Additions to Silurian Stratigraphy, Lithofacies, Biogeography and Paleontology of Bolivia and Southern Peru

Since publication of the South American Silurian correlation chart (Berry and Boucot, 1972), additional, significant information has become available regarding the Silurian lithostratigraphy, biogeography, paleogeography, and faunas of Argentina, Bolivia, and Peru. The new data help refine the position of the boundary between the North Silurian and Malvinokaffric Realms, as well as extend it far to the south of where it had been previously recognized (Text-fig. 1). Silurian lithofacies in the central Andean area emphasize regional change from a sand-size clastic facies with a shallow-water shelly fauna on the west, to a shaly, commonly graptolitic facies, on the east. The similarity in lithology and fauna between the upper part of the Lampa Formation of southern Peru and the Catavi Formation of Bolivia helps one recognize that the sandy lithofacies of the Upper Silurian extends across west-central Bolivia into southern Peru. This, together with the shaly, commonly graptolitic facies known further east, supports the concept of a western land area from which much of the sand-size material was derived. The Precambrian Arequipa Massif (Dalmayrac et al., 1977) may correspond, at least in part, to this source region. New data from southern Peru permits delineation of a western, north-south oriented, belt of rocks which lacks glacial deposits between the Ordovician and Silurian in contrast to an eastern belt characterized by glacially derived rocks at the Ordovician-Silurian boundary. Palynological evidence from the Pojo, Bolivia area substantiates the glacial-marine origin of certain eastern sediments. Combined with available biogeographic evidence, these data suggest a warm-water current to the west, and colder water to the east. Near Vila-Vila, south of La Paz, a small faunule from the Vila-Vila Formation enables the position of the Silurian-Devonian boundary to be more closely approximated than was previously possible. Trilete spores and spore tetrads support conclusions based on previous records from the Northern Hemisphere, about both the time of advent and environments of early Paleozoic land plants. Several Silurian-Early Devonian faunas from northern Bolivia and southern Peru are listed. Eight brachiopods are described, including one new species.