Succession Of Sandstones Research Articles

The Castlegate Sandstone of the Book Cliffs, Utah: Sequence Stratigraphy, Paleogeography, and Tectonic Controls

ABSTRACT Earlier stratigraphic work had predicted that at the type section of the Castlegate Formation, the Castle Gate, near Price, Utah, the unit consists of two sequences separated by a sequence boundary representing approximately one million years of unrecorded time. Although the type section is well exposed, it consists of a monotonous succession of braided fluvial sandstones and no obvious boundary can be identified using facies criteria--this is a good example of a cryptic sequence Petrographic data indicate, however, a significant change in detrital composition 20 m above the base of the section, at a through-going erosion surface that is therefore interpreted as the sequence boundary. Revised sequence correlations, together with other petrographic data and regional paleocurrent patterns, provide the basis for a model of the paleogeographic evolution of the area. Rocks assigned to the Castlegate Sandstone comprise two or possibly three sequences formed at times of slow regional subsidence. Erosional sequence boundaries and tilts in paleoslope between each sequence record thrust loading and unloading of the basin and the growing influence of intrabasinal upwarps, movement of which was beginning to be affected by Laramide movements toward the end of Castlegate sedimentation.

High resolution geochemical and petrographical correlation of deep marine Palaeocene sandstones from the Foinaven Field, West of Shetland

Cores from three wells in the Upper Palaeocene deep-marine reservoir interval of the Foinaven Field, West of Shetland have been analysed using trace element geochemistry and petrography. The 60 m thick turbidite-dominated succession is restricted to the T34 biozone and comprises a series of seismically defined channel and overbank deposits. Petrographic and trace element geochemical data indicate that wells 204/24a-3 and 204/25b-5 have strong similarities and that strata in the remaining well (204/24a-7) have distinctly different signatures. Of the two related wells, 204/25b-5 is dominated by a thick amalgamated sandstone succession, 204/24a-3 by more thinly bedded and finer grained sandstones considered to represent channel and associated overbank deposits respectively. Similarities in geochemical and petrographic data allow division of the biozone in the two related wells into four principal correlation zones with two additional sub-zones. The study has established a high resolution correlation scheme and constrained correlation lengths for reservoir and non-reservoir facies in a deep marine depositional system.

Tropical/subtropical Upper Paleocene–Lower Eocene fluvial deposits in eastern central Patagonia, Chile (46°45′S)

A succession of quartz-rich fluvial sandstones and siltstones derived from a mainly rhyolitic source and minor metamorphic rocks, located to the west, represent the first Upper Paleocene–Early Eocene deposits described in Chilean eastern central Patagonian Cordillera (46°45′S). This unit, exposed 25km south of Chile Chico, south of lago General Carrera, is here defined as the Ligorio Márquez Formation. It overlies with an angular unconformity Lower Cretaceous shallow marine sedimentary rocks (Cerro Colorado Formation) and subaerial tuffs that have yielded K–Ar dates of 128, 125 and 123Ma (Flamencos Tuffs, of the Divisadero Group). The Ligorio Márquez Formation includes flora indicative of a tropical/subtropical climate, and its deposition took place during the initial part of the Late Paleocene–Early Eocene Cenozoic optimum. The underlying Lower Cretaceous units exhibit folding and faulting, implying a pre-Paleocene–Lower Eocene contractional tectonism. Overlying Oligocene–Miocene marine and continental facies in the same area exhibit thrusts and normal faults indicative of post-Lower Miocene contractional tectonism.

Geocongress 2000: A new millennium on ancient crust 27 th Earth Science Congress of the Geological Society of South Africa

