Sheet-like Deposits Research Articles

Facies distribution and stratigraphic architecture of the Lower Cretaceous McMurray Formation, Lewis Property, northeastern Alberta

Abstract Within the Lewis study area (Townships 91–92, and Ranges 6-8W4), McMurray Formation strata comprise four facies associations that form a depositional continuum of braided-fluvial (FA1), tidally-influenced braided- or low-accommodation meandering-fluvial and meandering-tidal channel-fills (FA2), associated overbank (FA3), and open-estuarine tidal flat deposits (FA4). The primary reservoir occurs in transgressive FA2 deposits. Lower-FA2 channels incise older water-saturated FA1 sand and consist of medium to locally-coarse, bitumen-saturated sand with rare to locally common pin-stripe laminated and/or 1-5 cm thick mud beds. Channels were initially confined by steep valley walls formed along the sub-Cretaceous unconformity. As a consequence, coeval interchannel sediment (FA3) was cannibalized by lateral channel migration and occurs only as common mud-clasts. The net result was the accumulation of a sand-rich, sheet-like deposit with locally-preserved fine-grained interchannel deposits, suggesting a high rate of lateral versus vertical accretion. With the filling and elimination of the irregular paleotopography along the unconformity, upper-FA2 channels became unconfined and formed thick, areally extensive inclined heterolithic stratification (IHS) deposits. Similarly, coeval interchannel deposits are more widely-distributed and thickly preserved compared to underlying strata. The position of upper-FA2 channels was indirectly influenced by paleotopography along the unconformity because IHS strata are thickest and sandiest along trends that overlie earlier channel systems, suggesting that syn-depositional salt dissolution controlled channel position and infilling history. Stratigraphically-upward there is a gradational change to tidal-flat deposits (FA4), suggesting abandonment of the main north-south channel system and a condition of coarse-grained bedload sediment starvation. Although most point-bar deposits within the Lewis study area consist of 10-30 m thick vertically-stacked successions of incomplete IHS deposits, several complete successions from channel base to overbank deposits are preserved. The geometric similarity between meandering-fluvial channels of different scales, climatic regimes, and substrate conditions suggest that the geometry of meanders is governed by mechanical principles. Although the applicability of these relationships to meandering-tidal channels is equivocal, using these formulas on IHS deposits of the McMurray Formation can provide a first order approximation of channel width (W=1.5h/tanβ), depth (0.595h), meander wavelength (λ=10.9w1.01), and point-bar length (λ). For two examples of complete point-bar deposits, channel widths were estimated to be at 148 m and 336 m; channel depth 12.6 m and 18.7 m; meander wavelength 1718 m and 3930 m; and point-bar length 859 m and 1965 m.

Development of a late Viséan (Mississippian) mixed carbonate/siliciclastic platform in the Guadalmellato Valley (southwestern Spain)

The Peñas Rubias Syncline (southwestern Spain) exposes a well-preserved shallow-water platform succession containing a spectrum of facies corresponding to inner platform intertidal to supratidal environments, across to deeper-water middle to outer platform environments. Nineteen microfacies are recognized, which are grouped into seven facies association corresponding to: siliciclastic deltaic bars, mixed carbonate/siliciclastic shoals, carbonate mud mound boundstones, background platform carbonates, background platform siliciclastics, mixed tempestites and deep-water siliciclastic shales and sandstones. The age of the succession was determined mainly by foraminiferans and calcareous algae, which permit the succession to be assigned to the late Brigantian (latest Viséan). This upper Brigantian platform is the only record of sedimentation of this age in the region, and thus is key for interpreting the sedimentary and tectonic evolution of the Carboniferous rocks in Sierra Morena. Biotic and sedimentological features were analyzed in order to assess the controls on the sedimentation. Several factors have influenced sedimentological changes: turbidity, subsidence, siliciclastic discharges, storms and bioturbation. The siliciclastic discharges exerted a considerable control on the basal deposits, mostly in their percentage of quartz sand grains and as microconglomerates. However, they did not develop as large deltaic deposits, and their influence can be considered as virtually negligible in regards to the remaining part of the succession. Turbidity, as a result of higher percentage of silt and mud in suspension, seems to be the main factor controlling the change between the older intertidal deposits in the inner platform to the younger subtidal deposits of the middle and outer platform. As a result of the increase of the mud and silt in suspension, facies changed first to marlstones and nodular argillaceous limestones, and second, to predominantly calcimicrobial boundstones and shales in the uppermost part of the carbonate succession, as well as showing a marked change from photic-controlled benthic faunal and microfloral assemblages to assemblages more tolerant or better adapted to muddier dysphotic substrates. Bioturbation is also interpreted as one of the main controls influencing the different type of boundstones in the middle platform, permitting the vertical growth of dome-shaped mud-mounds or as sheet-like deposits. Storm influences seem to have exerted some control on the positive relief of the dome-shaped boundstones, which are usually capped by tempestites. The differential subsidence observed in the northwestern sector of the platform allowed the accumulation of many stacked dome-shaped mounds, a feature not recognized in southeastern parts. The general stratigraphical sequence seems to be controlled by eustasy and synsedimentary tectonics. The overall succession exhibits a pronounced deepening-upwards transgressive sequence from siliciclastic delta bars, mixed shoals, carbonates and shales of the middle platform with the growth of calcimicrobial boundstones and dark green shales, passing up into black shales at the top of the sequence, in the outer platform and, possibly, submarine slope settings. Although this transgressive sequence is in harmony with the 3rd-order glacioeustatic cycle defined for the late Brigantian in the western Palaeotethys, all these Brigantian rocks accumulated in a synsedimentary extensional regime, which is related to the initiation of a sinistral strike–slip regime, previously recognized as affecting only Serpukhovian and younger rocks in Sierra Morena. These synsedimentary faults allow us to recognize significant lateral variations in thickness over short distances.

