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

40 ILLINOIS BASIN COAL RESOURCES
between the coal beds, ~articularly the sandstones,
vary laterally in lithologic composition
and thickness so that they produce variable
electric log patterns which fail to establish
the position of coal beds.
Some of these beds appear to have good
thickness. The total thickness of the roof
shale and the coal combined in four of these
coal zones is known to range from 3 to 6
feet (Nos. 121, 378, 402) with coal-bed
thicknesses which appear to run up to at
least 4 feet. As shown in the cross-sections,
however, the thickness of these coal beds is
irregular and they appear to have the lenticular
characteristic of coal beds of these
groups elsewhere, particularly in the outcrop
areas in the Eastern Interior province.
The coal beds may, in short distances, decrease
in thickness to less than 2 feet, and
their positions may be indicated simply by
roof shale or underclay, or they may be split
into thin benches.
The locatio,ns, thicknesses, and intervals
to coal No. 5 of these thicker beds are as
follows: In Shell-R4oss No. 5 (No. 121),
5 feet of coal and roof shale were logged, of
which 4 are probably referable to the coal
proper, This bed occurs 227 feet below
coal No. 5 at a depth of 1141 feet. At an
additional depth of 102 feet, a bed of coal
having a black shale roof, with a combined
thickness of 4?,4 feet, was encountered. The
black shale is capped by a fusuline bearing
limestone 2 feet thick. This coal bed occurs
329 feet below No. 5 coal bed at a depth of
1243 feet.
In Ohio-Webster No. 3 (No. 402), what
is regarded as a split coal occurs 202 feet
below No. 5 coal bed at a depth of 1268
feet below the surface. The upper and
lower benches are each 3 feet thick and are
separated by 3 feet of underclay. On the
basis of the drilling time these may be only
1 foot of coal in the upper bench and 2 feet
of coal in the lower bench. Twenty-seven
feet further down in the same drill hole
were beds of black shale and coal which together
were 4 feet thick, of which 3 feet
represents the coal bed on the basis of the
drilling time. This bed occurs 229 feet
belo,w coal No. 5 at a depth of 1295 feet.
At a depth of 1522 feet below the surface,
a 3-foot layer of undifferentiated roof shale
2nd coal was logged in the same hole. The
interval between this coal bed and coal
No. 5 is 456 feet.
The tentative correlations of these thicker
coal beds encountered in the two control
drill holes (Nos. 121 and 402) of the eastwest
cross-section are shown in plate 2. The
close association of the two coal beds (the
upper one at 1118 and 1268 feet respectively
in holes Nos. 121 and 402, and the lower
one at 1141 and 1295 feet), indicates that
these may be the Dekoven and Davis beds.
Such a correlation is further supported by
the occurrence of a marine fusuline-bearing
limestone as the caprock of the next underlying
coal bed, which occupies a position
similar to that of the Stonefort limestone
and coal of southeastern Williamson
county.'" The eastward widening of the
interval between coal No. 5 and the coal
beds tentatively correlated with the Dekoven
and Davis beds conforms to the regional
thickening of the Pennsylvanian deposits
in this direction.
The coal and black shale zone 4fi feet
thick which occurs 329 feet below coal No.
5 in drill hole No. 121 (Shell-Moss No.
5) is tentatively correlated with a I-foot
coal and black shale zone 371 feet below
coal No. 5 in drill hole No. 402 (Ohio-
IVebster No. 3) (pl. 2). The lowermost
thick coal logged in drill hole No. 402,
456 feet below coal No. 5, was not reached
in the Survey logging of drill hole No. 121.
A general decrease in thickness of this coal
zone between drill holes Nos. 121 and 402
is indicated, however, by the electric logs of
-
intervening drill holes, which in some cases
fail to show a prominent coal-bed peak
at the appropriate stratigraphic position.
The only other control drill hole of the
north-south cross-section ( pl. I ) in which
coal beds of important thickness were encountered
is No. 375 (Krohn-King No. I),
In this drill hole a split coal bed was logged
at a depth of 1265g feet below the surface
and 238 feet below coal No. 5. The upper
14Henbest, L. G., Fuaulinellas from the Stonefort limestone
member of tlic Tradewater formation: Jour. Paleo., vol.
2. pp. 70-71, 1928.