Calcareous Siltstone Research Articles

99/02355 State-of-the-art and potential for development of raw mineral resources in northern Kazakhstan

The Xinji and Zhushadong formations are the oldest record of Cambrian deposits in the southwestern margin of the North China Platform. The two lithostratigraphic units are carbonate-dominated in lithology and are Cambrian Series 2 in age. The basal unit, the Xinji Formation yields diverse skeletal fossils, including sponge spicules, chancelloriid sclerites, hyoliths, micromolluscs, trilobites, echinoderm ossicles, cambroclaves, tianzhushanellids, protoconodonts and other small shelly fossils. The fossil assemblage, as the earliest fauna of the Cambrian in North China, provides an important window to investigate the Cambrian explosion in the region and contains fossil elements with biostratigraphical significance. Seven microfacies types of the Xinji and Zhushadong formations are recognized: (1) phosphorized and selectively dolomitized bioclastic grainstone, (2) dolomitized ooid grainstone, (3) dolomitized peloid wackstone, (4) lime mudstone preserved as dolomitic microsparite, (5) siliciclastic wackstone, (6) calcareous siltstone, and (7) breccia. The different microfacies are traced to three successive depositional environments, beginning with a bioclastic shoal, then a mixed-depositional back-shoal and finally an oolitic shoal. The system of the shoal and back-shoal is interpreted as representing the mixed siliciclastic-carbonate deposition during the initial transgression of the North China Platform as well as the early development of an epeiric environment.

Evidence for relative sea-level falls during deposition of the Upper Carboniferous Millstone Grit, South Wales

Detailed sedimentological facies analysis of the fluvio-deltaic Millstone Grit succession (Upper Carboniferous) of South Wales reveals that a number of cyclothems bounded by marine flooding surfaces (marine bands) in these strata exhibit facies architectures that represent erosion, non-deposition and/or deposition during periods of falling relative sea-level. A major fluvial complex below the Subcrenatum Marine Band, the Farewell Rock, lies within an incised valley, with a regional unconformity (sequence boundary) at its base. This unconformity is marked by deep erosional relief, an identifiable time gap and an angular discordance in bedding. The Cumbriense Quartzite, a correlative unit containing several mature palaeosols, records a depositional hiatus on a terrace-like interfluve that lay beyond the margins of the coeval Farewell Rock valley. Cyclothems in the underlying Middle Shales contain additional surfaces and units of subtler character. Beneath the Cancellatum Marine Band, a thin (15 cm), calcareous siltstone bed (the ‘Amroth Granule Bed’) that directly overlies prodelta shales contains reworked bioclasts, bored phosphorite clasts and quartz granules. Quartz granules in this bed are interpreted to represent relict lowstand, fluviatile? deposits, which were reworked during later transgression. Three further cyclothems contain sharp-based, storm-reworked mouth bars that record an abrupt lowering of wave base, most probably during periods of falling relative sea-level. One of these cyclothems also contains a distributary channel complex, which records an abrupt influx of coarse-grained sediment of ambiguous origin. The significance of these subtle surfaces and units for intracyclothem stratigraphy has rarely been considered; their prevalence in the Middle Shales provides evidence for numerous, high-frequency relative sea-level falls, which were previously unrecognized. These relative sea-level falls appear to alternate coherently with the widespread sea-level rises recorded by the marine bands, suggesting that glacio-eustasy is their most likely driving mechanism. The notion of glacio-eustatic sea-level falls is supported by the correlation of the basal Farewell Rock sequence boundary with sequence boundaries documented in adjacent basins. The angular unconformity and a change in sediment provenance at the base of the Farewell Rock, however, suggest an additional tectonic control on stratigraphic architecture here, namely a short-lived phase of rifting or inversion prior to widespread fluvial incision. Copyright © 1998 John Wiley & Sons, Ltd.

