Quartz Fragments Research Articles

Petrology and Diagenesis of Pocono-Price Big Injun Sandstone, Granny's Creek Field, West Virginia: ABSTRACT

Two cores of the Lower Mississippian Pocono-Price Big Injun sandstone were studied megascopically and petrographically to determine the effects of mineral content and diagenesis on porosity. Most of the samples studied are fine-grained sublitharenites. Quartz, polycrystalline quartz, and argillaceous rock fragments comprise up to 95% of the framework grains. Many samples also are characterized by abundant pseudomatrix resulting from the squeezing and degradation of mica flakes, argillaceous rock fragments, and feldspars. Initial porosity probably was very high because of the well-sorted and angular nature of the sand grains, although reservoir porosities generally range only up to 15%. Mechanical compaction, diagenetic cements, and degradation of sand grains have significantly reduced initial porosity. The general sequence of postdepositional events includes: (1) mechanical compaction, (2) illite and chlorite grain coatings, (3) quartz and barite pore filling and replacement, (4) calcite pore filling and replacement, (5) dolomite, siderite, and pyrite replacement, and (6) dissolution of feldspar grains. Chlorite coatings are abundant in some samples and, where present, help to preserve primary porosity by inhibiting the growth of quartz cement. Minor secondary porosity has resulted from dissolution of feldspar. Clean, high quartz sandstones with well-developed chlorite coatings form the best reservoirs in this area.

Stress estimates and fault history from quartz microstructures

Fault plane structures and microstructures have been examined from a fault in Cambro-Ordovician quartzites exposed on the north coast of the Cantabrian zone, N.W. Spain. Along the fault there is a 2–3 cm wide cataclasite zone bounded by plastically deformed intact quartzite, which is restricted to a few metres from the fault plane. In thin section, the cataclasite consists of angular fragments of quartz in a fine-grained matrix of quartz cement. Cathodoluminescence reveals extensive intra- and transgranular microcracking. Deformation lamellae (D.L.) are widespread in the intact quartzite; electron microscopy shows that they are defined by a banded dislocation substructure. The microstructures indicate a fault history consisting of plastic deformation restricted to a metre-scale zone, followed by cataclasis and dilational fracture. The early plasticity is interpreted as evidence for the propagation of a process zone. The D.L. are used as an indicator of power-law-breakdown creep, which occurs above a critical normalized stress, estimated as at least 170 MPa from available data. Dislocation densities also indicate a stress higher than 116 MPa. The process zone as defined by formation of D.L. is modelled using fracture mechanics theory. For the likely size of the zone, the remote applied stress for fault propagation is shown to be less than 100 MPa.

Concentration of gold in in situ laterites from Mato Grosso

The gold concentration studied is located in lateritic soils overlying Precambrian schists of the Cuiaba Group in Mato Grosso, Brazil. The following five horizons may be recognized from bottom to top: (1) a gray-blue altered schist horizon, (2) a red argillaceous alterite, (3) a horizon characterized by iron oxihydroxide-rich pebbles and quartz fragments in an iron oxihydroxide-rich matrix and clays, (4) an iron crust, and (5) the present soil. The most significant gold content is found in the third horizon just below the iron crust. According to geological study and morphological observations of the gold particles, the gold ore mined today is the result of two combined processes, i.e., the ferrallitic alteration of quartz lodes enclosed in schists and the effect of the red argillaceous alterite which acts as an impervious structure preventing the largest gold grains from migrating downward during their mechanical concentration.

Preliminary Statistical Analysis and Provenance Trends in Desmoinesian Sandstones from Central and Eastern Oklahoma: ABSTRACT

