Using sandstone and conglomerate petrofacies to unravel multiple provenance areas in broken-foreland basins: The vinchina formation (Miocene, NW Argentina) as a study case

Abstract

The Vinchina Formation is a very thick succession (up to 6400 m-thick) of red-beds deposited in fluvial, fluvio-eolian, eolian, and lacustrine environments. This unit constitutes part of the Miocene sedimentary record of the broken-foreland Vinchina Basin, cropping out along the Sierra de Los Colorados, La Rioja Province, northwestern Argentina. The aim of this paper is to provide information regarding the provenance areas that supplied detritus to the Vinchina Formation. For this purpose, modal compositions of thirty-three conglomerates and one hundred and three sandstone samples were studied. On the basis of the registered modal compositions, five petrofacies were defined in both the gravel and the sand grain-size. Compositional variations both in time and along depositional strike allowed recognizing subtle changes in the source areas. The Nv petrofacies is defined by neovolcanic (andesitic) contents greater than 90% in conglomerates or 65% in sandstones, and it represents a synchronous volcanic supply, most likely intrabasinal in origin. The Bas petrofacies is defined by clasts with plutonic/metamorphic affinities contents greater than 85%, representing the advance of a crystalline clastic wedge from north to south, resulting from the tectonic uplift of the Sierra de Toro Negro. The MxB petrofacies comprises mixtures of plutonic/metamorphic and volcanic components with dominance of the first. The plutonic/metamorphic contents in this petrofacies vary between 50% and 85% in conglomerates and between 60% and 85% in sandstones. It results from combined detrital contributions from the crystalline basement and the Cordillera, where the former dominates over the latter. Similarly, the MxPv petrofacies indicates modal compositions where the mixtures are dominated by paleovolcanic clasts. Their contents vary between 50% and 85% in conglomerates and between 60% and 85% in sandstones. This mixed petrofacies results from the combined detrital supply from the Cordillera and the crystalline basement, with dominant cordilleran contributions. Finally, the Pv petrofacies is defined by paleovolcanic clasts contents greater than 85% in conglomerates, whereas it was not registered in the sand fraction. This petrofacies represents a highly dominant paleovolcanic supply from the cordilleran area. The identification of internal compositional variations allowed interpreting an evolutionary model for the basin: the first stage is characterized by paleovolcanic cordilleran detrital contributions almost exclusively, with a minor presence of crystalline detritus from the barely uplifted Sierra de Toro Negro; the second evolutionary stage represents an important uplifting pulse of the Sierra de Toro Negro and the advance of the crystalline clastic wedge (axial drainage); the third stage is marked by the withdrawal of the clastic wedge and the recovery of the paleovolcanic cordilleran supply (transverse drainage), combined with minor crystalline contributions; the last evolutionary stage represents the second uplifting pulse of the Sierra de Toro Negro, which led to a higher presence of crystalline detritus mixed with the paleovolcanic supply. The neovolcanic contributions indicate discrete volcanic effusions in the basin vicinity geographically related to the cordilleran area, or most likely intrabasinal.