Coniacian/Santonian boundary in Riu de Carreu valley and Prats de Carreu, South-Central Pyrenees (N Spain)

Abstract

A conspicuous level containing Platyceramus undulatoplicatus (Roemer) occurs in several stratigraphic profiles measured and sampled in a 15 km wide area along Riu de Carreu valley and Prats de Carreu. This is the index species for the Coniacian-Santonian boundary following the criterion suggested for the definition of the Coniacian-Santonian boundary by the Santonian working group (Lamolda & Hancock, 1996). Its presence, not only allows the identification in this area of the Coniacian-Santonian boundary, but also facilitates the re-examination of previous biostratigraphic framework. It becomes also evident that the presence of Platyceramus undulatoplicatus is relatively facies independent, since it occurs from the calcareous platform of the western part of the area to the talus marls of the eastern one. The area is located at the northern flank of the Sant Corneli Anticline, a southward verging South Pyrenean thrust propagation fold, and the outcrops, of well developed platform and basin facies, range from the Coniacian to the Santonian and are extremely well exposed. Stratigraphic units of the area were formally named by Gallemí et al. (1982). However, other authors have been inconsistently using informal names (Gili et al., 1996, Vicens et al., 1998). The biostratigraphic survey on these platform-basin outcrops is based mainly on inoceramids, rudists, ammonites, echinoids, brachiopods and planktonic foraminifera. Early biostratigraphic framework has been developed by Pons (1977), Martínez (1982), Gallemí et al. (1983), López (1986), Gómez-Garrido (1989), Gallemí (1994) and Muñoz (1995). Vicens et al. (1998) correlated, both stratigraphically and sequentially, the transit between basin and platform sequences in the western part of the area. High-resolution biostratigraphic framework on inoceramid-ammonite biozones is being currently developed to check accurate relationships between Platyceramus undulatoplicatus and other fossils record, especially that of Texanites species. The lowermost Santonian is precisely established by means of the very abundant and conspicuous Platyceramus undulatoplicatus (Roemer). This index inoceramid species co-occurs mainly with other bivalves, echinoids and sponges. Other inoceramid species showing divergent ribs, as Cordiceramus cordiinitialis ickernensis (Seitz), are also reported in the Lower Santonian of the studied area. Upper Coniacian inoceramids are characterised by Magadiceramus species, as Magadiceramus subquadratus subquadratus (Schlüter) and Magadiceramus soukupi (Macáck); they overlie or co-occur with Protexanites bourgeoisi (d’Orbigny), ammonite marker for the upper part of the Upper Coniacian. Rudist faunas co-occur just below the Platyceramus undulatoplicatus beds.

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