Middle Jurassic biostratigraphy of Argentina

RICCARDI, A.C.; DAMBORENEA, S.E.; MANCEÑIDO, M.O.; BALLENT, S.C.

Poitiers, Francia

Simposio; 3rd International Symposium on Jurassic Stratigraphy; 1991

The Middle Jurassic of west-central Argentina comprises several stratigraphic sequences. They have been characterized on the basis of the geographic and stratigraphic distribution of ammonoids, bivalves, brachiopods, foraminifers and ostracods. Aalenian ammonoids are relatively rare and belong in the Bredya manflasensis and Zurcheria groeberi Zones. Coeveal bivalves are quite diverse and typical of  near-shore facies. Few relictual brachiopods from Late Toarcian assemblages are also present. The ammonoid fauna becomes very abundant  and widespread close to the Aalenian-Bajocian boundary (Puchenquia malarguensis Zone). Throughout the Early Bajocian (Pseudotoites singularis, Emileia giebeli and Humphriesianum Zones) there is a declining trend in diversity, except for a minor rise in the giebeli Zone. Similar diversification peaks are shown by calcareous microfossils. For the same interval, diversity of bivalves and, to a lesser extent, of brachiopods exhibits a steady increase to attain its maximum for the whole Middle Jurassic in the singularis-giebeli Zones. Late Bajocian ammonoid faunas are poorly known, only the Megasphaeroceras rotundum Zone is well-represented containing a rich assemblage approximately equivalent to the Subfurcatum Zone. Bivalves become less varied and are confined to prevailing off-shore facies virtually  devoid of other invertebrates. Further restriction of platform facies characterizes most of the Bathonian and earliest Callovian. Ammonoids are missing of poorly represented in the Early and Middle Bathonian. They become comparatively more abundant in latest Bathonian and early Callovian (Steinmanni, Vergarensis and Bodenbenderi Zones), where two inoceramid zones are also recognized (Retroceramus patagonicus and R. stehni Zones). Late Early to Middle Callovian regressive near-shore facies usually contain relatively common bivalves, brachiopods, foraminifers and ostracods, whereas ammonoids are rarer (Proximum Zone and Loczyceras patagoniensis Horizon).