Paper No. 2
Presentation Time: 8:30 AM
TRANSITION FROM COLD CLIMATE ENDEMIC FAUNAS AND GLACIATION TO GLACIATION-PROXY CARBONATES (DEVONIAN-PERMIAN), CENTRAL ANDES
Late Paleozoic events in western Gondwana show a shift from continental scale glacial (Late Devonian), and glacio-fluvial (Mississippian?), yielding to Pennsylvanian-Permian carbonate settings. Continuous stratigraphic sections at Lake Titicaca and northern Bolivian subsurface record these dramatic changes, which cannot be found in a single section elsewhere in Gondwana. At Lake Titicaca, latest Devonian highly organic matrix diamictites contain very large granitic, metamorphic and rare sedimentary clasts. Also, local highs probably had alpine glaciers. Successive glacio-fluvial systems have feldspathic sandstone. Faunally, the Early and early Middle Devonian are dominated by highly endemic shelly fossils of the Malvinokaffric Realm, with many taxa originating in Bolivia. However, by late Middle Devonian time, cosmopolitan immigrants from North Africa began to replace endemic taxa. This is supported by scarce plant assemblages and more prolific palynoassemblages from the same time interval in South America. The Late Devonian through Early Pennsylvanian succession is dated exclusively through palynomorphs, calcareous microfossils and conodonts. Accompanying the Late Devonian glaciation is a sealevel drop, with marine acritarch dominance supplanted by miospores, including the cosmopolitan Retispora lepidophyta at the D/C boundary. Palynological results show Mississippian rocks are limited. Warm-water carbonates appear during the Mississippian and above limited coal beds in the Early Pennsylvanian (Bashkirian) in northern through central Bolivia. Lithologies include paleosols, dolomites, burrowed limestones, and open marine packstones. Pennsylvanian and Permian carbonates contain “west Texas” warm-water fusulinid faunas. Carbonate sequence stratigraphic work that includes good biostratigraphic control gives sequence boundaries and proxies for eight LPIA events, 306.5-283 Ma. The carbonates are correlated to cooler water siliciclastics in southern Bolivia into Argentina, with Pennsylvanian diamictites. Temporal and paleogeographic proximity of cold-water clastics near warm-water carbonates are a paleoclimatic enigma. One suggestion is that Gondwana rotated into lower paleolatitudes, but the sharp Pennsylvanian paleoclimatic gradients remain unexplained.