Rip-up Clasts Research Articles

Stratiform tourmalinites in metamorphic terranes and their geologic significance

Stratiform tourmalinites are significant minor rock types in many regional metamorphic terranes of the world. Tourmalinites are more widespread than previously recognized and are especially common in Proterozoic and early Paleozoic sequences dominated by clastic metasedimentary rocks. They consist of conformable layers made up primarily of quartz and abundant tourmaline, the latter typically exceeding 15% to 20% of rock volumes. A few tourmalinites display striking sedimentary structures such as graded bedding, cross-laminations, slump and flame structures, and rip-up clasts. These and other geologic features provide important constraints for assessing the origin of these rocks, and they suggest that tourmalinites form by the early diagenetic modification of a primary boron-rich chemical precipitate. Tourmalinites are significant in preserving a valuable record of unusual chemical and paleogeographic conditions in clastic sedimentary basins, and in their close association with a variety of stratabound mineral deposits.

Petrography, Diagenesis, and Depositional Setting of Glenn (Bartlesville) Sandstone, Berryhill Unit, Glenn Pool, Oklahoma: ABSTRACT

Petrography and physical stratigraphy of the (Bartlesville) sandstone in the 160-acre Berryhill unit, Glenn Pool, were established from 10 cores and more than 60 modern well logs. The reservoir mostly is sublitharenite to litharenite; lithic constituents chiefly are metamorphic rocks and rip-up clasts. Principal diagenetic minerals are kaolinite, chlorite, and illite. This evidence and data from the regional and local stratigraphic framework indicate that sands were upper delta-plain deposits. Logs of the closely spaced wells show moderately complex short-distance change in geometry of the sandstone and attendant reservoir heterogeneity. All this information is integral in ongoing plans for enhanced recovery and in current research on enhancement of well logs by s gnal processing. End_of_Article - Last_Page 498------------

Sedimentary Petrology and Depositional Environment of Sandstones of Scow Bay, Indian and Marrowstone Islands, Northwestern Washington: ABSTRACT

The unnamed middle Eocene sandstones of Scow Bay that are well exposed on the beaches of Indian and Marrowstone Islands, northwestern Washington, document a previously unknown local basement high that shed sediment south onto a subsea fan from the San Juan Islands terranes. The sandstones are lithic arenites with a variety of lithic grains, predominantly volcanic and sedimentary (including chert). Rare quartz-plagioclase plutonic and low-grade metamorphic (mainly chlorite-rich) lithic grains are also present. Quartz and plagioclase comprise most of the nonlithic grains. Rare heavy minerals and potassium feldspar are also present. The San Juan Islands terranes were the source area. Paleocurrent directions obtained from flute casts support this interpretation. The sandstones are thin to very thick-bedded with minor shale interbeds and at least two 20 to 30-m (65 to 100-ft) thick shale beds. The sandstone beds are commonly structureless, although dish structures, poorly developed parallel lamination, load casts, and shale rip-up clasts are abundant locally. Soft sediment deformation, including slumps, is locally evident. Amalgamation of thinner sandstone beds into very thick (3-5 m, 10-16 ft) beds is common but often difficult to recognize owing to their structureless nature. The exposure along the east coast of Indian Island reveals at least 11 thinning- and fining-upward sequences. The sandstones were deposited as channel-fill sequences on the midfan region of a subsea fan. The thick shale beds were deposited between active channels. End_of_Article - Last_Page 506------------

Reservoir Characterization of Mesaverde (Campanian) Bedload Fluvial Meanderbelt Sandstones, Northwestern Colorado: ABSTRACT

