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

100 ILLINOIS BASIN COAL RESOURCES
bed. Because actual proof of the presence OTHER PENNSYLVANIAN
of this coal bed is not established by diamond STRATA OF STRATIGRAPHIC
drilling, it is a less satisfactory key bed than
INTEREST
those described above. Although tentatively MCLEANSBORO GROUP
designated as "No. 4" coal bed, correlation
Approximately
with either the S~rnmurn'~ (No. 4)
125 feet above the Shoal
bed of
Creek limestone in the upper part of the
western Illinois or Indiana No. IV coal bed
~IcLeansboro group a thin bed of black
has not been definitely established. These
shale and an underlying thin coal bed are
three coal beds all lie at about the same
reported. An overlying limestone bed, 1
stratigraphic position.
to 2 feet thick, is locally present. Drill
cuttings indicate that this is a mediumgray
shaly f ossiliferous limestone. This
and other Pennsylvanian beds described in
the report are shown graphically in figures
21 and 22.
At varying positions 370 to 420 feet
above the No. 6 coal bed a bed of black shale
or "slate," accompanied by a coal bed or a
thin bed of limestone, or both, is commonly
reported in drill records or indicated in
A knowledge of the genesal character
and spacing of the key beds in the upper part
of the Pennsylvanian succession is necessary
to interpret correctly the electric logs. 'The
relatively strong resistivity of the Shoal
Creek, Bankston Fork, and Herrin limestones
in the McLeansboro group and of
the No. 6, No. 5 and "No. 4" coal beds in
the Carbondale group makes it possible to
identify the position of this series of strata
with reasonable accuracy in any electric
log." The resistivity and potential curves
may change somewhat with variation in the
thickness of the individual members, but
the general pattern remains essentially the
same.
BEDS IN THE LOVV'ER FART
PENNSYLVANIAN
OF THE
Because there has been no diamond drilling
below No. 5 coal bed, no individual
member of the lower Carbondale, the
Tradewater or the Caseyville groups, except
"No. 4" coal bed, can be regarded as a
key bed. Drilling of seven sotary-drill
control wells was observed by Survey field
parties and the cuttings were studied in the
laboratory. Even these drill holes have not
provided information of much value in interpreting
electric logs through these lower
Pennsylvanian strata.
t4 Wanless, H. R., Pennsylvanian cycles in western Illi-
1101s : Illinois Geol. Survey Bull. 60, pp. 179-193, 1931.
l5 Taylor, op. cit.
electric logs. A similar thin bed of black
shale and a coal bed are also commonly reported
270 to 300 feet above the No. 6
bed."
The West Franklin limestone17 is an important
member of the lower part of the
McLeansboro group in certain counties in
sou theastern Illinois and in adjacent coun-
ties in southwestern Indiana. Limestone
beds suggestive of the West Franklin limestone
were penetrated in only two control
drill holes. In a control drill hole logged
by the Survey in sec. 15, T. 5 S., R. 7 E.
(County No. 498) a 4-foot limestone was
penetrated 270 feet above No. 6 coal bed.
The presence of limestone at about the
same position in drill holes to the north and
south is indicated by electric logs. A control
drill hole in sec. 26, T. 4 S., R. 7 E.
(County No. 478) penetrated a white to
light gray, fine to crystalline, fossiliferous
limestone 310 feet above No. 6 coal bed.
This limestone bed can be traced westward
to a diamond drill hole in sec. 27, T. 4 S.,
R. 6 E. (County No. 3) in which a 4-foot
10 Cady, G. H., Coal resources of District VI: Illinois
Geol. Survey Min. Inv. Bull. 15, p. 31.
17 Collett, J., Thirteenth annual report: Ind. Dept. Geol.
and hiat. Hist., pp. 61-62, 1884.