Arkell.1956.Jurassic..

CORDILLERAS OF NORTH AMERICA 533
Bajocian, Callovian, and locally Oxfordian, faunas. Farther south, at
least in the United States, it became separated from the coastal geosyncline
by a belt of uplift, a geanticline which suffered erosion and supplied much
sediment eastwards. In, probably, Kimeridgian times an extensive basin
became cut off from the sea in the Western Interior of the United States,
and in it was formed the continental Morrison formation with its land
animals. Finally, in the Upper Cretaceous and early Tertiary, the Rocky
Mountains trough and much of its shelf-facies upon the edge of the
continental platform were compressed by the Laramide orogeny, with
intense folding and thrusting.
Thus the outer (Coast Ranges) and inner (Rocky Mountains) belts of
the Cordillera in the main are structurally of different ages, separated in
time by most of the Cretaceous period: the outer ranges were first buckled
in the Nevadan orogeny, the inner in the Laramide. In between was
something of a no-man's-land. It is characterized mainly by high-angle
block faulting of late Tertiary date, at least in places even later. To these
last movements of cordilleran evolution are due the Basin Ranges and
their continuation northward in the plateaux and fault troughs of the
'Interior System' of Canada.
For all the Cordilleras of North America (except Alaska) a broadly
consistent evolution has been worked out, but the variation is almost
infinite and the complexity formidable. The broad structural and orographical
features of the present day by no means coincide with those of
the Jurassic. The unravelling of the tangle has proceeded much farther
in North America than in eastern Asia, but although a great amount of
detail has been published for many areas, there are still large regions almost
unknown—particularly in NW. Canada and Alaska—and a clear picture
is still a dream for the future. Even the outcrop sketch-maps for the
Americas (figs. 85-88), though probably much more accurate than those for
east Asia, are still of necessity mere sketches. The latest available geological
maps of Canada and the United States still show vast regions as
'undifferentiated Mesozoics', or as 'Trias-Jura', or 'Jurassic-Cretaceous'.
In some of these areas Jurassic has been marked on the accompanying
sketch-maps, in others it has not, according to the present author's personal
assessment of the literature. Nevertheless such sketches are believed to
be more useful than no maps at all, and less misleading than attempts at
palaeogeographical maps, constructed from shaky inferences elaborated
by guesswork.
Since the Jurassic of North America has an extensive literature in the
English language which is readily accessible to English-speaking readers,
greater compression has been considered justified than for other parts of
the world where the literature is more difficult to discover and obtain
and is largely in foreign languages. American and Canadian users of this
book are asked on this account to excuse what may seem a disproportionate
brevity or even perfunctory treatment of one of the greatest series of
Jurassic outcrops in the world.
http://jurassic.ru/