Abstract: Detailed Lithostratigraphy and Age Determinations of La Luna Formation in Two Sections of SW Tachira State (Venezuela)

F. G. Yoris, M. Ostos, M. Boujana, J. Perez, G. Booth, S. Packer, F. Galea, R. Lander

The La Luna Formation crops out almost completely at Las Delicias Bramon road, near the Colombia-Venezuela border, and at La Ortiza creek near San Cristobal, capital city of Tachira State. The distance between these two sections is close to 35 km, and we have found similar thicknesses in both of them: 192 m (Las Delicias) and 188 m (La Ortiza). The lower contacts with the Capacho Formation seem to be concordant in mesoscopic scale but we do not discard some condensation. The upper contacts in both sections are transitional with the black shales of Colon Formation, increasing slowly the proportion of fine clastic sediments and diminishing the proportion of chert and siliceous mudstones; we cannot correlate the Tres Esquinas Member with any specific phosphatic bed or interv l; the presence of primary structures with SW migration trend of bedforms and paleocurrent indicators along the cherty beds (Ftanita del Tachira Member, close to 97 m thick at both sections) and upper-transitional interval to Colon Formation, reveals that neither the "cherts" nor the phosphatic/glauconitic beds are all deposited in "quiet" environment and that any conclusion about the possibility of "slow rate" deposition environments need still more sedimentological, geochemical and biostratigraphic support. The general age for the La Luna Formation is Late Cenomanian to Campanian (Las Delicias) and Coniacian to early Maastrichtian(?) (La Ortiza).

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela