Arkell.1956.Jurassic..

THE JURA MOUNTAINS
miles, while the cover was held down by the weight of the Molasse in
the midland valley, stretches credulity too far. As argued by Cadisch
(1934), Aubert (1945), and Lees (1952), it is more probable that the
basement under the fold Jura was weaker than in the horsts and that
the main anticlines and thrust faults in the cover reflect strike-thrusts
or thrust-anticlines in the basement. Support for this hypothesis has
been provided by closer study of the Jurassic stratigraphy. The main
facies-changes in some of the Upper Jurassic stages occur along lines
roughly parallel to the chain (Aubert, 1947); the migrating coral reefs
of the Upper Oxfordian and Kimeridgian occur in bands parallel to the
present strike of the folds (Bourgeat, 1887); and according to Carozzi
(1948) and Donze (1950) the freshwater beds of the Purbeckian occur
in separate ellipses which to a considerable degree coincide with existing
anticlinal culminations. It therefore appears that embryonic folding
of the basement was already taking place in the Jura in Upper Jurassic
times, long before there was any weight over the midland valley to hold
down the cover. Moreover, there is a series of tear-faults which disjoint
the otherwise even sheaves of folds and must surely be controlled by tears
in the basement. Nevertheless these modifications do not seriously
detract from the pre-eminence of the Jura as an illustration of disharmonic
folding of a mighty sedimentary cover (the Jurassic alone is up to 1700 m.
thick) shortened to accommodate itself to a yielding substratum within
a semicircular frame of resistant horsts. Since the Miocene molasse
and Pontian freshwater beds are intimately involved in the structures,
while the earliest Quaternary deposits (Deckenschotter) lie across their
eroded edges, the main phase of folding and thrusting must have taken
place in the late Miocene to Pliocene (Favre & Jeannet, 1934, p. 56).
No doubt much of the disharmonic detail was controlled by the distribution
and behaviour of Triassic salt deposits (Bonte, 1943; Lees, 1952,
p. 19).
The floor of the midland valley did not remain entirely free from
folds. The ridge of Mont Saleve, south of Geneva, and some other
folds farther south, although separated by most of the narrowing
midland valley, can only be classed with the Jura, tectonically and
stratigraphically.
During the Jurassic, as usual, the trough now occupied by the Jura
became progressively more differentiated. In Liassic times it was merely
a continuation of the Rhone valley and Paris Basin, with which it remained
at all times in direct communication through the broad Langres portal.
But in the Middle Bajocian coral reefs began to grow, and during the
Upper Oxfordian and Lower Kimeridgian these assumed great importance,
building up in elongated bands parallel to the present strike of the
folds, as remarked above, but also migrating south-eastwards as time
progressed, as if advancing out from the Langres portal towards the
Alpine geosyncline. The latest reefs are in the Tithonian in the most
south-easterly outcrops, at Mont Saleve and Echaillon. Dating of
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