Preliminar correlation of the Pleistocene sequences of the Tarija valley (Bolivia) with the Pampean chronological standard

Abstract

Most of the mammal collections of the Pleistocene from the Tarija valley (Bolivia) lack precise stratigraphic data. Some fossil collections were made under stratigraphic control but have not been described. However, mammals were used for correlation and comparison with the pattern established for the Pampean region of Argentina. The bearing units have been referred either to the Ensenadan and Bonaerian stages, the "Belgranan", the Ensenadan, or the Lujanian in a broad sense, corresponding to the middle-upper Pleistocene or the upper Pleistocene. Studies during the last twenty five years accomplished by American authors and based on magnetostratigraphy and absolute dating suggest that the Tarija sequence corresponds to the Ensenadan (early-middle Pleistocene) of the Pampean standard. Recently, Italian scientists, based on radiometric dating, discarded this age and considered that the whole sequence belongs to the late Pleistocene. New biostratigraphic and systematic studies allowed the recognition of Glyptodon munizi, Arctotherium angustidens, and Hippidion devillei, exclusive taxa of the Mesotherium cristatum Biozone, the biostratigraphic base for the Ensenadan of the Pampean region. However, collections include mostly putative endemic species or those characteristic of the Equus (Amerhippus) neogaeus Biozone, the biostratigraphic base for the Lujanian of the Pampean area. In sum, from a biostratigraphic point of view, the Tarija sequence has taxa from the lower to middle Pleistocene (Ensenadan) and the upper Pleistocene-lower Holocene (Lujanian) of the Pampean area. From a paleofaunistic standpoint, the association recovered from the Tarija valley shows significant differences with the typical Pampean association. The high frequency of remains of Gomphotheriidae and Equidae and, in lower proportion, of Hydrochoeriidae and Tapiridae, are noteworthy. Members of these two latter families are restricted today to warm and wet climates. Most Glyptodontidae records belong to Glyptodon, and in the Pampean region this family is much more diversified.