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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]

Abstract The stratigraphic response to marine shoreline transgressions have been debated for the last six decades. A particularly interesting aspect is that transgressive ravinement often removes evidence of the shoreline itself, until there is turnaround to regression. To elucidate this topic, we present a high-resolution chronostratigraphic study of an overall transgressive coastline, documented with evidence from over 1450 wells covering an area of 6200 km2 to define the fundamental sandbodies and relevant stratigraphic surfaces. The mapping of 15 elongate sandstone units stacked into 6 reservoirs shows that there was a spectacular westward rising of back-stepping barrier spits, sourced by long-shore currents mobilizing sand from a delta to the north. Each fundamental barrier spit unit (BSU) is a 6–12 metres thick, regressive, southward elongate and narrowing sandbody generated by longshore drift of sandy sediment. Each barrier sand body and its related overlying transgression is estimated to represent some 20 Ky. Strong transgressive ravinement events periodically flooded the barrier spits and shifted the deposition westward in four to 35 km steps. Westward transgressive-stepping across and into a structurally-generated embayment progressively dampened wave energy with each backstep. Morphologically, there were two endmember sandbody types: 1) narrow, drum stick-like sandbodies built mostly by longshore accretion, i.e. true barrier bars, and 2) broader and more extensive regressive belts of sandstone that resemble strandplains. The former display typically longer backsteps across lagoonal areas whereas the latter show a more aggradational stacking arrangement which evidences a quasi-balance between accommodation and sediment supply. There is no evidence of a smooth and continuous transgression controlling sandstone deposition and preservation. A hybrid punctuated transgression model, that considers the alternation of long-term coastal retrogradation and short-lived barrier spit development by longshore accretion and seaward progradation, is the most appropriate model for creating and preserving Upper Almond sandbodies. Highlights • This research involves the analysis of 1450 wells over an area of 6600 km2 in the Upper Almond Formation. • The individual sandbody mapping shows a spectacular and landward rising of back-stepping barrier spits. • The paper reviews the various accepted mechanisms for barrier migration during transgression. • Authors conclude that there is no evidence of continuous transgression controlling sandstone deposition and preservation. • An "hybrid punctuated" transgression the most appropriate model for creating and preserving Upper Almond sandbodies. [ABSTRACT FROM AUTHOR]

Some potentially attractive reservoirs, containing anomalously thick (10s to a few 100 m), cross-stratified sandstone, have been locally encountered within both the classic regressive (lower Brent) and the transgressive (upper Brent) segments of the Brent Delta. Three documented cases of these sandstone bodies are re-examined. They are internally dominated by simple or compound dunes, and typified by two types of deepening-upward succession, recording a retrogradational or transgressive shoreline history. Type I is expressed as a single estuarine succession changing upwards from erosive, coarse-grained channelized deposits into outer estuary tidal bar deposits. The estuary is underlain and overlain by deltaic deposits. Type II lacks significant basal river deposits but is composed by stacked mixed-energy and tide-dominated estuarine deposits. It is underlain by deltaic deposits and overlain by open marine sediments. Considering the structural evolution in the northern North Sea basin, we suggest (as did some earlier researchers) that these sandstone bodies were local, but sometimes broad transgressive estuaries, formed at any time during large-scale Brent Delta growth and decay. The estuary generation was likely triggered by fluvial incision coupled with active faulting, producing variable accommodation embayments, where tidal currents became focused and deposition became transgressive. The spatial variations of the interpreted estuary deposits were linked with variable, fault-generated accommodation. The relatively simple, lower Brent estuarine units were created by short-lived, fault activity in places, whereas the complex, stacked upper-Brent estuarine units were likely a result of more long-lived, punctuated fault-induced subsidence leading into the northern North Sea main rifting stage. The thick cross-stratified units potentially accumulated in the hangingwall of large bounding faults. [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]

Shelf ridges are sedimentary bodies formed on the continental shelf due to transgressive reworking (tidal or storm) of lowstand deposits. Common on modern shelves, they are under-represented in the geological record due to a lack of recognition criteria and facies model. This article proposes a new facies and architectural model for shelf ridges, linked to their inception-evolution-abandonment cycle and the process regime of the basin. The model is mainly based on new outcrop data and interpretations from three sandstone bodies of the Almond Formation, an overall transgressive interval during the infill of the Campanian Western Interior Seaway. Building from the case study, and ancient and modern examples, six characteristics are proposed for the recognition of ancient shelf ridges. Shelf ridges: (i) are encased between thick marine mudstone intervals; (ii) have a basal unconformity that erodes into marine muds or into the remnants of a previous shoreline; (iii) have a non-erosional upper boundary that transitions into marine muds; (iv) are characterized by clean and well-sorted sandstones, often cross-bedded; (v) contain fully marine ichnofauna; and (vi) present compound architectures with large accretion surfaces and lower order structures. Although shelf ridges have been described in previous studies as generated exclusively by either tidal or storm currents, it is clear, from modern examples and the case study, that these two processes can be recorded and preserved in a single shelf ridge. The stratigraphy of these sandstone bodies is therefore much more complex than previously recognized, bearing the signature of changing tidal and storm intensity through time. Because they are developed during transgressions, shelf ridges are commonly subject to strong changes in process regime as sea-level changes can easily affect the oceanographic conditions and the morphology of the basin. For this reason, shelf ridges can provide the best record of shelf process variability during transgressions. [ABSTRACT FROM AUTHOR]

