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1.
Sarbani Patranabis Deb 《Journal of Earth System Science》2005,114(3):211-226
The Neoproterozoic Kansapathar Sandstone of the Chattisgarh basin, a shallow marine shelf bar sequence, consists of mineralogically
and texturally mature sandstones with subordinate siltstones, mudstones and conglomerates. The sediments were transported,
reworked and deposited in subtidal environments by strong tidal currents of macrotidal regime as well as storms, and accumulated
as discrete shoaling-upward features, separated from each other by muddy to low-energy sandy deposits. The sandbodies developed
into shoaling up linear bars, often more than a kilometre in length, through accretion of thick cross-stratified units in
transverse directions under the influence of ebb and flood tidal currents, as well as in longitudinal direction affected by
southeasterly flowing along-shore currents. The aggrading upper surfaces of the bars experienced protracted reworking by strong
oscillatory wave currents leading to extensive development of subaqueous 2D or 3D dunes mantled with lag pebble deposits at
different points. With continued shoaling and progradation, the bars amalgamated into large sandstone sheets with the development
of high energy beach deposits and coastal sand flats in the uppermost part of the sequence. The presence of rill marks, flat-topped
ripples, wrinkle marks, desiccation cracks and adhesion warts point to intertidal conditions with intermittent exposure. The
high energy sandstone bars overlie a thick mudstone-dominated shelf sequence across a sharp interface indicating rapid change
in the sea-level, provenance, rate of sediment generation and sediment input, and circulation condition in the shelf. A quiet
muddy shelf was replaced by a major sand-depositing environment with strong, open marine circulation. An interplay of tidal
currents, oscillatory wave currents and storm currents generated a complex flow pattern that varied in time and space from
bimodal-bipolar to strongly unimodal flows.
Close parallelism of wave ripple crests, trend of linear bars and unidirectional flows suggest that the elongate bars were
parallel to sub-parallel to the coastline, and were strongly influenced by along shore drift. The inferred coastline was broadly
N-S. The large-scale structures in the bar sandstones, emplacement of vast amount of sand and migration of large bedforms
under strong macrotidal currents collectively indicate that the Kansapathar shelf was intimately connected with an open ocean
basin towards north-northwest. 相似文献
2.
The lower part of the Cretaceous Sego Sandstone Member of the Mancos Shale in east‐central Utah contains three 10‐ to 20‐m thick layers of tide‐deposited sandstone arranged in a forward‐ and then backward‐stepping stacking pattern. Each layer of tidal sandstone formed during an episode of shoreline regression and transgression, and offshore wave‐influenced marine deposits separating these layers formed after subsequent shoreline transgression and marine ravinement. Detailed facies architecture studies of these deposits suggest sandstone layers formed on broad tide‐influenced river deltas during a time of fluctuating relative sea‐level. Shale‐dominated offshore marine deposits gradually shoal and become more sandstone‐rich upward to the base of a tidal sandstone layer. The tidal sandstones have sharp erosional bases that formed as falling relative sea‐level allowed tides to scour offshore marine deposits. The tidal sandstones were deposited as ebb migrating tidal bars aggraded on delta fronts. Most delta top deposits were stripped during transgression. Where the distal edge of a deltaic sandstone is exposed, a sharp‐based stack of tidal bar deposits successively fines upward recording a landward shift in deposition after maximum lowstand. Where more proximal parts of a deltaic‐sandstone are exposed, a sharp‐based upward‐coarsening succession of late highstand tidal bar deposits is locally cut by fluvial valleys, or tide‐eroded estuaries, formed during relative sea‐level lowstand or early stages of a subsequent transgression. Estuary fills are highly variable, reflecting local depositional processes and variable rates of sediment supply along the coastline. Lateral juxtaposition of regressive deltaic deposits and incised transgressive estuarine fills produced marked facies changes in sandstone layers along strike. Estuarine fills cut into the forward‐stepped deltaic sandstone tend to be more deeply incised and richer in sandstone than those cut into the backward‐stepped deltaic sandstone. Tidal currents strongly influenced deposition during both forced regression and subsequent transgression of shorelines. This contrasts with sandstones in similar basinal settings elsewhere, which have been interpreted as tidally influenced only in transgressive parts of depositional successions. 相似文献
3.
