Palaeoenvironmental implications and diagenesis of inoceramid shells (Bivalvia) in the mid-Maastrichtian beds of the Sopelana, Zumaya and Bidart sections (coast of the Bay of Biscay, Basque Country)

Inoceramid bivalve shells from the mid-Maastrichtian beds of the Sopelana, Zumaya and Bidart sections in the Basque Country, were tested for their post-depositional diagenetic alteration. Petrography, cathodoluminescence (CL) and scanning electron microscope (SEM) observations together with geochemical analyses, indicate that the coarsely prismatic calcite of the shells from the Sopelana and Zumaya sections have undergone diagenetic alteration, without there being substantial textural changes in the original prismatic microstructure. A generalized bright yellowish to red colour is seen in all the samples observed by CL. The delta(18)O values obtained from the shell prisms range from -2.9 parts per thousand to -5.18 parts per thousand, with a mean value of -3.6 +/- .0.5 parts per thousand in the Sopelana section, whereas in the Zumaya section they range from -1.88 parts per thousand to -4.71 parts per thousand, with a mean value of -3.2 +/- 0.3 parts per thousand. These values are clearly lower than those from the Bidart section (from -0.32 parts per thousand to -1.48 parts per thousand; mean value -0.7 +/- 0.3 parts per thousand). Other evidence of diagenetic alteration is inferred from both whole-shell geochemistry and microprobe analysis. A complementary zonation in the distribution of 'diagenetic tracers' can be detected from the inner shell layer (OSL) to the outer shell layer (OSL) of inoceramid shells, together with a rare earth element (REE) enrichment in the shells which have lighter delta(18)O values. In contrast, the inoceramid bivalve shells and their host-rock from the Bidart section were less altered. The calculated mean palaeotemperature (15 degrees C) and the value of 13 degrees C estimated from the non-luminescent middle shell layer seem to be consistent with palaeolatitudes about 30 degrees N for deep marine waters of mid-Maastrichtian time, as the last step of the long-term climatic cooling of the Late Cretaceous 'greenhouse'. (C) 1998 Elsevier Science B.V. All rights reserved.