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1.
《Sedimentology》2018,65(4):1170-1212
Barrier‐island system evolution is controlled by internal and external forcing mechanisms, and temporal changes in these mechanisms may be recorded in the sedimentary architecture. However, the precise role of individual forcing mechanisms is rarely well understood due to limited chronological control. This study investigates the relative role of forcing conditions, such as antecedent topography, sea‐level rise, sediment supply, storms and climate changes, on the evolution of a Holocene wave‐dominated barrier‐island system. This article presents temporal reconstruction of the depositional history of the barrier‐island system of Rømø in the Wadden Sea in unprecedented detail, based on ground‐penetrating radar profiles, sediment cores, high‐resolution dating and palynological investigations, and shows that ca 8000 years ago the barrier island formed on a Pleistocene topographic high. During the initial phase of barrier evolution, the long‐term sea‐level rise was relatively rapid (ca 9 mm year−1) and the barrier was narrow and frequently overwashed. Sediment supply kept pace with sea‐level rise, and the barrier‐island system mainly aggraded through the deposition of a ca 7 m thick stack of overwash fans. Aggradation continued for ca 1700 years until sea‐level rise had decreased to <2 mm year−1. In the last ca 6000 years, the barrier prograded 4 to 5 km through deposition of a 10 to 15 m thick beach and shoreface unit, despite a long‐term sea‐level rise of 1 to 2 mm year−1. The long‐term progradation was, however, interrupted by a transgression between 4000 years and 1700 years ago. These results demonstrate that the large‐scale morphology of the Danish Wadden Sea shoreline influences the longshore sediment transport flux and the millennial‐scale dispersal of sediment along the shoreline. On decadal to centennial timescales, major storms induced intense beach and shoreface erosion followed by rapid recovery and progradation which resulted in a highly punctuated beach and shoreface record. Major storms contributed towards a positive sediment budget, and the sustained surplus of sediment was, and still is, instrumental in maintaining the aggradational–progradational state of the barrier island.  相似文献   

2.
This study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr?1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr?1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply.  相似文献   

3.
This study employs facies analysis and basic principles of sequence stratigraphy to correlate isolated outcrop sections and reveal the depositional history of the Chmielnik Formation – a prominent mid‐Serravalian clastic wedge formed on the basinward forebulge flank of the Polish Carpathian Foredeep. The coarse‐grained clastic wedge, up to 30 m thick and spanning ca 1·1 Ma within biozone NN6, consists of fluvio‐deltaic, foreshore and shoreface deposits with a range of large littoral sand bars, all enveloped in muddy offshore‐transition deposits. Its dynamic stratigraphy indicates rapid shoreline shifts and environmental changes due to the interplay of forebulge tectonism, sediment supply and third‐order eustatic cycles. A similar interplay of tectonism and eustasy is recognizable in the whole middle Miocene sedimentary succession deposited on the forebulge flank, demonstrating an extreme case of an accommodation‐controlled shelf and indicating tectonic cycles of the forebulge uplift and subsidence spanning ca 800 to 900 ka. The episodes of forebulge uplift correlate with the main pulses of orogen thrusting. The resulting composite peripheral unconformity differs markedly from the idealized model of a ‘steady‐state’ stepwise onlap driven by forebulge continuous retreat. It is concluded that the foredeep peripheral unconformities, instead of being simplified in accordance with this idealized model, should rather be studied in detail because they bear a valuable high‐resolution record of regional events and give unique insights into the local role of tectonics, eustasy and sediment supply.  相似文献   

