Abstract: Cyclicity in the Cretaceous Organic-Rich Facies of Colombia, and Its Predictive Potential for Evaluating Source-Rocks

Tomas Villamil, Claudia Arango, Erle Kauffman, Carlos Jaramillo

Late Cretaceous organic-rich rocks of Colombia show remarkable depositional cyclicity in 100 and 400-500 ky frequencies. Cyclicity was tested with two time-series analysis equations and with random peridogram techniques. Cycles were measured between parasequence boundaries as defined in distal limestone-shale facies and in proximal calcareous shale facies with bedding thicknesses and color variations were used to distinguish the cycles. Analyses were also conducted on faunal abundance and diversity. Population variations of planktic foraminifers, ribbed ammonite morphotypes and dinoflagellates and pollen were evaluated for cyclicity. Closely-spaced geochemical analyses, particularly the total content of barium, phosphorous and calcium record 400-500 ky and 40 ky cyclicity whereas faun l indicators and facies exhibit a strong 100 ky signal. Statistically tested depositional cyclicity was probably the result of Milankovitch climate variations that affected the rate of sediment supply and the amount of upwelling and productivity in the upper water column. Cyclicity directly controls the organic carbon content of the strata; warm and dry periods were characterized by high relative sea level, high upwelling and productivity and lower sedimentation rate. Cool wet periods were characterized by an increase in sedimentation rate, relatively lower sea level and lower upwelling and productivity. In very thin cycles (<.3 m) TOC quality is low; in thicker cycles (1-5 m) TOC content and quality are optimal, and in thick (>20 m) cycles TOC is lower.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994