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Bureau Seminar, January 16, 2009

Tectonostratigraphic Significance of The Morichito Subbasin, Eastern Venezuelan Basin

Lorena Moscardelli, Migdalys Salazar, William Fisher, and Maria Antonieta Lorente
Bureau of Economic Geology

The Morichito Subbasin is a SW-NE oriented depocenter, located in the Eastern Venezuelan Foreland Basin (EVFB). It covers an approximate area of 1 000 km2 between the Serrania del Interior fold and thrust belt and the Pirital High. This basin was formed during the Neogene as the result of complex transpressional interactions between the Caribbean and South-American plates. Previous studies have tried to address the tectonostratigraphic significance of the Morichito Subbasin but 630 km2 of recently acquired 3D seismic data allowed us to expand the understanding of this subbasin. The Morichito Subbasin occupies a relatively small area of the EVFB. However, its relevance lies in the fact that it provides a valuable stratigraphic record that can be used to unveil the timing of the main deformational events that took place in the rest of the foreland basin.

This work presents the tectonostratigraphic evolution of the Morichito Subbasin by defining four tectonostratigraphic sequences (Units I to IV). Each sequence was defined based on the integration of well logs, biostratigraphic information and geomorphological interpretations that were performed using well and 3D seismic data. Unit I (Late Oligocene to Middle Miocene) was deposited in shallow marine environments (Carapita Formation) and its areal coverage extends to the south, beyond the boundaries of the Morichito Subbasin. Its configuration was highly influenced by the accommodation space generated due to the tectonic load of the Serrania del Interior range and it suggests that the unit pre-dates the formation of the Pirital High. Unit II (Middle Miocene) is composed of alluvial fan deposits (Morichito Formation) that were derived from the Serrania del Interior Range. The internal configuration of the unit indicates that it was deposited during the period of greatest activity of the Pirital Fault. Moreover, the shallowest section shows onlap relationships against an incipient high that indicates that the Pirital High was generated at that time. During the latest part of the Middle Miocene, the Pirital High was uplifted and the Morichito Subbasin was isolated from the southern portions of the foredeep. Units III (Middle Miocene to Early Pliocene) and IV (Early Pliocene to Recent) are composed of shallow marine and fluvial deposits (Las Piedras and Mesa Formations). These two units represent the final phases of basin infilling when tectonic activity and subsidence were at lower rates.


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