Trough Cross-bedded Sandstone Research Articles

A sedimentação proterozóica (esteniana-toniana) na borda oeste do Craton Amazônico

The Serra dos Pacaas Novos, located in the part of Rondonia, near the town of Guajara Mirim, and inserted in the Amazonian Craton, is built up mainly by conglomerates and sandstones of the Upper Proterozoic Palmeiral Formation. The facies analysis allows to distinguish six lithofacies: (1) massive or crudely bedded ortoconglomerate (Gm); (2) sandstone with horizontal bedding (Sh); (3) trough cross-bedded sandstone (St); (4) planar crossbedded sandstones (Sp); (5) sigmoidal cross-bedded (Ss) and (6) massive sandstones (Sm). The conglomerate facies was interpreted as longitudinal bars or stacked sheets formed during events of high discharge. The interbedded lenses of sandstone, on the other hand, was deposited during low water in minor channels. This interpretation points to a fluvial style with a braided channel patern similar to the Scott type. However, it is uncertain if this depositional pattern may be related to a proximal fluvial system or to a alluvial fan.The provenance of the Palmeiral deposits is shown by their different types of lithic fragments including quartz, quartzite, sandstone and volcanic rocks, which only in part are derived from the crystalline basement. Rock fragments from the nearby Jamari Complex (basement) were not found.The Palmeiral Formation went through several diagenetic processes which include quartz overgrowths, formation of kaolinite, illitization of kaolinite and pressure solution well documented by cavities in gravels. In the Serra of Pacaas Novos, the Palmeiral Formation is confined to a graben structure which, possibly during Cenozoic, suffered relief inversion leading to the present morphological configuration of the graben. The structural analysis resulted in the separation of three strutural sectors.

Depositional controls on petrophysical properties and reservoir characteristics of Middle Miocene Miri Formation sandstones, Sarawak

Rock exposures around the town of Miri, Sarawak, belonging to the Miri Formation (Middle Miocene strata), represent the uplifted part of the subsurface sedimentary strata of the Miri oilfield. Data derived from outcrop studies concerning facies and sand body characteristics, and petrophysical properties are crucial for subsurface reservoir characterization and modeling of hydrocarbon reservoirs deposited in similar settings. The aim of this paper is to integrate lithofacies and petrophysical properties of Miri sandstones, in order to characterize and quantify the Miri reservoirs. The Miri Formation consists of a wide range of siliciclastic, tide-generated and storm-and wave- generated facies. Twelve lithofacies have been identified and grouped into two major facies associations: (i) the estuarine, tide-dominated, and (ii) the shoreface-offshore transition, storm-and wave-dorninated facies associations. The estuarine lithofacies are characterized by distinct and diagnostic tidal signatures; tidal dune cross-bedding with mud draped cosets and foresets including mud couplets, bidirectional (herringbone) cross-bedding, rhythmic stratifications, flaser bedding, wave bedding and lenticular bedding. Shoreface-offshore transition, storm-and-wave facies association is represented by sandstone bodies with evidences of storm and wave generated sedimentary structures; swaley cross-stratified sandstones, amalgamated hummocky cross-stratified sandstones, bioturbated sandstones and mudstone inter-bedding with parallel stratified to hummocky cross-stratified sandstone. Petrophysical properties were determined for six sandstone lithofacies: (i) Lithofacies A (multiple stacked trough cross-bedded sandstone, tidal channels and bars, estuary mouth), (ii) Lithofacies B (parallel-bedded sandstone of estuary upper flow sand flat), (iii) Lithofacies F (homogeneous coarse sandstone, outer estuarine tempestites), (iv) Lithofacies G (swaley cross­ stratified, upper-to-middle shoreface sandstone), (v) Lithofacies I (fine-grained bioturbated sandstone of the lower shoreface) and (vi) Lithofacies L (fine-grained, hummocky cross stratified sandstone, offshore transition). These lithofacies are characterized by a wide range of permeability values, which vary by several orders of magnitude (0.35 to 287 md), while porosity vary by only a few percent. Lithofacies A and F recorded the best reservoir properties; porosities are 23.3-29.7% and permeabilities are 9.64-287 md. Lithofacies G shows a wide range of porosity and permeability values that range from high to low reservoir properties; porosities are 23.5-27.5% and permeabilities are 3.4-45 md Lower shoreface and offshore transition (lithofacies I and L respectively) display the lowest reservoir properties; porosities are 13.5-24.5 % and permeabilities are 035-3.4 md In general, high reservoir quality of Miri sandstones is associated with coarser grain size, low clay contain and better sorted grains. Extensive clay drapes, bioturbation, and increasing proportion of very fine grains content result in significant decrease in permeability in both tide-generated and wave-generated lithofacies.

