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

108 ILLINOIS BASIN COAL RESOURCES
In southern Saline and Gallatin counties
the base of the Palzo sandstone lies upon,
or a short distance above, the Dekoven coal
bed, which is a short distance above the
Davis coal. Formerly these beds were incorrectly
thought to represent the Murphysboro
coal bed of Jackson County. In places
the Palzo sandstone apparently extends below
the level of the Dekoven coal bed so
that the Oavis coal bed is the first underlying
coal bed. In other places thick shale
may separate the Palm sandstone from the
Dekoven coal bed.
A coal bed commonly underlies or is a
short distance below the Palzo sandstone
in drill holes in Hamilton County. It map
represent the Davis or the Dekoven coal
bed. In one of the seven control drill holes
a thin bed of limestone is present a short
distance below the coal bed.
Two Hamilton County coatrol drill holes
were studied to positions below the top of
the Tradewater group ; one continued
to the base of the Pennsylvanian system.
In one drill hole three coal beds were penetrated
below the zone of the Dekoven and
Davis coal beds, and in the o'ther four coal
beds. No correlation of these coal beds is
attempted, but one map represent the Murphysboro
coal bed.
STRUCTURE OF HERRIN
(NO. 6) COAL BED
The structure of the Herrin (No. 6) coal
bed in Hamilton County is indicated graphically
by the accompanying structure contour
map, which shows distribution of
variations in the altitude of the top of the
coal by contour lines spaced at 25-foot intervals
(pl. 9).
The general regional dip of the No. 6
coal bed is at an average sate 03 25 feet per
mile fl-on1 the highest recorded altitude, 166
feet below sea level, in sec. 17, T. 7 S., R.
7. E. (County No. 496) to the lowest recorded
altitude, 638 feet below sea level,
in sec. 16, T. 4 S., R. 6 E., (County No.
528) a distance of about 18% miles.
The structure map m7as constructed from
unevenly distributed datum points, so that
different parts of the map vary in detail
and accuracy depending upon the spacing
of the control points. The contours are
drawn mechanically assuming a constant
slope between adjacent datum points.
Irregularities in the direction and amount
of dip of the coal bed produce some reversals
of the regional dip. The following structural
features are noteworthy: the Dale-
Hoodville domes in the south-central part
of the county, the Rural Hill anticlinal
nose in the southwestern part, the Dahlgren
anticline in the northwestern part, the
Blairsville and Bungay domes in the northeastern
part, and a structural low in the
north-central part of the county. Some oil
has been produced from each of the structural
highs.
Where drilling is closely spaced the regional
dip is modified locally to such an extent
that local rather than regional conditiom
dominate the structure. Where datum
points are widely spaced, additional drilling
would probably change the generalized
structure mapping. Unfortunately it has
not been possible to discover these structural
irregularities from surface outcrops.
The present structure map is a revision of
the Hamilton County portion of the blueline
print map of No. 6 coal bed in Hamilton
and White and parts of Saline and
Gallatin counties dated October 1, 1938.22
COAL BEDS OF MINEABLE
THICKNESS
Herrin (No. 6) and Harrisburg (No. 5)
ccal beds are the only coal beds believed to
be of mineable thickness over any great extent
of Hamilton County. No. 6 coal bed
r2nges from 5 to between 9 and 10 feet
thick in the 30 diamond-drill holes in the
county. In seven holes the thickness is
hetween 5 and 6 feet; in eight holes between
6 and 7 feet; in ten holes between 7 and 8
feet; in two ho'les 8 feet ; and in three hdes
between 9 and 10 feet. In one of the control
rotary-drill holes, the No. 6 coal bed
was found to be 6 feet thick in an area
'Tacly, G. H.. et al., Structure of No. 6 coal bed in
I-Iamilton and White and parts of Saline and Gallatin
colintic~: Illinois Geol. Survey Cir. 42, 1939.