Cenomanian events in the deep western Basque Basin: the Leioa section

Analysis of the microfaunas (foraminifera, ostracods) and the stable isotop e values (delta(13)C, delta(18)O) of the Leioa section, as representative o f the deep Basque Basin, has allowed us to propose a detailed palaeoenviron mental reconstruction of this region during the Cenomanian, as well as to r egister global chronostratigraphic reference levels to facilitate interregi onal correlations. During the Cenomanian, part of the basin, the Plentzia T rough, was occupied by intermediate water masses, as deduced by the relativ e percentages of planktonic (Rotalipora) and benthonic foraminifera. A noti ceable change is observed at the middle-late Cenomanian transition: the rep lacement of the dominance of keeled (rotaliporids) by incipiently-keeled (d icarinellids, praeglobotruncanids) planktonic foraminifera, indicative of t he new influence of the upper intermediate waters. The temporary effect of shallow waters is deduced in one interval of the latest early Cenomanian an d two more of the middle Cenomanian, as indicated by the dominance of globu lar planktonic foraminifera (hedbergellids). These water masses were modera tely to strongly hypoxic (<4 to <2 ml/l of dissolved oxygen) after the ostr acod platycopid signal and benthonic foraminiferal hypoxic indicators. The dysaerobia may have been particularly strong (almost anoxia?) during pan of the middle Cenomanian. Micronutrient availability was also restricted duri ng several intervals of the middle Cenomanian, as indicated by the sudden d ecrease in the species diversity of the calcitic benthonics during the peri od when increased trends of the delta(13)C isotopic signal are observed. Th e combination of both hypoxia and nutrient depletion produced drastic chang es in the microfaunal assemblages, with emigrations and local extinctions, showing benthonic perturbations from the time indicated by the base of the Rotalipora reicheli Zone onwards. From the beginning until the end of the m iddle Cenomanian, eleven of these perturbations are recorded as regional bi oevents, using as bioevent-markers, intervals where microfauna was absent ( including benthic-free intervals, B-FI; benthonic calcitic-free intervals, BC-FI; and ostracod-free intervals, O-FI). These changes led to the renewal of the microfaunas: benthonic foraminifera renewed their specific stocks d uring the early to early middle Cenomanian, with planktonic foraminifera an d ostracods undergoing renewal at the end of the middle Cenomanian. Isotope values of delta(18)O and delta(13)C are consistent with the palaeoenvironm ental changes detected by the microfaunas; their maximum and minimum shifts coincide with the bioevents. The double-peaked positive shift of delta(13) C for for the mid-Cenomanian of northwest Europe (Jenkyns et al., 1994; Pau l et.al., 1994a) has been recognized in this series of the Basque Basin. Th e palaeoenvironmental perturbations deduced in the Cenomanian of the Leioa section are attributed essentially to palaeoceanographic changes, where int ermediate water masses profoundly influenced the planktonic and benthonic e cosystems. The influence of other local causes, such as volcanic activity a t that time, or tectonics between the Iberian and European plates, are more difficult to prove. Several of the bioevents defined in the middle Cenoman ian of this basin could probably be global in nature, and thus may be usefu l for establishing interregional correlations. (C) 1998 Academic Press Limi ted.