Reservoir characterization of Mesaverde meanderbelt sandstones is used to determine directional continuity of permeable zones. A 500-m (1,600-ft) wide fluvial meanderbelt in the Mesaverde Group is exposed as laterally continuous 3-10-m (10-33-ft) high sandstone cliffs north of Rangely, Colorado. Forty-eight detailed measured sections through 3 point bar complexes oriented at right angles to the long axis of deposition and 1 complex oriented parallel to deposition were prepared. Sections were tied together by detailed sketches delineating and tracing major bounding surfaces such as scours and clay drapes. These complexes contain 3 to 8 multilateral sandstone packages separated by 5-20 cm (2-8 in.) interbedded siltstone and shale beds. Component facies are point bars, crevasse splays, chute bars, and floodplain/overbank deposits. Two types of lateral accretion surfaces are recognized in the point bar facies. (1) Gently dipping lateral accretions containing fining-upward sandstone packages. Large scale trough cross-bedding at the base grades upward into ripples and plane beds. (2) Steeply dipping lateral accretion surfaces enclose beds characterized by climbing ripple cross laminations. Bounding surfaces draped by shale lags can seal vertically stacked point bars from reservoir communication. Scoured boundaries allow communication in some stacked point bars. Crevasse splays showing climbing ripples form tongues of very fine-grained sandstone which flank point bars. Chute channels commonly cut upper point bar surfaces at their downstream end. Chute facies are upward-fining with small scale troughs and common dewatering structures. Siltstones and shales underlie the point bar complexes and completely encase the meanderbelt system. Bounding surfaces at the base of the complexes are erosional and contain large shale rip-up clasts. End_of_Article - Last_Page 493------------

Red Bluff, Marion County, Mississippi: A Citronelle Braided-Stream Deposit: ABSTRACT

Red Bluff is an erosional escarpment located on the western margin of the Pearl River flood plain in northwestern Marion County, Mississippi. The bluff shows approximately 30 m (100 ft) of relief and is composed of alternating units of red-to-yellow sand and sandy gravel. The sand grains are composed primarily of quartz, with small amounts of heavy minerals and feldspar. The gravel is composed of varying percentages of chert, flint, jasper, rip-up clasts, quartz, and tripoli, including a small fraction of silicified Paleozoic fossils. Grain-size analysis of the sediment and investigation of the sedimentary structures suggest a braided-fluvial environment of deposition. Particle sizes in the medium sand to pebble range predominate in all units; very little silt and clay is present. The largest particles present are boulder-size rip-up clasts. The most conspicuous sedimentary structures at Red Bluff are graded bedding, low-angle to medium-angle cross-bedding, and well-developed paleochannels. A statistical comparison (discriminant analysis) of the seven most abundant heavy minerals of Red Bluff, with the same suite of heavy minerals found at the type section of the Citronelle Formation (Pliocene-Pleistocene), and outcrops of a known Miocene coarse clastic unit indicates a correlation of Red Bluff to the Citronelle Formation. These heavy minerals are kyanite, staurolite, rutile, tourmaline, zircon, black opaques (primarily ilmenite and magnetite), and white opaques (primarily leucoxene). The suite of heavy minerals present at Red Bluff belongs to the east Gulf province. This metamorphic assemblage of heavy minerals implies the source area of the sediments at Red Bluff to be the southern Appalachians. The silicified pebble-size Devonian-Mississippian fossils were derived most likely from formations flanking the southern Appalachians in northern Alabama. End_of_Article - Last_Page 1472------------

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

Possible Tectonic Influence on and Facies Distribution of Shannon Ridge Sandstones, Wyoming: ABSTRACT

In the Powder River basin of Wyoming, Upper Cretaceous Shannon Sandstone shelf sand ridges are formed along the crest of a broad, subtle, southwest to northeast-trending paleoarch. During Shannon deposition, relief on the arch was great enough to alter shelf energies and cause sand ridges to develop within a predominantly silty shale interval. Possible recurrent movement in the Salt Creek anticline created a paleohigh which strongly localized development of thick sand ridge complexes in the Shannon Sandstone. During Shannon deposition, relief on the paleohigh apparently was strong enough to cause ridges to build laterally as well as vertically. Shannon ridge complexes at Salt Creek are more oblate, bigger, thicker, and more closely spaced than most central Powder River basin ridges. Also, there are two vertically stacked ridge systems developed within the Shannon Sandstone. While the lower ridge system is coeval with the Shannon ridge system in the central basin, the upper ridge system is only developed locally and, we believe, is related to active growth on the paleohigh during Shannon deposition. At no time, however, did he paleohigh cause ridges to be subaerially exposed. Eleven Shannon shelf ridge and ridge-associated facies were defined in outcrops on the Salt Creek anticline. Vertical and lateral changes in facies are relatively abrupt where observed in closely spaced outcrop sections and, in general, facies are stacked in coarsening-upward sequences with central bar facies commonly immediately overlying interbar sandstone facies. Porous and permeable potential reservoir facies include: central bar facies, a clean, cross-bedded sandstone; bar margin facies (Type 1), a highly glauconitic, cross-bedded sandstone containing abundant shale and limonite (after siderite) rip-up clasts and lenses; and bar margin facies (Type 2), a cross-bedded to rippled sandstone. These facies were formed by sediment transported and deposited in the form of medium to larg -scale troughs and sand waves on and across the tops of ridges by moderate to high-energy shelf currents. End_of_Article - Last_Page 1358------------

