Siliciclastic Sandstones Research Articles

Upper Cretaceous Bioturbated Fine-to-Medium Sands: a Problem for Deep-Water Exploration in Campos Basin, Offshore Brazil: ABSTRACT

The presence of bioturbation in oil-prone turbidite deposits has detained a better exploration of upper Cretaceous siliciclastic reservoirs in Campos Basin. Bioturbated sandstones have degraded their permo-porosity with impact in oil production. They occur associated to unbioturbated sandstones with similar seismic amplitude, becoming difficult to separate them in seismic mapping. A comprehensive study in order to reduce the exploratory risk must consider firstly the recognition of genetic facies association through cores, and secondly the calibration of sonic well-logs and seismic velocity sections with rock data. This study deals with sedimentary facies association. The range of main facies in such reservoirs includes: (1) medium-to-coarse siliciclastic sandstone with cross stratification; (2) fine-to-medium massive sandstone with thin traction carpets, bioturbated by opportunistic ichnofabrics and, (3) bioturbated, muddy, fine-to-medium, quartz-feldspatic and glauconitic sandstone over 60 in thick. The genetic facies associations suggest that the sandwich reservoirs were formed by high density turbidite currents deposited en masse or by thin traction carpets. The bioturbated sandstone was originated by reworking of bottom currents rich in nutrients and oxygen. Detritic and biogenic glauconite covered and/or filled bioturbations indicate a basinward movement of the bottom currents. The successive alternation of high-energy with low-energy ichnofabrics reflects cyclic variations inmore » current velocities.« less

Pliocene‐Pleistocene marine cyclothems, Wanganui Basin, New Zealand: A lithostratigraphic framework

Abstract The Rangitikei River valley between Mangaweka and Vinegar Hill and the surrounding Ohingaiti region in eastern Wanganui Basin contains a late Pliocene to early Pleistocene (c. 2.6–1.7 Ma), c. 1100 m thick, southward‐dipping (4–9°), marine cyclothemic succession. Twenty sedimentary cycles occur within the succession, each of which contains coarse‐grained (siliciclastic sandstone and coquina) and fine‐grained (siliciclastic siltstone) units. Nineteen of the cycles are assigned to the Rangitikei Group (new). Six new formations are defined within the Rangitikei Group, and their distribution in the Ohingaiti region is represented in a new geologic map. The new formations are named: Mangarere, Tikapu, Makohine, Orangipongo, Mangaonoho, and Vinegar Hill. Each formation comprises one or more cyclothems and includes a previously described and named d stinctive basal horizon. Discrete sandstones, siltstones, and coquinas within formations are assigned member status and correspond to systems tracts in sequence stratigraphic nomenclature. The members provide the link between the new formational lithostratigraphy and the sequence stratigraphy of the Rangitikei Group. Base of cycle coquina members accumulated during episodes of sediment starvation associated with stratigraphic condensation on an open marine shelf during sea‐level transgressions. Siltstone members accumulated in mid‐shelf environments (50–100 m water depth) during sea‐level highstands, whereas the overlying sandstone members are ascribed to inner shelf and shoreface environments (0–50 m water depth) and accumulated during falling eustatic sea‐level conditions. Repetitive changes in water depth of 50–100 m magnitude are consistent with a giacio‐eustatic origin for the cyclothems, which correspond to an interval of Earth history when successive glaciations in the Northern Hemisphere are known to have occurred. Moreover, the chronology of the Rangitikei River section indicates that Rangitikei Group cyclothems accumulated during short duration, 41 ka cycles in continental ice volume attributed to the dominance of the Milankovitch obliquity orbital parameter. The Ohingaiti region has simple postdepositional structure. The late Pliocene formations dip generally to the SSW between 4° and 9°. Discernible discordances of c. 1° between successively younger formations are attributed to synsedimentary tilting of the shelf concomitant with migration of the tectonic hingeline southward into the basin. The outcrop distribution of the Rangitikei Group is strongly influenced by this regional tilt and also by three major northeast‐southwest oriented, high‐angle reverse faults (Rauoterangi, Pakihikura, and Rangitikei Faults).

