BIODIVERSITY AND PALEOECOLOGY OF AN EXCEPTIONAL LATE MIOCENE MARINE ECOSYSTEM, URUMACO FORMATION, FALCóN STATE, VENEZUELA

The Falcón Basin of Venezuela comprises a 9000 m thick succession of Neogene continental, marginal marine, and shallow marine deposits. This thick sequence preserves an important record of faunal and paleoenvironmental change in an area critical to understanding changes influenced by climate change, the rise of the Isthmus of Panama, and regional fluvial changes. The Urumaco Formation is particularly noteworthy for its rich vertebrate assemblage, which includes giant crocodiles, rodents and turtles. Despite containing a rich terrestrial vertebrate record, the Urumaco Formation is marine in many of its facies, and its rich mollusc diversity was the subject of several studies in the first part of the 20^th century. The age of the Urumaco Formation has been poorly understood due to the scarcity of well-preserved microfossils and the lack of modern revision to the mollusc fauna. The terrestrial mammal fauna has pointed to a middle to late Miocene age.

Preliminary sampling (bulk and field observations) of marine invertebrate assemblages was conducted to assess the viability of constructing a high-resolution paleoenvironmental and chronological framework through the formation. A particularly intensive effort was made to sample the El Hatillo site, found near the top of the middle member of the Urumaco Formation. Despite limited sampling, a clear biotic gradient of assemblages representing estuarine (brackish) through lower inner shelf habitats is reconstructed. More than 200 species were recorded from the preliminary survey, including over 154 species from El Hatillo, where two stacked sedimentary sequences of shell beds, organic mudstones, and trough cross-bedded sandstone are especially well exposed.

Neogene marine and terrestrial paleoenvironments of the Miocene-Pliocene Falcón basin were likely controlled by regional tectonics, basin subsidence, and overprinted by sea-level fluctuations. Parts of the Urumaco Formation exhibit characteristics associated with a prograding strandplain-deltaic depositional environment, although much of the lower and middle member are more reminiscent of accumulation on the inner continental shelf and coastal plain during rapid fluctuations in relative sea-level. The molluscan fauna suggests a latest Miocene age, which is corroborated by ⁸⁷Sr/⁸⁶Sr geochronology.