159 8. Hydro-stratigraphic study of the Hydrogeological Experimental Site of Poitiers, France from 0.60 to 0.80 m thick, underlies a hardground surface associated with a bivalverich horizon. The upper bed is correlated with the Galeolaria socialis (serpulidae) bed described by Gabilly (1978) and is interpreted as marking the base of the Laeviuscula Biozone (Fig. 8). Based on this interpretation, the Laeviuscula Zone begins at 95.59 m (M01), 95,92 m (M09), 96.61 m (M10), and 96.26 m (M14). The bioclastic horizon beneath the hardground forms part of a 0.80 m thick fossiliferous bed, whose basal contact is a planar to undulating erosional surface. This surface is interpreted as the Aalenian–Bajocian unconformity (D7) and is observed at 96.41 m (M01), 97.52 m (M10), and 97.20 m (M14). The lower stratum is tentatively assigned to the Ovalis Biozone (Gabilly, 1978, p. 148) with the identification of a Sonninidae in the Saint Benoit cliff by Patrick Branger, 4 kilometers south of the HES. D7 discontinuity delimits a vacuolated texture interval with a dark hue, referred to as the Middle Dolomitized Zone. Below the D7 discontinuity, a packstone unit can be correlated with the oolitic and oncolitic facies of the Concavum Biozone (Mourier, 1986, p. 13). At its base, chert nodules and bands following bedding planes are present, particularly at 98.56 m (M14). These cherts, typically 20–30 cm thick, are attributed to the Murchisonae Biozone of the Aalenian, though their stratigraphic value is limited due to their occurrence across multiple Biozones (Benvel, 1978; Mourier, 1986). Two hardgrounds are recognized at the base of this formation, at 105.86 m (M10) and 105.21 m (M14), and are cautiously correlated—due to dolomitization effects— with the D6bis discontinuity. Another hardground, observed at 110.15 m (M10) and 110.85 m (M14), marks the top of a decimetric-bedded limestone unit. The base of this unit is poorly defined on OPTV logs because of intense dolomitization (Lower dolomitized zone). This hardground is correlated with the D6 discontinuity. A change in sedimentation is observed at 119.10 m (M14) and likely corresponds to the top of the Toarcian marls. In another OPTV borehole (M09), this boundary Figure 8 Bajocian lower boundary.
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