Transition from storm wave‐dominated outer shelf to gullied upper slope: The mid‐Pliocene Orinoco shelf margin, South Trinidad

Abstract

AbstractShelf‐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.