Geologic Studies in Alaska by the U.S. Geological Survey, 1992

144 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992
the Orca Group have been interpreted as a deep-sea fan
by Winkler (1976). Bedding and cleavage generally dip
northward in both the Valdez and Orca Groups, with dips
to the northeast or northwest on the east and west sides,
respectively, of the hinge of the orocline. The study area
includes a segment of the Contact fault, which according
to Plafker and others (1977) separates higher grade Late
Cretaceous rocks of the Valdez Group to the north, from
lower grade Paleocene and Eocene rocks of the Orca
Group to the south (Plafker and others, 1985). However,
several workers have noted that the change from inboard,
more deformed and metamorphosed rocks, to outboard,
less deformed and less metamorphosed rocks, in northern
and western Prince William Sound is locally complex
with no clear lithologic break, which indicates that the
Orca and Valdez Groups represent parts of a continuously
evolving accretionary prism (Dumoulin, 1987; Gibbons,
1988; Bol and Gibbons, 1992). Therefore, following the
suggestion of Bol and Gibbons (1992) and the terminol-
ogy of Bol and others (1992), we use the term "Chugach-
Prince William terrane" for the terrane designation of
these rocks.
In addition to the turbidites, a belt of ophiolites that
stretches from Elrington Island northeast to the Copper
Mountain-Ellamar area is found in Prince William Sound
(Crowe and others, 1992) (fig. 1). One of these ophiolites
is located on Glacier Island in the southern part of the
study area (fig. 2); we will not consider these ophiolites
further.
Plutons of two ages intrude the accretionary prism in
the Prince William Sound region. Granite to granodiorite
plutons of the Sanak-Baranof belt (Hudson and others,
1979) that are dated by K-Ar as approximately 50-55 Ma
in age (Nelson and others, 1985; Tysdal and Case, 1979)
are located in eastern Prince William Sound, also near
Girdwood, as well as further to the southwest on the Kenai
Peninsula (fig. 1). Granite to diorite and gabbro plutons
dated by K-Ar at -35 Ma (Nelson and others, 1985;
Tysdal and Case, 1979) are located only in the western
part of Prince William Sound.
Figure 1. Regional geologic map of south-central Alaska showing present study area and study area of Bol and Gibbons (1992).
Rocks north of Contact fault belong to Valdez Group and those to the south belong to Orca Group. In general, strike of bedding and
cleavage is subparallel to Contact fault.