Wednesday, 8 October 2008
George R. Brown Convention Center, Exhibit Hall E
Bolivia experienced a sharp paleo-climatic gradient during Carboniferous time. While the altiplano area witnessed departure of glacially-influenced deposition, southern Bolivia recorded high volumes of siliciclastic deposition under high flow and discharge regimes, including many glacio-fluvial and glacial (dropstone and diamictite) beds. The siliciclastic Macharetí and Mandiyutí Groups (Carboniferous) of southern Bolivia contain this evidence of the late Paleozoic glaciation recording glacial influence at distinct intervals. Current palynological dating of the two groups places them in the Pennsylvanian (di Pasquo, 2003), juxtaposing them between tills and glacial pavements in northern Argentina and warm water carbonates in northern Bolivia. Other work suggests truly Mississippian units in southern Bolivia. Resolution of the period of high climatic gradient and its succession in Bolivia is a remarkable near-field juxtaposition of glacial and warm water carbonate units. These groups consist of seven formations: Itacua, Tupambi, Itacuami, Chorro/Tarija and Taiguati formations in the Macharetí Group and Escarpment and San Telmo formations of the Mandiyutí Groups. Sandstones within these groups appear compositionally and texturally similar in outcrop, although they are variable (both compositionally and texturally) in thin section. Thickness of the groups varies greatly from 0 to 1000m each due in part to their deposition in paleovalleys and erosional events throughout the succession. Five sections measured within a 10 km2 area showed thickness variations of 0 to 84m for the Macharetí group and 0 to 522 m for the Mandiyutí group; not all formations were present at all sections (i.e. the Itacua and Taiguati fms. were not present). Given these significant thickness variations and outcrop similarity of sandstones, correlation is difficult. Formational names do not apply to thinner sections where many are missing; therefore, it is suggested that formational names of these sandstones not be used as they do not apply to thinner sections.