Desmoinesian sandstones from the northeast Oklahoma platform and from the Anadarko and McAlester basins record a complex interaction between mid-Pennsylvanian source-area tectonism and cyclic sedimentation patterns associated with transgressions and regressions. Framework grain summaries for 67 thin sections from sandstones of the Cherokee Group (Bartlesville, Red Fork, Skinner, and Prue) were subjected to multivariate statistical analysis to establish regional compositional trends for provenance analysis. R-mode cluster and correspondence analyses were used to determine the contributing effect (total variance) of key framework grains. Fragments of monocrystalline and polycrystalline quartz, chert, metamorphic rock, and limestone contribute most to the variation in the grain population. Q-mode cluster and correspondence analyses were used to identify three distinct petrofacies. Petrofacies I is rich in monocrystalline quartz (86 to 98%) and contains rare mica and rock fragments. Petrofacies II is also rich in monocrystalline quartz (66 to 86%) and contains as much as 15% metamorphic and sedimentary rock fragments. Petrofacies III is compositionally heterogeneous and contains fragments of polycrystalline and monocrystalline quartz, mica, chert, and metamorphic and sedimentary rocks. Quantitative analyses indicate that Desmoinesian sandstones were derived from complex sedimentary and metamorphic source areas. Petrofacies I sandstones are restricted to the southwestern part of the Anadarkomore » basin and the northeast Oklahoma platform, whereas petrofacies II and III sandstones are distributed throughout the study area. The distribution of petrofacies within the region suggests a model of source-area interaction and cratonic sediment recycling.« less

Petrology and Provenance of Upper Cretaceous Sandstone, Southern San Rafael Mountains, Santa Barbara County, California: ABSTRACT

Petrologic analysis of 24 medium to coarse-grained sandstone samples, collected from a 2950-m submarine fan complex of late Campanian-early Maestrichtian age exposed within Mono Creek Canyon, reveal commonly calcite cemented, poorly sorted, subangular biotic arkoses. Framework averages 86.0%. Matrix - primarily detrital quartz, feldspar, and lithic fragments finer than 0.03 mm and mechanically and chemically altered phyllosilicates and labile aphanites - averages 8.9%. Calcite cement averages 4.2%. Porosity averages 0.9%. Gazzi-Dickinson point counts of 400 framework grains per slide yield modal averages of Q/sub 37.7/ F/sub 49.8/ L/sub 12.5/; Qm/sub 27.4/ F/sub 49.8/ Lt/sub 22.8/; Qm/sub 35.6/ P/sub 43.7/ K/sub 20.7/; and Qp/sub 49.4/ Lv/sub 22.1/ Ls/sub 28.5/. P/F averages 0.68, Lv/L averages 0.45, Qp/Q averages 0.27, and detrital phyllosilicate, predominantly biotite, averages 5.7% of total framework. Neither primary nor secondary parameters vary systematically with stratigraphic position. Miscellaneous constituents average 1.3% of framework and include epidote, garnet, amphibole, pyroxene, zircon, and tourmaline as well as carbonaceous blebs, opaque minerals, and unidentifiable lithic fragments. Separate analysis of 100 medium sized quartz grains per slide indicates a mean population of 63.0% non-undulatory monocrystalline quartz, 9.1% undulatory monocrystalline quartz, 10.1% polycrystalline quartz of 2 to 3 crystals, and 17.9% polycrystalline quartz composed ofmore » more than 3 crystals. Modal data, plotted upon provenance discrimination diagrams, indicate a plutonic provenance transitional between a dissected magmatic arc and uplifted basement terrane. Paleocurrent data, neglecting possible clockwise rotation, indicate sediment transport from the north.« less

Eocene Sediment Dispersal Pattern Records Asymmetry of Laramide Green River Basin, Southwestern Wyoming: ABSTRACT

Provenance and paleocurrent data from synorogenic fluvial sandstones can be used to constrain theories about the timing and structural style of Laramide foreland uplifts and associated basins. The Green River basin of southwestern Wyoming is a large ellipsoidal basin bounded by uplifts with diverse orientations and basement rock compositions. Sandstone from the main body of the Eocene Wasatch Formation in the Green River basin was sampled along the south and west flanks of the Rock Springs uplift. Petrographic examination and paleocurrent measurements reveal two main facies. The first facies is rich in feldspar and metamorphic rock fragments derived from the Wind River Mountains to the north. The second facies is dominated by quartz and sedimentary rock fragments, reflecting a source in the Uinta Mountains to the south. Distribution of these facies indicates that a sediment lobe extends 15 km into the basin from the Uinta Mountains. Another sediment lobe originates from the Wind River Mountains and extends approximately 100 km south into the basin. This pattern suggests that the topographic axis of the depositional basin was an east-west-trending trough about 15 km north of the Uinta Mountains. The position of the basin axis is inferred to reflect greater subsidence nearmore » the Uinta Mountains. The asymmetric distribution of the two facies of the Wasatch Formation thus supports the model of basin subsidence caused by thrust-loading and indicates the Uinta Mountains were the dominant active thrust block bounding the Green River basin during the early Eocene.« less

