ABSTRACT: Devonian Paleogeography of the West-Central Precordillera Region, San Juan Province, Argentina

SESSAREGO, HORACIO L., Independent Geologist, Calgary, Alberta, Canada

A study of the Devonian outcrops in the Western Precordillera of San Juan Province, Argentina, was undertaken in order to obtain a better understanding of the tectonic-sedimentary evolution of the Devonian Cuyo basin, located on a passive continental margin. Detailed regional mapping and facies analysis have led to a stratigraphic and depositional model for the sequence.

During the Lower to Middle Devonian, deep-sea fan sediments named El Planchon Formation were deposited in the outer areas of the basin. They are coarse conglomerates, sandstones, and minor mudstones, arranged in a large-scale, fining-upward sequence. The coarse-grained facies were deposited from various types of sediment gravity flows. The paleocurrent pattern and the sandstone composition (dominantly basalt and sedimentary rock fragments) suggest a local, eastern provenance attributed to the faulting and uplifting of the Zonda arch. This north-south-trending submarine arch, composed almost exclusively by an Ordovician ophiolite complex, separated the eastern main basin from the western El Planchon deposition site. It had a dominant influence on sedimentation until the Late Permian.

In Late Devonian time, while the eastern main basin was filled with submarine fan deposits (Punta Negra Formation), fine-grained turbidites called Codo Formation were confined to the western area of the Zonda arch. These sediments were deposited from high- to low-density turbidity currents. An eastern cratonic complex indicates the sandstone composition and the paleocurrents of both Codo and Punta Negra formations, and suggests that the arch has not acted as a significant sediment supplier.

At the beginning of the Carboniferous, a major diastrophic episode occurred and the arch became a paleogeographic high called Proto-Precordillera. It was the setting of the Alpine type glaciers, which generated the tillites along the eastern and western flanks of the Precordillera.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)