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1.
Seismic reflection profiles (3.5 kHz) were obtained along more than 3500 km of shiptrack in Lake Superior within the last 2 yr. The acoustic character of profiles is categorized as: (I) a single, strong reflector at the lake floor, (II) a thick, acoustically transparent layer overlying a strong reflector, and (III) relatively thick sediment with internal acoustic reflectors. These profiles, in conjunction with sediment cores from the area, reveal that varved glacial-lacustrine sediment settled out preferentially in a trough between Isle Royale and the north shore, and to a lesser extent in other topographic depressions; bottom currents generated by storm waves prevent clay accumulation on till or bedrock in the open lake wherever the bottom is shallower than 100 m; bottom currents prevent deposition or erode bottom sediment in certain deep-water (> 200 m) valleys; and lacustrine sediment is disturbed by creep or slumping off Grand Portage, Minnesota, and by other processes such as dewatering in many other areas. These factors complicate sedimentation in Lake Superior, and must be considered when investigating any aspect of the lake sediment. 相似文献
2.
Seismic reflection profiles (3.5 kHz) were obtained along more than 3500 km of shiptrack in Lake Superior within the last 2 yr. The acoustic character of profiles is categorized as: (I) a single, strong reflector at the lake floor, (II) a thick, acoustically transparent layer overlying a strong reflector, and (III) relatively thick sediment with internal acoustic reflectors. These profiles, in conjunction with sediment cores from the area, reveal that varved glacial-lacustrine sediment settled out preferentially in a trough between Isle Royale and the north shore, and to a lesser extent in other topographic depressions; bottom currents generated by storm waves prevent clay accumulation on till or bedrock in the open lake wherever the bottom is shallower than 100 m; bottom currents prevent deposition or erode bottom sediment in certain deep-water (> 200 m) valleys; and lacustrine sediment is disturbed by creep or slumping off Grand Portage, Minnesota, and by other processes such as dewatering in many other areas. These factors complicate sedimentation in Lake Superior, and must be considered when investigating any aspect of the lake sediment. 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.
The intracratonic Canning Basin is Western Australia's largest sedimentary basin (>400 000 km2 ) and has experienced repeated episodes of Phanerozoic extension and subsidence, resulting in deposition of a number of first-order 'megasequences'. A major phase of basin extension and sedimentation (Grant Group) occurred in the Late Carboniferous/Early Permian when Australia lay at high palaeolatitudes. Facies analysis of 5000 m of drill core from 25 continuously cored wells in Grant Group strata on the fault-bounded Barbwire Terrace in the northern Canning Basin identified three facies associations (FAs). These record the predominance of fault-generated, subaqueous mass flow and sediment reworking. The lowest association (FA I; up to 355 m thick) rests unconformably on tilted older strata and consists of coarse-grained, subaqueously deposited, sediment gravity flow facies. These include fault-generated breccias, massive and graded sandstones and conglomerates deposited by turbidity currents and diamictites generated by mixing of different textural populations during downslope remobilization. FA I is overlain abruptly by relatively fine-grained deposits of FA II (up to 140 m thick), which consist of laminated to thin-bedded mudstone and sandstone turbidites, recording an abrupt increase in relative water depths. In turn, these facies coarsen upwards and are transitional into shallow-water, swaley cross-stratified and rippled sandstones of FA III (up to 125 m thick). The overall stratigraphic succession probably records an initial phase of faulting and accommodation of coarse sediment (FA I), a subsequent phase of rapid subsidence, increasing water depths and 'sediment underfilling' (FA II) and, finally, a regressive phase of shoreface progradation. The occurrence of rare striated clasts in FA I suggests reworking of glacial sediment, but no direct glacial influence on sedimentation can be identified. 相似文献
5.
