Geodynamic Control on the Deposition of Siliciclastics and Carbonates in West-Directed Subduction Settings: An Example From Eastern Offshore of Tobago Island, Southeastern Caribbean

Abstract

Tectonic settings characterized by west-subducting slabs typically show compressional structures formed in their accretionary prisms. These are progressively transferred to an extensional regime, as a result of the eastward roll-back of the lower plate. This transition may cause major events in a sedimentary basin: changes in accommodation, subsidence, and redistribution of depositional patterns. The implications are strong for exploration of hydrocarbons since, among others, these factors may be critical for their generation, migration and trapping. Understanding the geodynamics of these settings is thus essential to gain insights into the tectono-sedimentary evolution of a basin and its exploration strategies. We present a case study located eastern offshore of Tobago Island. The tectono-stratigraphy of the area is controlled by the west-directed subduction of the Atlantic Plate beneath the Caribbean Plate and sediment supply from the Orinoco River. Interpreted seismic and gravity data suggest that in the middle Miocene the area transitioned from compression to extension. Evidence of the latter is the present-day architecture of an eastward-dipping, angular erosional unconformity. The unconformity is the result of a period of subaerial exposure, during which part of the accretionary prism was eroded. Both, the compressional structures of the accretionary prism and the erosional unconformity, are cut by normal faults related to the subsequent collapse of the region controlled by the eastward slab retreat. At this time, horsts and grabens were developed and, based on the seismic geometries, optimal conditions for the deposition of shallow-water carbonates existed in the area. Subsequently, the basin experienced a major transgression and a rapid eastward tilting of the strata. This is inferred to be caused by the ongoing slab retreat, along with the effects of the lithosphere flexure due to the large load of Neogene sediments. On this sedimentary package, the main interpreted seismic facies are mud drapes, mass transport deposits, sinuous basin-floor leveed-channels and terminal lobes. Altogether, the basin has been rapidly subsiding. Finally, the Neogene sediments appear detached and collapsed over a continuous surface, which is considered a regional detachment. This level is interpreted to be composed of mobile shale that favored the development of folds, mud-diapirs and normal faults, suggesting the persistence of an extensional regime.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017