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1.
The Lower Jurassic Mashabba Formation crops out in the core of the doubly plunging Al-Maghara anticline, North Sinai, Egypt. It represents a marine to terrestrial succession deposited within a rift basin associated with the opening of the Neotethys. Despite being one of the best and the only exposed Lower Jurassic strata in Egypt, its sedimentological and sequence stratigraphic framework has not been addressed yet. The formation is subdivided informally into a lower and upper member with different depositional settings and sequence stratigraphic framework. The sedimentary facies of the lower member include shallow-marine, fluvial, tidal flat and incised valley fill deposits. In contrast, the upper member consists of strata with limited lateral extension including fossiliferous lagoonal limestones alternating with burrowed deltaic sandstones. The lower member contains three incomplete sequences (SQ1-SQ3). The depositional framework shows transgressive middle shoreface to offshore transition deposits sharply overlain by forced regressive upper shoreface sandstones (SQ1), lowstand fluvial to transgressive tidal flat and shallow subtidal sandy limestones (SQ2), and lowstand to transgressive incised valley fills and shallow subtidal sandy limestones (SQ3). In contrast, the upper member consists of eight coarsening-up depositional cycles bounded by marine flooding surfaces. The cycles are classified as carbonate-dominated, siliciclastic-dominated, and mixed siliciclastic-carbonate. The strata record rapid changes in accommodation space. The unpredictable facies stacking pattern, the remarkable rapid facies changes, and chaotic stratigraphic architecture suggest an interplay between allogenic and autogenic processes. Particularly syndepositional tectonic pulses and occasional eustatic sea-level changes controlled the rate and trends of accommodation space, the shoreline morphology, the amount and direction of siliciclastic sediment input and rapid switching and abandonment of delta systems.  相似文献   

2.
The Campanian Cliff House Formation represents a series of individually progradational shoreface tongues preserved in an overall landward-stepping system. In the Mancos Canyon area, the formation consists of four, 50- to 55-m-thick and 10- to 20-km-wide sandstone tongues, which pinch out landwards into lower coastal plain and lagoonal deposits of the Upper Menefee Formation and seawards into offshore shales of the Lewis Shale Formation. Photogrammetric mapping of lithofacies along the steep and well-exposed canyon walls was combined with sedimentary facies analysis and mapping of the detailed facies architecture. Two major facies associations have been identified, one comprising the mostly muddy and organic-rich facies of lagoonal and lower coastal plain origin and one comprising the sandstone-dominated facies of shoreface origin. Key stratigraphic surfaces were identified by combining the mapped geometry of the lithofacies units with the interpretation of depositional processes. The stratigraphic surfaces (master ravinement surface, shoreface/coastal plain contact, transgressive surface, maximum flooding surface and the sequence boundary) allow each major sandstone tongue to be divided into a simple sequence, consisting of a basal transgressive system tract (TST) overlain by a highstand system tract (HST). Within each sandstone tongue, a higher frequency cyclicity is evident. The high-frequency cycles show a complex stacking pattern development and are commonly truncated in the downdip direction by surfaces of regressive marine erosion. The complexities of the Cliff House sandstone tongues are believed to reflect changes in the rate of sea-level rise combined with the responses of the depositional system to these changes. Synsedimentary compaction, causing a thickness increase in the sandstone tongues above intervals of previously uncompacted lagoonal/coastal plain sediments, also played a role. This study of the facies architecture, geometry and sequence stratigraphy of the Cliff House Formation highlights the fact that there may be some problems in applying conventional sequence stratigraphical methods to landward-stepping systems in general. These difficulties stem from the fact that no single stratigraphic surface can easily be identified and followed from the non-marine to the fully marine realm (i.e. from the landward to the basinward pinch-out of the sandstone tongues). In addition, the effects of synsedimentary compaction and changes in the shoreface dynamics are not easily recognized in limited data sets such as from the subsurface.  相似文献   

