UPPER-SLOPE TO SHELF-EDGE DELTA ARCHITECTURE, MIOCENE CRUSE FORMATION, ORINOCO SHELF MARGIN, TRINIDAD

摘要

Trinidad is an uplifted segment of the Neogene Orinoco shelf, created largely by the paleo-Orinoco River and delta. The Atlantic-facing shelf-margin sediment prism has an internal clinoform architecture, with both sandy marine and nonmarine topsets and muddy deepwater slope deposits that contain turbidite channels and collapsed shelf-edge blocks. The shelf prism is. 10 km thick and. 200 km wide at present, built from late Miocene to present. The shelf margin had an irregularly rising trajectory towards the Atlantic, with very thick topset aggradation and rapid progradation of the fronting deepwater slope. The 18-33 km/My progradation rate for the shelf margin with exceptionally high shelf-subsidence rates (up to 1000 m/My) documents a high sediment supply, despite very high shelf subsidence, and strongly suggests prominent sand bypass from shelf into deepwater areas for much of the Neogene interval. The early proximal, onshore south Trinidad part of this sediment prism, the late Miocene and early Pliocene Cruse Formation, irregularly outcrops for tens of kilometers along southern Trinidad in a disrupted but near-downdip (shelf to basin) direction from west to east. This distribution allows the late Miocene shelf-break position in the paleo-Columbus Channel to be identified in the outcrops, 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, collapsed from the shelf edge, that are disoriented and embedded in deformed slope mudstones. These eastward distorted outcrops are interpreted as within the headward reaches of a Columbus Canyon system, otherwise known from seismic interpretation offshore in the Columbus Channel. In this eastward outcrop area below the shelf edge, the facies are mainly large, shelf-edge collapse mass-transport blocks, as well as in situ turbidite-filled slope channels, thin-bedded turbidites, debrites, and abundant slope mudstones. Most of the sandstone blocks and associated chaotic beds contain highly deformed parallel-laminated and hummocky cross strata, betraying their former shelf location. Landward of the identified shelf-edge location, there are stacked parasequences (each 3-15 m thick) of undeformed, upward-coarsening shelf-edge delta deposits, in places sharply truncating (erosional truncation) the slope mudstones and mass-transport deposits. The significant downcutting of most of the topset Orinoco channels, their position so close to the shelf edge, as well as the generally erosional contact between base of topsets and underlying slope mudstones suggests that the topset deltas were forced regressive, probably driven across the shelf by falling relative sea level, despite high subsidence rates at this time. The outcropping facies architectures, both vertically and laterally (from shelf to deepwater slope) generate a hypothesis that the early paleo-Orinoco shelf margin grew by an alternation of upward-and basinward-growing clinoforms. Particularly marked is the earliest Pliocene period of marked forced regression of the Orinoco Delta, possibly when short periods of icehouse eustatic sea-level fall outpaced subsidence rate. This lesson from the outcrop area is used to further understand the flat-rise-flat-rise, shelf-edge trajectory pattern that persisted throughout the Neogene margin development, as seen on seismic lines across the Columbus Basin.