LA FORMACION GUALCAMAYO EN LA SIERRA DE VILLICUM: SUS GRAPTOLITOS Y FAUNAS ASOCIADAS.

PERALTA, SILVIO H.

Boletín Academia Nacional de Ciencias

Academia Nacional de Ciencias

Lugar: Córdoba; Año: 1995 vol. 33 p. 61 - 69

     The Gualcamayo Formation is recognized in the eastern slope of the Villicum range, eastern Precordillera of San Juan Province (Argentina).This Formation is 30 m thick, overlay transitionally limestones of the San Juan Formation (Early Ordovician), is divided in this work into two units that crops out in the Don Braulio Creek:- Lower Member homologous to the so-called Transition Zone composed of a cyclic alternation of dark muddy limestones beds and black shales, approximately 12 m thick, and bears fossil assemblages of shelly faunas, graptolites, and conodonts of early Llanvirnian age. The shelly faunas and conodonts assigned to the Amorphognathus variabilis-Eoplacognathus suecicus Zone, occur in the dark muddy limestones beds, and the black shales bears a  graptolitic association composed of Paraglossograptus tentaculatus Hall, Glossograptus hincksii (Hopkinson), Oelandograptus austrodentatus americanus (Bulman), Psudotrigonograptus ensiformis (Hall), Zigograptus sp., Isograptus caduceus Salter, Amplexograptus aff. A. arctus Elles & Wood, Isograptus sp., Tetragraptus sp. and Pterograptus sp. -The upper Member of 20 m thick is made up of black shales and muddy shales, and bears graptolites assemblage composed of Paraglossograptus tentaculatus (Hall), Glossograptus hincksii (Hopkinson), Pseduotrigonograptus ensiformis (Hall), Oelandograptus austrodentatus americanus (Bulman), Amplexograptus cf.  A. arctus Elles & Wood, Isograptus caduceus primulus Harris, Iso-graptus caduceus australis Cooper, Isograptus sp., Pterograptus sp., and Zigograptus sp. This assemblage and those from the Lower Member are assigned to the Paraglossograptus tentaculatus Zone of the Ordovician bioestratigraphic sequences of North America.     The correlation of fossiliferous Llanvirnian deposits from Villicum range with the Gualcamayo Formation on its type locality (Guandacol) and with the Los Azules Formation on the Jáchal area, is discussed  on the basis of the  lithostratigraphic, stratigraphic relations and biostratigraphic characters. However, the age  of the Transiton Zone (Lower Member in this paper) between San Juan and Gualcamayo Formations in Guandacol, is assigned to the Middle Arenigian age based on its graptolite and conodont assemblages, thus that this Member could be postulated as a diachronic unit.