Fine-grained Facies Research Articles

Catastrophic mass failure of a Neoproterozoic glacially influenced continental margin, the Great Breccia, Port Askaig Formation, Scotland

Abstract The Great Breccia is a thick (over 50 m) diamictite that occurs close to the base of the Neoproterozoic Port Askaig Formation in Scotland, and contains matrix-supported clasts up to 100 m across in a poorly sorted dolomitic matrix. Detailed logging and mapping of cliff and shoreline exposures through the Great Breccia on the Garvellach Islands show that it is not a simple ‘megabreccia’ but consists of three sedimentary units including a megabreccia (Unit 1), diamictites interbedded with dolomitic conglomerate, sandstone and mudstone (Unit 2) and a diamictite with metre-sized dolomite clasts (Unit 3). The sedimentological characteristics of the Great Breccia indicate that it formed by a variety of subaqueous sediment gravity flow processes, including matrix-rich debris flows and turbidites. Clasts up to 100 m across contained within the megabreccia unit record catastrophic collapse and failure of local calcareous siliciclastic deposits; other diamictite units were deposited by debris flows. Conglomerates were deposited from high-concentration turbulent flows, and sandstones and fine-grained facies also exhibit characteristics typical of turbidites. Continued downslope remobilization of sediment in the basin following deposition of the Great Breccia is recorded by overlying deformed diamictites, detrital dolomites and siltstones of the Disrupted Beds. The Great Breccia and associated facies within the lowermost part of the Port Askaig Formation are similar to those found in carbonate megabreccia successions that record catastrophic mass failure of carbonate platforms. Failure events in the Port Askaig Formation most likely relate to localized faulting and subsidence of the Dalradian Basin. Although the Port Askaig Formation is considered a Neoproterozoic ‘glacial’ succession, there is little direct evidence for glacial influence on sedimentation in the Great Breccia, making it difficult to fully assess paleoclimatic conditions.

Sedimentologic and sequence stratigraphic model of a Neocomian marine carbonate–siliciclastic ramp: Neuquén Basin, Argentina

Facies analysis of the upper member of the Agrio Formation (Hauterivian–Barremian) in Loma La Torre, central Neuquén Basin, allowed the definition of an open marine ramp mainly characterised by the accumulation of fine-grained sediments of the basinal to outer ramp settings, with subordinated mid to inner ramp silts, carbonate sands, and carbonate buildups. Accumulation conditions for the fine-grained facies match with a low energy environment, below storm-wave base level, poor to null oxygenation of the substrate, and alternating conditions of siliciclastic input and micritic carbonate production. Carbonate-rich facies (wackestones, packstones, grainstones, and biolitites) as well as siltstones and heterolithic intervals may have accumulated under more oxygenated and moderate energy conditions, with some evidence of distal orbital flows. An oxygenation curve was traced from the combined analysis of ichnofacies and sedimentary facies. This curve shows that diversity of ichnofossils is controlled not only by the oxygenation of the substrate, but also by its consistency or even the energy of the environment. Vertical association of facies and stratal geometry allowed the definition of five stratigraphic sections, which were grouped into three depositional sequences related to third order cycles of sea-level change. Each sequence is characterised by a basal transgressive interval (TST) followed by a highstand systems tract (HST). The TSTs are relatively thick and show an aggradational stacking of outer ramp to basinal deposits. The HSTs are characterised by shallower deposits that may show either a strong progradational arrangement, evidenced by low angle clinoforms, or a subhorizontal stratal pattern with a shallowing upward trend. The progradational arrangement of HSTs indicates low rates of accommodation/siliciclastic input or accommodation/carbonate production. This contrasts with transgressive intervals, in which the increase of these rates cause a starved stage, especially in the deepest portions of the basin. The relatively shallower position of the HST deposits allowed the development of high-frequency cycles that seem to be related to high-order eustatic oscillations. In theses cycles, transgressive periods, characterised by anoxic conditions and saturated substrates, pass vertically into highstands represented by more oxygenated and looser substrates in which benthic organisms spread, supplying significant volumes of carbonate sand.

