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The Continental Shelf as a Sedimentary Conveyor or Filter? The Role of Topset Process Regime in Controlling Sediment Distribution Patterns in Clinothems

Abstract

Clinothem successions are valuable archives of basin-margin evolution; the trajectories of successive clinoform rollovers are widely applied to predict patterns of spatio-temporal sand distribution. However, quantitative changes in grain character (including grain size, grain shape, sorting, and sand-to-mud-ratios) across complete clinothem systems remains poorly constrained and largely unquantified; as such, predicting the distribution of reservoir quality sand remains challenging. Novel, quantitative grain character datasets are presented for core (Miocene intrashelf clinothems, offshore New Jersey) and outcrop (Eocene clinothems, Sobrarbe Deltaic Complex, Spain) case studies, which target topset, foreset and bottomset deposits of individual clinothems. The examples presented here suggest that across successive clinothems, the distribution of sand is partly determined by the dominant process regime in operation at the shelf edge, not clinoform trajectory alone. Under river dominated process regimes, coarse grained sediment is effectively conveyed downdip under both rising and falling rollover trajectories; under wave dominated process regimes, coarse grained sediment is effectively filtered and retained in the shelf setting. Within individual clinothems, autogenic variability in topset process regime exerts a quantifiable effect on grain character across the complete depositional profile; influencing flow-style beyond the clinoform rollover, the lateral distribution of facies, and the resulting reservoir quality of deposits. The grain character datasets comprise unique databases of grain size, grain shape, and sorting statistics; they have been used to: i) quantitatively define intra-clinothem surfaces (timelines) at higher resolution than is possible using chronostratigraphic techniques; ii) refine the placement of sequence boundaries; and, iii) develop a more comprehensive model of clinothem evolution, in which the nature of the flows and the dominant process regime in operation at the shelf edge control the downdip and temporal distribution of sand and mud. The results presented here can be applied to test and refine numerical forward models that seek to improve prediction of reservoir characteristics in both mature and frontier hydrocarbon basins.