Paper No. 5
Presentation Time: 2:25 PM


THOMAS, William A., Geological Survey of Alabama, P.O. Box 869999, Tuscaloosa, AL 35486-6999 and ASTINI, Ricardo A., CICTERRA-CONICET, Laboratorio de Análisis de Cuencas- Universidad Nacional de Córdoba, Av. Velez Sarsfield 1611, Córdoba, X5016GCA, Argentina,

The Precordillera microcontinental terrane (now in the eastern foothills of the Andes in northwestern Argentina) was rifted from the Ouachita embayment of southern Laurentia during the Early Cambrian and was accreted to western Gondwana during the Middle–Late Ordovician. These conclusions are based on multiple, independent lines of evidence. In contrast, existing alternative interpretations have used only selected parts of the data, not all of the data.

Evidence for Cambrian rifting history and provenance:

• a Cambrian-Ordovician carbonate bank like Laurentia, unlike Gondwana;

• Olenellus-Salterella; Laurentian Early Cambrian faunas;

• basement Pb isotopes similar to Laurentian Texas promontory;

• Precordillera synrift redbeds-evaporites coeval with 539–534 Ma Laurentian synrift igneous rocks along the Southern Oklahoma transform-parallel fault system;

• Early Cambrian synrift detrital-zircon population like Laurentian Alabama promontory;

• Sr isotopic similarity of Early Cambrian evaporites of Precordillera and Laurentian Alabama promontory;

• complementary asymmetry of passive-margin subsidence along conjugate margins of Precordillera and Laurentian Ouachita embayment; and

• Cambrian paleomagnetic pole of Precordillera consistent with palinspastic location in Laurentian Ouachita embayment.

Evidence for history of Ordovician accretion to Gondwana:

• faunal evolution from Laurentian (Early Cambrian) to endemic to Gondwanan (Middle Ordovician);

• foreland flexural subsidence and deposition of Middle Ordovician black shale;

• westward prograding clastic wedge (Middle to Upper Ordovician);

• Famatina volcanic arc (499–468 Ma) on Gondwanan upper plate;

• multiple (>300) bentonite beds in Middle Ordovician succession;

• 464 Ma metamorphic overprint along east side of Precordillera;

• detrital-zircon ages from synorogenic clastic wedge indicating basement sources along east side of Precordillera;

• Middle–Late Ordovician west-vergent thin-skinned thrust belt; and

• Gondwanan Hirnantian glaciation on Precordillera.

The integration of multiple independent lines of data supports the internally consistent interpretation of Early Cambrian rifting of the Precordillera from the Ouachita embayment of Laurentia and Middle–Late Ordovician accretion to western Gondwana.