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The three‐dimensional facies and architecture variability of shelf‐edge deltaic units cropping out at the transition between the Lower–Middle Jurassic Lajas and Los Molles formations of southern Neuquén Basin, Argentina, is presented here, as well as their stratigraphic relationship to uppermost deep‐water slope channel systems. Deep‐water, slope mudstones with thin turbidite beds merge upward with prodelta mudstones and thin sandstones, which are truncated by delta‐front to mouth‐bar sandstones. The latter sandstones are then downcut by large‐scale, trough cross‐stratified coarse‐grained sandstones and conglomerates of distributary channel systems and along‐strike, amalgamated with cross‐bedded sandy units showing evidence of tidal reworking. Proximal–distal facies and architecture variability within a shelf‐edge deltaic succession demonstrates that distributary channel‐complexes become wider and deeper basinward, forming channelized river‐dominated distributary fairways separated by tidally reworked inter‐distributary sand belts at the shelf edge. Evidence from depositional‐dip oriented outcrops shows a lack of collapsed and slumped strata at the shelf edge, and that the coarse shelf‐edge distributary channel fills continue far down the deepwater slope, and conglomerates transform to become high‐density turbidites to mainly thick‐bedded, sand‐matrix‐supported debrites. The interplay between flood tides and river currents is interpreted to have primarily modulated the focusing of river drainages, and consequently coarse‐grained sediment transport, along preferential routes on the outer‐shelf to shelf‐edge and down onto the slope. This contribution documents a unique example of coarse‐grained (mostly conglomeratic) shelf‐edge delta systems, tying bed‐scale facies and architecture data to a seismic‐scale shelf‐margin morphology, thus providing outcrop analogue data for the characterization of shelf‐edge delta systems in the subsurface. [ABSTRACT FROM AUTHOR]

The siliciclastic topset of a continental margin, or a shelf‐margin prism in subsiding nonplate‐margin deepwater basins, is the flat‐lying upper part of the margin succession; it is coeval basinwards with deepwater slope clinoforms. Topsets develop by the aggradation of repeated, cross‐shelf, shoreline regressions and transgressions, thereby hosting the shelf portion of stacked, fourth‐order stratigraphic sequences. Sediment spreading downdip and along strike during the cross‐shelf transit of the sediment delivery system, as well as process regime changes of deltas and shorefaces (regressive) and of estuaries, barrier–lagoon systems and shelf ridges (transgressive) are highly variable over short distances, so that correlation within a single stratigraphic sequence is far more difficult than correlation of the cross‐shelf maximum flooding surface boundaries. Thickness of individual regressive–transgressive, fourth‐order sequences is given by shelf accommodation, typically <10 m in embayment or on the inner shelf and up to 200 m on outer shelf. Tectonic subsidence and compaction will enhance this thickness only if rates are very high compared to shelf‐transit time. In very high subsidence rate settings, the transgressive tracts are well preserved and often thickly developed. Topset sequences in an Icehouse climate setting tend to have a high proportion and greater landward development of marine vs nonmarine deposits, compared to Greenhouse sequences, because of the importance of eustatic rise of sea level in the former. Previous numerical experiments show that even for very wide shelves and irrespective of Icehouse or Greenhouse conditions, deltas rarely take more than 10 s of ky to reach their shelf edge, suggesting that it is fourth‐order (or higher) sequences that are the fundamental ones in sequence stratigraphy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It is clear from multiple modeling and field studies that deltas can attain a shelf-edge position under conditions of rising sea level, and this is accepted as an alternative way to form submarine fans without relative sea-level lowstand conditions. However, the relative importance and the range of controls that generate this type of shelf-edge deltas are less well known. To investigate these controls, we input the natural range of seven accommodation and morphological variables to a geometric model that estimates the required sediment load to form the highstand shelf-edge delta. We conduct a one-at-a-time sensitivity analysis on each variable to determine their relative importance. Our results suggest that, other than sediment supply, shelf width is the most important control in forcing deltas to the shelf edge during rising sea level. Two of the traditional stratigraphic trinity, i.e., eustatic sea-level change and shelf subsidence, most likely play less important roles than implied in the literature. This result is consistent with a recent trend in sequence stratigraphic studies encouraging consideration of multiple controls in all interpretations, rather than assuming dominant control by relative sea level. [ABSTRACT FROM AUTHOR]