Deposits from dome-collapse and fountain-collapse pyroclastic flows at Soufrière Hills Volcano, Montserrat

abstract Pyroclastic flows were formed at Soufrière Hills Volcano by lava-dome collapse and by fountain collapse associated with Vulcanian explosions. Major episodes of dome collapse, lasting tens of minutes to a few hours, followed escalating patterns of progressively larger flows with longer runouts. Block-and-ash flow deposit volumes range from <0.1 to 25 x 10 6 m 3 with runouts of 1-7 km. The flows formed coarse-grained block-and-ash flow deposits, with associated fine-grained pyroclastic surge deposits and ashfall deposits. Small flows commonly formed lobate channelized deposits. Large block-and-ash flows in unconfined areas produced sheet-like deposits with tapering margins. the development of pyroclastic surges was variable depending on topography and dome pore pressure. Pyroclastic surge deposits commonly had a lower layer poor in fine ash that was formed at the current front and an upper layer rich in fine ash. Block-and-ash flows were erosive, producing striated and scoured bedrock surfaces and forming channels, many metres deep, in earlier deposits. Abundant accidental material was incorporated. Pyroclastic flow deposits formed by fountain collapse were pumiceous, with narrow sinuous, lobate morphologies and well developed levees and snouts. Two coastal fans formed where pyroclastic flows entered the sea. Their seaward extent was limited by a submarine slope break.

Alluvial-to-lacustrine systems in a pull-apart margin: southwestern Eumsung Basin (Cretaceous), Korea

Abstract In the pull-apart margin of pull-apart basins, sedimentation patterns are poorly known, compared with those in rift basins. The rhomb-shaped Eumsung Basin (Cretaceous), mid-west Korea, was formed in an overstep of the Kongju fault system and provides significant information on how sedimentary bodies dynamically filled the pull-apart margin. According to a detailed analysis of sedimentary facies, palaeoflow directions, clast compositions and downstream changes in clast size, the southwestern basin fills comprise three facies associations representing distinctive depositional environments: (1) facies association (F.A.) I (volcaniclastics-dominated alluvial fan); (2) F.A. II (alluvial plain); and (3) F.A. III (floodplain/lake). The matrix-supported and clast-supported conglomerate (F.A. I) is indicative of deposition in alluvial fans. The hollow-fill or sheetlike deposits encased in purple siltstone (F.A. II) are suggestive of channel or sheetflood deposition in an alluvial plain. The dominant purple and green mudstone (F.A. III) indicates deposition in floodplain and lacustrine environments. The distribution of facies associations represents a basinward change in depositional environments from alluvial-fan to lacustrine systems. The succession is divisible into three large-scale lateral and partly vertical units: the southernmost Ochang (∼1.5 km thick), the medial Baeti (∼1 km thick) and the northernmost Kugokri units (∼0.5 km thick), on the basis of basinal structure and changes in clast composition and lithofacies. The Ochang–Baeti transition reflects synchronous development of the alluvial-to-lacustrine systems in a pull-apart margin. This succession development is attributable to the basinal faults progressed during the basin development. The Baeti–Kugokri transition recorded the changes in clast composition and lithofacies, which was due to the progressive shifting in basinal setting from a pull-apart margin to a strike-slip margin. Unlike that of the strike-slip margins, clast composition of the pull-apart marginal fills reflects adjacent source-rock lithology. In the pull-apart margin, alluvial-to-lacustrine systems show a transverse-to-margin channel network (060°–135°), converging into the southwestern depocentre. Although the basin is of strike-slip origin, sedimentation patterns in the pull-apart margin are suggestive of relatively synchronous developments of the alluvial-to-lacustrine systems reflecting the lithology of the adjacent source rock and subsurface structure of normal-slip-dominant faults.