《Sedimentary Geology》2006,183(1-2):99-124
The snowball Earth hypothesis suggests that the Neoproterozoic was characterized by several prolonged and severe global glaciations followed by very rapid climate change to ‘hot house’ conditions. The Neoproterozoic Port Askaig Formation of Scotland consists of a thick succession of diamictite, sandstone, conglomerate and mudstone. Sedimentological and stratigraphic analysis of Port Askaig deposits exposed on the Garvellach Islands was carried out to establish the nature of Neoproterozoic palaeoenvironmental change preserved in this thick succession. Particular emphasis was placed on identifying and distinguishing between climatic and tectonic controls on sedimentation.Port Askaig Formation diamictite units are attributed to deposition by sediment gravity flow processes or ‘rainout’ of fine-grained sediment and ice-rafted debris in a glacially influenced marine setting. Associated facies record various depositional processes ranging from sediment gravity flows (conglomerate, massive sandstone and laminated mudstone) to deposition under other unidirectional currents (cross-bedded and horizontally laminated sandstone). The Port Askaig Formation is also characterized by abundant soft sediment deformation features that occur at discrete intervals and are interpreted to record episodic seismic activity.Stratigraphic analysis of the Port Askaig Formation on the Garvellach Islands reveals three phases of deposition. Phase I was dominated by sediment gravity flow processes and sedimentation was primarily tectonically controlled. Phase II was a transitional phase characterized by continued tectonic-instability, an increased supply of sand to the basin and the preservation of current-generated facies. In the third and final phase of deposition, the interbedded units of sandstone and diamictite are interpreted to reflect development of large sandy bedforms and ice margin fluctuations in a tectonically stable marine setting.Sedimentological and stratigraphic analysis of the Port Askaig Formation demonstrates that tectonic activity had a significant influence on development of the lowermost parts of the succession. Climatic influences on sedimentation are difficult to identify during such phases of tectonic activity but are more easily discerned during episodes of tectonic quiescence (e.g.,, Phase III of the Port Askaig Formation). The thick succession of diamictite interbedded with current-deposited sandstone preserved within the Port Askaig Formation is not consistent with deep freeze conditions proposed by the snowball Earth hypothesis. 相似文献
4.
TThe Roper Group is a cyclic, predominantly marine, siliciclastic succession of Calymmian (Early Mesoproterozoic) age. It has a distribution of at least 145 000 km2 and a maximum known thickness of ～5000 m. In the Roper River district the middle part of the Roper Group (～1300 m thick) is characterised by the cyclical alternation of mudstone and sandstone units, and can be divided into six third‐order depositional sequences. A typical sequence is broadly progradational in aspect, and comprises a lower, mudstone‐rich, storm‐dominated shelf succession (up to 330 m thick), and a sequence‐capping unit dominated by tidal‐platform cross‐bedded sandstone (up to 80 m thick); both are interpreted as highstand systems tracts. Transgressive strata are poorly represented but where present are characterised by paralic to fluvial redbed assemblages that include ooidal ironstone. Roper Group sequences lack a distinct condensed section and sequence boundaries are mostly conformable. Erosional contacts separate mud‐rich shelf facies from sequence‐capping sandstones. We infer that these erosion surfaces were generated by episodic flexural tectonism, which also generated the accommodation and sediment supply for Roper sequences. 相似文献
5.