Neogene Sedimentation and Tectonics in the Cibao Basin and Northern Hispaniola: An Example of Basin Evolution Near A Strike‐Slip‐Dominated Plate Boundary

Sedimentologic analyses, subsidence history interpretation, and a new gravity survey are synthesized to interpret basin evolution in a region of large strike‐slip offsets and oblique crustal convergence within the northern Caribbean plate boundary zone. We investigate the mid‐Miocene to mid‐Pliocene Yaque Group, which accumulated in the western Cibao basin of northern Hispaniola. Asymmetric subsidence of the Cibao basin was accompanied by southward transgression and deposition of an upward‐fining succession from shallow‐marine or brackish water conglomerate through sub‐wavebase siltstone to deep‐water calcareous siltstone and uncommon interbedded sandstone; bioclastic limestone is locally present and in most cases accumulated in a forereef location. In early Pliocene time (ca. 4 Ma), fine‐grained sediment deposition was briefly interrupted by widespread conglomerate accumulation; however, basin subsidence was uninterrupted, and water depths continued to increase. At this time, basinal subsidence accelerated as the Cordillera Septentrional was uplifted along the Septentrional fault zone at the basin's northern flank. The across‐strike gravity profiles require a S‐vergent thrust component along most of the Septentrional fault zone. Gravity variations indicate that the Septentrional fault zone dips northward (∼50°) in the central part of the basin, adjacent to the Septentrional fault zone, where the basin is deepest, and steepens eastward and westward. Contrasts in sedimentation patterns between the Cibao basin and elsewhere in northern Hispaniola reveal a complex history of dominantly strike‐slip faulting modified by varying amounts of convergence.

The Queenston Formation: shale-dominated, mixed terrigenous-carbonate deposits of Upper Ordovician, semiarid, muddy shores in Ontario, Canada

The Upper Ordovician Queenston Formation constitutes the westernmost part of the Taconic clastic wedge of eastern North America. In Ontario, the Queenston Formation consists of a mixed terrigenous-carbonate succession composed primarily of red silty shales, which are locally gypsiferous, alternating, in the middle and lower parts of the formation, with thin grey shales, calcareous siltstone and sandstone, and bioclastic limestone. Brachiopods, some ostracodes, and, in the lower beds, bryozoans constitute the body fossil assemblage. Diplocraterion, Skolithos, Chondrites, and occasional large vertical burrows of unknown origin form the ichnofauna. Plane beds, ripple marks, and occasional swaley and hummocky cross-stratifications are present in the calcareous units. Mudcracks, gypsum nodules, occasional halite hopper casts, and gilgai-like structures typically occur in horizons where shales alternate with siltstone and limestone. These features are indicative of prograding muddy and storm- and tide-affected shores. In southernmost Ontario, the upper part of the formation is mostly composed of a red shale, with local recognizable shale-filled channels. This shale is of paralic origin, and it is truncated by the unconformity separating the Ordovician from the Silurian units in this region. This last event may, in part, be related to a eustatic sea-level drop due to the contemporaneous Saharan glaciation, if not exclusively due to tectonic movements.

Hydrothermal manganese crusts from Enarete and Palinuro seamounts in the Tyrrhenian Sea

Manganese crusts were recovered from Enarete and Palinuro Seamounts in the Tyrrhenian Sea. The crusts consist of porous, black, layered Mn oxides up to 45 mm thick. In some cases, the surface has a black metallic sheen similar to that observed in crusts from the Tonga‐Kermadec Arc. The crusts overlie substrates such as calcareous sediment, siltstone, and oyster shells. They consist dominantly of 10‐Å manganate and 7‐Å manganate, with minor quartz, calcite, aragonite, illite, mont‐morillonite, plagioclase, and goethite. The sample having the highest Mn content contained 52.8% Mn, 0.15% Fe, 46 ppm Ni, 84 ppm Cu, 26 ppm Zn, 6 ppm Pb, and 2,130 ppm Ba, with a Mn/Fe ratio of 347. It also had a low rare‐earth element (REE) abundance (La 2.7 ppm) and a negative Ce anomaly (Ce/La ratio 0.5). A TV profile on Palinuro showed the occurrence of yellow (Fe oxyhydroxide or nontronite), black (Mn oxide), and white (sulfate) halos in the sediment and opaqueness in the overlying water. Nontronite and massive sulfides were also recovered at some locations on Palinuro. The morphology, mineralogy, and composition of these crusts as well as ancillary geological evidence confirm that the crusts are of hydrothermal origin. Similar crusts occur in a number of island‐arc environments, particularly in the Pacific, and are known to be formed by low‐temperature hydrothermal venting. The origin of the Tyrrhenian Sea is complex but it is believed to be a back‐arc basin formed above a NW‐dipping subduction zone. The Aeolian islands and seamounts consist of 16 volcanic edifices which form an anticlockwise arc around the Marsili abyssal plain. Palinuro is the youngest of these. Our data support the idea that the Mn deposits formed as a result of submarine hydrothermal activity associated with subduction‐related processes.

