首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Sediments exposed at low tide on the transgressive, hypertidal (>6 m tidal range) Waterside Beach, New Brunswick, Canada permit the scrutiny of sedimentary structures and textures that develop at water depths equivalent to the upper and lower shoreface. Waterside Beach sediments are grouped into eleven sedimentologically distinct deposits that represent three depositional environments: (1) sandy foreshore and shoreface; (2) tidal‐creek braid‐plain and delta; and, (3) wave‐formed gravel and sand bars, and associated deposits. The sandy foreshore and shoreface depositional environment encompasses the backshore; moderately dipping beachface; and a shallowly seaward‐dipping terrace of sandy middle and lower intertidal, and muddy sub‐tidal sediments. Intertidal sediments reworked and deposited by tidal creeks comprise the tidal‐creek braid plain and delta. Wave‐formed sand and gravel bars and associated deposits include: sediment sourced from low‐amplitude, unstable sand bars; gravel deposited from large (up to 5·5 m high, 800 m long), landward‐migrating gravel bars; and zones of mud deposition developed on the landward side of the gravel bars. The relationship between the gravel bars and mud deposits, and between mud‐laden sea water and beach gravels provides mechanisms for the deposition of mud beds, and muddy clast‐ and matrix‐supported conglomerates in ancient conglomeratic successions. Idealized sections are presented as analogues for ancient conglomerates deposited in transgressive systems. Where tidal creeks do not influence sedimentation on the beach, the preserved sequence consists of a gravel lag overlain by increasingly finer‐grained shoreface sediments. Conversely, where tidal creeks debouch onto the beach, erosion of the underlying salt marsh results in deposition of a thicker, more complex beach succession. The thickness of this package is controlled by tidal range, sedimentation rate, and rate of transgression. The tidal‐creek influenced succession comprises repeated sequences of: a thin mud bed overlain by muddy conglomerate, sandy conglomerate, a coarse lag, and capped by trough cross‐bedded sand and gravel.  相似文献   

2.
《Sedimentary Geology》1999,123(3-4):199-218
Gravelly shoreline deposits of the latest Pleistocene highstand of Lake Lahontan occur in pristine depositional morphology, and are exposed in gravel pits along Churchill Butte in west-central Nevada. Four environments differentiated at this site are alluvial fan/colluvium, lakeshore barrier spit, lake lower-shoreface spit platform, and lake bottom. Lakeshore deposits abut, along erosional wave headcuts, either unsorted muddy to bouldery colluvium fringing Churchill Butte bedrock, or matrix-supported, cobbly and pebbly debris-flow deposits of the Silver Springs fan. The lakeshore barrier spit is dominated by granule pebble gravel concentrated by wave erosion of the colluvial and alluvial-fan facies. The lakeward side of the barrier consists of beachface deposits of well-sorted granules or pebbles in broad, planar beds 1–10 cm thick and sloping 10–15°. They interfinger downslope with thicker (10–25 cm) and less steep (5–10°) lakeward-dipping beds of fine to medium pebble gravel of the lake upper shoreface. Interstratified with the latter are 10–40-cm-thick sets of high-angle cross-beds that dip southward, alongshore. Higher-angle (15–20°), landward-dipping foresets of similar texture but poorer sorting comprise the proximal backshore on the landward side of the barrier. They were deposited during storm surges that overtopped the barrier berm. Gastropod-rich sand and mud, also deposited by storm-induced washover, are found landward of the gravel foresets in a 15-m-wide backshore pond. Algal stromatolites, ostracodes, and diatoms accumulated in this pond between storm events. The lake lower shoreface, extending from water depths of 2 to 8 m, consists of a southward-prograding spit platform built by longshore drift. The key component of this platform is large-scale sandy pebble gravel in 16° southward-dipping `Gilbert' foresets that grade at a water depth of about 6–7 m to 4°-dipping sandy toesets. A shift from bioturbated lower-shoreface sand and silt, to flat and laminated lake-bottom silt and mud, occurs between water depths of 10–40 m and over a shore-normal distance of ≥250 m. This lake-bottom mud facies, unlike the others, is areally expansive.  相似文献   

