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

34 ILLINOIS BASIN C09L RESOURCES
lain by black sheety roof shale less than a
foot thick in three out of the four control
wells logged. The underclay limestone,
which ranges from 3 to 7 feet thick, immediately
underlies the coal bed except at
hole (No. 377) where 4 feet of light gray
to greenish gray calcareous underclay intervenes.
The limestone is light gray to
buff, finely crystalline to sublithographic,
less commonly light brown, or mottled
greenish gray from clay inclusions, and
slightly granular. Ostracods, which according
to C. L. Cooper are freshwater
forms, are sparingly distributed through the
cuttings. In one control well (No. 373)
the coal bed is capped by a marine limestone
that is 2 feet thick, buff to light brown,
mottled greenish gray, dense, argillaceous,
and contains marine invertebrate fossils.
3.) A thin coal bed, 75 to 96 feet below
the prominent underclay limestone described
in paragraph 2, is present in the control
drill holes of the north-south cross-section
(pl. 1). The coal bed is less than a foot
thick and, like the bed above, is capped by
black sheety shale up to 1 foot thick. In
one of the control wells (No. 403) the coal
bed appears to be absent but black roof shale
is present. An underclap is sporadically
developed and is light gray to greenish gray,
calcareous, pyritic, and slightly siltp.
In the Kenner oil pool (pl. 1, Nos. 495,
398), correlation from well to well is uncertain
because of the local presence in this
general zone of three coal beds lying about
10 feet apart. The upper bed is represented
by 1 foot of bony coal and black shale, the
lower one by 2 feet of coal with a thin light
gray silty clay shale underneath, and the
intermediate bed has the appearance of the
single bed usually present in this zone.
The close spacing, limited areal extent, and
the absence of evidence of intervening
marine deposits suggest local splitting of a
single bed.
4.) Also in the same north-south section
(pl. 1) a coal bed was encountered in drill
holes from 56 to 78 feet below the bed
described in paragraph 3. This bed ranges
from I to 2 feet in thickness and is also
commonly overlain by 1 foot of black sheety
roof shale. A marine impure caprock limestone
and an underclay were noted in one
drill hole (No. 378). The caprock consisted
of 3g feet of dark gray to black
calcareous pyritic and glauconitic shale in
which densely crowded fossil aggregates
form limestone lenses. The fossils include
crinoidal skeleton elements and brachiopods.
The underclay is gray, soft, and smooth,
and apparently less than 2 feet thick.
A coal bed 2 to 2>5 feet thick with black
roof shale was found from 41 to 41 feet
above the last described coal horizon in drill
holes Nos. 377, 378, T. I N., R. 5 E., but
not elsewhere. In one of the holes (No.
378) the coal bed is from 18 inches to 2
feet thick and is underlain by light to dark
gray slip-fractured underclay which contains
carbonaceous plant rootlets.
In the three southernmost wells (Nos.
495, 398, 403) of the north-south crosssection
(pl. I), a Cordaites-shale 1 foot
thick, with coaly laminae and coal streaks,
which lies frorn 14 to 19 feet below the
main coal horizon described in this section,
possibly represents a coal bed.
5.) The next lower horizon that can
usually be recognized is represented by beds
lying below the coal bed described in paragraph
4. The larger interval is believed to
be due to the local presence of a thick relatively
non-compactable sandstone in the
intervening strata. In four logs (Nos. 377,
378, 495, 103) of the north-south crosssection
(pl. I), 6 inches to 1 foot of coal
is present, the bed being capped in two
wells bj~ a black shale from 6 inches to 2
feet in thickness. In one of the coatrol
wells (No. 495) the sandstone overlying
the coal bed is calcareous, conglomeratic in
the basal five feet, and contains marine
fossils, as is indicated by the presence of
crinoidal columnals and articulate brachiopods.
The pebbles of the conglomerate
consist of buff dense fossiliferous limestone,
green clay, and siderite. In another drill
hole (No. 378) the coal bed is underlain
by a limestone two feet thick, which is buff
to gray, earthy, and contains ostracods of
the fresh-water type. In the southernmost
well (No. 103) a buff finely crystalline
fossiliferous limestone 3 feet thick is be-