Depositional Systems in Naknek Formation, Late Jurassic, Lower Cook Inlet, Alaska: ABSTRACT

The Naknek Formation is a thick wedge-shaped unit of Late Jurassic age. It accumulated along the western edge of a Jurassic fore-arc basin. The formation was examined by BP Alaska geologists along the western shore of Lower Cook Inlet between Chisik Island and Cape Douglas. Correlation of stratigraphic sections has identified three sandstone/conglomerate sequences separated by two siltstone units. The conglomerate units are thick, lenticular, and occur as both organized and disorganized conglomerate facies. The sandstone units exhibit the following sedimentary structures and bedding types: (1) massive, graded, and amalgamated beds; (2) thickening and coarsening-upward sequences; (3) slump structures; (4) load and flute casts; (5) soft-sediment faulting; (6) rip-up clasts; (7) ripple lamination; and (8) minor cross-bedding. The siltstone units are parallel laminated and have thin sandstone beds with partial and complete density current sequences. Bioturbation is not common although some grazing burrows are present. Ammonites, belemnites, plant debris, and radiolarians are the most common fossils, although forams, gastropods, and pelecypods (dominantly Buchia sp.) are also present. Previous workers have described the Naknet Formation as a shallow-marine deposit, but did not identify specific sedimentary facies. This study indicates that the Naknek Formation was deposited in moderate to deep water as a submarine fan complex. End_of_Article - Last_Page 551------------

Carbonate Cementation of Gulf Coast Barrier-Island Sands and Formation of a Stratigraphic Trap: ABSTRACT

Field investigations in the area of Grand Isle, Louisiana, indicate the presence of active carbonate precipitation. Modern shoreline facies are being cemented by high-magnesium calcite, ranging from 10 to 50 mole % magnesium carbonate. Cementation occurs between the beach and marsh environments as surface crusts or laminae. The environment is characterized by seasonal high temperature (up to 50°C), high salinity (40 to 90^pmil), supratidal stromatolites and organic decomposition. After burial, lithified crusts undergo chemical alteration, as indicated by a decrease in magnesium carbonate content (20 to 35 mole %). The cement eventually undergoes dissolution and reprecipitation, forming a well-cemented sandstone. Rip-up clasts of these sandstones are found along Gulf Coast beaches where a transgressive sequence occurs. These clasts are typically cemented by high-magnesium calcite with 10 to 15 mole % magnesium carbonate. Carbon isotope analyses of rip-up clasts indicate that dissolution and reprecipitation processes are methane-derived due to organic decomposition. Buried crusts may be preserved in the subsurface and form an effective barrier to fluid migration. Where cementation is extensive, porosity is occluded on the marsh-side of the barrier-island sequence. Cement distribution is, therefore, a primary controlling agent of hydrocarbon migration and subsequent entrapment. Migration of hydrocarbons from organic-rich marsh sediments would be prevented by the early formation of the permeability barrier. Cementation, then, would account for accumulation of hydrocarbons on the marsh side of the barrier-island sequence and the formation of a stratigraphic (or diagenetic) trap. End_of_Article - Last_Page 245------------

Provenance and Depositional Mode of Upper Cretaceous Chatsworth Formation Conglomerates, Simi Hills, California: ABSTRACT