3.
On the south‐west coast of Vancouver Island, Canada, sedimentological and ichnological analysis of three beach–shoreface complexes developed along a strait margin was undertaken to quantify process–response relations in straits and to develop a model for strait‐margin beaches. For all three beaches, evidence of tidal processes are expressed best in the lower shoreface and offshore and, to a lesser extent, in the middle shoreface. Tidal currents are dominant offshore, below 18 m water depth (relative to the mean spring high tide), whereas wave processes dominate sediment deposition in the nearshore (intertidal zone to 5 m water depth). From 18 to 5 m water depth, tidal processes decrease in importance relative to wave processes. The relatively high tidal energy in the offshore and lower shoreface is manifest sedimentologically by the dominance of sand, of a similar grain size to the upper shoreface/intertidal zone and, by the prevalence of current‐generated structures (current ripples) oriented parallel to the shoreline. In addition, the offshore and lower shoreface of strait‐bound beach–shoreface complexes are recognized ichnologically by traces typical of the Skolithos Ichnofacies. This situation contrasts to the dominantly horizontal feeding traces characteristic of the Cruziana Ichnofacies that are prevalent in the lower shoreface and offshore of open‐coast (wave‐dominated) beach–shorefaces. These sedimentological and ichnological characteristics reflect tidal influence on sediment deposition; consequently, the term ‘tide‐influenced shoreface’ most accurately describes these depositional environments.  相似文献   

4.
Alongshore variation in the rip current hazard at Pensacola Beach, Florida   总被引：2，自引：1，他引：1  
Many drowning and near drownings at Pensacola Beach, Florida are attributed to rip currents, the strong seaward-flowing currents that extend from the shoreline to the line of breakers and sometimes beyond. While surf forecasts assume that the rip hazard is uniform alongshore and that the (erosion) rips are ephemeral features, evidence is presented to suggest that the rip hazard at Pensacola Beach is not uniform alongshore. Rather the rip current “hotspots” develop as a consequence of an alongshore variation in the surf similarity parameter and nearshore state on the order of ~1,450 m. The variation is forced by transverse ridges on the inner shelf that force wave refraction and focusing at the ridge crests. This creates a more dissipative, rhythmic bar and beach morphology at the ridges and rougher surf. Between ridges, where wave heights and periods are smaller and the outermost bar is forced closer to the shoreline, the nearshore is in a (more reflective) bar and rip state during red flag conditions. Drownings between 2000 and 2009 are shown to be clustered between transverse ridges and in the years following a hurricane or tropical storm (2000–2003 and 2005–2008) when the bar and rip morphology first develops as the shore face recovers. This continues until the innermost bar attaches to the beach face unless the bar system is reset by another tropical storm or hurricane. It is argued that the rip hazard is dependent on the alongshore covariation of the environmental forcing with the individual and group behavior in both time and space, even on what appears to be a relatively uniform beach environment.  相似文献   