Hidden Cretaceous basins in Nova Scotia

Early Cretaceous unconsolidated quartz sand and kaolinitic clay deposits in the lowlands of Nova Scotia are preserved in narrow half-grabens obscured by glacial drift. The Chaswood Formation sediments can be subdivided into three members; upper and lower members dominated by cyclical sand–mud facies of fluvial origin and the middle member with lignitic clay of lacustrine origin. Ferruginous oxisols are common in the fine-grained facies of the upper and lower members. Seismic data indicate that Chaswood Formation strata in the Elmsvale Basin are deformed into steeply dipping faults and fault-related folds (Rutherford Road fault zone). An Aptian–Albian age for this tectonic event is inferred from synsedimentary deformation and from the angular unconformity spanning the Late Cretaceous and Tertiary that truncates the Chaswood Formation. Exhumation of a thick cover of Mesozoic sediment (1–2 km) is needed to account for the preservation of Chaswood Formation outliers after ~80 Ma of erosion. The half-grabens that host the Chaswood Formation were formed in the Mesozoic and were antecedent to the present-day structurally controlled lowlands.

Holocene glacimarine sedimentation, inner Scoresby Sund, East Greenland: the influence of fast-flowing ice-sheet outlet glaciers

Holocene glacimarine sedimentation associated with fast-flowing outlet glaciers draining the Greenland Ice Sheet is investigated using sedimentary and acoustic data from inner Scoresby Sund, East Greenland. Sedimentation in inner Scoresby Sund is dominated by three processes which are influenced by differences in proximity to fast-flowing outlet glaciers, extent of glacier-ice cover and fjord bathymetry: (1) sediment-gravity flow, principally in the form of turbidity currents and debris flows; (2) suspension sedimentation from turbid meltwater plumes; and (3) iceberg rafting. These processes result in texturally and sedimentologically heterogeneous lithofacies. Proportionally, fine-grained muds (laminated, stratified and massive facies) dominate cores recovered from inner Scoresby Sund, accounting for 80% of the total, whereas diamict facies account for only 15%. Abundant fine-grained muds demonstrate that meltwater flux and sedimentation is significant in this high Arctic glacimarine environment, in settings proximal to fast-flowing outlet glaciers. With increasing distance from these glacier termini, muds are replaced progressively by iceberg-rafted, coarse-grained sediment. The dominance of this iceberg-rafted sediment in outer Scoresby Sund reflects both its more distal location from fast-flowing glacier termini, and the high calving flux associated with these ice masses. Laminated muds deposited by turbidity currents and suspension settling from overflow plumes in inner Scoresby Sund are similar to lithofacies produced in temperate and subpolar glacimarine systems. This implies a similarity in sedimentation processes and resulting facies across a wide spectrum of climatically-, glaciologically- (fast-flowing and non fast-flowing ice-masses) and oceanographically-variable glacimarine settings. Recognition of laminated, fine-grained facies in the geological record therefore does not necessarily indicate a temperate palaeo-glacial setting. However, the predominance of iceberg-rafted diamict facies in ice-distal sedimentary records suggests the former presence of relatively cold environmental conditions in which iceberg-sedimentation played a dominant role.

Megaflute erosion surfaces and the initiation of turbidite channels

Research Article| February 01, 2000 Megaflute erosion surfaces and the initiation of turbidite channels Trevor Elliott Trevor Elliott 1University of Liverpool, Department of Earth Sciences, Liverpool L69 7GP, UK Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (2): 119–122. https://doi.org/10.1130/0091-7613(2000)28<119:MESATI>2.0.CO;2 Article history received: 21 Jul 1999 rev-recd: 24 Sep 1999 accepted: 05 Oct 1999 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Trevor Elliott; Megaflute erosion surfaces and the initiation of turbidite channels. Geology 2000;; 28 (2): 119–122. doi: https://doi.org/10.1130/0091-7613(2000)28<119:MESATI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Distinctive, laterally widespread surfaces, here termed megaflute erosion surfaces, provide evidence of widespread erosion and sediment bypass in a turbidite system and offer insights into the mechanisms by which small-scale turbidite channels can be initiated. The surfaces are erosional into sandstone, are ornamented by exceptionally large flutes, and are overlain by fine-grained facies that onlap the relief of the surface. These surfaces can be traced laterally into deeper erosional surfaces that define sandstone-filled turbidite channels. Once formed, the channels provided a pathway for subsequent turbidity currents while the flanking, slightly elevated megaflute surfaces accumulated fine-grained sediment via overbank and flow stripping processes. These observations imply that the channels were formed by single, high-magnitude, low-frequency turbidity currents and were filled by a succession of lower magnitude, higher frequency flows. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Depositional setting and implications of Paleoproterozoic glaciomarine sedimentation in the Hamersley Province, Western Australia

