by Maria Veronica Castillo, 2001
University of Texas PhD dissertation

Abstract
The Icotea and VLE1 faults are two of the major faults in the Maracaibo Basin, Venezuela, and are known from seismic and well data to have been most active during Paleogene time. Both faults have linear traces over distances up to 100 km, are deeply buried under largely unfaulted Neogene sedimentary rocks, and are associated with localized continental growth strata of Paleogene age along their traces. The origin and displacement history of both faults has remained controversial partly because of along-strike complexities in fault structure, use of highly exaggerated 2D seismic lines, and the lack of synoptic views of both fault systems. Previous interpretations range from east dipping basement-involved thrust faults to sub-vertical left-lateral strike-slip faults controlling pull-apart basins. I use regional 2D seismic data crossing both faults, and 3D seismic data covering a 1600 km2 area of the southern Maracaibo Basin to describe structures along the traces of both faults and fault termination structures at their southern ends. These seismic reflection data show that both faults are inverted normal faults that first formed during the late Jurassic-early Cretaceous. The abrupt termination of both faults in the southern part of the basin probably corresponds to the southern ends of two parallel rift structures. This study also uses 3D seismic reflection data to describe a karst horizon in the Aptian-Albian carbonate rocks that may have formed during a worldwide eustatic drop in Albian sea level that also produced the well-known mid-Cretaceous unconformity recognized in the Gulf of Mexico, France, and Middle East. The karst interpretation may allow a better understanding of reservoir characteristics at this level in the carbonate platform, which are generally attributed to fracturing rather than subaerial weathering. The presence of a regionally extensive karst surface at depth beneath other basins along the northern margin of South America may prove to be a useful oil exploration play concept. 1VLE fault: named by the Petróleos de Venezuela filial MARAVEN S.A., to describe a NS striking fault in Block V located in the central area of the Lake Maracaibo.

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