GONDWANAN LATE DEVONIAN GLACIATION: EFFECTS OF A SEVERE DRAWDOWN

Late Devonian glaciation from the Parnaiba through the Andean basins of South America has been well-demonstrated. In the Parnaiba Basin is 350m of glacial diamictite (Cabecas Fm.), as well as glacial pavement. The Amazonas Basin has two diamictite formations (Oriximina and Curiri fms.), totaling 600m. The Solimoes Basin has the 60m Jaragui Fm. The Madre de Dios Basin of Bolivia has 60m of diamictites (Toregua Fm.), while diamictites of the Cumana Fm. in Bolivia's altiplano are 60-200m. In Africa, additional evidence is seen in Ghana (Tokoadi Sandstone), Niger (75m. of "glacial conglomerates"), and the Central African Republic (Mambere Fm.). A conservative estimate of the area affected by glaciation is 16,000,000 square km (6450km E-W dimension, 2500km N-S dimension). Glaciation is coeval with Variscan, Antler, and other orogenies. Ice volume is difficult to assess, but even relatively thin ice sheets would give a significant sealevel drop of at least 150m. Effects of the glacioeustatic drawdown were severe. Subaerial erosion surfaces are prominent in many western USA carbonates, in which collapse evaporation breccias also occur. Cratonal eolian sands are abundant in Cordilleran (USA) carbonates. Combined nutrient inputs (orogeny) and drawdown (glacio-eustacy) produced euxinic basins in many locations (e.g., Appalachians, western Canada), and evaporites (western USA and Canada). Coincident with a shutdown of the productive Late Devonian (Frasnian) carbonate factory are lacunae, eolianites, and restricted carbonates. It is additionally suggested that a significant shift from skeletal to bacteria-generated organic buildups occurred in China (Shen, 1999), and local skeletal carbonates in Moravia shed subaerially derived debris during Famennian time (Hladil, 1996). Therefore, systems creating the well-known Carboniferous cyclothems were in place during Late Devonian time.