Dunes and bars are common elements in tide‐dominated shelf settings. However, there is no consensus on a unifying terminology or a systematic classification for thick sets of cross‐stratified sandstones. In addition, their ichnological attributes have hardly been explored. To address these issues, the properties, architecture and ichnology of compound cross‐stratified sandstone bodies contained in the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains are described here. In these transgressive sandstones, five types of compound cross‐stratified sandstone are distinguished based on foreset geometry, sedimentary structures and internal heterogeneity. These represent four broad categories of subtidal sandbodies: (i) compound‐dune fields; (ii) sand sheets; (iii) sand ridges; and (iv) isolated dune patches; tidal bars comprise a fifth category but are not present in the Gog Group. Compound‐dune fields are characterized by sigmoidal and planar cross‐stratified sandstone in coarsening‐upward and thickening‐upward packages (Type 1); these are mostly unburrowed, or locally contain representatives of the Skolithos ichnofacies, but are intercalated with intensely bioturbated sandstone containing the archetypal Cruziana ichnofacies. Sand‐sheet complexes, also composed of compound dunes, cover more extensive subtidal areas, and comprise three adjacent subenvironments: core, front and margin. The core is characterized by thick‐bedded sets of cross‐stratified sandstone (Type 2). A decrease of bedform size at the front is recorded by wedges of thinner‐bedded, low‐angle and planar cross‐stratified sandstone (Type 3) exhibiting dense Skolithos pipe‐rock ichnofabric. The margin is characterized by interbedded sandstone and mudstone, and hummocky cross‐stratified sandstone. Sand‐sheet deposits exhibit clear trends in trace‐fossil distribution along the sediment transport path, from non‐bioturbated beds in the core to Skolithos ichnofacies at the front, and a depauperate Cruziana ichnofacies at the margin. Tidal sand ridges are large elongate sandbodies characterized by large sigmoid‐shaped reactivation surfaces (Type 4). Sand ridges display clear ichnological trends perpendicular to the axis of the ridge, with no bioturbation or a poorly developed Skolithos ichnofacies in the core, a depauperate Cruziana ichnofacies in lee‐side deposits, and Cruziana ichnofacies at the margin. While both tidal ridges and tidal bars migrate by means of lateral accretion, the latter occur in association with channels while the former do not. Because tidal bars tend to occur in brackish‐water marginal‐marine settings, their ichnofauna are typically of low diversity, representing a depauperate Cruziana ichnofacies. Isolated dune patches developed on sand‐starved areas of the shelf, and are represented by lenticular sandbodies with sigmoidal reactivation surfaces (Type 5); they typically lack trace fossils, but the interfingering muddy deposits are intensely bioturbated by a high‐diversity fauna recording the Cruziana ichnofacies. The variety of sandbody types in the Gog Group reflects varying sediment supply and location on the inner continental shelf. These, in turn, governed substrate mobility, grain size, turbidity, water‐column productivity and sediment organic matter which controlled trace fossil distribution. 相似文献
6.
The role of wave reworking on the architecture of storm sandstone facies, Bell Island Group (Lower Ordovician), eastern Newfoundland 总被引：2，自引：0，他引：2
Stacked shallow marine cycles in the Lower Ordovician, Bell Island Group, of Bell Island, Newfoundland, show upward thickening and upward coarsening sequences which were deposited on a storm-affected shelf. In the Beach Formation each cycle has a facies sequence comprised, from base to top, of dark grey mudstones, light grey mudstones, tabular sandstones and mudstones, lenticular sandstones and mudstones, and thick bedded lenticular sandstones, reflecting a progressive increase of wave orbital velocities at the sediment surface. The mudstones and tabular sandstones reflect an environment in which the sea floor lay in the lower part of the wave orbital velocity field and in which tempestites were deposited as widespread sheets from weak combined flow currents. The lenticular sandstones in the succeeding facies are wave reworked sands, commonly lying in erosional hollows and having erosional tops and internal hummocky cross-stratification. Planar lamination is relatively uncommon and sole marks are mainly absent. In this facies oscillatory currents were dominant and accumulated sand in patches generally 10–30 m in diameter. The facies formed on the inner shelf where the oscillatory currents generated by storm waves had powerful erosional effects and also determined the depositional bedforms. Mud partings and second-order set boundaries within sandstone beds are believed to separate the products of individual storms so that many lenticular sandstone beds represent the amalgamation of several event beds. This interpretation has important implications for attempts to estimate event frequency by counting sandstone beds within a sequence and for estimates of sand budgets during storm events. The thick bedded lenticular facies appears to have been formed by erosion of the mud beds between the lenticular sands, leading to nearly complete amalgamation of several lenticular sand bodies except for residual mud partings. In the overlying Redmans Formation the process of amalgamation progressed even further so that nearly all the mud partings were removed, resulting in the formation of thick bedded tabular sandstones. Sequence stratigraphic analysis of the cyclical sequence suggests that the cycles were eustatically controlled. The rising limb of the sea level curve produced only the dark grey mudstone part of the cycle while the remainder of the cycle was deposited on the falling limb. There is a gradational but rapid facies transition from the tabular to the lenticular sandstone facies which is interpreted as occurring at the inflexion point on the falling limb. The thick bedded facies of the Beach Formation and the thick bedded tabular facies of the Redmans Formation represent periods of maximum sea level fall. The stacked cycles in the Beach Formation are interpreted as an aggradational, high frequency sequence or parasequence set bounded at the top by a sequence boundary and succeeded by the three aggradational parasequences of the Redmans Formation. The recognition of storm facies with sandstone beds of very different bed length has important implications for the reservoir modelling of such facies. 相似文献
7.
