Arkell.1956.Jurassic..

52 THE PARIS BASIN
TOARCIAN (up to 10 m.)
In the condensed succession of clays and shales all the main zones
and subzones are represented. The following sequence has been
recognized (Brasil, 1895a, 1896; Munier-Chalmas, 1892):
Niveau de Pleydellia aalensis
Niveau de Dumortieria pseudoradiosa
Niveau de Grammoceras thouarsense & G. fallaciosum
Niveau de Haugia variabilis
Niveau de Hildoceras bifrons & Dact. commune
Niveau de Harpoceras falcifer & H. levisoni
A list of ammonites from each level of the Upper Toarcian is given by
Brasil (1895a) and of the Lower Toarcian by Munier-Chalmas (1892).
At May the various zones and subzones overlap one another on to the
Palaeozoic platform and are overlapped by the Bajocian.
PLIENSBACHIAN
Thin remnants and wisps of deposits, limestone, sand and conglomerate
with various Upper and Lower Pliensbachian ammonite faunas, occur
at a number of places upon the Palaeozoics and sometimes contain blocks
of Ordovician Gres de May (Bigot, 1930, p. 380). A remarkable ammonite,
Canavaria mazetieri (Dubar sp., 1927), seems to be a link with
Mediterranean faunas; it was found in situ at Tilly in the limestone band
which contains Amaltheus and Pleuroceras and forms the top of the stage.
At this locality the thickness reaches 7 m. and three horizons are separable
(Dangeard, 1951, p. 97).
SINEMURIAN AND HETTANGIAN
Remnants of these stages also occur, but ammonites are rare; Psiloceras
johnstoni and Arietites bisulcatus are recorded (Bigot, 1930, p. 378).
Locally the Sinemurian reaches 30-35 m., consisting of clays and thin
argillaceous limestones with Gryphaea arcuata.
SOUTH-WEST OF THE PARIS BASIN, WITH POITOU AND
NORTHERN AQUITAINE
South of the Loire the Jurassic ring that encircles the Paris Basin sends
out an extension south-westward, between the crystalline rocks of
Armorica and the Plateau Central, to join up with Jurassic outcrops that
flank the northern rim of the basin of Aquitaine. The narrows between
the two crystalline massifs are known in geological literature as the
Straits of Poitiers. The Jurassic forms a low plateau, faulted and gently
folded in Tertiary times on lines parallel to the Armorican structures in
the basement.
In this south-western area the Lias is greatly reduced and incomplete
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