Sedimentary Rock Fragments Research Articles

Petrology and Hydrocarbon Reservoir Potential of Mississippian (Chesterian) Sandstones, Black Warrior Basin, Mississippi: ABSTRACT

ABSTRACT The character and reservoir quality of six different Mississippian (Chesterian) sandstone units in frontier areas of the Black Warrior Basin of Mississippi have been determined by inspection of cores, examination of thin-sections, and X-ray diffractometry. A total of 113 samples from ten wells was taken from cores of the following sandstones: the Lewis, the sandy Lewis Limestone, the Evans, the Rea, the Sanders, and the Carter. Hydrocarbon production from the basin, which is located in NE Mississippi and NW Alabama, is mostly shallow gas (with minor gas-condensate and oil) from these units. Sample depths range from about 2,500 ft in northern Chickasaw County to about 5,500 ft in Monroe and Lowndes Counties, with the deepest samples coming from almost 11,000 ft in northern Clay County. The sandstones are very fine- to fine-grained quartz arenites having framework grains that are, on average, 98.3% monocrystalline quartz, 0.8% polycrystalline quartz, 0.2% feldspar, 0.1% metamorphic rock fragments, 0.3% sedimentary rock fragments, and 0.2% muscovite. Remnant clay rims under secondary overgrowths generally outline sub-rounded to well-rounded grains. There is little variation in the character of the framework grains between the different sandstone units. Differences in degree of compaction and the percentage and type of cement lend some individuality to the sandstones, however. The most common cement is silica precipitated as secondary overgrowths on detrital quartz. Less common is a carbonate (dominantly dolomite with some calcite) cement, but in one highly oolitic Lewis Limestone sample, 54.8% is carbonate. In many samples, the carbonate cement has corroded adjacent framework grains. Authigenic clays, primarily kaolinite, and pyrite occur as post-gramework grain dissolution cements. Except where cemented by carbonate, the sandstones are highly compacted. This is indicated by sutured, concavo-convex, and long grain contacts as well as occasional microstylolites. Megascopic inspection of cores revealed numerous stylolites in four wells; this includes all three producing wells sampled. Petrographic porosity ranges from 0.0 to 9.3%, averaging 1.6%. Most porosity is secondary, resulting from the dissolution of carbonate and silica cement, and possibly some of the few original unstable framework grains. The Lewis Sand from a depth of about 11,000 ft has no porosity in the samples examined, all pore space being occluded by pyrite. Relative percentages of framework grains indicate that the provenance terrane was likely a stable craton interior, presumably to the north, having a sedimentary rock cover with very mature quartz sandstones as a major component. This is in contrast to the overlying Pottsville (Pennsylvanian) rocks of the Black Warrior Basin, which contain 10-18% metamorphic rock fragments, and which probably represent detritus shed from a rising Ouachita terrane to the south and southwest. Based on the very fine-grained nature of the sand and the presence of marine fossils, oolites, and thin shale interbeds, the Chesterian sandstones were likely deposited offshore from deltas recognized to the north and east of the study area.

Petrology and Depositional Environments of Lower Tuscaloosa Formation (Upper Cretaceous) Sandstones in North Hustler and Thompson Field Areas, Southwestern Mississippi: ABSTRACT