The Arak basin in the northern part of the Malayer-Esfahan metallogenic belt (MEMB) of Iran hosts 9 Zn-Pb sulfide deposits within a Jurassic sedimentary succession of black, fine-grained sandstone, siltstone and shale. This succession was deposited in an extensional back-arc environment between the Sanandaj- Sirjan Zone (SSZ) and the Central Iranian microcontinent. Two of the 9 deposits (Western Haft-Savaran and Hossein-Abad) are the focus of study. Here, three ore facies can be distinguished, based on the morphology of the orebodies and the sulfide mineralogy and textures: (a) stockwork, (b) massive, and (c) bedded mineralization. The stockwork and massive mineralization are interpreted to have formed in the subsurface, whereas bedded mineralization was sedimented on the seafloor. The Fe contents of sphalerites from massive and stockwork mineralization from the two deposits, together with the presence of pyrrhotite inclusions, indicate that sphalerite precipitated from relatively low fS2 (<10−11) fluid. Homogenization temperatures of fluid inclusions in sphalerite, quartz and siderite in massive mineralization range from 105 to 309 °C, and average 208 °C, with salinities between 4.03 and 11.1 wt% NaCl eq., and averaging 5.77 wt% NaCl eq. These widely ranging salinities and homogenization temperatures reflect mixing of shallow connate water with venting metalliferous fluid that is of deep basinal origin, and deposition of mineralization predominantly below the seafloor. Collectively, the host rock types, ore facies and textures, deposition in an reduced ambient environment, fluid inclusion temperatures and salinities, and mineralizing fluid behaviour at the seafloor are all compatible with a vent-proximal sedimentary exhalative (SEDEX) origin for the Western Haft-Savaran and Hossein-Abad deposits.

Stratigraphic evidence for an Early Aptian sea-level fluctuation: the Graua Limestone of south-eastern Ethiopia

The occurrence of a thin band of marine limestone (Graua Limestone) within a thick succession of fluviatile sandstones in south-eastern Ethiopia is direct evidence of flooding of part of the East African craton (Horn of Africa). According to the presence of abundant orbitolinid foraminifers (Palorbitolina lenticularis Blumenbach andPraeorbitolina cormyi Schroeder), the age of the Graua Limestone can be referred to the Early Aptian. Stratigraphy and palaeogeographic reconstructions for the Early Cretaceous in the surrounding regions (Kenya, Somalia, Yemen, Socotra, Oman, Syria, Lebanon, Israel, Egypt and Libya) show that the Early Aptian transgression was of regional extent. Our data seem to confirm that this transgression was of relatively short duration. This pulse cannot be related to tectono-eustatic mechanisms, which are too slow. A short-lived event should be invoked: either a regional tectonic pulse or the desiccation of the proto-South Atlantic.

The Lesotho Highlands Water Project: A Geotechnical and Engineering Geological Review of the Initial Phase

The purpose of Lesotho Highlands Water Project is primarily to supply water from Lesotho to Gauteng Province, the industrial region of South Africa. In addition, the hydroelectric scheme at Muela will supply enough electrical power to meet the needs of Lesotho. The project consists of a number of dams, reservoirs and tunnels, and a hydroelectric scheme which will be constructed in four phases. Construction operations, if the project runs to completion, will last until around 2020. Phase IA commenced in 1987 and consists of Katse Dam, a Transfer Tunnel from the impounded reservoir to the Muela Hydroelectric Power Station, the Muela Dam, and a Delivery Tunnel from the latter dam to the outfall on the Ash River, which is part of the Vaal River system. The tunnels total some 81 km in length. Except for the adits which were drilled and blasted, the tunnels were constructed by five tunnel boring machines. The Katse Dam and Transfer Tunnel were constructed in the basalts of the Drakensberg Formation. Many of these basalts have been subjected to deuteric alteration and consequently contain expansive clay minerals. Their presence can mean that the basalts disintegrate rapidly on exposure, which led to increased excavation at the dam site, and slabbing and spalling from the perimeter of the tunnel. The Muela Hydroelectric Power Station and Delivery Tunnel South were constructed primarily in sandstones of the Clarens Formation. These sandstones presented few problems during excavation and this tunnel was bored 20 months ahead of schedule. It was anticipated that the Delivery Tunnel North would be the most difficult to excavate as it runs through a succession of mudstones and sandstones. Hence, this tunnel was excavated by a double-shielded tunnel boring machine which could bore through varied ground and provide protection against rock fall, while simultaneously installing precast lining segments.

