Gondwana Glacial Paleolandscape, Diamictite Record of Carboniferous Valley Glaciation and Preglacial Remnants of an Ancient Weathering Front in Northwestern Argentina

Abstract

A record of glacier advance and retreat is preserved in Carboniferous strata exposed in an exhumed glacial paleovalley on the eastern side of the Paganzo basin. Previous investigations have focused on the sandstones in the paleovalley and inferred a glacial lacustrine history. New observations have demonstrated that remnants of a preglacial, ancient weathering front, developed under wet tropical conditions and composed of corestones, are found underneath the glaciogenic deposits. Delta and alluvial fan deposits were also recognized, but no inferences were made from the diamictites in the paleovalley regarding glacial events (Andreis et al., Bol Acad Nac Cienc Cordoba 57:3–119, 1986; Buatois and Mángano, J Paleolimnol 14:1–22, 1995; Sterren and Martínez, El Paleovalle de Olta (Carbonífero): Paleoambiente y Paleogeografía. 13º Congreso Geológico Argentino and 3º Congreso de Exploración de Hidrocarburos, Actas, 2, 89–103, 1996). This chapter focuses on the diamictites and provides a link between the sediment infill and the glacial origin of the paleovalley. We describe diamictites and associated sediments at three main locations: at La Chimenea, near the mouth of the paleovalley; at Mid-Valley, near the middle of the paleovalley; and at the Campsite near the head of the valley. We interpret some of the diamictites exposed at La Chimenea and at Mid-Valley to be subglacial tillite. Deformation in the sandstone underlying the tillite indicates warm-based conditions as the glacier advanced over soft deformable sediment. At the Campsite location, a diamictite bed, which is about 1.5 m thick, lies within a sequence of alternating sandstone and siltstone beds. The diamictite bed is interpreted to represent an ice-front readvance during a period of ice retreat. The diamictite may be a debrite originating off the ice front, or a subglacial deposit, i.e., a tillite, or a combination of both. Two additional diamictite beds, exposed higher in this sequence of alternating sandstone and siltstone beds, may also record minor ice-front advances into the flooded valley. Evidence of an ancient, preglacial weathering front (Late Devonian?–Earliest Carboniferous?) has been found in the granitic basement rocks which underlie the glaciogenic deposits, as large corestones included in a weathered regolith. This weathering front was developed under wet tropical conditions, before the onset of Carboniferous glaciations. The tillite and other diamictites overlying the corestones are composed largely of locally derived granitic basement rock. Features observed in the tillite and other diamictites are attributed to rapid rates of deposition, depositional processes, and the susceptibility of pre-weathered granitic basement rock to glacial and other erosional processes. Processes other than glacial erosion and deposition, including mass transport (slumping, rafting, sliding, and debris flow), also operated in the steep-sided valley and contributed large amounts of diamictite and other sediment to the valley fill. Corestones, weathered from the basement rock during a pre-Carboniferous period of intense weathering, constitute the larger clasts in the diamictite and associated deposits. The glacial paleolandscape is very well preserved in detail, after being buried during the Permian and later exhumed in the Cenozoic. The glacial valley was likely a transitional (fjord) environment, as micropaleontological material (Gutiérrez and Limarino, Ameghiniana 38:99–118, 2001) and clay mineral assemblages (Net et al., Sediment Geol 152:183–199, 2002) indicate a marine transgression into the area during the Middle Carboniferous.