Sedimentology and petrography of mass‐emplaced limestone (Orahiri Limestone) on a late Oligocene shelf, western North Island, and tectonic implications for eastern margin development of Taranaki Basin

Abstract The Te Kuiti Group in North Wanganui Basin, North Island, New Zealand, of Oligocene ‐ earliest Miocene (Whaingaroan‐Waitakian) age, is dominated by calcareous siltstone, calcareous sandstone, and skeletal limestone. Exposures in the southwestern corner of the basin at Awakino Tunnel are distinctive because, compared with elsewhere, the group is generally thicker (>300 m), has strong dips (25–45°E), exhibits an up‐section decrease in the amount of dip, and the capping Orahiri Limestone includes several thick (up to 3 m) mass‐emplaced units containing a variety of 1–10 cm sized calcareous lithoclasts of older Te Kuiti Group rocks. Petrographic and δI8O and δ13C data suggest that the source deposits of these lithoclasts were cemented at relatively shallow subsurface burial depths (100–500 m) before their uplift and erosion. The lithoclasts so produced were rounded by abrasion in shoal water, often bored profusely by pholad bivalves, and sometimes encrusted by coralline algae, before being periodical...

A study of some Nigerian carbonate rocks for the building construction industry

A number of geotechnical analyses were carried out on selected carbonate rock samples from ten locations in Nigeria. This was to assess the suitability of these rocks as building construction aggregates. The analyses included uniaxial compressive strength, tensile strength porosity, water absorption and dynamic fragmentation. Evaluation of these carbonate rocks on the basis of a quality index ( QI) scheme show that some lithofacies (i.e., calcareous siltstone, pisolitic limestone, marly limestone, massive limestone, sandy limestone, oncolitic limestone and fossiliferous limestone) which were obtained from Abini, Ashaka, Calabar Ikom road, Ikot Okpora, Mfamosing and Nkalagu are the best for construction purposes. However, it is recommended that the susceptibility of each deposit to solution should be investigated.

Geology and lithogeochemistry of the Ren gold prospect, Elko County, Nevada — the role of rock sampling in exploration for deep Carlin-type deposits

The Ren gold prospect, Elko County, Nevada, is in the northern part of the Carlin trend, two kilometers northwest of the recently-discovered, high-grade Purple Vein deposit. The Ren area is underlain mainly by Paleozoic sedimentary rocks, consisting of limestone, calcareous siltstone, and mudstone of the eastern (carbonate) assemblage, overlain in thrust contact by chert, quartzite, and mudstone of the western (siliceous) assemblage. Cretaceous(?) granodiorite porphyry and hornblende porphyry dikes have intruded the sedimentary rocks along north-striking faults. Three stages of mineralization include a pre- or syntectonic base metal-barite assemblage, a middle stage of Ag- and Sb-rich jasperoid, and a late Au-rich stage responsible for the potentially economic mineralization at the prospect. The latter two stages of alteration and mineralization were focused along steep east-dipping faults and dikes, and the nearly flat-lying contact between lower massive limestone and laminated calcareous siltstone. Mineralization is present between 380 and 500 m below the surface. Alteration includes decalcification and weak silicification in siltstone, and formation of massive jasperoid in the upper part of the limestone unit. Alteration of dikes is mainly sericite-quartz-pyrite, with late pyrite-quartz-kaolinite. The element suite characteristic of Au-stage mineralization includes Au, As, and Hg with minor Ag and Hg; Ag and Sb are most enriched in the earlier jasperoid event. Haloes of As and Hg extend at least 80 m above the Au mineralization, but no anomalies are present at the surface. Gold anomalies are more widespread, and extend to shallower depths, but are less coherent.

TRIASSIC STRATIGRAPHY AND SEDIMENTARY EVOLUTION OF THE ANNAPURNA TETHYS HIMALAYA (MANANG AREA, CENTRAL NEPAL)

