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.
The mode of channel‐bend transformation (i.e. expansion, translation, rotation or a combination thereof) has a direct bearing on the dimensions, shape, bedding architecture and connectivity of point‐bar sandstone bodies within a fluvial meander belt, but is generally difficult to recognize in vertical outcrops. This study demonstrates how the bend transformation mode and relative rate of channel‐floor aggradation can be deciphered from longitudinal outcrop sections aligned parallel to the meander‐belt axis, as a crucial methodological aid to the reconstruction of ancient fluvial systems and the development of outcrop analogue models for fluvial petroleum reservoirs. The study focuses on single‐storey and multi‐storey fluvial meander‐belt sandstone bodies in the Palaeogene piggyback Boyabat Basin of north‐central Turkey. The sandstone bodies are several hundred metres wide, 5 to 40 m thick and encased in muddy floodplain deposits. The individual channel‐belt storeys are 5 to 9 m thick and their transverse sections show lateral‐accretion bed packages representing point bars. Point bars in longitudinal sections are recognizable as broad mounds whose parts with downstream‐inclined, subhorizontal and upstream‐inclined bedding represent, respectively, the bar downstream, central and upstream parts. The inter‐bar channel thalweg is recognizable as the transition zone between adjacent point‐bar bedsets with opposing dip directions into or out of the outcrop section. The diverging or converging adjacent thalweg trajectories, or a trajectory migrating in up‐valley direction, indicate point‐bar broadening and hence channel‐bend expansion. A concurrent down‐valley migration of adjacent trajectories indicates channel‐bend translation. Bend rotation is recognizable from the replacement of a depositional riffle by an erosional pool zone or vice versa along the thalweg trajectory. The steepness of the thalweg trajectory reflects the relative rate of channel‐floor aggradation. This study discusses further how the late‐stage foreland tectonics, with its alternating pulses of uplift and subsidence and a progressive narrowing of the basin, has forced aggradation of fluvial channels and caused vertical stacking of meander belts.  相似文献   

2.
The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of delta‐scale (i.e. tens of metres high) subaqueous clinoforms in the upper Jurassic Sognefjord Formation of the Troll Field. Mud‐prone subaqueous deltas characterized by a compound clinoform morphology and sandy delta‐scale subaqueous clinoforms are common in recent tide‐influenced, wave‐influenced and current‐influenced settings, but ancient examples are virtually unknown. The data presented help to fully comprehend the criteria for the recognition of other ancient delta‐scale subaqueous clinoforms, as well as refining the depositional model of the reservoir in the super‐giant Troll hydrocarbon field. Two 10 to 60 m thick, overall coarsening‐upward packages are distinguished in the lower Sognefjord Formation. Progressively higher energy, wave‐dominated or current‐dominated facies occur from the base to the top of each package. Each package corresponds to a set of seismically resolved, westerly dipping clinoforms, the bounding surfaces of which form the seismic ‘envelope’ of a clinoform set and the major marine flooding surfaces recognized in cores. The packages thicken westwards, until they reach a maximum where the clinoform ‘envelope’ rolls over to define a topset–foreset–toeset geometry. All clinoforms are consistently oriented sub‐parallel to the edge of the Horda Platform (N005–N030). In the eastern half of the field, individual foresets are relatively gently dipping (1° to 6°) and bound thin (10 to 30 m) clinothems. Core data indicate that these proximal clinothems are dominated by fine‐grained, hummocky cross‐stratified sandstones. Towards the west, clinoforms gradually become steeper (5° to 14°) and bound thicker (15 to 60 m) clinothems that comprise medium‐grained, cross‐bedded sandstones. Topsets are consistently well‐developed, except in the westernmost area. No seismic or sedimentological evidence of subaerial exposure is observed. Deposition created fully subaqueous, near‐linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were probably fed by a river outlet in the north‐east and sculpted by the action of currents sub‐parallel to the clinoform strike.  相似文献   