Fluvial facies architecture in small-scale river systems in the Upper Dupi Tila Formation, northeast Bengal Basin, Bangladesh

The late stage basin-fill history of the fluvial Dupi Tila Group (Plio-Pleistocene) is described. These rocks have been deposited in the Sylhet trough, a sub-basin of the Bengal Basin, in a foreland basin setting. This outcrop study, carried out in Sylhet, Bangladesh, presents the first detailed facies analysis of the Upper Dupi Tila Formation. Four facies have been identified: trough cross-bedded sandstone (St), ripple cross-laminated sandstone (Sr), finely laminated mud with ripples (Fl), and massive mud with rootlets (Fm). Facies analysis supplemented with embedded Markov chain analysis, reveals small-scale fining-upward cycles (average 4.5 m thick). Facies architectural elements include channel (CH), lateral accretion (LA), sandy bedforms (SB), and overbank fines (OF) with limited vertical and lateral connectivity of the sand bodies. The average channel depth and width is 5 and 30 m, respectively. Sand body geometry ranges from tabular, to sheet, to shoestring with a 0.45 net to gross ratio. This study shows that the Upper Dupi Tila Formation is composed of small-scale, mudstone-reach meandering river deposits. In Bangladesh, the Dupi Tila Formation is the main aquifer presently being utilized. Understanding of facies architecture and sand body geometry of this Formation is crucial in examining the issue of arsenic and other contaminations of ground water in Bangladesh.

Long-term ENSO-like events represented in the Middle Pleistocene shelf successions, Boso Peninsula, Japan

Millennial-, century-, and decadal-scale stratigraphic cyclicities are described from transgressive shelf successions of the Middle Pleistocene Ichijiku and Kakinokidai Formations (ca. 0.7 Ma) in the Boso Peninsula, Japan. The Ichijiku Formation (ca. 400 m thick) represents a shelf sandridge complex developed under the influence of the large meanders of the paleo-Kuroshio Current, and intertongues with outer shelf muddy deposits of the Kakinokidai Formation (ca. 170 m thick) to the northeast. Most of the two formations are classified as a transgressive systems tract formed during the rise in glacioeustasy from oxygen isotope stages 18 to 17. Three different types of cyclicities are identified in the transgressive shelf deposits. The first cyclicity (ca. 3000 years duration) is represented by coarsening-upward units of the Kakinokidai Formation associated with the northeastward extension of the sandridge complex. Lower parts of each coarsening-upward unit are characterized by intercalations of tempestites more than the upper parts. The second cyclicity (ca. 250 years duration) is defined by master bedding surfaces within sandridge deposits, interpreted as formed in response to lateral migration of foresets of the sandridge complex. The third cyclicity (ca. a few decades or shorter duration) is defined as the units bounded by reactivation surfaces within large-scale trough cross-bedded sandstone units. The three different types of cyclicities are interpreted to document fluctuations in the paths and speeds of the paleo-Kuroshio Current in response to the large meanders. Oscillations of the large meanders of the modern Kuroshio Current every few to 10 years are interpreted to be in response to ENSO (El Niño-Southern Oscillation) events, and are also associated with the decrease in frequency of typhoons in the western Pacific. If our stratigraphic interpretation is true, the paleo-Kuroshio-Current-controlled shelf successions document high-frequency oscillations in paleoclimates responding to the long-term ENSO-like events in the western Pacific during the Middle Pleistocene.