ABSTRACT The lithology, diagenesis, and depositional environments of the Dykes, Denkman, and McComb sands of the Stringer Sand Member of the Lower Tuscaloosa Formation in the North Hustler and Thompson field areas of Amite County, Mississippi, have been determined by detailed description of cores from nine wells and examination of 61 thin-sections. The cores were recovered from driller depths of about 11,100 ft in the updip Tuscaloosa trend, which is being actively explored for stratigraphically trapped oil. Most of the sandstone samples (65%) are quartz arenites, and the remainder are sublitharenites: both types consist dominantly of fine- to very fine-grained sand. The average sandstone consists of 67.3% framework grains, 14.8% matrix, 9.2% cement, and 8.7% pore space. Monocrystalline quartz is by far the most abundant framework grain (88.7%), with polycrystalline quartz (6.6%), potassium feldspar (0.3%), plagioclase (0.1%), chert (1.9%), igneous (mostly volcanic) rock fragments (0.5%), metamorphic rock fragments (0.2%), and sedimentary rock fragments (1.5%) also present. The matrix for the most part consists of pore-filling clays, and includes organic matter and secondary pyrite. Quartz and ferroan dolomite (ankerite) are the most common cements; high concentrations of siderite and calcite occur as cement within some thin layers, and siderite is also found commonly as small concretions. Porosity is mostly secondary, resulting from partial dissolution of framework grains and cement. Quartz overgrowths probably formed during an early stage of diagenesis, followed by deposition of calcite cement which corroded and embayed quartz and other framework grains. Apparently the calcite was later replaced by ankerite. Late-stage pore-filling clays include vermicular kaolinite and chlorite, the latter occurring as rims and in dissolution-enlarged pore space. The depositional environment of the Dykes and Denkman sands in all wells is interpreted to be fluvial; stacked point bars are obvious in one well. The McComb sand, best developed in a wildcat well between Thompson and McComb fields, is marine, based on the occurrence of glauconite and shell fragments. The presence of a basal scour surface on shale indicates that this sandstone may be the product of marine reworking of a transgressed distributary mouth bar.

Sedimentology, Petrology, and Reservoir Characteristics of Lower Strawn Sandstone, Bent Tree Field, Hardeman County, Texas: ABSTRACT

Reservoir sandstones of the lower Strawn Formation (early Middle Pennsylvanian) in the Bent Tree field of Hardeman County, Texas, are coarse to fine-grained, texturally submature arkoses. Cores show the sandstones to have been deposited in 1.5-4.5 m thick fining-upward successions of aggraded or prograded bar units. Each bar unit has a sharp erosional base overlain by cross-bedded, coarse-grained, conglomeratic sandstone, which, in turn, is overlain by medium to fine-grained, horizontally bedded or ripple-bedded sandstone. The coarse conglomeratic sandstones are interpreted to represent deposition in main channels of a braided fluvial system that were progressively filled by aggrading and prograding bars. The interbedded, finer grained, more immature sandstones appear to have been deposited in auxiliary channels or swales, or in proximal overbank settings. The detrital framework grain suite of the reservoir sandstones averages 47% quartz, 30% feldspars, 19% igneous rock fragments, and 4% sedimentary rock fragments. The source of these sands was a plutonic/cratonic igneous massif with minor exposures of older sedimentary strata, and was probably the ancestral Wichita Mountains. Diagenesis has significantly affected the petrographic and reservoir properties of the lower Strawn sandstones, primarily through the in-situ alteration of detrital feldspathic grains and by the precipitation of authigenic quartz overgrowths,more » chlorite clay, and carbonate cements.« less

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

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

Losses of Porosity and Permeability in Lithic Sands Assessed with Compaction Experiments: ABSTRACT

Compaction experiments with Recent lithic sands indicate that in addition to grain rearrangement, ductile grain deformation can significantly reduce porosity and permeability in sandstones. Compacted artificial mixtures of quartz and potentially labile rock fragments demonstrate, as expected, that the lithic grain content can control the amount of compaction and porosity loss at depth. Using a triaxial deformation apparatus, simulated overburden experiments were conducted with a Recent lithic sand from the Eagle River, Alaska, containing approximately 55% argillaceous sedimentary rock fragments. Additional experiments were conducted with artificial mixtures of quartz grains and varying percentages of shale fragments. Water-saturated samples were compacted at a constant strain rate of 10/sup -4/ under maximum uniaxial stress levels ranging from 1500 to 9000 psi (equivalent to burial depths of 3000 to 18,000 ft). Porosity loss due to compaction was calculated from volume reduction. Vertical permeability for each experiment was measured in situ at the maximum simulated overburden stress level. Results indicate that the porosity of these lithic sands initially declines almost linearly with increasing overburden pressure. At higher pressures (in excess of 5000 psi) the porosity decline gradient is greater for sands with a greater percentage of lithic fragments. This relationship may be attributed tomore » continued porosity reduction due to mechanical deformation of ductile grains. Thin-section analysis of the Eagle River sands indicates that deformation of labile grains is important in reducing porosity to 7% and vertical permeability to 3.2 md at a simulated overburden pressure of 9000 psi.« less

