Mass-transport complexes and associated processes in the offshore area of Trinidad and Venezuela

Abstract

Mass-transport complexes (MTCs) form a significant component of the stratigraphic record in ancient and modern deep-water basins worldwide. One such basin, the deep-marine margin of eastern offshore Trinidad, situated along the obliquely converging boundary of the Caribbean and South American plates and proximal to the mouth of the Orinoco River, is characterized by catastrophic shelf-margin processes, intrusive and extrusive mobile shales, active tectonics, and possible migration and sequestration of hydrocarbons. Major structural elements found in the deep-water slope regions include large transpressional fault zones (i.e., Darien Ridge, Central Range, Los Bajos), along which mobile shales extrude to form sea-floor ridges; fault-cored anticlinal structures overlain by extrusive sea-floor mud volcanoes; shallow-rooted sediment bypass grabens near the shelf break; and normal and counterregional faults. A total of 10,708 km2 (4134-mi2) of merged three-dimensional (3-D) seismic surveys enable sub-sea-floor interpretation of several erosional surfaces that form the boundaries of enormous mass-transport complexes. The data show numerous episodes of MTC developments, which are characterized by chaotic, mounded seismic facies and fanlike geometry. Their extent (up to 2017 km2 [778 mi2]) and thickness (up to 250 m [820 ft]) is strongly influenced by sea-floor topography. Mass-transport flows show runout distances from the source area of 60–140 km (37–86 mi). Depositional architecture identified with these units includes (1) large-magnitude lateral erosional edges, (2) linear basal scours, and (3) side-wall failures. Mud volcanoes act as barriers to cross-slope mass sediment movements and form zones of shadowing on their downslope side that protect those regions from erosion. The subsequent erosional shadow remnants (ESRs) comprise preserved regions of older levee-channel complex sediments and are considered for the first time in this study as potential stratigraphic traps in deep-water deposits. Active tectonism in the region, high sedimentation rates associated with the Orinoco delta system, and abundant potential unstable hydrate suggest the viable presence of several higher frequency mechanisms at work for MTC generation than sea level fluctuations alone.