Sedimentology of archean greenstone belts: Signatures of tectonic evolution

Stratigraphic styles in Archean greenstone belts are compared to those of modern and Phanerozoic depositional basins in order to test the conformity of tectonic style through time. Six lithological associations in greenstone belts are recognized: (1) mafic-ultramafic volcanic, (2) calc-alkaline volcanic, (3) bimodal volcanic, (4) quartz arenite-iron-formation or carbonate, (5) conglomerate-wacke and (6) conglomerate-arenite. Examples of the associations are described from the Kaapvaal, Superior and Zimbabwe Provinces and the Pilbara Block. Each association differs only slightly between the four regions, thereby emphasizing a common tectonostratigraphic theme. Sedimentary rocks are a minor component of the mafic-ultramafic volcanic association. They were deposited in two types of volcanic-basin environments: one similar to barred lagoons and bays around oceanic volcanic islands, and the other similar to sediment-starved platforms adjacent to coalesced volcanoes in inter-arc, intra-arc and back-are basins. In contrast, sedimentary rocks in the calc-alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins, and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers. Sedimentary facies were identical to those of forearc-trench and marine volcanoplutonic-arc settings. Chemogenic lithofacies in both volcanic associations were deposited in shallow to deep-marine waters, although shallow-water profiles were selectively preserved in some terrains. Sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite, basalt, and rhyolite. Depositional environments ranged from braid-plain and braid-delta to coastal marine and submarine fan. Starved-basin deposits include banded iron-formation and sulfidic black shale. The association is identical to that of cratonic extensional basins in arc-continent and intracontinental rifts. Sedimentary rocks of the quartz arenite-iron-formation or carbonate association onlap weathered basement or bimodal-volcanic association rocks. Depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal-marine to offshore pelagic-hemipelagic, or from inshore intrabasinal-clastic and volcaniclastic to offshore pelagic-hemipelagic. Siliciclastic deposits are supermature. Two tectonic sites are represented: those with siliciclastic facies represent continent-adjacent syn- to post-rift stable shelves and those dominated by banded iron-formation, with volcaniclastic deposits, represent arc-adjacent post-rift stable shelves. Sedimentary rocks of the conglomerate-wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone, with minor volcanic components. These were deposited in braid-plain, braid-delta and submarine-fan environments within elongate sedimentary basins. Coastal-plain and pelagic basin facies are also present. Siliciclastic detritus was derived from syndepositional, magmatic and metasedimentary thrust-belts. Two stratigraphic styles are preserved: one that records a flysch-like stage of deep- to marginal-marine environments, and the other that records a molasse-like stage of marginal-marine to fluvial environments. The association is identical to that of compressional-foreland basins of arc-continent collisional and compressional-arc tectonic affinities. The conglomerate-arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units. It is dominated by thick wedges of siliciclastic sandstone, although bimodal volcanic and volcaniclastic rocks can also be present. Depositional environments ranged from talus and alluvial fan, and braid-plain to either lacustrine or coastal marine. Local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns. Longitudinal facies geometries are typical. The association is similar to that of strike-slip collisional graben in hinterland tectonic-escape and terrane-accretion orogens. The examples show that, despite all the ramifications of secular geophysical, geothermal and geochemical global change, the stratigraphic style of sedimentary rocks in Archean greenstone belts can be matched with modern tectonic analogues, thereby emphasizing the conformity of stratigraphic style through time. The inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique. Major differences exist with regards to the chemical composition of certain magmatic and sedimentary products, but these are compensated for by their constant stratigraphic function in terms of depositional processes, depositional environments and tectonic sites.

Variability in early vadose carbonate diagenesis in sandstones

Early carbonate diagenesis in calcareous and siliciclastic sandstones in the vadose environment is reviewed and shown to be more extensive than generally recognized and, more importantly, highly variable in character, both spatially and temporally. Spatial variability is classed into three hierarchical scales—macro (regional), meso (sediment body) and micro (individual and groups of pores). Dependent and independent variables relating to the diagenesis are shown to vary according to scale. At the macro scale the independent variables are tectonic activity, climate, sea level and lithofacies; at the meso scale they include location of the sediment body relative to the watertable and sea level, and exposure to sea spray, plant cover, water balance relationships, texture and allochem content. Micro-scale independent variables are the chemistry, amounts and mobility of the pore waters and individual particle microstructures and mineralogies. Dependent variables at one scale are shown to exist as independent variables at the next, more detailed, scale. The infinitely complex combinations of variables operating over different time scales explain the observed and characteristic variability in diagenetic processes and products within the vadose zone.

