New chronostratigraphy for a lower to upper Carboniferous strike-slip basin of W-Precordillera (Argentina): Paleogeographic, tectonic and glacial importance

Abstract

New biostratigraphic ages and the stratigraphic revision of a c. 4 km thick succession of Lower and Upper Carboniferous beds, exposed in the Del Salto creek, in the western Precordillera (San Juan, Argentina) are presented. Detailed mapping and analysis of the vertical and lateral variations of the sedimentary systems of the three units we are redefining here (El Planchón, Churupatí and Del Salto formations) allowed us to modify many pre-existing concepts. The palynological data provided for the El Planchón Formation corroborate it is not Devonian but Carboniferous, (mid-late Visean), since it conformably overlies the Del Ratón Formation (Early Visean). A distal glacial interval in El Planchón lower member, distal turbidites and hemipelagites in the middle member and non-glacial shallow-water heterolithics with small slumps and proximal turbidites for the upper member are interpreted. An angular unconformity defines the base of the alluvial conglomeratic Churupatí Formation (redefined herein), that shows dramatic lateral variations from a dominantly alluvial conglomeratic sequence to the north (green- purple conglomerates) and sandier fluvial to swampy sequences to the south. Chrupati palynological content suggests its late Visean – early Serpukhovian age. A new erosive unconformity separates Churupatí from Del Salto formations with an interpreted glacial origin, while the palynological content of basal Del Salto Fm indicates a Pennsylvanian (Bashkirian) age, not Permian. A dextral strike-slip basin is interpreted to explain this Carboniferous succession, based on geological, stratigraphical and sedimentological data. Evidence supporting this interpretation are: (1) its thickness (c. 4 Km) in a restricted geographical place as no traces of these units are found east or westwards except for Del Salto formation equivalents. (2) the amount of active-tectonically fed gravel, important lateral changes, and fast transitions to from gravel-to mud-dominated sequences, (3) the southward displacement of the entry-point from hinterland of coarse sediment observed between Churupatí and Del Salto formations, (4) the partial rotation of local structures, (5) its coincidence with the modern dextral Del Tigre fault. The entire sequence supports the existence of three main glacial episodes and does not support the existence of the Protoprecordillera, but a surrounding hummocky positive terrain that did not obstruct the drainage systems moving sediment eastwards.