Onshelf to Deepwater Architectures of the Late Miocene to Pliocene Orinoco Delta, Trinidad
Department of Geological Sciences, The University of Texas at Austin
Trinidad Island is an uplifted segment of the Neogene shelf built by the Orinoco River Delta. The shelf-margin sediment prism has an internal clinoform architecture, with both shallow-marine topsets (repeated delta transits on the shelf) and deepwater, channelized-slope. It is many km thick and 100s of km wide, built from late Miocene through present. The shelf-margin had an irregularly rising trajectory producing thick topset aggradation and relatively rapid progradation of the fronting deepwater slope. Very high sediment supply led to prominent sand bypass from the shelf into deepwater areas at times. Trinidad outcrops expose the proximal half of this prism including Miocene deepwater strata. We reconstruct the depositional architecture of the southern Trinidad paleo-Orinoco margin, from outcrop windows in the shallow and deepwater stratigraphy. The Neogene shelf-margin prism on southern Trinidad is composed by 4 basinward-expanding clinoform wedges and 3 intervening transgressive episodes that flooded landwards back across Trinidad for 100s of km. The integration of south-coast outcrop data (6 outcrop sites dispersed along 35 km of coast; ca. 1900 m measured sections) illustrates the stacking of sedimentary facies from deepwater slope to shallow shelf as the margin clinoforms advanced from W to E-NE: (1) Cruse Clastic Wedge outcrops along the southern coast illustrate Late Miocene, thin bedded levee systems, deepwater slope channels and thick basin-floor mudstones, as well as Early Pliocene shelf-edge deltas, slope deformation and associated collapsed pieces of shelf edge; (2) Forest/Gros Morne/Mayaro Clastic Wedge outcrops show Mid-Pliocene storm-wave dominated shelf-edge deltas, probably in thickened growth-fault compartments along the eastern reaches of the prism on SE Trinidad; (3) Lr. Morne L'Enfer Clastic Wedge outcrops illustrate Mid-Late Pliocene river-tide interaction delta lobes and distributary channels, along with overlying estuary valley fills that formed during transgression across the shallow inner shelf reaches of SW Trinidad; (4) Upper Morne L'Enfer Clastic Wedge outcrops also show tide-dominated deltas and overlying distributary channels and coaly delta plain deposits. Although these outcrops represent stratigraphic windows of different ages, the integrated data provides us with a linkage model of shelf to deepwater deposition as well as a sub-regional prediction of the larger scale onshore Trinidad to offshore Columbus Basin correlation.
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