Arkell.1956.Jurassic..

78 BORDERS OF THE MASSIF CENTRAL
0-20 cms. thick, under the Ciret. As stated above, the common ammonite
in the Ciret figured by Roman & Petouraud, 1927, pi. vi, as Strenoceras
subfurcatum is Pseudogarantiana dichotoma Bentz of the Garantiana
Zone; this throws doubt on other records of Strenoceras in the area.
MIDDLE BAJOCIAN (up to 70 m.)
Zones of Stephanoceras humphriesianum and Otoites sauzei. In the
Maconnais (Roche, 1939; Mouterde, 1953, p. 304) there is at the top
a yellowish or whitish limestone, 0-8 m., with Teloceras blagdeni; and
derived specimens of T. banksi Sow., T. blagdeniforme Roche and Normannites
orbignyi Buckman occur in the Subfurcatum Zone. Beneath come
up to 50 m. of crinoidal limestone (Calcaire a Entroques superieur)
with Stephanoceras humphriesianum and 5. brodiaei (Sow.) in the upper
part and Otoites sauzei in the lower part. Below again are limestones
and marls (0-8 m.) with O. sauzei, Emileia polyschides (Waagen), etc.
passing up locally into sandstones. At Mont d'Or, the so-called blagdeni
beds are somewhat older, containing various large Stephanoceras (but no
Cadomites), with Normannites latansatus (Buckman), Chondroceras
evolvescens (Waag.), Dorsetensia subtecta Buckman, etc. (Roche, 1943).
This bed rests disconformably on the Concavum horizon of the Lower
Bajocian. At Crussol the Sauzei and lower Humphriesianum horizons
are present (Roman, 1950, p. 40), but the Blagdeni and Subfurcatum
horizons seem to be absent.
Zone of Sonninia sowerbyi. Limestones and marls (1-4 m.) in the
Maconnais have yielded large Sonninids, Euhoploceras cf. acanthodes
(Buck.), E. aff. umbilicatum (Buck.), and E. aff. marginatum (Buck.), which
Roche (1939, p. 95) assigned to the Concavum Zone; but he was misinformed,
for these species, although figured by Buckman as from that
zone, came from Sonninia beds which he later designated Discites
[Subzone] and put at the base of the Sowerbyi Zone (see p. 33). This
date has been confirmed by subsequent discovery of Hyperlioceras
curvicostata Buckman (Mouterde, 1953, p. 301). The assemblage is the
same as that already recognized, with a much richer fauna, at Tremardieres
near Poitiers and identified as of Discites date by S. S. Buckman (Welsch,
1928a) (see above, p. 54). The middle and upper parts of the Sowerbyi
Zone (Trigonalis and Laeviuscula Subzones) are both missing, however.
In the Mont d'Or this gap widens and the whole Sowerbyi Zone is
missing (Mouterde, 1953, p. 335). It comes in again at Crussol and
Privas, with Sonninia cf. reformata Buckman (Roman, 1935, pi. iii, 6)
(Discites Subzone).
LOWER BAJOCIAN (up to c. 50 m.)
In the Maconnais four successive ammonite horizons have been
recognized, corresponding to the Zones and Subzones of Concavum,
Bradfordensis, Murchisonae, and Opalinum. Graphoceras concavum
occurs rather above the middle of the Calcaire a Entroques inferieur.
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