After initial opening of Neotethys in the Permian, thermal subsidence and deepening continued in the Triassic. In the Scythian, three pelagic nodular carbonate intervals of Early Griesbachian to Early Dienerian, Early to mid-Smithian and latest Smithian to earliest Aegean age are interbedded with shelfal shales (Tamba Kurkur Formation). Ammonoid-and conodont-rich condensed carbonates were deposited on the outer shelf, with maximum depths around 150+200 m reached during transgressive stages. The Anisian to lowermost Norian shelfal succession mostly consists of marly limestones and marls (Mukut Formation). Thick calcareous siltstones rapidly accumulated in the Carnian, testifying to a new stage of tectonic extension affecting the Tethys Himalayan passive margin. The thick Tarap Formation of largely mid-Norian age points to strong continuing subsidence. Interbedded siltstones and subarkosic sandstones (lower member) are locally overlain by coral-bearing patch reefs (middle member). Chamosite-bearing hybrid arenites deposited at transgressive stages characterize the upper member. The Triassic succession is capped by subarkoses and quartzarenites, interbedded with dolomitic to bioclastic hybrid sandstones and silty limestones (Quartzite Series), in turn overlain by the Kioto Limestone.

Geology and geochemistry of wall-rock alteration at the Carlin gold deposit, Nevada

The Carlin disseminated gold deposit occurs in an autochthonous sequence of Paleozoic sedimentary rocks exposed in a structural window in the Roberts Mountains thrust in north-central Nevada. The upper 175 m of the Silurian Roberts Mountains Formation hosts the majority of ore at Carlin and is characterized by laminated, fine-grained, calcareous and/or dolomitic argillaceous siltstone with local coarser grained siltstones and 50-cm-thick lenticular interbeds of sand- and granule-sized calcareous bioclastic debris or fossil hash. Detailed studies of drill core and exposures in the East pit of the Carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds, indicating that these two features were major inflow zones for hydrothermal fluid.In unoxidized rocks, unaltered calcareous siltstone (1) containing quartz, dolomite, calcite, illite, K feldspar, and pyrite is progressively converted to assemblages of (2) quartz + dolomite + calcite + illite + pyrite, (3) quartz + dolomite + illite-K mica + pyrite, (4) quartz + illite-K mica + pyrite, and (5) quartz + kaolinite-dickite + pyrite adjacent to inflow zones where jasperoids are developed. Gold most consistently enriches the zone of calcite and dolomite removal (3 and 4 above), though it occurs in all zones, locally in high concentrations. This zoned alteration was accomplished by a CO 2 -rich acidic fluid. This acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability.Oxidation is wholly a supergene effect related to deep weathering, because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry; it has produced low-temperature goethitic Fe oxides rather than higher temperature hematite and is not spatially related to Au distribution at the mine or on a district scale.Because of extensive carbonate removal leading to local volume reduction through collapse and/or compaction, geochemical effects are examined using ratios to relatively immobile elements such as Al and Ti. Extensive depletion of Ca, Mg, aud CO 2 and introduction of Si, Au, and S have occurred. Potassium is depleted in the conversion of illite to dickite-kaolinite in proximal silicified inflow zones, and Fe enriches some pyritized rock. Carbonate removal and silicification are two separate processes, both of which are spatially associated with mineralization. Mineralized decarbonated rocks and barren footwall rocks commonly are not silicified, and intensely silicified proximal alteration zones are generally low grade.

The Saint‐Daniel Melange: Evolution of an accretionary complex in the Dunnage Terrane of the Quebec Appalachians

The Saint‐Daniel Mélange is part of a series of mélanges located along the Baie Verte‐Brompton line in the Northern Appalachians. This line marks the suture between rocks of oceanic affinities and those of the ancient passive margin of North America with which they collided during the Taconian (Middle to Late Ordovician) orogeny. The Saint‐Daniel Mélange contains a wide variety of lithologies including well‐bedded to dismembered sedimentary sequences, pebbly mudstone, olistostromes, and slivers of igneous and metamorphic rocks. Black shales with interbeds of green shale, calcareous siltstone, or sandstone are the dominant units. They exhibit various stages of mélange formation such as those present in shallow parts of an accretionary complex. Units of oceanic origin include sediments derived from the forearc basin and slivers of an ophiolite and of a magmatic arc. Units derived from sediments of the passive margin of North America are also present. The ratio between these various lithologies changes greatly within the mélange on a kilometric scale along strike. The Saint‐Daniel Mélange is a structural complex in which the various units were assigned a sequential order mimicking a stratigraphic order. The Saint‐Daniel Mélange is interpreted as the relict of an accretionary complex because of its actual structural position within the Northern Appalachians and because all its lithologies and their structural fabric can be found in modern accretionary complexes.