Detection of sedimentary cyclicity and stratigraphic completeness by wavelet analysis; an application to late Albian cyclostratigraphy of the Western Canada Sedimentary Basin

ABSTRACT On the basis of the time-frequency scaling property of the wavelet transform, accumulation rates and stratigraphic completeness can be calculated for various observation time spans (ot) by using wavelet analysis. Wavelet analysis also allows automatic detection of high-frequency sedimentary cyclicity, and abrupt and gradual variations in sedimentation rate. The preservation of different frequency cycles is strongly dependent on variations of the intensity-to-noise ratio of the original cycles and accumulation rate through time, as demonstrated by computer models. Wavelet analysis was used to detect and correlate periodic-cyclic successions of marine sediments in the Western Canada Sedimentary Basin in observation time spans ranging from 100 kyr to 3.8 Myr. Cycles in gamma-ray data are in accordance with sonic logging, lithology, and biotic change. Cyclic units embedded in the upper Albian Westgate and Joli Fou Formations are almost periodic with 11 and 10 repetitions, respectively. Accumulation rates at ot = 100 kyr vary from 5-7 cm/kyr in mudstone successions to 29 cm/kyr in sandstone successions of the Rocky Mountains Foothills. These 100 kyr cycles are preserved from 30% in the Manitoba Escarpment to 100% in SE Alberta in the Westgate Formation. These cycles can be correlated throughout facies belts of siliciclastic sedimentation in the entire basin. In gamma-ray logging the cyclicity is most pronounced in the foredeep succession and almost without fluctuations in the basin center. Cyclic changes from wet to dry climate probably forced by Milankovitch cycles of 100 kyr eccentricity may have controlled the stable periodic sedimentary cyclicity.

Reservoir Architecture of Deep-Lacustrine Sandstones from the Early Cretaceous Rec&amp;ocirc;ncavo Rift Basin, Brazil

Synrift, lacustrine sedimentation took place along the entire eastern Brazilian margin in response to the Neocomian breakup of Gondwana. In the Reconcavo basin, a deep lake was developed and filled mainly by a thick (>2000 m) succession of dark-colored, organic-rich mudstones and sandstones, and by subordinate, oncolite- and Ostracod-rich carbonates. In the Fazenda Balsamo oil field, near the northeastern border fault margin of Reconcavo basin, deep-lacustrine reservoirs comprise a succession up to 424 m thick composed mainly of laminated (lower section) and massive (upper section), medium- to fine-grained sandstones. Their porosities and permeabilities typically range from 14 to 23%, and 100 to 500 md, respectively. The sandy reservoirs and interbedded source-rock mudstones comprise eight transgressive successions (mostly 20-120 m thick) bounded by basin-wide, mudstone marker beds. The average duration (about 83,000 yr) of each succession suggests that climatically driven lake level fluctuations may have modulated the cyclic, deep-lacustrine sedimentation in the study area; however, the interaction of changes in lake level with tectonic activity defined the dominant type of reservoir facies and the geometry and position of sand bodies. Laminated sandstones form sandstone bodies that are 600-1200 m wide, 1.5-4.5 km long, and up to 46 m thick, which fill northeast-oriented, fault-bounded troughs. These rocks probably were deposited by long-duration, sand-rich density underflows preferentially developed during rising lake level or highstands. These rocks may reflect an increasingly humid climate and more powerful and sediment-loaded influent streams. The more intense fault activity during the deposition of the lower reservoir section would have axially focused density underflows coming from the northeastern end of the basin. Massive sandstones comprise turbidite channel-fills that are 100-800 m wide, more than 2 km long, and up to 38 m thick. Fault activity diminished during the deposition of the upper reservoir section, allowing the cutting and filling of northwest-oriented channels by turbidity currents derived from the nearby border fault margin, preferentially during falling lake level or lowstands. Despite the local erosion associated with some turbidite channels, no unconformity or widespread erosion surface can be recognized within the studied succession. The relatively rapid and continuous tectonic subsidence along the eastern border fault margin of the Reconcavo basin provided virtually unlimited accommodation space for the aggradational stacking of climatically controlled, deepening-upward successions.

