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VILLAMIL, TOMAS, CLAUDIA C. JOHNSON, and ERLE G. KAUFFMAN, Department of Geological Sciences, University of Colorado, Boulder, CO

ABSTRACT: Molluscan Biozones, Event/Cycle Chronostratigraphy, and Sea Level History: A New Integrated Cretaceous Chronology for Northern South America

The Cretaceous marine basins of northern South America, in their evolution, record a dynamic interplay between regional and plate tectonics, sealevel history, changing paleo-ocean/paleoclimate systems, and the rates/patterns of sedimentation driven by both allocyclic and autocyclic processes. Regional interpretation of these complex interactions; and the evolution of the South American passive margin, requires a high-resolution stratigraphic system of dating and correlation. Because short-term events and dynamic processes shape the stratigraphic record, this chronology must have a resolution of <1 million years, ideally 10's-100's of ka correlation units. This resolution does not currently exist in South America, where Cretaceous radiometric dates and paleomagnetic analyses are few event/cycle chronostratigraphy and sequence stratigraphy are not yet well developed and where current biostratigraphic resolution based on mollusks averages 1- >2 Ma/biozone. New efforts to establish a more refined chronology for South America involve: (a) High-resolution (cm-scale) event/cycle chronostratigraphic analyses of key Colombian and Venezuelan sections; (b) search for datable ash/bentonite beds; (c) detailed paleontologic collecting to firmly establish biostratigraphic ranges of mollusks and microplankton with high biostratigraphic potential, and to formulate regional assemblage zones; and (d) integration of these diverse data through graphic correlation to provide a new chronologic standard for the region. Initial results are presented: Middle Cretaceous biozones are now esolved to <500 ka/zone; a fine-scale sequence stratigraphy and sealevel history has been determined; numerous regional physical and chemostratigraphic event beds are identified; and climate-driven cyclostratigraphy has been resolved to at least 50 ka within the passive margin sequences of Colombia and Venezuela.

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