Late Pleistocene–Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 1—Geophysical results

In the 1994, 1995, and 1998 austral field seasons, geophysical research efforts in the Ross Sea focused on acquiring a high-resolution database designed to permit (1) reconstruction of the maximum extent and configuration of the ice sheet during the Last Glacial Maximum (i.e., oxygen isotope stage 2); (2) reconstruction of conditions at the base of the ice sheet; and (3) assessment of the relative retreat history of the ice sheet following the Last Glacial Maximum. Five seismic facies are distinguished on the basis of external geometry, relationships among features, bounding surface amplitude, intensity of internal acoustic signature, and geometry of internal reflectors. Seismic facies 1 is a transparent draping unit correlated to diatomaceous mud, interpreted as being deposited under open-marine conditions. Seismic facies 2 and 3 are comprised of units displaying subdued massive internal signatures with smooth lower bounding surfaces and hummocky upper surfaces characterized by glacial lineations. These units are interpreted to be grounding-zone proximal deposits overridden by the expanded ice sheet. Seismic facies 2 may be a deforming-till unit. Seismic facies 4a is characterized by an internally massive to chaotic signature, an erosional, often flat, lower surface, and a hummocky upper surface displaying glacial lineations. This unit is interpreted to be till and is divided into deposits associated with the most recent glacial expansion (4a) and with an earlier (pre–Last Glacial Maximum) expansion (4b). Seismic facies 5 is an acoustically laminated, ponded, and draping deposit interpreted to contain proglacial and sub–ice-shelf materials deposited continuously since the last interglacial period. The associations of these units provide context for the interpretation of the ice-edge maximum position, conditions at the base of the ice sheet, and the relative retreat history of the region. There is compelling evidence for a much-expanded ice sheet in the Ross Sea during the Last Glacial Maximum. In the western Ross Sea, the maximum grounding position is marked by an isolated grounding-zone wedge, and placed in the vicinity of Coulman Island, approximately 150 km from the continental-shelf edge. In the central Ross Sea, the maximum grounding position is close to the continental-shelf break, based primarily on the presence of an extensive 60-m-thick sheet-like deposit with a fluted upper surface. Streaming ice may have occupied bathymetric lows on the continental shelf, as suggested by (1) the configuration of bathymetry; (2) the presence of glacially eroded troughs; (3) the concentration of sediment (interpreted to be deforming till) within the middle to outer shelf reaches of the troughs; and (4) the fluted nature of the upper surface. Absolute rates of streaming ice flow relative to inter-ice-stream areas are not implied. The western Ross Sea continental-shelf deposits record the retreat history of ice derived predominantly from the East Antarctic Ice Sheet and glaciers of the Transantarctic Mountains. Ice flow on the continental shelf is interpreted to have remained fixed in position, restricted by the walls of the troughs. On the inner shelf of the western Ross Sea, the ice flowed over and eroded lithified sedimentary strata. Slower-moving ice occupied flat-topped banks. A single grounding-zone wedge occurs on the western Ross Sea central shelf; no substantial deposits are observed on the inner continental shelf. This lack of grounding-zone features reflects a restricted sediment supply and a relatively steady, rapid retreat of the ice sheet. During retreat, ice flow acted independently in each major trough and on the bank tops. Ice retreated from Victoria Land Basin–Drygalski Trough before it retreated from JOIDES Basin. Ice remained on the bank tops, shedding material into the abandoned troughs. Grounded ice in the central Ross Sea was derived predominantly from an expanded West Antarctic Ice Sheet. The ice sheet remained grounded on the continental-shelf edge after retreat of ice from the western Ross Sea. Expanded ice eroded the inner shelf and deposited sheets of till on the central and outer shelf. Ice-stream drainage shifted laterally, as recorded by rounded, laterally accreting ridges that separate bathymetric troughs. The central Ross Sea is the repository for larger volumes of sediment, derived from ice flow across basins of relatively thick, unlithified sedimentary deposits. Two till sheets mark grounding-zone positions in the central Ross Sea, reflecting the higher sediment supply and a stepped deglaciation. Ice remained grounded on the Pennell Bank to the west; a series of moraines marks the retreat of ice across the bank. Mega-scale glacial lineations and other streamlined, subglacial geomorphic features occur across the continental shelf, primarily within the troughs. The lineations substantiate the maximum reconstruction and support the interpretation of a deforming substrate beneath the outer reaches of the expanded ice sheet. This deforming substrate may have contributed to the onset of deglaciation. Features associated with meltwater are rare or absent, suggesting that basal meltwater played a minor role in retreat of the ice sheet.

Nodular melanosis due to epithelial neoplasms: a finding not restricted to regressed melanomas.

