subsurface geology and coal resources of the pennsylvanian system ...

3 5 ILLINOIS BASIN COAL RESOURCES
13.) "Bankston Fork" limestone. A
limestone averaging 2 feet in thickness is
commonly encountered between "No. 7"
coal and the Herrin limestone; where the
Jamestown coal bed is developed, it lies
between the Jamestown and "No. 7" coal
beds. The limestone zone is discontinuous
but occurs in widely scattered drill holes
throughout the area studied. This limestone
is thought to be equivalent to the
Bankston Fork, since "No. 7" coal bed has
been tentatively correlated with the Cutler
coal bed of southwestern Illinois.
The in-
terval between the "Bankston Fork" limestone
and "No. 7" coal bed ranges from 3
to 14 feet. The limestoae lies from 8 to 28
feet above coal No. 6. This interval
widens locally, owing to the presence of a
lenticular sandstone, thought to represent
the Anvil Rock sandstone of southern
Illinois.
In well cuttings the limestone is buff, less
commonly light gray to brown, finely
crystalline or slightly granular, and contains
calcareous foraminifera including
fusulines, brachiopods, and ostracods.
1.) No. 5A coal bed is commonly encountered
between coal No. 6 and coal NG.
5. The combined thickness of the coal and
black roof shale ranges between 1 and 2
feet. A thin light to greenish-gray underclay
is always present below the coal. In
the control drill hole No. 377 (sec. 10, T.
4 W., R. 5 E.) a split of the No. 5A coal
bed is possibly represented by two thin beds
separated by 5 feet of light gray shale (pl.
1). The upper bed consists of undifferentiated
coal and black shale roof, 1 foot
thick, which rests on underclay 2 feet thick.
The lower bed consists of a coal bed, 1 foot
thick, accompanied by overlying black shale
1 foot thick and underclay 3 feet thick.
The underclay rests immediately on top of
the St. David limestone. This is the only
drill hole where two beds have been found
at this position. The relative position of
No. 5A coal bed varies considerably, ranging
from 2 to 20 feet above No. 5. Where
the coal bed is close to the top of the St.
David limestone, the sequence from No. 5A
through No. 5 coal bed closely resembles
the succession from the Jamestown through
No. 6 coal bed. In T. 2 N., R. 6 E.,
the interval between No. 5A and No. 5 coal
beds reaches 42 feet, part of which is represented
by a lenticular sandstone 25 feet
thick. In the Krohn-King No. 1 well (sec.
20, T. 4 N., R. 5 E., No. 378) the No.
5A coal bed is capped by dark gray calcareous
shale containing glauconite pseudomorphs
of foraminifera, crinoid columnals,
and ostracods.
2.) Sixty to 85 feet below No. 5 coal
bed a thin coal bed, "No. 4," was encountered
in all six control wells of the two
cross-sections (pls. 1, 2). The widespread
extent of this coal bed in Clay County is
further indicated by its presence in the eight
additional control drill holes which have
been logged to sufficient depth. In general
the interval between No. 5 and "No. 4"
coal beds ranges from 60 to 90 feet. Black
shale, 6 inches to 2 feet thick, is found in
all control wells; the shale is underlain by
a coal bed ranging from 6 to 1s inches thick
in 12 of the 15 drill holes logged. In drill
hole No. 261 (sec. 3, T. 3 N., R. 7 E.),
drilling time indicated the presence of 2g
feet of coal. One to 2 feet of white to gray
underclay, occasionally sideritic, calcareous,
and slightly pyritic, is commonly present.
In control drill hole No. 402, the underclay
is 8 feet thick and is underlain by 2
feet of buff, finely crystalline, slightly pyritic,
underclay limestone. An underclay
limesto,ne was noted in two additional control
wells. A capro,ck limestone is unusual,
but in drill holes Nos. 495 and 199 such
a limestone is 1 to 2 feet thick. The limestone
cuttings from drill hole No. 495 are
brown, fine-grained, slightly ferruginous,
glauconitic, and contain calcareous foraminifera,
crinoidal skeleton elements, and
brachiopods.
This coal bed is correlated with the
Summum (No. 4) coal bed of western
Illinois*2 only because its general stratigraphic
psition with reference to No. 5
coal bed is similar. It is the same as the
"No. 4" coal bed described in accompanying
12n'a~11es~. FI. R.. Pennqylvanian cycles in western Illinoi,;
Illinois Geol. Survey Bull. 60, pp. 82-181. 1931.