Search

Late Cretaceous continental redbeds, the Guifeng Group of the Yongchong Basin in SE China have been investigated to conduct detailed fan facies description and interpretation. Tectonic activities determined the alluvial fan development along the basin margin, but the alluvial facies was linked with paleoclimate changes. The Guifeng Group is divided into the Hekou, Tangbian and Lianhe formations in ascending order. The Hekou conglomerates are typically polymict, moderately sorted with erosional bases, cut-and-fill features, normal grading and sieve deposits, representing dominant stream-flows on alluvial fans during the initial opening stage of the basin infill. The Tangbian Formation, however, is characterized by structureless fine-grained sediments with dispersed coarse clasts, and couplets of conglomerate and sandstone or siltstone and mudstone, recording a change to a playa and ephemeral lake environments with occasional stream flooding, thus indicating a basin expanding stage. The hallmark of the Lianhe Formation is disorganized, poorly sorted conglomerates lack of erosional bases, and a wide particle-size range from clay to boulders together reflect mud-rich debris-flows accumulating on fans, likely related to reactivation of faulting along the northwestern mountain fronts during a post-rift stage. The depositional system changes from stream-flows up through playa with ephemeral streams to debris-flows during the accumulation of the three formations are thus attributed to different source rocks and climatic conditions. Therefore, the fluvial-dominated fans of the Hekou Formation recorded a subhumid paleoclimate (Coniacian-Santonian Age). The dominant semiarid climate during the Campanian Age produced abundant fine-grained sediments in the playa and ephemeral lake environments of the Tangbian Formation. A climatic change towards more humidity during the late stage of the Guifeng Group (Maastrichtian Age) probably yielded high deposition rate of coarse clasts in debris-flow dominated fans of the Lianhe Formation. Thus the Late Cretaceous climate changes are inferred to have influenced and preserved signals in the alluvial stratigraphy of the Yongchong Basin. [ABSTRACT FROM AUTHOR]

Relative sea-level fall and high sediment supply can be decisive in driving shelf-edge sands to form deep-water fans. However, a closer look at a series of lowstand shelf-edge deltas on flattish or downward prograding shelf margins yields two further important insights on this paradigm. Firstly, among 42 reviewed shelf-edge delta examples, only 24 river-, or wave-dominated shelf-edge deltas with supply of sand-dominated sediment fit sequence-stratigraphic models that link relative sea-level fall to submarine-fan growth. Contrary to conventional lowstand models, it is virtually impossible for lowstand shelf-edge deltas with supply of mud-dominated sediment (as evidenced by 18 reviewed examples without fan growth) to partition large volumes of shelf-edge sands into deep-water sites to form sandy basin-floor fans, even under the scenarios of river-dominated process regimes and sufficient sea-level fall. Grain size of the supply sediment (dominantly sandy or muddy, represented by the presence or absence of sandy upper delta fronts) therefore also plays a pivotal but underappreciated role in driving shelf-edge sands into deepwater, further modulating the conventional lowstand sand delivery concept. Secondly, contrary to recent suggestions that link basin-floor fan growth mainly to river-dominated shelf-edge process regimes, wave-dominated shelf-edge deltas with either high or low supply of dominantly sandy sediment (6 of 42 examples) can also foster sandy basin-floor fans, in conditions of direct linkage between deltas and slope-channel heads. These exceptional conditions, although long known, are less rare than believed, thus further modifying the evolving delivery paradigm. [ABSTRACT FROM AUTHOR]

