MARINE RECORD OF AN EARLY MIOCENE CATASTROPHIC VOLCANIC EVENT, MONTE LEON FORMATION, PATAGONIA, ARGENTINA

Preliminary analysis of an exposed planar surface of the Monte León Formation (earliest Miocene) of Santa Cruz Province, Argentina, suggests that the exposure records a catastrophic volcanic event impacting an offshore marine setting. Recent coastal erosion has exposed an assemblage consisting solely of the geryonid crab Chaceon peruvianus (d'Orbigny, 1842) which is present in remarkable numbers. Field mapping of a 50 m by 10 m area of the exposure has revealed over 70 crab specimens, fully articulated and in living position. Analysis of the crabs collected thus far has shown that the assemblage consists entirely of individuals that are small for the species, encrusted with epibionts, and exclusively female in gender. Although the articulated state of the crabs indicates relatively rapid burial, their upright position is not consistent with the random orientations that would be expected from a density-driven current. Petrographic analysis has revealed that the entombing sediment is a slightly compacted argillaceous volcanic tuff, consisting almost entirely of volcanic glass shards and euhedral plagioclase grains. Biogenic fragments are noticeably absent from the deposit, unlike sediments from elsewhere in the section. From the above evidence we have inferred that the crabs were migrating across the sea floor at the time of the pyroclastic event. The ashfall exterminated the crabs, either by poisoning or gill fouling, and subsequently buried their remains. Suprajacent layers record the re-establishment of normal marine conditions.