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1.
David J. Went 《Sedimentology》2013,60(4):1036-1058
Quartzites are especially characteristic of Proterozoic and Cambro‐Ordovician shallow marine strata, whereas equivalent age fluvial deposits are commonly arkosic. The absence of land vegetation in the pre‐Silurian influenced weathering processes and styles of fluvial deposition. It may also have had an impact on shallow marine sedimentation. Two field studies from the English Channel region are presented to investigate the processes leading to quartzite formation. On Alderney, nearshore marine and fluvial facies occur interbedded on a metre scale and are interpreted to represent deposition on the lower reaches of an alluvial plain, and in beach and upper shoreface environments. The marine and fluvial sandstones display marked differences in textural and mineralogical maturity, pointing to a process of sediment maturation by the destruction of feldspar and labile grains at the shoreline. At Erquy, fully mature, marine quartzites occur bounded above and below by alluvial deposits via sharp or erosional surfaces, and are interpreted to represent high energy, storm and tidally influenced lower shoreface and inner shelf deposits. A model for quartzite development is proposed where, under a cool climate, frequent storms in un‐vegetated, tectonically rejuvenated uplands provided an abundance of arkosic sand to fluvial basins and clastic shorelines. The model proposes that the marine basins were subject to high wave energies, frequent storm events and tidal currents. These were conditions conducive to transforming arkosic sand to quartz‐rich sand by the attrition of feldspar at the shoreline and in the shallow marine environment. On sediment burial, further feldspar destruction occurred during diagenesis. The proposed model highlights the potential for a step change in sediment maturity to occur at the shoreline in early Palaeozoic depositional systems tracts.  相似文献   

2.
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3.
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   

4.
The Moodies Group in the Dycedale Syncline, Barberton Greenstone Belt consists of a 100–130 m-thick upward-fining succession that exhibits a transition from fluvial to tide-modified sedimentation. A basal, 10–30 m-thick conglomerate–sandstone interval of alluvial origin is overlain by stacked upward-fining, decimeter- to meter-scale cycles within which three facies are recognized: 1) conglomerate lag; 2) cross-bedded sandstone; and 3) interlaminated sandstone–siltstone and mudstone. Within the cycles, the abundance of mudstone drapes increases upwards. Structureless conglomerates and cross-beds lacking mudstone drapes record braided-alluvial processes. In contrast, cross-beds with mudstone drapes and interlaminated sandstone–siltstone and mudstone are products of flows modified by various tidal beats. Sand and/or silt transport took place during the ebb and flood stages and mudstone accumulated during slack water phases. Alternating thick–thin laminations reflect dominant and subordinate, twice-daily tides. Thicker groupings of foresets and thicker intervals of vertically stacked sandstone–siltstone and mudstone laminations are interpreted as spring tide deposits whereas thinner groupings of foresets and thinner laminations record neap tides. Desiccated mudstone drapes on foresets indicate that bedforms rarely were locally exposed during some portion of the tidal cycle. Abundant exposure structures in the interlaminated sandstone–siltstone and mudstone facies indicate that the cycles are upward shoaling. The stacked upward-fining cycles are attributed to alternating subaerial exposure and fluvial influx followed by marine inundation, probably related to absolute sea level fluctuations. Lack of high-order vegetation on the Archean landscape promoted rapid lateral migration or avulsion of tidally influenced fluvial channels.

The association of facies within the 100–130 m-thick upward-fining succession is comparable to Holocene and ancient paleovalley fills characterized by basal alluvial gradational upwards into estuarine facies. However, in the absence of vegetation, the land–ocean interface in the Archean probably consisted of laterally extensive fan or braid deltas rather than point sources of sediment characteristic of most modern rivers. The abrupt up-section change from syntectonic, high-energy, alluvial–fluvial flash flood deposits to tide-influenced sedimentation implies a proximal source that provided sediment to a shoreline influenced by strong tidal action. Possible Holocene analogues are orogenic settings such as the Canterbury Plains of New Zealand, the Indo-Gangetic Plains of India and strike-slip settings such as the Gulf of Aqaba but all three examples lack a direct transition to tidally influenced sedimentation.  相似文献   


