Arkell.1956.Jurassic..

SOUTH-WEST OF THE PARIS BASIN 53
and, as in Normandy, the various stages overlap the crystalline rocks,
sometimes with basal conglomerates. The Toarcian is thin but complete.
Not far south of the Loire is its type-locality of Thouars. The Bajocian
and Bathonian are thicker and highly variable in facies. The Callovian
is in most places greatly reduced in thickness but complete and extremely
rich in ammonites. The Oxfordian is also fully represented. This is
usually the highest stage present, but in the south-west there is some
Lower Kimeridgian left beneath the transgressive Cenomanian. Excellent
detailed general accounts are given by Welsch (1903), Couffon & Dolfuss
(1928), and Glangeaud (1895).
KIMERIDGIAN (0-50 m.)
A broad band of limestones with marls and coral rags, up to 100 m.
thick, extends from the coast between La Rochelle and Marans inland
through Charente Inferieure and Charente, in which it is often difficult
to draw the line between Lower Kimeridgian and Upper Oxfordian.
The upper part, when in suitable cephalopod facies, yields Streblites
tenuilobatus (Oppel), Rasenioides thermarum (Oppel), Aspidoceras, Taramelliceras,
Lithacoceras, etc. To this period belong the fine coral reefs
exposed on the coast near La Rochelle and figured by Haug (1910, Traite,
pis. 107, 108). (Glangeaud, 1895, pp. 215-33).
OXFORDIAN (up to 90 m.)
Zone of Epipeltoceras bimammatum. The lower part of the formations
just described, where in suitable facies, yields the zone fossil and Ochetoceras
marantianum d'Orb. sp., used as alternative index by Oppel and
others, of which the type locality is Marans on this coast, together with
other Oppeliidae and Perisphinctidae (Glangeaud, 1895, pp. 222-3;
Welsch, 1903, p. 850; Gerard, 1937, with some new species of
Epipeltoceras).
Zone of Gregoryceras transversarium. This zone also exhibits rapid
changes of facies, from marls and marly limestones with cephalopods
to coral rags and lithographic limestones with pelecypods. In places
where the facies is suitable all the characteristic ammonites occur. The
beds have variously been called the transversarium, perarmatum, martelli
and canaliculatum zone (Glangeaud, 1895, pp. 195-213; Welsch, 1903,
pp. 847 ff.; Gerard, 1937, pis. xi-xiv, with new species).
Zone of Cardioceras cordatum. The ammonite fauna of this zone is
mostly pyritized (Glangeaud, 1895, p. 198). The small Cardioceras spp.
figured by de Grossouvre (1922, pi. xv) certainly belong to this zone
and seem to include C. costicardia Buckman (fig. 1) and other English
species. With them are Creniceras crenatum and a number of Taramelliceras
(Proscaphites), Trimarginites, Euaspidoceras, etc., including
interesting new species figured by de Grossouvre (1922) and Gerard (1937).
The Zone of Quenstedtoceras mariae seems to be missing.
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