The Jurassic of Kuhn Ø, North-East Greenland

The Middle–Upper Jurassic succession of Kuhn Ø, North-East Greenland accumulated in a major half-graben and is an excellent analogue for the subsurface of the mid-Norwegian shelf. On Kuhn Ø, peneplaned crystalline basement was incised by a drainage system during a major base-level lowstand, probably in late Early or early Middle Jurassic times. It was filled with fluvial conglomerates of the newly defined Middle Jurassic Bastians Dal Formation during subsequent base-level rise. As sea level continued to rise, precursor-peat of the coals of the Muslingebjerg Formation formed in swamps which covered the conglomerates and filled the remaining space of the incised valley system. The valley and interfluve areas were flooded in Late Bathonian – Callovian times and tidally-dominated, shallow marine sandstones of the Pelion Formation were deposited on top of the valley fill and over the adjacent basement peneplain. These sandstones are overlain by the newly defined shallow marine Oxfordian Payer Dal Formation which is subdivided into a lower unit and an upper unit, separated by a major drowning surface. The Payer Dal Formation sands were flooded in the Late Jurassic and organic-rich, offshore mudstones of the Bernbjerg Formation were deposited. The Jurassic succession of Kuhn Ø can thus be subdivided into large-scale sedimentary units separated by major drowning surfaces. They are of regional extent, and in combination with biostratigraphic and 87Sr/86Sr isotope data they allow the correlation of the sedimentary units on Kuhn Ø with more offshore deposits to the south in Wollaston Forland and more landwards successions to the north in Hochstetter Forland. Petrographically, the trough cross-bedded sandstones of the Pelion Formation and the lower unit of the Payer Dal Formation include both calcite-cemented and poorly cemented quartz sandstones. The calcite cement was derived from dissolution of abundant calcareous fossils and forms concretionary horizons. The upper unit of the Payer Dal Formation mainly consists of weaklycemented quartz sandstones with porosities around 30%. The sandstones of the Pelion and Payer Dal Formations on Kuhn Ø are petrographically very similar to Jurassic sandstones from the mid- Norwegian shelf and the Barents Sea with regard to original mineralogical composition, sorting and grain size. The Bernbjerg Formation mudstones are comparable to the Upper Jurassic source rock of the mid-Norwegian shelf and the Barents Sea, but have lower hydrogen index (HI) values due to terrigenous input in a relatively proximal setting. Coals of the Muslingebjerg Formation have significant source rock potential with measured HI values up to 700, kerogen types II–III and total organic carbon (TOC) values above 50%.

FLUVIAL ARCHITECTURE AND THE TECTONIC CONTROL ON DEPOSITION OF ONSHORE EUMERALLA FORMATION, OTWAY RANGES, VICTORIA: IMPLICATIONS FOR EXPLORATION IN THE EARLY CRETACEOUS OTWAY BASIN

Spectacular outcrops of the terrestrial Aptian-Albian Eumeralla Formation are exposed in the Skenes Creek– Wongarra region, located on the eastern margin of the Otway Ranges, Victoria. The succession comprises mudstone-dominated floodplain and lacustrine successions and fluvially-derived sandstones. Lithofacies observed in the study area comprise intraformational and exotically derived conglomerate, massive and planar laminated sandstone, trough and tabular cross-bedded sandstone, ripple laminated sandstone, interbedded sandstone and mudstone, massive to laminated mudstone and thin coal seams. Architectural analysis of the fluvial system reveals these lithofacies are arranged into architectural elements that include channel elements, sandy bedforms, downstream and lateral accretion elements, laminated sand sheets and overbank fines elements.The fluvial system is characterised by low-sinuousity, braided river channels with high width to depth ratios. Palaeocurrent data indicates that the generally westward palaeoflow is interpreted to have been diverted into local axial-through drainage patterns by active northeast trending normal faults. One of these, the Skenes Creek Fault, is also likely to have structurally isolated floodplain and lacustrine successions from the main channel belt, leading to the deposition of an anomalously thick coal measure sequence in the hanging wall of the fault. The local study therefore provides insight into regional lithofacies and potential source rock distributions, and the associated tectonostratigraphic evolution of the Eumeralla Formation in the eastern Otway Basin. While the nature of the Eumeralla Formation sandstone does not lend itself to good reservoir properties, the geometry and internal structure of the sands provide an excellent model for other fluvial sandstone reservoir reconstructions.

Sedimentological and Petrographical Characteristics of Coal- and Oil Shale-Bearing Strata from the Ilısılık Region, Central Anatolia, Turkey

The present study describes the sedimentology, coal petrology, and geochemistry of a Miocene coal- and oil shale-bearing succession in the Il L s L l L k region in central Anatolia, Turkey. Vertical sequence analysis within the 50-150 m thick coarse-grained succession of the Il L s L l L k oil shale field has allowed 4 sedimentary associations to be distinguished. The facies associations in the Il L s L l L k oil shale field are dominated by fluvial deposits, which document abrupt facies transitions along the axis of the basin. Fluvial poorly-sorted cobble conglomerates of facies association 1, located along the basin margin grade basinward into well-sorted conglomerates of facies association 2. These deposits grade basinward into the laterally continuous sheets of trough cross-bedded fine-grained sandstone and massive siltstone of facies association 3. Further basinward the fine clastics, oil shale, and coal of facies association 4 are common. Reflectances measured on vitrinite are between 0.54 and 0.67% Rmax. This indicates a coalification equivalent to low volatile subbituminous coal rank of the American Society for Testing and Materials classification for the Il L s L l L k coals. The maceral composition makes the Il L s L l L k oil shales of the lamosite type.