3.
Tide‐dominated deltas have an inherently complex distribution of heterogeneities on several different scales and are less well‐understood than their wave‐dominated and river‐dominated counterparts. Depositional models of these environments are based on a small set of ancient examples and are, therefore, immature. The Early Jurassic Gule Horn Formation is particularly well‐exposed in extensive sea cliffs from which a 32 km long, 250 m high virtual outcrop model has been acquired using helicopter‐mounted light detection and ranging (LiDAR). This dataset, combined with a set of sedimentological logs, facilitates interpretation and measurement of depositional elements and tracing of stratigraphic surfaces over seismic‐scale distances. The aim of this article is to use this dataset to increase the understanding of depositional elements and lithologies in proximal, unconfined, tide‐dominated deltas from the delta plain to prodelta. Deposition occurred in a structurally controlled embayment, and immature sediments indicate proximity to the sediment source. The succession is tide dominated but contains evidence for strong fluvial influence and minor wave influence. Wave influence is more pronounced in transgressive intervals. Nine architectural elements have been identified, and their internal architecture and stratigraphical distribution has been investigated. The distal parts comprise prodelta, delta front and unconfined tidal bar deposits. The medial part is characterized by relatively narrow, amalgamated channel fills with fluid mud‐rich bases and sandier deposits upward, interpreted as distributary channels filled by tidal bars deposited near the turbidity maximum. The proximal parts of the studied system are dominated by sandy distributary channel and heterolithic tidal‐flat deposits. The sandbodies of the proximal tidal channels are several kilometres wide and wider than exposures in all cases. Parasequence boundaries are easily defined in the prodelta to delta‐front environments, but are difficult to trace into the more proximal deposits. This article illustrates the proximal to distal organization of facies in unconfined tide‐dominated deltas and shows how such environments react to relative sea‐level rise.  相似文献   

4.
Dune stratification types, which include grainfall, grainflow and ripple lamination, provide a record of the fine‐scale processes that deposited sediment on palaeo‐dune foresets. While these facies are relatively easy to distinguish in some cross‐bedded sandstones, for others – like the Permian Coconino Sandstone of northern and central Arizona – discrete stratification styles are hard to recognize at the bedding scale. Furthermore, few attempts have been made to classify fine‐scale processes in this sandstone, despite its renown as a classic aeolian dune deposit and Grand Canyon formation. To interpret depositional processes in the Coconino Sandstone, cross‐bed facies were characterized using a suite of sedimentary textures and structures. Bedding parameters were described at multiple scales via a combination of field and laboratory methods, including annotated outcrop photomosaics, strike and dip measurements, sandstone disaggregation and laser‐diffraction particle analysis, high‐resolution scans of thin sections, and scanning electron microscopy. Cross‐beds were observed to be laterally extensive along‐strike, with most dip angles ranging from the mid‐teens to mid‐twenties. While some cross‐bed sets are statistically coarser near their bases, others exhibit no significant vertical sorting trends. Both massive and laminated textures are visible in high‐resolution scans of thin sections, but laminae contacts are commonly indistinct, making normal and reverse grading difficult to define. Diagenetic features, such as stylolite seams and large pores, are also present in some samples and might indicate alteration of original textures like detrital clay laminae and carbonate minerals. Observed textures and sedimentary structures suggest that the cross‐beds may consist of grainflow and grainfall deposits, but these remain difficult to differentiate at outcrop and thin‐section scales. This characterization of fine‐scale processes will play a critical part in the development of depositional models for the Coconino Sandstone and elucidate interpretations for similar cross‐bedded formations.  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.
Falling‐stage deltas are predicted by sequence stratigraphic models, yet few reliable criteria are available to diagnose falling‐stage deltaic systems in surface exposures. Recent work on the Upper Cretaceous (Turonian) Ferron Sandstone in the western Henry Mountains Syncline of south‐central Utah has established its environment of deposition as a series of modest‐sized (5 to 20 km wide), probably asymmetrical, mixed‐influence deltas (‘Ferron Notom Delta’) that dispersed sediment eastwards from the rising Sevier orogenic hinterland into the Western Cordilleran Foreland Basin. Analysis of sandstone body stacking patterns in a 67 km long, depositional strike‐parallel (north–south) transect indicates that the growth of successive deltas was strongly forced by synsedimentary growth of a long wavelength (ca 100 km), 50 m amplitude fold structure. Herein, two discrete areas within this transect, superbly exposed in three dimensions, are documented in order to determine the details of stratal stacking patterns in the depositional dip direction, and thereby to assess the stratigraphic context of the Ferron Notom Delta. In the two study areas, dip transects expose facies representing river mouth bar to distal delta front environments over distances of 2 to 4 km. Key stratal packages are clinothems that offlap, downlap, and describe descending regressive trajectories with respective to basal and top datums; they are interpreted as the product of relative sea‐level fall. The vertical extent of clinoforms suggests that deltas prograded into <30 m of water. Furthermore, these deltaic successions preserve abundant evidence of delta front slope failure, growth faulting, and incision and filling of deep (<15 m) slope gullies. Gully fills are composed of chaotic intraformational breccia and/or massive sandstone, and constitute linear, ‘shoestring’ sandbodies in the distal portions of individual palaeodelta systems. They are interpreted to have been cut and filled during the late falling‐stage and lowstand of relative sea‐level cycles. The north–south distribution of the stratal style described above seems to be focused on the flanks of the growth anticline, and so the numerous falling‐stage systems tracts preserved within the Ferron Notom Delta probably owe their origin to synsedimentary structural growth, and the unstable fluid pressure regime that this growth imposed on the sea floor and shallow subsurface.  相似文献   

