OSTRACODS FROM THE LOWER MEMBER OF THE DEVILS GATE FORMATION (FRASNIAN) IN THE TYPE LOCALITY (NEVADA) - RELATION WITH THE ALAMO EVENT

The presence of entomozoacean ostracods belonging to the genus Franklinella in the Alamo Breccia Member exposed on the west slope of the Tempiute Mountains (Nevada), also in the punctata Zone in Belgium (Europa) and probably at the same level in Algeria (North Africa), has suggested that the Alamo Event must be related to a marine hypoxic event. Since hypoxic events are frequently associated with period of extinction, we have undertook the study of ostracods from the Late falsiovalis to the hassi Zones, in the Devils Gate Pass section located close to Eureka, in Nevada. For Sandberg et al. (1997), the Alamo Event is probably related in that section to a 29-cm thick bed of slightly sandy, graded calcareous diamictite with granule-size carbonate clasts.

About 2000 ostracods have been extracted from 36 samples, and 26 taxa were identified in the lower member of the Devils Gate Fm. Ostracods are generally rare or absent, and they belong to the Eifelian Mega-Assemblage indicative of shallow environments. No ostracods indicative of hypoxic water conditions (Myodocopid Mega-Assemblage) or of deep and (or) cold environments (Thuringian Mega-Assemblage) has been found. The distribution of ostracods is greatly influenced by strong salinity variations in the lower member of the Devils Gate Fm. They are poorly diversified, generally indicative of semi-restricted water conditions, and their absence in several samples may indicate real lagoonal conditions.

The rarity and poor diversification of ostracods in samples collected in the base of the lower member of the Devils Gate Fm is not favorable to display any extinction close to the bed related to the Alamo Event. Nevertheless their greater abundance and diversity above this bed seems to indicate the absence of extinction in shallow setting.

The ostracod fauna displays affinities with fauna described from Western Canada and, however in a smaller extent, from the Russian Platform. The exchanges with western Europa were, on the contrary, reduced.