Comparative Lower-Middle Ordovician conodont oxygen isotope palaeothermometry of the Argentine Precordillera and Laurentian margins

ALBANESI G. L.; WILLIAMS, I. S.; TROTTER J. A.; BARNES C. R.; BERGSTRÖM S. M.

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2020

0031-0182

Stratigraphic, palaeontologic, and palaeomagnetic data support a hypothesis that argues for the ArgentinePrecordillera rifting from the southwestern margin of Laurentia in low latitudes during the Cambrian, migratingacross the Iapetus Ocean, colliding with the Gondwanan margin in the late Middle Ordovician, and receivingglaciogenic sediments in the Late Ordovician. An alternative model proposes that the Precordillera originated asa low-latitude segment of Gondwana, migrated southward through major transform faulting toward high latitudesin the late Middle Ordovician, to reach its present position in the Devonian. New conodont oxygen isotopecompositions (δ18Ophos) have been determined by ion microprobe SHRIMP II using samples from both thePrecordillera and Laurentia (Marathon area of Texas, Wilcox Pass in Alberta, and western Newfoundland).Significantly, the δ18O values of conodonts from all four widely separated areas show a consistent pattern of acyclic but overall increasing trend in δ18O (ca. 16 to 18?) hence ocean cooling through the Early and MiddleOrdovician. An apparent change occurs at the basal Late Ordovician, where δ18O values obtained from conodontsin the uppermost sample from the Precordillera are significantly higher (+1.5?) than those fromLaurentia. Albeit from a single sample, this higher value implies significantly cooler conditions, as would beanticipated with a southerly (poleward) migration of the Precordillera (irrespective of either hypothesis). Thevirtual absence of conodont-bearing carbonates in most of the Precordilleran Upper Ordovician precludedanalysis of younger samples. When combined with existing macrofaunal and palaeomagnetic data, the oxygenisotope data would tend to favour the model of a drift of the Precordillera from tropical to higher latitudesduring the Ordovician; however, further studies are needed to determine unequivocally whether thePrecordillera originated from southern Laurentia (Ouachita embayment). These new oxygen isotope valuesprovide the best and regionally most consistent data through the Early-Middle Ordovician.