A combination of facies analysis and the identification of dropstones as well as striated and faceted clasts in diamictites and associated sedimentary rocks from the Hamersley Province confirms earlier suggestions of their glacial origin. Ten lithofacies, comprising two diamictite, four sandstone, and four fine-grained facies, have been identified and arranged into three glaciomarine facies associations. Massive diamictite interbedded with laminated siltstone in the southern part of the province is interpreted to have been deposited peripheral to the margin of rapidly melting tidewater glaciers emanating from more extensive ice cover in highlands to the south. Distal diamictites, sandstones, and laminated siltstones preserved north of the Hardey syncline were deposited from sediment gravity flows, sediment plumes, and rain-out from rare icebergs. Background marine sedimentation consisted of banded iron formation and siltstone. Using these glacial deposits as an event marker, it can be shown that ∼2000 m of turbidites and shallow-marine sedimentary rocks in the Turee Creek Group, on the southern margin of the Hamersley Province, are lateral facies equivalents of as much as 400 m of banded iron formation and mudrock to the north. This correlation demonstrates the markedly asymmetric nature of subsidence in the McGrath trough, which has previously been interpreted as a foreland basin with an orogenic margin to the southwest. New geochronologic data constrain the age of these glacigenic deposits to 2.45–2.20 Ga, which is within the range of present constraints for similar deposits in North America, Fennoscandia, and South Africa. Available paleomagnetic data suggest low-latitude (5°–11°) glaciation for the North American, South African, and West Australian deposits.

Incised valley fill sandstone bodies in Upper Carboniferous fluvio–deltaic strata: recognition and reservoir characterization of Southern North Sea analogues

Multistorey fluvial sandstone bodies in Upper Carboniferous fluvio–deltaic strata are proven reservoirs in the Southern North Sea. Several of these bodies have been interpreted as incised valley fills. Analogous valley fill sandstone bodies have been studied in outcrop and subsurface datasets from Upper Carboniferous successions in the onshore UK and Germany. Using techniques and tools applicable to Southern North Sea datasets, we review the diagnostic criteria of these valley fills: (1) stratigraphic context, characterized by deep erosion into underlying strata, lateral correlation to an interfluve surface and intimate association with an initial flooding surface in overlying strata; (2) basinward facies shift at the base of the sandstone body; (3) erosion of underlying time markers; and (4) distinctive internal architecture, which reflects increasing accommodation space during valley filling. Several features of onshore valley fill sandstones have been characterized in order to constrain the properties of analogous valley fill reservoirs in the Southern North Sea. (1) Valley geometry and extent. Studied onshore valley fills generally have a uniform thickness of 20–25 m, thickening locally to 30–45 m, and widths of 5–25 km. A small number of valley fills, associated with turbidite-fronted deltas, thicken to 50–80m in the 2–5 km adjacent to their mouths. Sheet-like fluvial sandstones overlying sequence boundaries have thicknesses comparable to conventional valley fills, but exceed 35–70 km in width. (2) Valley connectivity; typically, valley fills occur as discrete sandstone bodies, but they may amalgamate laterally, to form sheet-like sandstones, and vertically, producing thick (up to 200 m) sand-prone stratigraphic packages. The internal geometry of these sand-prone packages is complex. (3) Valley fill character. Valley fills generally lack an overall fining-upwards trend and are dominated by coarse- to medium-grained sandstone of low-sinuosity (braided?) fluvial facies throughout. Where present, fine-grained facies are preferentially preserved in the upper part of a valley fill. (4) Detrital mineralogy and provenance. At least some valley fill sandstones exhibit a different composition and provenance to surrounding strata. Such valley fill sandstones are likely to be anomalously feldspathic in the Southern North Sea.

