Middle Jurassic Biostratigraphyof Argentina

The Middle Jurassic of west-central Argentina comprises several stratigraphic sequences characterized on thebasis of ammonoids, bivalves, brachiopods, foraminifers and ostracods. Aalenian and Bajocian ammonoids belong in the Bredyia manflasensis, "Zurcheria" groeberi Puchenquia malarguensis, Pseudotoites singularis, Emileia giebeli, Stephanoceras humphriesianum and Megasphaeroceras rotundum Zones. Three independent series of four biozonal units each, respectively based on bivalves, brachiopods and microfauna, are recognized in this interval. Bathonian and Callovian ammonoids are allocated to the Cadomites--Tulitidae, Lilloettia steinmanni, Eurycephalites vergarensis, Neuqueniceras (Frickites) bodenbenderi, Hecticoceras proximum Zones and Rehmannia (Loczyceras) patagoniensis Horizon. At the same time, further biostratigraphic subdivisions are distinguished by other taxa, totalling two for bivalves, three for brachiopods and one for calcareous microfossils. Diversity trends of the different faunal groups and sequence distribution are explained by changes in relative extension of platform and basinal areas.