Sedimentation in epithermal veins of the Bohemia mining district, Oregon, USA: Interpretations and significance

Open spaces in epithermal veins of the Bohemia mining district, Oregon, USA, filled with sediments during hydrothermal activity. These sediments consist mainly of chalcedony, rock fragments, and vein quartz fragments. In addition, hematite is deposited during stage three of the vein development. Observed sedimentary features include draping laminae, erosion surfaces, slumping, and graded bedding. Such sediments can be used for reconstruction of the original orientation of vein systems, because the sediment laminae are initially deposited horizontally. Vein sediments record variations in fluid flow due to self-sealing, fracturing, and cessation of hydrothermal activity. Investigation of vein sediments therefore provides an additional tool to unravel the geologic history of epithermal systems. The chalcedonic vein sediments record large temperature drops and highly silica supersaturated waters, probably due to fracturing and pressure release. Hematitic vein sediments indicate sulfide deficient hydrothermal fluids.

Experimental Frost Shattering of Some Precambrian Rocks, Finland

The intensity of frost shattering and the amount and type of debris produced from quartzite, granulite and rapakivi granite samples were studied with laboratory experiments. The experiments were run for 1103 temperature cycles from + 15°C to -8°C. The amounts of debris ranged from 0.01 to 0.8 percentages of debris relative to dry weight of the original samples. Grain size of frost weathered material was mainly in sand and gravel fraction classes. Surface texture of quartz grains was studied with SEM. Shape of the grains was irregular and their edges were sharp. Many of the surfaces were rather smooth. Irregular conchoidal fractures are also characteristic. Very often the surfaces were covered by very small, flat fragments of quartz. Some opened microfissures were also found in the grains. The Finnish rocks seem to be very resistant to frost weathering and postglacial frost shattering has had rather limited importance in landform formation in Finland.

Ore petrography of a sedimentary uranium deposit, Live Oak County, Texas

Samples from the McLean 5 open-pit uranium mine, a small high-grade deposit located along a normal fault in the Miocene Oakville sandstone of Live Oak County, Texas, have been studied for uranium abundance, distribution, and nature of occurrence on the microscopic level. The host sandstone is composed of quartz, feldspars, and volcanic rock fragments, cemented by sparry calcite. Authigenic minerals include iron disulfide minerals (dominantly pyrite and some marcasite) and small amounts of clays, Ti oxides, and opal. High-grade ore (to 3% U) occurs along the fault, decreasing to less than 1,000 ppm within 10 m from the fault. The ore mineral is amorphous pitchblende and exhibits botryoidal morphology.The microscopic occurrence of uranium, documented by fission-track mapping of petrographic thin sections, is presented in detail. Uranium occurs abundantly as grain coatings and fillings in intergranular spaces in samples with high uranium content, where calcite cement has been partially or totally leached as mineralization proceeded. Lesser amounts are adsorbed onto leucoxene (microcrystalline anatase), mud clasts, and altered igneous rock fragments. Adsorbed uranium is the major mode of occurrence in samples, with lower uranium contents farther from the orebody.Textural relations indicate that iron sulfides formed both before and after mineralization. Initial mineralization was by adsorption onto aggregates of fine particles of Ti oxide and clay minerals of various origins. With dissolution of cement and continued uranium influx, uranium precipitated as grain coatings and pore fillings.

