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

7 4 ILLINOIS KASIAT COAL RESOURCES
though such a shale is common at this position.
A limestone outcropping along Saline
River about 2 miles northwest of Ridgewajin
the NWS SW>$, sec. 23, T. 8 S., 111. 8
E., is tentatively assigned to the West
Franklin rather than to the Shoal Creek, as
previously thought."
The West Franklin limestone is recorded
by one, two, or three characteristic limestone
peaks in the normal resistivity curve. When
the presence of three limestone beds is indicated,
the middle peak is most prominent,
having the greatest relief in both the normal
and third resistivity curves, and is the peak
most likely to go off scale (greater than
100 ohm-meters) in both resistivity curves.
The peaks representing the other two
benches rarely extend beyond the 70-ohmmeter
value in the case of the normal resistivity
curve, and the third resistivity
curve usually shows a valley. Small negative
potential usually occurs at the positions
of the thinner upper and Ion-er beds; whereas
the position of the thicker intermediate
bed is marked by a negative potential of as
~nuch as 50 millivolts. The position of the
variegated clay shale between the two lower
beds is marked by a prominent valley 01-
re-entrant of the normal curve, which generally
characterizes underclay-like shales, and
by a positive peak of the potential curve.
""No. 7" coal bed.-The "No. 7" coal
bed, usually only 2 to 3 feet thick, is commonly
present 150 to 249 feet below the
West Franklin liniestone and 40 to 50 feet
above Herrin (No. 6) coal bed. Its position
is generally indicated on the normal
resistivity curve by a peak extending about
10 ohm meters and by a re-entrant in the
third resistivity curve, and the potential
curve shows a slight negative value. The
position of the coal bed is fairly definitely
indicated because the pattern noted above
terminates a rather long and monotonous
pattern of low relief below the position of
the West Franklin limestone. The coal
bed is the first fairly distinct irregularity in
the group of irregu!arities that mark the
nosition of the beds near the No. 6 coal bed,
T~ady. G. I-I., Structure of l3cr1-in (No. 6) coal bed ill
I-In~nilion. Saline. and Gallatin Cor~n~iec. Illinois. north nf
Sl~an-neetown fault: Illinois Geol. S~tr-vey Cir. 12 (Tah~ilatcd
Conl Data, Gallatin Count?;. Itenl No. 102), 1939.
Herrin 1inzestone.-This is the caprock
of the No. 6 coal bed from which it is usually
separated in Gallatin County by a black
shale a few feet thick. The limestone, 2
to 6 feet thick, often 3 feet, is almost as
widespread in this county as the coal bed
lying from 1 to 10 feet below. Except for
the Jamestown limestone, a few feet higher,
which it resembles, Herrin limestone is
distinctive in its lithology among Pennsylvanian
limestones in this part of the State.
It varies in color from gray to brownishgray
to almost black and in texture from
earthy to finely granular and dence. The
black varieties are particularly fine grained.
Bedding is usually poorly developed. The
Herrin limestone is usually fossiliferous ;
Fusulina girtyi is generally present in anjoutcrop
and is ccrnraon in drill cuttings."
In a strip pit in sec. 16, 7'. 9 S., R. 8 E., the
rock contains dark gray chest in large irregular
masses.
The pattern produced by Herrin limestone
in the electric log will be considered
in the section on the Herrin (No. 6) coal
bed.
CARBONDALE GROUP
Herrin (No. 6) coal bed.-The uppermost
member of the Carbondale group is
the Herrin (No. 6) coal bed. This bed,
4 to 5 feet thick in Gallatin County, carries
here, as elsewhere in the Illinois coal field,
a clay band called the blue band about 1lh
inches thick, 12 to 15 inches above the base
of the bed. This band. however, is not
usually picked up in logging rotary-drill
holes. Blxk "slate," 2 to 3 feet thick,
commonly separates the coal bed from the
Herrin limestone or capi-ock, and in places
an additional few feet of gray shale map
intervene between the black "slate" and
the caprock. Beneath the coal bed are a
few feet of underclay, the lower part of
which in places is nodular and calcareous
or sandy.
A characteristic pattern in electric logs
occurs opposite the No. 6 coal bed, particularly
when grouped with the overlying
shales and limestones and the underclay.
" Dunbar, Carl 0.. and Ilenbest. Llord G., Pennsvlvania
Iii~i~linid~r nf Tllinnic. Illinois Stace Gcol. Surrey Bull. 67,
p 21, plate TT, 1912.