Chronology and geology of an Early Miocene mammalian assemblage in North of South America, from Cerro La Cruz (Castillo Formation), Lara State, Venezuela: implications in the ‘changing course of Orinoco River’ hypothesis.

Abstract

In general the geology of paleontological sites in Venezuela is poorly known. With the purpose of improving this knowledge we describe the geology of the Castillo Formation (Late Oligocene to Early Miocene) at Cerro La Cruz locality, in Lara state, Venezuela, that contains several records of vertebrate and invertebrate fauna. Lithologically, the Cerro La Cruz sequence is composed by alternating packages of siliciclastic and carbonate sediments, with a predominance of mudstone. The paleoenvironment is inferred as a mainly near-shore marine complex that could be associated with regressive and transgressive phases. Nevertheless, into the middle part of the Cerro La Cruz outcrops two levels containing at least six mammalian remains were found, confirming the early continental mammal assemblage in Venezuela. The continental vertebrate assemblage includes Xenarthra, Notoungulata and Litopterna mammals, fresh water fishes, the turtle Chelus, wood and leaves, allows us to interpret the paleoenvironment of the continental episodes of this locality as a mosaic composed of an humid forest and lowland savanna. Four isotopic dates using Strontium (87Sr/86Sr), from the Cerro La Cruz locality were obtained, which bracket of the sediments to between 17.21 to 19.27 Ma, confirming an Early Miocene age for this locality. Biochronologically, the mammalian assemblage so far recovered does not permit to assign this fauna to any of the recognized South American Land Mammal Ages, however the isotopic ages suggest affinities with a Pansantacrucian mammalian subcycle. Our interpretation of the geology at Cerro La Cruz shows no evidence for the presence of a major river system crossing over that zone during the Early Miocene and does not support the hypothesis of the possible draining of a ‘Proto-Orinoco’ river into Maracaibo or Falcón basin during the Early Miocene.