Eocene Stratigraphy of Southeastern Tierra del Fuego Island, Argentina

Recognizing folded and thrusted Eocene foreland strata of the oil-producing Austral Basin provides new insights on the Paleogene evolution of Tierra del Fuego Island and the adjacent Atlantic shelf. Field mapping and petrographic and micropaleontologic studies at the Punta Torcida anticline depict an Eocene composite section about 1600 m thick assigned to the La Despedida Group. This group is divided into three formations: Punta Torcida (215 m), Leticia (520 m), and Cerro Colorado (855 m). These formations are subdivided into informal members. In the Punta Torcida Formation the lower member is composed of dark-gray mudstones with thin sandstone beds. The middle member of the Punta Torcida contains mudstones with small sandstone lenses. Both the lower and middle members have a high radiolarian content. The upper member contains mudstones. Lithologic succession, presence of pyrite, and a well-preserved benthic assemblage having little diversification suggest a regressive sequence formed in shelf settings with a dysaerobic to anoxic bottom. Planktonic foraminifera indicate a Ypresian age (equivalent to upper Zone P6 to basal Zone P9 tropical planktonic zones). In the Leticia Formation the lower and upper members are glauconite rich, highly bioturbated sandstones; in the middle member, rich in pyrite and carbonized plant material, the glauconite is rare or absent. Large channels, filled with cross-stratified sandstone lenses and having a general absence of planktonic foraminifera, suggest estuarine, restricted-marine settings. Planktonic foraminifera indicate a late-middle Eocene age (basal Zone P12 to middle of Zone P14) indicating a hiatus with the underlying Punta Torcida Formation. The Cerro Colorado Formation, a coarsening- and thickening-upward succession of mudstones and sandstones, is a major, shelfal transgressive-regressive sequence punctuated by minor deepening and shallowing. Its four members record different benthic assemblages that reflect cooling waters. The uppermost member bears abundant radiolarians and planktonic foraminifera typical of an oxygen-minimum habitat and indicating a late Eocene age (upper Zone P15 to upper Zone P16). The microfauna and stratigraphic position of the lower three members are consistent with a latest middle Eocene age (middle of Zone P14 to lower Zone P15). In preliminary observations on the outcrop area of Late Cretaceous-Paleogene strata between Cabo San Pablo and Bahia Thetis, two important angular unconformities were recorded: one at the contact between the Estratos de Policarpo and the Rio Bueno-La Despedida Group, and the other at the contact between the La Despedida Group and the Cabo Pena-Rio Leona formations.

Reservoirs of the Dinantian (Lower Carboniferous) play of the Southern North Sea

Critical to the successful exploitation of the northern margin of the Southern Gas Basin is the development of the Lower Carboniferous (Dinantian, Viséan) play. Whilst the exploration for intra-Carboniferous traps has been unsuccessful, the presence of large Base Permian evaporite sealed closures still holds opportunities. The discovery of hydrocarbons within one of these closures on the edge of the traditional gas basin suggests that migration of gas from Westphalian and Namurian coals and shales has occurred. Thus in this area, the primary remaining risk is the presence and distribution of reservoir. Two potential reservoirs are examined in the subsurface, the Fell Sandstone Formation and the Whitby Member. Onshore analogue data has been combined with the sparse offshore data to explore the distribution of these sandstones. The Fell Sandstone Formation was deposited at a time of high sediment supply resulting in a thick (300 m) succession of fluvial sandstones whose distribution is closely linked to the accommodation space developed in the hanging walls of contemporaneous faults. The Whitby Member (thickness 30 m) was deposited at a time of lower sediment supply which enabled relative sea-level changes to control the vertical distribution of sandstone bodies. It demonstrates some of the features which suggest it could infill a palaeovalley. If active faulting continued throughout this time then a similar hanging wall concentration of sandbodies, as is seen in the Fell Sandstone Formation, could be expected.

Reservoir quality and petrophysical evaluation of Paleocene sandstones in the West of Shetland area

A series of regionally correlatable unconformity bounded sequences has been identified within the Paleocene interval of the West of Shetland area. These sequences contain a succession of sandstones deposited in deep-water environments within the Early to ‘mid’ Paleocene (Danian to early Thanetian) with a gradual shallowing upwards to shelf and deltaic deposits in the Late Paleocene. Reservoir properties of these sandstones vary widely, with porosities ranging from &gt;30% to below 10%, and permeabilities from &gt;2D to &lt;0.1 mD. Although there is a severe reduction of reservoir quality with depth of burial, certain sandstone units have retained high porosities (&gt;20%) and permeabilities (10–100 mD) at below 3 km burial depth. This preservation of reservoir quality is due in part to depositional facies, but mineralogical/diagenetic variability has also allowed the preservation of porosity and permeability. In particular, the presence of grain rimming chlorite in these sandstones is fundamental in preserving porosity. Log and core analyses of these sandstones has shown that interpretation of reservoir potential and fluid saturations is not straightforward. The effect of diagenetic minerals on log responses and in particular the salinity of formation waters (both in water- and hydrocarbon-bearing zones) were poorly known at the start of the study, and had to be accurately quantified. The integration of all available data, including intensive study of core material, allowed the definition of these effects and thus allowed the adequate modelling of hydrocarbon saturations.