Subsurface Study of Lower Tuscaloosa Formation at Olive Field, Pike and Amite Counties, Mississippi: ABSTRACT

ABSTRACT Olive Field is located on the Amite-Pike County line in southwestern Mississippi. The field was discovered by Shell Western Exploration and Production Inc., and produces from a sandstone within the Lower Tuscaloosa Stringer Member; the trap is stratigraphic. The Stringer Member consists of alternating sandstones, siltstones, and shales, which have an average thickness of 230 ft (76 m) at Olive Field. The Stringer Member is a transgressive sequence deposited on eroded Washita-Fredericksburg sediments and is overlain by Middle Tuscaloosa marine shales. The coarse-to fine-grained sandstones are quartz-rich with a high percentage of sedimentary rock fragments. The majority of sandstones are shale arenites. However, a large number of rock fragments were badly altered; these fragments could not be identified with confidence; many may be of volcanic origin. The sandstones are cemented by quartz overgrowths, Fe-rich carbonate cement, and chlorite. Sandstones commonly have a high preserved interparticle porosity that is characteristic of Lower Tuscaloosa sandstones. This interparticle porosity is attributed to chlorite, which occurs as an isopachous cement that coats grains and reduced nucleation sites for quartz overgrowths. The Stringer Member was subdivided into five zones in this study based on lithology and electric log responses. The lower three zones were deposited in a fluvial environment. The sandstones were deposited as point bars and channel sands in a meandering system. Siltstones and shales represent overbank, flood basin, and channel fill deposits. The upper two zones consist of sediments deposited at or near sea level. Finely laminated shales and siltstones represent marsh and bay deposits. The topmost zone is a thin continuous sand body that represents a remnant beach deposited before major marine transgression.

Diagenesis of Middle to Upper Devonian Catskill Facies Sandstones in Southeastern New York: ABSTRACT

Middle to Upper Devonian Catskill facies sandstones exposed in southeastern New York form part of a regressive alluvial plain sequence, 1.5 km thick, deposited in a foreland basin as a result of the Middle Devonian Acadian orogeny. Petrologically, most Catskill sandstones are litharenites whose detrital composition was important in determining the course of diagenesis. Foliated metamorphic rock fragments, sedimentary rock fragments, monocrystalline quartz, and polycrystalline quartz volumetrically comprise 80% or more of the Catskill sandstones. Metamorphic rock fragments are by far the most important rock fragment type. Both sandstone grain size and the modal percentages of monocrystalline and polycrystalline quartz increased stratigraphically upsection in the facies. These two trends reflect progradation and a possible change in sandstone provenance during Catskill facies deposition. Point counting using cathodoluminescence petrography demonstrated that these sandstones are characterized by abundant pseudomatrix. Ductile-grain deformation and alteration of metamorphic and sedimentary rock fragments contributed heavily to the formation of pseudomatrix. Compaction and chemical diagenesis have effectively reduced Catskill sandstone porosity to irreducible levels. Porosity ranges from 7.1 to 0.9%, whereas permeability ranges from 0.06 to less than 0.001 md (unstressed measurements). Much of the porosity is present as microporosity. Chemical diagenetic events recognized in the Catskill sandstonesmore » include (listed in general paragenetic order): (1) authigenic quartz overgrowths; (2) chlorite and illite clay recrystallization, replacement, and pore filling; (3) calcite replacement; (4) dolomite pore filling/replacement; and (5) secondary porosity formation and kaolinite replacement/pore filling.« less

Petrologic and stable isotope study of the gold-bearing breccia pipe at the Golden Sunlight Deposit, Montana