4.
Sequence developments in rift basins are considered to be influenced largely by tectonics and to a lesser extent by eustatic sea‐level and climate. Studies indicate that in passive margin basins, climate can mask the effects of tectonics and eustasy by modulating the sediment supply. It is, however, less understood how the sedimentary sequence in rift basins might respond to strong climatic fluctuations where tectonic pulses generate rapid accommodation space. Here a case study has been provided to assess the effect of climate vis à vis sea‐level and tectonics on sequence development in the Cambay rift basin, western India, during the Early Palaeogene (Late Palaeocene to Early Eocene) super greenhouse globe. Facies analysis of this shale–lignite sequence suggests deposition in a lagoon/bay, developed over the Deccan Trap basement. Detailed sequence stratigraphic analysis using basin‐wide representative composite sections, marker lignite seam, event bed and high‐resolution carbon isotope (δ13C) chemostratigraphy suggest an overall transgressive motif. Among the three prominent Early Eocene eustatic highstands, only the one at ca 53·7 Ma is expressed by the thickest coal accumulation throughout the basin. Expression of the other sequence stratigraphic surfaces is subdued and can be due to the overall finer grain size of the sediment or local variation in the subsidence rate at different fault‐controlled mini‐basins. Enigmatic presence of a maximum flooding surface coinciding with the 53·7 Ma climate event (Eocene Thermal Maximum 2), manifested by negative carbon isotope excursion, indicates possible influence of climate over and above tectonics in developing the rift sequence. Qualitative rainfall variation assessed using the magnitude of the carbon isotope excursion and pollen abundance show that a relatively dry/low precipitation climatic phase during the Eocene Thermal Maximum 2 hindered the siliclastic supply to the basin. Thus, it has been inferred that climate‐induced high siliciclastic supply possibly enhanced the autocyclic reorganization and hindered the development of the key sequence stratigraphic surfaces across the basin during climate extremes.  相似文献   

5.
Clastic, depositional strandplain systems have the potential to record changes in the primary drivers of coastal evolution: climate, sea‐level, and the frequency of major meteorological and oceanographic events. This study seeks to use one such record from a southern Brazilian strandplain to highlight the potentially‐complex nature of coastal sedimentological response to small changes in these drivers. Following a 2 to 4 m highstand at ca 5·8 ka in southern Brazil, falling sea‐level reworked shelf sediment onshore, forcing coastal progradation, smoothing the irregular coastline and forming the 5 km wide Pinheira Strandplain, composed of ca 500 successive beach and dune ridges. Sediment cores, grab samples and >11 km of ground‐penetrating radar profiles reveal that the strandplain sequence is composed of well‐sorted, fine to very‐fine quartz sand. Since the mid‐Holocene highstand, the shoreline prograded at a rate of ca 1 to 2 m yr?1 through the deposition of a 4 to 6 m thick shoreface unit; a 1 to 3 m thick foreshore unit containing ubiquitous ridge and runnel facies; and an uppermost beach and foredune unit. However, the discovery of a linear, 100 m wide barrier ridge with associated washover units, a 3 to 4 m deep lagoon and 250 m wide tidal inlet within the strandplain sequence reveals a period of shoreline transgression at 3·3 to 2·8 ka during the otherwise regressive developmental history of the plain. The protected nature of Pinheira largely buffered it from changes in precipitation patterns, wave energy and fluvial sediment supply during the time of its formation. However, multiple lines of evidence indicate that a change in the rate of relative sea‐level fall, probably due to either steric or ice‐volume effects, may have affected this coastline. Thus, whereas these other potential drivers cannot be fully discounted, this study provides insights into the complexity of decadal‐scale to millennial‐scale coastal response to likely variability in sea‐level change rates.  相似文献   

