Arkell.1956.Jurassic..

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THE JURA MOUNTAINS
occur in South America and Mexico (see table 24, p. 581); and it is
consistent with the widespread pre-Upper Tithonian movements known
in many parts of the world (for instance, overstep of Upper Tithonian
on to Palaeozoics in parts of Andalusia : p. 246).
Such an extensive uplift accounts for the failure of the uppermost
Kimeridgian and Portlandian ammonites to penetrate into these southern
basins. Contemporary uplifts elsewhere would also explain the
independent and divergent evolution of the Volgian ammonites in northeastern
seas which must have been more or less shut off from the
Portlandian basin of NW. Europe. Later uplifts completely isolated
the north-western basin and introduced the freshwater regime of the
Purbeck and Wealden Beds, just when in the south uplift had given
place to subsidence and initiated the Upper Tithonian-Berriasian cycle
of deposition in the Tethys. If the ostracods and freshwater mollusca
are to be relied upon as even approximate zonal guides, the hingeing
movement took place in the Lower and Middle Purbeckian; for this
facies and its peculiar fossil assemblage is alone common to the two
areas, and it ends the gap in the south and begins the gap in the north
(see table 5, p. 91).
[BERRIASIAN ?
THE SUCCESSION IN THE JURA PROPER
Under indubitable Valanginian and resting on the Purbeckian are
15-45 m
- °f limestones and marls. The lower part consists of coarsely
oolitic grey marls and limestones, the upper part of 'bastard marble'.
These beds are considered on insecure evidence to represent a littoral
facies of the Berriasian, but the only ammonites that appear to have
been found are both doubtful: one seems to be an Acanthodiscus misplaced
from the Hauterivian, and the other was compared to a Valanginian
ammonite (Am. hystrix Phillips) from the Speeton Clay (Baumberger,
1903-10, pi. xxi, fig. 1; and part vi, p. 39). The supporting fauna is
meagre and undiagnostic, but some ingredients, and also the same
'bastard marble' facies, occur in undoubted Berriasian farther south, in
the subalpine ranges.]
PURBECKIAN (12-25 m.)
The Purbeckian is most complete and fossiliferous in the territory
west of Lake Neuchatel and the Lake of Bienne, to beyond the River
Doubs. The interesting molluscan fauna has been monographed by
de Loriol & Jaccard (1865) and Maillard (1884, 1886), and their stratigraphical
results have been checked and amplified by Carozzi (1948),
from whose detailed monograph the following succession emerges :—•
Couches saumtaes supeneurs (brackish beds) . . Upper Purbeck
Couches lacustres (freshwater beds) with 'Intercalation
marine' in the middle . . . . . Middle Purbeck
Couches dolomitiques inferieures, in part replaced
laterally by Marnes a gypse . . . . Lower Purbeck
Ostracods collected by Carozzi and by Heap are being studied by F. W.
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