Relating eolian bounding-surface geometries to the bed forms that generated them: Etjo Formation, Cretaceous, Namibia

The Cretaceous Etjo Formation is a 200-m-thick eolian and fluvial sandstone succession exposed in the Huab Basin of northwestern Namibia. Eolian sedimentation was terminated abruptly by the emplacement of flood basalts of the Etendeka igneous province across much of the basin at 132 ± 1 Ma. The lavas “drowned” the dunes and, in doing so, preserved bed forms with heights and wavelengths of up to 100 m and 1.3 km, respectively. Subsequent erosion has resulted in the exposure of these bed forms and provides a rare opportunity to accurately reconstruct the three-dimensional geometry of an ancient eolian system and to relate bed-form morphology to bounding-surface geometries. Some bed forms lacked a single active slipface but instead were characterized by a dome form over which smaller transverse dunes migrated. Other bed forms were characterized by an active slipface and a stoss slope over which smaller, superimposed bed forms migrated. One bed form records a transitional evolution between these two end-member types. Preserved bed sets exhibit a positive (but stoss-erosional) angle of downwind bed-form climb of 1°. Bed forms of the Etjo Formation provide an opportunity to directly relate bed-set thickness to original bed-form height via the angle of climb.

3-D seismic imaging and interpretation of Brushy Canyon slope and basin thin‐bed reservoirs, northwest Delaware Basin

A study was done at Nash Draw field, Eddy County, New Mexico, to demonstrate how engineering, drilling, geologic, geophysical, and petrophysical technologies should be integrated to improve oil recovery from Brushy Canyon reservoirs at depths of approximately 6600 ft (2000 m) on the northwest slope of the Delaware basin. These thin‐bed reservoirs were deposited in a slope‐basin environment by a mechanism debated by researchers, a common model being turbidite deposition. In this paper, we describe how state‐of‐the‐art 3-D seismic data were acquired, interpreted, integrated with other reservoir data, and then used to improve the sitting of in‐field wells and to provide facies parameters for reservoir simulation across this complex depositional system. The 3-D seismic field program was an onshore subsalt imaging effort because the Ochoan Rustler/Salado, a high‐velocity salt/anhydrite section, extended from the surface to a depth of approximately 3000 ft (900 m) across the entire study area. The primary imaging targets were heterogenous siltstone and fine‐grained sandstone successions approximately 100 ft (30 m) thick and comprised of complex assemblages of thin lobe‐like deposits having individual thickness of 3 to 6 ft (1 to 2 m). The seismic acquisition was complicated further by (1) the presence of active potash mines around and beneath the 3-D grid that were being worked at depths of 500 to 600 ft (150 to 180 m), (2) shallow salt lakes, and (3) numerous archeological sites. We show that by careful presurvey wave testing and attention to detail during data processing, thin‐bed reservoirs in this portion of the Delaware basin can be imaged with a signal bandwidth of 10 to 100 Hz and that siltstone/sandstone successions 100 ft (30 m) thick in the basal Brushy Canyon interval can be individually detected and interpreted. Further, we show that amplitude attributes extracted from these 3-D data are valuable indicators of the amount of net pay and porosity‐feet in the major reservoir successions and of the variations in the fluid transmissivity observed in production wells across the field. Relationships between seismic reflection amplitude and reservoir properties determined at the initial calibration wells have been used to site and drill two production wells. The first well found excellent reservoir conditions; the second well was slightly mispositioned relative to the targeted reflection‐amplitude trend and penetrated reservoir facies typical of that at other producing wells. Relationships between seismic reflection amplitude and critical petrophysical properties of the thin‐bed reservoirs have also allowed a seismic‐driven simulation of reservoir performance to be initiated.