Stratigraphic hierarchy of organic carbon–rich siltstones in deep-water facies, Brushy Canyon Formation (Guadalupian), Delaware Basin, West Texas

The first systematic test for a predictive relationship between organic carbon content and stratigraphic hierarchy in a deep-water slope to basin-floor deposit was performed. The studied section includes the Pipeline Shale, the Brushy Canyon Formation, and the lower part of the Cherry Canyon Formation of the Delaware Mountain Group, West Texas. This interval represents one large-scale, 3rd-order genetic sequence within which 4th- and 5th-order stratigraphic cycles are recognized. Samples of fine-grained facies throughout the section were collected from outcrop and analyzed for organic carbon content and hydrogen index. Degree of pyritization was also determined for a subset of the samples. The results indicate that organic enrichment is closely correlated to the stratigraphic hierarchy at the 3rd-, 4th-, and 5th-order levels. The data suggest that quantity and quality of preserved organic matter are controlled by changes in bulk sedimentation rate (dilution vs. condensation), which affect organic matter inputs to the sediment, as well as the balance between (1) burial and preservation of organic matter and (2) its degradation on the sea floor during times of sediment starvation.

Facies and facies architecture of Paleogene floodplain deposits, Willwood Formation, Bighorn Basin, Wyoming, USA

Paleogene deposits of the Willwood Formation were analyzed in two areas of the Bighorn Basin to provide a better understanding of the facies and facies arrangement of floodplain deposits and the various processes that influence floodplain construction and facies variability. Despite similar facies and facies organization, floodplain deposits in the two areas differ in grain size, hydromorphy and maturity of the paleosols, and scale and organization of ribbon sandstones in the avulsion deposits. These differences appear to be controlled by basin position of the study areas and differences in avulsion between the areas. Two kinds of cumulative floodplain paleosols, red and purple paleosols, formed on overbank deposits. The red paleosols were better drained and formed on less clay-rich parent material. Intense mottling and iron-oxide nodules indicate that the purple paleosols, which formed on more clay-rich parent material, were poorly drained. The grain size differences reflect basin position, in particular, distance from a local sediment source. Avulsion deposits are volumetrically important, comprising about half of the floodplain deposits in both areas. The avulsion deposits resemble the Stage III splays described by Smith et al. (1989) [Smith, N.D., Cross, IA., Dufficy, J.P., Clough, S.R., 1989. Anatomy of an avulsion. Sedimentology 36, 1–24.] in terms of characteristics of their ribbon sandstones and the presence of thin, sheet sandstones or siltstones. Paleosols that formed on the fine-grained facies show morphologic differences interpreted as developmental differences and attributed to differing rates of avulsion in the two study areas. This study shows that avulsion-belt paleosols provide a key to understanding avulsion rate in the stratigraphic record. Other differences between the Willwood avulsion deposits in the two areas, and between these ancient deposits and the modern avulsion deposits described by Smith et al. may reflect: (1) problems in clearly recognizing ancient avulsion deposits in the field, and (2) sampling bias resulting from the areally restricted view provided by the stratigraphic record of an areally extensive modern depositional feature. At the same time, because exposures are three-dimensional, the Willwood strata reveal aspects of the facies and facies arrangement of avulsion belts not readily observed in the modern ones.

Lower Eocene alluvial paleosols: Pedogenic development, stratigraphic relationships, and paleosol/landscape associations

Abstract Two kinds of cumulative floodplain paleosols, red and grey paleosols, formed on overbank deposits of the Willwood Formation in the Sand Coulee area of the Bighorn Basin, Wyoming. Although both kinds experienced down-profile iron movement, due to seasonal saturation of a clay-rich A horizon, the red paleosols were better drained and their B horizons were rubified. Various redoximorphic features indicate that the grey paleosols were poorly drained. The red paleosols show progressive increases in the degree of profile development away from an associated channel sandstone, a paleosol/landscape relationship termed pedofacies by Bown and Kraus (1987). Although the grey paleosols show relatively systematic changes in hydromorphy, consistent pedofacies changes were not recognized. Furthermore, no lateral relationship between the red and grey paleosols was observed. These features suggest that, because of the retarding effects of poor drainage on soil weathering, poorly drained soils are not amenable to pedofacies modelling and that the landscape associations of well drained and poorly drained soils may be difficult to document without unusually extensive exposures. Results of this study also show that the pedofacies model is limited by sediment accumulation rates and the kind of fine-grained facies on which paleosols developed. Although readily observable in Sand Coulee, pedofacies are difficult to recognize in the Elk Creek area, where accumulation rates were approximately half as rapid as in Sand Coulee. The attainment of steady-state conditions is believed to have obscured pedofacies in the Elk Creek area. Finally, pedofacies are only developed on true overbank deposits, which, in the Willwood Formation and probably many other ancient alluvial sequences, make up only a fraction of the fine-grained deposits. Only immature soils formed on the remainder of the fine-grained facies because they were deposited very rapidly, as a result of channel avulsion.

