Graptolites of the Nemagraptus gracilis Zone in the black shale sequence of the San Juan Precordillera, Argentina: Its biostratigraphic and paleoenvironmental significance.

PERALTA, S.H.

Madrid

Conferencia; International Graptolite Conference (GWG-IPA) & 1998 Field Meeting IUGS Subcommission on Silurian Stratigraphy; 1998

ITGE, Univ. Complutense Madrid, Int. Commission on Stratigraphy, IUGS

ABSTRACT: Graptolite-bearing black shale sequence of the Los Azules Formation (Upper Ordovician, Caradoc), from the western flank of the Cerro La Chilca, at the Central Precordillera, in San Juan Province (Argentina), is described in this paper. Graptolite assemblages are adscribed to the Nemagraptus gracilis Zone, which is widely distributed overall Precordillera. On the basis of the biostratigraphic significance of the collected faunas, and sedimentological studies, the black shales of the Los Azules Formation were deposited during the basal Caradoc epoch, in agreement with the occurrence of graptolites of the lower part of the Nemagraptus gracilis Zone. The whole euxinic sequence of the Los Azules Formation, very rich in organic remains, may be related to the world wide transgresive and anoxigenic event, recognized on that time, and its facies sequence is interpreted as a main rise in the sea level. Such sequence in the Ordovician of Precordillera, is followed by a lowering at the Late Ordovician, during the Gondwana glacial episode.