Siltstone Clasts Research Articles

The Steepbank Formation: a paleokarst diamictite deposit in the Athabasca Oil Sands region of northeastern Alberta, Canada

Abstract The name “Steepbank Formation” is proposed for a paleokarst diamictite deposit that is present along the margins of the Middle Devonian Prairie Evaporite Formation in the Western Canada and Williston sedimentary basins, including the eastern portion of the Athabasca Oil Sands region. This poorly lithified diamictite forms a mappable unit that is distinct in both age and lithology from all contiguous formations. The Steepbank consists of clasts of dolostone, limestone, and siltstone up to the size of boulders with a matrix of silty to sandy calcareous mudstone. The material shows little or no evidence of bedding or sorting. Most clasts are angular and their orientation is commonly random. The diamictite formed in response to the dissolution of thick (up to 300 m) sequences of halite, anhydrite, and gypsum in the Prairie Evaporite Formation and the subsequent failure and collapse of interbedded and overlying insoluble strata. The top contact occurs where the intact strata of an overlying formation can be identified and is commonly gradational. The basal contact with the underlying Keg River Formation is sharp. Both contacts are unconformable. Evaporite dissolution and diamicton deposition likely began in late Middle Devonian time, moving down dip toward the west, and are continuing today near the Athabasca River.

Geometry and lithofacies of coarse-grained injectites and extrudites in a late Pliocene trench-slope basin on the southern Boso Peninsula, Japan

This study investigates the geometry and internal structures of coarse- to very coarse-grained volcanic sandstones and volcanic breccias with many siltstone clasts (interpreted to be sill-like injectites and extrudites) occurring in an upper Pliocene trench-slope basin succession on the southern Boso Peninsula, Japan. The injectites occur in the uppermost Shiramazu Formation, and pinch out laterally into siltstone-dominated deposits of the Mera Formation. Their thicknesses vary from a few centimeters to ~2m. The basal and upper contacts of the injectites with host muddy deposits are sharp and/or erosional, and are locally discordant with the bedding of the host deposits. Siltstone clasts, which were ripped up or ripped down from the host muddy deposits, are commonly incorporated into the injectites, although some siltstone clasts have geological ages older than those of the host deposits. Seven lithofacies have been identified in the injectites based on the internal structures. The combinations of internal structures are different from those of high-density turbidity current deposits and debrites, and suggest that injection was promoted by a combination of turbulent and laminar flow conditions.The extrudites show an overall convex-up geometry and possess lithological features similar to those of the injectites. They have been identified in the Rendaiji Conglomerate Member, which is encased in the Mera Formation, and which rests on the uppermost Shiramazu Formation. The extrudites are characterized by gently undulating waveforms that show upstream migration and climbing stacking patterns similar to the cross-sectional geometry of cyclic steps or upstream-migrating antidunes. The active eruption of solid–liquid mixtures onto the seafloor and sedimentary piles may have subsequently collapsed to produce supercritical high-density gravity currents down the flanks of a neptunian volcano.The injectites and extrudites locally contain Calyptogena shells and shell fragments, as well as fragments of carbonate concretions that exhibit low carbon isotopic ratios, suggesting that fluidization of the source sediments was triggered by a combination of seepage of cold methane-bearing water into the source sediments and seismic shaking. Volcaniclastic deposits older than the host muddy deposits are present in the trench-slope basin deposits and these are the likely source of the injectites and extrudites.

Downfan Transformation from Turbidity Currents to Debris Flows at a Channel-to-Lobe Transitional Zone: The Lower Pleistocene Otadai Formation, Boso Peninsula, Japan

