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

28 ILLINOIS BASIN COAL RESO UKCES
to 730 feet below the surface but is generally
encountered in wells at depths between
550 and 650 feet. The interval between
the Shoal Creek limestone and No. 6 coal
bed ranges from 283 to 455 feet. The
Shoal Creek limestone is continuous except
in the southeastern part of the county where
it is cut out in small areas by a sandstone
which, in the immediate area, commonlj~
overlies the limestone.
In drill cuttings the Shoal Creek limestone
appears white to buff, finely crystalline,
and dense. In several drill holes, the
basal part of the limestone is grayish brown
to light brown and in one hole was found
to be slightly glauconitic. Fossils, distributed
sparingly through the cuttings, commonly
consist of calcareous foraminifera,
skeletal elements of crinoids and occasional-
ly of fenestelloid bryozoa.
The thickness
of the limestone ranges from 5 to 8 feet.
It is always underlain by 1 to I feet of black
sheety shale, which in turn rests on underclay
or clay shale that is gray to greenish
gray in color, slip-fractured, and occasionally
slightly pyritic. A thin coal, as much
as one foot thick, lies between the underclay
and black shale in several wells.
The Shoal Creek limestone is readily recognizable
in electric logs by a narrow high
peak in the normal resistivity curve and a
high negative self-potential. A re-entrant
in the normal resistivity curve is commonlv
shown in the position of the black shale
and underlying underclay which represents
an additional feature in the recognition of
the 'Shoal Creek pattern. At some drill
holes in the cut-out areas the electric logs
indicate that a locally developed sandstone,
noted above, rests directly on the black
sh a1 e.
The West Franklin limestone occurs
generally from 180 to 230 feet below the
Shoal Creek limestone. The interval between
the West Franklin and No. 6 coal
bed ranges from 103 to 233 feet. The
West Franklin limestone is a less reliable
stratigraphic marker than the Shoal Creek
limestone because its identification is possible
only in electric logs of drill holes in scattered,
widely separated areas.
Drill cuttings from areas where the West
Franklin limestone cannot be identified in
electric logs reveal the presence, at its appropriate
position, of a thin limestone or of
a marine shale 6hich map contain abundant
limestone nodules. The lithologic variability
at the West Franklin horizon in
Clay County is well illustrated in the two
cross-sections (pls. 1 and 2). Despite the
limited areal development of the typical
TVest Franklin sequence, the horizon is
continuous over most of the county. The
lithologic variations represent marginal
facies changes along the western border of
the main area in which the limestone is
characteristically developed.
In the areas of characteristic West Franklin
development, the electric logs show the
presence of one, rarely of two, well-devel-
oped limestone benches. In several drill
holes logged by the Coal Division in these
areas, the sequence consists of a single upper
bench, 6 to 13 feet thick, which is separated
by shale from a lower bench only one
foot thick. Only the position of the upper
bench is distinguishable in the electric logs.
The local presence of a double rather
than a single upper bench is indicated by
drilling time and by cuttings from wel!s
in the Iola oil pool. A slight drop in the
drilling time within the formation, combined
with the occurrence of some black
shale in hole No. 30 and greenish argillaceous
limestone in hole No. 121, suggests
that the West Franklin limestone map consist
in places of as many as three benches.
In drill cuttings the lithology of the lime-
.tone is similar throughout and cannot be
med to identify the individual benches.
In the drill cuttings the limestones are
commonly buff to white, finely crystalline,
dense, rarely light bro,wn or gray and more
coarsely crystalline, or greenish gray and
argillaceous. Fossils, distributed sparingly,
consist 'of calcareous foraminifera, crinoidal
skeleton remains, brachiopod fragments,
and gastropods.
The interbedded shales are variable in
lithology and range from 2 to 10 feet in