Internal architecture and sedimentary evolution of coarse‐grained, turbidite channel‐levee complexes, Early Eocene Regência Canyon, Espírito Santo Basin, Brazil

ABSTRACTSuccessions of Early Eocene coarse‐grained turbidites up to 400 m thick fill fault‐controlled canyons along the eastern Brazilian continental margin. They form part of a Late Albian to Early Eocene transgressive succession characterized by onlapping, deepening‐upward sedimentation. In the Lagoa Parda oil field (Regência Canyon, Espírito Santo Basin) the turbidite facies consist mostly of unstratified conglomerate and sandstone, with interbedded bioturbated mudstone and thin‐bedded, stratified sandstone. Within the main Regência Canyon, the coarser grained facies occur within 38 deeply incised channels. The fills are 9 to &gt;50 m thick, 210 to &gt;1050 m wide and &gt;1 km long. The finer grained facies build asymmetrical levees that are higher and thicker on the left side (looking downstream) of their channels, probably as an effect of the Coriolis force (to the left in the Southern Hemisphere). Nine levee successions up to 50 m thick are associated with the 20 youngest channels. The deposits filling the low‐sinuosity Lagoa Parda channels record successive channel abandonment through relatively rapid avulsions. Avulsions of unleveed channels took place randomly, but channels with well‐developed levees show preferential avulsion to the right (looking downstream), opposite to the direction of preferential levee growth.Lagoa Parda channels can be grouped into three complexes 20–100 m thick. These complexes have an estimated duration of about 140 000 years. It is suggested that control of the development of individual channel complexes was related to variation in sediment supply, in turn probably related to climatic changes. The deposition of each channel complex would have followed an increase in sediment supply into the Regência Canyon through delta/fan‐delta and littoral drift systems, which in turn would have responded to phases of higher denudation rates in the high‐relief, ancestral coastal ranges of south‐eastern Brazil.Overall, the three Lagoa Parda channel complexes form a turbidite succession characterized by channel fills that become narrower, thinner and finer grained upward. These trends were induced mostly by a longer term (&gt;400 000 years) decrease in sediment supply, which in turn resulted from the combined effects of a long‐term (second‐order) trend of sea‐level rise, and the decreasing fault activity at the basin margin and source area.

Stratigarphy and Sedimentology of the Kuşçular Formation (Lower Paleocene), the West of Elazığ, Turkey

Stratigraphic and sedimentological aspects of Kuşçular Formation has been firstly recognized in this study which represent in the west of Elazığ. The sediments of this formation consist of seven lithofacies. they are; 1- Massive conglomerates, 2-Stratified conglomerates, 3-Massive sandstones, 4-Horizantel bedded sandstones, 5-Red mudstones, 6-Nodular gypsum, 7-Stratified gypsums. Fine-grained facies are dominant at the lower levels of sequences represent in the prograding alluvial fan envirenments. These successions represent a depositional basin which rapidly filled. Ions necessary for gypsium occurances in the playa deposits are thought to come from the sulfide minerals of Elazığ Complex, crystalline limestone of Keban Metamorphics and Harami Formation.

Accommodation-Based Controls on Fluvial-Deltaic Reservoir Compartmentalization: Examples from the Oligocene Frio Formation, South Texas: ABSTRACT

Accurate prediction of compartment architecture and intracompartment heterogeneity is necessary to locate and recover the estimated 15 billion barrels of mobile oil remaining in U.S. fluvial-dominated deltaic reservoirs. To improve this prediction, facies-specific relationships between accommodation trends and sand-body architecture that were established by outcrop studies in the western interior were Successfully applied in a study of Oligocene Frio Formation reservoirs along the Vicksburg Fault Zone of South Texas. Compartment architecture and internal heterogeneity were determined for mature upper and lower delta-plain reservoirs within two 4th-order genetic units to document reserve growth potential. In lower delta-plain settings, low-accommodation conditions at the base of genetic units produce narrow distributary channels that feed many small mouth bars, whereas high-accommodation conditions near the top of genetic units yield broader distributaries arid an expansive wave-dominated delta front. In proximal upper delta-plain settings, low accommodation produces few narrow fluvial channels that are commonly internally trough cross-strata predominate, with the finer grained silty upper point-bar deposits being removed by successive scour events during which base level has not risen substantially. In contrast, high accommodation yields many broad internally heterogeneous channels in which high and erosion of fine-grained channel-top facies. This study documents the profound effect thatmore » accommodation conditions can have suiting production behavior. Similar studies in other settings are needed to identify these relationships for the entire spectrum of depositional systems.« less

