Paleogeography and biostratigraphy of the Austral basin in Argentina and Chile : An appeal for sound systematics

Abstract

The Austral basin of Patagonia in southern South America contains one of the most complete Cretaceous sedimentary sections in the world. Many years of study have determined the biostratigraphic history of the Lower Cretaceous marine sediments, which have been divided into 10 assemblage zones. Lithological differences and numerous ammonoid fossil assemblages from two groups of sections in the northern part of the basin provide a history of the evolution of these Lower Cretaceous sedimentary sections. During Hauterivian time, the transgressive sea had its maximum advance, and thick sequences of black shales accumulated in pelagic environments under euxinic conditions. The fossil fauna was of very low diversity and had a distinctively endemic character. In the Barremian, a regressive cycle began that was characterized by maximum progradation of littoral facies and very low faunal diversity. This changed in the late Barremian when the faunal assemblage was linked to coeval South African faunas through free marine connections between southern Patagonia and southeastern Africa. The Aptian recorded the highest faunal diversity as a result of the open circulation of the South Atlantic Ocean. This was followed by abrupt faunal changes at the beginning of the Albian, when the basinal facies were limited to a small part of the central Austral basin. The fossil fauna first became more cosmopolitan, and then the sediments became poorly fossiliferous.