The stratigraphic relationship between the Waterberg and Soutpansberg Groups in Northern Province, South Africa: Evidence from the Blouberg area

Research Article| September 01, 2001 The stratigraphic relationship between the Waterberg and Soutpansberg Groups in Northern Province, South Africa: Evidence from the Blouberg area A.J. Bumby; A.J. Bumby Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby@postino.up.ac.za Search for other works by this author on: GSW Google Scholar P.G. Eriksson; P.G. Eriksson Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby@postino.up.ac.za Search for other works by this author on: GSW Google Scholar R. van der Merwe; R. van der Merwe Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby@postino.up.ac.za Search for other works by this author on: GSW Google Scholar W.D. Maier W.D. Maier Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby@postino.up.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information A.J. Bumby Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby@postino.up.ac.za P.G. Eriksson Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby@postino.up.ac.za R. van der Merwe Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby@postino.up.ac.za W.D. Maier Department of Earth Sciences, University of Pretoria, Pretoria 0002, Republic of South Africa. ABumby@postino.up.ac.za Publisher: Geological Society of South Africa First Online: 03 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2001 Geological Society of South Africa South African Journal of Geology (2001) 104 (3): 205–216. https://doi.org/10.2113/1040205 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.J. Bumby, P.G. Eriksson, R. van der Merwe, W.D. Maier; The stratigraphic relationship between the Waterberg and Soutpansberg Groups in Northern Province, South Africa: Evidence from the Blouberg area. South African Journal of Geology 2001;; 104 (3): 205–216. doi: https://doi.org/10.2113/1040205 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 SocietySouth African Journal of Geology Search Advanced Search Abstract The relative stratigraphic position of the mid-Proterozoic Waterberg and Soutpansberg Groups in Northern Province has long been a subject of debate. These two major Proterozoic groups are juxtaposed in the Blouberg area, although the nature of the contact between the two sedimentary units is generally regarded as being faulted along the southern strand of the Melinda Fault. The Blouberg area is also the type locality for yet another small mid-Proterozoic succession, that of the Blouberg Formation, which locally outcrops beneath both the Waterberg and Soutpansberg strata.Existing maps of the Blouberg area show the Wyllie’s Poort Formation of the Soutpansberg Group to unconformably overlie rocks of the Blouberg Formation in the western foothills of Blouberg mountain. However, recent mapping suggests that these “Blouberg” strata themselves unconformably overlie the Blouberg Formation. This, therefore, raises important questions regarding the stratigraphic placement of these intermediate strata in the western foothills of Blouberg mountain, which outcrop unconformably above the Blouberg Formation and unconformably below the Wyllie’s Poort Formation. The intermediate strata are characterised by a thin basal conglomerate, consisting of cobbles of quartz, quartzite and B.I.F. This basal conglomerate grades vertically into trough cross-bedded sandstone and granulestone, which are characterised by heavy mineral concentrations on foresets.Although this facies compares poorly with strata of the Mogalakwena Formation in adjacent areas, which are generally more conglomeratic, they are identical with strata from more distal outcrops of the Mogalakwena Formation further to the south-west Palaeocurrent directions recorded from the intermediate strata are unimodal towards the west-south-west, in common with those recorded from the Mogalakwena Formation. This suggests that the intermediate strata, unconformably overlain by the Wyllie’s Poort Formation of the Soutpansberg Group, can be correlated with strata of the Mogalakwena Formation. The Waterberg Group thus appears to pre-date the Soutpansberg rocks.Additional evidence for the age relationships proposed here can be gained from an east-north-east-trending dyke swarm, which locally intrudes the basement, Blouberg and Waterberg rocks. This dyke swarm does not intrude Soutpansberg strata in adjacent areas. Patterns of spider diagrams of incompatible trace elements recorded from these dykes are very similar to those from the basaltic Sibasa Formation of the Soutpansberg Group. This suggests that the dyke swarm may have acted as feeders to the Sibasa lavas, and also suggests that the Soutpansberg Group post-dates the Waterberg Group. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Upper Triassic (Keuper) non-marine trace fossils from the Haßberge area (Franconia, south-eastern Germany)