6.
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   

7.
High-arctic fan delta recording deglaciation and environment disequilibrium   总被引：1，自引：0，他引：1  
Study of a Holocene fan delta in Adventfjorden, Spitsbergen, provides new insight into the nature of high‐arctic coastal sedimentation and deglaciation dynamics. The fjord‐side, gravelly Gilbert‐type fan delta began to form at the local marine limit c. 10 ka BP, supplied seasonally with sediment by meltwater from a cirque glacier left behind by the retreating Late Weichselian ice sheet. Relative sea level had fallen by 63 m, and the fan delta reached a radius of c. 1 km by 6 ka BP, when the relic glacier eventually melted down and fluvial activity declined. A strong influence of marine processes is recorded by the fan‐delta foreset facies, overlain by alluvium. Supplied with sediment by longshore drift, the fan‐delta front continued to advance at a lower rate, while relative sea level fell further by 5 m and ceased to fall around 5·4 ka BP. The following transgression was countered by longshore sediment supply until 4·7 ka BP, when the delta‐front beach aggraded and a spit platform began to climb onto the delta plain, recording a relative sea‐level rise of 4 m. The subsequent regression was initially non‐depositional, with the relative sea level falling by > 4 m in 200 years, outpacing fluvial supply, and the re‐emerging fan delta being swept by longshore currents. A regressive beach began to form c. 4·3 ka BP, while relative sea level gradually reached its present‐day position. The feeder braided stream was wandering across the delta plain during this time, but incised once the fan‐delta shoreline began to retreat by wave erosion and turned into a receding modern escarpment. The stream has since been adjusting its profile by gradually eroding the pre‐existing alluvium and distributing the coarse sediment supplied from catchment slopes by debrisflows and snow avalanches. Modern snowflows have also spread debris onto the abandoned fan surface. The erosional retreat of the fan delta has been accompanied by lateral shoreline accretion on both its sides. The study has important regional implications and demonstrates that Holocene fan deltas can provide a valuable record of the deglaciation history in high‐arctic terrains, where glacial deposits are scarcely preserved on land.  相似文献   

8.
Cyclothemic sedimentary rocks of the Plio-Pleistocene Petane Group outcrop extensively in the Tangoio block of central Hawke's Bay, New Zealand. They are products of inner to mid-shelf sedimentation and were deposited during glacio-eustatic sea level fluctuations along the western margin of a shallow, pericontinental seaway located in a forearc setting. The succession consists of five laterally continuous cyclothems, each containing a fine grained interval of silt and a coarse grained interval of siliciclastic sand ± gravel or limestone. Five sedimentary facies assemblages comprising 20 separate facies have been recognized. Coarse grained intervals of cyclothems were deposited mostly during relative sea level lowstands and contain up to four facies assemblages: (1) a non-marine assemblage (with three component facies, representing braided river and overbank environments); (2) an estuarine assemblage (with three component facies, representing tidal flat and mud-dominated estuarine environments); (3) a siliciclastic shoreline assemblage (with six component facies, representing greywacke pebble beach, shoreface and inner shelf environments); and (4) a carbonate shelf assemblage (with four component facies, representing tide-dominated, inshore and shallow marine environments). Fine grained intervals of cyclothems were deposited during sea level highstands when the Tangoio area was generally experiencing mid-shelf sedimentation. This produced an offshore assemblage consisting of four component facies. The distribution of facies assemblages during relative sea level lowstands was dependent upon proximity to the shoreline, the type and rate of sediment supply to the basin, and shelf hydrodynamics. Carbonate shelf facies dominate coarse grained intervals in Cyclothems 3–5, but siliciclastic shoreline and non-marine facies dominate in Cyclothems 1 and 2. The abrupt change from siliciclastic to carbonate sedimentation during relative sea level lowstand deposition is thought to have been induced by rapidly falling interglacial to glacial sea level accentuated by regional tectonic shoaling. This caused most of the terrigenous sediment supply to bypass the Tangoio area. Consequently, carbonate sediment accumulated in inshore and shallow marine settings. Facies assemblages rarely show lateral interdigitation, but are vertically stratified over the entire Tangoio block. Facies successions in each cyclothem preserve a record of relative sea level change during deposition of the Petane Group and are consistent with a Plio-Pleistocene sea level change in eastern New Zealand of c. 75–150 m, i.e. approximately the magnitude suggested for Late Quaternary glacio-eustatic sea level changes.  相似文献   

