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

32 ILLINOIS BASIAT COAL RESOURCES
to distinguish the caprock of No. 5 coal bed
from the Herrin limestone, which was found
to be slightly glauconitic in only one of the
wells. Fossils, fairly abundant in the cuttings,
consist of calcareous foraminifera,
including fusulines, crinoidal remains, brachiopods,
and ostracods.
The roof shale is black, sheety, and pyritic.
The upper part of the roof shale is
locally calcareous, and occasionally pyritic,
or glauconitic, and commonly carries fusulines.
The coal bed is underlain by 1 to 4
feet of underclay, which is light gray to
greenish gray, commonly calcareous, partly
pyritic, and contains siderite in the form of
individual spherules or spherulitic aggregates.
An underclay limestone is occasionally
developed either in the form of
nodules or as a solid bed. The limestone is
buff to white, dense, and reaches a thickness
of 2 feet. Ostracods miere observed in so'me
of the limestone cuttings.
The electric log pattern of No. 5 coal
bed is characterized by a fairly high peak
in the normal resistivity curve, a reverse
peak in the third resistivity curve, and a
fairly high negative self-potential. The
presence of the caprock is commonly indicated
by a more pronounced peak in the
normal-resistivity curve which is separated
either by a reverse peak omr by only a slight
negative indentation frorn the coal bed peak
(pls. 1 and 2).
OTHER PROMINENT BEDS
In the control drill holes, particularly
those which penetrated all Pennsylvanian deposits,
a number of coal beds or coal-bed
markers and some limestones were encountered
in addition to the prominent key beds.
As a rule they are thin, ranging from 6
inches to 2 feet thick, and rarely as thick as
the prominent stratigraphic markers. Most
of these thinly developed beds occur either
above the Shoal Cseek limestone in the Mc-
Leansboro group or below "No. 2" coal bed
in the Tradewater and probably uppermost
Caseyville groups.
cross-sections were re pared for those parts
of the county where either the entire or the
greater part of Pennsylvanian sequence had
been logged and samples had been studied.
The drill holes represented in the northsouth
cross-section (pl. 1 ) are located in the
western tiers of townships, T. 5 N., R. 5
E., to T. 2 N., R. 6 E., and those in the
east-west cross-section (pl. 2) are in the
northern tiers of townships, T. 5 N., R. 5
E., to T. 5 N., R. 7 E. The control drill
hole Shell-Moss No. 5 in sec. 14, T. 5 N.,
R. 5 E. (No. 121) is common to both
diagrams.
Five closely spaced control drill holes,
extending with one exception through the
entire local McLeansboro succession, permit
the tracing from north to south (~1. 1)
of coal beds above the Millersville ( ?)
limestone. The correlations in the eastwest
cross-section (pl. 2) are based on the
logs of two control drill holes (Nos. 121,
402), of which only one included the upper
beds of the local McLeansboro successien
(No. 402). For this reason no attempt has
been made to correlate beds higher than the
Millersville ( ?) limestone in this cross-
section. Although the correlations shown
in the two cross-sections are only tentative,
they indicate the possibilities of a better
stratigraphic analysis using the sort of information
made available as a result of
these investigations.
The following data on the stratigraphic
position, lithologic character, thicknesses,
and distribution of the beds discussed are in
general obtained from the records upon
which the cross-sections are based.
1.) The Omega limestone, found in
scattered exposures in northwestern Clay
County, has previously been used as a
structure datum.' It has been penetrated
in a single control dsill hole in sec. 10, T.
4- N., R. 5 E. (No. 377), where logging
was started at a depth of 70 feet below the
surface. According to the driller, a 5-foot
li~nestone was encountered at a depth of
In order to determine their areal persist- "Velier, J. M., and Bell, A. I-I., The geology and oil
and gas possibilities of parts of Marion and Clay counties:
ence and identification in electric logs, two Tllinoi; Geol. Survey Rept. Inv. 10, pp. 29-32, 1937.