The Golden Sunlight, a gold-silver deposit in the Whitehall mining district, Jefferson County, Montana, is centered on a silica-cemented breccia pipe with fragments of both Belt Supergroup sedimentary rocks and a latite porphyry. Native gold and gold tellurides occur finely disseminated within gangue minerals that have filled open spaces in the breccia pipe.Alteration in and around ( approximately 0.8 km radius) the pipe is principally quartz-sericite. Within the pipe, breccia fragments are also characterized by a highly silicified outer rind. Openspace filling is divided into four periods as follows according to the presence of the following minerals: (I) quartz and pyrite, with minor hematite, chalcopyrite, bornitc, and gold minerals; (II) chalcopyrite, with lesser amounts of galena, sphalerite, and martasite; (III) additional gold-silver tellurides and tennantite; and (IV) barite, quartz, and sericite with minor carbonates and pyrite. Isotopic study shows delta 34 S values of sulfides between -12.2 and +3.1 per mil, whereas barite values range from 1.2 to 5.9 per mil. Oxygen isotope values of whole-rock breccia samples range from 6.5 to 13.8 per mil, quartz from 10.5 to 14.3 per mil, sericite from 9.7 to 15.5 per mil, and unaltered country rock from 12.3 to 16.6 per mil. Fluid inclusion homogenization studies indicate a temperature of deposition for period I mineralization of approximately 200 degrees C. The coexistence of sylvanitc and petzite in period III suggests a temperature less than 170 degrees C. Values of delta 18 O (sub H 2 O) computed from quartz and temperature data and sericite delta 18 O values range from -2.4 to +4.0 per mil. Hydrogen isotope values of fluid inclusions in period I quartz and water in equilibrium with late sericite fall betweem -56 and -33 per mil. Fluid inclusion freezing study of one sample of period I quartz suggests a fluid salinity of <1 equiv. wt percent NaCl.Isotopic and geologic evidence is consistent with tbrmation of a breccia pipe caused by exsolution of a vapor from a crystallizing magma. Oxygen and hydrogen isotope data suggest that extensive isotopic exchange between a magmatic fluid and igneous rocks occurred prior to open-space filling. Initial brecciation was accompanied by an isoenthalpic temperature decrease, which coupled with conductive cooling led to the precipitation of large amounts of silica. Textural features indicate that brecciation and fracturing, followed by fluid infiltration, occurred episodically. Sulfur isotope data is also consistent with derivation of sulfur from a magmatic source characterized by a delta 34 S (sub Sigma s) value between 0 and 5 per mil. Evidence from mineral assemblages and sulfur isotopes suggests that gold was most likely transported as a chloride complex and was deposited due to a pH increase accompanied by decreases in f (sub O 2 ) and possibly temperature.

Fluvial Systems of Upper Cretaceous Mesaverde Group and Paleocene North Horn Formation, Central Utah: Record of Transition from Thin-Skinned Deformation in Foreland Region: ABSTRACT

Nonmarine strata of the upper part of the Mesaverde Group and North Horn Formation exposed between the Wasatch Plateau and the Green River in central Utah record a late Campanian tectonic transition from thrust-belt deformation to basement-cored uplift. Mesaverde Group sediments were deposited by synorogenic braided and meandering rivers. During most of Campanian time, sediment transport was east and northeast away from the thrust belt across a fluvial coastal plain. Subsequent development of the San Rafael swell, a basement uplift, between western and eastern localities caused erosional thinning of the section. Sandstones within the upper part of the Mesaverde Group form two distinct compositional suites, a lower quartzose petrofacies and an upper lithic petrofacies. Lithic grain populations of the upper petrofacies are dominated by sedimentary rock fragments on the west and volcanic rock fragments on the east. Sedimentary lithic grains were derived from the thrust belt, whereas volcanic lithic grains were derived from a volcanic terrane to the southwest. Tributary streams carrying quartzose detritus from the thrust belt entered a northeast-flowing trunk system and caused a basinward dilution of volcanic detritus. Disappearance of volcanic grains and local changes in paleocurrent directions in latest Campanian time reflect initial growth of the San Rafael swell and development of an intermon ane trunk-tributary fluvial system. Depositional onlap across the Mesaverde Group by the post-tectonic North Horn Formation indicates a minimum late Paleocene age for uplift of the San Rafael swell. End_of_Article - Last_Page 854------------