Loch Macumber (early Carboniferous) of Atlantic Canada

Following the Acadian Orogeny, Atlantic Canada accommodated several, large, relatively deep lakes within a wrench-fault basin complex called the Maritime Basin. Late Devonian and Tournaisian lakes were hydrologically open, shallow to deep, mainly fresh water bodies. Middle Visean lakes, here collectively called Loch Macumber, were closed, deep, and meromictic. Their deposits comprise the first and thickest of five sequences in the Maritime Basin. Salinity in the loch increased with time from restricted marine or penesaline, to saline. Basin-centre facies consist of a thin, but extensive, sheet of black, peloidal laminated lime mudstones and an overlying thick evaporite complex. The carbonate sheet grades laterally into both laminated to thinly bedded marlstones, siliciclastic sandstones, and microbial, biocementstone mounds. Laminae consist of alternating carbonate and either silty carbonaceous shale or siliciclastic clay and silt. The mudstone and marlstone are locally interbedded with siliciclastic and carbonate turbidites, resedimented (?deep water) breccias, and olistostromes. Seasonal changes in anoxia and/or carbonate production produced rhythmic laminae of carbonate and carbonaceous shale. Carbonate grains consist of silt-sized microbial clots and rare arthropod carapaces and brachiopod shells. The mounds originated as tufa precipitated around subaqueous hydrothermal springs that supported chemosynthetic communities. Resedimentation processes including incipient brecciation, sliding, slumping, debris flows, and turbidity currents were common. The mounds trapped hydrocarbons from the surrounding laminite and sulphides from underlying hydrothermal vents. Increasing salinity with time resulted in sulphate and chloride precipitation that filled the basins and ended the life of Loch Macumber. After the deposition of thick evaporites the topography became less accentuated, the seas less saline, and the faunas more normal marine.

Diagenetic Processes Along a Basin-Wide Marker Bed as a Function of Burial Depth

ABSTRACT The Contessa megabed is a basin-wide turbidite bed in the Marnoso-arenacea Formation (Miocene) of the northern Apennines (Italy). It has a mixed siliciclastic/carbonatoclastic composition and was derived from the southern end of the elongate, NW-SE-trending Periadriatic foreland basin. Heavy-mineral contents of fifteen Contessa samples taken across the whole outcrop area vary systematically. Zircon, tourmaline, rutile, garnet, chloritoid, picotite and monazite are ubiquitous. Staurolite and, to a lesser degree, titanite are absent along the southwestern margin of the basin and become increasingly present toward the foreland to the northeast, where the basin fill becomes progressively thinner. Other petrologic parameters of the Contessa light-mineral composition depict a similar geographic trend. Intrabasinal carbonate clasts are progressively more abundant northeastward toward the shallowest part of the basin, whereas the relative percentage of sparite cement increases toward the southwest, where the basin fill is thickest. Among the clay minerals, mixed-layer illit /smectite is more abundant in the shallowest part of the basin and decreases progressively toward the deepest part of the basin. Petrologic parameters of mostly siliciclastic sandstone beds stratigraphically close to the Contessa megabed and derived from the Alps to the northwest (non-Contessa samples) depict similar trends, in spite of their different provenance. Assuming that the respective compositions of the Contessa and non-Contessa samples were initially homogeneous across the entire basin, these areal differences in mineralogy are interpreted to result from varying degrees of diagenesis due to differential sedimentary/tectonic load in the asymmetrical Periadriatic basin. Provenance studies must consider this possibility to avoid erroneous reconstructions of sediment paleodispersal systems.

The Paleozoic Petroleum Geology of Central Arabia (1)

Exploratory drilling in central Saudi Arabia has established significant Paleozoic petroleum potential in this region. In recent discoveries, the Permian interval of the Unayzah Formation has been identified as the primary oil and gas reservoir. The Unayzah Formation consists of alluvial and fluvial to shallow marine siliciclastic sandstones and siltstones that were deposited on a major pre-Unayzah unconformity. Saudi Arabian Paleozoic oil is a high-gravity (43-53 degrees API) crude with low sulfur content (usually less than 0.07%) that is classified as Arabian Super Light oil. Gas with condensate has also been discovered in the Unayzah Formation. The identified Paleozoic traps are moderate-relief, fault-generated structures, with generally 30-100 m of closure. Unayzah structural traps developed primarily during the Triassic to Early Jurassic; however, an earlier phase of significant pre-Unayzah structural development is also indicated between the Devonian and Late Carboniferous, and is related to a Hercynian structural event. Post-Triassic structure in central Arabia is mainly regional and results principally from Late Cretaceous to Tertiary development of the Central Arabian arch. The established source rock is the basal Qusaiba Shale, a widespread organic-rich shale that was deposited following regional deglaciation in the Early Silurian. Migration occurred regionally updip through Unayzah Formation reservoirs from the underlying Qusaiba Shale subcrop and/or vertically along fault-bounded zones on the flanks of structures from the underlying Qusaiba Shale. Interbedded Upper Permian shales and evaporites provide a top seal for the Unayzah and form the basal sequence of the overlying transgressive Khuff carbonate. Pre-Qusaiba reservoir targets include the Cambrian-Ordovician Saq Sandstone, the Ordovician Qasim Formation, and Upper Ordovician to Lower Silurian periglacial clastics of the Zarqa and Sarah formations. High-gravity, low-sulfur oil has also been discovered in this pre-Qusaiba clastic section. The reservoirs of the pre-Qusaiba structures are fault bounded and sources laterally by the down-faulted Qusaiba Shale.

