Paper No. 3
Presentation Time: 9:00 AM-6:00 PM


ANDERSON, Heidi, Department of Geosciences and Natural Resources, Western Carolina University, Cullowhee, NC 28723,

The Carboniferous Macharetí and Mandiyutí groups, deposited in the Tarija-Chaco Basin in southern Bolivia and northern Argentina, consist of repeating packages of sandstone and diamictite units measuring up to 1000 meters for each group. Facies analysis of the Macharetí and Mandiyutí groups resulted in the identification of 27 lithofacies and 10 lithofacies associations.

Diamictite units are dominated by Dmm, Dmm(c), Dms(c), and Dml with minor Dmm(r), Sm, St, and Scr. These lithofacies are common in the Macharetí Group and the upper Mandiyutí Group (San Telmo Formation). Glacial pavements occur in Dmm lithofacies and dropstones and Diplopodichnus biformis are described in Dml lithofacies, especially in the southern part of the basin. Faceted and striated clasts are found in all diamictite lithofacies. Four lithofacies associations identified from these lithofacies support deposition in proximal to distal glacial settings. Thickness of diamictite units and presence of glacial pavements in southern Bolivia support the presence of an ice sheet in northern Argentina and southern Bolivia that periodically entered the basin.

Sandstone-dominated units vary greatly in lithofacies, most commonly including St, Se, Sh, Sd, and Scr and minor Dmm, Dmm(r), Sm, Su, Sr, Fm, and Fl. Paleosols and plant fragments occur within some of these lithofacies. Six lithofacies associations identified in sandstone-dominated units are interpreted to be associated with fluvial and deltaic systems. Interbedding of diamictite lithofacies with sandstone-dominated lithofacies associations demonstrates a glacial influence on deltaic systems in the Macharetí Group and San Telmo Formation. The Escarpment Formation (lower Mandiyutí Group), however, is dominated by St, Se, and Sh with minor Fm and Fl without any diamictite facies; it represents a fluvial system likely deposited during an interglacial period.

Facies analysis supports that the repeating packages of sandstone and diamictite units are due to glacial advance and retreat cycles with the strongest glacial influence in the southern part of the basin. Reemergence of thick deposits of Dmm with faceted and striated clasts in the northern Tarija-Chaco basin reflects either variable preservation or addition of glacial sediments from other sources (mountain glaciers?).