5.
Although modern wave‐dominated shorelines exhibit complex geomorphologies, their ancient counterparts are typically described in terms of shoreface‐shelf parasequences with a simple internal architecture. This discrepancy can lead to poor discrimination between, and incorrect identification of, different types of wave‐dominated shoreline in the stratigraphic record. Documented in this paper are the variability in facies characteristics, high‐resolution stratigraphic architecture and interpreted palaeo‐geomorphology within a single parasequence that is interpreted to record the advance of an ancient asymmetrical wave‐dominated delta. The Standardville (Ab1) parasequence of the Aberdeen Member, Blackhawk Formation is exposed in the Book Cliffs of central Utah, USA. This parasequence, and four others in the Aberdeen Member, record the eastward progradation of north/south‐trending, wave‐dominated shorelines. Within the Standardville (Ab1) parasequence, distal wave‐dominated shoreface‐shelf deposits in the eastern part of the study area are overlain across a downlap surface by southward prograding fluvial‐dominated delta‐front deposits, which have previously been assigned to a separate ‘stranded lowstand parasequence’ formed by a significant, allogenic change in relative sea‐level. High‐resolution stratigraphic analysis of these deposits reveals that they are instead more likely to record a single episode of shoreline progradation characterized by alternating periods of normal regressive and forced regressive shoreline trajectory because of minor cyclical fluctuations in relative sea‐level. Interpreted normal regressive shoreline trajectories within the wave‐dominated shoreface‐shelf deposits are marked by aggradational stacking of bedsets bounded by non‐depositional discontinuity surfaces. Interpreted forced regressive shoreline trajectories in the same deposits are characterized by shallow incision of fluvial distributary channels and strongly progradational stacking of bedsets bounded by erosional discontinuity surfaces that record enhanced wave‐base scour. Fluvial‐dominated delta‐front deposits most probably record the regression of a lobate delta parallel to the regional shoreline into an embayment that was sheltered from wave influence. Wave‐dominated shoreface‐shelf and fluvial‐dominated delta‐front deposits occur within the same parasequence, and their interpretation as the respective updrift and downdrift flanks of a single asymmetrical wave‐dominated delta that periodically shifted its position provides the most straightforward explanation of the distribution and relative orientation of these two deposit types.  相似文献   

6.
The late Barremian succession in the Agadir Basin of the Moroccan Western High Atlas represents wave-dominated deltaic deposits. The succession is represented by stacked thickening and coarsening upwards parasequences 5–15 m thick formed during fifth- or fourth-order regression and building a third-order highstand systems tract. Vertical facies transitions in parasequences reflect flooding followed by shoaling of diverse shelf environments ranging from offshore transition interbedded mudstones, siltstones and thin sandstones, lower shoreface/lower delta front hummocky bedforms to upper shoreface/upper delta front cross-bedded sandstones. The regional configuration reflects the progradation of wave-dominated deltas over an offshore setting. The maximum sea-level fall led to the development of a sequence boundary that is an unconformity. The subsequent early Aptian relative sea-level rise contributes to the development of an extensive conglomerate lagged transgressive surface of erosion. The latter and the sequence boundary are amalgamated forming a composite surface.  相似文献   

7.
Ice scouring of lake and sea-floor substrates by the keels of drifting ice masses is a common geological process in modern northern lakes and continental shelves, and was widespread during the Pleistocene. Nonetheless, the importance of scouring as a geological process is not yet matched by many sedimentological studies of scour structures exposed in outcrop. This article presents an integrated study combining outcrop sedimentology and subsurface ground-penetrating radar (GPR) data from a relict late Pleistocene ice-scoured glacial lake floor now preserved below beach sediments in Ontario, Canada. Scours occur along a regressive sequence boundary where deep-water muddy facies are abruptly overlain by shallow-water sands resulting from an abrupt drop in water levels. This has allowed the keels of drifting ice masses to scour into muds. Three-dimensional data gained from the GPR survey show that scours are as much as 2.5 m deep and 7 m wide; they have berms of displaced sediment and are oriented parallel to the former shoreline. Scoured shoreface sediments that fill scours show abundant liquefaction structures, indicating substrate dewatering during repeated scouring events similar to that recently reported in the modern Beaufort Sea in Canada's far north. Marked changes in water depths are typical of glacially influenced lakes and seas, creating opportunities for drifting ice to scour into offshore muddy cohesive facies and be preserved. The data presented here may aid identification in ancient successions elsewhere.  相似文献   

8.
Abstract Physical stratigraphy within shoreface‐shelf parasequences contains a detailed, but virtually unstudied, record of shallow‐marine processes over a range of historical and geological timescales. Using high‐quality outcrop data sets, it is possible to reconstruct ancient shoreface‐shelf morphology from clinoform surfaces, and to track the evolving morphology of the ancient shoreface‐shelf. Our results suggest that shoreface‐shelf morphology varied considerably in response to processes that operate over a range of timescales. (1) Individual clinoform surfaces form as a result of enhanced wave scour and/or sediment starvation, which may be driven by minor fluctuations in relative sea level, sediment supply and/or wave climate over short timescales (101?103 years). These external controls cannot be distinguished in vertical facies successions, but may potentially be differentiated by the resulting clinoform geometries. (2) Clinoform geometry and distribution changes systematically within a single parasequence, reflecting the cycle in sea level and/or sediment supply that produced the parasequence (102?105 years). These changes record steepening of the shoreface‐shelf profile during early progradation and maintenance of a relatively uniform profile during late progradation. Modern shorefaces are not representative of this stratigraphic variability. (3) Clinoform geometries vary greatly between different parasequences as a result of variations in parasequence stacking pattern and relict shelf morphology during shoreface progradation (105?108 years). These controls determine the external dimensions of the parasequence.  相似文献   

