CLIMATIC AND VOLCANIC EVENTS RECORDED IN ATMOSPHERIC DUST OF THE LATE PENNSYLVANIAN COPACABANA FORMATION (BOLIVIA) OF WESTERN GONDWANA

Extending previous research on loess deposits of the late Paleozoic Ice Age (LPIA) from western equatorial Pangea, this study documents dust from Pennsylvanian (early Moscovian) carbonates of the Copacabana Formation, recovered in a drillcore (the “Manuripi X-1”) from the Madre de Dios basin (Bolivia). The Copacabana Formation spans Pennsylvanian and Early Permian time, so it formed coeval with ice centers and associated glacial deposits in adjoining regions of Gondwana (e.g. the Paraná and Tarija basins), at paleolatitudes of 36-28 ˚S. During Pennsylvanian time, this unit formed isolated from fluvial-deltaic influx, therefore, siliciclastic material within the carbonate is interpreted to reflect atmospheric input. The 27.5 m study interval comprises open-marine fossiliferous wacke/packstone to restricted carbonate mudstone arranged in upwardly shallowing cycles 1 – 3 m thick. Cycles are bounded by surfaces recording either an abrupt basinward facies shift, or inferred subaerial exposure surfaces marked by, e.g., reddened microkarst with a commonly silty matrix. Siliciclastic material extracted at 20 cm intervals (avoiding obvious ashes) varies from 0.56 – 43.89 wgt % in carbonate facies and 15.13 – 63.94 wgt % in microkarsted surfaces. The extraction process removed carbonate, organics, pyrite, and iron; any authigenic silica was removed manually. Grain size modes range from <1 to 97 µm, with coarser intervals generally correlating to peak dust content (wgt%), and cycle bounding surfaces. Inverse correlation of K/(Fe+Mg) and Na/K indicates ash-rich horizons. These data combined with La-Th-Sc and Zr/TiO₂ versus Nb/Y patterns enable discrimination of arc-derived from cratonal-sourced dust. The results indicate a western source of atmospheric volcanic influx consistent with this mid-latitude location and an easterly derived cratonal dust influx. The non-volcanic dust peaks at cycle boundaries, coincident with lowstand (glacial) conditions, suggests the existence of katabatic wind regimes originating from coeval ice centers of Gondwana.