9.
Deltas are commonly classified according to their plan‐view morphology as either river‐dominated, tide‐dominated or wave‐dominated. However, most deltas form under the mixed influence of these processes, commonly with laterally varying process regimes. It has also become clear that there is a mismatch between the plan‐view morphology and internal facies composition in some deltas. Combined outcrop and subsurface data from the Eocene Battfjellet Formation, Spitsbergen, provide an example of ancient shelf deltas that formed under mixed influence. Internally, these shelf deltas are characterized by wave‐dominated facies that are normally associated with strike‐extensive, nearly linear shoreline sandstones. However, the formation comprises partially overlapping sandstone bodies of limited lateral extent (<20 km in any direction). This stacking pattern is attributed to frequent autogenic lobe switching that caused localized and rapid transgressions. Such processes typify fluvial‐dominated deltas and occur less commonly in wave‐dominated ones. Thus, there is an apparent mismatch between inferred plan‐view morphology and internal facies composition. It is argued that the Battfjellet deltas were flood‐dominated and prograded mainly during periods of high fluvial discharge. However, reworking of the fluvial‐flood facies by fair‐weather and storm waves, as well as longshore currents, resulted in a wave‐dominated facies character. Delta lobes undergoing auto‐retreat were particularly prone to reworking by basinal processes, including tidal currents. It is suggested that repeated delta progradation from inner shelf settings towards the outer shelf and shelf edge was aided by high sediment supply rather than relative falls in sea‐level as previously suggested. This interpretation is supported by: (i) the lack of major facies dislocations and extensive sub‐aerial unconformities; and (ii) an overall relative rise in sea‐level as evidenced by an overall low‐angle (0·8 to 1·2°) ascending shoreline trajectory. The latter results from the combined effect of basin subsidence, eustatic highstand and sediment compaction.  相似文献   

10.
This paper presents an overview of the evolution of the Nile deep‐sea turbidite system during the last 200 kyr, over a series of glacial to interglacial cycles. Six individual deep‐sea fans were identified from an extensive field data set. Each fan comprises a canyon, channel system and terminal lobes. Two of these fan systems were possibly active at the same time, at least during some periods. Large‐scale slope failures destroyed channel segments and caused the formation of new submarine fan systems. These slope failures thus played an important role in the overall evolution of the turbidite system. During the last glacial maximum (ca 25 to 14·8 ka) the central and eastern parts of the Nile deep‐sea turbidite system were relatively inactive. This inactivity corresponds to a lowstand in sea‐level, and a period of arid climate and relatively low sediment discharge from the Nile fluvial system. Rapid accumulation of fluvial flood‐derived deposits occurred across the shallower part of the submarine delta during sea‐level rise between ca 14·8 and 5 ka. The most recent deep‐sea channel–lobe system was very active during this period of rising sea‐level, which is also associated with a wetter continental climate and increased sediment and water discharge from the Nile. Increased sediment deposition in shallower water areas led to occasional large‐scale slope failure. The Nile deep‐sea turbidite system was largely inactive after ca 5 ka. This widespread inactivity is due to retreat of the coastline away from the continental shelf break, and to a more arid continental climate and reduced discharge of sediment from the Nile. The Nile deep‐sea turbidite system may be more active during periods of rising and high sea‐level associated with wetter climates, than during lowstands, and may rapidly become largely inactive during highstands in sea‐level coupled with arid periods. These acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing. This large‐scale sedimentary system responded to monsoon‐driven climate change and sea‐level change in a system‐wide and contemporaneous manner.  相似文献   

11.
From new data on coastal and continental shelf morphology, sediments, stratigraphy and chronology, it is possible to formulate a general model of late Quaternary marine sedimentation, for New South Wales and southern Queensland. This model integrates various factors influencing deposition in coastal and shelf environments, in relation to glacio‐eustatic sea level oscillations.