Frictional melting of sedimentary rock during high-speed diamond drilling: an analytical SEM and TEM investigation

High-speed drilling at 900 rpm with a downhole motor-driven natural diamond drag bit has resulted in the extreme alteration of sedimentary rock in an offshore Canadian wildcat well. These effects, as revealed in the rock cuttings retrieved from the drilling fluid at surface, include discolouration (darkening), cataclasis, slickensiding and partial fusion. Calcareous siltstones are the most susceptible to alteration, especially melting, whereas sandstone and limestone cuttings mainly exhibit fracturing and surface glazing. Analytical scanning electron microscopy reveals that the calcareous siltstones have undergone decalcification via decarbonation and the loss of CaO to the drilling fluid. Decarbonation, combined with dehydration, resulted in the vesiculation and explosive fragmentation of certain cuttings. Analytical transmission electron microscopy shows that glass is present in the calcareous siltstone cuttings. Most of the modifications exhibited by the sedimentary rocks can be attributed to the effects of high strain-rate deformation and frictional heating at the bit-rock interface. That melting took place is supported by thermal calculations which indicate that, for a coefficient of friction of 0.5, temperatures of approximately 1200°C were realized at diamond-rock contacts. Such high temperatures can accelerate the wear of diamond and its substrate and hence shorten bit life, as well as obliterate potentially important geological information in the rock cuttings.

Paleozoic to early Mesozoic stratigraphy and sedimentary evolution of central Dolpo (Nepal Himalaya)

The stratigraphic succession of central Dolpo (Tarap-Atali area) begins with thick calcsilicate marbles of inferred Cambrol-Ordovician age, injected by pegmatitic dykes. The strongly deformed mid-Paleozoic sequence comprises fossiliferous marls, dolomitic quartzarenites and dolomites. The latter are unconformably followed by earliest Late Devonian biocalcarenites, capped by a maior ironstone layer. The overlying offshore black pelites contain quanzose sandstones and bioclastic limestones at several intervals, indicating deposition in shelfal environments at latest Devonian times. The Carboniferous is poorly represented in the surveyed area. A major disconformity, cutting deep into Early? Carboniferous fenestellid-rich biocalcarenites at Tarap, is overlain by white quartzarenites interbedded with dark pelites, sharply followed by a fossiliferous marker horizon yielding a large brachiopod fauna of mid-Permian age ( Costiferina arenites); the overlying fossiliferous shelfal pelites are unconformably followed by coarsening-upward quartzarenite sequences deposited in estuarine environments (Thini Chu Formation). In the Late Permian, another disconformity is overlain by conglomeratic to glauconitic arenites and shelfal pelites (Kuling Formation). The Triassic Tamba Kurkur Formation, with dolomitic subarkoses ar the base, consists of two condensed pelagic carbonate horizons of Dienerian and Smithian age, separated by dark pelites. Marly limestones and marls were deposited from the Spathian through the Carnian (Mukut Formation), and are overlain by the thick Tarap Shale. A thin basal condensed bed is followed by dark pelites yielding Early Norian ammonoids, by thick siltstones with phosphatic nodules and next by dark shales and calcareous siltstones with large Zoophycos -type burrows. The upper part of the unit contains up to fine-grained quartzo-feldspathic sandstones, nodular marly limestones and ironstone horizons (upper assemblage). The Late Triassic shallowing- upward succession is capped by dolomitic and oolitic quartzarenites with spectacular herringbone structures (Quartzite Series). The largely Lower Jurassic shallow-water Kioto Limestone, still containing metric intervals of up to medium-grained hybrid quartzarenites in the lower part, is followed by Middle Jurassic lumachelles (Laptal Formation). Younger terms of the succession are not exposed in Dolpo.

Hydrogeological and environmental impact of coal mining, Jharia coalfield, India

The Jharia coalfield is the most important and active minig region; it experiences groundwater inflow and affects groundwater levels in overlying aquifers, and it provides the basis for a conceptual model of the hydrogeological impacts of coal mining. The several sandstone aquifers of the overburden are separated by aquitards that limit vertical hydraulic connection, but the inflow responds to seasonal events and seems to be linked to shallow groundwater behavior. The mine drainage behavior suggests a hydraulic connection between the mine and the shallower groundwater system. The greatest declines are directly above the panels, with an immediate response to coal mining. The inflow is localized by natural and induced fracture zones and is mostly into recent workings. The groundwater behavior is controlled by hydraulic property changes caused by mine-induced fracturing. The hydrological and chemical qualities of the shallow groundwater regime in 13 mining collieries in Mukunda Block have been investigated. Water samples collected from 30 shallow monitoring dug wells were chosen for the study. Rainfall, runoff, and infiltration rates have been calculated in the area. The water-quality plottings were used to interpret the distribution of individual chemical parameters and in predicting the water quality. The underground mine water has been classified as: (1) unconfined groundwater in the calcareous siltstone and sandstone—its composition is Na, Ca, SO4 and Na-MgHCO3 with moderate total dissolved solids (TDS) 200–1480 ppm; (2) the deep groundwater originating from the coal seams and associated sediments in the near-surface environments—this is a Na-HCO3 water with higher TDS; and (3) spoil dump waters are essentially Na-HCO3 with high TDS. This article presents some hydrologic results and conclusions relating to the hydrogeological and environmental impacts of the coal mining in the Jharia coalfield.