Transgressive systems tracts and onlap shellbeds from mid-Pleistocene sequences, Wanganui Basin, New Zealand

ABSTRACT Sequences in the Castlecliff section (mid-Pleistocene, Wanganui Basin, New Zealand) are composed of transgressive and highstand systems tracts, and were deposited in response to fifth-order (c. 100 ky) and sixth-order (c. 40 ky) glacio-eustatic fluctuation on the paleo-Taranaki Shelf. Castlecliff transgressive systems tracts (TST), the topic of this paper, are ideally represented by fining-upward facies successions up to 22 m thick. Above a basal sequence-bounding ravinement surface, these successions usually comprise a subtidal cross-bedded shell-rich gravel (onlap shellbed) which is overlain by a fining-upward succession of inner-shelf siltstones and silty fine sandstones. TST successions are capped by mid-cycle condensed shellbeds (discussed in detail elsewhere) which represent terrigenous sediment starvation at the seaward feather-edge of TST deposition. Castlecliff TST facies architecture differs substantially from other examples of transgressive stratigraphy. First, Castlecliff TST are relatively thick in comparison to overall cycle thickness. Second, sequence-bounding ravinement surfaces at the base of Castlecliff TST everywhere rest directly on shelf siltstone constituting the truncated highstand systems tract of the underlying sequence; this indicates erosion of (1) coeval paralic TST landward of the ravinement surface, (2) the lowstand coastal-plain comprising the sequence boundary sensu stricto as well as any incised fluvial channels, and (3) the presumably coarsening-upward upper part of the underlying sequence. Such differences can be reconciled by consideration of local factors relating to the active tectonic setting of Wanganui Basin. Deposition occurred in a storm-influenced shelf embayment adjacent to a rising hinterland. In this setting, high-energy transgressive shorefaces scoured 10-20 m. At the same time, the embayed coastline provided a trap for the abundant sediment supply.

Relative sea-level changes recorded by paralic liptinite-enriched coal facies cycles, Middle Jurassic Muslingebjerg Formation, Hochstetter Forland, Northeast Greenland

The Middle Jurassic Muslingebjerg Formation of Hochstetter Forland, Northeast Greenland, consists of a succession of shallow marine sandstones encasing four coal seams formed in low-lying coastal mires. The seams are up to 3.45 m thick and contain coal facies cycles with high liptinite contents (up to 70 vol%). The cycles generally start with huminite-rich bright or banded coal and end with dull coal or clayey coal/coaly claystone. The dull coals are characterized by a high proportion of liptinite, commonly resinite, and often mineral matter. Liptodetrinite, resinite and cutinite dominate the liptinite maceral group. Similarity in the relative proportions of the liptinite macerals in the bright and dull coals and the general association of bright coal and dull coal with a high resinite concentration suggest, that the dull coals represent a residue after selective removal of ligno-cellulosic tissues in the precursor peat of primarily the bright lithotype. The clayey coal/coaly claystone lithotypes have high contents of mineral matter and commonly contain alginite and pyrite. Occurrence of pyrite in levels with high diasterane C 27 C 29 ratios and sometimes also alginite in the clayey coal/coaly claystone lithotypes, indicates a marine influence during flooding of the mires. This suggests a causal link between base-level rise in the coastal mires and relative sea-level rise. Accommodation space available for thick peat accumulations was governed by overall relative sea-level rise and the dulling-upward cycles record outpacing of peat accumulation due to accelerated rise in base level/relative sea level. The Muslingebjerg Formation consists of four depositional sequences, beginning with a thick coal seam resting on a sequence boundary. Peat accumulation reflects onset of base-level rise in the coastal area and the coals represent the lower part of the transgressive systems tract. The overlying shoreface sandstones form the upper part of the transgressive systems tract and a progradational highstand systems tract. Alternatively the two lower seams in the succession may form a transgressive-regressive couplet separated by a lagoonal sandstone split. In this case a sequence boundary occurs at the top of the upper regressive seam. The dulling-upward cycles within the coal seams represent parasequences or possibly higher order sequences. Minimum age-estimates suggest that they accumulated over 4000–11000 yr.

