PRECISE CHITINOZOAN DATING OF ORDOVICIAN IMPACT EVENTS IN BALTOSCANDIA

The chitinozoan biostratigraphy of four Ordovician impact craters has been investigated. Three of these (Tvaren, Kardla and Lockne) contain complete sequences of early Caradoc age, while the Granby crater conta ins rocks of late Arenig age yielding two bentonitic horizons at their top. Chitinozoans, together with graptolites and other planktic organ isms, were the first to invade the craters after the impact event. It has therefore been possible to date the impact events with a precision of less than one million years. An immigration of graptolites from Au stralia during the late Arenig corresponds to an immigration of chitin ozoans from Gondwana at this time. Two stratigraphically important tax a, not previously described or discussed, Lagenochitina sp. A aff. cap ax and Laufeldochitina sp. A aff. striata are commented upon. Three sp ecies, Cyathochitina hunderumensis, Spinachitina tvaerenensis and Tanu chitina granbyensis, are described as new.