5.
The Upper Permian Bijori Formation of the Satpura Gondwana basin comprising fineto coarse-grained sandstone, carbonaceous shale/mudstone and thin coal bands was previously interpreted as the deposits of meandering rivers. The present study documents abundance of wave ripples, hummocky and swaley cross-stratification and combined flow bedforms in the Bijori Formation, suggesting that a significant part of the formation was deposited in a wave-agitated environment. Evidence of near-emergent depositional conditions provided by repeated occurrence of rootlet beds and hydromorphic paleosols, local flooding surfaces denoting rapid fluctuation of water level, occurrences of temnospondyl vertebrate fossils, and absence of tidal signatures and marine fossils suggest a lacustrine rather than marine depositional regime. Five facies associations recognised within the Bijori Formation are inferred to represent fluvial channels and associated floodplains (FA1), lake shorelines (FA2), subaqueous distributary channels and associated levees (FA3), waveand storm-affected delta front (FA4), and open lacustrine/lower shoreface (FA5) deposits. The planoconcave fluvial channel-fill sandbodies with unidirectional cross-beds are clearly distinguishable from the delta front bars that show a convexo-plan or bi-convex sandbody geometry and dominance of wave and combined flow bedforms. Some of the distributary channels record interaction of fluvial and wave-dominated basinal processes. Major distributary sandbodies show a north to northwest flow direction while wave-affected delta front sandbodies show very complex flow patterns reflecting interaction between fluvial discharge and wave processes. Wave ripple crest trends show that the lake shoreline had an overall east-northeast to west-southwest orientation. The lack of documented contemporaneous lacustrine or marine sediments in the Satpura Gondwana basin posed a major problem of basin-scale palaeogeographic reconstruction. The existence of Bijori lake solves the problem and the lake is inferred to have acted as repository for the contemporaneous alluvial drainage. Development of the large Bijori lake body implies generation of accommodation space exceeding the rate of sediment supplied and thus represents locus of high tectonic subsidence. Transition of fluvial sediments with red mudstone and calcareous soil profile in the lower part of the succession to carbonaceous shale and coal-bearing lacustrine sediments in the upper part, denote a change from a warm semi-arid climate with seasonal rainfall to a more humid one.  相似文献   

6.
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   

7.
The Magallanes‐Austral Basin of Patagonian Chile and Argentina is a retroforeland basin associated with Late Cretaceous–Neogene uplift of the southern Andes. The Upper Cretaceous Dorotea Formation records the final phase of deposition in the Late Cretaceous foredeep, marked by southward progradation of a shelf‐edge delta and slope. In the Ultima Esperanza district of Chile, laterally extensive, depositional dip‐oriented exposures of the Dorotea Formation contain upper slope, delta‐front and delta plain facies. Marginal and shallow marine deposits include abundant indicators of tidal activity including inclined heterolithic stratification, heterolithic to sandy tidal bundles, bidirectional palaeocurrent indicators, flaser/wavy/lenticular bedding, heterolithic tidal flat deposits and a relatively low‐diversity Skolithos ichnofacies assemblage in delta plain facies. This work documents the stratigraphic architecture and evolution of the shelf‐edge delta that was significantly influenced by strong tidal activity. Sediment was delivered to a large slump scar on the shelf‐edge by a basin‐axial fluvial system, where it was significantly reworked and redistributed by tides. A network of tidally modified mouth bars and tidal channels comprised the outermost reaches of the delta complex, which constituted the staging area and initiation point for gravity flows that dominated the slope and deeper basin. The extent of tidal influence on the Dorotea delta also has important implications for Magallanes‐Austral Basin palaeogeography. Prior studies establish axial foreland palaeodrainage, long‐term southward palaeotransport directions and large‐scale topographic confinement within the foredeep throughout Late Cretaceous time. Abundant tidal features in Dorotea Formation strata further suggest that the Magallanes‐Austral Basin was significantly embayed. This ‘Magallanes embayment’ was formed by an impinging fold–thrust belt to the west and a broad forebulge region to the east.  相似文献   