3.
The Cutro Terrace is a mixed marine to continental terrace, where deposits up to 15 m thick discontinuously crop out in an area extending for ca 360 km2 near Crotone (southern Italy). The terrace represents the oldest and highest terrace of the Crotone area, and it has been ascribed to marine isotope stage 7 (ca 200 kyr bp ). Detailed facies and sequence‐stratigraphic analyses of the terrace deposits allow the recognition of a suite of depositional environments ranging from middle shelf to fluvial, and of two stacked transgressive–regressive cycles (Cutro 1 and Cutro 2) bounded by ravinement surfaces and by surfaces of sub‐aerial exposure. In particular, carbonate sedimentation, consisting of algal build‐ups and biocalcarenites, characterizes the Cutro 1 cycle in the southern sector of the terrace, and passes into shoreface and foreshore sandstones and calcarenites towards the north‐west. The Cutro 2 cycle is mostly siliciclastic and consists of shoreface, lagoon‐estuarine, fluvial channel fill, floodplain and lacustrine deposits. The Cutro 1 cycle is characterized by very thin transgressive marine strata, represented by lags and shell beds upon a ravinement surface, and thicker regressive deposits. Moreover, the cycle appears foreshortened basinwards, which suggests that the accumulation of its distal and upper part occurred during forced regressive conditions. The Cutro 2 cycle displays a marked aggradational component of transgressive to highstand paralic and continental deposits, in places strongly influenced by local physiography, whereas forced regressive sediments are absent and probably accumulated further basinwards. The maximum flooding shoreline of the second cycle is translated ca 15 km basinward with respect to that of the first cycle, and this reflects a long‐term regressive trend mostly driven by regional uplift. The stratigraphic architecture of the Cutro Terrace deposits is the result of the interplay between regional uplift and high amplitude, Late Quaternary glacio‐eustatic changes. In particular, rapid transgressions, linked to glacio‐eustatic rises that outpaced regional uplift, favoured the accumulation of thin transgressive marine strata at the base of the two cycles. In contrast, the combined effect of glacio‐eustatic falls and regional uplift led to high‐magnitude forced regressions. The superposition of the two cycles was favoured by a relatively flat topography, which allowed relatively complete preservation of stratal geometries that record large shoreline displacements during transgression and regression. The absence of a palaeo‐coastal cliff at the inner margin of the terrace supports this interpretation. The Cutro Terrace provides a case study of sequence architecture developed in uplifting settings and controlled by high‐amplitude glacio‐eustatic changes. This case study also demonstrates how the interplay of relative sea‐level change, sediment supply and physiography may determine either the superposition of cycles forming a single terrace or the formation of a staircase of terraces each recording an individual eustatic pulse.  相似文献   

4.
The upper portion of the Cuyo Group in the Zapala region, south‐eastern Neuquén Basin (Western Argentina), encompasses marine and transitional deposits (Lajas Formation) overlain by alluvial rocks (Challacó Formation). The Challacó Formation is covered by the Mendoza Group above a second‐order sequence boundary. The present study presents the stratigraphic framework and palaeophysiographic evolution of this Bajocian to Eo‐Calovian interval. The studied succession comprises the following genetic facies associations: (i) offshore and lower shoreface–offshore transition; (ii) lower shoreface; (iii) upper shoreface; iv) intertidal–subtidal; (v) supratidal–intertidal; (vi) braided fluvial to delta plain; (vii) meandering river; and (viii) braided river. The stratigraphic framework embraces four third‐order depositional sequences (C1 to C4) whose boundaries are characterized by the abrupt superposition of proximal over distal facies associations. Sequences C1 to C3 comprise mostly littoral deposits and display well‐defined, small‐scale transgressive–regressive cycles associated with fourth‐order depositional sequences. Such high‐frequency cycles are usually bounded by ravinement surfaces associated with transgressive lags. At last, the depositional sequence C4 delineates an important tectonic reorganization probably associated with an uplift of the Huincul Ridge. This is suggested by an inversion of the transport trend, north‐westward during the deposition of C1 to C3 depositional sequences (Lajas Formation) to a south‐west trend during the deposition of the braided fluvial strata related to the C4 depositional sequence (Challacó Formation).  相似文献   