Diagenetic Variations in Permo-Triassic Ivishak Sandstone in Prudhoe Bay Field and Central-Northeastern National Petroleum Reserve in Alaska (NPRA): ABSTRACT

Porosity in the Ivishak sandstone on the North Slope is frequently cited as an example par excellence of secondary, or leached, porosity. In an attempt to corroborate and expand this contention, 100 thin sections were examined from representative wells in the Prudhoe Bay field and in NPRA. Monocrystalline quartz and biogenic chert are dominant framework grains in the two areas. Minor amounts of polycrystalline quartz, and sedimentary and metamorphic rock fragments are also present. Most of the chert in NPRA is nonporous, whereas Prudhoe Bay chert contains microscopic pores where siliceous biogenic debris, chalcedony, or dolomite formerly existed. The diagenetic sequence of the NPRA samples includes: (1) initial porosity loss by either compaction or siderite, chlorite, or pedogenic calcite cementation; (2) minor carbonate dissolution; (3) localized chert dissolution; and (4) quartz overgrowth formation. The diagenetic sequence of the Prudhoe Bay samples includes: (1) chert dissolution; (2) quartz overgrowth, kaolin, pyrite, and siderite formation; and (3) siderite dissolution. Some workers have suggested that an early carbonate cement maintained the large intergranular pore volumes in the Ivishak at Prudoe Bay. A more likely possibility is that a long, initial period of shallow burial maintained a loose packing density without early cementation. Later chert and siderite dissolution increased porosity. By contrast, the Ivishak in NPRA was subjected to deeper burial early in its history. This caused porosity reduction by relatively early compaction. The later diagenetic events, which ultimately control porosity and permeability variations between the two areas, are, in part, also controlled by the different burial regimes. End_of_Article - Last_Page 675------------

基礎試すい十勝沖の地質層序 道東地域における中新世タービダイト相の考察

The Kisoshisui Tokachi-Oki Well at a total depth of 4, 457.06m which is located offshore Tokachi in eastern Hokkaido, Japan was penetrated into the alternation of conglomerate, sandstone and mudstone, exceeding 2, 000m in thickness.This paper is a study of the stratigraphy, the depositional environment and the distribution of the above sequence in this well.The sequence is correlative with the Okoppezawa Formation of middle Miocene age. Numerous coarse grained sand beds are abundant in older sedimentary rock fragments and contain primary sedimentary structures such as graded bedding, lamination and rip-up clast. The formation in this well is characterized by a rapid sedimentation rate of about 1, 330m/Ma, a lack of microplanktonic fossils and a trace of arenaceous benthonic foraminifers.The geological evidences indicate the Okoppezawa Formation in this well is typical turbidite facies in stagnant deep sea environment of deposition during middle Miocene age. The turbidite facies are distributed in a limited area along the western part of the Doto magnetic belt offshore Tokachi.

Comparison of Two Suites of Okpikruak Formation: A Point-Count Analysis: ABSTRACT

The Lower Cretaceous Okpikruak Formation lies unconformably on the Etivluk group in the Foothills thrust belt of the central Brooks Range. These deep marine sediments were shed northward during the first phases of Brookian deformation. The formation, as sampled, contains two petrographically distinct populations: the upper Kurupa-Oolamnagavik River drainage samples (KOR) of the Picnic Creek allochthon, and the Cobblestone Creek samples (CC) of the Endicott Mountains allochthon. The KOR samples have an average Q:F:L of 49:21:30 and Qp:Lv:Ls of 46:37:17. Variations in total detrital grain populations are greater for End_Page 682------------------------------ quartz (40-50%) and rock fragments (22-39%) and less for feldspar (17-27%). The KOR population is characterized by high proportions of polycrystalline quartz and low proportions of sedimentary rock fragments. Interpretation of detrital modal analyses suggests a complex source terrane involving orogenic recycling and mixing of sediments from a magmatic arc and a subduction system. In contrast to KOR samples, CC samples have an average Q:F:L of 16:11:73 and Qp:Lv:Ls of 11:34:55. Total grain populations vary most for rock fragments (56-85%) and quartz (6-27%) and to a lesser degree for feldspar (8-17%). The high variation in rock fragments suggests variable volcanic and sedimentary sources. Interpretation of detrital modal analyses also suggests complex sources but with a significant arc-orogen component. Both populations are characterized by varying proportions of diagenetic products, which include albite, calcite, chlorite, sericite, and epidote. End_of_Article - Last_Page 683------------

