SR ISOTOPE EVOLUTION OF MAASTRICHTIAN SEAWATER, DETERMINED FROM THE CHALK OF HEMMOOR, NW GERMANY

In 140 metres of Maastrichtian White Chalk (nannofossil chalk) exposed near Hemmoor, NW Germany, values of Sr-87/Sr-86 increase from 0.70776 0 in the Belemnella sumensis Zone (Lower Maastrichtian) at the base of the section (- 54.5 m; referred to 0 m at a prominent marl, M900) to 0.707821 in the Belemnella baltica/danica Zone (Upper Maastrichtian) a t the top of the section (+84.5 m). A plateau in Sr-87/Sr-86 occurs be tween -5 m and +50 m in the section, probably as a result of a very hi gh rate of sedimentation in this interval. A belemnite and associated nannofossil chalk have similar Sr-87/Sr-86 values, suggesting that the re has been little diagenetic alteration of the Sr-87/Sr-86 ratios in the chalk, which therefore preserves its original S-87/S-86. Compariso n of Sr-87/Sr-86 and nannofossil zonations for sequences at Bidart, Fr ance, and DSDP Sites reveals discordance and so possible diachronism o f the basal boundaries of nannofossil Zones CC25B and CC25C.