Nodular or tumoral melanosis consists of nodular or sheetlike deposits of melanophages in the dermis. When nodular melanosis is present, a completely regressed malignant melanoma is a major diagnostic consideration. We present a case of nodular melanosis due to regression of a pigmented basal cell carcinoma with pilar differentiation. In addition to this case, we present five additional cases of epithelial neoplasms with melanin deposition in the stroma. In each case, the source of the melanin was non-neoplastic dendritic melanocytes intermingled among the tumor cells. Therefore, if nodular melanosis is found, pigmented epithelial neoplasms should also be considered in the differential diagnosis.

Seismic patterns of a muddy contourite fan (Vema Channel, South Brazilian Basin) and a sandy distal turbidite deep-sea fan (Cap Ferret system, Bay of Biscay): a comparison

The aim of this paper is to discriminate the depositional facies, geometries and mechanisms of deposition of contouritic fans from those of turbidite distal fans, with a view to provide better resolution of reservoir prediction. Two examples are analysed: the Vema contouritic drift in the South Brazilian Basin and the Cap Ferret turbiditic fan in the Bay of Biscaye. The Vema contourite fan is a Neogene mud-rich accumulation (200–400 m thick), fed by Antarctic Bottom Water bottom currents and located downstream of the Rio Grande Rise. It forms one single-mound fan-shaped body in between two major channels, and where the main part of the deep circulation is funnelled into. As a result of the morphological and hydrological background, the comourite drift progrades mostly downstream. The accumulation was built as several depositional units (‘channel-levee’ systems) bounded by widespread discontinuities showing erosional patterns. This resulted from episodes of strong and/or unstable current activity producing the discontinuities, alternating with periods of relatively weak and stable currents and major deposition. The Pliocene-Quaternary Cap Ferret distal deep-sea fan is a thick (500 m) sand-rich turbiditic accumulation fed directly by an uplifting mountain range. The accumulation is developed downstream of a main turbiditic feeder channel and the volume of sediment involved is much higher than for the Vema contourite fan. It shows a complex network of shallow channels and low-relief levees which merge downstream into thin, sandy, sheet-like deposits. Several depositional units are stacked vertically. Each unit is built by the lateral migration of a ‘channel-levee’ system. The stacking pattern of the successive units is prograding towards the basin. The presence of major discontinuities cutting throughout a whole accumulation, and the fairly irregular geometry of the ‘channel-levee’ deposits (absence of any obvious migrating trend), appear to be the most distinctive features of a contourite fan. On the other hand, the lateral migration of the ‘channel-level’ geometries and the presence of erosional surfaces of limited extent and restricted to the channels, are the main diagnostic features of a distal turbidite fan. Otherwise the seismo-facies are fairly similar in both sedimentary bodies.

Three-Dimensional Seismic Stratigraphic Expression of Pliocene-Pleistocene Turbidite Systems, Northern Green Canyon (Offshore Louisiana), Northern Gulf Of Mexico

Interpreting three-dimensional (3-D) seismic data is of great value in high-grading prospects in frontier exploration areas in the Gulf of Mexico. This technique is now routinely used to reduce the exploration and production risks associated with turbidite reservoirs. A subregional 3-D seismic data set was used in this study to describe the Pliocene- Pleistocene turbidite systems in three intraslope minibasins in the north-central Green Canyon protraction area of the Gulf of Mexico. The stratal geometries and lithology of the different turbidite elements were interpreted based on the integrated analysis of well-log data, seismic facies observed on the vertical seismic profiles, and amplitude distribution recognized on series of horizon slices and amplitude extraction maps (horizontal seismic facies). The lithologic interpretation, however, was limited by the lack of seismic velocity data; the wells were tied to the seismic data through synthetic seismograms created by integrating the sonic and density logs. Three turbidite elements were mapped in the selected upper Pliocene-lower Pleistocene interval of the study area: basin-floor fan (amalgamated depositional lobes), erosional and depositional channels, and overbank sediments. Lobe-shaped high-amplitude areas (onlapping high-amplitude continuous reflections on seismic profiles) indicate laterally continuous, sheetlike deposition, interpreted as turbidite lobes. Elongated sinuous high-amplitude areas (high-amplitude discontinuous reflections) correspond to depositional channel systems. Sinuous low-amplitude areas in generally high-amplitude areas (e.g., sequence boundaries) reflect the erosional channels. The depositional channels are interpreted to be filled with sands; the shale-filled erosional channels are interpreted as conduits for sediments bypassing farther down slope. The detailed 3-D seismic interpretation allowed description of the characteristics of the channel systems, analysis of their evolution through 2 m.y., and determination of the controls on the variations in turbidite deposition. Widespread low- to moderate-amplitude areas without dominant amplitude patterns are interpreted as overbank deposits. The turbidite elements identified in this study, including basin-floor fans (depositional lobes) and channel systems, form the reservoirs in the fields of the study area and in many other fields and discoveries of the northern Gulf of Mexico. The geometries of the sand-rich turbidite elements mapped in this study are considered to be good analogs for these producing fields and some exploration targets. The results of this study also can be used as analogs for the description of reservoir architecture in the subsalt trend of the Gulf of Mexico and in other deep- water exploration areas.

