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

58 ILLINOIS BASIAT COAL RESOURCES
Harrisburg (No. 5) coal bed.-No. 5 and
No. 6 coal beds are separated by 74 to 131
feet (table 6) of strata, consisting mainly
of shale and sandstone and commonly including
one bed of coal. The township
average interval varies from 81 to 105 feet,
the average being below 95 feet north and
above 91 feet south of the base line. The
No. 5 coal bed varies from about 2 to about
4 feet in thickness in the control drill holes.
A brownish to brownish-gray, finely textured
limestone, the St. David (or Absher)
limestone, 2 to 5 feet thick, lies on or a
short distance above No. 5 coal bed, usually
with a bed of black "slate" intervening between
the limestone and the coal bed. The
St. David limestone is not present at the
position of two control drill holes (Nos. 7,
8, fig. 10) or was not recognized by the
observers.
In the electric logs the position of No. 5
coal bed is generally marked by a relatively
conspicuous resistivity peak in the normal
curve (No. 9, fig. 8; Nos. 5, 91, fig. 9;
No. 8, fig. lo), which, however, is not as
pronounced as that at the position of the
Herrin limestone and coal beds. It is
usually opposed by a small negative peak
on the self-potential curve. The "third"
curve may follow the normal curve or be
reversed, presumably because the bed is
thinner than the electrode spacing of 53
inches. This is not usually the case, as
probably in most drill holes the limestone,
black "slate," and coal bed are all represented
by the pattern. In some of the
control drill holes (Nos. 1, 11, 34, fig. 8 ;
No. 3, fig. 9) the position of the highest
resistivity peak at this general position fails
to occur at the recorded depth of the No. 5
coal bed. In some holes this may be due to
inaccurate depth mea-surement during drilling
(Nos, 11, 31, fig. 8; No. 3, fig. 9; No.
7, fig. lo), and in other holes to the presence
of a resistant bed below the underclay,
probably an underclay limestone (No. 1,
fig. 8). In interpreting electric logs the
position of No. 5 coal is generally regarded
as coincident with at least the lower part
of the prominent peak in the normal curve
at the appropriate interval be lo^ No. 6 coal
bed, with suitable consideration of the
records of adjacent control drill holes.
The underclay of No. 5 bed appears to
have the usual low resistivity of such beds.
OTHEK PROMINENT
PENNSYLVANIAN BEDS
Beds abozle the West Franklin limestone.-The
limestone member of the R4c-
Leansboro group referred to as Shoal Creek
in the present series of studies is probably
an equivalent of the one designated as Shoal
Creek and used as a key bed in neighboring
Wayne County.Vn parts of that county
it lies 350 to 475 feet above No. 6 coal bed
(200-225 feet above the West Franklin
limestone), but on the east side of the
county it becomes unrecognizable in the
drill records.
Of the 14 control drill holes in Edwards
County only three (No. 9 1, fig. 9 ; Nos. 6,
7, fig. 10) are reported to have penetrated
a limestone between 450 and 475 feet above
No. 6 bed. Correlation of these limestones
with the Shoal Creek limestone of Wayne
County is not too remote a possibility.
All of the control drill holes passed
through thin limestones, black shales, coal
beds, and wnderclays above the West Franklin
limestone. In some holes only one member
of such a group was penetrated, in others
two, and in still others an entire "cyclical"
sequence, but usually at only one or two
positions. In general the beds are thin,
and it is quite possible that some occurrences
might have escaped observation during
logging of the well and also in the examination
of drill cuttings.
At a position about 565 feet (t 25 feet)
above No. 6 coal bed and about 200 feet
above the West Ft-gnklin limestone, most of
the control drill holes (Nos. 1, 34, fig. 8 ;
Nos. 2, 4, 5, 9, fig. 9; Nos. 6. 10, fig. 10)
penetrated a thin bed of limestone ly' llng on,
or a short distance above, a thin coal bed,
usually with an intervening black shale.
This grour, of beds appears to be generally
nresent throughout the county. Its position
*' Sims, Paul K.. Pane. T. Norman. 3rd Cady. Gilbert
TT.. Pennsylvanian key beds in Wavne County, etc.: Illinois
Crr1;l. Survey Rept. Inv. 93. p. 27, 1941.