The model involves several components, including (i) very slow to negligible continental margin subsidence during the Quaternary, (ii) an inherited geomorphic framework; (iii) oscillations of sea level of c 100 m amplitude every 100 000 years, with interglacial high sea levels being close to present and only the Last Interglacial being significantly higher; and (iv) a wave climate that induces a potential south to north littoral sand transport at all sea level positions.

Terrigenous sediment that is moved from the hinterland through embayments to the shelf is either stored as barrier, estuarine or inner shelf deposits, or lost to depositional sinks on the continental slope or into coastal dune fields. Over many glacial‐interglacial cycles, sand has been progressively moved northward and has accumulated in vast aeolian sand deposits in southern Queensland. Littoral sand transport was especially effective during sea levels lower than present. The relatively shallow and lower gradient shelf north of Newcastle (33°S) has encouraged preservation at the coast of a wide range of depositional morphologies, including Pleistocene barriers, whereas the steeper southern shelf has induced net sediment loss seawards and shoreline erosion, excpt in the Holocene. To account for Holocene barrier development in the southern region, the model invokes reworking of sand deposits stranded high on the inner shelf at the end of the Pleistocene Epoch. These were in disequilibrium with Postglacial marine processes that operated at a lower level of the sea than did those during the Last Interglacial maximum.  相似文献   

12.
Coastal lagoons and beach ridges are genetically independent, though non‐continuous, sedimentary archives. We here combine the results from two recently published studies in order to produce an 8000‐year‐long record of Holocene relative sea‐level changes on the island of Samsø, southern Kattegat, Denmark. The reconstruction of the initial mid‐Holocene sea‐level rise is based on the sedimentary infill from topography‐confined coastal lagoons (Sander et al., Boreas, 2015b). Sea‐level index points over the mid‐ to late Holocene period of sea‐level stability and fall are retrieved from the internal structures of a wide beach‐ridge system (Hede et al., The Holocene, 2015). Data from sediment coring, georadar and absolute dating are thus combined in an inter‐disciplinary approach that is highly reproducible in micro‐tidal environments characterised by high sediment supply. We show here that the commonly proximate occurrence of coastal lagoons and beach ridges allows us to produce seamless time series of relative sea‐level changes from field sites in SW Scandinavia and in similar coastal environments.  相似文献   

13.
The Kaskapau and Cardium Formations span Late Cenomanian to Early Coniacian time and were deposited on a low‐gradient foredeep ramp. The studied portion of the Kaskapau Formation spans ca 3·5 Myr and forms a mudstone‐dominated wedge thinning from 700 to <50 m from SW to NE over ca 300 km. In contrast, the Cardium Formation spans about 2·1 Myr, is about 100 m thick, sandstone‐rich and broadly tabular. The Kaskapau and Cardium Formations are divided, respectively, into 28 and nine allomembers, each bounded by marine flooding surfaces. Kaskapau allomembers 1 to 7 show about 200 km of offlap from the forebulge, accompanied by progradation of thin sandstones from the eroded forebulge crest. In contrast, Kaskapau allomembers 8 to 28 and Cardium allomembers C1 to C9 show overall onlap onto the forebulge of about 350 km, and contain no forebulge‐derived sandstones. This broad pattern is interpreted as recording a latest Cenomanian pulse of tectonic loading which led to shoreline back‐step in the proximal foredeep and coeval uplift of the forebulge, leading to erosion. The advance of the sediment wedge after Kaskapau allomember 7 is attributed primarily to the isostatic effect of a distributed sediment load; the advance of the orogenic wedge had a subordinate effect on subsidence of the forebulge. For Kaskapau allomembers 1 to 6, isopachs trend north to south, suggesting a load directly to the west; allomembers 7 to 28 show an abrupt rotation of isopachs to NW–SE, suggesting that the load shifted several hundred kilometres to the south. This re‐orientation might be related to a change from an approximately orthogonal to a dextral transpressive stress regime. Within the longer‐term offlap–onlap cycle recorded by the Kaskapau and Cardium Formations, individual allomembers are grouped into packages reflecting higher‐frequency onlap–offlap cycles, each spanning ca 0·5 to 0·7 Myr. Offlap from the forebulge tends to be accompanied by more pronounced transgression in the foredeep, whereas onlap onto the forebulge is accompanied by progradation of tongues of shoreface sandstone. This relationship suggests that changes in deformation rate in the orogenic wedge modulated proximal subsidence rate, enhancing or suppressing shoreline progradation, and also causing subtle uplift or subsidence of the forebulge region. Wedge‐shaped allomembers in the Kaskapau Formation contain shoreface sandstone and conglomerate that prograded, respectively, <40 and <25 km from the preserved basin margin; progradation of coarse clastics was limited by rapid flexural subsidence. Tabular allomembers of the Cardium Formation imply a low flexural subsidence rate and contain sandy and conglomeratic shoreface deposits that prograded up to ca 180 km from the preserved basin margin. This relationship suggests that low rates of flexural subsidence promoted steeper alluvial gradients, more vigorous gravel transport and more extensive shoreface progradation. Overall, observed stratal geometry and facies distribution is explained readily in terms of current elastic flexural models. Most shoreface sandstones in the proximal foredeep show evidence of forced regression. Eustasy provides the most plausible explanation for relative sea‐level rise–fall cycles on the 125 kyr allomember timescale. Geometric relationships suggest eustatic oscillations of about 10 m. Forced regressive shoreface development was suppressed during Kaskapau allomembers 1 to 10 when the rate of flexural subsidence was at its highest.  相似文献   