9.
The Ordovician System, cropping out in southern and west-central Jordan, consists entirely of a 750 m thick clastic sequence that can be subdivided into six formations. The lower Disi Formation starts conformably above the Late Cambrian Umm Ishrin Formation. According to Cruziana furcifera occurring in the upper third of the Disi Formation, an Early Ordovician age is confirmed. The Disi Formation, consisting mainly of downstream accretion (DA) fluvial architectural element, was deposited in a proximal braidplain flowing N–NE from the southerly-located Arabian–Nubian Shield towards the Tethys Seaway. The braidplain depositional environment evolved into a braidplain-dominated delta through the middle and upper parts of the Disi Formation and the lower part of the overlying Um Saham Formation. The delta was replaced by siliciclastic tidal flats, that in turn evolved into an upper to lower shoreface environment through the upper part of the Um Saham Formation. The depositional environment attained the maximum bathymetric depth during the deposition of the lower and central parts of the third unit, the Hiswa Formation, where offshore graptolite-rich mudstone with intercalated hummocky cross-stratified tempestites were deposited. The Tethys Seaway regressed back through the upper part of the Hiswa Formation promoting a resumption of the lower–upper shoreface sedimentation. Oscillation between the lower to upper shoreface depositional environment characterized the entire fourth unit, the Dubaydib Formation, as well as the Tubeiylliat Sandstone Member of the fifth unit, the Mudawwara Formation. The depositional history of the Ordovician sequence was terminated by a glaciofluvial regime that finally was gradually replaced by a shoreface depositional environment throughout the last unit, the Ammar Formation.  相似文献   

10.
The Lower Permian Wasp Head Formation (early to middle Sakmarian) is a ～95 m thick unit that was deposited during the transition to a non‐glacial period following the late Asselian to early Sakmarian glacial event in eastern Australia. This shallow marine, sandstone‐dominated unit can be subdivided into six facies associations. (i) The marine sediment gravity flow facies association consists of breccias and conglomerates deposited in upper shoreface water depths. (ii) Upper shoreface deposits consist of cross‐stratified, conglomeratic sandstones with an impoverished expression of the Skolithos Ichnofacies. (iii) Middle shoreface deposits consist of hummocky cross‐stratified sandstones with a trace fossil assemblage that represents the Skolithos Ichnofacies. (iv) Lower shoreface deposits are similar to middle shoreface deposits, but contain more pervasive bioturbation and a distal expression of the Skolithos Ichnofacies to a proximal expression of the Cruziana Ichnofacies. (v) Delta‐influenced, lower shoreface‐offshore transition deposits are distinguished by sparsely bioturbated carbonaceous mudstone drapes within a variety of shoreface and offshore deposits. Trace fossil assemblages represent distal expressions of the Skolithos Ichnofacies to stressed, proximal expressions of the Cruziana Ichnofacies. Impoverished trace fossil assemblages record variable and episodic environmental stresses possibly caused by fluctuations in sedimentation rates, substrate consistencies, salinity, oxygen levels, turbidity and other physio‐chemical stresses characteristic of deltaic conditions. (vi) The offshore transition‐offshore facies association consists of mudstone and admixed sandstone and mudstone with pervasive bioturbation and an archetypal to distal expression of the Cruziana Ichnofacies. The lowermost ～50 m of the formation consists of a single deepening upward cycle formed as the basin transitioned from glacioisostatic rebound following the Asselian to early Sakmarian glacial to a regime dominated by regional extensional subsidence without significant glacial influence. The upper ～45 m of the formation can be subdivided into three shallowing upward cycles (parasequences) that formed in the aftermath of rapid, possibly glacioeustatic, rises in relative sea‐level or due to autocyclic progradation patterns. The shift to a parasequence‐dominated architecture and progressive decrease in ice‐rafted debris upwards through the succession records the release from glacioisostatic rebound and amelioration of climate that accompanied the transition to broadly non‐glacial conditions.  相似文献   

