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

EARLY CENOZOIC DEPOSITIONAL SYSTEMS, MATANUSKA COAL FIELD
METERS
O KILOMETERS
109
overthickened Midway coalbed, which abuts against a con-
glomeratic sandstone immediately to the east. This
conglomeratic sandstone, in turn, merges upward with a
conglomeratic sandstone that elsewhere overlies the Mid-
way coalbed (fig. 7). This lateral-facies variation is re-
corded where a left-lateral, oblique-slip fault is developed.
The Premier coal zone contains six coalbeds
interbedded with thin carbonaceous shale, mudstone, iron-
stone, and tonsteins. These coalbeds and coal-free
interbeds are, in turn, separated by similar but thicker
noncoaly intervals. In general, the lower four coalbeds
contain more thick coal-free interbeds than the upper two
coalbeds, which comprise more bone and bony coal
interbeds (Barnes and Payne, 1956). Laterally, the six
coalbeds of the Premier coal zone merge to the west. The
thickness of intervening noncoaly interbeds is expanded by
erosional-based, fining-upward sandstones and coarsening-
upward sandstones (fig. 7). The eastward overthickening
of the Premier coal zone and splitting of the coalbeds coin-
cides with the occurrence of left-lateral, oblique-slip faults.
In the erosional-based, fining-upward, conglomeratic
sandstone above the Jonesville coal zone, the conglomer-
atic sandstone consists of an amalgamated, multistory, and
multilateral architecture over the 800-m lateral extent
shown in figure 8. However, the complex is interrupted
by erosional-based bodies of interbedded mudstone, silt-
stone, and sandstone. The internal organization of the
sigmoidal macroforms within this conglomeratic-sandstone
also varies laterally. Sigmoidal macroforms, which are
well developed in all the conglomeratic-sandstone bodies,
generally show west-northwest dip orientation at the west
end of the cross section and east-southeast dip orientation
at the east end. Laterally to the west, the conglomeratic-
sandstone complex grades into interbedded rooted mudstone
and rippled siltstone and sandstone. The conglomeratic sand-
stone; kconformably overlie the Jonesville coal zone, which
consists of four coalbeds interbedded with carbonaceous shale,
mudstone, ironstone, and tonstein units (fig. 8). The lower-
most coalbed and interbedded noncoaly units are separated
from the upper three coalbeds by very thick (1.5 to 4.5 m)
interbedded mudstones, siltstones, and sandstones that lo-
cally thicken from east to west. However, in the section
shown in figure 9 these clastic interbeds regionally thicken
eastward in the direction of occurrence of the left-lateral,
oblique-slip faults. The uppermost coalbed and thin
noncoaly units (fig. 8) are separated from the middle
coalbeds by thin to thick (15 cm to 1.5 m) mudstones and
Figure 7. Lateral-facies architecture of interval from below
Eska coal zone to Premier coal zone in the Chickaloon
Formation. Architecture is restored from vertical component of
movement of left-lateral, oblique-slip faults whose locations are
shown in upper part of diagram.