Shelf-edge deltas are a key depositional environment for accreting sediment onto shelf-margin clinoforms. The Moruga Formation, part of the palaeo-Orinoco shelf-margin sedimentary prism of south-east Trinidad, provides new insight into the incremental growth of a Pliocene, storm wave-dominated shelf margin. Relatively little is known about the mechanisms of sand bypass from the shelf-break area of margins, and in particular from storm wave-dominated margins which are generally characterized by drifting of sand along strike until meeting a canyon or channel. The studied St. Hilaire Siltstone and Trinity Hill Sandstone succession is 260 m thick and demonstrates a continuous transition from gullied (with turbidites) uppermost slope upward to storm wave-dominated delta front on the outermost shelf. The basal upper-slope deposits are dominantly mass-transport deposited blocks, as well as associated turbidites and debrites with common soft-sediment-deformed strata. The overlying uppermost slope succession exhibits a spectacular set of gullies, which are separated by abundant slump-scar unconformities (tops of rotational slides), then filled with debris-flow conglomerates and sandy turbidite beds with interbedded mudstones. The top of the study succession, on the outer-shelf area, contains repeated upward-coarsening, sandstone-rich parasequences (2 to 15 m thick) with abundant hummocky and swaley cross-stratification, clear evidence of storm-swell and storm wave-dominated conditions. The observations suggest reconstruction of the unstable shelf margin as follows: (i) the aggradational storm wave-dominated, shelf-edge delta front became unstable and collapsed down the slope; (ii) the excavated scars of the shelf margin became gullied, but gradually healed (aggraded) by repeated infilling by debris flows and turbidites, and then new gullying and further infilling; and (iii) a renewed storm wave-dominated delta-front prograded out across the healed outer shelf, re-establishing the newly stabilized shelf margin. The Moruga Formation study, along with only a few others in the literature, confirms the sediment bypass ability of storm wave-dominated reaches of shelf edges, despite river-dominated deltas being, by far, the most efficient shelf-edge regime for sediment bypass at the shelf break. [ABSTRACT FROM AUTHOR]

Many modern deltas show complex morphologies and architectures related to the interplay of river, wave and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed-energy systems and their reservoir heterogeneities. The Lajas Formation, a sand-rich, shallow-marine, mixed-energy deltaic system in the Neuquén Basin of Argentina, previously defined as a tide-dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross-stratified, sheet-like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean-water (i.e. very little mud is present in the system), sand-rich deltaic system, very different from the majority of mud-rich, modern tide-influenced examples. The sand-rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal-current reworking, which produced amalgamated sandstone bodies in the delta-front area, and a final stratigraphic record very different from the simple coarsening-upward trends of river-dominated and wave-dominated delta fronts. [ABSTRACT FROM AUTHOR]

Palaeo shoreline is a commonly used proxy for palaeo sea level, but only if deposition is continuous and constant will shoreline trajectory T(l) completely capture sea-level time-ser-ies E(t). Artificial deltas were generated in the Eurotank flume facility under stepwise tectonic subsidence, periodic sea-level fluctuation and two periodic water-discharge scenar- ios, one in-phase and the other out-of-phase with sea level. Independent input variables tectonic subsidence Y, sea level E and water discharge Q (controlling sediment supply S) were varied and dependent output variable shoreline trajectory T was monitored. These experiments confirm that deposition is discontinuous even for continuous sediment supply, and this hinders the inference of sea-level curve from shoreline trajectory. These results justify the here-developed methodol-ogy for converting shoreline trajectory from the space domain to the time domain, thereby improving the accuracy of the inferred sea-level curve. [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]

