STRATIGRAPHIC ARCHITECTURE IN EARLY STAGES OF INTERMONTANE BASIN: THE MIOCENE CALCHAQUÍ FORELAND, NW ARGENTINA

CECILIA EUGENIA DEL PAPA; PATRICIO PAYROLA; HONGN FERNANDO; PINGEL, HEIKO; DO CAMPO, MARGARITA; LAPIANA AYELEN; STRECKER, MANFRED R.

Congreso; 25° Latin- American Colloquium.; 2019

Foreland basins are sensitive recorders of spatiotemporal variations of tectonic and climatic forcing concerning to processes associated with an approaching orogenic front. The analysis of depositional systems, and paleoflows coupled with stratigraphic arrangement of the foreland deposits allows a fairly accurate assessment of extra and intra basin deformational processes. The Calchaquí region, located at ~24-26°S in the Eastern Cordillera of NW Argentina, was once part of the contiguous Andean foreland basin that evolved structurally into a compartmentalized broken foreland during the Mio-Pliocene. This region is ideal for the study of the variations in tectonic, climatic and sedimentary processes, due to very well exposed of basin strata that contain a rich record of flora and vertebrate fossils, and radiometrically datable volcanic ashes. We combined traditional sedimentological methods, U-Pb zircon and K-Ar geochronology, clay mineralogy, and geochemical weathering/climate indices with structural field data and fault modeling to document the stratigraphic response of the former foreland deposits to basin fragmentation. The 14-9 Ma Las Flechas Member (upper Angastaco Formation) and the 9-5 Ma Palo Pintado Formation comprise three depositional systems and transitioned from gravelly braided, sandy braided to anastomosing-river systems. The slow transition from sandy to gravelly braided and the rapid transition from gravelly to anastomosing reveal a prograding-retrograding pattern of the sedimentary sequences highlighting an expansion surface. Thus, the regional sedimentary succession recording the transition between an unrestricted to a compartmentalized foreland is represented by the sharp contact between gravelly braided and the anastomosing that must have occurred by ca. 9 Ma. The new data suggest that fluvial depositional systems changed several times in terms of location and style and were associated with pro- and retrograding gravel wedges, most likely in response to multiple fault reactivations of the basement and inversion of normal faults most likely in response to structural growth of the eastern basin-bounding ranges of the Eastern Cordillera.