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MAP SYMBOLS
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Figure 1. Map showing the geology ojeMe Palisades Slate Park. Rel'isedjrom Peterson
and Groh (1970).
field guides (Peterson and Groh, 1970: Taylor
and Smith, 1987). Most of the material
printed in the regular format is for the lay
reader who wants to learn about the area
and its fascinating geology. However, addi·
tional and more technical infonnation has
been added in brackets for readers who want
to learn more about the geologic details.
Fonnation basalt suggests that an accreted
Precambrian greenstone Icrrane may underlie
the Cascades, the Deschutes Basin. or both
(Conrey, 1985).
Two Tertiary volcanic fonnations appear
to extend from the Blue Mountains westward
toward the Cascades beneath the Deschutes
Basin. The Eocene Clarno Fonnation (5240
million years [m.y.]) (Vance. 1988) consists
primarily of calc·alkaline andesites, basaltic
andesites, and minor rhyolitic domes, along
with de bris-flow deposits and other products
of a stratovolcano terrane (Bishop, 1989b),
These volcanic rocks extend from near Baker
City in eastern Oregon along the axis of the
Blue Mountain anticline to the east edge of
the Deschutes Basin and in the Mutton Mountains.
The John Day Fonnation of Oligocene
and early Miocene age is also present in the
Deschutes Basin, principally as isolated
buttes and highlands that punctuate the landscape.
These include Juniper Butte, Powell
Buttes, Cline Butte. Forked Hom Butte, and
possibly Gray BUlle.
Rhyolitic flows and ignimbrites of
Oligocene age that are often correlated with
the John Day volcanic rocks -{Robinson,
1975) are exposed in Haystack Butte and
Juniper Butte (Figure 3). Powell Butte, east
of Redmond, is Oligocene in age and possibly
a source of John Day tuffs. Cline Butte. to
the west, is of similar composition and may
be of similar age.
The best studied of these is Smith Rock
and the Gray Butte complex just west of
Terrebonne (Figure 4) (Obenniller, 1987:
Bishop, 1989a). This bimodal volcanic center
is associated with a thin veneer of olivine
basalts dated at 18 m.y. (Obenniller, 1987)
and a variety of younger rhyolitic volcanic
rocks, ranging from the rhyolite of Gray Butte
(12 m.y.) to the now-eroded lUff cone of Smith
Rock (11.5 m.y.) (Obermiller, 1987), The age
of the Gray BUlle complex is uncertain. Stratigraphy
(Smith. 1986a,b,c) and fossil evidence
(Ashwill, 1983) indicate an Oligocene
to early Miocene age and probable correlation
with Powell Buttes and similar centers. Map·
ping by Robinson (1975) shows them to be
older than the Columbia River basalts. How·
ever, studies indicate radiometric ages of
PRE-MIOCENE GEOLOGY OF THE
DESCHUTES BASIN
The Deschutes Basin lies between the
west·plunging nose of the Blue Mountains
and the nonh-south Cascade arc (Figure 2).
Its volcaniclastic rocks and flows cover ap·
proximately 1,900 mi 2 (5,000 km 2 ), extend·
ing approximately from Tumalo (9 mi [15
km] nonh of Bend) to Gateway (12 mi 120
km] nonh of Madras) in a crescent-shaped
band approximately 15 mi (25 km) wide.
The nature of the basement beneath the
Deschutes Basin is unknown. Feldspar·rich
anonhosite clasts recovered from a Deschutes
Figure 3. Juniper BUlle is an inlier oj the Clarno and John Day Formatiolls.
4
OREGON GEOLOGY, VOLUME 52, NUMBER I, JANUARY 1990