5.
The Bridport Sand Formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic‐carbonate shallow‐marine depositional system. At outcrop and in subsurface cores, conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces. The geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3D seismic data. Facies analysis suggests that the Bridport Sand Formation represents progradation of a low‐energy, siliciclastic shoreface dominated by storm‐event beds reworked by bioturbation. The shoreface sandstones form the upper part of a thick (up to 200 m), steep (2–3°), mud‐dominated slope that extends into the underlying Down Cliff Clay. Clinoform surfaces representing the shoreface‐slope system are grouped into progradational sets. Each set contains clinoform surfaces arranged in a downstepping, offlapping manner that indicates forced‐regressive progradation, which was punctuated by flooding surfaces that are expressed in core and well‐log data. In proximal locations, progradational shoreface sandstones (corresponding to a clinoform set) are truncated by conglomerate lags containing clasts of bored, reworked shoreface sandstones, which are interpreted as marking sequence boundaries. In medial locations, progradational clinoform sets are overlain across an erosion surface by thin (<5 m) bioclastic limestones that record siliciclastic‐sediment starvation during transgression. Near the basin margins, these limestones are locally thick (>10 m) and overlie conglomerate lags at sequence boundaries. Sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces, to form composite erosion surfaces. In distal locations, oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces. Over most of the Wessex Basin, clinoform sets (corresponding to high‐frequency sequences) are laterally offset, thus defining a low‐frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation. Aggradational stacking of high‐frequency sequences occurs in fault‐bounded depocentres which had higher rates of localized tectonic subsidence.  相似文献   

6.
This work presents the first detailed facies analysis of the upper Nyalau Formation exposed around Bintulu, Sarawak, Malaysia. The Lower Miocene Nyalau Formation exposures in NW Sarawak represent one of the closest sedimentological outcrop analogues to the age equivalent, hydrocarbon-bearing, offshore deposits of the Balingian Province. Nine types of facies associations are recognised in the Nyalau Formation, which form elements of larger-scale facies successions. Wave-dominated shoreface facies successions display coarsening upward trends from Offshore, into Lower Shoreface and Upper Shoreface Facies Associations. Fluvio-tidal channel facies successions consist of multi-storey stacks of Fluvial-Dominated, Tide-Influenced and Tide-Dominated Channel Facies Associations interbedded with minor Bay and Mangrove Facies Associations. Estuarine bay facies successions are composed of Tidal Bar and Bay Facies Associations with minor Mangrove Facies Associations. Tide-dominated delta facies successions coarsen upward from an Offshore into the Tidal Bar Facies Association. The Nyalau Formation is interpreted as a mixed wave- and tide-influenced coastal depositional system, with an offshore wave-dominated barrier shoreface being incised by laterally migrating tidal channels and offshore migrating tidal bars. Stratigraphic successions in the Nyalau Formation form repetitive high frequency, regressive–transgressive cycles bounded by flooding surfaces, consisting of a basal coarsening upward, wave-dominated shoreface facies succession (representing a prograding barrier shoreface and/or beach-strandplain) which is sharply overlain by fluvio-tidal channel, estuarine bay or tide-dominated delta facies successions (representing more inshore, tide-influenced coastal depositional environments). An erosion surface separates the underlying wave-dominated facies succession from overlying tidal facies successions in each regressive–transgressive cycle. These erosion surfaces are interpreted as unconformities formed when base level fall resulted in deep incision of barrier shorefaces. Inshore, fluvio-tidal successions above the unconformity display upward increase in marine influence and are interpreted as transgressive incised valley fills.  相似文献   

