Lateral variability of shelf-edge and basin-floor deposits, Santos Basin, offshore Brazil

ABSTRACT Construction of continental margins is driven by sediment transported across the shelf to the shelf edge, where it is reworked by wave, tide, and fluvial processes in deltas and flanking clastic shorelines. Stalling of continental-margin progradation often results in degradation of the outer shelf to upper slope, with resedimentation to the lower slope and basin floor via a range of sediment gravity flows and mass-movement processes. Typically, our understanding of how these processes contribute to the long-term development of continental margins has been limited to observations from broadly two-dimensional, subsurface and outcrop datasets. Consequently, the three-dimensional variability in process regime and margin evolution is poorly constrained and often underappreciated. We use a large (90 km by 30 km, parallel to depositional strike and dip, respectively) post-stack time-migrated 3D seismic-reflection dataset to investigate along-strike variations in shelf-margin progradation and outer-shelf to upper-slope collapse in the Santos Basin, offshore SE Brazil. Early Paleogene to Eocene progradation of the shelf margin is recorded by spectacularly imaged, SE-dipping clinoforms. Periodic failure of the outer shelf and upper slope formed ca. 30-km-wide (parallel to shelf-margin strike) slump scars, which resulted in a strongly scalloped upper-slope. Margin collapse caused: 1) the emplacement of slope-attached mass-transport complexes (MTCs) (up to ca. 375 m thick, 12+ km long, 20 km wide) on the proximal basin floor, and 2) accommodation creation on the outer shelf to upper slope. This newly formed accommodation was infilled by shelf-edge-delta clinoforms (up to 685 m thick), that nucleated and prograded basinward from the margin-collapse headwall scarp, downlapping onto the underlying slump scar and/or MTCs. Trajectory analysis of the shelf-edge deltas suggests that slope degradation-created accommodation was generated throughout the sea-level cycle, rather than during base-level fall as would be predicted by conventional sequence-stratigraphic models. Our results highlight the significant along-strike variability in depositional style, geometry, and evolution that can occur on this and other continental margins. Coeval strata, separated by only a few kilometers, display strikingly different stratigraphic architectures; this variability, which could be missed in 2D datasets, is not currently captured in conventional 2D sequence stratigraphic models.

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The arrival of the paleo–Orinoco Delta at Trinidad: The Cruse Formation delta lobes and delivery to deepwater Atlantic

Journal of Sedimentary Research ◽

10.2110/jsr.2020.47 ◽

2020 ◽

Vol 90 (8) ◽

pp. 938-968

Author(s):

Ariana Osman ◽

Ronald J. Steel ◽

Ryan Ramsook ◽

Cornel Olariu ◽

Si Chen

Keyword(s):