Various trace fossils from the Hassberge Formation and the Lowenstein Formation (Middle Keuper, Upper Triassic) of the Hasberge region are described. Twenty-three different forms have been identified, 17 of which are named, includingCruziana pascens n. isp.,Lockeia cunctator n. isp., andRusophycus versans n. isp.Lockeia siliquariaJames, 1879,L. amygdaloides (Seilacher, 1953),L. triangulichnusKim, 1994, andL. elongata (Yang, 1984) are revised and synonymized under the oldest available name,L. siliquariaJames, 1879.Rusophycus eutendorfensis (Linck, 1942) andR. carbonariusDawson, 1864 are revised. The diagnosis ofPolykladichnusFursich, 1981 is emended, and a diagnosis forHelminthoidichnitesFitch, 1850 is given for the first time. Among the described ichnotaxa,Skolithos ispp.,Rusophycus carbonarius, andTaenidium barretti are the most common forms. The trace fossil association is typical of theScoyenia ichnofacies, which indicates non-marine, periodically or completely inundated environments, such as floodplains and lake margins. Two palaeoichnocoenoses are identified. One ichnocoenosis, dominated byCruziana problematica, cf.Polykladichnus isp., andSkolithos isp. B characterizes margins of trough cross-bedded sandstones. Another ichnocoenosis, dominated byRusophycus versans n. isp.,Taenidium barretti,Scoyenia gracilis andSkolithos isp. A is related to ephemeral lake deposits. Taxonomic recommendations for the use of hitherto described and figured invertebrate Keuper trace fossils from Germany are given.

Detailed internal architecture of a fluvial channel sandstone determined from outcrop, cores, and 3-D ground-penetrating radar: Example from the middle Cretaceous Ferron Sandstone, east-central Utah

Ideally, characterization of hydrocarbon reservoirs requires information about heterogeneity at a submeter scale in three dimensions. Detailed geologic information and permeability data from surface and cliff face outcrops and boreholes in the alluvial part of the Ferron Sandstone are integrated here with three-dimensional (3-D) ground-penetrating radar (GPR) data for analysis of a near-surface sandstone reservoir analog in fluvial channel deposits. The GPR survey covers a volume with a surface area of 40 x 16.5 m and a depth of 12 m. Five architectural elements are identified and described in outcrop and well cores, using a sixfold hierarchy of bounding surfaces. Internally, the lower four units consist of fine-grained, parallel-laminated sandstone, and the upper unit consists of medium-grained, trough cross-bedded sandstone. The same sedimentary architectural elements and associated bounding surfaces are distinguished in the GPR data by making use of principles developed in seismic stratigraphic analysis. To facilitate comparison of geologic features in the depth domain and radar reflectors in the time domain, the radar data are depth migrated. The GPR interpretation is carried out mainly on migrated 100 MHz data with a vertical resolution of about 0.5 m. Measures of the spatial continuity and variation of the first- and second-order bounding surfaces are obtained by computing 3-D experimental variograms for each architectural element (each radar (Begin page 1584) facies). The maximum correlation length of the dominant internal features ranges between 4 and 6 m, and the anisotropy factor ranges between 0.6 and 0.95.

Depositional and structural setting of the (?)Lower Old Red Sandstone sediments of Ballymastocker, Co. Donegal

Abstract Approximately 250 m of unfossiliferous sediments attributed to the Old Red Sandstone outcrop onshore at Ballymastocker, Donegal in northwest Ireland. Massive sandstones and trough cross-bedded sandstones pass upwards into coarse purple-brown basal conglomerates. The majority of the clast types, which include quartzite, marble, pelite, schist, vein quartz and metadolerite, are derived from the local Dalradian basement. Acid to intermediate porphyry clasts have no local source and may indicate a source to the northeast in the vicinity of the Devonian Lorne Plateau volcanic rocks in Argyll. The rocks are believed to represent an intermontane alluvial fan environment. The sediments occur in an outcrop of 3 km length against the northern flank of Knockalla Mountain, bounded to the southeast by the NE-SW-trending Caledonian Leannan Fault and resting on fractured Argyll Group Dalradian basement. No unconformable contact is observed and all contacts may be tectonic. Coarsening-up cycles within the sediments may reflect syn-sedimentary tectonic activity. The Devonian rocks are transected by a fracture system implying dextral transpressional activity on the Leannan Fault. The Dalradian basement rocks preserve evidence of sinistral strike-slip movement in late Caledonian times. Slickenfibres observed within the sediments and Dalradian rocks provide evidence for oblique dip-slip movement to the northwest. It is considered that these structures represent the last movement of the ORS basin, dropping it below denudation level, which resulted in its preservation. Movement along the Leannan Fault played a major role in the preservation of the offshore basins in the Islay-Donegal Platform region.

