THE OCLOYIC (ORDOVICIAN) PERIPHERAL FORELAND BASIN AND ACCRETION OF THE ARGENTINE PRECORDILLERA MICROCONTINENT TO WESTERN GONDWANA

THOMAS, William A., Department of Geological Sciences, University of Kentucky, Lexington, KY 40506-0053, [email protected] and ASTINI, Ricardo A., Cátedra de Estratigrafía y Geología Histórica, Universidad Nacional de Córdoba, Pabellón Geología, Ciudad Universitaria, 2º Piso, Oficina 7, Córdoba, X5016GCA, Argentina

The Argentine Precordillera in the southern Central Andes is an accreted Laurentian terrane of Grenville-age basement and Cambrian-Ordovician carbonate-bank cover. Above the carbonate bank, a Middle-Late Ordovician synorogenic clastic wedge documents subsidence of a peripheral foreland basin in response to tectonic loading as the Precordillera microcontinent entered an east-dipping subduction zone beneath western Gondwana.

A regionally diachronous, westward progressing, upward transition from the carbonate bank to graptolite-rich black shales denotes initial subsidence of the foreland basin in early Middle Ordovician time. Above the black shales, westward-prograding conglomeratic turbidites include rounded clasts (~15-cm-scale) of igneous rocks and quartzite from an extrabasinal orogenic source and large (~5-m-scale) blocky olistoliths of limestone and black shale from a thin intrabasinal stratigraphic interval at the top of the Precordillera carbonate bank. Thin-skinned thrust sheets, detached near the top of the carbonate-bank stratigraphy, propagated into the foreland basin during deposition of the coarse turbidites, and supplied the large intrabasinal olistoliths. Progradation of turbidites continued through Middle and early Late Ordovician. West-flowing (transverse) paleocurrents in the east diverge westward into north-and-south-flowing (longitudinal) paleocurrents, consistent with basin-scale turbidity flows from an orogenic provenance on the east in the internides of the Ocloyic orogen, where top-to-west shear zones are associated with Ordovician and older metamorphic and igneous rocks. In the distal foreland to the west, shallowing-upward carbonates, coeval with the clastic wedge, mark the location of a peripheral forebulge.

The Famatina volcanic arc east of the Precordillera documents Ordovician subduction volcanism on the western Gondwana upper-plate margin. Famatina-derived K-bentonite beds extended throughout the Ocloyic foreland basin; however, no Famatina detritus has been recognized in the foreland-basin sediment, similar to provenance indicators in other foreland basins. Petrographic and isotopic data indicate a primary provenance in crystalline rocks of the orogenic hinterland, now exposed between the Precordillera and Famatina.