The first sandstone unit of the Esdolomada Member of the Roda Formation (hereafter referred to as 'Esdolomada 1') was formed by a laterally-migrating, shelf tidal bar. This interpretation is based on detailed mapping of the bedding surfaces on the digital terrain model of the outcrop built from light detection and ranging data and outcrop photomosaics combined with vertical measured sections. The Esdolomada 1 sandbody migrated laterally (i.e. transverse to the tidal currents) towards the south-west along slightly inclined (1.6° to 4.6°) master bedding surfaces. The locally dominant tidal current flowed to the north-west. This current direction is indicated by the presence of stacked sets of high-angle (average 21°) cross-stratification formed by dunes that migrated in this direction, apparently in an approximately coast-parallel direction. The tidal bar contains sets and cosets of medium-grained cross-stratified sandstone that stack to reach a thickness of about 5·5 m. Individual cross-bed sets average about 50 cm thick (with a range of 10 to 70 cm) and have lengths of ca 130 to 250 m in a direction perpendicular to the palaeocurrent. Set thickness decreases in the direction of migration, towards the south-west, and the degree of bioturbation increases, so that the cross-bedded sandstones gradually change into highly bioturbated finer-grained and thinner-bedded sandstones lacking any cross-stratification. The rate of thinning of individual dune sets as they are traced down any obliquely-accreting master surface is some 40 cm per 100 m (0·004) for the older, thicker sandstones, whereas the younger, thinner beds thin at a rate of 15 cm over 100 m (0·0015). The tidal bar has a sharp base and top and is encased in finer-grained bioturbated, marine sandstones. The Esdolomada bar crest was oriented north-west to south-east, parallel to the tidal palaeocurrents and to the nearby palaeoshoreline, but built by lateral accretion towards the south-west. Lateral outbuilding generated a flat-topped bar with a measured width of about 1700 m, and a preserved height of 5·5 m. The bar, disconnected from a genetically related south-westward prograding delta some 2 km to the north-east, developed during the transgressive phase of a sedimentary cycle. The tidal bar was most probably initiated as a delta-attached bar at the toesets of the delta front and during transgression evolved into a detached tidal bar. [ABSTRACT FROM AUTHOR]

Abstract: Demonstration of shelf-margin accretion by shelf-edge deltas during rising and highstand of relative sea level has important consequences for deepwater sand depositional models. Although highstand shelf-edge deltas are conceptually feasible and have been recently argued from subsurface data, we describe here the first outcrop example, thus providing facies and architectural data on this important category of delta. Deltas are able to reach the shelf-edge during rising sea level, if one or more of the key conditions of sediment supply, shelf width/gradient, or basinal processes are such as to allow complete cross-shelf progradation before the onset of delta auto-retreat. Such highstand deltas promote the retention of high volumes of sand on the aggrading shelf and coastal plain, and thus potentially have a reduced sand budget available for delivery to the deeper water areas. Clinoform 17, one of a series of eastward-prograding, shelf-margin clinoforms from the Eocene Battfjellet Formation on West Spitsbergen, contains a sand-rich delta complex sited near the clinoform shelf-slope rollover, and is argued to be a highstand (rising relative sea level) shelf-margin delta based on: (1) its highly aggradational architecture shown by an unusual (compared to other clinoforms) regressive unit thickness and its marked stacking of parasequences, (2) coeval accumulation of delta-plain and lagoonal deposits that are well-preserved in the landward reaches of the same clinoform, and (3) its context within a mappable, longer-term rising shelf-edge trajectory (through 5 clinoforms). It is likely that the delta reached its shelf-edge location because the shelf was narrow (less than 20 km), and not because of high sediment supply or relative sea-level fall. The delta system was markedly wave-dominated as might be predicted at a shelf-edge site. The sand-rich, shelf-edge portion of Clinoform 17 consists of (1) a 30–35 m thick regressive deltaic unit with offshore mudstones and thin tempestite layers, wave-dominated delta-front sandstones, and tidal–fluvial-distributary channels on the delta topsets, (2) an overlying 15–23 m thick, aggrading-to-transgressive shoreface/barrier unit with associated tidal-inlet/estuarine channel-fill deposits, and (3) an uppermost, <20 m thick regressive deltaic unit similar to (1). The slope successions of the units described in (1) and (3), beyond and below the shelf-edge, contain thin upper-slope tempestite sheet sandstones, within an otherwise shale-dominated environment. Neither sandy slope channels nor basin-floor fans are observed within the otherwise shale-prone deepwater segments of the clinoform. [Copyright &y& Elsevier]