Conglomerates of the Upper Cretaceous Chatsworth Formation occur as lenses of concentrated clasts in channels and as clasts dispersed at the base of thick, coarse-grained, graded sandstone beds. The matrix of the conglomerates consists of grains ranging from silt to granule size (4 mm) and comprises between 20 and 78% of any one conglomerate unit. The matrix composition ranges from 30 to 80% quartz, 25 to 60% feldspar, and 5 to 20% lithic fragments, with accessory biotite up to 5%. The conglomerate clast population is composed principally of clasts in the pebble size range. Five distinct rock types are recognized within the conglomerate clast population: blue-black argillite, 4 to 20%; felsic volcanites, 4 to 28%; felsic plutonites, 16 to 24%; arkosic sandstones and silts one, 0 to 12%; and a group of genetically related quartz-rich clasts, 17 to 46%. The quartz-rich clasts include sandstones and siltstones with continuous textural gradation from well-preserved sandstone through partially recrystallized sandstone with sutured grains, into metamorphosed, foliated quartz sandstone and quartz schist types. In addition, a conglomerate unit may contain between 0 and 14% authigenic rip-up clasts. The Chatsworth Formation, as a whole, is recognized to be a deep-sea fan complex upon which the primary depositional agent for sand was turbidity currents. Lenses of concentrated pebble conglomerates originated as debris flow, whereas beds of dispersed pebble clasts are of turbidity current origin. Paleocurrent data and the conglomerate clast composition for the Chatsworth Formation indicate that its detritus was derived from a source terrane to the south of the Simi Hills. The Santa Monica Mountains basement complex contains a large mass of argillite and felsic plutonite, but contains no felsic volcanite or quartz-rich suite of rocks. The basement in the northern Peninsular Ranges includes representatives of the principal rock types recognized in the clasts of the Chatsworth Formation conglomerates and, therefore, it is the best possible choice for the provenance. Extensive Franciscan terrane also lies south of the Chatsworth conglomerates, but no Franciscan detritus is recognized in the Chatsworth Formation. End_of_Article - Last_Page 1686------------

Cherts in Wishart Formation (Aphebian) of Labrador: Example of Rapid Shallow-Water Silica Sedimentation: ABSTRACT

The Wishart Formation of the Labrador trough (early Proterozoic) End_Page 992------------------------------ is a high-energy, terrigenous shelf deposit. Chert is a minor but widespread component of the Wishart and occurs in three main forms: (1) interstitial cements consisting of a mosaic of fine quartz crystals; (2) thin layers and lenses of very fine-grained chert lutite with finely disseminated impurities; and (3) peloids to flat pebbles of fine-grained chert lutite. The clasts of chert lutite are clearly intraformational; they are closely associated with and texturally identical to the in-situ chert layers. Chert cements are also found in intraclasts. All of the chert types are restricted to a few thin intervals which form good marker beds, and none show any signs of replacement. These observations indicate the cherts are primary siliceous deposits. Based on their textures and the sedim ntary structures in which they are found, the cements must have formed as rigid precipitates; the lutites as mud-like, slack-water accumulations; and the peloids to pebbles as rip-up clasts. The silica was probably hydrothermal in origin because the stratigraphic distribution of the chert is independent of facies and there is no evidence of basin restriction or evaporative conditions. The Fleming Formation, which lies directly beneath the Wishart, may have been the source of the postulated silica-rich hydrothermal waters. The Fleming consists mostly of brecciated and silicified rocks and an abundance of crystalline, void-filling quartz. The Wishart cherts offer proof that siliceous sediments and cements can accumulate rapidly in terrigenous marine environments. They provide an example of one mechanism for making the similar chert peloids which are abundant in early Proterozoic iron-formations. End_of_Article - Last_Page 993------------

The Orientation of Small Muddy Clasts in Turbidites: Paleocurrent Indicators on a Jurassic Carbonate Slope, Tunisia

The Orientation of Small Muddy Clasts in Turbidites: Paleocurrent Indicators on a Jurassic Carbonate Slope, Tunisia

Depositional Environment of Poison Canyon Sandstone in Gulf Mariano Lake Mine: ABSTRACT

