Ordovician cycles and Sea level fluctuation in the Precordillera Terrane, western Argentina

BERESI, MATILDE S.

Global Perspectives on Ordovician Geology

Balkema,

Lugar: Rotterdam; Año: 1992; p. 337 - 344

Global Perspectives on Ordovician Geology. Webby & Laurie (eds) e 1992 Balkema. Rotterdam. ISBN 90 5410 048 6 Ordovician cycles and sea-level fluctuation in the Precordillera Terrane, western Argentine Ordovician cycles and sea-level fluctuation in the Precordillera Terrane, western Argentine Ordovician cycles and sea-level fluctuation in the Precordillera Terrane, western Argentine M. S. Beresi Centro Regional de Investigaciones Cientificas y Tecnicas, Mendoza, Argentine ABSTRACT: The Ordovician facies development in the Precordillera terrane shows clear changes as respooses to relative sea-level fluctuation. The Precordillera Basin is located in western Argentina, in the Andean Arc. The Ordovician sequences comprise three cycles: firstly a carbonate cycle developed from the Upper Cambrian-Tremadoc to the beginning of the Middle Ordovician (lower Llanvirn); secondly, a clastic cycle developed from the upper Llanvirn to the lower Caradoc and thirdly, a clastic cycle developed from the Hirnantian to the Llandovery. These cycles appear to be controlled by sea­level changes rather than by tectonic influences. I INTRODUCTION Precordillera Argentina (Fig. 1) is unique in the Andes Mountains in its tectonic style and in that an almost complete sequence of Paleozoic platform sediments permits the identification of the Early Paleozoic margin of ancient Pangea. The Precordillera terrane (Baldis et aI., 1984) is a Cambrian-Devonian shelf-slope-oceanic basin assemblage bounded by melanges on both sides (Ramos et aI., 1986). This terrane was postulated as either having a para-autochthonous origin or an allochthonous derivation from a North American platform fragment (Ramos et aI., 1984). The sedimentary sequence consists of carbonates, siliciclastics and minor oceanic mafic volcanics. The basement below the Precordillera has been regarded as rigid and of Pampean and pericratonic character by Baldis et aI. (1985). The Cambro-Ordovician sediments were deposited from the Late Cambrian-Tremadoc to the late Ashgill in the Precordillera Basin between the Pampean Range craton and the western margin of Gondwana. The post­Ordovician sequences comprise siliciclastic sediments of the Silurian, Devonian, Carboniferous and Tertiary. 2 ORDOVICIAN SEDIMENTARY CYCLES The Ordovician facies development shows changes which have been interpreted as responses to relative sea-level fluctuation. The Ordovician sequences comprise three cycles: flTstly, a carbonate cycle developed from the Late Cambrian-Tremadoc to the beginning of the Middle Ordovician (early Llanvirn); secondly, a clastic cycle from the late Llanvirn to the early Caradoc and thirdly, a clastic cycle which was developed from the Hirnantian to the Llandovery . These cycles appear to reflect sea-level changes rather than local tectonic influences and were developed on the western passive continental margin of Gondwana. Analysis of the cycles of the Precordillera will demand more intensive study using a wide array of stratigraphic, sedimentological and paleoecological methods. 2.1 Carbonate I Cycle Carbonate deposition in the Precordillera terrane took place from the Early Cambrian (Olenellus Zone; Bordonaro, 1986) to the early Llanvirn (Amorphognathus variabilis Zone; Sarmiento, 1985). The carbonate cycle corresponds to the sediments of the Matagusanos Group (Baldis et aI.. 1981), comprising the La Flecha Formation in the south, and its equivalent San Roque Formation in the north and the overlying San Juan Formation limestones. This group ranges from the Upper Cambrian-Tremadoc to the lower 337