The 600 m thick prograding sedimentary succession of Wagad ranging in age from Callovian to Early Kimmeridgian has been divided
into three formations namely, Washtawa, Kanthkot and Gamdau. Present study is confined to younger part of the Washtawa Formation
and early part of the Kanthkot Formation exposed around Kanthkot, Washtawa, Chitrod and Rapar. The depositional architecture
and sedimentation processes of these deposits have been studied applying sequence stratigraphic context.
Facies studies have led to identification of five upward stacking facies associations (A, B, C, D, and E) which reflect that
deposition was controlled by one single transgressive — regressive cycle. The transgressive deposit is characterized by fining
and thinning upward succession of facies consisting of two facies associations: (1) Association A: medium — to coarse-grained
calcareous sandstone — mudrocks alternations (2) Association B: fine-grained calcareous sandstone — mudrocks alternations.
The top of this association marks maximum flooding surface as identified by bioturbational fabrics and abundance of deep marine
fauna (ammonites). Association A is interpreted as high energy transgressive deposit deposited during relative sea level rise.
Whereas, facies association B indicates its deposition in low energy marine environment deposited during stand-still period
with low supply of sediments. Regressive sedimentary package has been divided into three facies associations consisting of:
(1) Association C: gypsiferous mudstone-siltstone/fine sandstone (2) Association D: laminated, medium-grained sandstone —
siltstone (3) Association E: well laminated (coarse and fine mode) sandstone interbedded with coarse grained sandstone with
trough cross stratification. Regressive succession of facies association C, D and E is interpreted as wave dominated shoreface,
foreshore to backshore and dune environment respectively.
Sequence stratigraphic concepts have been applied to subdivide these deposits into two genetic sequences: (i) the lower carbonate
dominated (25 m) transgressive deposits (TST) include facies association A and B and the upper thick (75m) regressive deposits
(HST) include facies association C, D and E. The two sequences are separated by maximum flooding surface (MFS) identified
by sudden shift in facies association from B to C. The transgressive facies association A and B represent the sediments deposited
during the syn-rift climax followed by regressive sediments comprising association C, D and E deposited during late syn-rift
stage. 相似文献
6.
A sequence stratigraphic model for the Lower Coal Measures (Upper Carboniferous) of the Ruhr district, north-west Germany 总被引：3，自引：0，他引：3
Upper Carboniferous Coal Measures strata have been interpreted traditionally in terms of cyclothems bounded by marine flooding surfaces (marine bands) and coal seams. Correlation of such cyclothems in an extensive grid of closely spaced coal exploration boreholes provides a robust stratigraphic framework in which to study the Lower Coal Measures (Namurian C–Westphalian A) of the Ruhr district, north-west Germany. Three distinct types of cyclothem are recognized, based on their bounding surfaces and internal facies architecture. (1) Type 1 cyclothems are bounded by marine bands. Each cyclothem comprises a thick (30–80 m), regionally extensive, coarsening-upward delta front succession of interbedded shales, siltstones and sandstones, which may be deeply incised by a major fluvial sandstone complex. The delta front succession is capped by a thin (<1 m), regionally extensive coal seam and an overlying marine band defining the top of the cyclothem. (2) Type 2 cyclothems are bounded by thick (≈1 m), regionally extensive coal seams with few splits. The basal part of a typical cyclothem comprises a thick (15–50 m), widespread, coarsening-upward delta front or lake infill succession consisting of interbedded shales, siltstones and sandstones. Networks of major (>5 km wide, 20–40 m thick), steep-sided, multistorey fluvial sandstone complexes erode deeply into and, in some cases, through these successions and are overlain by the coal seam defining the cyclothem top. (3) Type 3 cyclothems are bounded by regionally extensive coal seam groups, characterized by numerous seam splits on a local (0·1–10 km) scale. Intervening