14.
Numerical modelling of depositional sequences in half-graben rift basins   总被引：1，自引：0，他引：1  
ABSTRACT A three‐dimensional numerical model of sediment transport and deposition in coarse‐grained deltas is used to investigate the controls on depositional sequence variability in marine half‐graben extensional basins subject to eustatic sea‐level change. Using rates of sea‐level change, sediment supply and fault slip reported from active rift basins, the evolution of deltas located in three contrasting structural settings is documented: (1) footwall‐sourced deltas in high‐subsidence locations near the centre of a fault segment; (2) deltas fed by large drainage catchments at fault tips; and (3) deltas sourced from drainage catchments on the hangingwall dip slope. Differences in the three‐dimensional form and internal stratigraphy of the deltas result from variations in tilting of the hangingwall and the impact of border fault slip rates on accommodation development. Because subsidence rates near the centre of fault segments are greater than all but the fastest eustatic falls, footwall‐sourced deltas lack sequence boundaries and are characterized by stacked highstand systems tracts. High subsidence and steep bathymetry adjacent to the fault result in limited progradation. In contrast, the lower subsidence rate settings of the fault‐tip and hangingwall dip‐slope deltas mean that they are subject to relative sea‐level fall and associated fluvial incision and forced regression. Low gradients and tectonic tilting of the hangingwall influence the geometry of these deltas, with fault‐tip deltas preferentially prograding axially along the fault, creating elongate delta lobes. In contrast, broad, sheet‐like delta lobes characterize the hangingwall dip‐slope deltas. The model results suggest that different systems tracts may be coeval over length scales of several kilometres and that key stratal surfaces defining and subdividing depositional sequences may only be of local extent. Furthermore, the results highlight pitfalls in sequence‐stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.  相似文献   