The Devonian Donghetang Formation is one of the important extensive marine sandstone units for oil/gas exploration on Tabei Uplift of the Tarim Basin, particularly in the Hade and Halahatang oilfields. However, not all of this sandstone is good quality reservoir, and so the sedimentary facies are critical to prediction of reservoir in the area. Based on an analysis of logs, core, outcrop and seismic data, a new hypothesis is proposed for the Donghetang Formation as a 2nd or 3rd order stratigraphic sequence (SQ_Ddh), formed between unconformities Tg22 (top) and Tg3 (bottom), consisting of local transgressive (braided delta deposits) and extensive high-stand (foreshore and shoreface deposits) system tracts. Through networked seismic data, integrated with erosion and decompaction correction, the palaeogeomorphology during development of SQ_Ddh has been reconstructed and several geomorphological units can be distinguished. The sedimentary facies and distribution of SQ_Ddh were related closely to the depositional geomorphology. Between the margin of the northern paleo-uplift zone and the northwestern and northeastern depressions there developed braided deltas during the period of the early TST, and wave-dominated shorelines during the HST period when extensive shorefaces developed across the central low-uplift zone. Beyond the southeastern shoreline, there developed offshore and marine-shelf deposits. The foreshore and delta-front mouth bar sandstones constitute the most favorable reservoir, surrounding the paleo-uplift and the marginal uplift zones. [ABSTRACT FROM AUTHOR]

Abstract: The Lower Eocene Baronia Formation in the Ager Basin is interpreted as a series of stacked compound dunes confined within a tectonically generated embayment or tidal seaway. This differs from the previous interpretation of lower Baronia sand bodies as tidal bars in the front of a delta. The key architectural building block of the succession, the deposit of a single compound dune, forms a 1–3m-thick, upward coarsening succession that begins with highly bioturbated, muddy, very fine to fine grained sandstone that contains an open-marine Cruziana ichnofacies. This is overlain gradationally by ripple-laminated sandstone that is commonly bioturbated and contains mud drapes. The succession is capped by fine- to coarse-grained sandstones that contain both planar and trough cross-strata with unidirectional or bi-directional paleocurrent directions and occasional thin mud drapes on the foresets. The base of a compound dune is gradational where it migrated over muddy sandstone deposited between adjacent dunes, but is sharp and erosional where it migrated over the stoss side of a previous compound dune. The cross strata that formed by simple superimposed dunes dip in the same direction as the inclined master bedding planes within the compound dune, forming a forward-accretion architecture. This configuration is the fundamental reason why these sandbodies are interpreted as compound tidal dunes rather than as tidal bars, which, in contrast, generate lateral-accretion architecture. In the Baronia, fields of compound dunes generated tabular sandbodies 100s to 1000s of meters in extent parallel to the paleocurrent direction and up to 6m thick that alternate vertically with highly bioturbated muddy sandstones (up to 10m thick) that represent the low-energy fringes of the dune fields or periods of high sea level when current speeds decreased. Each cross-stratified sandstone sheet (compound-dune complexes) contains overlapping lenticular “shingles” formed by individual compound dunes, separated by 10–30cm of bioturbated muddy sandstone, which migrated over each other in an offlapping, progradational fashion. Each compound-dune complex (the best reservoir rock) thins as it downlaps, at average rates of 3–4m/km in a dip direction. These reservoir units can be comprised of discrete compartments, each formed by a single compound dune, that extend for 500–1000m in the direction of the current, and are at least 350–600m wide in a flow-transverse direction. Distinguishing between tidal bars and tidal dunes in an ancient tidal succession can be difficult because both can contain similar cross-bedded facies and have overlapping thicknesses; however, the internal architecture and sandbody orientations are different. Tidal bars have their long axis almost parallel both to the tidal current direction and to the strike of the lateral-accretion master surfaces. In inshore areas, they are bounded by channels and fine upward. Large compound tidal dunes, in contrast, have their crest oriented approximately normal to the tidal currents and contain a forward-accretion architecture. Coeval channels are uncommon within large, sub-tidal dune fields. The above distinctions are very important to reservoir description and modeling, because the long axis of the intra-reservoir compartments in the two cases will be 90° apart. [Copyright &y& Elsevier]

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]