Facies and Architecture of the Tide-Dominated Late Pliocene Orinoco Delta (Upper Morne L'Enfer Formation) SW Trinidad
Chen, Si, Steel, Ronald J., and Dixon, Josh
Exceptionally high shelf-subsidence rates (0.8-6.0+mm/yr), a marked basinward stepping (to East and Northeast) of the paleo-Orinoco shelf prism and post-Pliocene uplift of Trinidad all allow the sedimentary facies, process regime and the evolution of the paleo-Orinoco Delta to be evaluated from extensive outcrops along the southwest, south and east coasts of Trinidad as well as at scattered small outcrops inland. The easterly growth (some 200km from late Miocene through present) of the continental shelf-to-slope clinoform and accompanying sedimentary prism was caused by repeated cross-shelf regressive-transgressive transits of the Orinoco delta system. This, in turn, caused a notable cyclic aggradation of the topsets of the shelf prism and a clear progradation of the deepwater slope, producing a stratigraphy reflecting interaction of Icehouse sea level changes, sediment flux across the shelf and rapid subsidence of the Orinoco margin. The studied outcropping strata of the Upper Morne L’Enfer Formation (Late Pliocene) along Cedros Bay and Erin Bay in southwest Trinidad documents facies and process regime of the successions in Cedros Bay (ca. 300m) and Erin Bay (ca. 450m). The study sites are some 100 km landward of the coeval shelf-slope break on the shelf margin.
Regressive facies associations including tide-dominated delta-front deposits (FA1), whereas those that are regressive lie within upward coarsening units, and include fluvial distributary channels (FA2), and rare shoreface deposits (FA3). Transgressive facies associations show an overall upward fining of grain size and include inner estuary fluvial-distributary channel sandstones (FA4), central estuary fluvial-tidal distributary channel sandstones (FA5), tide-dominated, estuarine channel-bar sandstones (FA6), tidal flat heterolithics (FA7). There is also a muddy-coaly association interpreted as areas of embayment or flood basin between the distributary channels (FA8). The tidal signals in both deltaic and estuarine units include bi-directional paleocurrents (channels), extensive and frequent mud drapes within sets of cross-strata from thickly-stacked, delta-front cross-bedded intervals, and flaser and lenticular bedding (tidal flats). There are very few signals of open-coast storm waves in the succession, suggesting embayed shorelines. The overall vertical change from distal deltaic up to coaly delta plain deposits in both successions show the clear progradational character of the Upper Morne L’Enfer deltaic prism, despite the superimposition of some 3 megasequences and 12 high-frequency regressive-transgressive units. A prediction of the larger scale, continuation of the succession to the outer shelf and shelf edge in the east is suggested.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013