Sequence-stratigraphical interpretation of organic facies variations in marine siliciclastic systems: general principles and application to the onshore Kimmeridge Clay Formation, UK

Abstract The occurrence and stratigraphical distribution of source rock potential in distal fine-grained facies is strongly related to the prevailing palaeo-oxygenation regime, especially during the deposition of the condensed section (CS). The CS will tend to correspond to organic facies B-AB, with good to excellent source-rock potential, only when conditions are dysoxic-anoxic, and organic facies D-CD, with poor source rock potential, when conditions are oxic. The applicability of the Creaney and Passey organic facies model is limited by its assumption that palaeo-oxygenation remains constant within the basinal facies. The CS commonly shows decimetre (Milankovitch)-scale redox cyclicity characterized by total organic carbon (TOC) and organic facies variations whose amplitude increases as sediment accumulation rates decrease. The greater frequency of dysoxic-anoxic conditions in the CS may be related to changes in accommodation space via the relationship between bottom water volume and the rate of deoxygenation, as observed in modern settings. The onshore Kimmeridge Clay diverges from the classic Exxon basin model as it was deposited in an intra-shelf setting where focusing and ponding of clayey sediment resulted in the basinal sections being both thicker and more complete. However, the best source rock intervals still coincide with the maximum flooding surfaces. The long-term Late Jurassic (shallow to deep to shallow) sea level curve appears to influence the mean (oxic to dysoxic-anoxic to oxic) palaeo-oxygenation regime in the basin and the background trend in sediment accumulation rate; it is expressed in a symmetrical increase in TOC to 4–6 wt% from background values of about 1 wt%. The superimposed decimetre- to metre-scale redox cycles appear to be controlled by Milankovitch climatic controls on water mass stratification. The relative spacing of these cyclic organic-rich interbeds decreases during relatively condensed (maximum flooding surface) intervals associated with the short-term sea-level curve, and the interval TOC values also increase and broaden. Intervals deposited during or shortly after lowstands are less organic-rich and have fewer and more widely spaced organic-rich beds. The individual organic-rich beds also appear to be condensed relative to adjacent mudstones; palynofacies and TOC versus hydrogen index data suggest the origin of these beds is due more to cyclically decreased dilution and redox conditions rather than enhanced palaeoproductivity.

Tidal rhythmites infine-grained Carboniferous limestones, U.S.A.

Analyses of fine-grained limestones reveals that many exhibit fine-scale laminations. Laminations can be normally graded and consist of a coarser-grained lower part and a finer-grained upper part. The upper part can also contain finely disseminated organic material. Despite the similarities of such graded laminae to yearly varves and turbidites, it can be demonstrated by use of laminae-thickness periodicities that some graded laminae are reasonably interpreted as the product of tidal processes. Within siliciclastic systems, modern analogues of such processes are available for comparisons. In fine-grained facies of the Salem Limestone (Visean; Indiana, U.S.A.), periodicities observed within sequential-laminae thicknesses indicate a dominant control by neap-spring tidal processes. Similarly, laminae within limestones of the vertebrate-bearing Hamilton paleochannel (Stephanian; Kansas, U.S.) exhibit similar features, including fine-scale tidal bundles. This limestone is noted for the abundance of articulated fish fossils. Carbonates containing articulated fish from the Wild Cow Formation (Stephanian; New Mexico, U.S.), exhibit diffuse laminations; however, closely associated siliciclastic mudstones contain laminae that exhibit tidal periodicities. There are many similarities between tidal periodicities and patterns of lamination thicknesses of these rocks. A tidal interpretation for these rocks allows for localized, very rapid rates of deposition. Such rapid deposition may, in part, help to explain how articulated fish and other vertebrates can become preserved within such fine-grained limestones.