Shelf-margin or shelf-edge deltas are a common constituent of Quaternary shelves, where their genetic link to falling and lowstand sea level has been well established, but they are rarely reported from older successions. Recognition of this delta type in the ancient record is nevertheless important, because (1) it helps to position the fossil shelf edge, (2) it provides insight into how sand budgets were partitioned between shelf edge, slope and basin-floor settings, and (3) it can contribute to the debate concerning three versus four systems-tract systematics and the positioning of a sequence boundary.Shelf-margin deltas create wide (tens of kilometers), high (hundreds of meters) and steep (3–6°) clinoforms, because they build across the relatively deepwater shelf margin. Such bodies tend to form strike-elongate wedges that initially thicken basinwards, attaining a maximum thickness of 50–200 m just outboard of the shelf edge, then thin on the middle/lower slope, whereas they commonly pinch out landwards by lapping back onto shelf shales. Prior to reaching the shelf edge, small-scale (tens of meters), tangential foresets (usually <3° slope) of the shelf delta downlap onto the preexisting outer-shelf surface, whereas below the shelf edge, the shelf-margin deltaic foresets become longer, steeper (3–6°) and more turbidite prone, and they downlap onto the preexisting slope of the shelf margin. The top of the shelf-edge delta package is a sharp, planar to incised surface that truncates the delta increasingly landwards, merging eventually with the basal downlap surface. The clinoform series created by this transition from outer-shelf to shelf-margin deltas tends to have a horizontal-to-downward trajectory and, eventually, in the most distal bundles, an aggrading to backstepping trajectory as relative sea level begins to rise.The key facies association in shelf-edge deltas is the mouth bar-to-delta front association. Mouth-bar facies, landwards of the shelf edge, consist mainly of thick, clean, flat to low-angle and ripple-laminated medium to fine sands. Basinwards from the shelf edge, such sands commonly alternate with heterolithic slumped unit, creating characteristic slumped to laminated couplets. Because the delta front is superimposed on a preexisting, steep and extended shelf margin, it commonly contains (beyond the mouth bars) thick successions (up to many tens of meters) of sandy, slope turbidites. Shelf-margin deltas differ from inner shelf deltas in showing: (1) an order of magnitude higher clinoforms (hundreds instead of tens of meters), and strike-elongated, locally pod-like sand bodies commonly affected and augmented by growth faulting; (2) paleoecological evidence of abrupt shallowing (foreshortened stratigraphy); (3) turbidite-prone delta-fronts; (4) larger scale and greater abundance of slope-controlled soft-sediment deformation and (5) the general absence of a paralic ‘tail’ along the trailing edge of the delta front.Shelf-margin deltas can be classified into two types. Stable shelf-margin deltas are usually tens of meters thick and are not associated with shelf-edge incision or major slope collapse/disruption features. Slope turbidites are common and occur as unconfined sheets and lobes. Such deltas form when relative sea level falls no lower than the level of the shelf platform or, after longer sea-level falls, when rivers reestablish at the shelf edge on the rise. Unstable shelf-margin deltas are associated with large-scale slope collapse, listric growth faults and sometimes with salt diapirs. Slides are common on the upper slope, whereas imbricated sediment packages and compressional ridges affect the slope toe. Slope turbidites can be ponded within structurally controlled mini-basins on the slope. Such deltas are often related to unusually great sand influx to the shelf edge, or simply to prolonged or large fall of relative sea level below the shelf edge.The diachronous erosional unconformity atop the shelf-to-shelf-edge deltaic wedge is viewed by some researchers as the most easily recognizable and persistent surface on the shelf and slope. This surface contrasts with the downlap erosional surface developed from the beginning of sea-level fall, which is viewed by other researchers as the key boundary surface within this complex. It should be noted that sea level can fall for a long period of time (tens of thousands of years) before deltas even reach the shelf margin and, therefore, before significant volumes of sand are delivered across the shelf break. Hence, the time of incision of the shelf edge and emplacement of deepwater sand is commonly long after the initial fall so the time of (maximum) relative lowstand of sea level may be a more practical choice for the timing of the sequence boundary.Recognition of shelf-margin deltas and analysis of their architecture help in the prediction of presence or absence of basin-floor fans. Deltaic complexes that are aggradational to backstepping, downlap onto disrupted or complex slopes, and overlie an incised shelf and shelf edge, predict that there should be basin-floor fans present. In contrast, delta complexes that show a prolonged and preserved progradational to downstepping architecture implies the presence of turbidite accumulations on the slope but not on the basin floor. [Copyright &y& Elsevier]