strata vary in thickness (15–60 m) and are characterized by strong local facies variability. Root-penetrated, aggradational floodplain heteroliths pass laterally into single-storey fluvial channel-fill sandstones and coarsening-upward, shallow lake infill successions of interbedded shales, siltstones and sandstones over distances of several hundred metres to a few kilometres. Narrow (<2 km) but thick (20–50 m) multistorey fluvial sandstone complexes are rare, but occur in a few type 3 cyclothems. Several cyclothems are observed to change character from type 1 to type 2 and from type 2 to type 3 up the regional palaeoslope. Consequently, we envisage a model in which each cyclothem type represents a different palaeogeographic belt within the same, idealized delta system, subject to the same allogenic and autogenic controls on facies architecture. Type 1 cyclothems are dominated by deltaic shorelines deposited during a falling stage and lowstand of sea level. Type 2 cyclothems represent the coeval lower delta plain, which was deeply eroded by incised valleys that fed the falling stage and lowstand deltas. Type 3 cyclothems comprise mainly upper delta plain deposits in which the allogenic sea-level control was secondary to autogenic controls on facies architecture. The marine bands, widespread coals and coal seam groups that bound these three cyclothem types record abandonment of the delta system during periods of rapid sea-level rise. The model suggests that the extant cyclothem paradigm does not adequately describe the detailed facies architecture of Lower Coal Measures strata. Instead, these architectures may be better understood within a high-resolution stratigraphic framework incorporating sequence stratigraphic key surfaces, integrated with depositional models derived from analogous Pleistocene–Holocene fluvio-deltaic strata. 相似文献
7.
8.
Kathryn M. Stack John P. Grotzinger Michael P. Lamb Sanjeev Gupta David M. Rubin Linda C. Kah Lauren A. Edgar Deirdra M. Fey Joel A. Hurowitz Marie McBride Frances Rivera‐Hernndez Dawn Y. Sumner Jason K. Van Beek Rebecca M. E. Williams Robin Aileen Yingst 《Sedimentology》2019,66(5):1768-1802
Recent robotic missions to Mars have offered new insights into the extent, diversity and habitability of the Martian sedimentary rock record. Since the Curiosity rover landed in Gale crater in August 2012, the Mars Science Laboratory Science Team has explored the origins and habitability of ancient fluvial, deltaic, lacustrine and aeolian deposits preserved within the crater. This study describes the sedimentology of a ca 13 m thick succession named the Pahrump Hills member of the Murray formation, the first thick fine‐grained deposit discovered in situ on Mars. This work evaluates the depositional processes responsible for its formation and reconstructs its palaeoenvironmental setting. The Pahrump Hills succession can be sub‐divided into four distinct sedimentary facies: (i) thinly laminated mudstone; (ii) low‐angle cross‐stratified mudstone; (iii) cross‐stratified sandstone; and (iv) thickly laminated mudstone–sandstone. The very fine grain size of the mudstone facies and abundant millimetre‐scale and sub‐millimetre‐scale laminations exhibiting quasi‐uniform thickness throughout the Pahrump Hills succession are most consistent with lacustrine deposition. Low‐angle geometric discordances in the mudstone facies are interpreted as ‘scour and drape’ structures and suggest the action of currents, such as those associated with hyperpycnal river‐generated plumes plunging into a lake. Observation of an overall upward coarsening in grain size and thickening of laminae throughout the Pahrump Hills succession is consistent with deposition from basinward progradation of a fluvial‐deltaic system derived from the northern crater rim into the Gale crater lake. Palaeohydraulic modelling constrains the salinity of the ancient lake in Gale crater: assuming river sediment concentrations typical of floods on Earth, plunging river plumes and sedimentary structures like those observed at Pahrump Hills would have required lake densities near freshwater to form. The depositional model for the Pahrump Hills member presented here implies the presence of an ancient sustained, habitable freshwater lake in Gale crater for at least ca 103 to 107 Earth years. 相似文献
9.
