Arkell.1956.Jurassic..

THE JURA MOUNTAINS 8 7
1939, p. 263) of B. richteri in the Lower Tithonian, but considering the
complete break in the ammonite fauna between the Lower and Upper
Tithonian in the south-east of France, it is probable that this record
results from some error. In any case, there can be no doubt that beds
containing these two Berriasellids, and no other ammonites, are Upper
Tithonian in age.
The general limits of deposition of the Jura Purbeckian were sketched
on a map by Maillard (1883, pi. A), but his subsequent discovery at the
Cluse de Chaille, north of Les Echelles (1886), and others by Moret,
extended the known distribution of the Purbeckian freshwater facies
considerably farther south, to the right-angle bend of the River Isere,
NW. of Grenoble. Moret's revised map of this southern extension of
the Purbeckian swamps or lake is reproduced here (fig. n). Like
Maillard, he interpreted the boundary as the original limit of deposition,
south of which the Purbeckian must pass laterally into the Upper
Tithonian (Moret, 1933, p. 81).
The succession of the topmost Jurassic and basal Cretaceous in this
critical region is as follows :—
[BERRIASIAN
In the Bauges and Chartreuse groups the following composite section
has been inferred by piecing together a number of separate exposures
(Donze, 1951):—
Upper Berriasian: marly limestones; Chara absent, and Dasycladacean
algae dying out (Clypeina jurassica Favre).
Middle Berriasian: 'Calcaire grossier', a detrital limestone largely
composed of echinoderm fragments, comminuted shells and pseudoooliths,
locally oolitic, always rich in Dasycladacean algae, often with
Chara; there are also some black limestone pebbles containing Chara
and supposed to be derived from erosion of Purbeck Beds (Moret &
Pachoud, 1948). From a lenticular development on this horizon at
Nivolet, east of Chambery, have been obtained (Gidon, 1948) Protacanthodiscus
euthymi (Pictet), Neocosmoceras aff. curelense (Kilian), Berriasella
boissieri (Pict.), Spiticeras sp., Neocomites occitanicus (Pict.), Lytoceras
honoratianum (d'Orb.), an assemblage of the main or middle Berriasian
horizon (Boissieri Zone) of Mazenot (1939, p. 25). This bed is about
15 m. thick around Chambery but thins southwards to about 30 cms.
in the middle of the Chartreuse group, and is replaced locally by marly
limestones similar to others in the Berriasian.
Lower Berriasian: marly limestones locally rich in foraminifera, and
with Dasycladacean algae.]
UPPER TITHONIAN (ARDESCIAN) AND PURBECKIAN
In the Bauges and Chartreuse groups this consists normally of
Calpionella and cephalopod limestone, locally pseudo-oolitic and sometimes
developing a feeble reef-facies towards the top (Donze, 1951).
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