In the upper part of the transgressive depositional systems, beds of calcrete and carniolar limestone are common, indicating a decreased sedimentation rate. The generation of accommodation space begins to slow down, leading to the enlargement of sandbodies corresponding to terminal-fan deposits (Friend, 1978). These developed over a flat topography where the flow scattered and expanded laterally. In the highstand phase, the accommodation space reduced even more and the sedimentation rate is lower, favouring a great development of pedogenic carbonates. Furthermore, sand deposition was reduced or nil and the gradient of the slope was low, so that marine floods over the coastal plain were more frequent, permitting the precipitation of carbonates and sulphates in coastal salt pans.

The Utility of Small Scale Sedimentary Structures in Shales: ABSTRACT

The Utility of Small Scale Sedimentary Structures in Shales: ABSTRACT

Fractal properties of simulated bed profiles in coarse-grained channels

Bed roughness characteristics in coarse-grained channels are fairly complex. A hierarchy of roughness elements can be observed, ranging from variable particle sizes and shapes and small-scale sedimentary structures, to large-scale bedforms such as riffle-pool sequences. The effects of these scales of roughness on the flow geometry still remain to be thoroughly investigated. The semivariogram has been suggested in the past as a means of quantifying bed roughness effects on streamflow, as well as for distinguishing between scales of roughness. However, field measurements are rather time-consuming. The low number of bed profiles measured in the field precludes the identification of generally applicable relationships between the statistical properties derived from the semivariograms (such as the Hausdorff dimensions and the scale of autocorrelation corresponding to each fractal band) and the bed configuration itself (geometrical and sedimentological properties). Simulation results of gravel-bed profiles are, therefore, presented in order to complement the original investigation of Robert (1988a). The simulation experiments, based on grain characteristics of sizes and shapes and on morphological properties of small-scale bedforms, yield very significant information on boundary roughness at the microscale and give insight into the interpretation of empirical semivariograms (derived from field measurements). Bed-material sorting, variable grain shapes, and height and spacing of cluster bedforms control the fractal dimensions obtained from the semivariograms, as well as the location of the break of slope and the range of the process.