Due to the apparent rhythmic characteristics of the stratigraphically complex unit formed of deltaic and mostly lacustrine deposits in Qianjiang depression (Jianghan basin), we used the seismic pattern proxies inside a seismic sequence, by analyzing the character of reflections (amplitude, continuity, and configuration) to detect the different seismic facies and to predict their depositional environmental settings. The depression fills are characterized by a distinct upward change in seismic facies; beginning with a fan facies in the bottom, followed by free facies occurring where thick salt sediments were presented, then convergent base-lapping facies, succeeded by chaotic facies and overlain by parallel to sub-parallel facies. The convergent base-lapping facies is the most common and the parallel and draping facies is restricted to slope areas shallower than 1000 m in water depth. Three depositional environments that range from fluvial, delta, to marine (lake) are predicted too. This paper lays the foundation for the development of a seismic sequence stratigraphic framework, and contributing to better understanding of the potential evaluation of hydrocarbon occurrence in the Eastern center of China. 相似文献
10.
Sergio Rocchi Alfredo Mazzotti Michele Marroni Luca Pandolfi Paolo Costantini Bertozzi Giuseppe Davide di Biase Francesco Federici Papa Goumbo L 《地学学报》2007,19(4):232-239
The Atlantic margin offshore Senegal has been explored by seismic reflection and GRAV‐MAG surveys. High‐amplitude, laterally transgressive seismic reflectors are found to coincide with gravimetric and magnetic highs. Once seismic data are integrated with potential fields modelling, these reflectors can be safely interpreted as saucer‐shaped igneous sills, up to some hundreds of metres thick, some km wide. The occurrence of hydrothermal vent complexes and forced folds in the stratigraphic sequence above the sills constrain the intrusion age to the Miocene. Field observations and in‐situ magnetic susceptibility measurements of Oligocene–Miocene and Quaternary igneous rocks emplaced in coastal Senegal support this interpretation. 相似文献
11.
Late Miocene nonmarine sedimentation and formation of magnesites in the Acıgöl Basin, southwestern Anatolia, Turkey 总被引：1，自引：0，他引：1
Hülya Aliek 《Sedimentary Geology》2009,219(1-4):115-135
The Late Miocene lacustrine Acıgöl Basin, SW Turkey, formed as an orogen-top, extensional half-graben, with the subaqueous accommodation controlled by the lake level and the bulk accommodation provided by active subsidence along a WSW-trending normal fault at the basin's southern margin. The basin-fill sedimentary succession consists of terminal alluvial-fan facies overlain by ephemeral lake-margin facies and perennial lake facies, with widespread fluvial facies at the top. The distal alluvial-fan facies include massive to stratified sandstones and massive mudstones with intervening nodular dolostones and incipient pedogenic horizons. The lake-margin facies are micritic magnesites passing laterally into peloidal, irregularly laminated magnesites towards the palaeolake margin and overlain by marlstones and dolostones, all with abundant evidence of episodic subaerial exposure (desiccation cracks, pedogenic features, and tepee structures). The perennial lake facies are micritic magnesites passing upwards into clayey dolostones and dolomitic or clayey marlstones. The fluvial facies capping the succession include planar cross-stratified conglomerates (channel-fill deposits), planar parallel-stratified, planar cross-stratified and rippled cross-laminated sandstones (crevasse-fill and crevasse splay deposits), and assemblages of mudstones intercalated with thin sandstone beds (overbank floodplain deposits).The sedimentological, mineralogical and geochemical data reveal large variations in the basin's hydrological regime, including short-term oscillations and bulk rise of the lake level, periodical changes in the Mg/Ca ratio and terrigenous mud supply, and a negative covariance of δ18O and δ13C fluctuations. The composition of terrigenous sediment and the chemistry of water supplied to the lake were controlled by the weathering, chemical leaching and erosion of the ultramafic–dolomitic bedrock in the catchment area. The bedrock yielded Mg-rich carbonate solutions that caused the deposition of Mg-carbonates in the lake.Despite short-term lake-level fluctuations, the lake's net water budget remained positive. It is suggested that the region's present-day climate and Mg-rich alkaline lakes can serve as an analogue for the climatic and hydrological conditions in the Late Miocene Acıgöl Basin. 相似文献
12.
