Onset of the Miocene Chaco foreland basin: depositional conditions, provenance and paleogeographic implications of the Tranquitas Formation (Argentina)

Abstract

The Oligocene–early Miocene Tranquitas Formation in the southern Chaco foreland basin records the onset of Central Andean shortening. The aims of this contribution are to: (1) provide original data of the depositional conditions of the Tranquitas Formation, (2) to determine the role of tectonic and climate influence on the depositional conditions and on the composition of these deposits and (3) propose an evolutionary model of accumulation based on a sequence stratigraphic approach. The detailed analysis of these deposits resulted in the definition of six facies associations, including: complex sandy narrow sheets (FA1), sandy narrow sheets with lateral accretion surfaces (FA2), sandstone ribbons (FA3), tabular muddy-sandstone deposits (FA4), sandstone lobes (FA5), and fine-grained deposits (FA6). Based on the spatial distribution and the vertical stacking of these associations, the Tranquitas Formation was divided into two major intervals. Interval 1 (lower section) is characterized by braided fluvial deposits and represents low accommodation conditions with a high proportion of channel deposits. Interval 2 (upper section) tends to an increase in the accommodation space characterized by the better preservation of floodplain deposits in moderate to high sinuosity meandering fluvial systems. The integration of paleocurrent, petrography, and clay mineralogy analyses indicates that detrital components come from the orogenic region located to the west, in an overfilled stage of the foreland basin. Clay mineral assemblage analysis suggests a long-term climate change from drier to more humid conditions and reflects the climate effect on the depositional paleoenvironments developed in the distal position of the basin. In addition, the resultant vertical and lateral stacking of sedimentary sequences representing different paleoenvironments of the Tranquitas Formation could be correlated with the Oligocene–early Miocene Petaca and lower-Yecua formations located in the center and north of the Chaco foreland basin.