Gravel and Sand Flotation: A Sediment Dispersal Process Important in Certain Nearshore Marine Environments

ABSTRACT Small rafts of floating sediment, predominantly of granule and small pebble size, were observed along a tidewater coast on James Ross Island, Antarctica. The sediment was lifted from the beach by the advancing tidewater and kept afloat by surface tension. The current transported the rafts at least 100-150 m off the beach, where the rafts broke up due to wind agitation of the sea surface and the gravel sank. This process can transport considerable volumes of sediment from the beach: it is estimated that flotation at the Naze launch ca. 500 kg per kilometer of shoreline per tidal cycle during calm and dry weather conditions, which for one summer season might end up in a total of 0.022 tons of sediment per meter of shoreline. The resulting sediments and sedimentary structures are conclud d to be similar to those resulting from iceberg-, sea ice-, and algae rafting, and sediment gravity flows. Floating can he an important way of rafting gravels into fine-grained sublittoral or shallow-marine facies, and should be included when dealing with processes related to sedimentation in the nearshore marine environment.

Microfacies of Cambrian Limestones in Jordan

The limestones of the Wadi Nasb Formation of the uppermost Lower Cambrian of Jordan are under- and overlain by massive sandstones of a near-shore facies. Facies analysis is based on samples from an outcrop at the northeastern shore of the Dead Sea and two oil test wells in the Wadi Sirhan Depression in eastern Jordan. Limestones were deposited in the shallow sea and within the coastal tidal area. Cyanobacteria, algae, echinoderms, trilobites and hyoliths have contributed the bulk of the carbonate and phosphatic material composing the Wadi Nasb limestone. Fine-grained facies types are composed of peloidal carbonate muds with laminar and nodular algal and cyanobacterial mats. They formed within a quiet tidallagoonal environment. The coarse grained facies types consist of carbonate sands with layers of sheell debris deposited in crossbeds in an environment with a rich endobenthic fauna. Here most particles were coated by cyanobacterial crusts. Ooids, oncoids and various coated grains are present. Consolidated sediments were commonly eroded within or near to this environment and their remains were integrated within the sands. Diagenesis is reconstructed step by step with deposition, first cementation, aragonite dissolution, compaction, pore filling, formation of pressure solution, growth of dolomite and anhydrite within the calcitic limestone and final fissure formation and filling.

Sedimentology of pennsylvanian quartzose sandstones of the lee formation, central appalachian basin: fluvial interpretation based on lateral profile analysis

Lateral profiling, including bounding surface and architectural-element analyses, are utilized to demonstrate a fluvial origin for the quartzose sandstones of the Lee Formation. Internal architecture of the sandstones consists primarily of multistorey and multilateral channel bodies. Individual channel elements, up to 20 m thick, contain a complex hierarchy of bedform deposits. The primary depositional elements within the channels were downstream-accreting (mid-channel?) macroforms; channel elements frequently contain deposits of more than one macroform. Reconstruction of the macroforms reveals accretion primarily by superposed bedforms that migrated down a low-angle front. Steeper, giant foresets, transitional along flow with the low-angle facies, indicate that the macroform episodically developed a steep slipface. The uppermost channels within the Rockcastle Member contain macroform elements with components of lateral accretion, interpreted as deposits of alternate bank-attached macroforms. Other depositional elements recognized within channel-fill deposits are minor-channel, sandy-bedform, gravity-flow (attributed to bank slumping), and channel-bottom elements. The latter element is contained within a facies sequence that suggests rising- to flood- to waning-stage deposition. In general, deposition probably occurred during relatively high stage; little evidence of low-stage flow was recognized. Subordinate fine-grained facies are interpreted as levee and overbank deposits. The greater part of the Lee Formation was deposited in deep, low-sinuosity, bedload-dominated fluvial channels. Strongly unimodal paleocurrents, lack of facies that suggest low-stage reworking and paucity of lateral-accretion features indicate deposition in a single-channel system (i.e., a low braiding index).

