Upper-Slope to Shelf-Edge Delta Architecture of the Miocene Cruse Formation Orinoco Shelf-Margin, Trinidad

Abstract

Trinidad is an uplifted segment of the Orinoco Neogene shelf. The Atlantic-facing shelf-margin sediment prism has an internal clinoform architecture, with both sandy marine topsets and muddy deepwater slope deposits with turbidite channels and collapsed shelf blocks. The shelf prism is several km thick and >200km wide, built from late Miocene to present. The shelf-margin had an irregularly rising trajectory with very thick topset aggradation and rapid progradation of the fronting deepwater slope. High sediment supply (18–33km/My progradation rate) and exceptionally high shelf-subsidence rates (up to 600m/My) led to prominent sand bypass from shelf into deepwater areas. Trinidad outcrops expose the proximal half of this sediment prism including Miocene deepwater strata. Cruse Fm. outcrops expose 10s of km in a downdip (shelf to basin) direction from west to east along southern Trinidad. This outcrop distribution allows the shelf-break position to be identified, separating a western shelf and shelf-edge delta segment from an eastern highly deformed segment with very large (house size) blocks of shallow-water facies that are disoriented and embedded in deformed mudstones. Below the shelf break, the facies are mainly very fine sandstone shelf-edge collapse blocks, as well as turbidite-filled slope channels and slope mudstones. Most of the sandstone blocks and associated chaotic beds contain highly deformed parallel-laminated and hummocky cross strata. Landward of the shelf-edge area, the facies are mainly stacked parasequences of undeformed, upward-coarsening shelf-edge delta deposits (3–15m thick), in places sharply truncating (toplap) the slope mudstones and mass transport deposits. The great thickness and downcutting of most of the topset channels and the repeated transits of the 30–40m thick delta cycles may indicate forced regressive deltas that were driven across the shelf by falling relative sea level at this time. The facies architectures, both vertically and laterally (from distal to proximal) provide two hypotheses for the Cruse Fm. clinoform sets morphology as well as facies patterns: (1) a simple rising trajectory with upward-growing clinoforms, or (2) a rising and then flat (forced regressive) trajectory combination. The Cruse Fm. clinoform sets record not only the position of the growing shelf edge, but also provide a quantitative indicator that up to two-thirds of the total sediment flux budget was by-passing the shelf edge into deepwater areas.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015