Provenance of Conglomerate Clasts from Upper Cretaceous Kuskokwim Group, Southwest Alaska: ABSTRACT

The predominantly Upper Cretaceous (Albian to Coniacian) Kuskokwim Group consists of marine turbidites and subordinate fluvial and shallow marine strata, deposited in an elongate southwest-trending basin covering over 70,000 km2 in southwestern Alaska. In the Sparrevohn and Cairn Mountain areas of the Lime Hills A-7 and A-8 quadrangles, fluvial, inner fan, middle fan, and outer fan facies are stacked with distal facies over proximal facies by northwest vergent thrust faults. Inner fan pebble-to-cobble conglomerate and pebbly sandstone were deposited as submarine grain flows up to 10 m thick containing reversely graded bases and normally graded tops. Clasts from these conglomeratic deposits are predominantly sedimentary rock fragments--particularly sandstone, si tstone, and argillite--originally thought to have been derived from the nearby Jurassic and Lower Cretaceous flysch (Kihiltna terrane). Detailed examination of these clasts, however, indicate that they contain as minor constituents Paleozoic coral, oolitic limestone, algal boundstone, radiolarian chert, and mafic, intermediate, and felsic volcanic rocks, most likely derived from the adjacent Nixon Fork, Dillinger, and Mystic terranes. Sandstone clasts are arkosic (Q25-F37-L38, n = 7) and contain subequal amounts of K-feldspar and plagioclase. Sand grains within these clasts are moderately sorted, subrounded, and have presolved contacts. Similar arkosic rocks have been described from the Dillinger terrane of the McGrath quadrangle and are the most likely source for the pebbles and cobbles within the Kuskokwim Group. End_of_Article - Last_Page 660------------

Tree-Ring Analysis of Movement of a Rock-Glacier Complex on Mount Mestas, Colorado, U.S.A.

Snow-avalanche debris, talus, and other slope deposits derived from a Tertiary pluton and surrounding Cretaceous and Tertiary sedimentary rock supply rock fragments to 23 rock glaciers on Mount Mestas, Colorado. One prominent ice-cemented rock-glacier complex studied since 1910 has a previously unreported extensive and flow in shale in part of its source region. Movement of this rock-glacier complex since the 15th century is indicated by the 283 trees sampled. Data derived from tree-rings show periods of movement in two directions, one to the northwest down the slump and flow, and the other southwest down the icecemented rock glacier. Movement was caused both by loading at the head of the rock-glacier complex and steepening of the toe by removal of mass during recent excavation.

Provenance and Diagenesis of Ivishak Sandstone, Northern Alaska: ABSTRACT

The Ivishak sandstone of northern Alaska is a regressive sequence of Lower Triassic fluvial and paralic deposits that constitutes an important hydrocarbon reservoir in the Prudhoe Bay area. A petrographic study of the formation, utilizing samples from wells from both reservoir and non-reservoir rocks, was undertaken to determine the provenance and diagenetic histories of the formation. The Ivishak sandstone can be characterized as a low-rank or lithic graywacke. The major detrital species it contains include: (a) quartz (46%), dominantly reworked sedimentary and volcanic monocrystalline quartz and metamorphic polycrystalline quartz; (b) chert (22%), containing variable amounts of inclusions (clay and carbonate); (c) sedimentary rock fragments (10%), largely mudstones and silty mudstones; and (d) metamorphic End_Page 504------------------------------ rock fragments (4%), mostly slate and phyllite. Feldspars are conspicuously absent in the sandstone. This detrital suite, and the overall decrease to the southwest in the grain size of the sandstone, indicate that the sands of the formation were derived from a northern landmass, exposed in the Late Permian and Early Triassic, which consisted of Precambrian schists and quartzites overlain by early Paleozoic marine sandstones and deep-water cherts and argillites. After deposition, the reservoir facies of the Ivishak sandstone underwent 4 consecutive diagenetic phases: (1) early carbonate cementation that prevented mechanical compaction, (2) dissolution of pore-filling carbonate and carbonate inclusions within chert grains, (3) precipitation of quartz as overgrowths, and (4) precipitation of authigenic kaolinite. At present, the intergranular porosity of the formation is high, averaging 11.5%, and is present in the forms of elongate and oversized secondary pores. Porosity also occurs as micropores associated with leached chert grains and with the kaolinite. In the more matrix-rich nonreservoir facies of the Ivishak, the clay matrix prevented complete carbonate cementation which allowed for greater mechanical compaction of the sandstones; in som places, this mechanical compaction, coupled with the precipitation of quartz overgrowths, reduced porosity to irreducible levels. End_of_Article - Last_Page 505------------