HOWARD D. JOHNSON 《Sedimentology》1977,24(2):245-270
Five coarsening upward shallow marine sandstone sequences (2–10 m thick), are described from the late Precambrian of North Norway, where they occur in a laterally continuous and tectonically undeformed outcrop. The sequences consist of five facies with distinct assemblages of sedimentary structures and palaeocurrent patterns. Each facies is the product of alternate phases of sedimentation during relatively high- and low-energy periods. Facies 1 to 4 are interpreted as representing prograding, subtidal sand bars. Sand bar progradation occurred during the highest energy periods when unidirectional currents flowed to the northwest, depositing trough cross-bedded sandstones (facies 3 and 4) on the bar crests and flanks, and sheet sandstone beds (facies 1 and 2) in the offshore environments. Weaker northwesterly flowing currents continued during moderate energy fair weather periods. Low energy fair weather periods were dominated by wave processes, which formed largescale, low-angle, westerly inclined surfaces on the bar flanks (facies 4) and wave rippled sandstone beds (facies 2) and flat laminated siltstone layers (facies 1) in the offshore environments. One sand bar was dissected by channels and infilled by tabular cross-bedded sandstones (facies 5). Bipolar palaeocurrent evidence, with two modes separated into two laterally equivalent channel systems, suggests deposition by tidal currents in mutually evasive ebb and flood channels. The inferred processes of these sand bars are compared with those associated with modern storm-generated and tidal current generated linear sand ridges. Both are influenced by the interaction of relatively low and high energy conditions. The presence of the tidal channel facies, however, combined with the inferred strong bottom current regime, is more analogous to a tidal current hydraulic regime. 相似文献
8.
The Kaskapau Formation spans Late Cenomanian to Middle Turonian time and was deposited on a low‐gradient, shallow, storm‐dominated muddy ramp. Dense well log control, coupled with exposure on both proximal and distal margins of the basin allows mapping of sedimentary facies over about 35 000 km2. The studied portion of the Kaskapau Formation is a mudstone‐dominated wedge that thins from 700 m in the proximal foredeep to 50 m near the forebulge about 300 km distant. Regional flooding surfaces permit mapping of 28 allomembers, each of which represent an average of ca 125 kyr. More than 200 km from shore, calcareous silty claystone predominates, whereas 100 to 200 km offshore, mudstone and siltstone predominate. From about 30 to 100 km offshore, centimetre‐bedded very fine sandstone and mudstone record along‐shelf (SSE)‐directed storm‐generated geostrophic flows. Five to thirty kilometres from shore, decimetre‐bedded hummocky cross‐stratified fine sandstone and mudstone record strongly oscillatory, wave‐dominated flows whereas some gutter casts indicate shore‐oblique, apparently mostly unidirectional geostrophic flows. Nearshore facies are dominated by swaley cross‐stratified or intensely bioturbated clean fine sandstone, interpreted as recording, respectively, areas strongly and weakly affected by discharge from distributary mouths. Shoreface sandstones grade locally into river‐mouth conglomerates and sandstones, including conglomerate channel‐fills up to 15 m thick. Locally, brackish lagoonal shelly mudstones are present on the extreme western margin of the basin. There is no evidence for clinoform stratification, which indicates that the Kaskapau sea floor had extremely low relief, lacked a shelf‐slope break, and was probably nowhere more than a few tens of metres deep. The absence of clinoforms probably indicates a long‐term balance between rates of accommodation and sediment supply. Mud is interpreted to have been transported >250 km offshore in a sea‐bed nepheloid layer, repeatedly re‐suspended by storms. Fine‐grained sediment accumulated up to a ‘mud accommodation envelope’, perhaps only 20 to 40 m deep. Continuous re‐working of the sea floor by storms ensured that excess sediment was redistributed away from areas that had filled to the ‘accommodation envelope’, being deposited in areas of higher accommodation further down the transport path. The facies distributions and stratal geometry of the Kaskapau shelf strongly suggest that sedimentary facies, especially grain‐size, were related to distance from shore, not to water depth. As a result, the ‘100 to >300 m’ depth interpreted from calcareous claystone facies for the more central parts of the Interior Seaway, might be a significant overestimate. 相似文献
