CARBON-ISOTOPE EVENT MARKERS THROUGH THE WENLOCK-PRIDOLI SEQUENCE AT OHESAARE (ESTONIA) AND PRIEKULE (LATVIA)

The Ohesaare borehole, in the northern part of the Silurian Baltic Gul f, penetrates shallow and deep shelf marlstones, limestones and dolost ones. The Priekule borehole in the deep shelf belt of the basin was dr illed through marlstones and graptolitic mudstones. Most of the succes sion; (except the upper part) are characterized by a high Sr/Ca ratio, close to marine calcites, suggesting good preservation of carbonate m aterial. Conodont alteration indices are low (1-2). Carbon isotopes we re measured using whole-rock samples. In the Ohesaare core section two distinctive delta(13)C positive excursions were established - one in the early Wenlock riccartonensis and low rigidus-ellesae standard grap tolite zones (+4.2 parts per thousand) and the second in the late Wenl ock nassa Zone (+4.6 parts per thousand) Before and after the latter t here are also intervals with low delta(13)C values, the first just abo ve the lundgreni ? Zone (-1.8 parts per thousand) and another in the l udensis (perhaps reaching into nilssoni) Zone (-38 parts per thousand) . In the Priekule section the early Wenlock delta(13)C peak is well re presented (+3.1 parts per thousand), but in the late Wenlock-early Lud low delta(13)C values show little variation (from 2.0 to -1 parts per thousand). In this section a high peak of delta(13)C values (+5.9 part s per thousand) was determined in the Nova Beds (correlated with the t opmost Gorstian and lowermost Ludfordian, Ludlow). The carbon isotope positive excursions show good correlation with lows of the global sea level curve and with most of the biotic and supposed climatic events ( glaciation, ocean state). (C) 1997 Elsevier Science B.V.