11.
The dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.  相似文献   

12.
Analysis of an 18-day time-lapse film record of shoreface ripple development, with concurrent measurements of near-bottom flow and surface waves, provides new insight on equilibrium bedform conditions, adjustment of ripple planform to variable hydrodynamics, and ripple migration behaviour. The study was conducted in approximately 10 m water depth, 1 km off Martinique Beach on the Atlantic coast of Nova Scotia (Canada), under low-energy summer wave conditions. Significant wave-height and peak period during the study averaged 0–7 m and 8 s, respectively, with extremes up to 1–7 m and 11 s during passage of three weak weather disturbances. Six mutually exclusive ripple types have been defined: (1) short-wavelength regular ripples; (2) variable bifurcated ripples; (3) variable terminated ripples; (4) short-crested ripples; (5) long-wavelength regular ripples; and (6) chaotic ripples. Ripple wavelength ranged from 0–07 m to 0–24 m and displayed a strong Reynolds number dependence. Together with other published field data, the results suggest a lower limit of γ=0–06 m for the wavelength of wave ripples in ocean shoreface environments. Ripple orientation ranged through 38° and responded rapidly to changes in wave approach direction, but did not conform to the orientation of the adjacent shoreline. Ripples were observed to migrate both on- and off-shore (with and against the wave advance direction) at rates up to ±0–1 m h-1, associated with net flows other than wave-induced onshore asymmetry and mass transport. Migration (mainly of ripple types 1 and 2) occurred during the peak of storm events, but showed no obvious correlation with measured near-bottom flow magnitude or direction. Ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight-crested rippie types 1 and 5 are present. Disequilibrium in orientation or dimensions is expressed by increasing sinuosity, bifurcation and crest termination in types 2,3,4 and 6.  相似文献   

13.
Delta asymmetry occurs where there is strong wave influence and net longshore transport. Differences in the morphology and facies architecture between updrift and downdrift sides of asymmetric deltas are potentially significant for exploration and exploitation of resources in this class of reservoirs. Although delta asymmetry has been recognized widely from modern wave‐influenced deltaic shorelines, there are few documented examples in the ancient record. Based on an integrated sedimentological and ichnological study, the along‐strike variability and delta asymmetry within a single parasequence (Ps 6) is documented in continuously exposed outcrops of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation near Hanksville in southern Utah. Two intra‐parasequence discontinuity surfaces are recognized which allow subdivision of the parasequence into three bedsets, marked as Ps 6‐1 to Ps 6‐3. Four facies successions are recognized: (i) wave/storm‐dominated shoreface; (ii) river‐dominated delta front; (iii) wave/storm‐reworked delta front; and (iv) distributary channel and mouth bar. Dips of cross‐strata within distributary‐mouth bars and shorefaces show a strong downdrift (southward) component. Ps 6‐3 predominantly consists of river‐dominated delta‐front deposits, whereas Ps 6‐1 and Ps 6‐2 show an along‐strike facies change with shoreface deposits in the north, passing into heterolithic, river‐dominated delta‐front successions south to south‐eastward, and wave/storm‐reworked delta‐front deposits further to the south‐east. Trace fossil suites correspondingly show distinct along‐strike changes from robust and diverse expressions of the archetypal Cruziana Ichnofacies and Skolithos Ichnofacies, into suites characterized by horizontal, morphologically simple, facies‐crossing ichnogenera, reflecting a more stressed, river‐dominated environment. Further south‐eastward, trace fossil abundance and diversity increase, reflecting a return to archetypal ichnofacies. The overall facies integrated with palaeocurrent data indicate delta asymmetry. The asymmetric delta consists of sandier shoreface deposits on the updrift side and mixed riverine and wave/storm‐reworked deposits on the downdrift side, similar to that observed in the modern examples. However, in contrast to the recent delta asymmetry models, significant paralic, lagoonal and bay‐fill facies are not documented in the downdrift regions of the asymmetric delta. This observation is attributed to a negative palaeoshoreline trajectory during delta progradation and subsequent transgressive erosion. The asymmetric delta was induced by net longshore transport from north to south. The forced regressive nature of the delta precludes significant preservation of topset mud.  相似文献   