7.
To elucidate the signature of isostatic and eustatic signals during a deglaciation period in pre‐Pleistocene times is made difficult because very little dating can be done, and also because glacial erosion surfaces, subaerial unconformities and subsequent regressive or transgressive marine ravinement surfaces tend to amalgamate or erode the deglacial deposits. How and in what way can the rebound be interpreted from the stratigraphic record? This study proposes to examine deglacial deposits from Late‐Ordovician to Silurian outcrops at the Algeria–Libya border, in order to define the glacio–isostatic rebound and relative sea‐level changes during a deglaciation period. The studied succession developed at the edge and over a positive palaeo‐relief inherited from a prograding proglacial delta that forms a depocentre of glaciogenic deposits. The succession is divided into five subzones, which depend on the topography of this depocentre. Six facies associations were determined: restricted marine (Facies Association 1); tidal channels (Facies Association 2); tidal sand dunes (Facies Association 3); foreshore to upper shoreface (Facies Association 4); lower shoreface (Facies Association 5); and offshore shales (Facies Association 6). Stratigraphic correlations over the subzones support the understanding of the depositional chronology and associated sea‐level changes. Deepest marine domains record a forced regression of 40 m of sea‐level fall resulting from an uplift caused by a glacio‐isostatic rebound that outpaces the early transgression. The rebound is interpreted to result in a multi‐type surface, which is interpreted as a regressive surface of marine erosion in initially marine domains and as a subaerial unconformity surface in an initially subaerial domain. The transgressive deposits have developed above this surface, during the progressive flooding of the palaeo‐relief. Sedimentology and high‐resolution sequence stratigraphy allowed the delineation of a deglacial sequence and associated sea‐level changes curve for the studied succession. Estimates suggest a relatively short (<10 kyr) duration for the glacio‐isostatic uplift and a subsequent longer duration transgression (4 to 5 Myr).  相似文献   

8.
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   

9.
Facies, geometry and key internal stratigraphic surfaces from eight Cretaceous and Eocene clastic shoreline tongues have been documented. The regressive parts of all the studied tongues represent storm‐wave influenced strandplains, deltas or fan‐deltas, and the regressive shoreline trajectories varied from descending to ascending. The transgressive parts of the tongues are dominated by either estuarine or coastal‐plain deposits. The distance from the coeval, up‐dip non‐marine deposits to the basinward pinchout of amalgamated shoreface sandstones, measured along depositional dip, is here termed the sand pinchout distance. The study shows that the angle of regressive‐to‐transgressive turnaround (defined by the angle between the regressive and subsequent transgressive shoreline trajectories) and the process regime during turnaround largely control the sand‐pinchout distance. The amount of transgressive erosion can also partly control the pinchout distance, but this parameter was comparable for the different examples presented here. If the type of depositional system at turnaround and the depth of transgressive erosion are constant, small angles of turnaround are associated with large pinchout distances, whereas larger angles of turnaround result in smaller pinchout distances. The model developed allows sand‐pinchout distance to be predicted, using data for the landward parts of shoreline tongues. The dataset also shows that steeply rising (aggrading) shoreline trajectories tend to produce more heterolithic sandstone tongues than those formed by lower‐angle trajectories.  相似文献   

10.
Well‐cuttings, wireline logs and limited core and outcrop data were used to generate a regional, three‐dimensional sequence framework for Upper Mississippian (Chesterian), Greenbrier Group carbonates in the Appalachian foreland basin, West Virginia, USA. The resulting maps were used to document the stratigraphic response of the basin to tectonics and to glacio‐eustasy during the transition into ice‐house conditions. The ramp facies include inner ramp red beds and aeolianites, lagoonal muddy carbonates, mid‐ramp ooid and skeletal grainstone shoal complexes, and outer ramp wackestone–mudstone, that grades downslope into laminated silty lime mudstone. The facies make up fourth‐order sequences, a few metres to over 90 m (300 ft) thick. The sequences are bounded along the ramp margin by lowstand sandstones and calcareous siltstones. On the ramp, sequence boundaries are overlain by thin transgressive siliciclastics and aeolianites, and only a few are calichified. Maximum flooding surfaces on the outer ramp lie beneath deeper water facies that overlie lowstand to transgressive siliciclastic or carbonate units. On the shallow ramp, maximum flooding surfaces overlie siliciclastic‐prone transgressive systems tracts, that are overlain by highstand carbonates with significant grainstone units interlayered with lagoonal lime mudstones. The fourth‐order sequences are the major mappable subsurface units; they are bundled into weak composite sequences which are bounded by red beds. In spite of differential subsidence rates across the foreland basin (1 to 3 cm/k.y. up to 25 cm/k.y.), eustatic sea‐level changes controlled regional sequence development. Thrust‐load induced differential subsidence of fault‐blocks, coupled with in‐plane stress, controlled the rapid basinward thickening of the depositional wedge, whose thickness and facies were influenced by subtle structures such as arches trending at high angles as well as parallel to the margin.  相似文献   

