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

30 ILLINOIS BASIN COAL RESOURCES
1 and 2). Drilling-time logs and sample
studies indicate that the coal bed and overlying
black sheety shale combined are from
I to 6 feet thick. The drilling time as a
rule shows no appreciable change as a basis
for estimating the shale and coal-bed ratio.
In only one hole (No. 108) did the drilling
time provide a satisfactory basis for separating
the two strata, to show a coal-bed
thickness of 2 feet.
The beds between the West Franklin
limestone and '"0. 7" coal bed range from
80 to I80 feet in thickness; they consist
predominantly of shale and subordinately
of interbedded siltstones 2nd sandstones.
Coal beds are usually absent. In a single
well logged by the Coal Division (No.
495) a thin plant-bearing shale with coal
partings and an underclay were noted 16
feet above "No. 7" coal bed.
Absence of a caprock limestone is a usual
characteristic of the succession above "No.
7" coal bed in this county.
In electric logs the position of "No. 7"
coal bed is commonly indicated by a small
but abrupt increase in resistivity as shown
in the normal resistivity curve as compared
with the overlying low-resistivity shales.
Because of its relative thinness, the "No.
7" coal bed is always indicated bj7 a negative
deflection of the third ~esistivity curve,
A negative self-potential peak is common.
For many holes in ~vhich the coal and b!ack
shale are less than 18 inches thick, the electric
logs show a negative peak in the normal
resistivity curve. This bed is tentatively
correlated with the Danville (No. 7) coal
bed.
No. 6 COAL BED
Herrin (No. 6) coal bed forms the
youngest key bed of the Carbondale group.
It lies from 816 to 1113 feet below the
surface in Clay County. Because of its relative
ease of identification and commercial
importance, No. 6 coal bed has been used
as a principal stratigraphic marker in most
of the previous stratigraphic and structural
studies of the Pennsylvanian deposits in the
Illinois basin. It also forms the datum
plane for reference to other key beds in the
overlying and underlying Pennsylvanian
strata.
No. 6 coal bed is widespread in Clay
County except in T. 5 N., R. 5 E., where
it is almost wholly absent. The two control
drill holes in this township encountered in
one well a thin streak of black shale less
than one foot thick, and in the other a streak
of coal probably 6 inches thick. Electric
logs indicate the sporadic development of
black shale and coal from less than 18 inches
up to 4 feet thick in this area. The position
of the horizon throughout the township can
be determined, however, from the position
of the persistent Herrin limestone, the base
of this limestone being used as the datum
plane in the structure map of No. 6 coal
bed (pl. 3) in the township.
Because No. 6 coal bed forms the principal
datum plane for reference to other key
beds, the evidence on which its identification
is based is briefly summarized. Control
was established by matching the logs secured
2nd compiled by the Coal Division in southern
Clay County with those of northern
tVayne County, where the horizon had been
definitely identified4 by subsurface tracing
from the southern Illinois mining districts.
Furthermore, the examination5 of the microspore
assemblages found in coal cuttings
from two drill holes (Nos. 377 and 30)
definitely identified Nos. "7", $A, and 5
coal beds. The spore assemblage from a
thin coal bed thought to be No. 6 in sec. 10,
'1'. 4 N., R. 5 E. (No. 377), although not
quite characteristic, was nevertheless thought
best referable to No. 6 coal. The stratigraphic
position of this coal bed with reference
to "No. 7" above and Nos. 5A and
5 coal beds below is in agreement with the
interpretation adopted. This correlation
substantiates the interpretation that No. 6
coal bed is cut out in most of T. 5 N., R.
5 E., and is thinly developed in a large part
of the adjacent township to the south (T.
4 N., R. 5 E.) (fig. 7).
In all of the control drill holes situated
outside the cut-out area, the coal is overlain
bv black shale. In the few holes in which
the drilling time permitted separatio~l of
Sim- Paul I