Exceptionally high shelf-subsidence rates (0.8–6.0+ mm/yr), a marked basinward stepping (to east and northeast) of the paleo-Orinoco shelf prism and post-Pliocene uplift of Trinidad all allow the sedimentary facies, process regime and the evolution of the Late Miocene Orinoco Delta to be evaluated from extensive outcrops along the southwest, and south coasts of Trinidad. The ca. 200 km easterly growth (late Miocene to present) of the Orinoco shelf-margin was generated by repeated cross-shelf, regressive–transgressive transits of the Orinoco Delta system. The studied Late Pliocene segment of this shelf-margin prism allows insight to how this margin was built. The Morne L'Enfer Formation (Late Pliocene) along Cedros Bay and Erin Bay in SW Trinidad, provides a window into the facies and process regime of the ca. 850 m-thick deltaic succession at an inner-shelf location some 100 km landward of the coeval shelf edge. Regressive facies associations include tide-influenced delta-front to prodelta deposits (FA1) within upward coarsening units, shoreface to offshore deposits, possibly with prograding mud cape deposits (FA2), and fluvial distributary channel infills (FA3), as well as muddy sediments of floodbasins and coastal embayments between the distributary channels (FA4), and tide-influenced bay-head delta deposits (FA5). Transgressive facies associations show an overall upward fining of grain size and include inner estuary distributary channels with minimal brackish-water or tidal influence (FA6), transition zone fluvial-tidal distributary channels (FA7), tide-dominated mid-outer estuary channel-bars (FA8), and intertidal to supratidal flat units (FA9). The tidal signals in both deltaic and estuarine units include bi-directional paleocurrents (channels), frequent mud drapes within stacked sets of cross-strata (delta-front), fluid mud layers, flaser, wavy and lenticular bedding, and ubiquitous spring-neap stratal bundling. The tide dominated nature of the paleo-delta in SW Trinidad was likely due to its location within an embayed proto-Columbus Channel, though by analogy with the modern Orinoco Delta, it is predicted that the same succession becomes wave dominated to the east as the delta emerged to the open ocean and approached the outer shelf and shelf-edge region. It is difficult to estimate how much of the abundant mud in the Pliocene deltaic sequences was derived from inner-shelf littoral currents with suspended Amazon River mud. The studied Late Pliocene Morne L'Enfer succession contains some 17 high-frequency transgressive–regressive sequences, each ca. 40–60 m thick, estimated to have an average time duration of 90–120 Ky. By analogy, the last glacial cycle on the Orinoco shelf saw the delta prograding across the 200 km-wide shelf to the shelf edge in ca. 100 Ky, then transgressing back to its present position in 20 Ky. A predicted model of the linkage between the study succession on SW Trinidad and its eastward continuation offshore towards the outer shelf and shelf edge in the Columbus Basin is suggested. [ABSTRACT FROM AUTHOR]

A simple inversion scheme for estimating sediment flux from ancient shelf-margin successions is presented here by treating shelf-margin clinothems as the product of deposition associated with migration of a shelf-edge clinoform with constant shape at a rate equal to the shelf-margin progradation rate. Assuming sediment conservation, deposition can be broken into components of (1) response to subsidence and sea-level changes, and (2) basinward migration of the clinoform profile. Sediment flux can therefore be estimated with knowledge of progradation rate, subsidence/sea-level change rate, and clinoform dimensions. An advantage of this methodology is that it requires only two-dimensional data (i.e., dip-oriented cross sections) rather than three-dimensional volumes, making it ideal for use with sparse data sets as well as with outcrops. This methodology is also useful for analyzing areally limited data sets because it can predict the flux of sediment transported beyond the area of data coverage. The approach is able to accurately reproduce the sediment-flux estimates of previous workers from several margins (the Fox Hills-Lewis, Zambezi, New Jersey, and North Slope margins) using both volumetric and forward-modeling methods. Not only are the predicted distributions for sediment flux across ancient shelf-margins similar to distributions predicted by more data-intensive theoretical models, the estimated magnitudes for paleofluxes favorably compare with measured loads from modern rivers. Growth of continents is achieved in part by accretion of sediment on shelf margins. The rates and patterns of continental expansion are therefore partially dependent on the magnitude and distribution of mass transfer from eroding hinterlands to continental margins, fluxes which also play a critical role in global biogeochemical cycles. Flux estimates cast into a mass-balance framework suggest that approximately two-thirds of continental-margin sediments are exported past the shelf edge into deeper water at long-term geologic time scales. This finding implies that two-thirds of the terrestrial-derived, particulate organic carbon (POC) delivered from rivers to the ocean can be stored in deep water over geologic time scales. The observations presented here indicate that repetitive delivery of sediment to margins by shelf-edge deltas is fundamental to the long-term process of margin accretion. [ABSTRACT FROM AUTHOR]

