Architecture and seismic geomorphology of shelf edge deltas along an active tectonic margin, eastern offshore Trinidad
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The South American continental margin, offshore eastern Trinidad and Venezuela, has a Tertiary and near-modern history of shelf margin sedimentation, with architecture and morphology influenced by margin tectonics. Influences include strike-slip faulting; compressional folding; extensional, counter-regional faulting; and upper-slope-graben development. A [similar to]2,600 km² 3D-seismic data volume containing nearly 75 km of shelf margin shows that the Orinoco Delta prograded to the ancient shelf edge, developing significant shelf edge deltas. The last Plio/Pleistocene deltaic regression and transgression were mapped using seismic tied to 14 well logs. Key surfaces were mapped and packages defined. Intervals were isopached to examine thickness changes along the shelf. Attributes (amplitude, semblance, root-mean-squared amplitudes, etc.) were analyzed to map seismic geomorphology of shelf and slope systems. Analysis included deltaic clinoform trajectory, slope-channel sinuosity and density, and fault character. We wanted to examine shelf-edge-structural influence on sources of deep-water sands and the relationship between delta architecture, fault character, and slope morphology. Observations showed that delta sediment bound for deep water during the last glacial maximum became trapped in upper-slope minibasins formed by down-to-the-basin normal and counter-regional growth faults. Only in the northernmost end of the study area where the counter-regional fault dies out did shelf edge deltas prograde onto the upper slope. Three types of shelf edge delta trajectories were identified: aggradational, progradational, and over-steepened. Over-steepened is associated with the highest density of bypassing channels and aggradation with the lowest. Most channels are straight—except on lower-angled slopes downdip of the progradational province, where channels show sinuosity. Channel densities from aggradational and over-steepened provinces decrease basinward of the shelf break and fault; progradational province channel densities remain constant or increase. Areas of higher fault throw correspond to downdip development of large numbers of bypass channels. However, areas downdip of over-steepened provinces show high concentrations of bypass channel development regardless of fault-throw associations. These data suggest that slope-channel frequency is closely tied to clinoform trajectory, with high clustering associated with progradational/over-steepened regions. Tectonics affect these more-unstable-trajectory regions to increase downslope sediment movement. Lower channel frequencies downdip suggest more aggradational, tectonically stable clinoforms proximally.