Tectonic Controls for Three Main Subsidence Phases of the Eastern Venezuelan Foreland Basin in the Region of the Orinoco Delta
We use subsidence analysis of three deep wells to basement combined with sequence stratigraphic mapping of seismic data to show that a 6684 km2 area of the Eastern Venezuelan foreland basin in the region of the Orinoco Delta underwent three main stages of foreland-related subsidence that followed a protracted Cretaceous - Late Oligocene period of precollisional, passive margin formation. Phase 1 consists of increased foreland basin subsidence in the late Oligocene to middle Miocene (23 - 13 Ma) at average sedimentation rates of 0.09 mm/yr. Clastic rocks of Phase 1 include the Freites Formation, a 1.2 km-thick section of greenish-gray fissile shale and shaly sandstone deposited in shallow marine - neritic environments. Seismic facies show progradation of Phase 1 clastic rocks as a wedge from the NE and NNE. Clastic rocks deposited during the accelerated Phase 2 in the middle to late Miocene (13 -11 Ma at sedimentation rates of 0.92 mm/yr) include the La Pica Formation, a 2.7 km-thick section of gray silt and fine-grained sandstone deposited in shallow marine/coastal proximal environments. Seismic facies show progradation of Phase 2 clastic rocks as a wedge to the NE. Phase 3 consists of decelerating foreland basin subsidence in the period of Late Miocene-Mid Pliocene (11-6 Ma at average sedimentation rates of 0.36 mm/yr). Sedimentary rocks deposited during this period include the Las Piedras Formation a 1.45 km-thick section of sandstone, carbonaceous siltstone and shale deposited in deltaic environments. Seismic facies show a progradation of Phase 3 clastic rocks as a wedge to the NE and ENE. Deeper marine environments and more rapid subsidence rates of Phases 1 and 2 are interpreted as an underfilled foreland basin stage related to active thrusting along the Serrania del Interior along the northern flank of the basin. Deltaic environments and slower rates of Phase 3 are interpreted as an overfilled foreland stage related to rapid seaward progradation of the Orinoco delta and its filling of the former, dynamically- maintained interior seaway.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009