Late-middle Miocene to Pliocene siliciclastics in the Northern Carnarvon Basin, Northwest Shelf of Australia, are interpreted as having been deposited by deltas. Some delta lobes deposited sediments near and at the shelf break (shelf-edge deltas), whereas other lobes did not reach the coeval shelf break before retreating landward or being abandoned. Shelf-margin mapview morphology changes from linear to convex-outward in the northern part of the study area where shelf-edge deltas were focused. Location and character of shelf-edge deltas also had significant impact on along-strike variability of margin progradation and shelf-edge trajectory. Total late-middle and late Miocene margin progradation is ca. 13 km in the south, where there were no shelf-edge deltas, vs. ca. 34 km in the north where shelf-edge deltas were concentrated. In the central area, the deltas were arrested and accumulated a few kilometres landward of the shelf break, resulting in an aggradational shelf-edge trajectory, in contrast to the more progradational trajectory farther north. This illustrates a potential limitation of shelf-edge trajectory analysis: only where shelf-edge deltas occur, there is sufficient sediment available for the shelf-edge trajectory to record relative sea-level fluctuations reliably. Small-scale ( ca. 400 m wide) incisions were already conspicuous on the coeval slope even before deltas reached the shelf break. However, slope gullies immediately downdip from active shelf-edge deltas display greater erosion of underlying strata and are wider and deeper (>1 km wide, ca. 100 m deep) than coeval incisions that are laterally offset from the deltaic depocenter ( ca. 0.7 km wide, ca. 25 m deep). We interpret this change in slope-gully dimensions as the result of greater erosion by sediment gravity flows sourced from the immediately adjacent shelf-edge deltas. Similarly, gullies also incised further (up to 6 km) into the outer shelf in the region of active shelf-edge deltas. [ABSTRACT FROM AUTHOR]

Shelf-edge deltas record the potential magnitude of sediment delivery from shallow water shelf into deep water slope and basin floor and, if un-incised, represent the main increment of shelf-margin growth into the basin, for that period. The three-dimensional complexity of shelf-edge delta systems and along-strike variability at the shelf edge in particular, remains understudied. The Permian-Triassic Kookfontein Formation of the Tanqua Karoo Basin, South Africa, offers extensive three-dimensional exposure (>100 km2) and therefore a unique opportunity to evaluate shelf-edge strata from an outcrop perspective. Analysis of stratal geometry and facies distribution from 52 measured and correlated stratigraphic sections show the following: (i) In outer-shelf areas, parasequences are characterized by undeformed, river-dominated, storm-wave influenced delta mouth-bar sandstones interbedded with packages showing evidence of syn-depositional deformation. The amount and intensity of soft-sediment deformation increases significantly towards the shelf edge where slump units and debris flows sourced from collapsed mouth-bar packages transport material down slope. (ii) On the upper slope, mouth-bar and delta-front sandstones pinch out within 2 km of the shelf break and most slump and debris flow units pinch out within 4 km of the shelf break. (iii) Further down the slope, parasequences consist of finer-grained turbidites, characterized by interbedded, thin tabular siltstones and sandstones. The results highlight that river-dominated, shelf-edge deltas transport large volumes of sand to the upper slope, even when major shelf-edge incisions are absent. In this case, transport to the upper slope through slumping, debris flows and un-channellized low density turbidites is distributed evenly along strike. [ABSTRACT FROM AUTHOR]

Twenty-seven marine and 23 lacustrine clinothem pairs were recognized and analyzed in Qiongdongnan Basin and Pannonian Basin, respectively. A comparison of their architecture highlighted some critical differences in sediment delivery and partitioning between the marine and closed lacustrine basins. In marine Qiongdongnan Basin, rising but forward-moving shelf-edge trajectories and clinoforms with a height of hundreds of meters commonly link downdip to absent or limited sandy bottomsets, whereas the opposite occurs in the lacustrine Pannonian Basin, where very thick bottomsets developed in front of highly aggradational clinoforms. A further comparison of strongly progradational clinothem sets with fairly flat shelf-edge trajectories suggests that relatively thick versus thin or absent sandy bottomsets occur in the marine and lacustrine settings, respectively. The main reason for this marked contrast in sediment partitioning across marine versus lacustrine clinoforms is climate, where a wet-dry climate model needs to be used to explain sediment dispersal to the floor of deep Lake Pannon. During the humid half-cycle, catchment precipitation and sediment flux into Lake Pannon were probably high, and coeval lake-level rise and reduced salinity likely increased shelf accommodation and caused topset aggradation of the forward-moving clinothems. This, in turn, resulted in enhanced hyperpycnal flows and thick bottomsets on the lake-basin floor. During the semiarid half-cycle, lake level was fairly stable, and both the catchment precipitation and sediment flux into Lake Pannon were probably reduced, causing some increase in the salinity, less frequent hyperpycnal flows, and resultant short downlapping clinothems lacking or with only thin bottomsets. Critical differences in sediment delivery and partitioning between marine and lacustrine basins, as highlighted by a comparison of aggradational to progradational clinothem pairs documented herein, draw attention to the pitfall of mechanically applying conventional marine sequence stratigraphy to lacustrine basins. [ABSTRACT FROM AUTHOR]