New early Triassic Bryozoa (Trepostomata) from Spitsbergen, with some remarks on the stratigraphy of the investigated horizons

AbstractNew trepostomatous bryozoan species, Paralioclema winsnesi sp. nov., Paralioclema mariaholmensis sp. nov. and Paralioclema sp. cf. mariaholmensis are described from the Lower Triassic succession of Spitsbergen. The investigated bryozoan faunas are associated with coarse sandstones, calcareous siltstones and shelly limestones. The bryozoan colonies are mostly well preserved in situ, and show limited evidence of transportation. Some zoarial fragments show bioerosion. The conodont species Neogondolella elongata, Ellisonia triassica, Neospathodus dieneri and Neospathodus svalbardensis extracted from the investigated horizons are Dienerian–Smithian in age.

3T sericite (dioctahedral mica) from the Porgera mine, Papua New Guinea.

3T sericite has been found in altered wall rocks of the Porgera gold deposit in Papua New Guinea. It occurs as fine-grained, clayey material forming aggregates of minute flakes replacing both argillaceous matter in calcareous siltstone and igneous components in breccia. It is closely associated with quartz, dolomite and/or calcite, subordinate pyrite and fluorite. Chemically the material is K-dominant dioctahedral mica with appreciable octahedral Fe and Mg (exactly intermediate between ideal muscovite and phengite). Although intermixed with 2M1 sericite in small quantities, the X-ray powder diffraction pattern is satisfactorily indexed with a hexagonal cell: refined cell parameters by least squares calculation are a=5.203(1)A, c=29.945(7)A and V=702.04(26)A3, which are in good agreement with the phengitic composition. The chemical composition is correlated with infrared and thermal properties.Crystallinity, chemical composition (phengitic substitution) and modes of occurrence are variable among common 1M and 2M1 and rarer 3T sericite minerals in this deposit, reflecting diversity of genesis and origin. Local activity of high temperature hydrothermal to pneumatolytic fluid may be responsible for the formation of the 3T sericite, which is extremely well crystallized and closely associated with fluorite.

Depositional Facies and Hydrocarbon Reservoir Compartmentalization of the Mississippian Aux Vases Formation at King Field, Jefferson County, Illinois

King field has produced over 4 million bbl of oil from the Mississippian Aux Vases Formation along a north-south-trending anticlinal structure 3 mi long and 1.5 mi wide. The porous and permeable quartz sandstone, which comprises the principal hydrocarbon reservoir, can grade laterally into nonporous calcareous sandstone, siltstone, shale, or limestone within one well location (660 ft). The Aux Vases Formation at King field was deposited in an intertidal to subtidal, mixed siliciclastic-carbonate environment, and the resulting mosaic of depositional facies significantly affects reservoirs continuity. Impermeable to only slightly permeable siltstones, shales, and limestones of the various offshore facies, and shales and siltstones of the tidal-flat facies separate the producing sandstones of a tidal channel-offshore bar facies into distinct reservoir compartments. This reservoir heterogeneity results in an oil-water contact at various depths across the field. Lateral compartmentalization of the reservoir has allowed large sections of the Aux Vases reservoir to remain unswept by waterflooding. Volumetric analyses suggest that untapped reservoir compartments may contain an additional 1-2 million bbl of oil recoverable by primary and waterflood methods.