Sequential Development of Alluvial/Lacustrine System: Southeastern Eumsung Basin (Cretaceous), Korea

ABSTRACT In a strike-slip setting, depositional history of marginal basins depends on the fault movements. In order to constrain controlling factors for the sequence changes of alluvial/lacustrine systems in the southeastern part of Cretaceous Eumsung Basin, a pull-apart basin, southwestern Korea, we made a detailed analysis of sedimentary facies, paleoflow directions, clast compositions, and downstream changes in clast size. The conglomeratic sequence ( 2.2 km thick) in the study area can be organized into five facies associations representing distinctive depositional environments: (1) facies association (FA) I (debris-flow-dominated alluvial fan), (2) FA II (stream-dominated alluvial fan), (3) FA III (alluvial-fan fringe), (4) FA IV (alluvial plain), and (5) FA V (floodplain/lake). The entire succession can be divided into two stratigraphic units, the Dootasan sequence below and the upper Berjae above, on the basis of facies associations and stratigraphic architecture of channel-fill and floodplain deposits. The Dootasan sequence ( 1.2 km thick) is characterized by a basinward change from matrix-supported onglomerate (lower part) to ribbon-shaped channel fills and sheetlike beds encased in purple siltstone with a thinning-upward trend (upper part). The matrix-supported conglomerate facies is indicative of deposition in a debris-flow-dominated alluvial fan. The hollow-fill or sheetlike deposits in purple siltstone are suggestive of channel or sheetflood deposition on an alluvial plain. The Berjae sequence ( 1 km thick) is transitional from superimposed channel-fill/sheetflood deposits with infrequent intercalation of purple siltstone wedges to conglomerate beds encased in siltstone. The stacked channel-fill or sheetflood deposits with rare siltstone beds are indicative of deposition in stream-dominated alluvial fan. Each sequence shows a basinward spatial change in facies. Between the two sequences, green siltstone beds (max. 100 m thick) with conglomerates are intercalated, suggesting deposition in a lacustrine environment. The sequence transition from the Dootasan to the Berjae sequence along the basin-margin fault was most likely due to the changes in fault movements. The Dootasan sequence was formed during sinistral-fault movement with a relatively high rate of fault slip (early stage of fault activation), whereas the Berjae sequence was deposited in the late stage with a relatively low rate of fault slip.

The success potential of international R&D cooperation : Thorsten Teichert Technovation, 13(8) (1993) 519–532

Coronary artery calcification is concomitant with the development of advanced atherosclerosis. Coronary artery calcification pathologically begins as microcalcifications (0.5 to 15.0 μm) and grows into larger calcium fragments, which eventually result in sheet-like deposits (>3 mm). This evolution is observed to occur concurrently with the progression of plaque. These fragments and sheets of calcification can be easily identified by radiography as well as by computed tomography and intravascular imaging. Many imaging modalities have proposed spotty calcification to be a predictor of unstable plaque and have suggested more extensive calcification to be associated with stable plaques and perhaps the use of statin therapy. We will review the pathology of coronary calcification in humans with a focus on risk factors, relationship with plaque progression, correlation with plaque (in)stability, and effect of pharmacologic interventions.

Holocene Paleofloods of the Ross River, Central Australia

The rivers of central Australia rise in the MacDonnell Ranges and flow out across broad, low-relief plains into the surrounding desert. The stratigraphy of the Ross River plain records the areal extent and frequency of Holocene floods. This floodout plain is underlain by deeply weathered alluvial deposits, characterized by red earth soils dated by thermoluminesence at >59,000 yr. This old alluvium is covered by a sheet-like deposit of very silty sand of probable eolian origin dated by thermoluminesence at 9200 ± 900 yr. The oldest Holocene alluvium occurs as broad, low-relief bars and levee deposits flanking the modem channel and as low-relief long-wavelength bedforms that fan out across the plain. This deposit resulted from a flood flow, up to 10 km wide, that covered the entire plain. Evidence for several large floods between 1500 and 700 yr B.P. is also preserved in a 500- to 1500-m-wide paleochannel. Thus, the surface features on the floodout plains are the product of a few rare large flood events. This paleohydrologic record is additional evidence of the dynamic nature of the hydrometerological regime of central Australia.