Abstract: Interpretation of the latest Pleistocene stratigraphy of the outer New Jersey shelf has evolved over the last three decades with addition of new geological and geophysical data. During the last decade, new results have described and interpreted various depositional elements within the latest Pleistocene that overlie the regional reflector known as the R-horizon. On the basis of observations made from detailed mapping of the R-horizon and the overlying sediment wedge(s) using high resolution shallow seismic (CHIRP) data acquired in several phases between 2001 and 2006, we revise the depositional model for the offlapping, multi-clinothem sediment wedge on the latest Pleistocene outer New Jersey Shelf as strike-elongated lobes of a falling-stage, wave-influenced delta system fed by a pre-LGM paleo-Hudson River Channel(s) that probably reached the late Pleistocene shelf-edge off New Jersey. This sediment wedge is composed of a series of prograding and offlapping clinothems of varying thickness. Within our data, which does not extend to the basinward termination of the wedge, we seldom observe preserved topsets of these clinothems. Parts of the foresets of some clinothems are also not preserved, particularly in the proximal part of the wedge. The sediment wedge is elongated in strike direction (NNE–SSW), which suggests along-strike sediment movement and possible redistribution of sediment in that direction, by ambient shore-parallel currents. Our data do not record any time-equivalent fluvial channels at the proximal end of the sediment wedge, though there are channel features that cap and cut down into several of the clinothems that make up the wedge farther seaward. We suggest that this can be explained in two possible ways: 1) relatively large spatial separation between the prograding front of the delta system and the delta-plain topset deposits, as is very common under a falling-stage sea-level condition; and 2) lack of preservation of the stratigraphic record of the delta-plain channels because of the likely vigorous falling-stage erosion of topsets during the fall of sea level, and possibly also during the subsequent transgressive ravinement and flooding after the LGM. The clinothems within the sediment wedge show systematic variation in thickness, indicating a northward shift of the main depocenter. This shift might also be an indication of gradual northward migration of the principal fluvial feeder. Such a possibility is supported by previously reported late Pleistocene paleo-Hudson Channel deposits south of the modern Hudson Shelf Valley. [Copyright &y& Elsevier]

Abstract: Some of the principles of tidal-wave theory and examples of mega-, macro-, meso- and microtidal coasts are reviewed, as well as sedimentary successions showing general tidal signals (tidalites) and thinly-laminated, cyclically stacked tidal strata (tidal rhythmites). Although tidalites are well known for their muddy stratification, some of the most spectacular tidal deposits are the sand-rich, cross stratified successions that accumulated as tidal dunes, compound dunes and tidal bars in deltas, estuaries, shelves and straits. Recent progress has been made on modelling of ancient tidal strata, (1) in relation to sea-level rise and fall, (2) in recognition of the systematic changes occurring within the important fluvial–marine transition zone, (3) in the prediction of ancient tidally influenced deposits using shoreline morphology, shelf width and accommodation to supply ratios, and in (4) generation of palaeo-ocean models and the computation of tidal dynamics in ancient seas and seaways. Recent key insights into ancient tidal strata include the recognition of fluid-mud deposits, the realization of the significance of tidal bars versus tidal dunes, the use of palaeogeographic data for prediction of tidal sediments and the recognition of ancient tidal-strait deposits. [Copyright &y& Elsevier]

Long-lived, multi-event bedforms hundreds of meters in wavelength in outcrop are interpreted as deepwater antidunes, the first recognized of this nature. Deepwater sediment gravity flows commonly reach a Froude supercritical flow state, but interpretation of their deposits largely excludes antidunes, which are commonly assumed to be ephemeral. Well-exposed, extensive slope turbidites of the Fish Creek-Vallecito Basin of Southern California (USA) are organized into 3-10-m-thick bedsets of 20-30 distinguishable lenticular backset beds that build low-angle (<10°), undulating geometries and accrete opposite to paleoflow. Bedsets lack high-angle geometries, deep scour surfaces, and structureless facies intrinsic to cyclic steps. Instead, bedsets are differentiated by rhythmic down-dip transitions from thin, subparallel fine-grained beds into thicker, inclined coarser-grained beds and back into thinner, flattening, and, in cases, downflow-dipping finer-grained beds. Within bedsets, compensationally stacked waveforms have ~3-7 m amplitudes and ~75-215 m wavelengths that increase upsection and are comparable to modern upstream-migrating sediment waves. Bioturbated fine-grained caps of each sand bed indicate that antidune bedforms evolved across multiple flow events. Recognition of antidunes in deep water can have important implications for paleoflow reconstruction. [ABSTRACT FROM AUTHOR]