Tectonic and volcanic controls on Early Jurassic rift-valley lake deposition during emplacement of Karoo flood basalts, southern Namibia

The Karoo Igneous Province of southern Africa is one of the classic Mesozoic flood basalt provinces of the world. In the case of the early Jurassic Kalkrand Formation of Namibia the succession comprises three major flood basalt units that are separated by two stratigraphically important fluvio-lacustrine interlayers. These horizons preserve a record of the complex interplay between sedimentation, effusion of Karoo flood basalts and extensional tectonics that predated and accompanied the break-up of Gondwanaland. Both sediment layers start with the dominantly local derivation of weathered and eroded lava debris, followed by the emplacement of subaqueous mass flows and subsequent deposition of chemical sediments. The latter are characterised by interbedded stromatolitic carbonates, grass-like structured gypsum, and plane-bedded sandstones and mudstones containing euhedral displacive gypsum crystals that grew in the subsurface as well as rosettes which nucleated on the sediment surface. The central parts of the lacustrine units are overlain by thin deltaic sandstones showing bottomset, foreset, and topset layering and, finally, braided fluvial, trough cross-bedded sandstones. Evidence of subsidence synchronous with the formation of lake bodies can be explained by two principal mechanisms. The first acted in localised areas only and is reflected by the development of small, centrally subsiding basins. From the repeated occurrence of onlapping geometries within such a pool, a multiphase history of sagging is deduced, being most likely related to periodic magma withdrawal and reduction in magmatic pressures in subsurface lava feeders beneath the basin floor. Abundant syn-subsident fracture features at lava–sediment contacts, such as sediment-, hydrothermal calcite-, and sometimes basic lava-filled fissure systems indicate the pronounced interaction between the underlying volcanics and these small areas of pronounced subsidence. Fluids passing through the volcanic pile exhaled into the lake, giving it the characteristics of alkaline lake systems described from more recent flood basalt areas associated with the modern African Rift System. On the regional scale, however, northerly trending extensional fault systems controlled half-graben basin geometries and both facies and thickness variations across faults indicate that tectonism operated contemporaneously with volcanism and lacustrine sedimentation. The analysis of faults and associated structures, such as regularly aligned sediment-filled fissures, sets of micro-faults, folds and basaltic dykes constrains the extensional opening direction for the Karoo graben structures in this area that heralded the opening of the South Atlantic and thus provides a basis to discuss the extensional history of the Namibian coastal margin within the regional tectonic framework.

Early Cretaceous deltaic sedimentary environment of the Côte d'Ivoire-Ghana transform margin as deduced from deep dive data

During the Equanaute cruise (June 1992), 14 submersible deep dives were conducted along the steep southern slope of the Cote d’Ivoire–Ghana marginal ridge. In situ diving observations and sample analysis were done to specify the clastic lithology of a sedimentary sequence about 2200 m thick. Most of the rocks collected consist of dark shales, greenish lenticular- to wavy-bedded siltstones and large-scale trough cross-bedded sandstones. Lithofacies analysis favors a deltaic origin for this sedimentary synrift succession, which is probably of Early Cretaceous age.

Point McIntyre Field, Alaska: An Integrated Reservoir Study

The geologic model of the Kuparuk Formation at Point McIntyre Field is the result of an integrated reservoir study. Data from 7000 feet of evenly distributed cores are the foundation for a lithofacies model, chronostratigraphic correlations, 2-D and 3-D mapping, and the geostatistical distribution of reservoir parameters. The resulting cellular model provides improved input to the simulator model used for reservoir management. The depositional lithofacies model is divided into 14 categories. These range from bioturbated, very fine-grained sandstone to conglomerates with abundant lithic mudstone clasts. Depositional lithofacies control reservoir quality, with the highest potential in massive or trough cross-bedded sandstones. The reservoir sandstones represent the marine portion of a fan-delta. No shales thicker than one inch are preserved within the Kuparuk Formation in the Point McIntyre cores. Accurate correlation and mapping of chronostratigraphic surfaces is essential to preserve porosity and permeability trends within the model. Sequence stratigraphic correlations show a complex depositional history and define twenty-one mappable zones with five major truncation surfaces. New zonal mapping methodologies honor the chronostratigraphy. Isochore maps contain strong depositional trends that influence reservoir management. A 3-D cellular model captures onlap and truncation relationships within the zones, preserving internal geometries. Lithofacies distributed within the 3-Dmore » model form a template tied to core and well control. The lithofacies template guides the geostatistical distribution of reservoir parameters.« less

Models for Tidal Sandstone Reservoir Characterisation from High-Resolution Sequence Stratigraphic Study of the Miocene Marine Molasse in the Digne-Valensole Basin, France: ABSTRACT