9.
The Frewens sandstone is composed of two elongate tide-influenced sandstone bodies that are positioned directly above and slightly landward of a more wave-influenced lobate sandstone. The 20-km-long, 3-km-wide Frewens sandstone bodies coarsen upwards and fine away from their axes, have gradational bases and margins and have eroded tops abruptly overlain by marine shales. These sandstones are superbly exposed in large cliffs on the banks of the South Fork of the Powder River in central Wyoming, USA. The deposits change upwards from thinly interbedded sandstones and mudstones to metre-thick heterolithic cross-strata and, finally, to metres-thick sandstone-dominated cross-strata. There is abundant evidence for tidal modulation of depositional flows; however, palaeocurrents were strongly ebb-dominated and nearly parallel the trend of sandstone-body elongation. Detailed mapping of stratal geometry and facies across these exposures shows a complex internal architecture. Large-scale bedding units within sandstone bodies are defined by alternations in facies, bed thickness and the abundance of shales. Such bedsets are inclined (5°–15°) in walls oriented parallel to palaeoflow and gradually decrease in dip over hundreds of metres as they extend from the sandstone-dominated deposits higher in a sandstone body to muddier deposits lower in the body. Where viewed perpendicular to palaeoflow, bedsets are 100-metre-wide lenses that shingle off the sandstone-body axis towards its margins. The sandstone bodies are interpreted as sand ridge deposits formed on the shoreface of a tide-influenced river delta. Metres-thick cross-strata in the upper parts of sandstone bodies resemble deposits of bars (sandwaves) formed where tidal currents moved across shallows and the tops of tidal ridges. Heterolithic deposits lower in sandstone bodies record fluctuating currents caused by ebb and flood tides and varying river discharge. Erosion surfaces capping sandstone bodies record tidal ravinement. The tidal ridges were abandoned following transgression and covered with marine mud as waters deepened. 相似文献
10.
EDWARD COTTER 《Sedimentology》1985,32(2):195-213
Within the Kinsale Formation (Lower Carboniferous) of southern Ireland are pebbly sandstones and conglomerates contained in what is known locally as the Garryvoe conglomerate facies. In this facies there are three main groups of lithologies: (a) heterolithic mudrocks and sandstones characterized by a wide variety of wave-produced structures; (b) sandstones dominated by swaley cross-stratification (SCS), parallel lamination, and rare hummocky cross-stratification (HCS); and (c) pebbly sandstones and conglomerates occurring as discrete beds or as gravel clasts dispersed through SCS sets. Successions of the facies comprise units of heterolithic mudrock and rippled sandstone alternating repeatedly with coarsening-upward units of SCS pebbly sandstone capped by top-surface granule and pebble lags. The Garryvoe conglomerate facies accumulated in a system of offshore bars on a muddy shallow-marine shelf that was dominated by waves and currents generated by storms. Sands and gravels were bypassed from a contemporaneous northerly coastal zone to the shelf, where they were moulded by the storm-generated flow into low, broad, sand ridges (offshore bars). The elongate bars were spaced kilometres apart, oriented obliquely to the coast, and separated by muddy interbar troughs. Their surfaces were largely covered by hummocky and swaley forms. Long-term, gradual seaward migration of the offshore bars concentrated gravels on landward flanks from the dispersed pebbly sands that were on the crests and seaward flanks. Exceptionally intense storms could form laterally extensive winnowed gravel lags above thinned bar sequences. Such storms could also flush gravel-bearing turbidity currents into muddy interbar trough areas. 相似文献
11.