A study of the depositional environment of the Poison Canyon Sandstone (Brushy Basin Member of the Upper Jurassic Morrison Formation) in the Mariano mine was undertaken to determine the relation of uranium occurrence to the depositional environment. An analysis of the sedimentary structures, paleocurrent measurements, isopach data, and electric-log shapes has led to the interpretation that the Poison Canyon Sandstone in the Mariano Lake mine vicinity is a sequence of bed-load deposits of a fluvial system. Paleocurrent measurements taken in the mine and at three outcrops of the Poison Canyon confirm findings from earlier work that indicated eastward current flow. The dominance of tabular cross-beds, channel scours, and trough cross-beds, the geometry of the Poison Canyon s ndstones, and the chaotic sedimentary structures corroborate deposition in a braided-stream environment. The Poison Canyon sandstones may be subdivided into two lithofacies. A cross-bedded unit is characterized by large-scale tabular cross-beds, scour surfaces, medium to coarse grain size, poor sorting, angular to subangular grains, an abundance of feldspar grains, and clay rip-up clasts. A second unit is dominated by laminar bedding, fine grain size, moderate sorting, subrounded grains, and an abundance of quartz. The source area for the sands had been dominated by plutonic rocks. These facies cannot be separated on the basis of electric-log shape. Uranium ore occurs in both facies but is more abundant in the cross-bedded unit. The complexities of the braided-stream system greatly complicate any study of the interrelation of the uranium and the depositional environment. Obvious conclusions are that the grain size does not greatly influence the emplacement of the uranium, that the orebody at the Mariano mine is epigenetic, and that the sheetlike nature of the Poison Canyon sandstones suggests that the limiting factors on the ore boundaries are geochemical rather than stratigraphic. End_of_Article - Last_Page 690------------

Eocene Conglomerate Sedimentology and Basin Analysis, San Diego and the Southern California Borderland

ABSTRACT Eocene conglomeratic strata in the San Diego area were deposited as a narrow, west-trending progradational system that changed facies from fluvial channel, to alluvial fan, to coastal plain-fan delta, to paralic, to shelf and subsea channels. Continuing this system westward are the large Eocene subsea fan deposits that include inner, middle, and outer subfan and basin plain facies of the southern California borderland. In all environments, 81 to 96 percent of the conglomerate clasts are distinctive, well-rounded, siliceous metavolcanic stones (Poway clasts) that reach 60 cm in size and average 6 cm; 2 to 13 percent are quartzite; and 0 to 12 percent are locally derived (Peninsular Ranges) crystalline basement (granitic and volcanic) rocks. The composition, shape, and average size of t e metavolcanic and quartzite clasts are constant in all environments, while the less resistant granitic clasts show the effects of progressive abrasion by a decrease in size and abundance with increased transport distances. Three major sedimentologic trends characterize the Eocene conglomerate. (1) Conglomerate uniformly decreases, and finer grained rocks increase, in abundance and thickness from east to west. (2) The nonmarine and shallow marine conglomerate facies each contain disorganized and organized beds with no grading. Organized conglomerates are framework supported, and have imbricated clasts that dip up-current. The long axes of the clasts are oriented perpendicular or parallel to the paleoflow or are distributed randomly. Some nonmarine sandstone beds contain caliche horizons. (3) The subsea fan conglomerate is both disorganized and organized, and the beds are graded, or inversely graded. The clasts are matrix-supported, imbricated, and dip up-current. Long axes of clasts are oriented both par llel to and perpendicular to paleocurrent trends, and angular mudstone rip-up clasts and reworked marine fossils are present.

Tourmaline concentrations in Proterozoic sediments of the southern Cordillera of Canada and their economic significance

Local concentrations of tourmaline occur in the lower Proterozoic (Helikian) Aldridge Formation of southeastern British Columbia, in some places in association with stratiform lead–zinc mineralization as at the Sullivan, Stemwinder, and North Star orebodies. The amount of boron in the rock is as much as two orders of magnitude above average levels reported for the Aldridge Formation or other similar types of sedimentary rocks. The concentrations are not detrital, but are caused by an anomalously high boron level, in a local area, at the time of sedimentation. The appearance of tourmaline within rip-up clasts, and in laminae within pebbles, is evidence of syngenetic introduction of boron.Three populations of tourmalines, on the basis of composition as determined by microprobe analyses, are described from the area:(1) A Proterozoic stock intruding the Aldridge Formation contains abundant schorl.(2) A tourmaline intermediate in composition between dravite and schorl is typical of Aldridge metasediments. Texturally this type occurs as (a) fine felted aggregates in the footwall of the Sullivan orebody, (b) disseminated through waste beds, and (c) in local concentrations removed from the Sullivan orebody.(3) Coarse-grained recrystallized tourmaline associated with the Sullivan ore is magnesium-rich. Recrystallization is erratic, and is probably related to uneven heat flow during metamorphism and to differences in bulk composition.The economic importance of tourmaline concentrations in the Aldridge Formation is their association in both space and time with stratiform sulfides.