Transformation of turbidity currents into debris flows across a channel-to-lobe transition zone was recognized on the basis of lateral mapping of a falling-stage-systems-tract deposit in the lower Pleistocene Otadai Formation submarine-fan sequence on the Boso Peninsula, Japan. The most proximal outcrops comprise 1 to 4 m thick turbidites, which are laterally equivalent to debrites encased in turbidites in outcrops farther down flow. Such turbidites and encased debrites were deposited from single flow events. The most distal outcrops comprise only thinner (2 to 40 cm thick) turbidites. Debrites contain many siltstone clasts and finer-grained sediments, which are characterized by deep-water faunas and clay-mineral composition similar to those of interbedded hemipelagites. Therefore, the transformation of turbidity currents into debris flows is interpreted to have occurred in response to the incorporation of many siltstone clasts and finer-grained sediment particles into the precursor turbidity currents through erosion of muddy substrates. Subsequently, turbulence in the precursor turbidity currents was suppressed and near-bed flow with higher sediment concentration developed. Intense erosion of muddy substrates is interpreted to have occurred in response to the increase in intensity of turbulence in the precursor turbidity currents at the mouths of middle-fan channels. The precursor turbidity currents (or turbidity currents generated by dilution of debris flows) produced a turbidite deposit, which was subsequently overrun by the debris flow in a proximal lobe environment. The debris flows are interpreted to have subsequently been transformed into turbidity currents, which are recorded by turbidite deposition in the more distal-lobe environment. Debrites can characterize the proximal portion of an attached-lobe deposit in a channel-to-lobe transition zone. Such debrites can be significant heterogeneities for fluid flows in sand-prone successions from middle-fan channel to attached-lobe deposits.

Lithofacies characterization and channel development in the outcrops of cretaceous sedimentary rocks, Dahomey basin, southwestern Nigeria.

Outcrop exposures of Cretaceous sedimentary rocks in Benin Basin southwestern Nigeria, allow the recognition of slope related sediments and slope valleys. Detailed outcrop logging permits the delineation of massive to cross bedded loose sand with lateral facies changes, normally graded sandstone, clay with injected sand, clayey silt with abundant clasts, channel complexes associated with different lithologies, loose sand, very fine to coarse grained sandstone and conglomerate with clasts of sandstone and siltstone. These lithofacies revealed history of erosion and deposition in various proportions. Four sets of channel development were delineated; the initial valley cut, complex reincision and fill, regional erosion and channel abandonment. These four categories of channel development are related to significant erosion and are possibly related to periods of lowstands. The outcrop exposures show that the first set of channel infill probably commenced with deposition of turbidites accompanied by marine transgression associated with contemporaneous slumping. This progress vertically to amalgamated (network of channels) channels filled with diverse lithologic units while the topmost channel is filled with channel lag and conglomerate. Channel infills are composed essentially of sediments derived from turbidites, slumps and debris flows. Two types of architectural elements recognized are single-story channel and multistory channel complex. The different stages of channel development can be considered in terms of low efficiency and high efficiency flows, which are related to slope equilibrium. Keywords: Outcrop, slope, channel development, channel architecture, process model, lowstandJournal of Mining and Geology Vol. 43 (2) 2007: pp. 131-146

Zircons and clay from morainal Permian siltstone at Mt Rymill (73°S, 66°E), Prince Charles Mountains, Antarctica, reflect the ancestral Gamburtsev Subglacial Mountains–Vostok Subglacial Highlands complex

Clasts of red siltstone with Glossopteris from moraine around Mt Rymill in the southern Prince Charles Mountains, East Antarctica, can be traced upslope in the Lambert Graben system to the nearby Gamburtsev Subglacial Mountains (GSM). The clasts contain zircons with SHRIMP U–Pb ages of 620–460 Ma and 1300–970 Ma from host rocks of intermediate to mafic rocks, and a clay fraction with a T DM Nd model age of 2.72 Ga and εNd of − 18.3, derived from upper continental crust. East of the GSM, at Lake Vostok, clasts of siltstone in accreted ice can be traced to the Vostok Subglacial Highlands (VSH). The clasts contain zircons and monazites with SHRIMP U–Pb ages [Leitchenkov, G.L., Belyatsky, B.V., Rodionov, N.V., Sergeev, S.A., 2007. Insight into the geology of the East Antarctic hinterland: a study of mineral inclusions from ice cores of the Lake Vostok borehole. In: Cooper, A.K., Raymond, C.R. (Eds.), Online Proceedings of the 10th ISAES, USGS Open-file Report 2007-1047, Short research Paper 014, 4 pages.] broadly similar to those from Mt Rymill, and a T DM Nd model age of 1.88 Ga and εNd of − 15 [Delmonte, B., Petit, J.R., Basile-Doelsch, I., Lipenkov, V., Maggi, V., 2004. First characterization and dating of East Antarctic bedrock inclusions from subglacial Lake Vostok accreted ice. Environmental Chemistry 1, 90–94.]. Other Mesozoic and Paleozoic sediments deposited in a radial pattern about central Antarctica contain detrital zircons dominated by populations aged 700–500 Ma and 1300–1000 Ma, which we interpret as reflecting a GSM-VSH provenance dominated by rocks generated during the Pan-Gondwanaland and Grenville events of supercontinental assembly.