7.
Sandstone bodies in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, previously considered as point bars formed in meandering rivers and other types of fluvial bars, are herein interpreted as delta mouth‐bar deposits. The sandstone bodies have been examined in a 2300 m long cliff section along the Argyle and Nine Mile Canyons at the southern margin of the Uinta lake basin. The sandstone bodies occur in three stratigraphic intervals, separated by lacustrine mudstone and limestone. Together these stratigraphic intervals form a regressive‐transgressive sequence. Individual sandstone bodies are texturally sharp‐based towards mudstone substratum. In proximal parts, the mouth‐bar deposits only contain sandstone, whereas in frontal and lateral positions mudstone drapes separate mouth‐bar clinothems. The clinothems pass gradually into greenish‐grey lacustrine mudstone at their toes. Horizontally bedded or laminated lacustrine mudstone onlaps the convex‐upward sandstone bars. The mouth‐bar deposits are connected to terminal distributary channel deposits. Together, these mouth‐bar/channel sandstone bodies accumulated from unidirectional jet flow during three stages of delta advance, separated by lacustrine flooding intervals. Key criteria to distinguish the mouth‐bar deposits from fluvial point bar deposits are: (i) geometry; (ii) bounding contacts; (iii) internal structure; (iv) palaeocurrent orientations; and (v) the genetic association of the deposits with lacustrine mudstone and limestone.  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.
近10年来,国内外的沉积构型研究从之前的以精细表征为主扩展到成因机制分析。文中以同生逆断层控制的冲积扇、可容空间影响下的曲流河点坝、浅水缓坡背景下的三角洲指状砂坝以及大陆斜坡微盆地背景下的重力流沉积等为例介绍相关研究进展。(1)挤压盆地边缘复杂的同生逆断裂构造活动控制了冲积扇的构型要素类型、叠置样式、分布演化及定量规模,表现出有别于构造稳定条件下的冲积扇构型模式;(2)曲流河在可容空间较小的情况下可发育顺流迁移型点坝,表现出特殊的微相类型、分布样式及旋回特征,而随着A/S值的不断增大,曲流河点坝可由鳞片状逐步演变为条带状,最后变为点状;(3)浅水三角洲中可发育类似于河控较深水三角洲中的指状砂坝沉积,其平面形态、微相组合样式、定量规模等特征受气候、沉积物供给、沉积水体等多因素的影响;(4)大陆斜坡微盆地内部可发育重力流水道、朵叶体、块状搬运体等多种构型要素类型,不同构型要素的空间分布样式、定量规模及构型演化模式受复杂地形地貌及构造活动的影响较为明显。综合原型模型分析、沉积物理模拟及数值模拟开展系统的定量化研究,建立定量的、可预测的碎屑岩沉积构型模式,是今后碎屑岩沉积构型研究的发展趋势。  相似文献   

