CALDERA-LAKE SEDIMENTARY SUITES FROM THE EARLY EOCENE OF PATAGONIA, ARGENTINA

Early Eocene deposits in NW Patagonia Argentina (Tufolitas Laguna del Hunco), which bear the world-widely recognized Laguna del Hunco biota, characterize by a deltaic-lacustrine distal volcaniclastic succession. The re-sedimented succession, 160-170 m thick, was deposited in a caldera-lake and shows five distinct sedimentary facies associations (FA), as follows: A) interdistributary bays and proximal-bottomset deposits, present along the entire section; B) distributary channels, present in the lower and upper section, C) proximal turbidite deposits, mostly restricted to the middle section; D) debris flow deposits, characteristic of the upper section; and E) medium to large-scale slump deposits, present in the middle section. The measured section shows a deepening-shallowing cycle, strongly influenced by eruptive volcanic processes of rhyodacitic nature. Distributary channels (FA-B) and interdistributary laminated fine-grained material (FA-A) would record the pre-eruption time near-shore deposits. The initial deepening of the lake would be the product of the blockage of the outlet of the basin immediately following the eruption. Aggrading rivers, caused by the rising level, are reflected as a high discharge of sediment in the offshore areas. This would favor the origin of the slump events (FA-E) after accumulated unconsolidated unstable sediments on the delta front slope slumped down most likely triggered by seismic shocks. This process would also favor the generation of density currents (FA-C, D) towards the bottom of the lake. The steadiness of the basin, reached later after the eruption, culminated in the filling of the basin, showing again delta plain near-shore facies (FA-A, B). The accumulation of plant remains, within deposits of the FA-A, was favored by the action of overflows and the following deposition either in subaqueous interdistributary plains or proximal-bottomset. This resumes the importance of understanding the sedimentary dynamics that favored the fossilization of one of the most diverse Paleogene biotas in the world.