Distribution patterns of siltstone clasts in turbidites and their relation to grain fabric of host turbidites

Siltstone clasts are commonly known from turbidites in deep-water successions and have been interpreted to document information regarding depositional processes of host sandstones. Here we studied formative processes of siltstone clasts on the basis of size and fabric of siltstone clasts and of host sandstones.Although siltstone clasts generally exhibit imbrication dipping to upslope directions regardless of depositional features of host sandstones, their size, density, and position in a turbidite bed are variable in response to vertical variations in grain size and grain fabric of host sediments. Therefore, the variation in size, density, and position of siltstone clasts in a single turbidite bed is interpreted to be controlled by temporal variations in the relative rates of suspended-load fallout during a single depositional process of sandstone beds from turbidity currents.

Stable isotope compositions of serpentinite seamounts in the Mariana forearc: Serpentinization processes, fluid sources and sulfur metasomatism

The Mariana and Izu–Bonin arcs in the western Pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge. We present stable isotope (O, H, S, and C) data for serpentinites from Conical seamount in the Mariana forearc and S isotope data for Torishima seamount in the Izu–Bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge, and to investigate the transport of sulfur from the slab to the mantle wedge. Six serpentine mineral separates have a restricted range of δ18O (6.5–8.5‰). Antigorite separates have δD values of − 29.5‰ to − 45.5‰ that reflect serpentinization within the mantle wedge whereas chrysotile has low δD values (− 51.8‰ to − 84.0‰) as the result of re-equilibration with fluids at low temperatures. Fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300–375 °C. Two late cross-fiber chrysotile veins have higher δ18O values of 8.9‰ to 10.8‰ and formed at lower temperatures (as low as ∼100 °C). Aqueous fluids in equilibrium with serpentine at 300–375 °C had δ18O = 6.5–9‰ and δD = − 4‰ to − 26‰, consistent with sediment dehydration reactions at temperatures < 200 °C in the subducting slab rather than a basaltic slab source. Three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have δ18O = 16.7–24.5‰, consistent with the serpentinizing fluids at temperatures < 250 °C. δ13C values of 0.1–2.5‰ suggest a source in subducting carbonate sediments. The δ34S values of sulfide in serpentinites on Conical Seamount (− 6.7‰ to 9.8‰) result from metasomatism through variable reduction of aqueous sulfate (δ34S = 14‰) derived from slab sediments. Despite sulfur metasomatism, serpentinites have low sulfur contents (generally < 164 ppm) that reflect the highly depleted nature of the mantle wedge. The serpentinites are mostly enriched in 34S (median δ34Ssulfide = 4.5‰), consistent with a 34S-enriched mantle wedge as inferred from arc lavas.

深海底堆積物研究の進展：低海水準期堆積モデルの再構築

A falling-stage deep-water succession is characterized by sandstone-dominated channel-and-overbank deposits and age-equivalent depositional-lobe deposits. In contrast, a lowstand deep-water succession is muddier and is characterized by a thickening-upward pattern of distal overbank deposits. The base of the falling-stage deposits is characterized by a distinct erosional surface compared with the base of the lowstand deposits, which is commonly represented by a gradational contact to the underlying falling-stage deposits.Deep-water channels commonly exhibit meandering patterns similar to fluvial channels, and the channel-fill deposits are encased in muddy overbank deposits. The distribution patterns and volumes of coarse-grained channel-fill deposits developed mainly during falling- and lowstand stages can be estimated from outcrop and seismic data on the basis of several empirical relationships among width, depth, and sinuosity of deep-water channels. Hyperpycnal flows have recently been interpreted to be one of the important mechanisms for developing thick to very thick-bedded, deep-water massive sandstones (DWMSs). DWMSs commonly exhibit sheet-like geometry and abruptly thin out and fine to the downslope direction. These abrupt changes in thickness and grain size of DWMSs are interpreted to be a result of deposition of a sufficient volume of sand particles from precursor hyperpycnal flows, which are subsequently transformed into buoyant plumes with finer-grained sediment particles associated with residual lower-density turbidity currents flowing to the further downslope direction. Modulation and transformation of turbidity currents into debris flows at a channel-to-lobe transition zone were recognized. The transformation of turbidity currents into debris flows is interpreted to have occurred in response to incorporation of many siltstone clasts and finer-grained sediment particles into the precursor turbidity currents from muddy substrates. The finding indicates that a laterally continuous sandstone body from middle-fan channel to depositional lobe deposits internally contains muddy baffles, which develop heterogeneity of fluid flows in reservoir sands and sandstones.