Kuwaiti dolocrete: petrology, geochemistry and groundwater origin

Near-surface sediments in southern Kuwait show extensive development of duricrusts. The host materials are siliciclastic sandstones of the post-Eocene Kuwait Group. Inland, the duricrusts are dominantly pedogenic calcrete (with some silcrete and gypcrete), whereas within 10–20 km of the coast, dolomite is the dominant duricrust mineral. Both these dolocretes and the inland calcretes display a similar maturation sequence in which carbonate-rich nodules develop and coalesce, carbonate progressively replacing and displacing detrital grains. The dolomite of the dolocretes forms mosaics of crystals typically 10–70 μm in size, varying from simple rhombs to spherulites. An intermediate morphology, named artichoke dolomite from its appearance in SEM, is particularly abundant. Authigenic palygorskite is associated with the dolomite. Dissolution of cores or zones within dolomite crystals has occurred. Calcite is present as sparry crystals (always post-dating dolomite) and is the expected precipitate from present-day soil and groundwaters. Chemical analyses of dolomite show highly negative values of δ 13C (− to − 10.7‰ PDB) and δ 18O varying from + 0.6 to + 3.3‰ PDB. The oxygen isotope values are interpreted as reflecting evaporation of a marine-based fluid. Manganese values of around 1000 ppm show that this fluid was reducing. Strontium data show variability reflecting mineral-fluid reactions. The general absence of metastable carbonates and presence of zoning in dolomite crystals suggests that meteoric dilution of seawater also occurred. Given that the duricrusts lack biogenic features, the light carbon isotope values are taken to indicate oxidation of seeping hydrocarbons. The dolocretes are interpreted as groundwater precipitates near the water table of a brackish water body formed at a time of higher relative sea level than today.

Comparative Stratigraphy and Subsidence History of Mesozoic Rift Basins of North Atlantic (1)

Mesozoic rift basins around the North Atlantic all possess many of the following Upper Jurassic to Lower Cretaceous (Oxfordian to Albian) stratigraphic elements: (1) an early Oxfordian hiatus, (2) backfill deposits in Upper Jurassic paleovalleys, (3) multiple Tithonian transgressive-regressive sequences, (4) early Berriasian regressive sequences, (5) a sub-Valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates, (6) Valanginian to Hauterivian fining-upward sequences, (7) condensed Barremian sequences, and (8) prominent Aptian unconformities. More local features include abrupt Kimmeridgian deepening (Lusitanian basin, Portugal), Albian deepening (Cantabrian and Aquitaine basins, northern Spain and southern France), and late Albian-early Cenom nian source rejuvenation (Cantabrian basin, northern Iberia). Decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence. Relatively high rates (50-100 m/m.y.) characterize onset of rift phases in the Triassic and the Kimmeridgian. Locally, in the Lusitanian basin of Portugal, Kimmeridgian rates reach about 250 m/m.y. In the Cantabrian and Aquitaine basins, a third phase of rapid subsidence characterizes the Aptian-Albian interval after the main phase of continental breakup around Iberia. This phase may be due to transtensional stresses as Iberia continued to rotate relative to Europe, tensional intraplate stresses, or gravity driven partial collapse of the new continental marg n. The North Atlantic Mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of Europe and North America. Pulses of extension generally are reflected in the stratigraphy of all of the basins. Superimposed on this record are the effects of eustatic sea level variations. The main difference between the Canadian Jeanne d'Arc basin and Iberian basins is the significantly greater Early and Middle Jurassic basement subsidence rate in the former (about 40 m/m.y. vs. 0-10 m/m.y.). This difference may result from prolonged extension on the Canadian side, perhaps due to northward transfer of shear stress from left-lateral transform motion between the Grand Banks and northwestern Africa.

Arenig sedimentation and basin tectonics in the Harlech Dome area (Dolgellau Basin), North Wales

AbstractArenig (Ordovician) clastic sediments crop out in the Harlech Dome region (North Wales), and are placed in a single stratigraphic unit: the Allt Lwyd Formation. This unit records a marine transgression onto an erosion surface produced during late Tremadoc arc volcanicity. Four discrete petrofacies are denned, and reflect differing proportions of detritus derived from Tremadoc-type basic-intermediate igneous rocks, and the local sedimentary basement. Initial shallow marine siliciclastic sandstones and conglomerates are overlain by extensive deep water mud-rich units. These generally shallow up into a complex arc-apron deposit, with sediments derived from the eroding Tremadoc arc, as well as from similar, synchronous volcanics. Predominantly epiclastic sandstones and conglomerates were deposited in deltaic and tidal environments in an arc-apron complex, and capped by condensed mudstones and an ironstone, deposited as sea level rose across these systems. Sediments were ponded in north–south orientated troughs and derived from uplifted blocks. Facies and petrofacies distribution were controlled by syn-sedimentary north-south and northeast–southwest faults. The Allt Lwyd Formation was ponded in a fault-controlled basin (the Dolgellau Basin), one of a series of interconnected sub-basins flooded by the Arenig transgression. The sediments preserved reflect deposition during the transgression of a volcanic arc, prior to the extrusion of marginal basin-type volcanics.