Textural Variation and Composition of Bentonite Derived from Basaltic Ash

The Rokle bentonite deposit is part of an accumulation of argillized volcanoclastic rocks in the Tertiary stratovolcanic complex of the Doupovske Mountains east of Karlovy Vary (Carlsbad), about 100 km westnorthwest of Prague, Czechoslovakia. The arenite basalt ash was originally composed of hyaloclasts and subordinate biotite. The following types of montmorillonite aggregates were produced during the alteration of the ash in a stagnant, lacustrine environment: (1) extremely fine-grained micro-crystalline aggregates that have honeycomb textures and that replace the original hyaloclasts, and (2) coarse crystalline aggregates that have more open honeycomb textures and that fill pores and cracks in altered hyaloclasts and in pumice vesicles. Both types of aggregates have the same chemical composition according to energy dispersive X-ray analysis. Montmorillonite, separated as the <1-¼m size fraction from olive gray bentonite, has the following crystallochemical formula: (K0.09Na0.02Ca0.29Mg0.10) (Al2.43Fe3+1.05Fe2+0.005Mn2+0.005Mg0.50) (Si7.62Al0.38) O20(OH)4. Biotite was apparently stable during the alteration of the hyaloclasts. Anatase and possible accessory heulandite-clinoptilolite were also formed in small amounts, but were not observed by scanning electron microscopy. Goethite is the youngest oxidation product in some parts of the bentonite. Minute fragments of sodium-rich plagioclase, potassium feldspar, quartz, and muscovite are ubiquitous accessories of the original hyaloclasts. Together with kaolinite, they formed from the underlying fresh or kaolinized orthogneiss.

Premineralization radial dikes of tourmalinized fluidization breccia, Julcani District, Peru

Dikes of fiuidization breccia composed largely of fragments of dacite lava are exposed over an area of about 3 km 2 in the central part of the Julcani volcanic center, central Peru. The dikes, which predate later quartz-alunite-pyrite and argillic alteration stages and economic enargite-tetrahedrite mineralization, exhibit a crude but definite radial pattern centered on the zone of later W-Au mineralization. Fragments of Paleozoic rock in dikes in outer areas of the pattern roughly constrain the location of an inferred early central vent structure 1.0 to 1.5 > or = km in diameter now buried beneath the dacite domes that host the breccia dikes.The microfragmental matrix of the dikes has been subject to strong quartz-tourmaline-pyrite alteration that did not affect dacite wall rock or larger dacite fragments within the dikes. The alteration was probably produced by fluids closely related to those that fluidized the dike material during emplacement.Fluid inclusions containing cubes of halite + or - sylvite are present in some quartz fragments within the breccia dikes and in phenocrystic quartz in dacite lava adjacent to the dikes. Estimated salinities require minimum trapping temperatures of about 260 degrees to 450 degrees C and show that fluid pressures during dike emplacement at times were appreciably greater than hydrostatic pressures. A few inclusions contain relatively large grains provisionally identified as hematite, anhydrite and/or gypsum, and carbonate, indicating the existence of solutions rich in these components.The high boron concentrations and highly saline fluid inclusions in the breccia dikes, the radial dike pattern, the high f (sub O 2 ) and f (sub S 2 ) of the magmas, and the general magmatic-hydrothermal history all suggest that the breccia dikes at Julcani were produced by the explosive release of magmatic fluids from a pluton crystallizing beneath the central part of the district.Areally restricted elongate zones of shattered dacite postdate the fiuidization breccia dikes. Tourmaline is absent and only minor particle rotation, rounding, and transport are evident. These zones probably resulted from the locally explosive formation of steam by the intrusion of late dikes into dacite lavas saturated with ground water.

Stimulation Results in the Low-Permeability Wasatch Formation: An Evolution to Foam Fracturing