Inter‐tuff correlation and architecture within the Moor Cliffs Formation, Lower Old Red Sandstone, southwest Wales: a multi‐disciplinary approach for reservoir characterization

AbstractThe laterally continuous coastal exposures of the Moor Cliffs Formation of the Anglo‐Welsh Basin have allowed a highly detailed 2D reconstruction of the floodplain. Whole‐rock trace element geochemical and spectral gamma‐ray (SGR) correlation are combined with traditional sedimentological analysis allowing outcrop and subsurface techniques to be compared. Micaceous, quartz‐rich and well‐sorted fluvial channel sandstones form up to 10% of the thickness in this low net‐to‐gross succession. High Th/K ratios derived from SGR analysis distinguish sandstones from the surrounding fine‐grained sediments, but lateral facies variations prevent the correlation of fluvial channel intervals using SGR without outcrop control. Red‐brown micaceous siltstone and claystone constitute 65 to 95% of the outcrop thickness and contain abundant pedogenic features characteristic of modern Vertisols. The most mature Vertisol horizons and the non‐pedogenically altered siltstones/claystones do not show significant differences in their SGR response. Intraformational conglomerates comprising carbonate and siltstone clasts within a siltstone matrix are common above laterally extensive erosion surfaces, and can be clearly recognized from a sharp spike in Nb/Y and Zr/Y ratios. Major tuff horizons, which form chronostratigraphic marker horizons throughout the Moor Cliffs Formation, are also characterized by a distinctive geochemical signature, enabling calibration of the geochemical and sedimentological datasets. Geochemical and SGR analytical techniques are complementary and assist in enhancing the confidence of correlations created by traditional techniques in the Moor Cliffs Formation. Where multiple correlation scenarios have been interpreted due to a lack of diagnostic marker beds or biostratigraphic control, the SGR and geochemical data can increase confidence in a particular interpretation and hence decrease uncertainty in geological models of low net‐to‐gross systems in the subsurface. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Mud volcanoes in the Alboran Sea: evidence from micropaleontological and geophysical data

During the BASACALB-TTR9 cruise of the R/V Professor Logachev (1999), two mud volcanoes (called Marrakech and Granada) were discovered in the southern sector of the mud diapir province in the West Alboran Basin (WAB). This paper presents micropaleontological and geophysical data on these mud volcanoes from gravity core samples, sidescan sonar (OKEAN) images and high-resolution seismic lines. Mud breccia recovered from the Granada mud volcano is matrix-supported with well-consolidated clasts of limestone, marlstone, claystone, siltstone, sandstone and mudstone, whereas mud breccia from the Marrakech mud volcano contains unconsolidated clasts. The mud breccia matrix contains abundant Miocene calcareous nannofossils (CN), together with Pliocene–Pleistocene species and reworked late Cretaceous and Paleocene–Eocene species. CN dating indicates that clasts in the mud breccia derive from late Cretaceous, Paleocene, Eocene, and probable Miocene sediments. These data suggest that the mud diapirs and mud volcanoes in the WAB can be derived from the olistostromes of Unit VI, the basal stratigraphic sequence in the Alboran Sea basin. Unit VI consists of lower Miocene sediments that incorporated late Cretaceous and Paleocene–Eocene materials and basement-derived rock fragments. The mud volcanic deposits are covered by a thin drape of pelagic marls, suggesting that these two volcanoes are currently inactive. Structures determined on high-resolution seismic profiles across mud volcanoes and surrounding diapirs correspond to the late-stage, Pliocene-to-Quaternary diapir development. This stage is thought to have developed during a compressional tectonic setting that produced folding and wrench tectonics throughout the basin. Mud ascent at that time resulted in active diapirism and mud volcanoes on the seafloor.

