Arkell.1956.Jurassic..

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THE JURA MOUNTAINS
In the valley of the Isere NW. of Grenoble are celebrated localities—
Aizy, Noyarey, Le Chevallon (for summary and bibliography see Mazenot,
1939, pp. 18-21), which have yielded the rich ammonite fauna of the
uppermost Upper Tithonian, characterized by Berriasella chaperi (Pictet),
B. aizyensis Maz., Dalmasiceras progenitor (Oppel), D. djanelidzei Maz.,
Neocomites suprajurensis Maz., etc. This assemblage is believed (Mazenot,
1939) to be higher than the main Upper Tithonian assemblage of Chomerac,
or Delphinensis Subzone, which contains Proniceras pronum (Oppel)
and (important in the present context) the Berriasellids without a ventral
groove, B. lorioli (Zittel) and B. richteri (Oppel).
The Cluse de Chaille section, with the Purbeck Beds, is on the northwest
side of the Grande Chartreuse group. The detailed profile described
by Maillard (1886, pp. 6-7) shows just over 10 m. of marls and thin beds
of limestone, underlying supposed 'Valanginian' [Lower Berriasian]
limestones. Less than a metre from the top is a 0-7 m. band of
shelly limestone with Berriasella cf. lorioli (Zittel), B. richteri (Zittel)
and marine gastropods identified as Tylostoma and Chemnitzia. At
2-37 m. below this is a thin seam of greenish grey marl (0-15-0-2 m.)
enclosing fragments and nodules of limestone and filling pockets in
the bed below; in this bed occur Physa and other indeterminable
freshwater shells.
From here northwards in the ranges that link up with the Jura proper
the Purbeck Beds are fully developed but no sections are known to show
clear relations to dateable Tithonian or Berriasian. In the mountains
on the north-east side of Lake Bourget the beds have been studied by
Donze (1950) and confirm Carozzi's conclusion in the Jura proper (see
p. 83) that anticlines were already rising in Purbeckian times. On the
flanks of the anticlines the Purbeckian is up to 50 m. thick, but the
proportion of freshwater to marine beds is low; while on top of the
anticlines the thickness is much less and the proportion of freshwater
beds much higher. There is in places on the anticlines a conglomerate
of pebbles derived from destruction of underlying limestones which are
undated but are customarily known in the Jura as 'Portlandian'.
Finally, on the ridge of Mont Saleve is the series of sections described
in detail by Joukowsky and Favre (1913, pp.310-18). Here the Purbeckian
is 40 m. thick and overlain by about 100 m. of limestones and other beds
assigned to the Berriasian but without ammonites. The Purbeckian
consists of lithographic limestones, marly limestones and marls, with
several bands of conglomerate or breccia; and Chora, ostracods and
foraminifera occur throughout. The fragments of Berriasella lorioli
and B. richteri were found in a marl parting in a limestone band 0-9 m.
thick, at a level 3-2 m. below the top of the section, and marine Jurassic
gastropods (Nerinea, Aphanoptyxis, Exelissa, Natica) and Corbula overlie
them. Only 0-75 m. below the ammonite bed is again a pebbly marl with
freshwater shells : here Valvata helicoides and Planorbis loryi, with Chara
and Cypris. This particular section was at Aiguebelle. At other sections
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