Paper No. 1-8
Presentation Time: 10:35 AM
A RECORD OF MARINE TRANSGRESSIONS DURING PENNSYLVANIAN GLACIATION, SOUTHERN BOLIVIA
Pennsylvanian 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 sandstones and 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-occurrence of numerous gastropods (Mourlonia balapucense) at this location, 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. Previous studies described the brachiopod Levipustula Levis in the upper Tarija Formation (upper Macharti Group) in south central Bolivia. These brachiopods record two marine transgressions within the Tarija-Chaco Basin, the first of which is followed by a return to glacial conditions. These marine transgressions correspond to short-term global events that documents an interlude in the Gondwana glaciations. The regionally extensive marine transgression recorded in the San Telmo Formation, which is 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. Accommodation within the basins was not eliminated by isostatic rebound. 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 deposition through the succession. The transgression from the north that produced the Copacabana Formation carbonates, in northern and western Bolivia, may have continued to the south, where sedimentary rocks were deposited in colder water. Alternatively, the presence of Levipustula Levis in the Tarija Formation (upper Machareti Group) in southern Bolivia may suggest that the transgression recorded in southern Bolivia came from the southwest.