10.
Shelf‐edge deltas play a critical role in shelf‐margin accretion and deepwater sediment delivery, yet much remains to be understood about the detailed linkage between shelf edge and slope sedimentation. The shelf edge separates the flat‐lying shelf from steeper slope regions, and is observable in seismic data and continuous outcrops; however, it is commonly obscured in non‐continuous outcrops. Defining this zone is essential because it segregates areas dominated by shelf currents from those governed by gravity‐driven processes. Understanding this linkage is paramount for predicting and characterizing associated deepwater reservoirs. In the Tanqua Karoo Basin, the Permian Kookfontein Formation shelf‐slope clinothems are well‐exposed for 21 km along depositional strike and dip. Two independent methods identified the shelf‐edge position, indicating that it is defined by: (i) a transition from predominantly shelf‐current to gravitational deposits; (ii) an increase in soft‐sediment deformation; (iii) a significant gradient increase; and (iv) clinothem thickening. A quantitative approach was used to assess the impact of process‐regime variability along the shelf edge on downslope sedimentation. Facies proportions were quantified from sedimentary logs and photographic panels, and integrated with mapped key surfaces to construct a stratigraphic grid. Spatial variability in facies proportions highlights two types of shelf‐edge depositional zones within the same shelf‐edge delta. Where deposition occurred in fluvial‐dominated zones, the slope is sand rich, channelized with channels widening downslope, and rich in collapse features. Where deltaic deposits indicate considerable tidal reworking, the deposits are thin and pinch‐out close to the shelf edge, and the slope is sand poor and lacks channelization. Amplification of tidal energy, and decrease in fluvial drive on the shelf, coincides with a decrease in mouth bar and shelf‐edge collapse, and a lack of channelization on the slope. This analysis suggests that process‐regime variability along the shelf edge exercised significant control on shelf‐edge progradation, slope channelization and deepwater sediment delivery.  相似文献   

11.
The Lower Cretaceous Fortress Mountain Formation occupies a spatial and temporal niche between syntectonic deposits at the Brooks Range orogenic front and post‐tectonic strata in the Colville foreland basin. The formation includes basin‐floor fan, marine‐slope and fan‐delta facies that define a clinoform depositional profile. Texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge‐front that included older turbidites and mélange. These new interpretations, based entirely on outcrop study, suggest that the Fortress Mountain Formation spans the boundary between orogenic wedge and foredeep, with proximal strata onlapping the tectonic wedge‐front and distal strata downlapping the floor of the foreland basin. Our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load‐induced flexural subsidence of the foreland basin.  相似文献   

12.
Existing facies models of tide‐dominated deltas largely omit fine‐grained, mud‐rich successions. Sedimentary facies and sequence stratigraphic analysis of the exceptionally well‐preserved Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) aims to bridge this gap. The succession was deposited in a structurally controlled, shallow, macrotidal embayment and deposition was supplemented by fluvial processes but lacked wave influence. The succession contains two stacked, progradational parasequence sets bounded by regionally extensive flooding surfaces. Within this succession two main genetic elements are identified: non‐channelized tidal bars and tidal channels. Non‐channelized tidal bars comprise coarsening‐upward sandbodies, including large, downcurrent‐dipping accretion surfaces, sometimes capped by palaeosols indicating emergence. Tidal channels are preserved as single‐storey and multilateral bodies filled by: (i) laterally migrating, elongate tidal bars (inclined heterolithic strata, 5 to 25 m thick); (ii) forward‐facing lobate bars (sigmoidal heterolithic strata, up to 10 m thick); (iii) side bars displaying oblique to vertical accretion (4 to 7 m thick); or (iv) vertically‐accreting mud (1 to 4 m thick). Palaeocurrent data show that channels were swept by bidirectional tidal currents and typically were mutually evasive. Along‐strike variability defines a similar large‐scale architecture in both parasequence sets: a deeply scoured channel belt characterized by widespread inclined heterolithic strata is eroded from the parasequence‐set top, and flanked by stacked, non‐channelized tidal bars and smaller channelized bodies. The tide‐dominated delta is characterized by: (i) the regressive stratigraphic context; (ii) net‐progradational stratigraphic architecture within the succession; (iii) the absence of upward deepening trends and tidal ravinement surfaces; and (iv) architectural relations that demonstrate contemporaneous tidal distributary channel infill and tidal bar accretion at the delta front. The detailed facies analysis of this fine‐grained, tide‐dominated deltaic succession expands the range of depositional models available for the evaluation of ancient tidal successions, which are currently biased towards transgressive, valley‐confined estuarine and coarser grained deltaic depositional systems.  相似文献   