Recent Advances in Sequence Stratigraphy: The Lowstand and Transgressive Systems Tracts

On basin margins characterized by a ramp physiography, relative sea level fall induces basinally isolated, shelf-perched, forced-regression shoreline and deltaic deposition rather than deep-water submarine-fan turbidite deposition. These deposits comprise the early lowstand systems tract. In proximal locations, the sequence boundary at the base of these lowstand deposits is expressed as an erosional unconformity, whereas in distal settings this surface occurs as a correlative conformity. The stratigraphic discontinuity at the top of these lowstand deposits is a ravinement surface and in places can be more striking than the sequence-bounding surface at the base. significant erosion due to fluvial processes during forced regression and subsequent shoreface processes during transgression may be common at the tops of these lowstand deposits. The transgressive systems tract commonly comprises, proximally to distally, backstepped barrier beach deposits, sheet-like lag deposits, and [open quotes]healing phase[close quotes] deposits. This latter unit has not been described widely, but volumetrically may contain a significant part of the transgressive systems tract. Healing phase deposits comprise a wedge of sediment that onlaps the last clinoform of the underlying progradational phase. These sediments commonly are derived by cannibalization from the top or edge of the subjacent progradational phase deposits (i.e., either highstand ormore » lowstand), during and immediately after transgression. These deposits are referred to as the healing phase because their deposition tends to [open quotes]heal over[close quotes] the relatively steep clinoform slope of the underlying progradational phase after transgression has resulted in landward shift of the depocenter. Although sometimes misinterpreted as lowstand deposits, these depositional units commonly do not contain significant reservoir facies.« less

Chemical fluxes and origin of a manganese carbonate-oxide-silicate deposit in bedded chert

Lens-like rhodochrosite-rich bodies within interbedded chert and shale are associated with basalt and/or graywacke in ophiolitic and orogenic zones. The Buckeye manganese mine in the Franciscan Complex of the California Coast Ranges is associated with metagraywacke. Despite blueschist-facies metamorphism, this deposit preserves the compositions and some textural features of its sedimentary protoliths. For this reason, it is a suitable deposit with which to compare more intensely altered deposits, or deposits originating in different paleoenvironments. Six Mn-rich and three Mn-poor minerals form monomineralic layers and mixtures: rhodochrosite, gageite, Mn-oxides (hausmannite, braunite), divalent Mn-silicates (caryopilite, taneyamalite), chlorite, quartz (metachert) and aegirine-augite. The Mn-rich protoliths have high Mn/Fe combined with relatively low concentrations of Ca, Al, Ti, Co, Ni, Cu, Th and REE. REE patterns of various protoliths are distinct. Rhodochrosite and gageite layers are depleted (seawater × 5 · 10 4) and flat, whereas patterns of metachert and the Mn-silicate-rich layers mimic the patterns of metashale and metagraywacke (seawater × 10 6). Hausmannite layers have flat patterns (seawater × 7 · 10 4) whereas braunite-rich layers are more enriched (seawater × 2 · 10 5) and show a distinct positive Ce anomaly. Factor analysis reveals components and fluxes attributed to sub-seafloor fluids (Ni, As, Zn, Sb, W, Mn), seawater (Mg, Au, V, Mo), detritus and veins (Ca, Ba, Sr). Silica is negatively correlated with the sub-seafloor factor. The observed variances indicate that water from the sediment column mixed with seawater, that deposition occurred near the sediment-seawater interface before mixtures of subsurface fluid and seawater homogenized, and that the system was not entirely closed during metamorphism. The variations in REE enrichment can be related to kinetics of deposition: rhodochrosite and gageite were precipitated most rapidly, and therefore were the protoliths that most effectively diluted the REE-rich background resulting from fine clastic material (derived from distal turbidites). The variation of the Ce anomaly and U/Th among diverse lithologies and the differences in Mn oxidation states are consistent with progressive dilution of reduced subsurface fluids with oxidized seawater. By this scheme, rhodochrosite, gageite and hausmannite were deposited from the most reduced fluids, braunite from intermediate mixtures, and Mn-silicates from the sub-seafloor fluids most diluted with fresh seawater. Comparison of the Buckeye with other lens-like and sheet-like deposits having high Mn/Fe and containing Mn 3+ and/or Mn 2+ suggests that each had three essential fluxes: a sub-seafloor source of Mn, a local source of very soluble silica and a source of relatively fresh, oxygenated water. Additional fluxes, such as clastics, appear to be more characteristic of the paleoenvironment than the three essential fluxes.

Quartz latite rheoignimbrite flows of the Etendeka Formation, north-western Namibia