Summary The Wasatch formation of the Uinta basin in eastern Utah is typical of many formations in the Rocky Mountains, having low permeability and high sensitivity to water. Stimulation treatments with several types of fracturing fluids, including oil-in-water emulsion fluids, complex get fluids, and foam fluids, have been generally successful. Production decline curves from 24 wells in the field were used for comparison of the different stimulation methods. Although foam fracturing has been used for the shortest period of time, comparison of the production histories shows the relatively higher efficiency of the foam fracturing treatments compared to other stimulation methods in the Wasatch formation. Foam fluids gave higher production rates and higher flowing pressures than offset wells fractured with complex gel fluids. A simulation model for oil and gas production was used to match the production history from this reservoir. The model allowed a projection of gas production based on early production from the wells and knowledge of the reservoir. Introduction There are many tight gas reservoirs in the Rocky Mountains that have low permeability and high water sensitivity and require stimulation to make them commercially productive. Low-permeability sandstones often have a productive. Low-permeability sandstones often have a small capillary network that tends to retain fluid. Fluid sensitivity may result from the presence of clays that can swell or migrate after water contact. The Wasatch is such a typical formation. In an attempt to make the formation commercially productive, stimulation efforts have undergone an productive, stimulation efforts have undergone an evolutionary process. The predominantly sandstone formation gives no initial production. Deep, propped fracture treatments served to improve and extend productivity. Oil-in-water emulsions were used initially to create fracture geometry and place proppant. But exact blending requirements and high frictional losses in pumping the treatments gave disappointing results. Complex gels produced high-viscosity fluids with lower frictional losses produced high-viscosity fluids with lower frictional losses resulting in better stimulation at lower cost. But the water sensitivity of the low-permeability Wasatch made it desirable to reduce the liquid burden on the formation. Foam fracturing made it possible to drastically reduce the liquid placed on the formation, place higher proppant concentrations with less water than complex gel, and provide an energy-assisted rapid cleanup of the fracture. provide an energy-assisted rapid cleanup of the fracture. Characteristics of the Wasatch Formation The Wasatch formation of the Uinta basin in north-eastern Utah originated in the Tertiary period of the Cenozoic era. Sediments were deposited within alluvial, deltaic, and lacustrine environments. The vertical extent of the formation varies from about 2,000 ft [610 m] along the south portion to near 4,000 ft 11,220 m] in spots on the nonhem end. Cross flow as well as under-pressured thief zones are common to the formation. The formation is composed of fractured sandstones, limestones, and dolomites lying within a massive shale section. The natural fractures often are filled with acid-soluble material such as calcite. The low permeability of the rock renders these fractures important to production. Dilute hydrochloric acid solubility ranges from 6 to 18% within the field of this study. Petrographic thin-section analyses were performed on cores from several wells. The following analyses were typical. Cherty Sandstone, 5,839 ft [1780 m]. These cores showed moderately sorted, medium-to-finely grained quartz, chert. feldspar, and detrital mica as framework grains. Intergrain pore space had been coated initially with mixed-layer clay then the overgrowth cemented. Remaining pore space was partially infilled with calcite, dolomite, and kaolinite material. Sandstone, 6,325 ft [1928 m]. These cores revealed moderately sorted, medium-to-very-fine sand of quartz, altered feldspar (altered to mixed-layer clays), chert, and mica fragments as the framework grains. Some detrital chlorite is present. but most chlorite occurs as rims on crystal outlines. Calcite cements many pores, but the remaining pore space is clean. JPT P. 1545

Copper-zinc deposits associated with basic volcanism, Goldstream area, southeastern British Columbia

Small stratabound deposits occur in the Selkirk Mountains. Goldstream consists of a thin sheet of massive sulfides in dominantly calcareous and graphitic schists of Paleozoic age. The massive sulfide layer consists primarily of intimately intermixed pyrrhotite, sphalerite, and chalcopyrite, with numerous subrounded inclusions of quartz, carbonate, and phyllite fragments. The sulfides are locally swirled around the gangue inclusions to produce a durchbewegung fabric, a texture common to many deformed and metamorphosed massive sulfide deposits. Contacts with the hanging wall and the footwall range from sharp to gradational over a few meters. Metal values have a simple and regular distribution in plan, with increasing grades toward the central thicker part of the massive sulfide layer. Goldstream and other copper-zinc deposits in the Goldstream camp are interpreted to be exhalative massive sulfide deposits that formed in an unstable subsiding basin, near the continental margin. They are similar to the besshi-type deposits of Japan.--Modified journal abstract.

