Arkell.1956.Jurassic..

55°
CORDILLERAS OF NORTH AMERICA
closely bound together palaeontologically and form a unit which is by no
means too large to equate with the immensely thick Macrocephalus Zone
of, for instance, Cutch (p. 391). Even 'Procerites' can be Callovian, as
Imlay points out; and apart from this doubtful determination there is no
evidence for anything Bathonian. In this connexion the excellent discussion
of the Greenland evidence by Donovan (1953, pp. 130-3) should be
read; though it does not seem to me to be necessary to doubt the generic
identity of the American Gowericeras which are at least as like the European
forms as are other North American ammonites such as the Cardioceratidae
and Stephanoceratidae.
Some of the ammonites figured by Imlay, as Cadoceras muelleri, C.
tetonense, C. piperense, from the Gowericeras beds, are astoundingly like
European Morrisiceras of the Middle Bathonian, and seem to go best in
the South American Callovian genus Eurycephalites, which Burckhardt in
1903 mistook for Morrisiceras (see p. 584).
BATHONIAN?
The Gypsum Spring formation of the Western Interior region contains
in its upper part gastropods likened by Imlay (1947, p. 242) to forms from
the English Great Oolite. The position of this horizon is below the
Callovian faunas just discussed and above the Middle Bajocian Stemmatoceras-Defonticeras
zone. No unequivocally Bathonian ammonites are
known, nor any datable to Upper Bajocian.
MIDDLE BAJOCIAN
The middle part of the Gypsum Spring formation of central Wyoming
and southern Montana, the lower part of the Sawtooth formation of
western Montana, and the lower part of the Twin Creek limestone of
western Wyoming and eastern Idaho contain a fauna characterised by
Chondroceras (Defonticeras) oblatum (Whiteaves), Zemistephanus vancouveri
McLearn and Stemmatoceras (Imlay, 1948). In Montana Teloceras is
also recorded (Imlay, 1952, p. 968). In many places this zone contains
limestones, and it represents a marine transgression from the west. The
fauna is clearly the same as that of the Middle Bajocian in Canada.
UNDATED (LIAS ?). (Up to 1000 m.)
The marine Middle Bajocian in Wyoming, Idaho and South Dakota
rests on the Nugget Sandstone, which may be a continental equivalent
of the Lower Fernie of Canada, but bears little resemblance to it. No
marine fossils occur, and there is much coarse current-bedding, with
polished sand grains and pebbles. In Arizona and parts of Nevada the
Navajo Sandstone, probably equivalent stratigraphically, has yielded
inconclusive reptilian remains. These sandstones are doubtless equivalent
in a general way to the Lias. (Imlay, 1952, pp. 664, 666.) Further thick
sandstones underneath (Wingate formation) may be basal Liassic or
Upper Triassic.
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