Stress State And Hydraulic Fracturing In the Athabasca Oil Sands

Abstract The geological history of the Athabasca Oil Sand (with respect to historical overburden or glacial load) suggests that the minor principal compressive stress is vertical to a depth of at least 350m. Features of natural slopes, hydraulic fracturing operations, and computer stress-strain modeling confirm the existence of higher lateral stresses in the Athabasca Oil Sand, Hydraulic fracture planes are created normal to the minor principal compressive stress direction, and are therefore horizontal above a depth of 850m, and probably vertical below a depth of 850m, Change of orientation of the minor principal stress may be brought about by fracturing operations, and in turn, this may result in an ability to control inter-well communication in areas of vertical hydraulic fracturing. INTRODUCTION Development of the Athabasca Oil Sands deposit, and other similar deposits in Alberta, eventually will require hydraulic fracturing or the opening of underground cavities. A crucial design parameter for both of these procedures is the orientation and magnitudes of the principal compressive stresses. Complete stress state data in oil sands are not available at present, but methods have been developed (e.g, Haimson, 1973) to assess the state of stress at depth by hydraulic fracturing. Because of the extremely low permeabilities of the oil sand, these methods may be successful in the oil sands despite their porous, uncemented nature. Qualitative data on the state of stress are available from natural slope observations, hydraulic fracturing data, and computer slope monitoring, Quantitative data may be obtained by computer back-analysis of prototype tunnels, and by carefully controlled hydraulic fracturing. GEOLOGICAL HISTORY OF THE OIL SANDS The Athabasca Oil Sands are contained largely (95 %) within the McMurray Formation. The McMurray Formation is of Lower Cretaceous age, is largely orthoquartzitic, and was deposited upon an undulating paleoerosion surface during regional transgression of an epicontinental sea (e,g, Carrigy, 1973). The McMurray Formation has many similarities to other "blanket sands" (Krumbein and Sloss,1963); for example, an argillaceous basal sequence, overlain by a conglomeratic channel facies, a general quartzose nature, a fining-upwards of grain size, and a distinct upwards progression to more marine facies. The basal argillaceous strata have been removed locally by erosion, but underlie at least 50% of the deposit in the surface-mineable area (Dusseault, 1977). The strata usually consist of argillaceous beds with associated lignites, occasional arenaceous strata, and zones of siderite- and pyrite-cemented materials. Paleosols have been observed upon paJeotopographic highs. The argillaceous strata, commonly called the "basal clays", have been noted in thicknesses of up to 30 m (Scotland and Benthin, 1954) and have a distinctive mineralogy. Illite, kaolinite, vermiculite and mixed-layer clays comprise the clay-mineral portion of the -2fJ- fraction (Dusseault, 1977). Smectite and chlorite are absent although smectite has been identified at higher elevations within the McMurray Formation (Carrigy, 1973). Oil-bearing strata may be found directly on and somewhat interspersed with the basal deposits. The lower beds are quartz-pebble conglomerates to coarse-grained sands, often with chert pebbles and rip-up clasts.

Petrology and Sedimentation of Early Paleozoic Island-Arc Deposits, Newfoundland: ABSTRACT

The Cambro-Ordovician New Bay Formation (2,000 m) of the Newfoundland Appalachians is a southward-thickening prism of conglomeratic, volcanogenic turbidites interbedded with sandstone and black argillite. Abundant sedimentary structures include tool and scour marks, cross-lamination, normal and multiple graded bedding, slump folds, and channels. The top of the formation is mostly agglomerate interbedded with red-and-green laminated silty argillite. Trails and burrows are the only fossils. The sandstones are lithic-feldspathic arenites and basaltic wackes composed of fresh, angular unstable mineral grains and little quartz. The conglomerates have volcanic and sedimentary clasts enclosed in an ashy matrix altered to chlorite. Volcanic clasts are basalt, andesite, and dacite porphyry. Sedimentary clasts include green cherty siltite, fine-grained feldspathic arenite, abundant rip-up clasts of argillite, and rare jasper. Analysis of Bouma sequences, amalgamated contacts, and trace fossils indicates abrupt regression late in New Bay deposition. Sole marks, slump folds, and channel axes indicate marginal sediment supply from turbidity currents and slumping down an east-facing submarine slope. On the northwest the formation thins and apparently passes into marine and subaerial pyroclastic strata of the Wild Bight Group. On the east it is in sedimentary and tectonic contact with the Dunnage melange which has been interpreted as an ocean trench deposit. The petrology and sedimentology of the New Bay Formation compares closely with certain Miocene volcanic arc deposits described from Malekula Island, New Hebrides. Therefore the New Bay is interpreted as a volcanogenic apron deposited in the arc-trench gap of a Cambro-Ordovician island arc. End_of_Article - Last_Page 779------------