Abstract The Kyokpori Formation (Cretaceous), south‐west Korea, represents a small‐scale lacustrine strike‐slip basin and consists of an ≈ 290 m thick siliciclastic succession with abundant volcaniclasts. The succession can be organized into eight facies associations representing distinctive depositional environments: (I) subaqueous talus; (II) delta plain; (III) steep‐gradient large‐scale delta slope; (IV) base of delta slope to prodelta; (V) small‐scale nested Gilbert‐type delta; (VI) small‐scale delta‐lobe system; (VII) subaqueous fan; and (VIII) basin plain. Facies associations I, III and IV together constitute a large‐scale steep‐sloped delta system. Correlation of the sedimentary succession indicates that the formation comprises two depositional sequences: the lower coarsening‐ to fining‐upward succession (up to 215 m thick) and the upper fining‐upward succession (up to 75 m thick). Based on facies distribution, architecture and correlation of depositional sequences, three stages of basin evolution are reconstructed. Stage 1 is represented by thick coarse‐grained deposits in the lower succession that form subaqueous breccia talus and steep‐sloped gravelly delta systems along the northern and southern basin margins, respectively, and a sandy subaqueous fan system inside the basin, abutting against a basement high. This asymmetric facies distribution suggests a half‐graben structure for the basin, and the thick accumulation of coarse‐grained deposits most likely reflects rapid subsidence of the basin floor during the transtensional opening of the basin. Stage 2 is marked by sandy black shale deposits in the upper part of the lower succession. The black shale is readily correlated across the basin margins, indicating a basinwide transgression probably resulting from large‐scale dip slip suppressing the lateral slip component on basin‐bounding faults. Stage 3 is characterized by gravelly delta‐lobe deposits in the upper succession that are smaller in dimension and located more basinward than the deposits of marginal systems of the lower succession. This lakeward shift of depocentre suggests a loss of accommodation in the basin margins and quiescence of fault movements. This basin evolution model suggests that the rate of dip‐slip displacement on basin‐margin faults can be regarded as the prime control for determining stacking patterns of such basin fills. The resultant basinwide fining‐upward sequences deviate from the coarsening‐upward cycles of other transtensional basins and reveal the variety of stratigraphic architecture in strike‐slip basins controlled by the changes in relative sense and magnitude of fault movements at the basin margins. 相似文献
13.
Fluvial sedimentation in a salt‐controlled mini‐basin: stratal patterns and facies assemblages,Sivas Basin,Turkey 
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下载免费PDF全文 Charlotte Ribes Charlie Kergaravat Cédric Bonnel Philippe Crumeyrolle Jean‐Paul Callot André Poisson Haluk Temiz Jean‐Claude Ringenbach 《Sedimentology》2015,62(6):1513-1545
The Sivas Basin, located on the Central Anatolian Plateau in Turkey, is an elongate Oligo‐Miocene basin that contains numerous salt‐walled mini‐basins. Through field analysis, including stratigraphic section logging, facies analysis and geological mapping, a detailed tectono‐stratigraphic study of the Emirhan mini‐basin and its 2·6 km thick sediment fill has been undertaken. Three main palaeoenvironments are recognized – playa‐lake, braided stream and lacustrine – each corresponds to a relatively long‐lived depositional episode within a system that was dominated overall by the development of a distributive fluvial system. At local scale, this affects the geometry of the succession and influences facies distributions within preserved sequences. Sequences affected by wedge geometries are characterized by localized channelized sandstone bodies in the area of maximum subsidence and these pass laterally to floodplain mudstone towards the diaper; several internal unconformities are recognized. By contrast, sequences affected by hook geometries display narrow and steep drape‐fold geometries with no evidence of lateral facies change and apparent conformity in the preserved succession. The sediment fill of the Emirhan mini‐basin records the remobilization of diapir‐derived detritus and the presence of evaporitic bodies interbedded within the mini‐basin, implying the growth of salt walls expressed at the surface as palaeo‐topographic highs. The mini‐basin also records the signature of a regional change in stratigraphic assemblage, passing from playa‐lake facies to large‐scale highly amalgamated fluvial facies that represent progradation of the fluvial system. The initiation and evolution of this mini‐basin involves a variety of local and regional controls. Local factors include: (i) salt withdrawal, which influenced the rate and style of subsidence and consequently temporal and spatial variation in the stratigraphic assemblage and the stratal response related to halokinesis; and (ii) salt inflation, which influenced the topographic expression of the diapirs and consequently the occurrence of diapir‐derived detritus intercalated within the otherwise clastic‐dominated succession. 相似文献
14.
