Evolving Source Areas for Upper Cretaceous--Lower Tertiary Marine to Continental Strata of Central Colombia

R. Ressetar, M. P. Segall, F. E. Laverde, R. B. Allen

Late Cretaceous marine conditions in the Upper Magdalena Valley (UMV) and the Sabana of Colombia ended with deposition of the regressive Guadalupe (Campanian-Maastrichtian) and Guaduas (Maastrichtian-Paleocene) Formations, followed by terrestrial deposition of the Gualanday Group (Eocene). X-ray diffraction, SEM, and thin-section analyses of samples from the southern UMV and the Sabana reveal compositional trends related to different depositional conditions and to evolving source terrains.

The upper Guadalupe Formation of the southern UMV is mineralogically distinct from the overlying units, containing high average concentrations of illite/smectite (40%) and clay-size quartz (20%) with little discrete illite. The Guaduas Formation in the UMV consists of weathered terrestrial strata that can be divided into two units: a more thoroughly weathered lower unit rich in chlorite (57%) and kaolinite (22%) with little illite/smectite (5%), and an upper unit with consistently higher illite/smectite concentrations (12%) and low but variable amounts of kaolinite (14%) and chlorite (36%). The Guaduas Formation of the Sabana has generally uniform relative mineral abundances, with illite/smectite concentrations similar to the lower Guaduas of the UMV but significantly more illite (52% . In contrast, the Gualanday Group in the UMV has extremely nonuniform mineral concentrations. Ranges in abundance of illite/smectite (0-92%), illite (0-28%), and chlorite (5-42%) indicate varying weathering processes and source areas. In general, however, the average concentrations in the Gualanday Group more closely resemble the Guadalupe Formation than the Guaduas.

The data indicate that after deposition of the Guadalupe Formation from a volcanic/metamorphic source, the sediment source changed or was completely cut off. Chemical weathering was enhanced during deposition of the Guaduas Formation in the UMV, but marine sedimentation, reflected in the uniform composition, continued in the Sabana. Anomalously high illite/smectite concentrations and overall variability in the Gualanday Group indicate a return to a volcanic source with deposition in an alluvial fan/fluvial environment.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.