Late Cretaceous Inoceramid Bivalves of the Kuskokwim Basin, Southwestern Alaska, and Their Implications for Basin Evolution

Upper Cretaceous rocks of the Kuskokwim Group are exposed in a large region of southwestern Alaska and are mainly composed of deformed turbidite deposits that contain few fossils other than inoceramid bivalves. This paper documents the taxonomy of the inoceramids in the Kuskokwim Group, develops an inoceramid biostratigraphy based on known ranges in other regions, and analyzes biogeographic patterns, paleoecology, and depositional history of the Kuskokwim Group.Most of the inoceramid bivalves present in the Kuskokwim Group are of Cenomanian and Turonian age, and an assemblage of species typical of late Turonian age rocks is particularly well developed. Only two localities appear to be as young as Santonian age. The following 16 species or subspecies are discussed and illustrated in detail:Birostrina tamuraiMatsumoto and Noda,Inoceramus virgatusSchlüter,I. pennatulusPergament,I. pictus minusMatsumoto,I.cf.I. yabeiNagao and Matsumoto,I.? sp. aff.I. costatusNagao and Matsumoto,I. hobetsensisNagao and Matsumoto,I. longealatusTröger,I. frechiFlegel,I. waltersdorfensis waltersdorfensisAndert,I.cf.I. waltersdorfensis hannovrensisHeinz,I. kuskokwimensisn. sp.,Mytiloidescf.M. opalensis(Böse),M. teraokai(Matsumoto and Noda),M.cf.M. incertus(Jimbo), andSphenoceramus naumanni(Yokoyama). In addition, a specimen with affinities toMytiloides striatoconcentricus carpathicus(Simionescu) and a specimen that may belong to theI. (Cremnoceramus?) rotundatus–I.(C.)erectuslineage are illustrated.Most of the taxa present in the Kuskokwim region are found in other regions of the North Pacific, particularly Japan and eastern Siberia, or are found throughout the Northern Hemisphere. Only one species,I. kuskokwimensisn. sp., is new and may be endemic. North Pacific taxa are predominant in the Kuskokwim region, but intervals near the Cenomanian–Turonian Stage boundary and in the upper Turonian contain taxa characteristic of Europe and the Western Interior basin of North America; some of these taxa have not been recorded previously in the North Pacific region. Turonian heteromorph ammonite assemblages associated with inoceramids in the finer grained facies of the Kuskokwim region are similar to those found in coeval rocks of Japan and Germany.The depositional area of the Kuskokwim Group can be broken into two northeast-trending subbasins, the Kuskokwim River subbasin to the northwest and the Mulchatna River subbasin to the southeast, connected by the Nushagak Hills corridor. Within the Kuskokwim River subbasin, deposition apparently started earlier in the north (middle Cenomanian) than in the south (late Cenomanian to early Turonian), and prograding deltaic sedimentation along the western margin also appears to have started earlier in the north. No marine fossils younger than latest Turonian to earliest Coniacian are known from the Kuskokwim River subbasin. The youngest fossils identified are Santonian in age and are from deep-water deposits in the Nushagak Hills corridor. Few fossils are known from the Mulchatna River subbasin and age control is limited.

Biostratigraphic and Paleoecologic Characterization of Cretaceous Sequences of the Western Interior Basin of North America

Five second-order and numerous third-order sedimentary sequences have been recognized in Cretaceous strata of the Western Interior basin, North America. Regional definition of the predominantly transgressive (TST) and highstand systems tracts (HST), from nearshore coarse clastics to basinal fine-grained facies, depends on a system of detailed biostratigraphy and chronostratigraphic correlation. A predominantly molluscan-based biostratigraphy utilizing composite assemblage zonation provides correlations across the basin at 0.1-0.3 Ma/biozone levels of resolution. High-resolution event chronostratigraphy provides regional correlation at finer (40-100 Ka) intervals, based on integrated bioevent, chemoevent, physical, and composite event units tied through graphic correlation to assemblage biostratigraphy and geochronology. Further, paleoecologic data that reflect changes in relative sea level can be used to better define systems tracts, especially in fine-grained basinal facies where sequence stratigraphic boundaries may be poorly expressed. These data include (1) taphonomic and community data reflecting condensation and offshore starvation during TST and TST-HST boundaries (e.g., shell lags and marine oyster beds); (2) systems tract-specific communities; (3) macrofaunal diversity trends; (4) temporal biofacies patterns where predominantly epifaunal communities characterize most of the basinal TST and early HST, whereas mixed epi- and infaunal communities characterize earliest TST and later HST and lowstand systems tracts; and (5) lateral biofaciesmore » patterns for each systems tract.« less