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The Aalenian/Bajocian and Bajocian/Bathonian boundaries in La Baume section near Castellane (Alpes de Haute Provence, SE France)

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De Beats K. , Cecca F. , Guyomar M. , Verniers J.

tom Vol. 4, no. 1

145-146

Jurassic and Lower Cretaceous formations are naturally exposed around the hamlet called La Baume, near Castellane (Alpes de Haute Provence, SE France). We have realized both a detailed log and a bed-by-bed sampling for ammonite biostratigraphy in a 68 metres-thick succession of pelagic marls and limestones that spans the uppermost Aalenian to lowermost Bathonian interval. This succession paraconformably overlies Lower-Middle Jurassic shallow-water limestones, whose topmost surface is marked by “pseudostromatolitic” crusts, and is overlain by the “Terres noires” Formation. The log has been subdivided in 122 beds; 400 ammonites have been collected. Benthic forams have been sought in the marly intervals that, however, are almost devoid of such microfossils. The pelagic succession studied can be roughly subdivided in a marl-dominated member 1, followed by a limestone-dominated member 2 that is overlain by a marl-dominated member 3. Zoophycos traces begins around meter 21 and disappear above metre 59. At the base of the pelagic succession Haplopleuroceras gr. mundum, Graphoceras limitatum, G. cf. decorum, indicate the Late Aalenian Concavum Zone. Representatives of Euhoploceras, Phylloceras and Lytoceras also occur. At metre 10 (bed 14) we have found Hyperlioceras cf. mundum, which allows us to draw the base of the Bajocian. This is reinforced by the occurrence of Hyperlioceras cf. aspera, H. cf. curvum, Euhoploceras gr. modestum around metre 13.5 (bed 17) and of H. cf. desori in the bed 22 (meter 21). The Laeviuscula Zone begins at metre 12.5 (bed 24) with the occurrence of representatives of the family Sonniniidae, which characterizes the ammonite fauna up to metre 45 (bed 75). A major gap of both Subfurcatum and Garantiana zones has been recognized at metre 59 (top of bed 109), at the boundary between members 2 and 3. The marl-dominated member 3, which contains ammonites of the Parkinsoni Zone (Parkinsonia cf. rarecostata, bed 114 and Parkinsonia cf. parkinsoni, bed 118a), overlies the top of member 2, ascribed to the top of the Humphriesianum Zone on the basis of the occurrence of Teloceras blagdeni. The Bajocian/Bathonian boundary has been drawn within bed 121 on the basis of the occurrence of Procerites cf. costulatosus. The main palaeontological interest of the ammonite fauna from La Baume is the richness and the diversity of the family Sonniniidae, with new forms that deserve an accurate palaeontological study. From the stratigraphical point of view our section could complementary to the candidate section for the Bathonian GSSP.