Arkell.1956.Jurassic..

PACIFIC BORDER OF THE UNITED STATES 555
{Gowericeras ?) occur in the Trowbridge Shale, thus correlating with
Canada and Alaska (Lupher, 1941, p. 264).
BAJOCIAN
In central Oregon a series of formations has been distinguished with
rich Middle and Lower Bajocian ammonite faunas as follows (Lupher,
1941):—
Izee group.
Snowshoe formation (840 m.). Sandstones grading to shales with
flinty nodules. Skirroceras cf. leptogyrale Buckman and S 1
. spp., Witchellia
aff. simulans (Buck.) and W. aff. felix (Buck.), Sonninia (Papilliceras)
stantoni Crickmay, S. cf. blackwelderi Crickmay, Hebetoxyites cf. hebes
Buck., H. cf. clypeus Buck. The age of the last two in England, according
to Buckman, is the upper part of the Sowerbyi Zone (Laeviuscula Subzone),
but all the others are of the Sauzei Zone. Stemmatoceras and
Normannites occur in a conglomerate derived from this formation.
Hyde formation (324 m.). Massive medium-grained sandstones with
Skirroceras and Sonninids as above.
Colpitts group: Warm Springs formation (30-90 m.) and Weberg
formation (30-67 m.). Limestones, sandstones and shales, with abundant
ammonites in both subdivisions. They include (not differentiated) a
profusion of Sonniniidae, many like those of the Sowerbyi Zone figured
in Buckman's monograph, including Euhoploceras, also Witchellia
('Zugophorites', 'Zugella'), Docidoceras spp., all of the Sowerbyi Zone, and
also Praestrigites ('Deltostrigites') cf. deltatus (Buckman) and Tmetoceras
cf. scissum (Ben.), of the Lower Bajocian.
The Sowerbyi and Sauzei Zones are also represented in the Mormon
formation (283 m.) at Mount Jura, California, as shown by Sonninia
schucherti, S. stantoni, S. blackwelderi, S. juramontanum, Normannites
reesidei and Chondroceras russelli, all of Crickmay (19336, pp. 909-913,
pis. xxvii-xxxi). Holcophylloceras occurs here.
TOARCIAN AND UPPER PLIENSBACHIAN
The next-lower formation in central Oregon, the Nicely Shale (40 m.),
contains fossiliferous nodules with Amaltheus cf. reticulatus (Simpson),
Amauroceras sp. nov., Harpoceras spp. nov., Platyharpites sp. nov. and
'early Hildoceratidae and Dactylioceratidae' (Lupher, 1941, pp. 240,
246). The same forms are found in the underlying Suplee formation
(12 m.). The age from this appears to be Upper Pliensbachian, perhaps
transitional to Toarcian. Harpoceras is also recorded from the Dunlap
formation of western Nevada, where it has been taken for basal Toarcian
(Muller & Ferguson, 1939, Table 3 and p. 1621). The Dunlap formation
contains thick conglomerates of local pebbles and its upper part is largely
volcanic; the conglomerates and local erosional unconformities are taken
to indicate uplift and folding during the Pliensbachian or possibly early
Toarcian (ibid. pp. 1616-7).
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