15.
Many pre‐Mesozoic records of Earth history are derived from shallow water carbonates deposited on continental shelves. While these carbonates contain geochemical proxy records of climate change, it is the stratal architecture of layered carbonate units that often is used to build age models based on the idea that periodic astronomical forcing of sea‐level controls the layering. Reliable age models are crucial to any interpretation of rates and durations of environmental change, but the physical processes that actually control this stratal architecture in shallow water carbonates are controversial. In particular, are upward‐shallowing stacks of carbonate beds bounded by flooding surfaces (‘parasequences’) truly a record of relative sea‐level change? The purpose of this study is to examine a tidal flat that is actively accumulating carbonate stratigraphy, and to determine the relative importance of tidal channel migration (poorly known, but investigated here) and Holocene sea‐level rise (well‐known) in controlling post‐glacial parasequence architecture. This work represents a field study of peritidal carbonate accumulation at Triple Goose Creek, north‐west Andros Island. By integrating surface facies maps with differential global positioning system topographic surveys, a quantitative relationship between facies and elevation is derived. Sedimentary facies are sensitive to elevation changes as small as 5 cm, and are responding to both internal (distance to nearest tidal channel) and external (sea‐level rise) controls. The surface maps also are integrated with 187 sediment cores that each span the entire Holocene succession. While flooding of the Triple Goose Creek area should have occurred by ca 4500 years ago, preservation of Holocene sediment did not begin until 1200 years ago. The tidal channels are shown to be stationary, or to migrate sluggishly at up to 6 cm per year. Therefore, while the location of tidal channels is responsible for the modern mosaic of surface facies, these facies and the channels that control them have not migrated substantially during the ca 1200 years of sediment accumulation at Triple Goose Creek. Once the region was channellized, vertical and lateral shifts in facies, such as the landward retreating shoreline, expanding mangrove ponds and seaward advancing inland algal marsh, are driven by changes in relative sea‐level and sediment supply, not migrating channels. While stratigraphic columns look different depending on the distance to the nearest tidal channel, the overall parasequence architecture everywhere at Triple Goose Creek records an upward‐shallowing trend controlled by the infilling of accommodation space generated by post‐glacial sea‐level rise.  相似文献   

16.
Autostratigraphy is the stratigraphy generated by large‐scale autogenesis, developed based on the full recognition of the non‐equilibrium behaviour of depositional systems in response to steady external forcing. The existing autostratigraphic concepts were derived mostly from studies of river deltas growing during a single rise or fall of base level (or relative sea level). The present study challenges to extend the autostratigraphic framework to the alluvial‐shelf system growing through steady base‐level cycles by two‐dimensional tank experiments. During each experimental run, the base level was changed symmetrically, wherein through cycles, the rise and fall had the same constant rate (|Rbl|) and period (Tbl), and thus the same constant amplitude (Abl), but with no basin tectonism. In total, nine runs with different combinations of |Rbl| and Abl were performed. The experimental results brought the following implications. (i) The shelf‐transiting active depositional system takes non‐equilibrium responses in earlier base‐level cycles, during each of which the system experiences episodes of degradation with base‐level fall. (ii) After the system has sufficiently grown through cycles, non‐equilibrium responses change into equilibrium responses, whereby the shelf‐transiting alluvial system, whether retrogradational or progradational, is free from degradation and continues to aggrade but with a gradually decreasing rate of aggradation. (iii) The alluvial topset river tentatively but autogenically attains a graded state during the falling limb of an intermediate cycle, which separates the earlier degradation‐inclusive and later aggradation‐sustainable cycles. (iv) The number (or duration) of cycles elapsed prior to this phase‐transition is linearly proportional to the amplitude (or the square of the period) of base‐level cycles, with a coefficient defined by the rates of base‐level change and sediment supply. Such a growth pattern does not necessarily hold when considering long‐term tectonic subsidence or uplift. These notions help to understand the stratigraphic architectures of natural alluvial‐shelf systems evolved through base level cycles.  相似文献   

