The Pico do Itapeva Formation: A record of gravitational flow deposits in an Ediacaran intracontinental basin, southern Brasilia Orogen, SE Brazil

摘要

The Pico do Itapeva Formation is a key metasedimentary unit to the understanding of the extensional events that occurred between the late stage of the southern Brasilia Orogen collision and the main collision in the central Ribeira Orogen. The formation crops out in a 20 km long NE-trending narrow belt in the Mantiqueira mountain range in eastern Sao Paulo State, Brazil. It is located in the interference zone of the southern Brasilia and the central Ribeira orogens and records deformation and greenschist facies metamorphism (biotite zone) related to the Brasiliano orogeny. The Pico do Itapeva Formation rests unconformably on a metaigneous substratum of the Socorro-Guaxupe Nappe/Embu Terrane and, on the southern side, is truncated by a steep SE-dipping dextral reverse shear zone. It consists of a coarsening and thickening-upward succession, with minimum thickness of 800 m, composed of lutite, arkose and conglomerate. These rocks constitute three distinct lithofacies associations: LAI-arkose, arkose-lutite composite beds, lutite and fine conglomerate beds; LAII-arkose, pebbly arkose and scarce lutite and; LAIII-conglomerate and pebbly arkose. Most of the beds are massive; graded beds, dish and convolute structures occur locally. Bed thickness varies from thin to very thick and amalgamated bodies constitute up to 30m thick strata. Three mappable units at scale 1:20,000 were recognized based on different proportions of the three lithofacies associations. The deposits are interpreted as the record of mass flows and associated processes in a fan delta setting developed in an intermontane rift basin. U-Pb LA-ICP-MS detrital zircon ages suggest the maximum depositional age at ca. 611 Ma and the basin evolution is interpreted in the range between 611 and 580 Ma during an inter-orogenic stage between the Brasilia and Ribeira orogenies. (C) 2018 Elsevier Ltd. All rights reserved.