Sea Level ◽

Late Miocene ◽

Sediment Supply ◽

Average Height ◽

Shelf Edge ◽

Sea Level Changes ◽

Orinoco Delta ◽

Basin Floor ◽

Shelf Margin ◽

High Sediment

ABSTRACT Icehouse continental-shelf-margin accretion is typically driven by high-sediment-supply deltas and repeated glacio-eustatic, climate-driven sea-level changes on a ca. 100 ky time scale. The paleo–Orinoco margin is no exception to this, as the paleo–Orinoco River Delta with its high sediment load prograded across Venezuela, then into the Southern and Columbus basins of Trinidad since the late Miocene, depositing a continental-margin sedimentary prism that is > 12 km thick, 200 km wide, and 500 km along dip. The Cruse Formation (> 800 m thick; 3 My duration) records the first arrival of the paleo–Orinoco Delta into the Trinidad area. It then accreted eastwards, outwards onto the Atlantic margin, by shallow to deepwater clinoform increments since the late Miocene and is capped by a major, thick flooding interval (the Lower Forest Clay). Previous research has provided an understanding of the paleo–Orinoco Delta depositional system at seismic and outcrop scales, but a clinoform framework detailing proximal to distal reaches through the main fairway of the Southern Basin has never been built. We integrate data from 58 wells and outcrop observations to present a 3-D illustration of 15 mapped Cruse clinoforms, in order to understand the changing character of the first Orinoco clastic wedge on Trinidad. The clinoforms have an undecompacted average height of 550 m, estimated continental slope of 2.5° tapering to 1°, and a distance from shelf edge to near-base of slope of > 10 km. The clinoform framework shows trajectory changes from strong shelf-margin progradation (C10–C13) to aggradation (C14–C20) and to renewed progradation (C21–24). Cruse margin progradational phases illustrate oblique clinothem geometries that lack well-developed topsets but contain up to 70 m (200 ft) thick, deepwater slope channels. This suggests a high supply of sediment during periods of repeated icehouse rise and fall of eustatic sea level, with fall outpacing subsidence rates at times, and delivery of sand to the deepwater region of the embryonic Columbus channel region. Also, evidence of wholesale shelf-edge collapse and canyon features seen in outcrop strongly suggest that deepwater conduits for sediment dispersal and bypass surfaces for Cruse basin-floor fans do exist. The change to a topset aggradational pattern with a rising shelf trajectory may be linked to increased subsidence associated with eastward migration of the Caribbean plate. The Cruse-margin topsets were dominated by mixed fluvial–wave delta lobes that were effective in delivery of sands to the basin floor. The preservation of a fluvial regime of the delta may have been impacted by basin geometry which partly sheltered the area from the open Atlantic wave energy at the shelf edge. Ultimately, understanding shelf-edge migration style as well as process-regime changes during cross-shelf transits of the delta will help to predict the location of bypassed sands and their delivery to deepwater areas.

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Upper-Slope To Shelf-Edge Delta Architecture, Miocene Cruse Formation, Orinoco Shelf Margin, Trinidad

Journal of Sedimentary Research ◽

10.2110/jsr.2016.7 ◽

2016 ◽

Vol 86 (2) ◽

pp. 87-106 ◽

Cited By ~ 16

Author(s):

Si Chen ◽

Ronald J. Steel ◽

Cornel Olariu

Keyword(s):

Shelf Edge ◽

Shelf Margin ◽

Upper Slope

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Basin margin sediment wedge build out of the Eastern Niger Delta: application of shelf-edge trajectory pattern studies

Journal of Petroleum Exploration and Production Technology ◽

10.1007/s13202-021-01100-w ◽

2021 ◽

Vol 11 (3) ◽

pp. 1093-1100

Author(s):

Okwudiri A. Anyiam ◽

Nicholas Hoggmascall ◽

Daniel K. Amogu

Keyword(s):

Niger Delta ◽

Sudden Change ◽

Sediment Flux ◽

Outer Shelf ◽

Shelf Edge ◽

Structural Style ◽

Shelf Slope ◽

Trajectory Pattern ◽

Shelf Margin ◽

Basin Margin

AbstractThe understanding of how basin margin sediment wedge builds out causes shelf-edge migration with time is approached based on shelf-edge trajectory pattern analysis using a high-resolution mega-merge seismic data from the eastern Niger Delta, Nigeria. The study focuses on a seismic dip transect traversing the Greater Ughelli, Central Swamp, Coastal Swamp and the Shallow Offshore Depobelts of the Niger Delta. On the regional dip transects, shelf-edge sediments occur as clinoform-bearing wedges at and immediately updip of the shelf-slope break. The shelf edge is deeply buried (> 2–4 s, twt), around the Greater Ughelli and Central Swamps. But with changing structural style, sudden change of ascending shelf edge around the Central Swamp was observed. The huge listric growth fault in the Coastal Swamp; around Bonny area, once again cut the shelf edge into half, rotated it along the listric fault and buried it distally. Several depositional packages show low to moderate ascending shelf-edge trajectory with progradational to aggradational clinoform growth that is characterized by thin sand sheets across most of the shelf and upper slope, though few are also characterized by progradational clinoform growth with thick sand on the shelf, upper-tolower slope and basin floor. The deposition is usually on the Outer Shelf Terrace (OST) which is regressive in a flat and rising trajectory style. This study has demonstrated that accommodation and sediment flux are the dominant controls on how the study basin’s sediment wedge built out, whereby limited accommodation promotes sediments with significant shelf-edge advance and descending trajectories, while increasing accommodation promotes ascending trajectories and increased deposition on the outer shelf. The greater sediments on the Outer Shelf Terrace and the shelf margin than on the slope gives more hydrocarbon prospectivity search around the outer shelf and shelf margin.

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