Sources, Distribution, and Mixing of Late Pleistocene and Holocene Sands on the South Texas Continental Shelf

ABSTRACT Gross grain shape and mineralogical analyses were used to determine the source, distribution, and mixing of late Pleistocene and Holocene sands on the South Texas continental shelf. Two major gross grain-shape types are found to be present in varying proportions in the sands of this shelf. The first sand type is associated with sands which are relatively enriched in plutonic-metamorphic quartz, volcanic quartz, feldspar and igneous and metamorphic rock fragments, and thus is considered to represent first-cycle sands derived from the Llano Uplift and the silicic volcanic rocks of the Southwest U.S. The second sand type is associated with more quartzose sands rich in sedimentary quartz and sedimentary rock fragments, and is considered to represent multicyclic sands eroded from the Texas Coastal Plain and High Plains. Variations in the relative proportions of these sand types in the sands of the nearshore/inner shelf region reflect both the changing sources of the sand and their transport, mixing, and convergence by modern nearshore processes. Three distinct sedimentary petrologic provinces, corresponding to three provinces previously defined by heavy mineral analysis (Van Andel and Poole 1960), are defined herein on the basis of their relative content of first- and multicyclic sand. Modern sands of the first of these provinces, the Rio Grande, are derived from the drainage basin of the Rio Grande and characteristically contain high proportions of first-cycle sand; such sands are presently found in the nearshore/inner shelf region between the mouth of the Rio Grande and the 27°N latitude to the north of the mouth of the river. Modern sands of the second province, the Texas Coast, are derived largely from the drainage basins of the South Texas coastal plain rivers and characteristically contain high proportions of multicyclic sand; such sands are presently transported southwestward by longshore currents to the 27°N latitude, where they converge with the northward-transported sands of the Rio Grande Province. Modern sands of the third province, the Western Gulf, are derived from the drainage basins of the Colorado and Brazos rivers and characteristically contain high proportions of first-cycle sands', such sands are also transported by longshore currents towards the southwest, where they are gradually diluted by Texas Coast Province sands. Sands of these three provinces are also found in the relict late Pleistocene fluvial and deltaic sediments on the outer part of the South Texas shelf. Previous studies had considered the fluvial and deltaic sediments on the northern part of the shelf to be entirely derived from the Western Gulf Province; however, this study shows that the relict fluvial deposits of certain South Texas coastal plain rivers which occur on the northern part of the shelf should be assigned to the Texas Coast Province. In all, multicyclic sands constitute approximately 48% of the late Pleistocene and Holocene sand of the South Texas continental shelf.

Carboniferous Terrigenous Clastic Facies, Hydrocarbon Producing Zones, and Sandstone Provenance, Northern Shelf of Black Warrior Basin: ABSTRACT

Deltaic and barrier-bar depositional systems from the Chester and Pottsville Groups of the Black Warrior basin produce natural gas and minor oil from 11 Mississippian sandstone reservoirs and four Pennsylvanian clastic units. Within the Chester, four major genetic sequences containing cratonic delta systems have been mapped. These include, in ascending stratigraphic position, the lobate, river-dominated Lewis system, the wave-dominated Evans and Hartselle systems, the river-dominated, elongate, and lobate deltas of the Muldon complex (Rea, Abernathy, Sanders, and Carter), and the hybrid Gilmer system. Chester deltas prograded southeastward onto the stable northern shelf from a cratonic source area, most likely the Ozark uplift. Net sandstone isolith maps for discrete genetic units demonstrate a northwest-to-southeast End_Page 1462------------------------------ progradational trend and a tendency for the sub-Millerella units to thicken updip to the line of post-Carboniferous erosional truncation. An isopach map of the Tuscumbia-Millerella interval shows thickening toward the north, away from the Ouachita orogenic source area. Limited petrographic evidence from Lewis and Carter sandstone bodies associated with the principal subsurface deltaic facies tract indicates a dominance of monocrystalline quartz and chert rock fragments, as well as an absence of lithologic indicators for an orogenic provenance. By way of contrast, outcrop studies of the Hartselle and Parkwood units in Alabama by W. A. Thomas et al describe an abundance of polycrystalline quartz in the Hartselle and both metamorphic quartz and phyllite rock fragments in the Parkwood. The Parkwood samples containing the orogenic indicators are restricted to the folded Appalachians. The Pottsville Group can be broken down into a maximum of 10 regionally mappable subsurface intervals. Widespread delta destructional coal seams and marine reworked sandstone bodies serve as marker units. Pottsville gas production derives from barrier bar facies in the lowest two genetic intervals and from the mixed barrier-bar and deltaic sandstone units of interval 3 (Nason). This Lower Pennsylvanian clastic wedge has its predominant source area to the south in the Ouachita orogenic belt. An isopach map of the total Pottsville documents significant thickening to an excess of 10,000 ft (3,000 m) in central Mississippi. All of the previously mentioned orogenic indicators are noted in the outcropping deltaic Pottsville facies. End_of_Article - Last_Page 1463------------