14.
Sellicks Beach, located on the eastern shore of Gulf St Vincent, South Australia, is subject to wave-dominated processes and northward longshore transport. During winter, when wave energy is typically vigorous, gravel deposits are exposed across most of the beach, and three step-like berms are well developed. Sand is restricted to a narrow strip that is exposed only at low tide. In contrast, during summer, when wave energy is generally moderate to low, much of the gravel is covered by a thin veneer of sand and only the high berm, on the landward edge of the beach, remains as an obvious feature. Steeply dipping Neoproterozoic to Cambrian strata that outcrop strongly across Sellicks Hill are the original source rocks for the beach gravel; distinctive sedimentary textures, structures and fossils in the cobble-size clasts can be confidently matched with those of the provenance rocks. Much of the sediment entered the modern beach environment as a consequence of coastal erosion of transitional alluvial fan sediments. The oldest alluvial fan sediments are of late Pliocene to earliest Pleistocene age. Mount Terrible Gully provides a conduit for the input of fluvial sediment at the mouth of Cactus Canyon, where clasts as large as boulders accumulate across the beach. Sellicks Beach gravels are subject to longshore transport northwards. Relatively softer clasts, such as those derived from the Heatherdale Shale, are rare beyond Cactus Canyon. In contrast, quartzite clasts are more abundant towards the north. This lithological differentiation is attributed to preferential survivorship of clasts that are physically harder and chemically less reactive. The change in the shapes of clasts northwards, from predominately shingle-like ‘very platy’ and ‘very bladed’ at Cactus Canyon, to more ‘compact’ towards the boat ramp, is in accord with the more massive fabric of the surviving quartzite clasts. At Sellicks Beach, preservation of uplifted, coarse gravels, with entire and comminuted marine molluscan shells, of last interglacial age, provides evidence of neotectonism. At the landward margin of the beach, imbricated gravels in which pore spaces have been infilled with mud, and which show no evidence of modern coastal erosion, may provide evidence of continuing uplift during the recent Holocene. The geological setting, geomorphic framework and modern sedimentary regime at Sellicks Beach combine to provide an exceptionally useful outdoor laboratory for education in field geology.  相似文献   

15.
安徽巢北地区五通组沉积环境分析   总被引：1，自引：0，他引：1  
下扬子地区广泛发育泥盆系五通组地层, 因为处于海侵初期, 地层中陆相和海相特征共存, 导致对该组沉积环境认识上的分歧;安徽巢北地区泥盆系五通组为一套厚层砂岩夹泥岩组合, 其沉积环境和沉积特征一直不清。为此, 在前人研究成果和野外剖面实测基础上, 采用沉积学、地层学等方法, 结合室内磨片和粒度分析方法, 对其厚层砂岩段进行了详细分析, 研究发现巢湖地区五通组形成于海相沉积环境, 即具滚动组分和跳跃组分组成的多段式的后滨-前滨, 跳跃组分和悬浮组分组成的两段式的前滨-上临滨, 有跳跃组分和悬浮组分组成的两段式的沙泥混合坪, 结合沉积序列特点和沉积特征, 最终确定五通组的观山段下部为滨岸相的后滨-前滨的砾石海滩沉积, 观山段上部为滨岸相的前滨-上临滨的砂质海滩沉积, 擂鼓台段下部为滨岸相的混合坪粉砂岩泥岩沉积, 擂鼓台段中部为滨岸相的泥坪、海湾、滨岸沼泽沉积间有前滨的砂质海滩沉积, 擂鼓台段上部为滨岸相的含铁质的混合坪沉积。该文不仅具有理论价值, 也具有一定的教学示范作用。  相似文献   