The Etendeka Formation of north-western Namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the Karoo Igneous Province in southern Africa. The age of the Etendeka Formation is approximately 130–135 Ma. The quartz latites make up a significant proportion of the stratigraphic succession (<25% of the total stratigraphic thickness) and form as much as 60% of the outcrop area in the southern Etendeka. Apart from some systematic differences between pitchstones and devitrified quartz latite, largely explained by alteration processes, individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation. Geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern Etendeka area enabling the reconstruction of the Etendeka Formation stratigraphy in this region. Individual quartz latite units occur as voluminous (400–2600 km3), widespread (up to 8800 km2), sheet-like deposits typically between 40 and 300 m thick. Each unit consists of basal, main and upper zones. The main zone generally constitutes over 70% of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite. In contrast the basal and upper zones of the flow are characterised by flow banding, pitchstone lenses and breccia, with rare occurrences of pyroclastic textures. The quartz latites are sparsely porphyritic (<10% phenocrysts) with glassy or devitrified groundmass textures. The phenocrysts consist of plagioclase, pyroxene, titanomagnetite and rare ilmenite. Pyroxene geothermometry indicates high (1000–1100°C) temperatures of crystallisation which, coupled with the absence or primary hydrous phases, indicates that the quartz latites were relatively hot, H2O-undersaturated magmas. The quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style. The local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high-temperature rheomorphic ignimbrites (i.e. rheoignimbrites). A combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the Etendeka area.

Quaternary fluvio-aeolian sand/silt and alluvial gravel deposits of northern Libya-event stratigraphy and correlation

Erosion surfaces and major palaeosols in Quaternary deposits of the eastern Jifarah Plain, allow subdivision of the sediment sequence into six aggradational units Q1–Q6. Comparison of Quaternary sequences in varying continental facies across northern Libya shows that the erosion surfaces and major palaeosols can be traced throughout the region and that they constitute major correlatable events. Correlation also confirms the presence of a seventh aggradational unit between units Q4 and Q5 in upland areas. The depositional units display an irregular but characteristic geometry which reflects changing basin architecture. In upland regions the deposits are typically linear wadi fill and coalesced fan apron passing out into more sheet-like deposits in lowland regions where valley fills are connected across interfluve areas by sheet-wash colluvial and aeolian deposits. Following overall vertical accretion through sequences Q1 to Q4, a major phase of downcutting and terrace formation commenced with the pre-Q5 erosional surface in the early Holocene. Age of the older units is as yet in doubt but there is evidence to assign unit Q1 to the early Pleistocene, unit Q2 to the late Riss and units Q3 and Q4 to the Wurm.

Sedimentologic Controls on Movement of Contaminants through Aquifers: Lessons Learned from Petroleum Geology: ABSTRACT

Most predictive hydrologic models for contaminant transport in unfractured sandstone aquifers assume sheet-like, uniform flow down a potentiometric gradient and ignore sedimentary heterogeneities. In contrast, petroleum exploitation effort document depositionally induced heterogeneities that control fluid movement through sandstone deposits. Sandstone deposits from many depositional environments contain heterogeneities capable of diverting contaminants outside the path predicted by potentiometric theory. Fluvial deposits can be linear sandstone bodies encased within impermeable mudstone, often oriented at high angles to the potentiometric gradient. Internally, fluvial sandstone bodies can be highly compartmentalized, resulting in complex but predictable flow paths. Similarly, predictable permeability relationships occur in other depositional environments. The internal stratification of eolianites causes directionally preferred flow of fluids and layered behavior. Eolianites have maximum preferred permeabilities, perpendicular to paleowind, that can exceed intermediate (parallel to paleowind) and vertical permeabilities by 2 to 3 orders of magnitude. Even sheet-like strandline deposits contain internal flow barriers that can isolate or divert contaminants. These predictable permeability relationships can have a dramatic impact upon fluid flow through aquifers, as well as hydrocarbon reservoirs. Prediction of heterogeneity requires a detailed understanding of the deposit in question. Core, well logs, and outcrop studies, rarely used in hydrogeology, are necessary to accuratelymore » predict flow characteristics. Although core and well logs are expensive, predictive sedimentology can offset these expenses by eliminating unnecessary monitor wells drilled large distances from the compartmentalized path of contaminant transport.« less

Subaqueous outwash deposits in the Ingraham ridge, Chazy, New York

A 1.5 km segment of Ingraham ridge, a stratified sand and gravel ridge in northeastern New York, is composed dominantly of subaqueous outwash deposits. The core of the ridge segment may include subordinate subglacial tunnel deposits. The subaqueous outwash is composed mainly of interbedded sediment gravity flow, bar, dune, and ripple deposits that occur in channels. Sheetlike cross-stratified and cross-laminated deposits also are common, particularly on the distal western margin of the ridge segment. The ridge segment is draped by glaciolacustrine laminated sand, silt, and clay and glaciomarine massive mud. Westward decrease in grain size and scale of sedimentary structures is attributed to confining ice walls of an asymmetric embayment. Sediments are deformed in part by the melting of adjacent and buried ice. The ridge was modified during Champlain Sea regression and the reworked deposits were colonized by shallow-water organisms. Reworking may have been minor, and the broad, low morphology of the ridge may be due to subaqueous outwash deposition along its full length during retreat of the ice margin.