Abstract Although shelf-edge deltas are well-imaged seismic features of Holocene and Pleistocene shelf margins, documented outcrop analogues of these important sand-prone reservoirs are rare. The facies and stratigraphic architecture of an outcropping shelf-edge delta system in the Eocene Battfjellet Formation, Spitsbergen, is presented here, as well as the implications of this delta system for the generation of sand-prone, shelf-margin clinoforms. The shelf-edge deltas of the Battfjellet Formation on Litledalsfjellet and Høgsnyta produced a 3–5 × 15 km, shelf edge-attached, slope apron (70 m of sandstones proximally, tapering to zero on the lower slope). The slope apron consists of distributary channel and mouth-bar deposits in its shelf-edge reaches, passing downslope to slope channels/chutes that fed turbiditic lobes and spillover sheets. In the transgressive phase of the slope apron, estuaries developed at the shelf edge, and these also produced minor lobes on the slope. The short-headed mountainous rivers that drained the adjacent orogenic belt and fed the narrow shelf, and the shelf-edge position of the discharging deltas, made an appropriate setting for the generation of hyperpycnal turbidity currents on the slope of the shelf margin. The abundance of organic matter and of coal fragments in the slope turbidites is consistent with this notion. Evidence that many of the slope turbidites were generated by sustained turbidity currents that waxed then waned includes the presence of scour surfaces and thick intervals of plane-parallel laminae within turbidite beds in the slope channels, and thick spillover lobes with repetitive alternations of massive and flat-laminated intervals. The examined shelf-edge to slope system, now preserved mainly below the shelf break and dominated by sediment gravity-flow deposits, has a threefold stratigraphic architecture: a lower, progradational part, in which the clinoforms have a slight downward-directed... [ABSTRACT FROM AUTHOR]

Outcrops of the early-mid Pliocene Orinoco Delta succession on the Atlantic-facing Trinidad continental margin demonstrate facies and stratigraphic architecture of a shelf margin developed in a setting of high sediment supply (Orinoco River) and unusually rapid, sediment accumulation/subsidence (averaging >1 km/My). The studied Moruga clastic wedge of the Orinoco margin consists of a series of topset to deepwater clinoforms with channelized sandstones, river- and mixed-process delta front, wave-dominated delta front, shoreface, prodelta-offshore transition-shelf, upper-slope turbidite channels and mass-transport deposits. The succession is sand-prone and generally consists of coarsening- and thickening-upward delta-front lobes with hummocky/swaley and wave-ripple stratified sandstones indicating dominant storm wave reworking of the delta, particularly when the delta lobes had reached outer shelf sites after long shelf transits. The lower parts of distributary channels are well preserved and characterized by cross-bedded to parallel-laminated sandstones with mud clast lags. The upper parts of channel fills were wave reworked during transgressions, creating hummocky/swaley cross-stratified strata. River-dominated and mixed-process (wave and tide) delta-front deposits occur commonly within stacked, coarsening-upward units. The river-dominated delta front deposits exhibit parallel-laminated to current ripple-laminated intervals with normally graded thin beds changing upward to parallel and low-angle cross lamination, but mixed-process delta front deposits also contain wave and bidirectional current ripples as well as discrete intervals of hummocky/swaley cross-stratification. The preservation of the channel-base as well as river- and mixed-process delta-front deposits in the otherwise storm wave-dominated Atlantic-facing setting suggests some sheltering of the coastal morphology, and possibly low-frequency of large storms, but it is more likely that the unusually rapid subsidence (at rates competing well with Icehouse rates of eustatic sea-level change) and burial of deposits may have prevented the fluvial and tidal signals from being completely reworked by the storm waves in the studied outcrop area. This study demonstrates an example of how the interactions of river-, tide- and wave-generated facies impacted stratigraphic architecture of a rapid subsiding shelf margin built by shelf-edge deltas with high sediment supply. [ABSTRACT FROM AUTHOR]