Mixed Carbonate and Clastic Sedimentation: North Insular Shelf of Puerto Rico: ABSTRACT

Light-colored carbonate sands and dark-colored muddy clastic sands are deposited in distinct and sharply defined bands across the narrow (less than 3 km, 1.9 mi) and steep insular shelf of northern Puerto Rico. The narrow bands of clastic sands are found adjacent to the mouths of each of the rivers. These bands are in textural equilibrium with the present-day physical conditions on the shelf and are deposited as a result of successive hurricane-related flooding events. Dominant sand-sized components are quartz, feldspar, and rock fragments. The rate of sedimentation in the intervening cross-shelf strips of carbonate sediment is much lower, and the shelf sediment cover here is not in textural equilibrium with the physical environment. Dominant sand-sized components are mol usk, algae, coral, and bryozoan fragments. The boundary between carbonate and clastic bands is very sharp, often occurring within 100 m (330 ft). The small carbonate fraction of the rapidly deposited clastic sediment and the particles making up the slowly deposited carbonate sediment exhibit strong differences in physical condition. The clastic calcareous fraction is fresh in appearance, highly angular if fragmented, and has original coloration. The relict calcareous material is old appearing, commonly stained and rounded, and has a dull luster. Polished surfaces and highly rounded grains characterize adjacent calcareous beach sands. The striking difference in the physical condition of the carbonate grains in clastic and carbonate sediments is a function of differing lengths of sea-floor exposure. End_of_Article - Last_Page 535------------

A Petrologic Examination of Some Resedimented, Coarse-Grained, Clastic Intervals in Ouachita Mountains, Arkansas: ABSTRACT

Several resedimented, coarse-grained clastic intervals exist in the strata of the Ouachita Mountains. A petrologic examination of selected units in the basal Stanley Shale, the Arkansas Novaculite, and the Missouri Mountain Shale yields information pertaining to provenance, transportation, and deposition. Twenty-five coarse-grained units sampled are categorized as massive sandstone, pebbly sandstone, clast-supported conglomerate, and matrix-supported conglomerate. Samples exhibit grading, imbrication, and stratification indicative of mass flow and fluidal flow-transport processes. Sedimentary rock fragments are the predominant clasts, with chert being the primary constituent. Other major constituents are detrital quartz, rip-up clasts, and shale fragments, of which the latter show signs of soft-sediment deformation. A metasedimentary source area is inferred. Paleoflow direction, estimated to be to the south and southwest, was ascertained by orientation of imbricated clasts and channel cuts. A vertical sequence change from a matrix-supported conglomerate to a bimodal, clast-supported conglomerate, both of which exhibit channeling, to a massive sandstone occurs at one locality. This fining-upward sequence is due to a change in source area, which may be the result of tectonism and/or glacio-eustatic sea-level fluctuations. End_of_Article - Last_Page 452------------

Origin of Thin, Siliceous Beds in Monterey Shale, Elk Hills Field, California: ABSTRACT