Berit Legler Howard D. Johnson Gary J. Hampson Benoit Y.G. Massart Chris A‐L. Jackson Matthew D. Jackson Ahmed El‐Barkooky Rodmar Ravnas 《Sedimentology》2013,60(5):1313-1356
Existing facies models of tide‐dominated deltas largely omit fine‐grained, mud‐rich successions. Sedimentary facies and sequence stratigraphic analysis of the exceptionally well‐preserved Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) aims to bridge this gap. The succession was deposited in a structurally controlled, shallow, macrotidal embayment and deposition was supplemented by fluvial processes but lacked wave influence. The succession contains two stacked, progradational parasequence sets bounded by regionally extensive flooding surfaces. Within this succession two main genetic elements are identified: non‐channelized tidal bars and tidal channels. Non‐channelized tidal bars comprise coarsening‐upward sandbodies, including large, downcurrent‐dipping accretion surfaces, sometimes capped by palaeosols indicating emergence. Tidal channels are preserved as single‐storey and multilateral bodies filled by: (i) laterally migrating, elongate tidal bars (inclined heterolithic strata, 5 to 25 m thick); (ii) forward‐facing lobate bars (sigmoidal heterolithic strata, up to 10 m thick); (iii) side bars displaying oblique to vertical accretion (4 to 7 m thick); or (iv) vertically‐accreting mud (1 to 4 m thick). Palaeocurrent data show that channels were swept by bidirectional tidal currents and typically were mutually evasive. Along‐strike variability defines a similar large‐scale architecture in both parasequence sets: a deeply scoured channel belt characterized by widespread inclined heterolithic strata is eroded from the parasequence‐set top, and flanked by stacked, non‐channelized tidal bars and smaller channelized bodies. The tide‐dominated delta is characterized by: (i) the regressive stratigraphic context; (ii) net‐progradational stratigraphic architecture within the succession; (iii) the absence of upward deepening trends and tidal ravinement surfaces; and (iv) architectural relations that demonstrate contemporaneous tidal distributary channel infill and tidal bar accretion at the delta front. The detailed facies analysis of this fine‐grained, tide‐dominated deltaic succession expands the range of depositional models available for the evaluation of ancient tidal successions, which are currently biased towards transgressive, valley‐confined estuarine and coarser grained deltaic depositional systems. 相似文献
15.
Paulo S.G. Paim 《Sedimentary Geology》2007,202(4):776-795
The upper portion of the Pedra Pintada Alloformation includes about 100 m of mostly eolian deposits. This paper emphasizes the vertical succession and lateral association of sedimentary facies, based on analysis of outcrop data and aerial photographs, as well as the hierarchy and origin of bounding surfaces. It aims to propose a high-resolution stratigraphic and depositional model that may be useful to exploitation of eolian reservoirs. The succession has been preserved due to basin subsidence, and is described in terms of four facies associations that constitute three dominantly eolian units. These units are sharply bounded by major flooding surfaces (super surfaces) that, in turn, are overlain by 1 to 2 m thick, dominantly water-laid facies (lacustrine, fluvial, deltaic and eolian). Both their internal organization and boundaries were controlled by changes in the base level rise rate. The basal Eolian Unit is composed of crescentic eolian dunes and damp interdune deposits ascribed to a wet eolian system. On the other hand, eolian units II and III, also characterized by crescentic eolian dunes (simple and compound) deposits, were related to dry eolian systems, since they comprise dry (eventually wet) interdune facies. Eolian Unit III is truncated by basinwide unconformity, which is then overlain by the ephemeral fluvial deposits (Varzinha Alloformation). This second type of super surface is related to climate-induced wind erosion (deflation) down to the water table level (regional Stokes surface) followed by fluvial incision linked to tectonic activity. 相似文献
16.