11.
Neogene strata of the northern part of the Pegu (Bago) Yoma Range, Central Myanmar, contain a series of shallow marine clastic sediments with stratigraphic ages ranging from the Early to Late Miocene. The studied succession (around 750 m thick) is composed of three major stratigraphic units deposited during a major regression and four major transgressive cycles in the Early to Late Miocene. The transgressive deposits consist of elongate sand-bars and broad sand-sheets that pass headward into mixed-flats of tidal environments. Marine flooding in transgressive deposits is associated with coquina beds and allochthonous coral-bearing sandy limestone bands. Major marine regressions are associated with lowstand progradation of thick estuary point-bars passing up into upper sand-flat sand bodies encased within the tidal flat sequences and lower shoreface deposits with local unconformities. The succession initially formed in a large scale incised-valley system, and was later interrupted by two major marine transgressions in the generally regressive or basinward-stepping stratigraphic sequences. Successive sandbodies were formed during a sea-level lowstand and early stage of the subsequent relative rise of sea level in a tide-dominated estuary system in the eastern part of the Central Myanmar Tertiary Basin during Early to Late Miocene times.  相似文献   

12.
The discovery of whale fossils from Eocene strata in the Fayum Depression has provoked interest in the life and lifestyle of early whales. Excellent outcrop exposure also affords the dataset to develop sedimentological and stratigraphic models within the Eocene strata. Previous work generally asserts that the thick, sand‐rich deposits of the Fayum Depression represent shoreface and barrier island successions with fine‐grained lagoonal and fluvial associations capping progradational successions. However, a complete absence of wave‐generated sedimentary structures, a preponderance of thoroughly bioturbated strata and increasingly proximal sedimentary successions upwards are contrary to accepted models of the local sedimentological and stratigraphic development. This study considers data collected from two Middle to Upper Eocene successions exposed in outcrop in the Wadi El‐Hitan and Qasr El‐Sagha areas of the Fayum Depression to determine the depositional affinities of Fayum strata. Based on sedimentological and ichnological data, five facies associations (Facies Association 1 to Facies Association 5) are identified. The biological and sedimentological characteristics of the reported facies associations indicate that the whale‐bearing sandstones (Facies Association 1) record distal positions in a large, open, quiescent marine bay that is abruptly succeeded by a bay‐margin environment (Facies Association 2). Upwards, marginal‐marine lagoonal and shallow‐bay parasequences (Facies Association 3) are overlain by thick deltaic distributary channel deposits (Facies Association 4). The capping unit (Facies Association 5) represents a transgressive estuarine depositional environment. The general stratigraphic evolution resulted from a regional, tectonically controlled second‐order cycle, associated with northward regression of the Tethys. Subordinate cycles (i.e. third‐order and fourth‐order cycles) are evidenced by several Glossifungites‐ichnofacies demarcated discontinuities, which were emplaced at the base of flooding surfaces. The proposed depositional models recognize the importance of identifying and linking ichnological data with physical–sedimentological observations. As such – with the exception of wave‐generated ravinement surfaces – earlier assertions of wave‐dominated sedimentation can be discarded. Moreover, this study provides important data for the recognition of (rarely reported) completely bioturbated sand‐dominated offshore to nearshore sediments (Facies Association 1) and affords excellent characterization of bioturbated inclined heterolithic stratification of deltaic deposits. Another outcome of the study is the recognition that the whales of the Fayum Depression are restricted to the highstand systems tracts, and lived under conditions of low depositional energy, low to moderate sedimentation rates, and (not surprisingly) in fully marine waters characterized by a high biomass.  相似文献   