8.
Mio-Pliocene deposits of the forebulge–backbulge depozones of the Beni-Mamore foreland Basin indicate tidally to fluvially dominated sedimentation. Seven facies assemblages have been recognized: FAA–FAG. FAA represents a distal bottom lake assemblage, FAB and FAD are interpreted as tidal flat deposits, FAC and FAG are interpreted as fluvial systems, FAE sediments are deposited in a subtidal/shoreface setting, and FAG represents a meandering fluvial system. The identification of stratigraphic surfaces (SU, MFS, and MRS) and the relationship among the facies assemblages permit the characterization of several systems tracts: a falling-stage systems tract (FSST) followed by a lowstand systems tract (LST), a transgressive systems tract (TST), and a highstand systems tract (HST). The FSST and LST may have been controlled by the uplift of the Beni-Mamore forebulge, whereas TST may result from a quiescent stage in the forebulge. Subaerial unconformity two (SU2) records the passage from a tide-influenced depositional system to a fully continental depositional system. The Miocene tidal-influenced deposits in the Beni–Mamore Basin suggest that it experienced a connection, either with the South Atlantic Ocean or the Caribbean Sea or both.  相似文献   

9.
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   

10.
Eighteen coastal-plain depositional sequences that can be correlated to shallow- to deep-water clinoforms in the Eocene Central Basin of Spitsbergen were studied in 1 × 15 km scale mountainside exposures. The overall mud-prone (>300 m thick) coastal-plain succession is divided by prominent fluvial erosion surfaces into vertically stacked depositional sequences, 7–44 m thick. The erosion surfaces are overlain by fluvial conglomerates and coarse-grained sandstones. The fluvial deposits show tidal influence at their seaward ends. The fluvial deposits pass upwards into macrotidal tide-dominated estuarine deposits, with coarse-grained river-dominated facies followed further seawards by high- and low-sinuosity tidal channels, upper-flow-regime tidal flats, and tidal sand bar facies associations. Laterally, marginal sandy to muddy tidal flat and marsh deposits occur. The fluvial/estuarine sequences are interpreted as having accumulated as a series of incised valley fills because: (i) the basal fluvial erosion surfaces, with at least 16 m of local erosional relief, are regional incisions; (ii) the basal fluvial deposits exhibit a significant basinward facies shift; (iii) the regional erosion surfaces can be correlated with rooted horizons in the interfluve areas; and (iv) the estuarine deposits onlap the valley walls in a landward direction. The coastal-plain deposits represent the topset to clinoforms that formed during progradational infilling of the Eocene Central Basin. Despite large-scale progradation, the sequences are volumetrically dominated by lowstand fluvial deposits and especially by transgressive estuarine deposits. The transgressive deposits are overlain by highstand units in only about 30% of the sequences. The depositional system remained an estuary even during highstand conditions, as evidenced by the continued bedload convergence in the inner-estuarine tidal channels.  相似文献   

11.
Current models of alluvial to coastal plain stratigraphy are concept‐driven and focus on relative sea‐level as an allogenic control. These models are tested herein using data from a large (ca 100 km long and 300 m thick), continuous outcrop belt (Upper Cretaceous Blackhawk Formation, central Utah, USA). Many channelized fluvial sandbodies in the Blackhawk Formation have a multilateral and multistorey internal character, and they generally increase in size and abundance (from ca 10% to ca 30% of the strata) from base to top of the formation. These regional, low‐resolution trends exhibit much local variation, but are interpreted to reflect progressively decreasing tectonic subsidence in the upper Blackhawk Formation and overlying Castlegate Sandstone. The trend may also incorporate progressively more frequent channel avulsion during deposition of the lower Blackhawk Formation. Laterally extensive coal zones formed on the coastal plain during shallow‐marine transgressions, and define the high‐resolution stratigraphic framework of the lower Blackhawk Formation. Large (up to 25 m thick and 1 to 6 km wide), multistorey, multilateral, fluvial channel‐complex sandbodies that overlie composite erosion surfaces occur at distinct stratigraphic levels, and are interpreted as fluvial incised valley fills. Low amplitude (<30 m) relative sea‐level variations are interpreted as the dominant control on stratigraphic architecture in the lower Blackhawk Formation, which was deposited up to 50 km inland from the coeval shoreline. In contrast, the high‐resolution stratigraphy of the upper Blackhawk Formation is poorly defined, and channelized fluvial sandbodies are poorly organized. Vertical and laterally offset stacking of a small proportion (<10%) of sandbodies produced ‘clusters’ that are not confined by ‘master’ erosion surfaces. Avulsion is interpreted to dominate the stratigraphic architecture of the upper Blackhawk Formation. This data‐driven analysis indicates that alluvial to coastal plain stratigraphic architecture reflects a combination of various allogenic controls and autogenic behaviours. The relative sea‐level control emphasized in sequence stratigraphic models is only rarely dominant.  相似文献   

