A Record of a Pennsylvanian glaciation, southern Bolivia.

ANDERSON, H.; DI PASQUO, M.M.; ISAACSON, P.

Moscow

Encuentro; GSA 68th Annual Meeting; 2016

University of Idaho

(Moscow, 18?19 May 2016) Rocky Mountain Section - T7. Late Paleozoic Ice Age: Gondwana Systems and Proxies in the U.S. Cordillera, Abstract #275802Pennsylvanian strata (Macharetí and Mandiyutí groups) in Bolivia and northern Argentina have long been considered to be wholly non-marine. Dominated by glacio-fluvial and glacial diamictites, much of the thick succession is peri-glacial in origin and deposited within a tectonically active basin. In southernmost Bolivia (Balapuca section), new discoveries of poorly preserved orthotetacean brachiopods (Derbyoides) in the San Telmo Formation (upper Mandiyutí Group), confirm an ephemeral marine influence in the depositional heriarchy. Before this discovery, co-occurring of numerous gastropods (Mourlonia balapucense), showing preservation of full ontogenetic growth stages, could not be confirmed as marine. Palynomorphs corresponding to the TB Zone di Pasquo ca. 200 m below the megafossil occurrence place the age of the assemblage in the Gzhelian Stage. All units exhibit erosion and high energy deposition, especially in diamictites of the Tarija Formation (upper Macharetí Group). Extensive recycling of Devonian and Mississippian palynomorphs (Retispora lepidophya, among others) occurs in both groups, and one reworked brachiopod found in the latter unit demonstrates pre-Gzhelian glacial and deglacial deposition through the succession. The apparently brief marine transgression corresponds to a short-term global event that documents an interlude in the Gondwana glaciations. The regionally extensive marine transgression characterized by the Tivertonia-Streptorhynchus fauna in western Argentina, would be linked with the beginning of the global major sea level rise that occurred during Gzhelian-Asselian times. Further, the marine transgression exceeded isostatic rebound. The lithologic facies and faunal composition differences between the north (Copacabana carbonates) and south (siliciclastics) is likely a result of thermal, and facies differences. The transgression from the North that produced the Copacabana Formation carbonates, in northern and western Bolivia, continued to the South, where sedimentary rocks were deposited in colder water.