Diagenesis of Sandstones in Permian Upper Minnelusa Formation, West Mellott Field, Powder River Basin, Wyoming: ABSTRACT

Dolomite-anhydrite-sandstone cycles of the Permian upper Minnelusa Formation, Wyoming, record multiple progradations of marine, siliciclastic sabkha, and eolian sediments. Heterogeneity in the sandstones, studied in five cores from present-day depths between 7,100 and 7,300 ft, is partly the result of diagenetic alterations. The earliest alternation was the formation of clay coatings on the detrital grains, particularly in siliciclastic sabkha and related eolian sandstones. Quartz cement formed small overgrowths, particularly in the siliciclastic sabkha sandstones. Carbonate clasts in the sandstones were pervasively dolomitized, as were the adjacent carbonate beds, possibly by reflux associated with brines that formed during the evaporitic stage of the next sedimentary cycle. The sandstones were then partially cemented with anhydrite (or gypsum). Clay coatings and partial cementation by anhydrite did not completely prevent a progressive loss of intergranular volume by pressure solution with burial. The illitization of early clay coatings was also a progressive process with burial. Late-stage dissolution of anhydrite produced irregularly distributed secondary sandstone porosity that ranged to approximately 30%; dissolution was most complete in the upper parts of eolian-dune and reworked-eolian sandstones, resulting in excellent reservoir properties. Late-stage dissolution of potassium feldspars also occurred, as evidenced by the lack of compaction of the partiallymore » dissolved grains. Hydrocarbons then migrated into and occupied the secondary porosity of the upper arts of eolian-dune sandstones and reworked-eolian sandstones at West Mellott.« less

Late Ordovician sand-wave complexes on Anticosti Island, Quebec: a marine tidal embayment?

Laterally discontinuous, mixed carbonate–siliciclastic sandstones in the upper Vaureal and lower Ellis Bay formations of Anticosti Island were deposited on an equatorial carbonate ramp with a slope of less than 1°. The 10–18 m thick sandstones are interpreted as subaqueous sand-wave complexes analogous to detached parts of modern shoreface-connected sand ridges. These record storm-enhanced, tidal modification of a northerly derived shoal retreat massif that may have formed in response to recovery from global sea-level lowstands in the Late Ordovician (Ashgill: late Rawtheyan – Hirnantian). The sand-wave complexes formed within a tidal embayment that was confined by the Precambrian Shield to the north and northwest by rising tectonic highlands of the Humber Zone in Newfoundland to the east, and by active tectonic highlands in the Quebec Appalachians (Gaspésie) to the south. Paleocurrent distributions, parallel to the western margins of the Strait of Belle Isle, suggest that the north end of the embayment was closed in Late Ordovician time. Low-diversity faunas within the sand units consist mostly of sowerbyellid, strophomenid, and rhynchonellid brachiopods, bivalves, gastropods, large aulacerid stromatoporoids, and large, domed favositid corals. These "sandy fades" faunas belong to communities significantly different from those found in the laterally interfingering and overlying carbonates and shales, suggesting that the sand waves played an important role in local community modification.

Cambrian Pisolites as Paleoenvironment and Paleotectonic Stress Indicators, Rattlesnake Mountain, Wyoming: ABSTRACT

Pisolitic-rich carbonates occur within the uppermost 0.5 m of the Meagher Limestone member of the lower Gros Ventre formation in exposures near Cody, Wyoming. The Meagher Limestone is overlain by 51 m, and underlain by 63 m of dark gray Gros Ventre shale. Pisolites range in size from 2.0 to 18 mm in diameter and occur in lime grainstones associated with trilobite fragments, peloids, glauconite, fine-grained subangular quartz, and minor oolites. Girvanella grainstones 15-20 cm thick directly underlie the pisolite strata and have contributed to some of the carbonate material within pisolite nuclei. Dolomite and ankerite may occur within pisolitic rocks as finely crystalline irregular patches. Pisoliths commonly show an oblate ellipsoid shape, with maximum flattening perpendicular to bedding. Long-axis to short-axis ratios of these grains in fracture planes perpendicular to bedding average between 2.5 to 3.5, with the long axis parallel or subparallel to bedding. Grains observed in bedding planes have ratios averaging between 1.5 to 2.0. A paleostress state has produced a strain ellipsoid with long-axis ratios ranging from 1.7 to over 3.0. There appears to be little or no tectonic strain on the bedding plane, so the strain can be described as uniaxial, with maximum compression perpendicularmore » to bedding. The majority of carbonate rocks in the Meagher Limestone were deposited in a normal marine subtidal setting, while ooid and pisolitic grain types are suggestive of subtidal-peritidal conditions. Because of the strain deformed pisoliths, a subaqueous versus subaerial environment of pisolite genesis is difficult to assess. A siliciclastic sandstone, 0.6 m thick with low-angle tabular crossbedding, is present immediately beneath the Meagher Limestone. The sandstone is composed of 94% fine to medium sand-size subangular quartz grains and is associated with glauconite, minor biotite, zircon, and ilmenite.« less