12.
Triassic strata of the Dockum Group in Texas comprise two major upward-fining alluvial-lacustrine depositional sequences. The two sequences are represented by the (1) Santa Rosa-Tecovas, and (2) Trujillo-Cooper Canyon Formations. The second sequence is much thicker than the first, and occupies a greater geographic part of the Dockum basin. Each sequence of alluvial and lacustrine sediment accumulation is characterized by sediment derivation from a different source terrain. The unconformable relationship between the two depositional sequences, the change in mineralogical composition and presumed source areas between these units, differences in paleocurrent orientation between units, and evidence for intervening episodes of local deformation indicate that the sequences are of tectonic origin. These strata are not the product of a single sediment dispersal system, such as the centripetally-drained lacustrine delta complex previously envisioned for the Dockum basin. Both Dockum sequences are comprised largely of two typical alluvial facies associations; stream channel facies, and overbank flood-plain facies, that are similar to those described in nearly all fluvial deposits. In addition, the Dockum Group contains a peculiar lacustrine facies that accumulated in local flood-plain depressions, and probably resulted from subsidence over areas of subsurface salt dissolution. Vertebrate fossil assemblages are found in all three Dockum facies associations. Five fossiliferous sites in the Dockum are discussed in the context of these three depositional settings. The Dockum tetrapod diversity is reviewed in a hierarchical phylogeny with remarks on the history of collection, stratigraphic distribution of genera, and their taxonomic status. The stratigraphic ranges of tetrapod taxa do not support the recently proposed successive Otischalkian, Adamanian, Revueltian, and Apachean biochrons within the Dockum Group. Instead, a few index fossils provide a broad framework for correlation of Late Triassic nonmarine strata of the Dockum with the Carnian and Norian Alpine marine stages.  相似文献   

13.
The Pennsylvanian to Permian lower Cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the Paradox foreland basin fill exposed in and around the Canyonlands region of south‐east Utah. Aeolian facies comprise: (i) sets and compound cosets of trough cross‐bedded dune sandstone dominated by grain flow and translatent wind‐ripple strata; (ii) interdune strata characterized by sandstone, siltstone and mudstone interbeds with wind‐ripple, wavy and horizontal planar‐laminated strata resulting from accumulation on a range of dry, damp or wet substrate‐types in the flats and hollows between migrating dunes; and (iii) extensive, near‐flat lying wind‐rippled sandsheet strata. Fluvial facies comprise channel‐fill sandstones, lag conglomerates and finer‐grained overbank sheet‐flood deposits. Shallow marine facies comprise carbonate ramp limestones, tidal sand ridges and bioturbated marine mudstones. During episodes of sand sea construction and accumulation, compound transverse dunes migrated primarily to the south and south‐east, whereas south‐westerly flowing fluvial systems periodically punctuated the dune fields from the north‐east. Several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds. These surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo‐water‐table. During episodes of aeolian quiescence, fluvial systems became more widespread, forming unconfined braid‐plains that fed sediment to a coastline that lay to the south‐west and which ran approximately north‐west to south‐east for at least 200 km. Shallow marine systems repeatedly transgressed across the broad, low‐relief coastal plain on at least 10 separate occasions, resulting in the systematic preservation of units of marine limestone and calcarenite between units of non‐marine aeolian and fluvial strata, to form a series of depositional cycles. The top of the lower Cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north‐eastward directed marine transgression into the basin prior to the onset of exclusively non‐marine sedimentation of the overlying Cedar Mesa Sandstone. Styles of interaction between aeolian, fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small‐scale autocyclic behaviour of competing, coeval depositional systems and larger‐scale allocyclic changes that record system response to longer‐term interdependent variations in climatic and eustatic controlling mechanisms. The architectural relationships and system interactions observed in the lower Cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea‐level, and that these two external controls probably underwent cyclical change in harmony with each other in the Paradox Basin during late Pennsylvanian and Permian times. This observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio‐eustatic driving mechanism.  相似文献   

