Cretaceous to Holocene structural and stratigraphic development in south Lake Maracaibo, Venezuela, inferred from well and three-dimensional seismic data

The subsurface of the southern part of the Maracaibo Basin has been studied in much less detail than the central and northern basin because hydrocarbon-rich rocks of Cretaceous and Paleogene age are deeply buried to a depth of 4.5 km (2.7 mi) by a thick Neogene clastic section eroded off of surrounding mountain ranges. In this article, we describe the results of the interpretation of 1600-km2 (617-mi2) three-dimensional (3-D) seismic reflection data centered on this horizontal to monoclinally southwestward-dipping middle Cretaceous to Holocene section adjacent to a zone of northwestward overthrusting along the mountain front of the Mrida Andes. To establish the structural and stratigraphic history of the 3-D seismic study area, a series of time slices were constructed at key horizons through the 3-D seismic volume. The ages of units in these time slices were correlated to stratigraphic formations known from outcrops at the basin edges, well data from the 3-D seismic study area, and subsurface correlation with better studied areas in the northern basin. The type and orientations of structures along each time horizon were mapped to constrain the successive structural and stratigraphic events that affected the different tectonosequences in the southern Maracaibo Basin. The main tectonic events recorded both by structures and unconformity-bound tectonosequences include the following events from youngest to oldest: (1) Pliocene–Holocene shortening produced by the uplift of the Mrida Andes and overthrusting at the mountain front has produced an approximately 6-km (3.7-mi)-deep localized foredeep basin; the axial traces of these folds trend northeast-southwest and parallel the strike of a set of normal faults; all structural features are consistent with regional northwest-southeast shortening. A low-angle thrust fault responsible for the formation of the main northeast-southwest fold is inferred to continue downdip as a flat-lying thrust that may be continuous with low-angle thrust faults observed along the northern Mrida Andes mountain front. The level of thrust detachment occurs at the Cretaceous Coln shale and accommodates a negligible amount of shortening (500 m; 1640 ft). (2) Late Miocene–Pliocene shortening caused by uplift of the Sierra de Perij and the Mrida Andes produced an inversion of north-south–striking normal faults and folds in a parallel orientation. The axial traces of these folds and the trend of inverted structures coincide with the trends of the most prolific Eocene sandstone reservoirs in the central and northern Lake Maracaibo areas. (3) Late Paleocene–middle Eocene formation of a foreland basin is related to the southward overthrusting at the northeastern margin of the basin; normal faults with an average trend of S37E record flexure of the basin during this event; faults also include the southern splayed termination of the left-lateral Icotea strike-slip fault. (4) The termination of the Icotea fault in the south lake area is consistent with the small amount of left-lateral strike-slip displacement previously inferred from the geometry of the Eocene Icotea pull-apart basin in central Lake Maracaibo.