Arkell.1956.Jurassic..

EAST OF THE PARIS BASIN 6 7
known as the Raricostatum limestone, which may be of different ages
in different places and as a whole combines the Oxynotum and Raricostatum
Zones and their various subzones. Its upper surface is eroded
and followed by a disconformity and pebble bed (Kliipfel, 1921; Frebold,
1927; Maubeuge, 1949a). Thus, although the poor development of
these three zones at the type-locality of Semur (see below) has been
adduced as an argument for detaching them from the Sinemurian although
d'Orbigny included them in it, the highly condensed form of two-thirds
of the beds is against taking Lorraine as type-area for a stage based upon
them. (For the development and lists of ammonites from other parts
of the eastern rim of the Paris Basin, see the same references as before,
plus Gerard & Gardet, 1938.)
LOWER SINEMURIAN (up to 50 m.)
At the type-locality, Semur, Cote d'Or, the Upper Sinemurian is
highly condensed and fragmentary, represented by 25 cms. only of
phosphatic nodules with ammonites of the Oxynotum and Raricostatum
Zones (Mouterde, 1953, p. 404). The Lower Sinemurian also is condensed,
but much more complete. It comprises 6 m. of grey, semi-crystalline
limestones full of Gryphaea, in undulating beds with shaly partings.
The succession of ammonites is somewhat different from that expected
from more northerly European and English sections (Mouterde, 1953,
p. 396; the standard zones are here given, and nomenclatural corrections
have been made):
Zone of Euasteroceras turneri. Gryphaea limestone with Microderoceras
birchi, present a short distance to the north of Semur, 1-2 m.
Zone of Arnioceras semicostatum:
Horizon of Arnioceras semicostatum (Y. & B.) and A. miserabile
(Quenst.)
Horizon of Arnioceras geometricum (Oppel) and Arietites falsani
(Dum.)
Horizon of Euagassiceras resupinatum (Simpson) (=sauzeanum
d'Orb.) and Metarnioceras
Horizon of Agassiceras scipionianum (d'Orb.)
Horizon of Arnioceras ceratitoides (Quenst.)
Zone of Arietites bucklandi:
Horizon of Arietites bucklandi (Sow.)
Horizon of Coroniceras kridion (Ziet.) and Megarietites meridionalis
(Reynes)
Horizon of Coroniceras rotiforme (Sow.)
Horizon of Metophioceras cordieri (Canav.)
Southward from Semur parts of the series are missing and the Semicostatum
Zone overlaps. West of the Loire the thickness increases to
25 m. and perhaps 50 m. at first, but in Indre it thins again. Northeastward
round the Paris Basin the facies remains constant. The thickness
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