14.
山东黄河北煤田石炭—二叠系太原组地层沉积特征   总被引：1，自引：1，他引：0  
为了研究黄河北煤田的沉积特征,查清研究区内太原组地层的岩性类型、沉积构造、粒度分布特征,并进一步明确其沉积体系及沉积相的特征和沉积演化,本文根据野外露头的观察和室内岩心的描述、薄片分析,结合钻井、测井资料,运用沉积学、古生物地层学、层序地层学、岩石粒度分析等方法,对研究区的太原组地层沉积特征进行了详细的研究.结果表明:研究区石炭-二叠系的含煤地层太原组沉积物中砂岩、粉砂岩、泥岩、石灰岩及煤岩均较发育,常见的含铁矿物主要是褐铁矿、黄铁矿、菱铁矿等;主要层理构造有水平层理、平行层理、交错层理、波状层理、韵律层理等,生物扰动构造非常发育;主要的生物化石发育于台地相灰岩中,包括蜓类、牙形石等个体较大化石;主要发育植物化石有轮叶、科达木、宽带羊齿等;太原组沉积环境比较动荡,为河流相或者浅海相环境,其陆表海盆地层序及内部单元的典型界面有最大海泛面、区域性海退界面和最大海退面,并依据其不同界面,进行了沉积相划分和层序地层分析;研究区内晚石炭世的太原组地层为陆表海沉积序列,其沉积环境演化主要是由海陆交互相沉积向陆相沉积的转换.  相似文献   

15.
A stratigraphic motif observed in many foreland basins is the development of basinward tapering siliciclastic wedges characterized by various scales of depositional cycles. The Middle Devonian (Givetian) Mahantango Formation in the central Appalachian foreland basin is such an example. It consists of both small-and large-scale thickening- and coarsening-upward cycles; the small-scale cycles are typically less than 10 m thick whereas larger-scale cycles are generally a few tens of metres thick and commonly contain several of the smaller-scale cycles. Outcrop-based facies analyses indicate that the depositional cyclicity resulted from episodic progradation of a regionally straight, tide-dominated shoreline onto a storm-dominated, shallow marine shelf. The depositional model for this ancient shallow marine system consists of a vertical facies succession in which storm-dominated offshore marine mudstone and fine sandstone pass gradationally upward into storm-dominated nearshore marine shelf and shoreface sandstone overlain by, in proximal sections, tide-dominated shoreline sandstone, pebbly sandstone and mudstone. Transgressively reworked lag deposits cap most of the thickening- and coarsening- upward packets. In this model, coarse-grained rocks, rather than implying basinward shifts of facies, are a consanguineous part of the stacked shoaling cycles. Lateral facies relationships show that the dominance of storm- vs. tide-generated sedimentary features is simply a function of palaeogeographical position within the basin; proximal sections contain tidally influenced sedimentary features whereas more distal sections only display evidence for storm-influenced deposition. These results suggest caution when inferring palaeoceanographic conditions from sedimentological datasets that do not contain preserved examples of palaeoshorelines.  相似文献   

