The ecologic and taphonomic effects of a Miocene volcanic ashfall on Atlantic inner-shelf communities

CRAWFORD, R. S.; CASADÍO, S.; FELDMANN, R.; GRIFFIN, M.; SCHWEITZER, C. E.

Buenos Aires

Congreso; Congreso del Regional Committee on Atlantic Neogene Stratigraphy (RCANS); 2008

Regional Committee on Atlantic Neogene Stratigraphy

Explosive eruptions were frequent throughout the Cenozoic in Patagonia. South of 45ºS this volcanism showed a maximum activity during the Miocene. This is evidenced in the thick tuffaceous deposits that make up the continental and marine rocks of that age. However, the effects that this volcanism had on Patagonian Miocene ecosystems remain poorly understood. Recognition within the early Miocene Monte León Formation of four paleosurfaces containing different invertebrate assemblages and in close geographic and stratigraphic proximity to each other allows assessment of the ecological and taphonomic impacts of volcanic ashfall on Miocene inner-shelf communities. The paleosurfaces are exposed within the boundaries of Monte León National Park (Paleosurface I, lat. 50°19´22.5 S, long. 68°51´31.6 W; Paleosurface II, lat. 50°19´22.6 S, long. 68°51´32.2 W; Paleosurface III, lat. 50°19´26.8 S, long. 68°51´43.1 W; and Paleosurface IV, lat. 50°19´32.4 S, long. 68°51´55.1 W) and are accessible during low tide on the wide abrasion platform along the beach there. The paleosurfaces were measured using a 1.5 m2 grid system, and described and mapped in plan view recording the precise location of all fossil specimens. Sedimentologic, stratigraphic, and taphonomic features suggest a shallow water habitat, above storm wave base. Two different invertebrate assemblages were recorded: 1) a relatively diverse, normal marine benthic fauna consisting of bivalves, gastropods, bryozoans, echinoids, and decapods; and 2) a unique assemblage consisting solely of numerous, exceptionally well-preserved, fully-articulated crabs. The crab-rich assemblage was stratigraphically below the surfaces containing the normal marine assemblage. The crabs are contained within a slightly compacted, argillaceous volcanic tuff. Biogenic fragments other than the crabs are noticeably absent from the deposit, unlike samples collected from the stratigraphically higher surfaces. Most of the crabs were preserved with their third maxillipeds in an open, gaping posture. This is consistent with postures observed in crabs suffering from respiratory distress. From the above evidence it is inferred that the crabs were suffocated, killed, and rapidly buried. The apparent faunal disparity observed on the four paleosurfaces within the Monte León Formation is interpreted as representing the initial decimation and eventual re-establishment of the benthic marine fauna following a catastrophic volcanic event.