Calcium carbonate cement (caliche) in Keweenawan sedimentary rocks (∼ 1.1 Ga), upper Peninsula of Michigan

Abstract A complex caliche profile, and numerous widespread zones of carbonate cement occur in a thick sequence of fluvial siliciclastic sandstones and conglomerates that were deposited in the Lake Superior basin during and after continental basaltic volcanism, as interflow conglomerates of the Portage Lake Volcanics and as the Oronto Group. In conglomerates the carbonate cement forms discontinuous zones, up to 60 cm thick and several meters in length, consisting of up to 25 vol.% carbonate. Whereas the caliche horizon at the Centennial Mine shows typical upward-grading features characteristic of pedogenic caliche, the widespread sparry carbonate cement is less diagnostic. It is interpreted also as caliche for the following reasons: (1) it predates regional epidote metamorphism; (2) a caliche profile has been described from the same strata; (3) textures characteristic of caliche such as carbonate-rimmed pebbles, pendulose calcite on pebbles, sand grains floating in a carbonate matrix, and exfoliated chlorite; (4) possible vadoids; (5) the carbonate-rich zones are widespread stratigraphically and geographically in basinward-inclined alluvial fans, and (6) the distribution of these zones, and the total quantity of contained carbonate requires other than a point source for the origin of the carbonate. Several lines of evidence suggest that the prevailing climate may have been arid, with a fluctuating, perhaps seasonal rainfall pattern required for caliche: (1) a paleolatitude about 30° north; (2) carbonate-filled shrinkage cracks denoting high evaporation; (3) intermittent, flashy, or ephemeral streams recorded locally in fluvial beds by numerous superimposed horizons of mud-chip conglomerate, enormous, previously cemented mudstone rip-ups; and desiccation-cracked mud drapes. Interpretation of the cement as vadose caliche is compatible with a sedimentation model for the region as a semi-arid intermontane basin, flanked by alluvial fans and with a central saline lake.

Sedimentologic Significance of Turbidite Ichnofacies in the Saraceno Formation (Eocene), Southern Italy

ABSTRACT Mixed siliciclastic-carbonate and siliciclastic turbidites of the Saraceno Formation (Eocene) in Calabria, southern Italy, contain diverse trace-fossil associations. The depositional setting appears to have been a small, tectonically isolated, rapidly subsiding borderland basin into which turbidity currents dumped a variety of sediments. The formation comprises four sedimentary facies, each of which is characterized by a distinctive ichnofacies (from bottom to top): Facies I (alternating calcarenite and thick-bedded shale), Chondrites-Muensteria-Rhizocorallium; Facies II (alternating calcarenite and thin-bedded shale), Chondrites-Muensteria-Zoophycos; Facies III (thin-bedded siliciclastic sandstone), Zoophycos-Helminthoida-Palaeophycus; Facies IV (alternating sili iclastic sandstone and shale), Zoophycos-Helminthoida-Phycosiphon. The trace-fossil associations are dominated by fodinichnia, such as Chondrites in the lower part of the formation (Facies I and II) and Zoophycos in the upper part (Facies III and IV). The agrichnia (graphoglyptids) and pascichnia that usually dominate turbidite ichnofacies are uncommon, perhaps owing to a) a fairly shallow water depth, b) an extremely high frequency of turbidity currents, and/or c) dysaerobic or anaerobic interstitial conditions within the sediment.

In search of the origin of cement in siliciclastic sandstones: An isotopic approach

In siliciclastic sediments only a minor amount of cement is internally derived. This implies that an external source has to be sought in sandstone diagenesis. Controversy exists about the external source of cement and the nature of the aqueous solution, viz. meteoric, hydrothermal and marine, and/or dewatering of shale. Isotope geochemistry may prove useful in solving this controversy. The oxygen isotopic compositions of neoformed clay minerals are related to the isotopic composition of the coexisting aqueous solution. Aqueous solutions of diverse origin such as meteoric, hydrothermal and marine, and dewatering of shale, in general, bear characteristic isotopic signatures. Thus, the δ 18O-values of authigenic clay minerals may be used to identify the nature of the original aqueous solution, provided the temperature of the environment is known and the extent and the nature of isotopic exchange between the minerals and fluids following crystallization can be evaluated. Oxygen isotopic compositions of early authigenic clay minerals in Permo-Triassic Gondwana sandstones of Peninsular India have been used to infer the cement source. Sandstones of Sakmarian age have an average δ 8O-value of +5.0%o, whereas a relatively high average δ 8O-value of +13.2%o is observed in sandstones of Rhaetic age. δ 18O-values of sandstones ranging in age between Sakmarian and Rhaetic have values between +5.0 and +13.2% 0. This gradual increase of δ 180-values with decreasing age shows a strong correlation with the changing latitudinal location of the sample site from a position around 60°S during Sakmarian time to a position around 38°S during Rhaetic time. The changing pattern of oxygen isotopic composition of suthigenic clay has been interpreted as a result of corresponding change in isotopic composition of meteoric water due to the northerly drift of the Indian plate during Gondwana sedimentation. This suggests that the aqueous solution, involved in the early diagenesis of Gondwana sandstones, is of meteoric origin.

