Andean Jurassic inoceramids as bioevent markers for the Austral Realm

DAMBORENEA, S.E.

Mendoza, Argentina

Congreso; 4th International Congress on Jurassic Stratigraphy and Geology; 1994

Subcomisión del Jurásico del CAE

The inoceramids are a group of Mesozoic bivalves most extensively used in biostratigraphy. Although occurring all over the world, their pattern of palaeogeographical distribution has revealed a number of mainly bipolar taxa throughout their history. Furthermore, the presence of some species has been used to characterize the Boreal and Austral realms at certain times. In polar regions  they sometimes were one of the commonest elements or marine faunas. They have just been used, for instance, to establish detailed biostratigraphic zonations (Far East Russia), or to originally define some local Jurassic stages (New Zealand). Andean Jurassic inoceramids were not so abundant as in polar regions but they are by far the most accurately dated by ammonites from the Southern Hemisphere. The zonation proposed for the Middle Jurassic of the Neuquén basin has proven to have wider relevance and is being successfully used to correlate local successions in other regions. Present knowledge on first appearances and disappearances of species is reviewed for the Neuquén basin in order to test these organisms as bioevent markers. The genus Parainoceramus (used in a broad sense) is known by one Pliensbachian species, P. apollo (Leanza), which has wide geographical range, one Aalenian undeterminable species and P.? westermanni Damb. from lower/upper Bajocian boundary. The first appearance of Retroceramus marwicki (Speden) is dated as late Bajocian (Rotundum Zone) in the Neuquén basin. The same lineage is later represented by R. patagonicus (Phil.) in late Bathonian times (Steinmanni Zone) and replaced by R. stehni Damb. during latest Bathonian-earliest Callovian. These last three species have a very wide geographical range including New Zealand and Antarctica. Retroceramus galoi (Boehm) was recently found in late Callovian (?) and early Oxfordian beds in southern Mendoza and northern Neuquén, showing that its first appearance in eastern and western Pacific was coeval.