A hot Miocene subaqueous scoria-flow deposit in the Shimane Peninsula, SW Japan

Abstract A subaqueous scoria-flow deposit occurs in the Miocene Takashibiyama Formation in SW Japan. Faunal assemblages in associated sediments suggest an outer shelf to upper bathyal environment, with water depth likely between 200 and 1500 m. The deposit, 5–7 m thick, is similar to subaerial scoria flow deposits, containing mainly non- to moderately vesiculated hyalopilitic basaltic andesite lapilli and coarse ash. The essential basaltic andesite clasts are variably vesiculated even within single clasts, and are commonly cracked with curviplanar surfaces, whereas the inner parts are cracked in a pattern similar to perlitic cracks. The clasts are blocky with curviplanar surfaces if non- to poorly vesiculated, and are fibrous with irregular shapes and flattened by compaction if more vesiculated. At one proximal location, fines-depleted, rootless pipes 5–90 cm wide and 1–3 m long occur, and spherical to amoebic vesicles 0.01–0.3 mm in diameter and filled with stilbite, quartz or greenish-brown clay are common in the fine-ash matrix and in siltstone clasts contained in the deposit. The vesicles represent open spaces once supported by gas, and the rootless pipes are interpreted as gas-escape pipes. Magnetization vectors of essential clasts are dispersed not at random but around a single direction, suggesting that the clasts gained thermoremanent magnetism mainly during cooling in the flow or during post-emplacement cooling and then rotated to some extent by differential movement of the flow or by gas-escape in the deposit. The vesicularity, shape and texture of essential clasts suggest a phreatomagmatic eruption, involving magmatic vesiculation and dynamic interaction of magma and ambient water. The perlitic cracks in the inside of the essential clasts and the paleomagnetic vectors of the essential clasts indicate that the flow was initially hot and that the deposit cooled slowly. Internal depositional features change rapidly over only 500 m. Rootless pipes and vesicles in the fine-ash matrix and siltstone clasts are therefore absent at distal locations, indicating that an initially cohesionless debris flow quickly became sufficiently turbulent to mix with ambient seawater.

Oncolites in Cretaceous Yucca Formation, Presidio County, Texas: ABSTRACT

Detailed stratigraphic work in the Yucca Formation, Presidio County, Texas, has revealed about 70 ft (21 m) of oncolite-bearing strata. The zone of oncolite occurrence is 180 ft (55 m) above the unconformity separating the Permian and Cretaceous rocks and 665 ft (203 m) below the initial occurrence of abundant Orbitolina. The zone varies both vertically and laterally from oncolite-supported conglomerate with micrite matrix to micrite with less than 10% oncolite content, to algal-encrusted, matrix-supported pebble conglomerate. Oncolites range from less than 0.5 in. (1.25 cm) to approximately 3 in. (7.5 cm) in diameter, and are commonly size sorted within the zone. The oncolites are most commonly nucleated about a fragmental piece of oncolite material and are classified as type-SS. In the pebble conglomerate facies, algal encrustations occur on very well-rounded, spheroidal clasts of siltstone, micrite, and chert. Very rare encrustations are present on clasts appreciably larger or smaller than the pebble size. Rarely is the algal material nucleated about organic remains such as pelecypod, gastropod, or brachiopod shells. The presence of this oncolite zone within an otherwise unfossiliferous section provides some control for interpreting depositional environments within the Yucca Formation. However, oncolites have been reported from several aquatic depositional environments both in the geologic record and as forming today. These environments include: lacustrine, fluvial; shallow, nearshore marine; and marginal marine brackish water systems. Local sedimentary features and broader stratigraphic relations effectively eliminate lacustrine and fluvial interpretation. End_of_Article - Last_Page 760------------

Early acquisition of characteristic magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona

Four discrete magnetizations associated with events occurring during deposition, early diagenesis, recent weathering, and cloud to ground lightning have been identified in rocks of the Moenkopi Formation at Gray Mountain, Arizona. A paleomagnetic study of more than 800 samples tied closely to the physical stratigraphy of the Moenkopi indicates that most of the magnetization remaining after partial alternating field and thermal demagnetization was acquired during and shortly after deposition. Directions of this remaining (characteristic) magnetization, in both normally and reversely magnetized rock, are closely similar to directions reported for the Moenkopi elsewhere on the Colorado Plateau. Sampling across the Moenkopi Formation at Gray Mountain has revealed two reversed magnetozones separating three normal magnetozones. Generally sharp boundaries of the magnetozones, their close concordance with physical stratigraphic units, and the rarity of anomalous directions of magnetization within the magnetozones indicate that most of the secondary components of the characteristic magnetization were acquired within 104–105 years after deposition. A conglomerate test based on siltstone clasts in sandstone shows highly scattered but not completely random directions of magnetization following partial thermal demagnetization. A secondary Triassic normal magnetization appears to be superimposed on some of the clasts, as suggested by the approximate coincidence of the resultant magnetic vector from the clasts with a Triassic normal direction. A fold test based on samples from highly deformed silty clay stone and sandstone in an intraformational landslide shows that these rocks acquired most of their stable magnetization before folding. A component of Triassic normal magnetization acquired after deformation also occurs in some of these rocks.

