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

EARLY CENOZOIC DEPOSITIONAL SYSTEMS, WISHBONE
HILL DISTRICT, MATANUSKA COAL FIELD, ALASKA
ABSTRACT
Vertical and lateral facies architecture of the coal-
bearing Chickaloon and conglomeratic Wishbone Forma-
tions in the Wishbone Hill district of the Matanuska coal
field reflect evolution of depositional systems from low- to
high-gradient streams during Paleocene and Eocene time.
Low-gradient, bedload meandering and mixed-load anasto-
mosed streams principally drained the Matanuska area dur-
ing the Paleocene. Low-lying mires formed on abandoned
belts of meandering streams during lateral aggradation, a
process influenced by tectonic stability and autocyclic pro-
cesses. In contrast, low-lying mires in anastornosed
streams developed during vertical aggradation, which was
controlled by regional basin subsidence. Growth faulting
promoted prolonged paludification of mires on the
upthrown area and fluvial-channel capture on the
downthrown area. Overthickened fluvial deposits and in-
tervening coal zones, and coal-zone splitting reflect epi-
sodic interaction of intrabasin allocyclic processes (basin
relative stability and subsidence), autocyclic processes
(avulsion and abandonment of stream courses), and effects
of growth faulting, all of which controlled basin dynamics.
As extrabasinal uplift of the provenance area influenced
the basin dynamics, steepening of gradients produced
braided streams and mountain-front alluvial fans during
the early Tertiary. Climate probably slightly controlled ba-
sin dynamics because megafloras of these fluvial deposits
represent subtropical and warm-temperate conditions.
INTRODUCTION
Historically, the Paleocene and Eocene Chickaloon
Formation is one of the major coal-producing rock units in
Alaska. The Chickaloon Formation is exposed in the
Matanuska coal field (fig. 1). Here the Wishbone,
Chickaloon, and Anthracite Ridge coal districts combined
produced as much as 6,130,000 metric tons from 1915 to
1967 (Memtt and Belowich, 1984). Surface and under-
ground coal mines were concentrated in the Wishbone Hill
district (Barnes, 1951; Barnes and Payne, 1956). Surface
mining in the Evan Jones Coal Mine between the Moose
By Romeo M. Flores and Gary D. Stricker
and Eska Creeks near Jonesville (fig. 2) and north of the
Wishbone Hill syncline created strip-mine highwalls that
are continuous for over 3 km. We carried out facies in-
vestigations along these highwalls. Although drill-hole
data near the strip mine was collected by Barnes and
Payne (1956), no description of the highwall geology was
ever recorded. Recent reclamation proposals led to an ur-
gent need to collect stratigraphic and facies data in the
strip mine, and plans to open an underground mine near
the Evan Jones strip mine (Bohn and Schneider, 1992, p.
32) necessitated a better understanding of the coal-bearing
facies. During the summer of 1992 we described 46 strati-
graphic sections and assembled photomosaics with
complementary sketches of seven highwalls at the Evan
Jones strip mine.
REGIONAL GEOLOGY
The Matanuska coal field is located in the Matanuska
River valley at the northeast head of Cook Inlet. The val-
ley is bounded to the north by the high-angle Castle
Mountain fault (Martin and Katz, 1912) and Talkeetna
Mountains, which contain granitic and gneissic rocks. To
the south, the valley is bounded by the Chugach Moun-
tains, which consist of greenstone, diorite, and interbedded
slate and graywacke (Barnes and Payne, 1956; Winkler,
1992). The Castle Mountain fault (fig. 2) was interpreted
by Grantz (1966) as a right-lateral fault during Mesozoic
through early Tertiary time and a reverse vertical fault
from Oligocene time to the present. Detterman and others
(1976) suggested that the Castle Mountain fault was active
for millions of years and recorded at least several kilome-
ters net displacement. Payne (1955) characterized the
Matanuska River valley as part of a geosynclinal trough 50
km wide, which extends beyond the Copper River to the
east and into Cook Inlet to the southwest.
Mapping of the Matanuska River valley by Capps
(1927), Barnes and Payne (1956), Grantz and Jones
(1960), and Barnes (1962) shows that Tertiary nonmarine
clastic sedimentary rocks are generally concentrated along
the central part of the geosynclinal trough. Subordinate
amounts of the Tertiary rocks are of hypabyssal intrusive and