Enigmatic Uppermost Permian-Lowermost Triassic Stratigraphic Relations in the Northern Bighorn Basin of Wyoming and Montana

Abstract Nineteen measured sections in the northern Bighorn Basin of Wyoming and Montana provide the basis for an analysis of Permian-Triassic stratigraphic relations. The Permian-Triassic unconformity is well-defined in north-central Wyoming where gray, yellowish-weathering, calcareous siltstones of the Lower Triassic Dinwoody Formation disconformably overlie carbonates of the Upper Permian Ervay Member of the Park City Formation with little physical evidence of a significant hiatus. The Dinwoody is gradationally overlain by red beds of the Lower Triassic Red Peak Formation of the Chugwater Group. The Dinwoody depositionally thins to zero near the Wyoming-Montana state line. Northward, the erathem boundary is enigmatic because fossils are absent, Permian rocks change facies to resemble the Dinwoody, and there is no record of a Permian-Triassic unconformity. Discontinuous exposures and the restrictive access policies of the Crow Indian Reservation also contribute to the problem. Some previous workers assigned these rocks to the Permian, while others considered them Triassic in age. Up to 12 m (37 feet) of Dinwoody-like Permian rocks unconformably overlie the Pennsylvanian Tensleep Sandstone and grade into the Lower Triassic Red Peak Formation in the westernmost surface exposures along the eastern flank of the Bighorn Basin in Montana. South and east of this area, but north of the Dinwoody termination, Ervay carbonate is overlain by up to 24 m (74 feet) of red siltstone capped by.25 (0.8 feet) to 0.5 m (1.6 feet) of laminated dolomitic limestone. This interval is assigned to the basal Red Peak. Thin Lower Triassic dolomitic mudstones within the Dinwoody and basal Red Peak are commonly laminated, with associated peloidal micrite and coarse quartz silt. These carbonates are also poorly fenestral, and may contain small quantities (1% or less) of small peloidal rip-ups and very fine-grained quartz sand. Carbonates within the Dinwoody-like Permian are similar, but contain variable quantities of sand-size quartz and little to no coarse quartz silt.

Short Paper: Hummocky cross-stratification in the Downton of the Welsh borderland

Hummocky cross-stratification is recorded from the classic Upper Silurian Whitcliffe Group (Ludlow Series)/Downton Group (Pridoli Series) boundary section at Ludlow, Shropshire. This necessitates a revision of previous interpretations of palaeoenvironments which infer either a tidal mudflat depositional setting. Evidence for tidal influence is lacking. Instead the importance of storm activity, probably on a shoreface, is emphasized. The absence of articulate brachiopods and other open marine faunal elements in the Downton strata is attributed to changes in salinity in the restricted Downton sea shortly before its elimination by Acadian uplift. The boundary between the Upper Whitcliffe Formation (Ludlow) and the overlying Downton Castle Sandstone Formation (Pridoli), formerly proposed as the stratotype section for the base of the fourth Series of the Silurian System, is designated at Ludford Corner, Ludlow, Shropshire [SO 51237413] (Fig. 1, Bassett et al. 1982 and references therein). It has received considerable attention, with particular emphasis on faunal changes through the section. In addition to its stratigraphic significance, the section occurs at a crucial position in the transition from marine to non-marine environments associated with the Acadian (late Caledonian) inversion of the Welsh Basin ( Woodcock et al. 1988 ). The Downton Castle Sandstone Formation in the Ludlow area overlies shelly calcareous siltstones of the top Ludlow, Upper Whitcliffe Formation and comprises a coarsening and cleaning-upward sequence, about 15 to 20 m thick ( Bassett et al. 1982 ). Coastal mudflat deposits of the Temeside Shale Formation lie directly above but are not seen at Ludford corner. The basal part of the

Early Paleozoic terranes of New Zealand

Abstract Lower Paleozoic rocks of New Zealand comprise two major assemblages each with their own distinct sedimentary tectonic, metamorphic and igneous history; they thus represent two distinct tectono-stratigraphic terranes. In Nelson-Westland, the western, or Buller, terrane consists of the Western Sedimentary Belt, together with Ordovician paragneiss at Charleston and in Victoria Range. The sedimentary sequence, ranging in age from basal to Upper Ordovician, comprises continentderived quartz-rich turbidites with black shales inferred to have been deposited in submarine fans and slope basins. The eastern, Takaka terrane (Central and Eastern Sedimentary Belts) is much more varied in lithofacies, composition and age (Cambrian to Silurian) and itself comprises several tectonic, probably thrust, slices. Volcanics, volcaniclastics, siliceous and calcareous siltstone, conglomerate and turbidites dominate the Cambrian part of the sequence and indicate proximity to a Cambrian island arc. The oldest sediments ar...