ABSTRACT

Progradation of deltaic systems that reach the shelf edge is considered a primary mechanism to deliver sand to continental margins. Sediment bypass processes can dominate the outer shelf to upper slope transition, causing poor preservation of reservoir quality sandstones. Turbidites can carry the bulk of the coarse fraction downdip where a thicker sand pile can be deposited once the channel-to-lobe transition is reached. Predicting this lateral and vertical variability along the slope is challenging. This configuration can be further complicated when mixed siliciclastic–carbonate systems are present. The Roseway–Missisauga case study from Nova Scotia is used here to explore potential implications associated with the development of deep-water turbidites that are time equivalent to outer-shelf mixed siliciclastic–carbonate units. Two scenarios are possible: (1) The carbonate factory is dominant, and the development of carbonate reefs and pinnacles on the outer shelf prevents the passage of siliciclastic systems beyond the shelf break; in this case, the siliciclastic component is sequestered within the inner-outer shelf. (2) Favorable conditions for carbonate production gradually deteriorate by the activation of fluviodeltaic systems that prograde outboard reaching the slope region. In this last scenario, low relief and lateral discontinuous carbonate shoals are ubiquitous in the outer shelf representing the last outboard remnants of the carbonate factory. Shelf-edge deltas circumvent or breach these carbonate shoals, establishing sedimentary pathways on the shelf-break region that connect with deep-water turbidites. Observations suggest that this last scenario is the most likely in this part of the Scotian margin during the Late Jurassic to Early Cretaceous.

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