Arkell.1956.Jurassic..

THE JURA MOUNTAINS
the lower part of the Murchisonae Zone, the 'Ancolioceras beds', and
part of the Scissum Zone of England.
Zone of Tmetoceras scissum, with T. scissum (Ben.), T. regleyi (Thiol.),
Costileioceras sinon (Bayle), Costileioceras spp., etc. This assemblage
unites the faunas of the Scissum Zone in England, Canada, and Argentina,
with the Sinon Zone of Germany, which are thus shown to be contemporary
(Lieb, 1950, p. 452). The same association also occurs in the
Swabian Jura (see below, p. 125).
Zone of Leioceras opalinum, with L. opalinum and Costileioceras costosum
(Qu.). These two forms also occur together in the Swabian Jura (p. 125),
so that costosum at most marks a subzone of the upper Opalinum Zone.
At Saint-Quentin la Verpilliere, at the southern extremity of the Jura,
Leioceras opalinum, Tmetoceras scissum and Ludwigia murchisonae occur
with Pleydellia aalensis in a condensed bed 15 cms. thick, directly overlain
by the Humphriesianum Zone (Riaz, Riche & Roman, 1913, p. 89).
TOARCIAN (up to 20 m.)
Throughout most of the Jura the Toarcian is highly condensed, consisting
of a few metres of clays, marls, or shales, sometimes with the usual
thin mudstone or micaceous sandstone bands, fish beds, or Posidonia
shales. The maximum thickness at outcrop seems to be about 20 m.,
reached in the Bernese Jura, but underground near Porrentruy a boring
proved greater thickening (Schmidt, 1924). The most celebrated locality
for fossils is Saint-Quentin, at the southern extremity of the He Cremieu,
where the Toarcian is about 3-5 m. thick and developed largely as an
oolitic iron ore, formerly worked at la Verpilliere mines (de Riaz, Riche
& Roman, 1913). From these mines were obtained most of the ammonites
figured by Dumortier in his fourth volume (1874), but they were not
accurately localised stratigraphically. They include a wealth of wellpreserved
species of both Lower and Upper Toarcian age, indicating
that nearly all the subzones are present, highly condensed, several often
mixed in the same thin beds. Some subzones represented in Dumortier's
collection (at Lyons: see Roman, 1937) cannot now be located in sections
still accessible and are therefore assumed to have been lenticular and
strictly limited in area. As remarked above, a single bed 15 cms. thick,
at the top yields Pleydellia aalensis, Leioceras opalinum, Tmetoceras
scissum and Ludwigia ?nurchisonae, and is overlain directly by the
Humphriesianum Zone in some places and the Garantiana Zone (Ciret)
in others. The Toarcian of the dept. of Doubs is described by Grosjean
(1922) and by Fallot, Corroy & Gardet (1933, p. 7).
Near Salins, the following ammonite succession is recorded (Piroutet,
1920), but no thicknesses are mentioned:—
Subzone of Pleydellia aalensis, with Dumortieria radiosa in the lower
part and Leioceras opalinum in the upper part
Subzone of Dumortieria radians
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