Palaeoecology of Cenomanian oysters of the northern margin of the Aquitaine Basin

The purpose of this paper is to understand which parameters control the palaeoenvironmental distribution of Middle Cretaceous oysters. To reach this objective, the following two step analysis has been carried out. First, ten major Upper Albian to Lower Turonian outcrops from the northern part of the Aquitain Basin (SW France) (fig. 1) were analysed with respect to their sedimentological and palaeontological features (see fig. 2, for oyster distribution). They represent a time interval corresponding to a 2(nd) order transgression [Flardenbol et al., 1997; Neraudeau et al., 1997], characterised in the Charentes (North Aquitaine Basin) by a great variety of depositional environments and very rich in oyster assemblages [Videt, 2004]. According to previous authors [Moreau, 1993; Neraudeau et al., 1997; Platel, 1989, 1996], this series can be divided into seven lithological units, A to G, four units (A, B, C and G) being subdivided into two or three subunits (A1 and A2, B1 to B3, C1 to C4, G1 and G2). Apart from the sub-units A1 and Tu, which correspond to the Late Albian and Early Turonian respectively, all the lithological sub-units A2 to G2 correspond to the Cenomanian series. A and B belong to the lower Cenomanian, C1 to C3 to the middle Cenomanian, and C4 to G2 to the Upper Cenomanian. In terms of palaeoenvironments, unit A is considered as deposits of a sandy estuary (with local lignite layers) [Neraudeau et al., 2002, 2003; Perrichot, 2003], and unit B as shallow subtidal sand dunes [Vullo et al., 2003]. Unit C corresponds to the optimal development of a carbonate platform with rudists [Chereau et al., 1997], unit D to a marly open shelf marly facies, unit E to an oyster bank mainly composed of Pycnodonte biauriculata [Dhondt, 1984], unit F to a moderately deep bioclastic facies colonised by rudist Ichthyosarcolites triangularis and, unit G, which forms progressively marly up-section, to progressive platform flooding at the Cenomanian-Turonian boundary. From the analysis of these different lithological units and subunits, the North Aquitaine Basin can be considered as a mixed siliciclastic-carbonate platform. Two main depositional systems have been identified, namely an open one and a closed one. The synthetic distribution of twelve kinds of oysters biofacies (bf1 to bf12) described in these units and subunits is summed up in figure 3 according to depositional type, lithology and depth. Based on the palaeoenvironmental distribution of Middle Cretaceous oysters in the northern part of the Aquitain Basin, the palaeoecological affinities of the nine marine species are discussed regarding seven main parameters i.e., oxygenation, water turbulence, salinity, turbidity, bathymetry, grain size, and substrate consistency (i.e. hardground VS softground) (fig. 4). Acutostrea aff. incurva (NILSON, t827) (figs 4, 5h) and Curvostrea rouvillei (COQUAND, 1862) (figs 4, 5i) are very rare species with a distribution that is still ambiguous. Apparently they preferred soft substrates and seem to have tolerated lowered oxygen levels. In addition, they are encountered in quiet, deep environments, i.e., the lower infralittoral to circalittoral zones sensu Neraudeau et al. [2001]. Ceratostreon flabellatum (GOLDFUSS, 1833) (figs 4, 5e) is not a prolific species but was widely distributed all over the carbonate platform. Nevertheless it is mainly marine and located in the infralittoral zone [sensu Neraudeau et al., 2001]. Gyrostrea delettrei (COQUAND, 1862) (figs 4, 5g) might have been very widespread but is very rare. It was most abundant in marginal marine environments where it was the sole oyster that tolerated brackish water conditions. Pycnodonte biauriculata (LAMARCK, 1819) (figs 4, 5d), in spite of its very short stratigraphic range (Naviculare Ammonite Zone), colonised a wide variety of environments. It is mainly a relatively medium water species (regarding to other species) [Stenzel, 1971; Harry, 1985; Freneix and Viaud, 1986], from the lower infralittoral zone [sensu Neraudean et al., 2001] but it needed food-laden currents. Pycnodonte vesicularis (LAMARCK, 1819) (figs 4, 5f) is also an ubiquitous species. However, in contrast to Pycnodonte biauriculata, it preferred deep, soft substrates (circalittoral and deeper ones? [Neraudeau and Villier, 1997]). Rastellum carinatum (LAMARCK, 1806) (figs 4, 5c) and Rastellum diluvianum (LINNE, 1767) (figs 4, 5b) exhibit an identical distribution pattern in spite of the fact that R. diluvianum is more selective than Rastellum carinatum. Carter [1968], Jablonsky and Lutz [1980] and Freneix and Viand [1986] have already demonstrate that these species do not tolerate turbulent conditions but are particularly adapted to quiet water and soft substrates. The two species also do not tolerate salinity variations. Rhynchostreon suborbiculatum (LAMARCK, 1801) (figs 4, 5a) is the most ubiquitous species in the Cenomanian of the Aquitain Basin. Videt and Neraudeau [2003] and Videt [20041 have already defined the parameters that affected its shape and its abundance. As the species does not occur in brackish water deposits, salinity seems to have been a major factor limiting its distribution.