EFFECTS OF VOLCANIC ASHFALL RECORDED IN ANCIENT MARINE BENTHIC COMMUNITIES - COMPARISON OF A NEARSHORE AND AN OFFSHORE ENVIRONMENT

Ecologic and taphonomic impacts of volcanic ashfall on a Pleistocene n earshore hard-bottom community and a Cretaceous offshore soft-bottom c ommunity are compared and contrasted. The hard-bottom (reef) community , dominated by sessile, epifaunal organisms, was devastated by rapid d eposition of volcanic sediment. In contrast, the offshore community, d ominated by motile, burrowing organisms, was considerably less affecte d, since many of these organisms could escape burial. These examples e mphasise the potential bias that volcanism may introduce into the foss il record, particularly the over-representation of sessile fauna. The new substrate provided by volcanic sediment inhibited the re-establish ment of the reef community, but was not sufficiently different from te rrigenous mud to alter the structure of the offshore community. Unlike other agents of rapid burial (e.g. storms, mass wastage), volcanic se dimentation in these settings was not accompanied by a significant inc rease in hydraulic energy. Consequently, faunal remains were buried in situ and rapidly isolated from the destructive effects of subsequent hydraulic disturbance, scavenging, bioerosion and bioturbation. In bot h of the examined settings, this contributed to exceptional preservati on of skeletal remains. Preservation was also promoted by early diagen etic cementation. In the reef setting, cementation of ash was laterall y continuous, whereas in the offshore setting, early diagenetic minera ls were precipitated as small concretionary bodies. Both styles of ear ly diagenetic cementation enhanced preservation by effectively armouri ng buried fauna from destructive taphonomic processes.