SEDIMENTOLOGY OF A LATE PALEOZOIC PALEOVALLEY IN THE EASTERN PAGANZO BASIN, LA RIOJA PROVINCE, ARGENTINA

As global climate transitioned from the icehouse of the Late Paleozoic Ice Age (LPIA) to greenhouse conditions of the Late Permian, glaciers vanished across Gondwana; however, not all ice centers responded synchronously. The Paganzo Basin, northwestern Argentina, was geographically adjacent to significant mid-Carboniferous ice centers that disappeared notably earlier than ice over much of Gondwana. Confining the extent of glacial ice during the LPIA and resolving the timing of its disappearance is of particular interest as this dramatic climatic transition is the closest analogue in Earth history to late Cenozoic climate change, potentially aiding in anticipating the response of modern climate to similar drivers.

The majority of the late Paleozoic glacial record in the region is from paleofjord deposits in the western basins, but in the eastern Paganzo Basin a paleovalley, only a few hundred meters wide, was cut into the predominantly granitic basement of the Sierra de Chepes. The basal fill of the valley was previously interpreted as glacigenic, with the valley carved by either local alpine glaciers or an outlet glacier draining westward from an ice sheet situated over the eastern part of the Paganzo Basin. This study tests the glacigenic setting for the paleovalley near Olta, in La Rioja Province.

The strata is dominated by medium- to thick-bedded, fine to coarse-grained sandstones deposited by lacustrine wave activity and underflow currents, and by massive or weakly graded conglomerates deposited by stream and debris flows associated with prograding alluvial fans. Lacustrine sediments interfinger with distal alluvial fan conglomerates along the valley wall, suggesting water was impounded by transverse progradation of the fans across the valley. Bulleted clasts, glacial abrasion marks, diamictites with clast fabrics, and other diagnostic glacial features were not observed. Diamictites were rare and associated with landslide deposits along the valley wall, and soft sediment deformation beneath boulder-rich conglomerates can be attributed to mass-transport deposits and rock fall off of the steep valley walls. The basal fill of the Olta Paleovalley is interpreted here as a lacustrine depositional environment in a narrow mountain valley and contains no evidence for glacial ice in the eastern Paganzo Basin.