13.
Amorosi  Colalongo  Pasini  & Preti 《Sedimentology》1999,46(1):99-121
Data from 17 continuously cored boreholes, 40–170 m deep, reveal the subsurface stratigraphy of the Romagna coastal plain. Sedimentological and microfaunal data allow the distinction of eight facies associations of Late Pleistocene–Holocene age, including 18 lithofacies and 16 faunal associations. Ten 14C dates provide the basis to establish a sequence stratigraphic framework for the succession corresponding to the upper part 35 ky BP of the last glacio-eustatic cycle. The eight facies associations can be grouped into lowstand, transgressive and highstand systems tracts. The upper part of the lowstand systems tract consists of alluvial plain deposits. These accumulated during the Late Pleistocene when the shoreline was ≈250 km south of its present-day position. A pronounced stratigraphic hiatus (between 25 and 8·8 ky BP) is invariably recorded at the upper boundary (transgressive surface) of these Pleistocene, indurated and locally pedogenized alluvial deposits. The succeeding postglacial history is represented by a well developed transgressive–regressive cycle. Transgressive deposits, interpreted to reflect the rapid landward migration of a barrier–lagoon system, include two wedge-shaped, paralic and marine units. These thicken in opposite directions and are separated by a ravinement surface. Above the transgressive deposits, the maximum flooding surface (MFS) marks the change from a transgressive barrier–lagoon complex to a prograding, wave-dominated delta system (early Po delta). The MFS can be traced landwards, where it constitutes the base of lagoonal deposits. An aggradational to progradational stacking pattern of upper delta plain (marsh), lower delta plain (lagoon/bay), and delta front (beach ridge) deposits reflects the progressive increase in the sediment supply/accommodation ratio during the following highstand. The alluvial deposits capping the sequence accumulated by the 13th century AD, in response to an avulsion event that caused abandonment of the former Po delta lobe and the northward migration of the Po River towards its present position.  相似文献   

14.
《Sedimentary Geology》2005,173(1-4):151-185
An Early Miocene (Early Burdigalian) incised valley-fill was produced through development of an alluvial system during active extension and block rotation in the Mut Basin. Five phases of alluvial activity have been recognized and are linked to specific tectono-stratigraphic factors. The entrenchment phase (phase 1) was a response to a rapid decrease in accommodation caused by a combination of sea-level fall and accelerated tectonism that occurred around the basin during active extension. A lacustrine depositional system that pre-dated entrenchment was abruptly succeeded by an erosional fluvial system. The initial erosion, the entrenchment phase was followed by the deposition of ephemeral meandering fluvial facies and later by high sinuosity sandy meandering fluvial facies. During the aggradational phase (Phase 2), coarser-grained, lower sinuosity meandering river facies were vertically stacked in response to successive periods of fault-block rotation and basinal subsidence. The thickest stratigraphic interval was deposited during this time. Simultaneously in a basinward position, finer-grained distal facies were deposited. The succeeding backfilling phase (Phase 3) was marked by further fault-block rotation and an increase in the catchment area that resulted in higher flow regime and more sediment input. A further increase in accommodation space due to block rotation resulted in the retreat of facies belts, and the deposition of a retrogradational stacked gravelly low-sinuosity meandering facies during the early transgressive phase (Phase 4). In the downstream part of the alluvial valley, fluvio-deltaic and non-marine transitional facies were deposited and progressively retreated landwards during marine flooding. Phase 5 marks the main interval of Early Miocene marine transgression (a combination of global eustasy and regional epeiric subsidence). During this time, the facies belts within the incised valley-fill were dominated by estuarine and lagoonal facies assemblages, while the distal parts of the alluvial valley became completely flooded with marine waters. At the end of the transgressive phase, in the uppermost early Burdigalian, the estuarine and lagoonal facies migrated further inland, while shallow-marine sediments (reefal limestones) were deposited in distal parts of the now-drowned valley, blanketing the pre-existing topography.  相似文献   