16.
《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   

17.
The upper portion of the Virgelle Member (Upper Cretaceous Milk River Formation) at Writing-on-Stone Provincial Park of southern Alberta preserves evidence of tidal processes along an otherwise wave-dominated, progradational shoreline in the Cretaceous Interior Seaway of North America. The upper Virgelle Member is underlain by offshore transition to lower shoreface deposits of the Telegraph Creek Member and the lower Virgelle Member, respectively, and is overlain by the non-marine shales and sandstones of the Deadhorse Coulee Member. The sediments of the upper Virgelle Member were deposited along a prograding shoreline and are interpreted here as those of a tidal-inlet complex. Most inlet sections consist of an erosional base overlain by a shale-pebble conglomerate, followed by cross-bedded sandstones which become finer-grained and decrease in scale upwards. Indicators of tidal processes include palaeocurrent distributions, mud couplets, tidal bundles, re-activation surfaces and herringbone cross-beds. The sequence through the tidal-inlet complex can be differentiated, according to prevalent palaeoflow directions and sedimentary structures, as ebb-dominated, flood-dominated, or mixed-tidal influence. Ebb-dominated sections commonly contain lateral accretion surfaces whereas flood-dominated sections contain tidal-ramp deposits. Back-barrier lagoon deposits are dominated by sandstones of an extensive flood-tidal delta with only thin shales preserved locally at the top of the inlet complex. Deposits of ebb-tidal deltas are absent, presumably due to the effective sediment dispersal by waves and wave-induced longshore currents acting on the regionally wave-dominated shoreline.  相似文献   

18.
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19.
《Sedimentology》2018,65(6):2171-2201
In modern siliciclastic environments terrestrial and aquatic vegetation binds substrate, controls weathering and erosion rates, influences run‐off, sediment supply and subsequent depositional architecture. This study assesses the applicability of modern depositional models that are impacted by vascular vegetation, as analogues for ancient pre‐land plant systems. A review of pre‐Devonian published literature demonstrates a paucity of described tidal successions; this is possibly due to the application of modern analogues for interpreting the record when there is a lack of tidal indicators. This paucity suggests a need for revised models of tidal deposition that consider the different environmental conditions prior to land plant evolution. This study examines the Ordovician–Silurian Tumblagooda Sandstone, which is exposed in the gorge of the Murchison River and coastal cliffs near Kalbarri, Western Australia. The Tumblagooda Sandstone comprises stacked sand‐rich facies, with well‐preserved bedforms and trace fossils. Previous interpretations of the depositional setting have proposed from a mixed sheet‐braided fluvial and intertidal flats; to a continental setting dominated by fluvial and aeolian processes. An enigmatic element is the rarity of mud‐rich facies preserved in the succession. Outcrop logging, facies and petrographic analysis record dominantly shallow water conditions with episodes of emergence. Abundant ichnotaxa indicate that marine conditions and bi‐directional flow structures are evidence for an intertidal and subtidal depositional environment. A macrotidal estuary setting is proposed, with evidence for tidal channels and repeated fluvial incursions. Physical and biogenic sedimentary structures are indicative of tidal conditions. The lack of clay and silt resulted in the absence of flaser or lenticular‐bedding. Instead cyclic deposition of thin beds and foreset bioturbation replaced mud drape deposits. Higher energy conditions prevailed in the absence of the binding activity of plants in the terrestrial and marine realm. This is suggestive of different weathering processes and a reduction in the preservation of some sedimentary features.  相似文献   

20.
河相层序地层研究进展述评   总被引：7，自引：0，他引：7  
将河流平衡剖面看成是陆区基准面的观点已受到怀疑，有迹象表明地貌基准面有可能是陆区基准面。河相沉积记录不仅与基准面变化有关，也与源区和流域盆地的许多因素有关，而且河相地层对基准在的响应也比过去理解的复杂得多。上游区由不整合面办公室的河相层序一般难以与海相层序对比。已可相层序模式仍以基准面为支撑理论，层序界面常按财相向盆迁移、复合砂体、古流向偏转和无沉积非作用面等识别，内部仍为三分。目前对可相层序发育  相似文献   

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