Sedimentation and Tectonics in Early Tertiary Continental Borderland of Central California: ABSTRACT

The most prominent Eocene paleogeographic elements of central California were the Sierran continental landmass on the east and the Salinian continental borderland on the west, both underlain by granitic crust and separated by a trough-shaped, deep-marine area underlain by oceanic crust. These linear elements trended approximately north-south, parallel with the edge of the continent. Erosion of the Sierran area yielded clastic detritus that was transported westward and deposited in successive fluvial, near-shore, shallow-marine, and deep-marine environments. The Salinian borderland, which probably had been detached from the Sierran landmass by pre-Eocene right-lateral faulting along its eastern edge, consisted of irregular island uplands interspersed with deep marine basin . Detritus eroded from these islands was deposited as submarine fans within the borderland basins and on the sea floor on the east and west. Microfossils indicate deposition in basins of bathyal to abyssal depths and unrestricted access to the ocean. The early Tertiary submarine-fan deposits are similar throughout the central Coast Ranges, and contain assemblages of proximal and distal turbidites, deep-sea conglomerates, grain-flow deposits, and hemipelagic shales. They differ from most modern deep-sea fan deposits and typical flysch sequences in that they are very thick, limited in areal extent, and consist mostly of coarse clastic detritus. The conglomerates contain coarse clasts, including boulders, and have very thick, irregular bedding. Proximal sandstones consist of Bouma ae to abc e sequences that are very thickly bedded and amalgamated, poorly graded to ungraded, commonly structureless, and contain large mudstone rip-up clasts and dish structures. Distal sandstones contain more complete Bouma sequences, a greater variety of sedimentary structures, and are graded. The borderland region was tectonically active during sedimentation, so that island source areas and depositional basins formed and changed abruptly. Submarine fan sedimentation was rapid, but the supply of sediment was probably not continuous for long periods of time. Paleocurrent directions are varied, reflecting the irregular distribution of source areas and depositional basins. End_of_Article - Last_Page 797------------

SEDIMENTARY HISTORY OF THE GILWOOD SANDSTONE (DEVONIAN) UTIKUMA LAKE AREA, ALBERTA, CANADA

ABSTRACT The Gilwood sandstone (Devonian), a member of the Watt Mountain Shale, is one of several sandstone units which were deposited on the flank of the Peace River Arch where it now forms an excellent stratigraphic trap containing hydrocarbons. The sandstone has an average composition of 75 per cent quartz, 16 per cent microcline, 1 per cent plagioclase, 1 per cent biotite, 15 per cent cementing material and an average porosity of 10 per cent. It exhibits several varieties of sedimentary structures such as cross-bedding, both trough and tabular, ripple drift cross-laminations, graded-bedding, rip-up clasts, laminations, convolute laminations and other minor structures. Grain size distribution of the sandstone is largely unimodal, except in certain places where it is bimodal. Most samples are positively skewed and moderate to poorly sorted. Paleocurrent analysis, texture of the sand and its geometry suggest that the Gilwood sandstone is, largely, a deltaic deposit derived from the west and modified slightly by long shore currents flowing from the north-northeast. The roundness of grains, suturing in quartz particles, high concentration of quartz and the remarkably low concentration of heavy minerals suggest that the sand is at least a second cycle deposit, indicating that the Peace River landmass had a sedimentary cover during Middle Devonian. Depositional environments, as inferred from texture and structures of the Gilwood sandstone, are divided into three realms: fluvial, transitional and nearshore marine. Several types of sedimentary structures are characteristic of each realm.