The late Barremian succession in the Agadir Basin of the Moroccan Western High Atlas represents wave-dominated deltaic deposits. The succession is represented by stacked thickening and coarsening upwards parasequences 5–15 m thick formed during fifth- or fourth-order regression and building a third-order highstand systems tract. Vertical facies transitions in parasequences reflect flooding followed by shoaling of diverse shelf environments ranging from offshore transition interbedded mudstones, siltstones and thin sandstones, lower shoreface/lower delta front hummocky bedforms to upper shoreface/upper delta front cross-bedded sandstones. The regional configuration reflects the progradation of wave-dominated deltas over an offshore setting. The maximum sea-level fall led to the development of a sequence boundary that is an unconformity. The subsequent early Aptian relative sea-level rise contributes to the development of an extensive conglomerate lagged transgressive surface of erosion. The latter and the sequence boundary are amalgamated forming a composite surface. 相似文献
12.
通过对南祁连盆地木里坳陷石炭系、二叠系、三叠系、侏罗系等4套层系5条剖面的野外测量及室内地质分析,明确了该区天然气水合物潜在气源岩的岩性特征、沉积相类型及沉积演化过程。石炭系—二叠系整体以出露中厚层砂岩夹薄层泥岩为主要特征,沉积浅海陆棚相、滨岸相和三角洲相,由于断层发育致使局部地层厚度减薄且泥质岩大部分缺失,可能难以成为天然气水合物潜在气源岩;上三叠统整体以发育中薄层泥岩与中薄至中厚层砂岩互层为主要特征,沉积相类型为潮坪相、湖泊相和河流相,泥岩累积厚度较大,是天然气水合物主要潜在气源岩;中侏罗统整体上以发育厚层泥岩、砂岩为主要特征,沉积相类型为辫状河相、三角洲相和湖泊相,是天然气水合物次要的潜在气源岩。研究结果为南祁连盆地木里坳陷天然气水合物气源岩研究提供了重要地质依据。 相似文献
13.
Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin 总被引：1，自引：0，他引：1
The relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the Lower Cretaceous Missisauga Formation was investigated using a combination of electron microprobe, bulk geochemistry and X‐ray diffraction data. The succession studied comprises several stacked parasequences. The delta progradational facies association includes: (i) fluvial or distributary channel sandstones (some with tidal influence); (ii) thick‐bedded delta‐front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms; and (iii) more distal muddier delta‐front and prodeltaic facies. The transgressive facies association includes lag conglomerate, siderite‐cemented muddy sandstone and mudstone, and bioclastic sandy limestone. Chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles. Good quality chlorite rims are well correlated with Ti in bulk geochemistry. Ti is a proxy for Fe availability, principally from the breakdown of abundant detrital ilmenite (FeTiO3). Under conditions of sea floor diagenesis, the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate‐depletion level and of the overlying Eh‐controlled diagenetic zones, resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones. This early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite. Thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation. This study thus shows that the preservation of porosity by chlorite rims is a two‐stage process. Rapidly deposited delta‐front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims, but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis. 相似文献
14.
Sten‐Andreas Grundvg Mads E. Jelby Snorre Olaussen Kasia K.
liwi&#x;ska 《Sedimentology》2021,68(1):196-237
The dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture. 相似文献
15.