Many modern deltas exhibit a compound geometry that consists of a shoreline clinoform and a larger subaqueous clinoform connected through a subaqueous platform. Despite the ubiquity of compound clinoforms in modern deltas, very few examples have been documented from the ancient sedimentary record. We present recognition criteria for shelf compoundclinoform systems in both tide- and wave-dominated deltas by integration of ancient and modern examples from multiple types of data. The compound clinothem can be identified by using a combination of: (1) the three-dimensional (3-D) configuration identified in bathymetric or seismic data, (2) bipartite stacked regressive units, consisting of a lower muddy coarsening-to-fining-upward (CUFU) or coarsening-upward (CU) unit (30-100 m thick) and an overlying sandier CU unit (5-30 m thick) (together they represent the subaqueous and shoreline clinoform pair), and (3) distinct facies described herein, though both types of delta have highly bioturbated mudstone and siltstone bottomsets. Tide-dominated deltas have muddy foresets with tidal scours containing tidal rhythmites or inclined heterolithic strata in the subaqueous clinothem overlain by river and tidal deposits of the shoreline clinothem. Wave-dominated deltas show mainly wave-enhanced sediment-gravity-flow (WSGF) beds and some thin hummocky/swaley cross-stratified (HCS/SCS) sandstones toward the top in the subaqueous muddy foreset, and upward-thickening HCS/SCS and trough/planar cross-bedded sandstones interbedded with siltstones in the shoreline clinothem. The subaqueous platform, which links the clinoform couplet, shows evidence of frequent tidal or wave reworking and redeposition. The platform in tide-dominated deltas is characterized by tide-generated heterolithic strata (e.g., bidirectional current-rippled and cross-stratified sandstones, spring and neap tidal bundles, tidal rhythmites) with occasional storm-wave--influenced strata. In contrast, the wave-dominated platform comprises small-scale swales with scours and mud clasts and some WSGF deposits. The proposed criteria can aid in the recognition of compound deltaic clinothems in other basins, particularly those with limited amounts and/or types of data. [ABSTRACT FROM AUTHOR]

Spatial and temporal changes in physical depositional processes of shallow-marine deltas affect facies distributions, which are critical for reconstructing paleogeography and predicting reservoir heterogeneities. The updated interpretation of the classical ancient 'river-dominated' Lower Wilcox delta in the Gulf of Mexico suggests more significant process variations than previously suggested. The dominance of the storm-wave process is indicated by wave-ripple laminae and combined-flow ripple laminae in the fair-weather intervals, and hummocky/swaley cross-stratification and low-angle laminae in the storm beds. However, the relatively simple alternation of fair-weather beds and storm beds is complicated by the presence of: 1) normal and inversely graded siltstone to very fine-grained sandstone beds, suggesting hyperpycnites generated during the river flooding season; 2) mud-dominated heterolithics with delicate mud rhythmites, suggesting tidal modulation between the storm events. The storm-wave generated succession, with preservation of both river-flood and tidal signals during fair-weather condition, represents regressive shoreline to-shelf outbuilding. The wave-dominated delta or shoreface deposits are commonly truncated by fluvial-dominated, tide-influenced distributary channels, then overlain by tidal heterolithics and gradually open-shelf deposits, suggesting the development of small estuaries during transgressive phases of the Wilcox shoreline. The interpreted mixed-process delta is linked to the last growth stage of the Lower Wilcox Group, prior to inundation of the Texas shelf by thick marine mudstones. Its depositional setting was characterized by the widest Lower Wilcox shelf (>50 km) and a weakening sediment supply, resulting in a stronger wave and tide influence, relative to a weak fluvial influence. This work thus provides a well-constrained example of the complex interplay of fluvial, wave, and tidal currents in a process stratigraphy where decreasing river flux and relative sea level rise allowed wave reworking to become the dominant theme, even though the same system has been previously interpreted as a classic river-dominated delta. Many existing interpretations of ancient deltas thought to have mainly been controlled by a single process may need to be revisited and revised to take into account the natural spatial and temporal variability inherent to the shallow marine realm. [ABSTRACT FROM AUTHOR]