Arkell.1956.Jurassic..

624 GENERAL SURVEY
that of the present day; in fact the volcanic rocks, batholiths and foldstructures
of the Pacific border of North America are largely of Jurassic
origin, as appears in the next two chapters. Batholiths such as characterize
the mobile belt in North America were not intruded in the Jurassic
period in any other part of the world. Active volcanicity is found also in
South America, in the Caucasus and Crimea, and on a smaller scale in
the Mediterranean. Jurassic orogenies occurred in North America but
not South America, and also in the Caucasus and Crimea.
Over the rest of the mobile belts, down most of the east side of Asia
(except NE. Siberia) and thence to New Zealand, the only feature distinguishing
the Jurassic deposits from those on the labile shelves, besides
occasional volcanicity, is the great thicknesses of sediment, and even that
criterion fails in the Himalayas, in SE. Asia and over much of the. Mediterranean
area. Geosynclines certainly existed in the Jurassic in NE.
Siberia, where thicknesses of 4000 to 6000 m. are recorded, and (the
Papuan geosyncline) from New Guinea, through New Caledonia to New
Zealand, where the thickness of the Jurassic approaches 5000 m. Something
like such thicknesses are reached in the Alps where, as Deecke (1912)
showed, small deeps and shallows and emergent islands and mobile ridges
existed side by side.
In the Himalayas and Tibet the Jurassic system shows signs of mobility,
such as disconformities, condensed formations and contrasting facies,
but there is no indication of a geosyncline between Western Yunnan and
the Pamirs. As has been pointed out before, the celebrated Spiti Shales
are similar in facies to the English Upper Lias and Kimeridge Clay,
but they are thinner, more heterogeneous, and indicative of less continuous
subsidence than the Kimeridge Clay of Kimeridge. The Spiti Shales
extend over an enormous area in Tibet (p. 413) without radical change of
facies. The Callovian in the Spiti area and the Bajocian in Tibet (Lungma
Limestone) are condensed and highly fossiliferous ironshot oolites, and in
the exotic blocks of the Kiogars are samples of red Adnet Limestone of
Hettangian and Sinemurian age and Upper Tithonian Calpionella oolite,
both matching their counterparts in southern Europe. Except for 200-
300 m. of possibly Lower Tithonian limestone (Kiogar Limestone), all
the formations, though varied, are thin; and there are no conglomerates,
unconformities, or Jurassic igneous rocks. The Jurassic features of this
part of the greatest Tertiary mobile belt on earth, therefore, are those of a
labile shelf. Here the Tethys was merely a shelf-sea comparable with that
of north-western Europe.
In the Mediterranean area outside the Alps (where acute tectonic
deformation in the Tertiary orogenies makes reconstructions difficult
and hazardous), North Africa (Barbary) provides the most detailed
information over the largest area, thanks to the work of many French
geologists in Algeria and Morocco, and especially to the brilliant analyses
and syntheses of G. Lucas (1942, 1952). Barbary forms the southern part
of the mobile belt of Tethys, adjoining the north margin of the African
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