Reservoir architecture of the upper Sherwood Sandstone, Wytch Farm field, southern England

Abstract The Sherwood Sandstone Group reservoir in the Wytch Farm field comprises a c. 150 m thick succession of arkosic sandstones deposited in a variety of fluvial, lacustrine and aeolian depositional systems. These systems show at least three orders of facies variability, which are interpreted to be the depositional response to climatic changes. These comprise a first-order evolutionary trend over the entire Sherwood Sandstone Group from perennial braidplain to ephemeral sheetflood systems to ephemeral lacustrine conditions. This trend culminated in deposition of the Mercia Mudstone Group, and reflects a long-term waning of sand supply and increasing ‘flashiness’ of the fluvial system. This trend is further subdivided into second-order cycles defined by five areally widespread floodplain and lacustrine deposits containing minimal development of fluvial sandstones. These represent widespread, episodic reductions in fluvial sediment supply and rising base level during more ‘humid’ climatic conditions. These horizons form the basis for the reservoir layering scheme. Each floodplain episode is increasingly more mud-rich upwards through the Sherwood section, and the sand-rich fluvial packages between become systematically more ephemeral in character. Third-order cycles are defined by thin (&lt;2 m), but alreally widespread floodplain and lacustrine horizons which are most readily identifiable in the upper half of the Sherwood section. The sandstones between these cycles are composed of aeolian and sheetflood deposits, but are incised by coarse-grained multistorey-multilateral channel deposits. The incisions are interpreted to be the result of fluvial erosion during dry climatic conditions when lake levels fell and the alluvial plain was devegetated. These incised fluvial deposits form the principal producing intervals in the upper part of the reservoir, particularly in the eastern part of the field. Higher frequency stratigraphic cycles are locally expressed by variations in ephemeral lake levels, palaeosol development and episodic development of wind-blown sand patches. At outcrop, the stratigraphically equivalent Otter Sandstone Formation ( c. 100 km to the west) shows comparable evolutionary patterns, albeit with a subtly different facies make-up. The recognition of a hierarchy of climatically driven cycles within the reservoir permits high-resolution correlation and the recognition of subtle, but important, changes in sandbody geometry and connectivity within successive cycles.

A playa deposit of pre-Yellow Sands age (upper Rotliegend/Weissliegend) in the Permian of northeast England

A complexly interbedded thin succession of sandstone, siltstone and mudstone beneath the Permian Yellow Sands Formation (upper Rotliegend/Weissliegend) was cored in a borehole 11 km off the coast of northeast England. The cored strata rest unconformably on fissured Westphalian B mudstone and mainly comprise low-dipping fine- to coarse-grained grey sandstone with unevenly spaced thinner dark-red to grey mudstone beds and laminae. Most of the sandstone beds are moderately to poorly sorted, fine- to very fine-grained subarkoses, with some sublitharenites, and are weakly to strongly cemented by dolomite (predominant), kaolinite and illite; sedimentary structures include subhorizontal plane to wavy lamination, ripple lamination, salt ridges and soft-sediment deformation. The siltstones and mudstones are more uniform in lithology and primary thickness; they contain sandstone-filled desiccation cracks, mudstone saucers and narrow sandstone dykes. Traces of possible former evaporite minerals are concentrated in the sandstones. By comparison with modem desert depositional environments, these strata are interpreted as the deposits of a playa occupying a depression on a stony, deflationary desert surface on a rock pediment or peneplain flanking the ancestral Pennines. The floor of the depression, which probably lay in the capillary fringe just above the contemporary water table, was periodically flooded so as to form a shallow playa lake. Comparable successions beneath the Yellow Sands have not been recorded from surface exposures in northeast England but mudstone beds at this stratigraphical level have been recorded in four other cored offshore boreholes and are presumed to be of similar origin to those described in this study. The stratigraphical position of these deposits suggests that they may be of pre-Yellow Sands age and coeval with or younger than the Basal Permian Breccia. Palynological analysis of mudstone samples from the core reveal the presence of plant material and indeterminate, pyrite-invested, possibly indigenous bisaccate pollen indicative of a vegetated hinterland with deposition occurring within a reducing, possibly sulphide-rich environment.

