Paleo-Orinoco Shelf-Margin Growth — Process Regimes and Delta Evolution

Abstract

We present the first quantitative sedimentological characterization of integrated onshore outcrops and offshore subsurface data for the paleo-Orinoco (late Miocene-Pliocene) shelf-margin prism, Trinidad. A mixed river, tide, wave and gravity flow system generated this 10km-thick sedimentary prism. The analysis includes: 1)8 clastic sedimentary facies that exhibit an intact linkage from river and deltaic shelf, across shelf edge to deepwater slope, which are evaluated from spectacular outcrops dispersed along southern coast of Trinidad Island. 2)well log correlation of the same strata across southern Trinidad to offshore Columbus Basin; 3)Seismic clinoform morphology analysis with quantitative sediment flux calculations and predictions. Shelf-margin growth was generated by repeated (100ky time scale) cross-shelf, regressive-transgressive transits with internal variability of clinoform architecture and process-regime changes. The reconstructed prism is composed by 3 progradational wedges, each with thickness up to 2km and separated by Trinidad-wide marine flooding intervals of similar extent, following by a final Pliocene retro-gradational wedge. The entire prism shows an overall rising trajectory, despite general falling sea level during the global icehouse period. The relationship among the paleoflux, shelf edge trajectory, and clinoform height and shape is discussed together with the impact of severe tectonic subsidence, high-frequency sea level changes, climate change, and sediment supply. The evolution of sediment flux percentage at the shelf edge shows a trend opposite to trajectory changes. The latter shows two rising-flat cycles followed by a back-step, whereas the sediment flux percentage decreased from the beginning and then increased again at a late stage (varies between 49%-84%) with nearly 63% of the sediments on average transported beyond the shelf edge to deepwater areas. This sediment flux calculation together with the flat trajectory segments predicts large slope or basin floor fans on the toesets. At short time scales, the variation in the percentage of bypassing sediment budget depends on a series of control factors. However, for relatively longer geologic time scale, the total sediment flux (varies between 20-60m2/yr) shows a tendency to maintain a constant bypassing percentage (around 60%), because the sum of the control factors tends to autogenically maintain an equilibrium across the clinoform system during long-term shelf margin growth.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016