Abstract: Sea Level Fall During the Late Miocene and Its Influence on the Sedimentation of the La Pica Formation, Eastern Venezuela

Sandra Crespo De Cabrera, George Allen

Well log, seismic, biostratigraphic and lithologic data in the La Pica formation indicate the occurence of an abrupt regressive facies shift in the late Miocene of the Monagas fore-land basin in eastern Venezuela. This regional facies transition comprises a change from outer neritic mudstones to coastal and inner shelf deposits and is interpreted as resulting from the 10.2 Ma eustatic sea level fall.

The La Pica Formation is a 2500 m thick stratigraphic unit which spans the upper; Miocene and extends across the entire basin. In the western, i.e. landward extremity of the basin, it unconformably overlies outer shelf mudstones of the mid-Miocene Carapita formation. In the more distal parts of the basin, the sequence boundary exhibits a conformable and more gradual facies transition. The upper limit of the La Pica is interpreted as the 5.5 Ma Messinian sequence boundary. The formation comprises several depositional sequences which can be traced regionally.

Paleontologically, this formation is characterized by abundant mollusks shell fragments: at the base and by a deep water, agglutinated foraminiferal assemblage at the top. The rest of the unit is barren of microfauna. No planktic or calcareous benthic foraminifera have been found associated with this deep water assemblage. For this reason it could be interpreted as a reworked microfauna or as an assemblage where dissolution has removed the calcareous portion. Alternatively, it could be suggested that paleoceanographic conditions affected considerably the assemblage diversity.

Active tectonic subsidence during the late Miocene enhanced the eustatic rises, whereas decreasing subsidence rates in the Pliocene appear to have amplified the effects of the Pliocene eustatic falls..

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France