43 2. The stratigraphy of the Middle Jurassic grainstones (Lussac-les-Châteaux and Le Vigean sectors, Mourier and Gabilly, 1985). Exceptionally, in the terminal part of the stage, green clay lenses with ostracods can be observed (La Tour-au-Cognum). D9 – Lower Bathonian In northern Aquitaine, the base of the Bathonian stage corresponds to what former quarry workers called the “rotten bank.” This is characterized by one or two multidecimeter limestone banks framed by greenish marl joints. In natural gamma-ray logs, this layer produces a marked peak that is easy to spot. It contains phosphate nodules and numerous fossils: bivalves, gastropods, sponges, and cephalopods typical of the Zigzag zone (units 28-29-30). As it approaches the Saint-Maixent graben, this level becomes much more carbonated but retains its paleontological characteristics (units 31 to 33). Above this, spotted limestones reappear over a thickness of about ten meters. Compared to those of the Upper Bajocian, there is an increase in the frequency of flints. In Poitiers, at a height of about 3 meters, gravelly limestones with numerous Ctenostreon and, more rarely, a few ammonites from the Zigzag zone can be observed. Above this are thick beds of gravelly limestone with isolated flints (4 m). Moving eastward, this succession varies little, except for the upper part, which then consists of a marker level of oolitic limestone. D10 – Middle and upper Bathonian At the base of the Middle Bathonian, in the basin area, ammonites become frequent again (unit 36). The limestone facies with tuberoids and sponges remains predominant (approximately 3 meters), generally without flint, with a few greenish clay interbeds. Higher up, and only from la Crèche sector onwards, flints return temporarily. Sponges remain abundant and can locally form bioherm-type accumulations. At the top of the stage, much finer limestones (4 meters) are deposited, again rich in ammonites from the Retrocostatum zone. The virtual absence of Clydoniceras and the problems of correlation between the Tethyan and Boreal domains make it impossible to assess the extent of the gap in the late Bathonian. On the other hand, the erosion surface (D11) that truncates these beds and precedes the first Callovian deposits is well marked and, in most cases, easily identifiable thanks to the presence of glauconitic coatings. As a result of their erosion, the Middle and Upper Bathonian deposits do not exist directly below the threshold. In Poitiers, this level consists of 5.50 m of gravelly and suboolithic limestone containing 4 to 5 continuous bars of flints, 0.15 to 0.30 meters thick. There are quite a few bivalves, polyps, and rare ammonites (Cadomites orbignyi). The stage ends with 7 m of gravelly to suboolithic limestone containing scattered flints. The whole is truncated by a weathering surface.
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