HOW-MANY PALYNOLOGICALLY CONSTRAINED GLACIAL EVENTS DURING THE LATE DEVONIAN AND MISSISSIPPIAN OF GONDWANA

Despite the generally warm climates of the Earth during the Late Devonian-Mississippian timespan, Western Gondwana (Argentina, Bolivia, Brazil and Peru) displays a few occurrences of glacial deposits which can be dated by miospore stratigraphy and therefore correlated to standard conodont zonation. They allow correlation with several known sedimentary events in North America and Europe.

Glacial and interglacial cycles are quite evident after the sharp climate change occurring during the Late Famennian, in the Middle expansa Zone, introducing a new, almost cosmopolitan vegetation belt characterized by the miospore Retispora lepidophyta. But the best documented part of the Late Devonian-Mississippian time-span is obviously the Latest Famennian age, when glacial deposits reached sealevel. Based on miospore (and locally on acritarch) quantitative data, cycles are very obvious in equatorial (Greenland) and tropical (Ardennes-Rhine) regions. They allow very detailed correlation of the Hangenberg Crisis, in the Middle-praesulcata Zone, with new geochemical data from tropical (Western Europe) and subtropical (Southern France and Morocco) regions.

Sealevel changes during the Latest Frasnian to Late Famennian time-span may correspond to tectonic events, since no dated glacial deposits are recorded in this interval. Middle or Late Tournaisian diamictites in South America are ambiguously characterized by their miospore content. But South American Late Viséan diamictites are palynologically constrained. In Western Gondwana diamictites are preceded by erosional or nondepositional gaps which are so far poorly explained.