17.
Two large (200 to 300 km), near‐continuous outcrop transects and extensive well‐log data (ca 2800 wells) allow analysis of sedimentological characteristics and stratigraphic architecture across a large area (ca 60 000 km2) of the latest Santonian to middle Campanian shelf along the western margin of the Western Interior Seaway in eastern Utah and western Colorado, USA. Genetically linked depositional systems are mapped at high chronostratigraphic resolution (ca 0·1 to 0·5 Ma) within their sequence stratigraphic context. In the lower part of the studied interval, sediment was dispersed via wave‐dominated deltaic systems with a ‘compound clinoform’ geomorphology in which an inner, wave‐dominated shoreface clinoform was separated by a muddy subaqueous topset from an outer clinoform containing sand‐poor, gravity‐flow deposits. These strata are characterized by relatively steep, net‐regressive shoreline trajectories (>0·1°) with concave‐landward geometries, narrow nearshore belts of storm‐reworked sandstones (2 to 22 km), wide offshore mudstone belts (>250 km) and relatively high sediment accumulation rates (ca 0·27 mm year?1). The middle and upper parts of the studied interval also contain wave‐dominated shorefaces, but coeval offshore mudstones enclose abundant ‘isolated’ tide‐influenced sandstones that were transported sub‐parallel to the regional palaeoshoreline by basinal hydrodynamic (tidal?) circulation. These strata are characterized by relatively shallow, net‐regressive shoreline trajectories (<0·1°) with straight to concave‐seaward geometries, wide nearshore belts of storm‐reworked sandstones (19 to 70 km), offshore mudstone belts of variable width (130 to >190 km) and relatively low sediment accumulation rates (ca ≤0·11 mm year?1). The change in shelfal sediment dispersal and stratigraphic architecture, from: (i) ‘compound clinoform’ deltas characterized by across‐shelf sediment transport; to (ii) wave‐dominated shorelines with ‘isolated’ tide‐influenced sandbodies characterized by along‐shelf sediment transport, is interpreted as reflecting increased interaction with the hydrodynamic regime in the seaway as successive shelfal depositional systems advanced out of a sheltered embayment (‘Utah Bight’). This advance was driven by a decreasing tectonic subsidence rate, which also suppressed autogenic controls on stratigraphic architecture.  相似文献   

18.
Excellent exposures of thick, multistorey, fluvial deposits from the deltaic Atane Formation on south‐east Nuussuaq, central West Greenland, show the architecture of up to 100 m thick continuously aggrading fluvial depositional complexes. The succession comprises vertically stacked channel belt sandstones separated by thin floodplain deposits, with little to no incision between storeys. Architectural elements and palaeocurrent patterns of channel deposits indicate deposition in large, relatively stable, low‐sinuosity rivers, probably located within an incised valley. Gradual transitions from channel to floodplain deposits accompanied by a gradual change from floodplain to spillover sand suggest avulsion on the floodplain as a possible mechanism for the vertically alternating channel and floodplain deposits. Despite its relative proximity to contemporaneous sea‐level (ca 35 km upstream from the palaeo‐shoreline) the depositional complex is entirely non‐marine. The aggrading nature of the deposits suggests a continuously rising base level coupled with a high and steady sediment supply. Vertical alternations between floodplain and channel deposits may be forced by subtle interruptions in this balance or autocyclic mechanisms on the floodplain. This study provides an example of aggrading lowstand/non‐marine transgressive systems tract deposits.  相似文献   

19.
By analyzing the balanced cross sections and subsidence history of the Longmen Mountain thrust belt, China, we concluded that it had experienced five tectonic stages: (1) the formation stage (T3x) of the miniature of Longmen Mountain, early Indosinian movement, and Anxian tectonic movement created the Longmen Mountain; (2) the stable tectonic stage (J1) where weaker tectonic movement resulted in the Longmen Mountain thrust belt being slightly uplifted and slightly subsiding the foreland basin; (3) the intense tectonic stage (J2-3), namely the early Yanshan movement; (4) continuous tectonic movement (K–E), namely the late Yanshan movement and early Himalayan movement; and (5) the formation of Longmen Mountain (N–Q), namely the late Himalayan movement. During those tectonic deformation stages, the Anxian movement and Himalayan movement played important roles in the Longmen Mountain’s formation. The Himalayan movement affected Longmen Mountain the most; the strata thrust intensively and were eroded severely. There are some klippes in the middle part of the Longmen Mountain thrust belt because a few nappes were pushed southeastward in later tectonic deformation.  相似文献   

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