Alastair H. Ruffell 《Cretaceous Research》1995,16(6)
Non-marine strata of Early Cretaceous age ('Wealden facies') are found at outcrop in the type localities of the Wessex Basins, southern England (and adjacent subsurface) and in extensive and thick successions filling the North Celtic Sea Basin. Sedimentology, paleontology, petrology and geochemistry have traditionally been used as evidence in determining the climatic, tectonic and sea level controls on Wealden facies, sedimentary processes and stratigraphy. Analysis of seismic data, through seismic facies and sequence stratigraphic analysis, allows direct comparison of the Wealden in these basins and new interpretations to be made of the tectonic and depositional influences. In the north-eastern end of the North Celtic Sea Basin, tectonic controls on seismic facies can be demonstrated and are related to coarse-grained fan-delta horizons documented in core. In the south-western North Celtic Sea Basin, adjacent to the Cretaceous proto-Atlantic, tectonic controls are less apparent, and changes in relative sea level were probably more important in controlling the preservation of stratigraphic sequences. Where the non-marine Lower Cretaceous succession is imaged clearly, the stratigraphic similarity between parts of the North Celtic Sea and the Weald and Channel Basins suggests a wider control by relative sea-level fluctuations. However, important variations in seismic facies within the basins indicate localized, dominant tectonic control. The recognition of a very distinctive 'lower' Wealden seismic facies, observed as undulatory (?channelized) and downlapping reflections, is contrasted with either the parallel or synsedimentary fault-dominated 'upper' Wealden facies. These seismic characteristics reflect the previously recognized climatic/tectonic change from Hastings to Weald Clay Group environments. The debate on tectonics versus eustacy is further complicated: palaeoclimate appears to be a third process responsible for stratigraphic variation. 相似文献
17.
The stratigraphic record of many cratonic carbonate sequences includes thick successions of stacked peritidal deposits. Representing accumulation at or near sea‐level, these deposits have provided insights into past palaeoenvironments, sea‐level and climate change. To expand understanding of carbonate peritidal systems, this study describes the geomorphology, sedimentology and stratigraphy of the tidal flats on the Crooked‐Acklins Platform, south‐east Bahamas. The Crooked Island tidal flats extend continuously for ca 18 km on the platformward flank of Crooked Island, reaching up to 2 km across. Tidal flats include four environmental zones with specific faunal and floral associations and depositional characteristics: (i) supratidal (continuous supratidal crust and pavement); (ii) upper intertidal, with the mangrove Avicennia germinans and the cyanobacteria Scytonema; (iii) lower intertidal (with the mangrove Rhizophora mangal) and (iv) non‐vegetated, heavily burrowed subtidal (submarine). These zones have gradational boundaries but follow shore‐parallel belts. Coring reveals that the thickness of this mud‐dominated sediment package generally is <2 m, with depth to Pleistocene bedrock gradually shallowing landward. The facies succession under much of the tidal flat includes a basal compacted, organic‐rich skeletal‐lithoclast lag above the bedrock contact (suggesting initial flooding). This unit grades upward into rhizoturbated skeletal sandy mud (subtidal) overlain by coarsening‐upward peloid‐foraminifera‐gastropod muddy sand (reflecting shallowing to intertidal elevations). Cores from landward positions include stacked thin indurated layers with autoclastic breccia, root tubules and fenestrae (interpreted as supratidal conditions). Collectively, the data reveal an offlapping pattern on this prograding low‐energy shoreline, and these Holocene tidal flats may represent an actualistic analogue for ancient humid progradational tidal flats. Nonetheless, their vertical facies succession is akin to that present beneath channelled belt examples, suggesting that facies successions alone may not provide unambiguous criteria for prediction of the palaeogeomorphology, lateral facies changes and heterogeneity in stratigraphic analogues. 相似文献
18.
