New glacial evidences at the Talacasto paleofjord (Paganzo basin, W-Argentina) and its implications for the paleogeography of the Gondwana margin

CAROLINA DANIELSKI AQUINO, JUAN PABLO MILANA, UBIRATAN F. FACCINI

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014 p. 278 - 300

0895-9811

The Talacasto paleovalley is situated in the Central Precordillera of San Juan, Argentina,where upper carboniferous-permian rocks (Paganzo Group) rest on Devonian sandstones of the Punta Negra formation. This outcrop is an excellent example of a glacial valley-fill sequence that records at least two high frequency cycles of advance and retreat of a glacier into the valley. The paleocurrents analysis shows transport predominantly to the south, indicating that in this site the ice flow differs from the other nearby paleovalleys. Evidence of glacial origin of this valley is indicated by the finding of glacial striae on valley sides, and the U-shape of the valley as indicated by very steep valley walls, locally overhanging. Deglaciation is indicated by a suit of retransported conglomerates deposited in a shallow-water environment followed by a transgressive succession, suggesting eustatic rise due to meltwater input to the paleofjord. The complete sedimentary succession records distinct stages in the evolution of the valley-fill represented by seven stratigraphical units. Those units are identified based on facies associations, and their interpreted depositional setting. Units 1 to 5 shows one cycle of deglaciation and the unit 6 marks the beginning of a new cycle of glacier advance characterized by different types of glacial deposits. All units show evidence of glacial influence such as dropstones and striated clasts indicating that the glaciers were always present in the valley or in adjacent areas during sedimentation. The Talacasto paleofjord provides a good evidence of the Late Paleozoic Gondwana glaciation in western Argentina and shows examples of sedimentary successions interpreted as deposited by a confined wet-based glacier in advance and retreat cycles, with eventual release of icebergs into the basin. The outcrop is also a key for reconstructing the local glacial paleogeography which suggests a new interpretation that disagrees with the present-day studies.