15.
The Lower Permian (Artinskian to Sakmarian) Pebbley Beach Formation (PBF) of the southernmost Sydney Basin in New South Wales, Australia, records sediment accumulation in shallow marine to coastal environments at the close of the Late Palaeozoic Gondwanan ice age. This paper presents a sequence stratigraphic re‐evaluation of the upper half of the unit based on the integration of sedimentology and ichnology. Ten facies are recognized, separated into two facies associations. Facies Association A (seven facies) comprises variably bioturbated siltstones and sandstones with marine body fossils, interpreted as recording sediment accumulation in open marine environments ranging from lower offshore to middle shoreface water depths. Evidence of deltaic influence is seen in several Association A facies. Facies Association B (three facies) comprises mainly heterolithic, interlaminated and thinly interbedded sandstone and siltstone with some thicker intervals of dark grey, organic‐rich mudstone, some units clearly filling incised channel forms. These facies are interpreted as the deposits of estuarine channels and basins. Throughout the upper half of the formation, erosion surfaces with several metres relief abruptly separate open marine facies of Association A (below) from estuarine facies of Association B (above). Vertical facies changes imply significant basinward shift of environment across these surfaces, and lowering of relative sea level in the order of 50 m. These surfaces can be traced over several kilometres along depositional strike, and are defined as sequence boundaries. On this basis, at least nine sequences have been recognized in the upper half of the formation, each of which is < 10 m thick, condensed, incomplete and top‐truncated. Sequences contain little if any record of the lowstand systems tract, a more substantial transgressive systems tract and a highstand systems tract that is erosionally truncated (or in some cases, missing). This distinctive stacking pattern (which suggests a dominance of retrogradation and progradation over aggradation) and the implied relative sea‐level drop across sequence boundaries of tens of metres are remarkably similar to some other studies of continental margin successions formed under the Neogene icehouse climatic regime. Accordingly, it is suggested that the stratigraphic architecture of the PBF was a result of an Icehouse climate regime characterized by repeated, high‐amplitude cycles of relative sea‐level change.  相似文献   

16.
During the early Middle Devonian in South China, an extensive carbonate platform was broken up through extension to create a complex pattern of platforms, and interplatform basins. In Givetian and Frasnian carbonate successions, five depositional facies, including peritidal, restricted shallow subtidal, semi‐restricted subtidal, intermediate subtidal and deep subtidal facies, and 18 lithofacies units are recognized from measured sections on three isolated platforms. These deposits are arranged into metre‐scale, upward‐shallowing peritidal and subtidal cycles. Nine third‐order sequences are identified from changes in cycle stacking patterns, vertical facies changes and the stratigraphic distribution of subaerial exposure indicators. These sequences mostly consist of a lower transgressive part and an upper regressive part. Transgressive packages are dominated by thicker‐than‐average subtidal cycles, and regressive packages by thinner‐than‐average peritidal cycles. Sequence boundaries are transitional zones composed of stacked, high‐frequency, thinner‐than‐average cycles with upward‐increasing intensity of subaerial exposure, rather than individual, laterally traceable surfaces. These sequences can be further grouped into catch‐up and keep‐up sequence sets from the long‐term (second‐order) changes in accommodation and vertical facies changes. Catch‐up sequences are characterized by relatively thick cycle packages with a high percentage of intermediate to shallow subtidal facies, and even deep subtidal facies locally within some individual sequences, recording long‐term accommodation gain. Keep‐up sequences are characterized by relatively thin cycle packages with a high percentage of peritidal facies within sequences, recording long‐term accommodation loss. Correlation of long‐term accommodation changes expressed by Fischer plots reveals that during the late Givetian to early Frasnian increased accommodation loss on platforms coincided with increased accommodation gain in interplatform basins. This suggests that movement on faults resulted in the relative uplift of platforms and subsidence of interplatform basins. In the early Frasnian, extensive siliceous deposits in most interplatform basins and megabreccias at basin margins correspond to exposure disconformities on platforms.  相似文献   

17.
Strata of the Bardas Blancas Formation (lower Toarcian–lower Bajocian) are exposed in northern Neuquén Basin. Five sections have been studied in this work. Shoreface/delta front to offshore deposits predominate in four of the sections studied exhibiting a high abundance of hummocky cross-stratified, horizontally bedded and massive sandstones, as well as massive and laminated mudstones. Shell beds and trace fossils of the mixed Skolithos-Cruziana ichnofacies appear in sandstone beds, being related with storm event deposition. Gravel deposits are frequent in only one of these sections, with planar cross-stratified, normal graded and massive orthoconglomerates characterizing fan deltas interstratified with shoreface facies. A fifth outcrop exhibiting planar cross-stratified orthoconglomerates, pebbly sandstones with low-angle stratification and laminated mudstones have been interpreted as fluvial channel deposits and overbank facies. The analysis of the vertical distribution of facies and the recognition of stratigraphic surfaces in two sections in Río Potimalal area let recognized four transgressive–regressive sequences. Forced regressive events are recognized in the regressive intervals. Comparison of vertical distribution of facies also shows differences in thickness in the lower interval among the sections studied. This would be related to variations in accommodation space by previous half-graben structures. The succession shows a retrogradational arrangement of facies related with a widespread transgressive period. Lateral variation of facies let recognize the deepening of the basin through the southwest.  相似文献   

