The collapse of the protoprecordillera and unification of the Paganzo and Calingasta-uspallata basins: Retamito Formation a key in the integrative biostratigraphic and palaeoenvironmental model

Abstract

The Retamito Formation is located at the southernmost of Oriental Precordillera, San Juan, Argentina. This contribution defines eight facies associations: FA1 (breccias and conglomerates) linked to initial valley fills by a chaotic basal colluvium fan and/or karst collapse. FA2 (sandstones and gravelly sandstones) is interpreted as deposits of coastal shoreface systems. The FA3 (coal and coaly mudstones) could be attributed to a paralic environment and represent the first record of plant remains of Argentina (NBG Biozone). FA4 (gravel-bed) is interpreted as a proximal alluvial system. FA5 (shales) contains the first record of brachiopods and a new taxa Komiellanetes cesariae sp. nov, probably representing an eastward extension of the TS fauna in a brief marine incursion. FA6 (mudstones and sandstones) is deposited in a delta front system. The FA7 (red mudstones) are linked to lagoons deposits, where they preserved a floristic assemblage of megaflora remains (Ferugliocladus Superbiozone) composed of seeds, stems, and leaves that are most likely of Permian age. Finally, the FA8 (interbedded sandstones and mudstones) is interpreted as deposited by an anastomosed fluvial system. We present paleoenvironmental and paleontological data of the Retamito Formation correlated with the neighboring Andapaico and Ansilta formations in an east-west transect. This correlation parsimoniously linked the Paganzo and Calingasta-Uspallata Basins in seven filling scenarios (temporal stages Ts). From Ts1 to Ts5, both basins were separated by the Protoprecordillera. Common paleoenvironments on both basins suggest unification of them in Ts6 and Ts7. These temporal stages are paleontologically controlled by two new fossil levels and the corresponding sedimentary paleoenvironments that contain them. Thus, it could be demonstrated that the positive element of the Protoprecordillera was active until the Upper Carboniferous and collapsed towards Early Permian, unifying both basins in this sector of Gondwana.