Abstract
The first sedimentological characterization and correlation of onshore outcrop and offshore subsurface data (southern Columbus Basin) are presented for the paleo-Orinoco (Upper Miocene–Pliocene) shelf-margin prism, Trinidad Island, Trinidad and Tobago. The paleo–Orinoco River delta system and associated continental slope, which generated the 10-km-thick sedimentary prism, was a mixed river, tide, wave, and sediment-gravity flow system that tracked down to deep-water submarine fans. The analysis here includes: (1) an evaluation of delta-plain to deep-water turbidite sedimentary facies, which are seen in spectacular outcrops along the southwest, south, and southeast coasts of Trinidad Island, (2) well-log correlation of the same Upper Miocene–Pliocene strata across southern Trinidad and out to the southern part of the offshore Columbus Basin along an off-axis transect, because the main fairway into the most rapidly subsiding part of Columbus Basin is structurally complex, and few detailed data have been released for publication; and (3) use of published seismic data for reconstruction of clinoform morphology across the relatively undeformed segment of the margin, with quantitative sediment flux calculations and predictions.
Paleo-Orinoco shelf-margin growth was generated by repeated (∼<100 k.y. time scale) cross-shelf, regressive-transgressive transits (>100 km) of the Orinoco delta system, with internal variability in clinoform architecture and process-regime changes during shelf-margin construction. The Upper Miocene–Pliocene prism, with its linkage to the southern Columbus Basin and Columbus Channel, is composed of four progradational clastic wedges, each with a thickness up to 2 km, separated by well-known, Trinidad-wide marine flooding intervals of similar extent, followed by an aggradational wedge with decreased clinothem thickness. The studied sedimentary prism spans an ∼4 m.y. period and shows an overall rising shelf-edge trajectory, despite an overall and periodic falling eustatic sea level during this global icehouse period.
The relationships among the Orinoco paleoflux, shelf-edge trajectory, and clinoform height and shape are discussed for a southeastern segment of the margin that is much less deformed than the main growth-faulted Columbus Basin. However, this off-axis segment was also subject to severe tectonic subsidence, as well as high-frequency sea-level, climate, and sediment supply changes. The estimated sediment flux percentage at the shelf edge shows a similar time trend to that of the well-imaged shelf-edge trajectory. The latter shows three long-term (∼0.5 m.y.) trajectory changes, whereas the estimated sediment flux percentage at the shelf edge shows three supply pulses (with shelf-edge flux varying between 40% and 91% of original sediment discharge) through the early Pliocene, with nearly 76% of the sediment on average transported beyond the shelf edge to the deep-water areas, despite a relatively wide shelf. This sediment flux calculation, and particularly the occurrence of flat segments along the shelf-edge trajectory, predicts significant sediment bypass to slope or basin-floor fans on or beyond the toesets of the paleo-Orinoco shelf margin. At short time scales, the variation in the percentage of sediment budget bypassing the shelf edge depends on a series of shelf parameters. However, at geologic time scale, the total sediment flux bypassing the shelf edge is two thirds to three quarters of the total sediment budget.