16.
JEAN‐FRANÇOIS GHIENNE FLAVIA GIRARD JULIEN MOREAU JEAN‐LOUP RUBINO 《Sedimentology》2010,57(5):1175-1198
Climbing dune‐scale cross‐statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium‐grained to coarse‐grained sandstones that typically involve 0·3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss‐depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing‐dune cross‐stratification related to the migration of lower‐flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion‐based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/subaqueous transition of related deltaic wedges, while the latter formed in an ice‐proximal segment of the outwash plain. In erosion‐based sand sheets, climbing‐dune cross‐stratification results from unconfined mouth‐bar deposition related to expanding, sediment‐laden flows entering a water body. Within incised channels, climbing‐dune cross‐stratification formed over eddy‐related side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing‐dune cross‐stratification is a combination between high‐magnitude flows and sediment supply limitations, which lead to the generation of sediment‐charged stream flows characterized by a significant, relatively coarse‐grained, sand‐sized suspension‐load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse‐grained sand fallout in sediment‐laden flows following flow expansion throughout mouth bars or in eddy‐related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss‐sides and generation of widespread climbing‐dune cross‐stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood‐dominated depositional systems of foreland basins. 相似文献
17.
《Sedimentology》2018,65(3):809-841
Degradation of basin‐margin clinothems around the shelf‐edge rollover zone may lead to the generation of conduits through which gravity flows transport sediment downslope. Many studies from seismic‐reflection data sets show these features, but they lack small‐scale (centimetre to metre) sedimentary and stratigraphic observations on process interactions. Exhumed basin‐margin clinothems in the Tanqua depocentre (Karoo Basin) provide seismic‐reflection‐scale geometries and internal details of architecture with depositional dip and strike control. At the Geelhoek locality, clinothem parasequences comprise siltstone‐rich offshore deposits overlain by heterolithic prodelta facies and sandstone‐dominated deformed mouth bars. Three of these parasequences are truncated by a steep (6 to 22°), 100 m deep and 1·5 km wide asymmetrical composite erosion surface that delineates a shelf‐incised canyon. The fill, from base to top comprises: (i) thick‐bedded sandstone with intrabasinal clasts and multiple erosion surfaces; (ii) scour‐based interbedded sandstone and siltstone with tractional structures; and (iii) inverse‐graded to normal‐graded siltstone beds. An overlying 55 m thick coarsening‐upward parasequence fills the upper section of the canyon and extends across its interfluves. Younger parasequences display progressively shallower gradients during progradation and healing of the local accommodation. The incision surface resulted from initial oversteepening and high sediment supply triggering deformation and collapse at the shelf edge, enhanced by a relative sea‐level fall that did not result in subaerial exposure of the shelf edge. Previous work identified an underlying highly incised, sandstone‐rich shelf‐edge rollover zone across‐margin strike, suggesting that there was migration in the zone of shelf edge to upper‐slope incision over time. This study provides an unusual example of clinothem degradation and readjustment with three‐dimensional control in an exhumed basin‐margin succession. The work demonstrates that large‐scale erosion surfaces can develop and migrate due to a combination of factors at the shelf‐edge rollover zone and proposes additional criteria to predict clinothem incision and differential sediment bypass in consistently progradational systems. 相似文献
18.
The Lower Tagus Valley in Portugal contains a well-developed valley-fill succession covering the complete Late Pleistocene and Holocene periods. As large-scale stratigraphic and chronologic frameworks of the Lower Tagus Valley are not yet available, this paper describes facies, facies distribution, and sedimentary architecture of the late Quaternary valley fill. Twenty four radiocarbon ages provide a detailed chronological framework. Local factors affected the nature and architecture of the incised valley-fill succession. The valley is confined by pre-Holocene deposits and is connected with a narrow continental shelf. This configuration facilitated deep incision, which prevented large-scale marine flooding and erosion. Consequently a thick lowstand systems tract has been preserved. The unusually thick lowstand systems tract was probably formed in a previously (30,000–20,000 cal BP) incised narrow valley, when relative sea-level fall was maximal. The lowstand deposits were preserved due to subsequent rapid early Holocene relative sea-level rise and transgression, when tidal and marine environments migrated inland (transgressive systems tract). A constant sea level in the middle to late Holocene, and continuous fluvial sediment supply, caused rapid bayhead delta progradation (highstand systems tract). This study shows that the late Quaternary evolution of the Lower Tagus Valley is determined by a narrow continental shelf and deep glacial incision, rapid post-glacial relative sea-level rise, a wave-protected setting, and large fluvial sediment supply. 相似文献
19.