16.
The shallow marine sedimentary sequence of the Jaisalmer Basin exhibits one of the important and well-developed Tithonian sedimentary outcrops for western India. The ichnology and ichnofabric of the lower part of Bhadasar Formation (i.e., Kolar Dongar Member) belonging to Tithonian age are presented and discussed. The Kolar Dongar Member represents a shallow marine succession that contains 16 ichnotaxa: Ancorichnus ancorichnus, Conichnus conicus, Gyrochorte comosa, cf. Jamesonichnites heinbergi, Imponoglyphus kevadiensis, Laevicyclus mongraensis, Monocraterion tentaculatum, Ophiomorpha nodosa, Palaeophycus tubularis, P. bolbiterminus, Phycodes palmatus, Planolites beverleyensis, Rhizocorallium isp., Rosselia rotatus, R. socialis, and Teichichnus rectus. The ichnofabric analysis divulges five distinct ichnofabrics, each typifying distinct depositional environment within shallow marine conditions. The ichnofabric Ophiomorpha 1 with syn-sedimentary faulting exemplifies high energy conditions typical of lower shoreface environment, whereas the Ophiomorpha 2 ichnofabric typifies upper shoreface environment. The Ancorichnus ichnofabric reflects lower offshore condition of deposition. The high ichnodiversity Ancorichnus–Rosselia ichnofabric is indicative of inner shelf conditions, while low ichno-diversity Teichichnus ichnofabric indicates prevalence of low energy brackish bay environment. Thus, Tithonian Kolar Dongar Member indicates depositional environment ranging from shoreface to offshore to inner shelf and finally to brackish bay environment.  相似文献   

17.
The Upper Cretaceous Twentymile Sandstone of the Mesaverde Group in NW Colorado, USA, has been analysed with respect to its pinch‐out style and the stratigraphic position of tidally influenced facies within the sandstone tongue. Detailed sedimentological analysis has revealed that the Twentymile Sandstone as a whole is a deltaic shoreface sandstone tongue up to 50 m thick proximally. Facies change character vertically from very fine‐grained, storm wave‐dominated shelf sandstones and mudstones to fine‐grained, wave‐dominated sandstones and, finally, to fine‐ to coarse‐grained tidally dominated sandstones. The pinch‐out style is characterized by a basinward splitting of the massive proximal sandbody into seven coarsening‐upward fourth‐order sequences consisting of a lower shaly part and an upper sandy part (sandstone tongue). These are stacked overall to reflect the regressive‐to‐transgressive development of the tongue. Each of the lower sandstone tongues 1–3 are gradationally based, very fine‐grained and dominated by hummocky cross‐stratification and were deposited on the lower to upper shoreface. Sandstone tongues 4 and 5 prograded further basinwards than the underlying tongues, are erosively based, fine‐ to coarse‐grained and mainly hummocky, herringbone and trough cross‐stratified. Especially in tongue 5, tidal indicators, such as bipolar foresets and double mud drapes, are common. These tongues were deposited as upper shoreface and tidal channel sandstones respectively. Sandstone tongues 6 and 7 retrograded in relation to tongue 5, are very fine‐ to fine‐grained and hummocky cross‐stratified. These tongues were deposited in lower shoreface to offshore transition environments. The two lower fourth‐order sequences were deposited during normal regressions during slowly rising or stable relative sea level and represent the highstand systems tract. The three succeeding fourth‐order sequences, which show succeedingly increasing evidence of tidal influence, were deposited during falling and lowstand of relative sea level and represent the falling stage (forced regressive) and lowstand systems tracts. The uppermost two fourth‐order sequences were deposited during rapidly rising sea level in the transgressive systems tract. The maximum tidal influence occurred during lowstand progradation, in contrast to most other published examples reporting maximum tidal influence during transgression.  相似文献   