The Marine Molasse of the Digne-Valensole Basin is a 1200 m thick, siliciclastic-dominated succession with minor bioclastic carbonate units. Lying between the continental Red Molasse below and Valensole Conglomerate above, it forms the major part of a broad transgressive - regressive basin-fill succession ranging from U. Eocene to Pliocene in age. In common with Marine Molasse deposits from other parts of the Western Alpine Foreland Basin, tidal currents strongly influenced the depositional character of the 1200 m thick succession. In addition, cyclic controls on relative sea level resulted in the formation of numerous stacked depositional sequences bounded by erosional unconformity surfaces lagged by cobbles and pebbles. Low order composite sequences are in the order of 100 m thick while high order sequences are a few tens of metres thick. Detailed logging of the succession at intervals of 1-2 km has identified a number of tidal sandstone body types. These are, primarily, estuarine incised valley fill deposits and tidal delta deposits. The incised valleys show a range of infill types including heterolithic tidal channel/flat deposits, trough cross-bedded sandstone deposits and steeply inclined point bar accretion deposits. This study aims to characterise the reservoir architecture of different sandstone body types at amore » range of scales, to study permeability distribution and to examine, in particular, the influence of cyclic changes in relative sea level on sandstone body scale, connectivity and type.« less

Planar cross-bedding associated with rip currents of Upper Cretaceous formations, northeast Japan

The sedimentary structure produced by rip currents is mainly characterized by seaward-dipping planar cross-bedding. Field observations from the Upper Cretaceous (Santonian) Kunitan and Taneichi formations in northeast Japan show that the occurrence of the cross-bedded units extends laterally from the ancient beachlines but is restricted in vertical extent. The planar cross-set bed of the Kunitan Formation overlies very coarse-grained, parallel-laminated and trough cross-bedded sandstone beds of longshore current origin and underlies a pebbly conglomerated bed that was probably formed by a storm-generated bottom current. Above is a zone of alternated beds of bioturbated and parallel-laminated or hummocky, cross-stratified, fine-grained sandstones, representing either lower shoreface or transitional depositional environments. The planar cross-bedding of the Taneichi Formation is intercalated with the underlying trough cross-bedding and with the overlying bioturbated or parallel-laminated, very fine-grained sandstone bed which also includes hummocky cross-stratification. Both planar cross-set beds of the Taneichi and Kunitan formations are present in the succession which fines upward, suggesting an increasing depth of deposition. Such field occurrences indicate that bedforms produced within deeply incised rip channels which extend towards the sea can be untouched by ravinement and remain well preserved during transgression.

Depositional facies and environments in the Permian Umm Irna formation, Dead Sea area, Jordan

The Permian Umm Irna Formation in central Jordan consists of a 60 m thick sequence of clastic sediments which can be divided into two fluvial sedimentary facies. The lowermost facies (1) is characterized by the presence of five sandstone-dominated fining-upward sequences, each sequence comprising an erosively based coarse- to fine-grained, trough cross-bedded sandstone, overlain and laterally intertonguing with maroon siltstone and silty-shale, containing locally abundant carbonaceous plant material. The overlying facies (2) also consists of up to five fining-upward sequences, each comprising an erosively based coarse-grained pebbly sandstone grading up through fine-grained sandstone and siltstone into silty-shale. The sandstones are tabular, and laterally persistent, internally complex units structured by erosively bounded trough cosets. The thickness and grain size of the sandstones and the proportion of siltstone and silty-shale is much greater than in facies (1); it also contains abundant ferruginous concretions (glaebules). Deposition occurred on an unconfined braidplain sloping northwestwards away from an elevated provenance (Arabo-Nubian Shield) in the south and east. The lowermost facies (1) is regarded as a distal environmental equivalent of the overlying facies (2). Deposition was largely controlled by period c shifts of the active channel tract, influenced by a gradual increase in the elevation of the source area, thereby promoting basinwide progradation and vertical stacking of the two major facies components. The ferruginous concretions (glaebules) in facies (2) are thought to have resulted from diagenetic processes in the vadose zone of a humid, possibly tropical climate, characterized by alternating wet and dry seasons and strong leaching. The fluvial depositional system drained basinwards into a shallow-water marine depository along the southern margin of the Tethys Ocean. The ocean may have exerted a maritime influence on the climate of the alluvial plain.