18.
This study focuses on Miocene sedimentation and stratigraphic evolution in a major transfer zone at the northern tip of the Thal Fault segment, Gulf of Suez. The succession generally shoals upwards from offshore mudstone containing pro-delta turbidites, into conglomeratic delta foresets and topsets, with sandstone-dominated shoreface facies coexisting laterally. Despite this upward shoaling, key stratal surfaces marking abrupt changes in relative sea-level allow the succession to be divided into four stratal units. The stacking pattern of the stratal units suggests an initial relative sea-level rise that generated a major marine flooding surface. A relative sea-level fall followed, resulting in widespread exposure and incision. During the ensuing relative sea-level rise a lowstand coarse-grained delta and coeval shoreface succession prograded several kilometres basinward. The stratigraphic development of the transfer zone delta is in marked contrast to that of aggradationally stacked deltas that occur near the centre of the Baba-Sidri fault segment, further south. At the transfer zone, low rates of subsidence and accommodation development coupled with a high sediment supply derived from a large fault tip drainage catchment have produced a strongly progradational delta subject to marked changes in relative sea-level. In the fault centre location, however, higher rates of accommodation development coupled with lower rates of sediment supply from footwall catchments have produced aggradationally stacked deltas. The results from this study have implications for sequence stratigraphic models and hydrocarbon exploration within extensional basins.  相似文献   

19.
Variability in accommodation and sedimentation rates within a basin generates significant deviations in the along-strike stratal stacking patterns of systems tracts. This variability can lead to coeval depositional units that record the juxtaposition of transgressive (retrogradational) and regressive (progradational) stratal stacking patterns. In scenarios where transgressive and regressive units are deposited concurrently, challenges arise when attempting to correlate and place systems tracts into a sequence stratigraphic framework. In these scenarios, the maximum flooding surface records a high level of diachroneity, with the position of the surface variable throughout the stratigraphic column. In this study, Viking Formation (late Albian) deposits in the Western Canada Sedimentary Basin, central Alberta, Canada, preserve significant along-strike variability of palaeoshorelines that developed in response to autogenic processes as well as allogenic controls that were active during deposition. Specifically, structural reactivation of Precambrian basement structures during Viking deposition led to significant variability in depositional environments along the palaeoshoreline. The incremental basement reactivation of the Precambrian Snowbird Tectonic Zone influenced sedimentation patterns and the creation of anomalous zones of accommodation in localized areas of the basin. Across fault boundaries and within the anomalously thick strata, both progradational and retrogradational stacking patterns occur within broadly contemporaneous deposits, complicating the correlation of stratigraphic units. While the concomitant deposition of transgressive and regressive units has been documented in a number of modern marine analogues, the concept is rarely applied to ancient successions. By identifying along-strike variabilities in shoreline geometries and incorporating the autogenic and allogenic controls that were active during deposition, a more accurate sequence stratigraphic framework can be proposed.  相似文献   

20.
Coastal strandplain deposits near Umiujaq, eastern Hudson Bay, Canada, were formed under falling relative sea level conditions resulting from postglacial isostatic uplift. Ground-probing radar profiles across the strandplain reveal a lower progradational unit (LPU) discordantly overlain by an upper progradational unit (UPU), which were correlated with stratigraphic sections exposed in incised valley walls. The discordance is a wave erosion surface (WES) that separates fine shoreface sands of the LPU from coarse-sand and gravel of the UPU. Major basal downlap surfaces can be traced updip into marine terraces and define downstepping wedges. The downstepping is interpreted as representing ‘autocyclic’ morphological reconfiguration rather than a response to changes in the rate of sea level fall. The internal architecture is strongly dependent on the accommodation and thus on antecedent topography. A conceptual model for strandplain deposition under falling sea level incorporates a bipartite shallowing-upward sandy succession when sufficient accommodation is available. Where accommodation space is limited, a sharp-based bar-and-beach sandbody directly overlies muddy deeper water deposits and the WES resembles a regressive surface of erosion.  相似文献   

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