REVISITING STRATIGRAPHY, GEOCHONOLOGY AND PALEOGEOOGRAPHY AT LA VENTA FORMATION, TATACOA DESERT, COLOMBIA

Since the 1940s, Miocene sedimentary strata in La Tatacoa Desert (LTD) Upper Magdalena Valley, Colombia) are recognized as a major repository of vertebrate fossils, particularly the Middle Miocene primate-rich beds at La Venta (LV) Formation; arguably the richest Neogene fossil assemblage of Tropical South America. The time frame proposed for LV is one of great paleobiotic (primate dispersal, floral changes) and paleogeogprahic changes (climate, mountain building, fluvial re-organization, etc.) in northern South America. We present new stratigraphic/cartographic information and the first geochronology U/Pb-detrital zircon dataset from 9 stratigraphic horizons at LTD. Our results contribute to elucidating the chronology, facies characteristics and sratigraphic relationships of strata exposed in this area, yielding more complexity to the time and sedimentstratigraphic perspective of this important section, while providing important clues about potential sources (provenance) of detrital material and volcanic products during the Miocene.

Traditionally, two Middle Miocene units are defined in the study area, La Victoria and Villavieja formations, both correletable with the Honda Group. We demonstrate the existence of three distinct continental units separated by unconformities. The oldest unit ~20 to 14 Ma dominates the eastern section of LTD, it is made of intercalated sandstone (large, silicified tree trunks xilopal in its middle portion) and mudstone, lacks conglomeratic facies, exhibits characteristic folding, it is fossil-poor and can be correlated with the Honda Group. A marked angular unconformity separates these basal units from the fossil-rich Grupo LV with ages from ~13 to 11 Ma, exposed as a SE tilted monocline. Conglomerates are found at the base and middle portion of the sequence (San Alfonso and Cerbatana “formations”), with grey (Neogene) and purple (Jurassic) andesite clasts. Sands and siltstone dominate the remaining strata (La Victoria, Villavieja and Mesitas “formations”). A subtle unconformity marks the beginning of the last Miocene cycle (10-6 Ma) dominated by typically red volcaniclastic horizons at the base ranging to pale (gray, blue, yellow) sands and agglomerates, in horizontal position, similar to the Mesa fomratino in the Middle Magadlena Valley.