13.
The Early Miocene Bílina Palaeodelta consists of fluvio‐deltaic and lacustrine clastics deposited along the south‐eastern margin of the extensional Most Basin, part of the Eger Graben in north Bohemia (Czech Republic). The Bílina succession shows evidence of repeated advances of an axial deltaic system across a thick accumulation of organic material and clay in the hangingwall of an active fault. Exposures up to ca 4·5 km long in the Bílina open‐cast mine help bridge the gap between seismic scale and typical outcrop scale of observation and thus allow the relationships between small‐scale and basin‐scale stratal geometries to be evaluated. The Bílina Palaeodelta deposits include sand‐dominated, fluvial channel fills and heterolithic sheets interpreted as delta plain strata, sand‐dominated mouth‐bar wedges and heterolithic sheets of prodeltaic deposits, passing distally into lacustrine clays. The depositional environment is interpreted as a fluvial‐dominated, mixed‐load, lacustrine delta with a high degree of grain‐size segregation at the feeder‐channel mouths. On the largest temporal and spatial scales, variable tectonic subsidence controlled the overall advance and retreat of the delta system. The medium‐term transgressive‐regressive history was probably driven by episodes of increased subsidence rate. However, at this temporal scale, the architecture of the deltaic sequences (deltaic lobes and correlative lacustrine deposits) was strongly affected by: (i) compaction of underlying peat and clay which drove lateral offset stacking of medium‐term sequences; and (ii) growth of a fault‐propagation fold close to the active Bílina Fault. At the smallest scale, the geometries of individual mouth bars and groups of mouth bars (short‐term sequences) reflect the interaction among sediment loading, compaction and growth faulting that produced high‐frequency relative lake‐level fluctuations and created local accommodation at the delta front.  相似文献   

14.
《Sedimentology》2018,65(5):1447-1481
  相似文献   

15.
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   

16.
《Sedimentology》2018,65(3):809-841
Degradation of basin‐margin clinothems around the shelf‐edge rollover zone may lead to the generation of conduits through which gravity flows transport sediment downslope. Many studies from seismic‐reflection data sets show these features, but they lack small‐scale (centimetre to metre) sedimentary and stratigraphic observations on process interactions. Exhumed basin‐margin clinothems in the Tanqua depocentre (Karoo Basin) provide seismic‐reflection‐scale geometries and internal details of architecture with depositional dip and strike control. At the Geelhoek locality, clinothem parasequences comprise siltstone‐rich offshore deposits overlain by heterolithic prodelta facies and sandstone‐dominated deformed mouth bars. Three of these parasequences are truncated by a steep (6 to 22°), 100 m deep and 1·5 km wide asymmetrical composite erosion surface that delineates a shelf‐incised canyon. The fill, from base to top comprises: (i) thick‐bedded sandstone with intrabasinal clasts and multiple erosion surfaces; (ii) scour‐based interbedded sandstone and siltstone with tractional structures; and (iii) inverse‐graded to normal‐graded siltstone beds. An overlying 55 m thick coarsening‐upward parasequence fills the upper section of the canyon and extends across its interfluves. Younger parasequences display progressively shallower gradients during progradation and healing of the local accommodation. The incision surface resulted from initial oversteepening and high sediment supply triggering deformation and collapse at the shelf edge, enhanced by a relative sea‐level fall that did not result in subaerial exposure of the shelf edge. Previous work identified an underlying highly incised, sandstone‐rich shelf‐edge rollover zone across‐margin strike, suggesting that there was migration in the zone of shelf edge to upper‐slope incision over time. This study provides an unusual example of clinothem degradation and readjustment with three‐dimensional control in an exhumed basin‐margin succession. The work demonstrates that large‐scale erosion surfaces can develop and migrate due to a combination of factors at the shelf‐edge rollover zone and proposes additional criteria to predict clinothem incision and differential sediment bypass in consistently progradational systems.  相似文献   

17.
《Sedimentology》2018,65(1):62-95
Despite advances in estuarine facies models, ancient bayhead delta deposits are not widely recognized or utilized, and their facies characteristics are poorly documented. Sedimentology of three well‐exposed, bayhead delta deposits within the well‐known stratigraphic framework of the Book Cliffs, Utah, and comparison to modern bayhead deltas provide insight into their variability, and a model for reservoir prediction. Bayhead deltas develop at the innermost part of bays within wave‐dominated and mixed‐energy estuaries on transgressive coastlines. The deposits of these deltas are valuable because they record the high‐frequency turnaround from transgression to regression; in turn, this delineates the long‐term transgression trajectory and informs reservoir prediction. This study uses outcrop data and synthesis of modern bayhead delta characteristics to define criteria for bayhead delta recognition, develop a detailed facies model and highlight their utility in a regional context. The following criteria for bayhead delta identification are proposed herein: (i) overall coarsening‐up pattern; (ii) small‐scale clinoform (5 to 15 m height; 100 to 1000 m length) showing a down‐clinoform decrease in grain size and palaeocurrent energy; (iii) increasing abundance of mud interbeds towards the clinoform toe; (iv) basinward‐directed palaeocurrents generated mainly by fluvial input with strong tidal overprint; (v) greatest tidal influence in the intertidal zone of the inactive delta front and better preservation of turbidites at clinoform toes; (vi) brackish trace and body fossils; and (vii) position within an overall backstepping stratigraphic succession. Ancient bayhead deltas described in this study are situated within a moderately rising to flatly rising transgressive interval over a 300 km transect. Steeply rising transgressive trajectories sequester sandy, thicker, better connected transgressive deposits in the inner part of the estuary where bayhead deltas tend to stack vertically. Flatter transgressive trajectories sequester sand in disparate, disconnected estuarine sandstone bodies with partial preservation of bayhead deltas. Proposed criteria aid recognition of bayhead delta deposits in other basins to reconstruct transgressive shoreline trajectories that inform reservoir models and volumetrics.  相似文献   

18.
The Miocene Gorgoglione Flysch Formation records the stratigraphic product of protracted sediment transfer and deposition through a long‐lived submarine channel system developed in a narrow and elongate thrust‐top basin of the Southern Apennines (Italy). Channel‐fill deposits are exposed in an outcrop belt approximately 500 m thick and 15 km long, oriented oblique to the palaeoflow, which was roughly south‐eastward. These exceptional exposures of channel‐fill strata allow the stacking architectures and the evolution of the channel system to be analyzed at multiple scales, enabling the effects of syn‐sedimentary thrust tectonics and basin confinement on the depositional system development to be deciphered. Two end‐member types of elementary channel architecture have been identified: high‐aspect‐ratio, weakly‐confined channels, and low‐aspect‐ratio, incisional channels. Their systematic stacking results in a complex pattern of seismic‐scale depositional architectures that determines the stratigraphic framework of the deep‐water system. From the base of the succession, two prominent channel complex sets have been recognized, namely CS1 and CS2, consisting of amalgamated incisional channel elements and weakly‐confined channel elements. These channelized units are overlain by isolated incisional channels, erosional into mud‐prone slope deposits. The juxtaposition of different channel architectures is interpreted to have been governed by regional thrust‐tectonics, in combination with a high subsidence rate that promoted significant aggradation. In this scenario, the alternating ‘in sequence’ and ‘out of sequence’ tectonic pulses of the basin‐bounding thrusts controlled the activation of coarse‐clastic inputs in the basin and the resulting stacking architectures of channelized units. The tectonically‐driven confinement of the depositional system limited the lateral offset in channel stacking, preventing large‐scale avulsions. This study represents an excellent opportunity to analyze the stratigraphic evolution of a submarine channel system in tectonically‐active settings from an outcrop perspective. It should find wide applicability in analogous depositional systems, whose stratigraphic architecture has been influenced by tectonically‐controlled lateral confinement and associated lateral tilting.  相似文献   