Comparison of Depositional System and Reservoir Characteristics of Selected Blanket-Geometry Tight Gas Sandstones: ABSTRACT

Future development of gas in tight sandstones is highly dependent upon price and the state of technology, including detailed understanding of internal and external reservoir geometry. Part of the tight gas resource lies in blanket-geometry, siliciclastic sandstones; over 30 such sandstones in 16 sedimentary basins and at depths mostly less than 10,000 ft (3,000 m) were reviewed for the Gas Research Institute. Emphasis was placed on depositional systems and resulting lithogenetic facies as an important control on sand body geometry. In contrast to lenticular sandstones, blanket-geometry tight gas sandstones were deposited as deltaic, barrier strand plain, and shelf systems. Overlap occurs between systems, as in the case of a strand plain developed marginal to a deltaic depocenter. Not all parts of all sandstones are tight (< 0.1 md permeability); tight areas vary from extensive (J Sandstone, Denver basin) to very limited (Hartselle Sandstone, Black Warrior basin). Five stratigraphic units were selected from which developments in reservoir characterization, fracture treatment, and other technologies can likely be extrapolated to a wider group of tight gas reservoirs. The Travis Peak Formation (East Texas basin/North Louisiana salt basin) is a fan delta system. It ranges in depth from 3,000 to 11,000 ft (900 to 3,350 m), with net pay of 30 to 86 ft (9 to 26 m) and post-stimulation gas flows of 500 to 1,500 MCFGD. The Frontier Formation (Greater Green River basin) is a wave-dominated End_Page 460------------------------------ deltaic system with associated barrier strand plain facies; shallower prospective areas range in depth from 7,000 to 12,000 ft (2,100 to 3,650 m). The Frontier has net pay of 10 to 90 ft (3 to 27 m) and post-stimulation gas flows up to 2,500 MCFGD. Characteristics of the Travis Peak have extrapolation potential to Tuscarora-Medina-Clinton sandstones of the Appalachian basin; deltaic facies of the Frontier may correspond to parts of the Davis Sandstone (Fort Worth basin), Olmos Formation (Maverick basin), and the Fox Hills Sandstone (eastern Greater Green River basin). Cozzette and Corcoran Sandstones (Piceance Creek basin) are predominantly barrier strand plain deposits; they range in depth from 2,500 to 8,000 ft (760 to 2,400 m), with net pay of 10 to 70 ft (3 to 21 m) each and pos -stimulation gas flows average 1,250 MCFGD. The upper Almond Formation (eastern Greater Green River basin) may contain more shallow marine and offshore bar than barrier strand plain facies. It occurs at depths of 6,000 to 15,000 ft (1,800 to 4,600 m), with net pay of 14 to 18 ft (4 to 5.5 m); and post-stimulation gas flows up to 1,700 MCFGD. The characteristics of the Cozzette, Corcoran, and upper Almond may be extrapolated to other marginal marine units in the Mesaverde Group and parts of the Dakota Sandstone in several Rocky Mountain basins. Shelf deposits include the Mancos B interval of the Mancos Shale (Piceance Creek and Uinta basins) at depths of 3,500 to 5,000 ft (1,000 to 1,500 m) in areas of recent drilling. Mancos B net pay ranges from 38 to 120 ft (11.5 to 36.5 m), and po t-stimulation gas flows range up to 350 to 1,200 and MCFGD. Extrapolation potential exists in the Anadarko basin and Northern Great Plains area. These five stratigraphic units have potential for increased commercialization. By understanding the initial properties derived from the depositional setting of a tight gas sand, the explorationist can better extrapolate successful exploration, stimulation, and production techniques between reservoirs in similar depositional settings. End_of_Article - Last_Page 461------------

Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming

Research Article| March 01, 1975 Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming HANS P. EUGSTER; HANS P. EUGSTER 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar LAWRENCE A. HARDIE LAWRENCE A. HARDIE 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar Author and Article Information HANS P. EUGSTER 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 LAWRENCE A. HARDIE 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (3): 319–334. https://doi.org/10.1130/0016-7606(1975)86<319:SIAAPC>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation HANS P. EUGSTER, LAWRENCE A. HARDIE; Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming. GSA Bulletin 1975;; 86 (3): 319–334. doi: https://doi.org/10.1130/0016-7606(1975)86<319:SIAAPC>2.0.CO;2 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 SocietyGSA Bulletin Search Advanced Search Abstract The Wilkins Peak Member of the Green River Formation of Wyoming has been examined in outcrop with the object of reconstructing its depositional environment. Based on their assemblages of sedimentary structures, seven rock units are described, six of which define depositional subenvironments. These units are (1) flat-pebble conglomerate, (2) lime sandstone, (3) mudstone, (4) oil shale, (5) tronahalite, (6) siliciclastic sandstone, and (7) volcanic tuff. Their respective subenvironments are (1) rapid transgression of a shallow lake, (2) lake shore oscillating over a mud flat (slow transgression), (3) playa mud flats, (4) shallow lake with occasional desiccation, (5) seasonally dry salt lake, (6) braided stream, and (7) not specific.These subenvironment deposits are arranged in depositional cycles. We have observed four types of cycles involving flat-pebble conglomerate (A), oil shale (B), mudstone (C), lime sandstone (D), and also trona. These cycles are I: A-B-C, II: D-B-C, III: D-C, and IV: B-trona-C. Individual cycles have been correlated over distances of up to 24 km.The Wilkins Peak Member is thought to have been deposited in a playa-lake complex, which consisted of a shallow, central playa lake that was surrounded by vast, normally exposed mud flats fringed by alluvial fans. Evaporative concentration of bicarbonate-rich inflow waters led to saturation with respect to calcite, most of which must have been deposited as cement within alluvial fans. Evaporation continued in the capillary zone of the mud flats, precipitating calcite first, then magnesian calcite, and eventually protodolomite. The carbonates accumulated as a soft micritic mud at the fringes of the playa mud flats.During periods of desiccation, the muds were subject to cracking, and the mud-crack polygons contributed sand- and silt-size dolomitic micrite intraclasts that were transported to the central lake by the next storm. When the central lake was large, oil shale accumulated in it, with the organic matter derived from a flocculent ooze consisting of bottom-dwelling blue-green algae and fungi. During dry periods the lake shrank, and trona and halite precipitated in the central portions.An understanding of the Wilkins Peak sediments can be achieved only by considering as inseparable the hydrologic, sedimentary, geochemical, and biologic processes responsible for their formation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lithofacies Variations in Green River Formation, Wyoming: ABSTRACT

The Green River Formation of Wyoming is a thick lens of sedimentary rocks which was deposited in a large playa-like complex. These sediments accumulated in three major depositional environments: (1) marginal (siliciclastic sandstone and siltstone); (2) mudflat (marlstone); and (3) lacustrine (oil shale, trona, fossiliferous limestone). In general, the basinward transition of lithofacies is from coarse-grained, cross-bedded, channel sandstones at the basin margins to algal and oolitic limestones to clastic and algal dolomitic marlstones and finally to oil shale and trona at the center of the basin. The lateral intertonguing of these lithofacies and the repetition of large- and small-scale lithologic cycles provide a basis for the reconstruction of the physical, chemical, a d biologic evolution of Eocene Lake Gosiute. Thin, but extensive, algal marlstone units reflect the basinward migration of Lake Gosiute's shoreline during periods of increased aridity. Widespread fossiliferous limestones suggest transgressive shoreline conditions. The occurrence of thin, extensive tuff beds establishes time-stratigraphic relations among the lithofacies. The correct interpretation of lithofacies changes in a closed basin can be an important tool for locating deposits of rich oil shale, trona, and perhaps oil- and gas-bearing channel sandstone reservoirs similar to those now being developed in the Uinta basin. End_of_Article - Last_Page 919------------

Depositional Environment of the Green River Formation of Wyoming: A Preliminary Report

A new model is proposed for the depositional environment of the Green River Formation of Wyoming, which is based on a combination of lake and playa environments. We propose that oil shales and trona beds accumulated in shallow lakes which were fringed by large playa flats. In these playa flats alkaline brines evolved through evaporation and precipitation of calcium carbonate and protodolomite in the capillary zone near the ground-water table. Dolomitic mudstones, marlstones, and calcareous and siliciclastic sandstones were the products of occasional floods on the playa; chief evidence consists of the assemblage of sedimentary structures found in these rocks, indicating frequent exposure to air, strong shallow-water currents, transport, and resedimentation. The playa-lake model leads to a more consistent picture of the hydrology, brine evolution, mineral formation, and sedimentation in the Green River basins than the previously accepted stratified lake concept.