18.
Shoreface architecture, evolution (mid-Holocene to present) and depths of transgressive ravinement were examined from Sabine Pass, at the Texas–Louisiana border, to South Padre Island, near the Texas–Mexico border, using 30 shoreface transects. Shoreface transects extend out to 16-m water depth, each created from an echo-sounding profile and, on average, seven sediment cores. The shoreface is composed of three broad sedimentological facies: the upper shoreface, composed almost entirely of sand; the proximal lower shoreface, composed of sand with thickly to medium-bedded (50–10 cm) mud; and the distal lower shoreface, composed dominantly of mud with medium- to thinly bedded (20–3 cm) sand. Shoreface architecture and evolution is extremely variable along the Texas coast. Shoreface gradients increase from 2·25 m km–1 in east Texas to 3·50 m km–1 in south Texas. Shoreface sands coarsen towards south Texas. East and south Texas shoreface deposits are thin and retrograding whereas central Texas shoreface deposits are thicker and prograding. Central Texas is characterized by stacked shoreface successions, whereas in east Texas, lower shoreface sands are preserved only in offshore banks. Preservation of shoreface deposits is low in south Texas. Although eustatic fluctuations and accommodation space have a strong impact on overall mid-Holocene to present shoreface evolution and preservation potential, along-strike variations in sediment supply and wave energy are the main factors controlling shoreface architecture. The transgressive ravinement surface varies from –6 to –15 m along the Texas coast.  相似文献   

19.
《Precambrian Research》2006,144(3-4):167-198
The Huqf Supergroup of Oman contains an excellently exposed succession from the presumed Marinoan-age Fiq glaciation (ca. 635 Ma) to the Precambrian–Cambrian boundary (542 Ma). Within this time interval, two major siliciclastic-to-carbonate cycles are present, starting with the transgression of basin margins following the deposition of the glacigenic, probably rift-related Fiq Member. The dominantly siliciclastic portion of the first cycle is termed the Masirah Bay Formation. In the Huqf region of east-central Oman, two quartz-arenitic sandstone bodies crop out, below the transition into the overlying Khufai Formation carbonates. In the Jabal Akhdar of northern Oman, only deep marine shales and siltstones are found.The Masirah Bay Formation in the Huqf area is divided into a number of members with constituent units containing distinctive facies assemblages. In Member 1, wave-rippled shoreface deposits are overlain by trough cross-stratified, coarse-grained sandstones deposited in proximal tidal sandsheets or estuarine tidal shoals. Member 2 represents essentially a repeat of this coarsening- and shallowing-up trend via a basin-wide flooding event. A second major flooding surface is overlain by the lower shoreface to offshore sedimentary rocks of Member 3, which pass gradationally upwards into the distal carbonate ramp of the Khufai Formation. The two major progradational cycles can be identified in nearby subsurface well penetrations such as Masirah-1 (SMP-1).The tidal sandstones of the Masirah Bay Formation were deposited as regressive-transgressive pulses in a longer term transgressive stratigraphic trend associated with both the demise of continental glaciation and the end of active extensional tectonics. As basin margins were flooded, compositionally and texturally mature but coarse-grained quartzose sand was swept by currents into tidal sandsheets and estuary-fills along a broad littoral margin situated in the eastern fringe of the study area. The development of meso-macrotidal conditions over extensive sandy shelves and coastlines appears to be a common but unexplained feature of the Ediacaran and Early Cambrian interval.  相似文献   

20.
A 1600-m-thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east-central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan-delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan-delta facies associations include: ungraded, matrix-and clast-supported conglomerate; normally graded, matrix- and clast-supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan-delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan-delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan-delta environment by debris flows, high- and low-density turbidity currents, and suspension fallout. The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High-energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore-directed currents produced uncommonly large (typically 1–2 m thick) trough cross-stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high-energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout. A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan-delta, subaqueous fan-delta and offshore lacustrine deposition. This model suggests that high-energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan-delta environment and promoting development of a well-defined nearshore lacustrine environment. Low-energy nearshore currents induced by shallow water along the  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号