Where eroding cohesive sediments are present, Lake Michigan bluffs range up to 40 m in height, exposing multiple glacial stratigraphic units. Following the model presented here, bluffs form as a wave-cut terrace erodes inland from a point near the original shoreline. The erosion plane is nearly horizontal, in contrast with the eastward dip of the glacial units inherited from underlying bedrock. Therefore, terraces eroding inland produce progressively higher bluffs and expose successively older units at the toe and beneath the lake. This process was repeated several times as lake levels sequentially dropped to their modern stage. The initial modern shoreline, and hence the width of the wave-cut terrace, was determined from four offshore seismic and bottom-sampling profiles. It was picked as an inflection point in the lake bed, occurring offshore of dipping reflectors intersecting the lake bottom. The calculated average recession rate over a 2500-year duration of the modern stage is 1.5 m/yr in contrast to average rates of approximately 0.6 m/yr measured over the last century. Thus rates decrease through time as the terrace widens and wave energy is dampened. By correlating bluff height to recession distance, a third rate of approximately 2.7 m/yr for the first 940 years of recession is calculated from relict Nipissing bluffs. The three rates define a steeply decaying exponential curve in early stages of bluff retreat, flattening into a nearly linear function after 1000 years. 相似文献
19.
鄂尔多斯盆地三叠系延长组发育多套生储盖组合,其中陇东地区是延长组主要的产油区之一。充分利用最新的三维地震资料,结合大量钻井和测井资料,针对研究区广泛发育的前积型地震连续强反射现象,开展地震地层对比划分、沉积演化及砂体展布规律研究。以湖泛期凝缩层对应的地震同相轴作为地层界面的对比标志,将研究区延长组中段划分出7个层序组,表现为震荡性湖退充填沉积特征,沉积单元呈透镜体依次向湖盆中心叠置,每个沉积单元均发育上部三角洲前缘、中部富泥斜坡和下部深水重力流的三段式沉积相组合,砂体主要分布在沉积单元的上部和下部,平面上具有明显的分带特征。本次研究成果与传统等厚地层对比方案存在明显差异,尤其在顺物源方向,单井之间的小层连通性和叠置关系差异较大,其认识更符合震荡性湖退沉积背景,对油田勘探开发更具有指导性。 相似文献
20.
在胜利油田,深层油气勘探已经成为寻找后备储量的主要方向,但是因为研究程度低而未能取得重大突破。因此,有必要跳出既成思路,针对孔店组作扎实的基础研究。本文以地震地层学的观点和方法,把东营-惠民凹陷孔店组的地震反射看作一个地震层序,并将其划分为三个亚层序(相当于层序地层学中的体系域),分别对应于地层分层上的孔一段、孔二段和孔三段。利用地震反射波的振幅、连续性、内部结构和外部形态等地震反射特征,在具有油气勘探前景的孔二段的地震反射中识别出了中振幅中连续亚平行席状相、杂乱充填相、杂乱-斜交前积相、丘状相、弱振幅中连续楔状相、弱变振幅低连续楔状相及帚状前积相等七种地震相。将地震相转变为沉积相,划分出了湖泊沉积环境的深湖-半深湖、滨浅湖、扇三角洲、近岸水下扇等沉积相,进而预测了有利相带,优选了部分深湖-半深湖相沉积区作为下一步勘探工作的重点目标区域,为今后的勘探工作提出了有益的建议。 相似文献