19.
The repetitive sedimentology of many Precambrian sheet‐dominated fluvial sandstones favoured their attribution to unconfined depositional processes. This article presents outcrop evidence for deep‐channelled drainage in the 1·9 Ga Burnside River Formation of Kilohigok Basin, Arctic Canada. On the ground, sheet‐like sandbodies with ubiquitous cross‐bedding are at first consistent with classic, unconfined depositional models. However, satellite and oblique‐aerial imagery of sections up to 15 km wide and 500 m thick reveals the occurrence of incised palaeovalleys hosting clustered, kilometre‐scale, channel bodies with attached large foreset bars pointing to downstream‐lateral accretion, sand sheets with aspect ratios (i.e. width to thickness) as high as 2500, and scattered aeolian intervals. The genetic association of these architectural elements points to aggradational fluvial piedmonts composed of low‐relief unit bars generated by braidplain channels several metres deep. Preservation of aeolianites was facilitated by fluctuating groundwater table and accommodation. Fluvial piedmonts were transected by weakly sinuous channel belts up to 25 m deep and characterized by through‐going or tributary planform. Aspect ratios comparable with those of late Palaeozoic to modern braided channels disprove the inference that all Precambrian streams readily widened in response to increased discharge. Previous facies models for large‐scale Precambrian sheet‐braided rivers failed to depict entire channel forms, possibly because they could not be resolved by ground‐based observations. Based on their limited geomorphic variability and abundance of architectural elements with very high aspect ratios, this study recommends that large sheet‐braided fluvial systems should still be considered separately from their post‐Silurian (i.e. vegetated) braided counterparts. Parallels between sheet‐braided and modern dryland rivers do not, however, reconcile with the deep, perennial, channelized processes described here. Yet, distal sand‐bed and perennial reaches of modern sandur plains remain the closest analogue to sheet‐braided rivers. This conjecture contradicts the assumption that all Precambrian rivers were prone to simulate seasonal behaviours independently from their actual climate regime.  相似文献   

20.
Spatial information on lithofacies from outcrops is paramount for understanding the internal dynamics, external controls and degree of predictability of the facies architecture of shallow‐water carbonate‐platform tops. To quantify the spatial distribution and vertical stacking of lithofacies within an outer‐platform shoal‐barrier complex, integrated facies analysis and digital field technologies have been applied to a high‐relief carbonate platform exposed in the Djebel Bou Dahar (Lower Jurassic, High Atlas, Morocco). The outer platform is characterized by subtidal, cross‐bedded, coarse grainstone to rudstone grading into supratidal, pisoidal packstone‐rudstone with tepees that together formed a 350 to 420 m wide shoal‐barrier belt parallel to the margin. This belt acted as a topographic high separating a restricted lagoon from the subtidal, open marine region. Low‐energy tidal flats developed on the protected flank of the barrier facing the lagoon. Lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model. The outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts. In addition, the spatial heterogeneity was captured in multiple time slices, i.e. during different phases of cyclic base‐level fluctuations. In general, the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top, establishing low‐energy subtidal conditions across the whole platform, and when the accommodation space was filled with tidal flat facies. Heterogeneity increased during deposition of the relief‐building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation. Cycles commonly are composed of a thin transgressive tidal flat unit, followed by coated‐grain rudstone bar facies. Lateral to the bar facies, pisoidal‐grainstone beach deposits accumulated. These bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand. There the bars either graded into supratidal pisoidal facies with tepees (when accommodation space was filled) or were capped by subaerial exposure (due to a sea‐level fall). Modified embedded Markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval (constant depositional mode). Analysis of individual sections did not reveal any ordering, which may be related to the limited thickness of these sections. Composite sections, however, rejected the null hypothesis of randomness. The addition of stratigraphically significant information to the Markov analysis, such as exposure surfaces and lateral dimensions of facies bodies, strengthens the verdict of unambiguous preferential ordering. Through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models, accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation.  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