Effects of Sedimentologic and Petrologic Heterogeneity on Reservoir Properties of the Upper Cretaceous Sussex Sandstone in the House Creek Field, Powder River Basin, Wyoming

Petrologic and sedimentologic heterogeneity in the Sussex Sandstone Member of the Upper Cretaceous Cody Shale in the House Creek field affects recovery of oil through a decrease in sandstone porosity and permeability and by causing isolation and compartmentalization of reservoir facies. The Sussex is an upward-coarsening bed of inter-ridge to central-ridge facies of a marine-ridge sandstone sequence. Oil is produced from fine- to medium-grained, porous, and mainly trough cross-bedded central-ridge and ridge-margin sandstones. Underlying and interbedded inter-ridge sandstones have low porosity and permeability and are laterally continuous, thinly-bedded, and generally tabular. Megascopic heterogeneity includes (1) compartmentalization of the lensate reservoir sandstones by interbedding with inter-ridge sandstones, (2) facies-related upward-increasing porosity and permeability in the marine-ridge sequence, and (3) bedform-related permeability boundaries that result from soft-sediment deformation of glauconite concentrated in trough cross-bedding laminae sets. Petrologic heterogeneities result mainly from highly variable distributions and amounts of cements and clays. Nonproductive inter-ridge and ridge-facies sandstones contain generally equivalent amounts of quartz cement (8% average) and greater amounts of calcite cement (12% average) and pore-occluding clays (14% average) than oil-productive sandstones. Reservoir sandstones contain averages of 9% carbonate cement and 10% clays, but the most productive and porous intervals are cemented by quartzmore » (8% average) with negligible calcite. Average thin-section intergranular porosity is 7% in oil-productive and 4% in nonproductive sandstones.« less

The sedimentology and accretionary styles of an ancient gravel-bed stream: the Budleigh Salterton Pebble Beds (Lower Triassic), southwest England

Three accretionary styles are present in the fluvial conglomerates and pebbly sandstones of the Lower Triassic Budleigh Salterton Pebble Beds (BSPB) of east Devon. The lower portions of this 20–30 m sequence are dominated by extensive (> 50 m) sheets or narrow lenses of planar cross-bedded conglomerate (sets 1–3 m thick). These abruptly pass, both downstream and laterally, into horizontally bedded conglomerates, cross-bedded sandstones and muddy sandstones. This accretionary style reflects deposition from linguoid-shaped macroforms whose distal margins were bounded by slipfaces. They accreted at anabranch confluences and as bank-attached bars in relatively confined and channelised gravel-bed streams. Lateral facies changes represent macroform evolution resulting from changing discharges or abandonment of a channel reach, followed by accretion of a new macroform. The upper portions of the BSPB mainly comprise couplets of horizontally bedded conglomerate overlain by large-scale trough cross-bedded sandstone. The conglomerate shows local channel-macroform relief and lateral and downstream transitions into sets of gently dipping conglomerate. This style of accretion represents deposition in less confined and less channelised fluvial systems by relatively low-relief gravel macroforms and large sand dunes. The third style recognised in the BSPB is much more localised. It consists of wedge-like sets of large-scale trough cross-bedded conglomerate. The sets are formed by erosion and subsequent fill of scours cut into a thick sandy substrate. Techniques of lateral profile analysis and hierarchical ordering of bounding surfaces can usefully be applied to some parts of the BSPB. This is important as the approach was originally developed for the deposits of sand-bed streams. However, this approach is more difficult to apply to fluvial conglomerates because of the cryptic nature of some gravel stratification. The appearance and character of some hierarchies of bounding surfaces may differ from their equivalents in fluvial sandstones.

High-energy ephemeral stream deltas; an example from the Upper Silurian Holmestrand Formation of the Oslo Region, Norway

The late orogenic deposits of the Upper Silurian Holmestrand Formation of the Oslo Region, Norway, comprises sharply alternating ephemeral stream and sandy beach face deposits. The formation is divided into three units reflecting the down-slope change on an extensive ephemeral stream delta system. The lower unit of the formation comprises ephemeral stream sediments as stacked sheet-sandstone bodies, deposited on a broad coastal floodplain. Each sheet is bounded by a lower intraclast strewn scoured surface overlain by massive, horizontally stratified, low-angle and trough cross-bedded sandstones. In the medial and upper units of the formation the ephemeral stream sediments alternate with beach face deposits, forming ephemeral stream deltas. The ephemeral stream deltas are marked by intraclast strewn scoured surfaces, and dominated by trough cross-bedded sandstones. They may contain escapement burrows, marine bioturbation and wave-working of the delta surfaces. The beach face deposits are composed of alternating irregular, horizontally stratified sandstones with current and wave ripple cross-lamination and tabular cross-bedded sandstones. Deposition took place in a coastal, non- to micro-tidal, shallow-water environment, influenced by longshore currents and low palaeo-wave energy. Seven ichnotaxa are recognized and support the sedimentary record of a transitional environment between a broad coastal floodplain and a shallow marine environment. The presence of calcretes indicate an arid to semi-arid palaeo-climate.