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

1 16 ILLINOIS BASIN COAL RESOURCES
thin limestone, brown to buff, fine to
medium grained, and fairly pure, overlies
the coal bed. The Jamestown coal bed of
southwestern Illinois occupies the same relative
position with respect to the Herrin
limestone, but the lithological characteristics
of this limestone differ somewhat from
those of the Jamestown limestone in the
type locality in Perry C~unty;~ there it
closely resembles the Herrin limestone.
No. 6 coal bed invariably has a moderately
thick underclay, averaging 2 feet, but
in many places as much as 5 feet thick. Like
many other Pennsylvanian underclays it is
very light gray, slip-fractured and massive,
and contains carbonaceous particles and
pyritized root remains. In most electric
logs the position of the underclay is marked
by a negative normal resistivity pattern.
Some drill holes penetrate a limestone 1
to 3 feet thick beneath the underclay, the
cuttings from which reveal a. light brown,
argillaceous, silty, non-fossiliferous, possibly
nodular rock. The general appearance of
the cuttings indicates that this is possibly
an underclay or "freshwater"Vype of limestone.
Since No. 6 and No. 5 coal beds are those
most likely to be mineable, the determination
of their thicknesses is of particular importance.
Unfortunately the determinations
cannot be made with desirable precision.
Most reliance is placed upon drilling
time, particularly when this is taken at
short intervals. In some drill holes the difference
in rate of drilling of the coal bed
and of the black shale that usually overlies
it makes it possible to differentiate the
coal from the shale (fig. 30). Commonly,
however, this is not possible. Nor can the
position of the base of the coal bed always
be definitely determined, although in general
it is believed that the underclay is somewhat
harder than the coal, so that the contact
of the two is indicated by a slight increase
in the drilling time.
The amount of coal delivered to the
sample box from each coal bed penetrated
is also an important means of estimating
'1 G. 13. Zady, personal communication.
J. Marvin, Cyclical sedimentation of the
~enns~~vahian period and its significance: Jour. Geol., vol.
38, No. 2, p. 102, 1940.
bed thickness. Continuous sieving at the
sample box is essential to recover the coal
cuttings, which, because of their low specific
gravity, tend to float off and get out of the
sample box.
Estimates of the thickness of No. 6 and
No. 5 coal beds made on the basis of the
pattern of electric logs are also unsatisfactory.
In some of the control drill holes for
which excellent stratigraphic logs are available,
the high resistivity peak of the normal
curve opposes both the roof shale and the
coal bed and, in some logs, even the Herrin
limestone; thus an estimate of the thickness
of the coal bed based entirely upon the width
of the high resistivity pattern would be excessive
(Nos. 3 and 9; figs. 29 and 30).
Yet in some electric logs irregularities in
the pattern appear to coincide with breaks
in the sequence and to provide a fairly definite
idea of the thicknesses of the various
beds (Nos. 8, 9, 59, and 91 ; fig 29).
In this report estimates of the thickness of
No. 6 and No. 5 coal beds are based upm
data provided by the control drill holes
(table 11).
No. 5A coal bed.-With respect to the
No. 6 and No. 5 coal beds, the coal bed
called No. 5A has a position similar to that
of the No. 5A (Briar Mill) bed in southern
Illin~is.~ It is 1 to 2 feet thick, is capped
by I to 2 feet of black "slate," and lies
about 20 to 30 feet above the Harrisburg
(No. 5) coal bed.
Harrisburg (No. 5) coal bed.-The position
of the No. 5 coal bed varies from 45 feet
below No. 6 bed in the northern part of
the county to 90 feet in the southern part
(fig. 29). In the control holes the No. 5
coal bed was usually as thick as No, 6 bed,
averaging 3 feet but attaining a thickness
of 5 feet. The bed is not continuous, and
where it is absent, 2 or 3 feet o,f black shale
probably marks its approximate position, as
1 to 2 feet of underclay usually underlies
the shale. Locally a thin capmck limestone
overlies the black shale. Cuttings from the
limestone consist in general o,f brown to
grayish, slightly argillaceous, very finely
"~~tts, Charles, Geology and mineral resources of the
Eqr~ali~~-Slia\vneetc~~ area: Illinois Geol. Survey Bull. 47,
1925.