ANOXIA AS THE CAUSE OF THE MID-EARLY CAMBRIAN (BOTOMIAN) EXTINCTION EVENT

New and revised Early Cambrian biostratigraphic data allow a quantitat ive analysis of changes in biotic diversity and extinction rate. The m id-Early Cambrian extinction can now be resolved into two distinct eve nts: the well-known early Toyonian Hawke Bay regression event, and a n ewly observed but more severe disruption during the early Botomian, he re named the Sinsk event. During the Sinsk event, the shallow-water be nthos of the so-called Tommotian fauna, together with archaeocyaths an d some trilobites, underwent a rapid decline. The Sinsk event is chara cterized by the significant accumulation of nonbioturbated laminated b lack shales in tropical shallow waters. Lamination is due to the fine alternation of clay- and organic-rich laminae with calcite-rich lamina e containing abundant monospecific acritarchs. These shales are enrich ed by pyrite and elements typical of anoxic conditions and support a b enthic biota of dysaerobic character. Our observations suggest that th e extinction during the early Botomian was caused by extensive encroac hment of anoxic waters onto epicontinental seas, associated with eutro phication and resultant phytoplankton blooms.