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

LEGEND
LlUESTONE /____/ BLACK SLATE AN3
SHALE
BLACK SHALE
/ 55r\NDSTOhE COAL
a CALCAREOUS
UNDERCLAY
SAND
Fit. 27.-Graphic representation of the partial succession and
of corresponding electric logs for seven drill holes in Ts. 3 and
-F N., R. 9 E., from a few feet "aove West Franklin limestone
to a few feet below No. 6 coal bed in Richland County.
sistivity peak opposed by a small negative
peak in the thisd curve and usually !;- n
small peak in the potential curve (fig. 29).
This bed is tentatively correlated with the
Danville (No. 7) coal bed.
From 4 to 10 feet below "No. 7" coal
bed there is in most places a bed of limestone
3 to 4 feet thick. Cuttings consist of fragments
of buff to brown, dense, slightly
argillaceous limestone. No fossils have
been o,bserved in the cuttings.
There are
insufficient data to permit correlation of
this limestone with any one of the limestones
found elsewhere between "No. 7"
coal bed and the Herrin limestone.
Merrin limestone and No. 6 coal bed.-
The No. 6 coal bed and its associated caprock,
the Heri-in limestone, have their usual
importance as key beds in Richland County.
No. 6 and No. 5 are usually the thickest
coal beds found in the county, No. 6 aver-
aging about 3 feet and reaching what is
believed to be a maximum of 5 feet. No.
6 coal bed has been traced into the Illinois
basin from the mining districts in southern
and southwestern Illinois by means of logs
of diamond-drill holes on the margins of the
mining districts, by carefully logged sotasy
holes, and by electric logs. The coal bed
is recognized in logs by its unusual thickness
and by its association with the Merrin
limestone, which has a distinctive lithol-
~gy.~ It is described as "an impure, earthy,
dark-gray to bluish-black limestone, finely
granular or sugary, and somewhat fossiliferous.
Among the characteristic fossils
sometimes seen in the cuttings are fragments
of certain fairly large robust fusulinids
(I;'usulina gil-tyi Dunbar and Condra) ."
The electric-log pattern for the limestone
and coal beds is quite variable. Some logs
show two separate peaks in the normal resistivity
curve, others one peak with a reverse
indentation, thereby supposedly diflerentiating
the limestone and the coal bed,
and in still others there is a combined peak
:zith no separation of limestone and coal
bed.
Almost immediately above the Herrin
Limestone, with 1 or 2 feet of intervening
underclay, in the northwest quarter of the
county, is a thin bed of cod overlain by 1
to 2 feet of black sheety shale. Locally a
Sims, Paul IT., et al., op. cit., p. 29.