Géologie MéiUterranéenne

Tome XXVm, n° 1-2, 2001, pp. 163 -168.

Anatomy of the carbonate bodies in relation to their position with respect to the maximun

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transgressive in the 2 -order Cycles of the upper Cretaceous from the Iberian Range

M. SEGURA* B. CARENAS**

J.GIL*

J.F. GARCIA-HIDALGO* A. GARCIA***

Introduction

At the end of the Jurassic, a complex graben of tectonic origin was formed in the Iberian Plate. The graben extended from the SSE continental margin, open to the Tethyan Ocean, to the Cantabrian-Pyrenean Basin at the north of the plate and open to the Atlantic (Fig. 1). Terrigenous sediments, locally very thick, infilled the graben during the lower Cretaceous with an irregular spatial distribution. Overlying, and during the upper Cretaceous, a sedimentary basin, the Iberian Basin, was originated, in which successive mixed platform sediments were deposited with an important development of carbonate facies and minor terrigenous facies related to a thin clastic wedge (GARCIA et al., 1989 b ; RUIZ et al, 1994).

Successive eustatic transgressions (upper Albian -Cenomanian) invaded this Iberian Basin first from the Tethyan margin (GARCIA et al ., 1993), but the correlative transgressions from the Cantabrian margin were limited to the northern margin of the Basin (FLOQUET, 1991). The tectonic evolution that during middle -upper Cretaceous experienced the Iberian Plate because of the opening of the Cantabrian-Pyrenean Basin, after an important event of extensional tectonics, originate a new oceanic basin (Gulf of Biscay or Gulf of Gascogne), and lead the development of an important continental platform in the northern margin of the Iberian Plate, which originates that Iberian Basin

Anatomy of the carbonate bodies in the 2nd order cycles (transgressive-regressive cycles)

sedimentation were linked to this new ocean (CARENAS et al, 1989 ; SEGURA et al, 1993 a). As consequence of this new palaeogeographic configuration, from the lower Turonian to upper Santonian there were an intermittent communication with the Tethys through a very shallow sedimentaiy platform, in which coastal deposits were accumulated or, even, it was emerging temporarily.

Eleven depositional sequences have been distinguished in the Iberian Basin during the upper Albian -lower Cenomanian (GARCIA et al., 1996 a ; GARCIA et al., 1996 b). These sequences belong to two 2nd order transgressive-regressive cycles.

Six depositional sequences are recognised in the lower 2nd order Cycle (upper Albian -middle Turonian) (Fig. 1), which the four basal sequences are located below the Cycle transgressive maximum (RUIZ et al, 1994). They are mainly composed of limestones and dolostones of tabular anatomy with green or brown marl beds intercalated (GARCIA et al., 1989a). The transgressive maximum is located at the base of the fifth depositional sequence, and it is composed of nodular biomicrites with marly intercalations, an abundant open marine fauna (ammonites, echinoderms, pelecypods, etc.) characterise this maximum (SEGURA et al, 1993 a). The upper part of the fifth sequence and the sixth sequence are considered the regressive deposits of the 2n order Cycle. They are poorly stratified, very thick bedded, dolostones (originally calcarenites) developing