The Miocene Monterey Shale consists of thinly interbedded black shale and siliceous beds in a section 1,800 ft (550 m) thick on the western anticline, Elk Hills field. Selected full-diameter cores were examined through the upper section west of, and partly equivalent to, the Stevens Oil Zone sandstones. The siliceous beds are commonly 1 to 5 cm, and rarely 8 to 10 cm, thick. The beds are generally structureless or contain a few indistinct laminae. Bases are in sharp contact with underlying shale, and some tops are gradational to overlying shale. In a few beds, the uppermost parts show curved laminae that represent low-amplitude ripples. Therefore, the beds seem to be distal turbidites composed of common, massive A divisions and rare, rippled C divisions. Many beds have a fine granular, graded texture with a thin basal zone of coarser detritus. The beds are composed of finely-crystalline, siliceous material, in some places partly replaced by dolomite(?). Petrographic study shows a significant content of fine sand to silt-size detritus. In a typical graded sequence, grains of quartz, plagioclase, and rock fragments form a thin lag at the base where they comprise more than 50% of the rock and have an average size of 0.13 mm. Detrital grains decrease upward to less than 3% at the top, and average size decreases to 0.05 mm. The thin, regularly bedded nature of the section, significant detrital content, and graded texture suggest that the siliceous beds are turbidity-current deposits. The siliceous component was probably pelagic, diatomaceous sediment from the basin floor that was incorporated in turbidity flows, transported a short distance, and redeposited with terrigenous detritus in massive A divisions of the turbidite sequence. Alternatively, subsequent recrystallization destroyed original lamination and produced the structureless beds. End_of_Article - Last_Page 900------------

Effects of Physical and Chemical Diagenesis on Low-Porosity, Low-Permeability Sandstones, Mesaverde Group, Uinta Basin, Utah: ABSTRACT

Examination of sandstones from core samples spanning a 110-m interval of Upper Cretaceous Mesaverde Group indicates that physical compaction of labile rock fragments and chemical diagenesis greatly modified the original lithology. These modifications reduced porosity (to less than 10%) and permeability (generally to less than 0.5 md at surface conditions). Local silty lamina further reduce vertical permeability. The sandstones are predominantly feldspathic litharenites composed of monocrystalline and polycrystalline quartz, igneous, metamorphic, and fine-grained sedimentary rock fragments, and small amounts of potassium and plagioclase feldspars. Cementing agents include quartz overgrowths, authigenic low-albite on plagioclase, intergranular calcite and dolomite, and authigenic clays (kaolinite, illite, and minor chlorite). Quartz overgrowths are more common than in the Mesaverde Group sandstones previously examined, and undoubtedly reduce porosity. The overgrowths are sometimes separated from the detrital-quartz host by an iron oxide-stained dust ring or by a thin (± 2 µ) film of chlorite. Locally, early formed intergranular calcite occupies as much as 40% of volume and, in additio to virtually eliminating porosity, has largely prevented compaction of labile rock fragments. Chert and fine-grained sedimentary rock fragments are commonly compacted between more competent framework grains. Following compaction and deformation, leaching of feldspars and rock fragments produced secondary porosity, which includes most measured porosity in these sandstones. End_of_Article - Last_Page 562------------

Reservoir Properties of Submarine-Fan Facies: Great Valley Sequence, California

Submarine-fan sandstones of the Great Valley sequence west of the Sacramento Valley, California, have low porosities and permeabilities (64 samples averaged 10.1% porosity and 0.87 millidarcies permeability). However, petrography and scanning electron microscope studies indicate that most sands in almost all submarine fan environments are originally porous and permeable. Thin turbidite sandstones deposited in areas dominated by shale in outer-fan, basin-plain, and overbank environments are cemented mainly by calcite; shale dewatering is inferred to contribute to rapid cementation early in the burial process. Sands deposited in inner- and middle-fan channels within interchannel and fan-fringe environments that contain only thin shale beds have small percentages of intergranular matrix or cement. The original porosity is substantially reduced mechanically at shallow depths and by pressure solution at deeper levels. Permeability decreases systematically with increasing age of the rocks, presumably as a result of increasing burial depths. Computer-run stepwise regression analyses show that the porosity is inversely related to the percentage of calcite cement. Such parameters as the contents of quartz, feldspar, and unstable rock fragments have no correlative effect on either porosity or permeability. The results reported here indicate original porosity and permeability can be high in deep-water submarine fans and that fan environments dominated by sand (with high sand/shale ratios) are more likely to retain higher porosity and permeability to greater depths than sand interbedded with thick shale sequences.