GLACIAL AND NON-GLACIAL DIAMICTITES: NEW INSIGHTS INTO THE MID-CARBONIFEROUS GLACIATION IN SOUTHWESTERN GONDWANA

One of most stratigraphically continuous and high fidelity archives of the mid-Carboniferous glaciation occurs in the Paganzo Group of northwestern Argentina. Existing interpretations of mid-Carboniferous glacigenic strata in the Paganzo Group argue for persistent glaciation, marked by several advance and retreat cycles of moraines associated with tidewater glaciers. A recent reinvestigation of outcrops hosting the mid-Carboniferous glaciation in NW Argentina reveals that the earliest Carboniferous sedimentary succession is not of glacial origin, but is most likely paleo-scree and breccia associated with karst topography. Furthermore, these deposits form a complex paleo-land surface developed on and within the Early Ordovician San Juan Limestone, characterized characterized by hummocky surfaces composed of paleo-scree and isolated blocks, angular channels filled with debris, and smooth and polished surfaces displaying nail head striations and roche moutonneé. In turn, these deposits are overlain by diamictite of glacial origin, containing large (>1m) dropstones, and faceted and striated clasts. Deformation and dewatering structures within the underlying paleo-scree and block deposits suggest that the basal non-glacial diamictite records the advance of continental ice prior to lithification. Directions of ice flow have been revised through careful examination of the nail head striations revealing that ice flow was likely east-west (present day orientation), which indicates that the mid-Carboniferous glaciation in NW Argentina was composed of alpine glaciation, as described elsewhere, plus an additional substantial volume of ice further inboard from the coast line. The composition of dropstones within the glacial diamictite combined with ice flow direction from nail head striations indicate that this additional volume of ice may have occupied the ancestral Sierra Pampeanas. These discoveries prompt a re-evaluation of the tectonic and glacial history of the southwest Gondwana margin during the late Paleozoic, and strongly suggest a coupling between the uplift of the Proto-Precodillera and the inception of glaciation during the mid-Carboniferous.