n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

intrusive rocks within tt tea but a short di
tance to the south is intruded and altered by them (,.
M. Woare, written commun., 1981). The diorite and
tonalite occur as narrow elongate bodies pardleling
the Chiroskey fault along the west side of the struc-
tural high. Both bodies are extensively sheared, and
their field relations uncertain; however, newly deter-
mined K-Ar ages (table 3) suggest thet the tonalite is
younger than the diorite.
.t fossils of Late Cretaceous age (Pal
~ ~ 1 3 ) .
Calc-alkalic subaerial volcanic rocks uncon for rnably
overlie the Cretaceous sandstone end shale unit in
the eastern and central parts of the map area. Between
the Anvik fault and the Yukon River, a volcanic
pile of more than 1,000 m of basalt and andesite flows
in its lower part and of rhyolite tuff, breccia, and
domes in its upper part is preserved in a large partly
Tab1 a 3. --Potsss!um-argon detegnln.btlons
LTUK decay constants: 1 -0.581~10'~~ yr"l- 1 =d.963~10-~~ yr-l; abundance ratio: 40~/~=1.167~10'd
atom percent. ~otassitm boslyses by ~aui ~lgck; argon dnslyses and age calcularfons by Krueger
Enterprises, Geochron Laboratorfes, and M. L. Si lbennan]
Map Fleld
No. No.
Map
symbol
Andesitic volcaniclastic rwk s of probt ible islandarc
effinities unconformabIy overlie . -- the pre-
Cretaceous rocks along the Chlroskey-Anvw structural
high. These volcaniclastic rocks are characterized by
cyclically repeated sequences that grade upward from
volcanic breccia and crystal-lithic tuff to fine-grained
cherty tuff and chert. Locally, along the south edge of
the map area, the volcanic unit also includes andeslte
and basalt flows. Radiolarians (D. L. Jones and others,
written commun., 1981) and poorly preserved fragments
of Buchia(?) indicate a probable Early Cretaceous
age for this volcanic unit.
The Cretaceous sedimentary section , which may
aggregate as much as 8,000 m in thickn ess, is composed
of a lower unit of graywackc ? and m udstone submarine-fan
turbidites and an upper unit of shallow
marine and fluvial deltaic deposits of sandstone and
shale. The graywacke and mudstone unit crops out
along the Norton Sound coast north of UndakIeet md in a small area southeast of UnalakIeet. Although no
fossils heve been found in the turbidite unit in the map
area, correlation with similar strata elsewhere in the
~ukon-Koyukuk basin suggests a late Early Cretaceous
(Albian) age (Patton, 1973). The sandstone and shale
unit Is widely exposed over nearly the entire map
area. In the vicinity of Unalakleet, it appears to overlie
the graywacke and mudstone unit, but along the
Chimskey-Anvik structural high it rests directly on the
andesitic volc~nicIastic rocks. 'these shallow-water
deposits contain abundant marine mollusks of late
Early and early Late Cretaceous (Albian-Cenommian)
K2°
40
Arrad
40
Ar,ad Calculated age
pcr' (1010 nei/g) 4hrtota, (rn.y.1
fault bounded southwest-plunging syncline. This pile is
intruded by fine-grained granodlorite of probable early
Tertiary age dong the axis of the syncllne. A single
K-Ar determination on hornblende from an andesite
flow near the base of the volcenic pile gave n latest
Cretaceous or earliest Tertiary age (table 3).
Tertiary sedimentary rocks are confined to a
small patch of variegated clay shale and lignitic coal
exposed In badly slumped beach bluffs south of Unalak-
leet on the shore of Norton Sound. These rocks have
been dated at late Oligocene or early Miocene on the
basis of their pollen flora.
Alkdi-olivine basalt of Quaternary and possible
late Tertiary age forms a broad lava field covering the
coastal lowlands that border Norton Saund in the
southwestern part of the map area. Locdly, this unit
includes fresh, unmodified cinder cones and flows of
probable Holocene age.
All the pre-latest Cretaceous rocks in the map
area are tightly folded and broken by cIosely spaced
high-angle faults. The latest Cretaceous or earliest
Tertiary volcanic rocks are broadly warped and dip
generally less than 40'; the late Tertiary or Quater-
nary basalt flows are essentlslly undeformed. The
Kaltag fotdt, a major strikeslip fault that can be
traced eastward across central Alaska at least es far
as the mouth of the Tanana River, is believed to have
between 100 and 130 km of right-lateral offset since
Cretaceous time (Patton and Hoare, 1968). 'Iho Anvik
fault, which may be a splay of the Kaltag fault, could
have as much as 35 km of right-lateral offset, judging