Middle Jurassic (Bajocian) Eastern Oregon

The remainder of the Warn1 Springs Member, as well
as the lower member of the Snowshoe Forma,tion in the
Izee area,, were probably deposited in deeper water than
was the Weberg Membe.r, as suggested by the absenee of
benthonic mollusks and by the fact that the sea transgressed
westward but the -..vater need not have been
deeper than neritic zone depth. Deposition in quiet
waters is clearly shown, however, by the homogeneous
character of the mudstones and siltstones, the presence
of fine laminations in these rocks, and an abundance of
well-preserved specimens of the fragile pelecypod
Bositra buchii (Romer) (equals Posidonia ornati
( Quenstedt) ) . As this species probably swam freely
(Jefferies and Minton, 1965, p. 156-179), its presence
is not an indication of depth.
The clastic volcanic rocks in the Suplee and Izee
areas, according to Dickinson and Vi grass ( 1965, p.
49-51, 54-56, 58-60), were derived from volcanic vents
GEOGRAPHIC DISTRIBUTION 37
to the east and west. They state that the volcanic. materials
of the Silvies Member of the Snowshoe Formation
were probably deposited by -..vestward-moving
turbidity currents, as indicated by the presence of wellsorted
graded layers, marked increase)n conglomeratic
la,yers eastward, and marked thickening eastwa,rd. They
state that the Basey Member of the Snowshoe Formation
contains well-sorted volcanic sandstone indicative
of deposition in shallow agitated waters. Nonetheless
the men1ber also contains layers of finely laminated siltstone
full of Bositra whose presence is indicative of
quiet, but not necessarily shallow, waters.
GEOGRAPHIC DISTRIBUTION
The geographic. occurrences of the ammonities desc.ribed
herein are shown on plate 48 and tables 5 to 10.
Detailed descriptions of the individual localities follow
table 10.