This paper presents examples of various large tidal sandbodies from the Eocene Roda Sandstone in the southern Pyrenees and the Late Pleistocene and Early Holocene in the East China Sea. An attempt is made to summarize the geometric variability of these large tidal sandbodies in relation to the sediment supply and tidal discharge of the depositional system. Transverse sand bars were developed in low-sinuosity, high-gradient channels with high influxes of coarse sediments and water from fluvial systems. Tidal point bars were formed in meandering low-gradient estuarine channel where tidal influence was stronger and sediment was finer than those of the transverse sand bar. A tidal delta complex was built up at the estuary mouth with an abundant sediment supply and an increased tidal discharge. Tidal sand ridges were formed when relict fluvial or deltaic sands were eroded and reworked by strong tidal currents during subsequent sea-level rise.
Since the sediment supply and the tidal discharge of the depositional system were closely related to the eustatic sea-level change and basin subsidence, i.e. the relative sea-level change, special attention will be given to the relationship between geometric variability of tidal sandbodies and the sequence stratigraphic framework in which various sandbodies occurred. Three orders of eustatic sea-level fluctuations can be recognized. The third-order eustatic sea-level cycle, together with basin subsidence, controlled the development of systems tracts and the occurrence of different tidal sandbodies, such as estuary and tidal flat facies during the late stage of a LSW systems tract (type 1 sequence) or a SM systems tract (type 2 sequence); tidal point bar facies, tidal delta facies or tidal sand-ridge facies during a TR systems tract; estuary facies during an early HS systems tract; and fluvial sand bar facies in a late HS systems tract and the early stage of a SM or LSW systems tract. There are also the fourth-order and fifth-order eustatic fluctuations, which are superimposed on the third-order eustatic changes and have important control on the build-up, abandonment and preservation of composite and single tidal sandbodies, respectively.
Since the deposition of tidal sandbodies is very sensitive to eustatic sea-level changes, recognition of various tidal sandbodies is important in sequence stratigraphy analyses of sedimentary basins and in the facies prediction of clastic sediments in basin modelling. 相似文献
20.
The Late Eocene-Early Oligocene sedimentary fill of the Lemnos Island, NE Greece, is represented by a submarine fan and shelf
deposits. Turbidites in the system occur as a laterally isolated body, with one sediment influx center present. The influx
center is a proximal distributary channel that occupies a position approximately in the fan’s center and displays the coarsest
sediment in the study area. It also suggests in association with the main palaeocurrent direction toward NE a curved shape
for the fan. The stratigraphic succession of the submarine fans indicates that their sedimentation started during the base
level fall and completed shortly after the base level rise. As a consequence, the study area was filled by turbidites that
correspond to forced regressive, lowstand normal regressive, and transgressive genetic units. The progradational bedsets,
within the basal part of the turbidite deposits, recorded the history of the base level fall. The mixed progradational/aggradational
style of the upper part of the submarine fan system suggests that the regression of the shoreline is driven by sediment supply
during a period of base-level rise at the shoreline, or at a time of baselevel stillstand. The overlying shelf facies consist
of thick to medium bedded sandstones and mudstones, which display a general thinning upward trend. The base of the mudstone
facies that overlie the thick-bedded, amalgamated sandstones corresponds to a transgressive surface. This surface separates
the low-stand deposits (thick-bedded sandstones) from the high stand deposits (mudstone facies), suggesting that deposition
of shelf facies occurred during a transgressive system tract. 相似文献