MID-ORDOVICIAN VOLCANISM IN THE WESTERN OUACHITA-CUYANIA BASIN

The W. Ouachita basin (Marathon basin, W. Texas) and Cuyania (greater Precordillera, W. Argentina) are underpinned by Laurentian Mesoproterozoic basement and evolved together, as evidenced by isotopic, litho-, bio- and chronostratigraphic data, and recent paleomagnetic determinations. Ages, lead-isotopic and geochemical data correspond well for Cuyania and for west-central Texas crystalline basement rocks, notably the Llano Uplift, Pecos layered mafic complex, and the W. Sierras Pampeanas with the anorthosite massifs of Sierras de Maz and Umango.

Proterozoic-Eocambrian outcrops around the northern basin rim supplied detrital zircons to Mid-Cambrian sandstones of Cuyania. Fully correlative carbonate successions developed on both the northern and southern platforms and hosted homologous sponge-algae-stromatoporoid reefs. Within the off-shelf calcarenite debris flows and bentonitic shales of the Marathon Fm. is a 20-km-long layer of mega-olistoliths (to 10s of meters) of shelf carbonates. Those and analogous mega-olistoliths in off-shelf deposits of the Precordillera were shed from fault-bounded blocks during extension/ transtension in the basin.

Volcanic ash beds occupy the same positions in both stratigraphic successions. Precordilleran metabentonites are dated at 469.5 ± 3.2 to 470.1 ± 3.3 Ma (U-Pb, SHRIMP, zircons) and are coeval with rhyolites of the Famatina complex. Pyroclastic zircons have been recovered from bentonitic intervals of the Marathon and Ft. Peña Fms., within identical faunal zones to those for ash beds within the San Juan and Gualcamayo Fms. of Cuyania. Meter-scale volcanic olistoliths and sand-sized volcaniclastics, were fed to the W. Ouachita (Marathon) basin from the south – possibly from the Famatina continental margin arc of W. Gondwana.

Extensional block faulting, variable shelf carbonate and outer-shelf/slope sedimentation, and explosive volcanism characterized the W. Ouachita-Cuyania basin in L. Cambrian- M. Ordovician time. Stratigraphy and structures of the basin support plate reconstructions based upon high-precision paleomagnetic data, which place S. Laurentia opposite W. Gondwana at low southern latitudes (~26ºS). The attenuated, thermally weakened Laurentian slab broke apart with continued right-oblique separation of Laurentia and Gondwana.