Stratigraphy and palynology of a late Paleozoic glacial paleovalley in the Andean Precordillera, Argentina

Abstract

A new locality where Carboniferous glacial diamictite appears confined to paleovalleys is described in the north of the Argentinian Precordillera. The glacial deposits of the lower part of the Carboniferous Quebrada Larga Formation are divided in three stratigraphic intervals, all of them confined to a paleovalley carved into the granitic and high-grade metamorphic basement. The lower section is composed of different types of massive and stratified diamictite interpreted to record reworking of previously deposited poorly sorted glacial sediments. The middle section comprises shale with dropstones, lenticular beds of diamictite and large-scale cross-bedded sets of sandstone and conglomerate. These rocks represent a more advanced stage of deglaciation that comprises the following succeeding steps: 1) amelioration of the climatic conditions, melting of glaciers, fluvial erosion, 2) the formation of a water body (onset of the fjord system) and 3) a later progradation of Gilbert-type deltas. Diamictite is missing in the upper section, which is chiefly formed by cross-bedded sandstone and conglomerate deposited in braided fluvial plains. A quantitative analysis of palynological assemblages through the sequence allowed a reconstruction of the dynamics of the vegetation developed during the filling of the paleovalley. The presence of monosaccate pollen grains supports an age not older than Serpukhovian and the recognition of the characteristic species of the Raistrickia densa–Convolutispora muriornata Biozone reinforces a late Serpukhovian–Bashkirian age for the assemblages.