Dispersion of bauxitic red mud in the Gulf of Corinth, Greece

Bauxitic red-mud slurry, discharged by an aluminium processing factory on the shelf of the Gulf of Corinth, Greece, is used as a tracer to examine sediment transport and dispersion processes in a tectonically active deep basin. The Gulf of Corinth is a silled elongate basin with a maximum depth of 860 m; it is characterised by high seismicity. The red mud is discharged through a pipeline at a depth of 100 m, where it forms an extensive deposit. Grab sampling has shown that the red mud extends over the slope and the abyssal plain up to a distance of 17 km from the mouth of the pipeline. It forms small scattered sheet-like deposits of up to 3 cm in thickness. The structures observed in the scattered patches of the red mud, which in some places are overlain by a thin veneer of grey/olive green (natural) sediments, suggest that the red mud is transported from the shelf to the abyssal plain by gravitative sediment flows. The flows are triggered by earthquakes. Granulometric and geochemical analyses of the red-mud samples show that the red mud, during its transport from the shelf to the abyssal plain, is subjected to mixing with the surrounding sediments. A different mixing ratio is determined, however, by reference to the various trace metals (Fe, Ni, Co, Pb).

Evolution and Stratigraphy of a Sandy Tidal-Flat Complex Within a Mesotidal Embayment: ABSTRACT

Sand-dominated intertidal environments in St. Helena Sound, South Carolina, a mesotidal estuarine system, can be divided into a continuum of barlike deposits dominated by tidal flows and sheetlike deposits of sandy tidal flats that are influenced by both waves and currents. A large sand flat attached to a marsh-island complex in the central interior of St. Helena Sound resembles a large flood-tidal delta and has been reworked by waves and migrating ebb-dominated tidal channels. The lower tidal flat (flood ramp) is composed of coarse to medium sand mixed with shell material. Large-scale planar cross-beds are formed by flood-oriented sand waves. Middle tidal-flat deposits consist of fine to very fine sand where burrowing by intertidal fauna disrupts structures of intermedia e to small-scale bed forms generated by both waves and tidal currents. The muddy, fine-grained sands of the upper tidal flat are reworked by wave-generated small-scale ripples and are partially bioturbated. A salt marsh-chenier complex, landward of the upper tidal flat, has prograded over older portions of the sand flat during earlier regressive phases. Evolution of the St. Helena Sound sand flats began with a transgressive phase marked by a transgressive lag deposit dated at 4,200 yr B.P., overlying Pleistocene estuarine, mud-flat, and salt-marsh deposits. Sediment for buildup of the sand flats was derived from reworking of surrounding barrier-island sands by migrating tidal channels. Sand was introduced into the lower tidal flat by swash-bar accumulation or strong flood-tidal currents. Building of the sand-flat sequence and development of the overlying salt marsh-chenier complex occurred episodically due to minor fluctuations in sea level. At present, the sand-flat sequence is in a transgressive phase and is being reworked by migrating tidal channels and the seawardmost chenier is subject to frequent overwashing. End_of_Article - Last_Page 285------------

Cyclicity and the facies model concept in fluvial deposits

ABSTRACT Since the fining-upward cycle model for meandering-river deposits was published in 1963, about a dozen facies models have been defined for rivers of varying sediment grain size, channel multiplicity and sinuosity. However, this suite of models is inadequate for the interpretation of many fluvial deposits because it does not incorporate the effects of a variety of outside-the-basin (allocyclic) sedimentary controls. The most important of these controls is tectonics, because differential movement between and source area, and changes in the rate of movement, can cause major changes in fluvial sedimentary environments. These changes give rise to fining- or coarsening-upward cyclic sequences which are superimposed on the cycles generated by within-basin (autocylic) mechanisms. Variations in climate can cause variations in sediment grain size and in depositional style as a result of alterations in weathering modes and discharge characteristics. These latter variations have been studied in hot arid, warm temperate and fluvioglacial regimes, but are poorly known in other climatic zones. As a result, many of our preconceptions about fluvial geomorphology and sedimentation may be inappropriate for other climatic conditions such as hot, humid environments. Channel morphology varies between four basic styles: braided, meandering, anastomosed and straight, but the sedimentological characteristics of intermediate morphological types have yet to be quantified. Many pre-Carboniferous rivers appear to have lacked clearly defined channels with cutbanks. The probable cause was low bank cohesion caused by an absence of land vegetation, a condition that cannot be modelled accurately from observations of modern rivers. The absence of vegetation would have favoured a flash-flood depositional process, which tends to generate sheet-like deposits rather than lenticular, channel-bound units. Sedimentological studies in modern rivers are beset by the problem of preservability, in that a river's long-term aggradation behaviour is commonly unclear. The problem is compounded by the fact that many studies have been carried out on rivers that are actively degrading. Cyclic sequences in fluvial sediments can be produced by a variety of mechanisms, and the interpretation of a given deposit solely in terms of a published facies model may be misleading. For most fluvial sequences it is necessary to adopt a multivariate basin analysis approach.