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HELENES, J., Corpoven, S.A., Puerto La Cruz, Venezuela

ABSTRACT: Chronostratigraphy of the Cretaceous-Oligocene Unconformity in Northern Monagas, Eastern Venezuela

Palynological analyses of several subsurface sections in the autochthonous block in Northern Monagas, eastern Venezuela, show a Cretaceous-Oligocene unconformity. The Maastrichtian to Paleocene sandstones of the San Juan Formation unconformably underlie the Oligocene to Miocene sandstones of the Merecure Formation. Log analyses indicate two stratigraphic sequences within San Juan and two more within Merecure, and integration with paleontological data indicates changes which can be correlated with Haq et al.'s (1988) Sequence Chronostratigraphic Charts as follows:

(1) The basal San Juan sandstones represent the late Maastrichtian (68 Ma) drop of sea level and the UZA-4.5 / TA-1.1 Sequence Boundary (SB). The Maastrichtian or Maastrichtian to Paleocene San Juan sandstones represent either the TA-1.1 and the TA-1.2? Third-Order Cycles.

(2) The late Oligocene sequence overlying the unconformity, represents the TB-1.3 Cycle starting at 26.5 Ma. Within this lower part of the Merecure sediments, an increase in the abundance and diversity of marine palynomorphs represents a maximum transgression correlatable with the 26 Ma Condensed Section contained in the TB-1.3 Cycle.

(3) The upper part of Merecure represents the TB-1.4 Cycle starting at 25.5 Ma. Within this cycle, the lowermost (Oligocene) sandstones contain almost exclusively terrestrial palynomorphs, while the (Miocene) alternating sands and shales of the upper

part contain diverse marine palynomorphs indicating a change from Low Stand Wedge to Transgressive deposits. This change is correlated with the Oligocene / Miocene limit at 25.2 Ma.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.