Thin-Skinned Deformation of the Cretaceous and Tertiary Sedimentary Section of Eastern Offshore Trinidad: ABSTRACT

Abstract

The major oil fields of the Monagas Fold-Thrust Belt of Eastern Venezuela are anticlinal structures related to thin-skinned thrusting that transported Tertiary clastic sediments up a series of ramps. The ramps are thought to arise from a flat-detachment surface located within Lower Cretaceous or Pre-Cretaceous sediments. The ramps merge upward into a flat-detachment located in soft plastic shales of the Miocene foredeep sequence. In Eastern Offshore Trinidad, similar deformation occurs within the easternmost extension of this fold-thrust belt along the Caribbean-South American Plate Boundary. Thrusting in Eastern Offshore Trinidad has produced a high-relief duplex structure that formed by interconnection of a basal detachment, ramp-thrusts, and a roof-thrust. Excellent 3-D seismic data show the geometry of this duplex structure, and well data show that the basal-detachment is located within the Late Albian-Cenomanian Gautier Formation. The Gautier is a very plastic organic-rich shale that forms an excellent surface for detachment. A series of ramps cut upward from this basal detachment through hard, competent siliceous rocks of the Turonian through Maastrichtian Naparima Hill Formation. The thrusts abruptly flatten into an upper detachment at the lithologic contact with soft ductile shales of the Oligocene Lower Cipero Formation. The geometry of the thin-skinned thrusting ismore » similar to that observed in the Monagas Thrust Belt, but differs in that the lower flat-detachment surface lies within Cretaceous sediments equivalent to the Querecual Formation, and not within earlier Cretaceous or Pre-Cretaceous Venezuelan equivalents.« less