Arkell.1956.Jurassic..

620 GENERAL SURVEY
seldom that we yet have accurate knowledge of their histories. As will
be seen from the brief remarks on the South African and South American
shields (pp. 343, 588), it is impossible to be sure whether any of the
continental deposits thereon are Jurassic, but the indications are that
the latest are all Triassic. The same applies to the Antarctic shield. The
North American, North African and Scandinavian shields definitely
carry no Jurassic deposits. The Indian shield has usually been supposed
to carry thick Jurassic deposits of continental facies, with plant-beds, but
on examination they all appear to be Cretaceous (like the Nubian Sandstone
of North Africa), and the Jurassic is left represented only by a disconformity
between Triassic Middle and Cretaceous Upper Gondwanas
(P- 384)-
It appears, therefore, that all these shields were above sea-level and the
scene of erosion during the Jurassic. They are recognizable only by the
sediments they supplied to surrounding shelf-seas and geosynclines.
For instance, on the south side of the Tethys in Barbary there are immense
wedges of clastic sediments which thin out northwards and were evidently
derived from erosion of the Saharan shield; in NE. England and Scotland
the Bajocian and Bathonian comprise unusual thicknesses of deltaic
deposits which can only have been derived from the Scandinavian shield;
and in Scania, close to the edge of the crystallines, similar elastics are
interbedded with and make up the greater part of the Lower Lias. In
America similar wedges of elastics, coarsening north towards the shield,
underlie the Gulf coastal plain, and in the Western Interior most of the
colossal pile of sandstones (p. 545) was presumably derived from the
shield—though here and in the south largely from Palaeozoic rocks rather
than direct from the crystallines. (The same secondary derivation was
independently inferred for the Nubian Sandstone.) From interfingering
of the elastics with marine faunas round the edges of the shields something
can be inferred of the periods of maximum erosion, and therefore
differential elevation, of the shields, and the picture gained is consistent
with the absence of deposits on the shield surface.
The shields of NE. Asia are less well defined and the region is hardly
known with sufficient geological detail to enable a clear picture to be
obtained. Angaria and Sinia were and are certainly covered to some
extent by continental deposits of Jurassic age, but a large part of the
outcrops of these deposits lies rather on folded, consolidated and eroded
Palaeozoic basement which should be classified with the shelves. Authors
disagree as to the number of shields that should be recognized at the
present day. Some are better classed as horsts in labile shelf areas, like
those in NW. Europe. It is not legitimate to admit existence of Jurassic
continental deposits as evidence of a shield, for, as just remarked, such
deposits are by no means characteristic of shields and hardly exist at all
on most of them.
There remains Australia. Only the western part of the continent is a
shield. The eastern part consists of Caledonian and Variscan fold ranges
http://jurassic.ru/