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.

Lithology-Dependent Diagenetic Control of Reservoir Properties of Conglomerates, Falher Member, Elmworth Field, Alberta

Conglomerates in the Falher Member of the Lower Cretaceous Spirit River Formation of Alberta are the reservoir rocks for the giant (2-3 Tcf or 5-8 × 1010 m3) Elmworth gas field. Three types of conglomerates are present: (a) unimodal--granules or pebbles lacking any matrix, (b) bimodal grain supported--a framework of pebbles with fine to medium sand in the interstices, and (c) bimodal sand supported--pebbles floating in fine to medium sandstone. The distribution of these three types is controlled by the original depositional environment. The granules and pebbles are composed of chert and silicified sedimentary and volcanic rock fragments. The sand grains are dominantly quartz. During diagenesis, very small (.01-0.1 mm) drusy quartz crystals form on the cherts and rock fragments, whereas quartz grains undergo heavy quartz overgrowth formation because of differences in nucleation and growth of quartz crystals. As a consequence, most unimodal conglomerates (low in quartz) have not experienced major reduction of porosity and permeability. Bimodal conglomerates, both sand and pebble supported, have lost significant amounts because of cementation of the quartz-rich matrix. Streaks and patches of kaolinite and calcite cements have also reduced porosity and permeability locally. Gas production from Falher wells, therefore, de ends not only on the amount of conglomerate, but also on the proportion of unimodal and bimodal types.

Alluvial Model for Eocene Wasatch Formation Coal, Powder River Basin, Wyoming: ABSTRACT

The Eocene Wasatch Formation in the Powder River basin, Wyoming, consists of a conglomerate facies (Kingsbury Conglomerate Member) on the western margin of the basin and a coal-bearing facies near the center of the basin. The conglomeratic facies consists of abundant, basally scoured, pebble to boulder conglomerates and sandstones, and minor rooted siltstones. The conglomerates contain abundant sedimentary and subordinate crystalline rock fragments derived from the adjoining Bighorn uplift. The coal-bearing facies comprises dominant coarse to conglomeratic sandstones and rooted siltstones and claystones. Minor constituents are fossiliferous limestones, carbonaceous shales, and coals. A thick, widespread coal bed (Felix coal) ranges from 10 to 28 ft (3 to 8.5 m) thick with n a 400 mi2 (1,035 km2) area and splits outward from this area into several beds. Where the coal is thick, it is underlain by sandstones and the coal splits are underlain by finer grained deposits. The conglomeratic facies represents wet alluvial-fan deposits consisting of graded gravel bars, channel sands, and finer overbank detritus. These sediments grade eastward into the coal-bearing facies that represents deposits of meandering streams and their adjoining flood plain and backswamp. The locations of the thickest, most widespread coal body and its splits in this facies are governed by depositional topography controlled by differential compaction of the substrate. Where the substrate is poorly compactible channel sandstones, the swamp surface was relatively high and free of sediment influx. Where the underlying deposits are fine grained and more compactible, the resulting low-lying swamp attracted water-borne sediments that interrupted peat accumulation. End_of_Article - Last_Page 953------------

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------------