An Early Cretaceous forearc basin in the Golfo de Penas region, southern Chile

A 4000 metre thick fining-up succession of conglomerates, sandstones and mudstones on the northwestern edge of the Golfo de Penas, southern Chile, is defined as the Puerto Barroso Formation. It probably formed in a forearc basin where a fan delta built across a narrow coastal plain into a shallow marine environment during the Barremian to Aptian. The basin is situated to the west of contemporaneous arc, back are and epicratonic basins and is the only documented forearc basin of this age in southern South America.

Sequence stratigraphic analysis of early and middle Triassic alluvial and estuarine fades in the Sydney Basin, Australia

Early to Middle Triassic sediments of the Sydney Basin (up to 1350 m thick) have hitherto been regarded as an alluvial and lacustrine succession unaffected by sea‐level induced base‐level changes. Here it is proposed that the dominantly alluvial sediments were deposited on an alluvial plain which graded seawards to an estuarine coastal plain. Hence, the succession had hydrological connection with the sea and, therefore, deposition may have been controlled by changes in relative sea‐level. A sequence stratigraphic framework, established in the downstream alluvial plain/coastal plain transition, can be carried landward to varying degrees, even to the basin margins. The intercalation of lowstand, coarse‐grained alluvial‐plain sediments with transgressive and highstand fine‐grained, coastal‐plain sediments was influenced by high‐frequency, 4th‐order changes in relative sea‐level which were responsible for the overall depositional character of the succession. These cycles constitute seven 3rd‐order sequences. During a low‐frequency, 2nd‐order highstand in the Early Triassic, dominantly fine‐grained coastal plain sediments were deposited, whereas during a lowstand in the Middle Triassic, dominantly coarse‐grained alluvial‐plain sediments were deposited. Interbedded alluvial and estuarine sediments were deposited in the more rapidly subsiding northeastern and central parts of the basin. Landwards, in the direction of slower subsidence towards the western basin margin, shaly coastal plain intervals thinned to zero and coarse‐grained alluvial‐plain sediments amalgamated vertically to form thick sandstone successions. Alluvial‐plain sediments are characterised by upward‐fining fluvial sandstones which intercalate seawards with grey, green, and red floodplain/estuarine shales. Further seawards these sediments grade to finer grained sandstones and multicoloured shales in upward‐coarsening estuarine bayhead deltas and central bay facies. Extensive redbed deposition, considered to indicate oligotrophy, commenced during the second 3rd‐order sequence above the Permian/Triassic boundary, continued during the deposition of three sequences, and finished at the end of the Early Triassic.

Reconnaissance geology in Upper Chitral, Baroghil and Karambar districts (northern Karakorum, Pakistan)

During the summer of 1992 a geological expedition crossed the northern Karakorum range in northern Pakistan, from the Chitral to Karambar valleys, from the villages of Mastuj to Imit. Some of the areas visited were geologically unknown. A number of structural units were crossed, belonging to the Karakorum block or to other crustal blocks north of it. They are: (a) the axial batholith, in which three plutonic bodies have been identified, and (b) the northern sedimentary belt (NSB), in which three major tectonostratigraphic units form thrust stacks dipping to the north. Their internal stratigraphy and structural style are partly different. The most complete contains a crystalline basement, transgressed by a marine succession during the Early Ordovician. The youngest strata are represented by the Reshun conglomerate, of inferred Cretaceous age. The northernmost unit of the NSB is tightly folded, whereas the central one forms a monocline. Vertical faults, mainly strike-slip, dissect the thrusted slabs. Metamorphic deformation is absent or reaches only the anchizone in the studied sector of the Karakorum NSB. To the north of the Karakorum proper there are several other tectonic units, separated by vertical faults. They are, from south to north: (a) the Tas Kupruk zone, with metavolcanics of basaltic to latibasaltic composition; (b) the Atark unit, mostly consisting of massive carbonate rocks of Mesozoic age; and (c) the Wakhan slates which consist of a thick widespread succession of dark slates, metasiltites and sandstones. The fine-grained elastic rocks are supposed to be Palaeozoic to Early Triassic in age. The Wakhan slates are intruded by plutons belonging to the East Hindu Kush batholith, from which a single K/Ar age on muscovite gave a Jurassic age.