Cleavage in Triassic rocks of southern New Brunswick, Canada

Cleavage is sporadically developed in Late Triassic redbeds of the Lepreau Formation in folds and in areas of uniform northwestward dip in the lower Lepreau River – Lepreau Harbour half-graben. The cleavage in shaly siltstone beds is a composite incipient fabric defined by patchily developed crenulated bedding laminae, pressure solution planes, and mineral orientation parallel to the cleavage direction; cleavage in sandstone beds is defined by alignment of submicroscopic cracks inferred from laboratory tests. The cleavage fabrics are subvertical trending northeast–southwest, and resulted from horizontal northwest–southeast compression. Closely spaced fractures subparallel to the cleavage fabric in both shaly siltstones and sandstones are interpreted as later structures formed during relaxation of tectonic stresses. The cleavage fabrics and fractures are also present in intraformational clasts of shaly siltstone and rare sandstone clasts within conglomerate beds. A distinct shape orientation of the shaly siltstone clasts parallel to their cleavage within relatively undeformed conglomerate is attributed to compression during folding of unconsolidated conglomerate beds. The mean cleavage orientation varies from coplanar to noncoplanar with fold axial surfaces. The folds and cleavage formed during lateral compression probably related to wrench faulting which postdates the formation of the Triassic grabens.

Eocene Paleocurrents and Sedimentation, San Nicolas Island, California

San Nicolas Island, off the coast of southern California, is the emergent part of a north-trending anticlinal ridge formed of middle Eocene sandstone, siltstone, shale, and conglomerate. The sedimentary units contain varied sedimentary structures, such as cross-bedding, oriented cobbles, and contorted lamination axes, which show that sediment-transporting paleocurrents were toward the southwest (vector mean of 193°). Other sedimentary structures such as flat diffuse laminations, dish structures, outsized siltstone clasts, together with the lack of well-developed graded bedding suggest that the sediment was emplaced by mass-flow or grain-flow deposition. The presence of a bathyal microfauna, the association of lenticular units with laterally contiguous units, the high percentage of sandstone in the section, channels with imbricated conglomerates, and pebbly mudstone indicate that the section was deposited on the middle or suprafan part of a submarine fan or cone.

Deep-Water Deposition of Upper Wilcox Sandstones, Katy Field, Texas: ABSTRACT

Lower Eocene Wilcox sandstones were studied in a continuous core from a depth of 10,357 to 10,607 ft. The sandstones are 6-30 ft thick, very fine grained (0.11 mm), and generally thinly laminated. They consist of quartz, 52%; feldspar, 16%; rock fragments, 7%; other grains, 7%; and clay matrix, 18%. Interbedded shales are dark gray to black, massive, and rarely silty and bioturbated. Sandstones are composed of thin beds that are 1-3 ft thick and commonly show a vertical bedding sequence of (1) a thin, basal zone that contains small, siltstone clasts 1-5 mm in long diameter, (2) a dominant middle zone of inclined laminae that dip at angles of about 5°, and (3) a thin upper zone of horizontally laminated siltstone. Grain size decreases upward from 0.16 mm to 0.05 mm within the thin beds. These units appear to represent a turbidite sequence corresponding appear to represent a turbidite sequence corresponding to a basal graded unit (B1), a middle laminated unit (B2), and an upper laminated unit (D). A thin pelite unit (E) may be present at the top. The ripple-laminated unit (C) is missing or poorly developed. Contorted bedding is present in everal intervals. A minor amount of sandstone also occurs in ripple lenses that are thinly interlaminated with dark-gray, pelagic shales. A deep-water origin is postulated for this section because of the turbiditelike bedding sequence and graded texture, scarcity of organic reworking, and regional location beyond the limits of mapped Wilcox deltas. End_of_Article - Last_Page 1827------------

DummyDistribution patterns of siltstone clasts in turbidites and their relation to grain fabric of host turbidites

dummyDistribution patterns of siltstone clasts in turbidites and their relation to grain fabric of host turbidites