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

oi the strata below the lower bench, but
this criterion should be used cautiously. -
The "No. 7" coal bed is a thin stratum
lying within 75 feet-usually less-of the
No. 6 coal bed. It usually lies at the base
of a monotonous succession of shales and
siltstones that give, in the electric log, a
shale pattern of prevailing low relief, usually
rising to less than 30 ohm-meters in the
"normal" curve. At the top of this succession
is the West Franklin limestone member.
Between "No. 7" and No. 6 coal
beds are several relatively thin beds of
limestone, shale, "slate," and sandstone
which produce a markedly irregular electric
log pattern, as described in the earlier
report of this serie~.~ The position of the
"No. 7" coal bed, when electric logs are
correlated with carefully prepared stratigraphic
logs, is usually found to mark the
position of change from a condition of prevailing
low resistivity to one of variable
resistivity (10 to 50 ohm-meters, AM =
18") extending down to the position of
Herrin limestone and No. 6 coal bed. The
actual position of the bed is usually marked
by a definite fluctuation in the pattern, but
there is no uniformity, the normal curve
indicating a resistivity in some logs less, in
others greater, than the average shale value.
The coal bed is designated "No. 7" because
its correlation with the Danville (No.
'7) bed, although it seems probable, has not
been definitely established. The Danville
(No. 7) bed is thought to be the same as
the bed known as Indiana VII. It is hoped
that studies of coal spores as index fossils,
now under way, will result in definitely
establishing the relationship of these beds,
as well as the relative position od the CutlerQoal
bed of southern Illinois which also
may represent the Danville (No. 7) bed.
Harrisburg (No. 5) coal bed seems to
-
have a wide but not a continuous distribution
in the Illinois basin. In places it seems
t~ be thicker than No. 6 coal bed.1° It
Taylor, Earle F., Pullen, At. William, Jr., Sims, Paul K.,
and Pavne, J. Norman, Methods of subsurface study of the
Pennsyfvanian strata encountered in rotary-drill holes: Illi-
nois Geol. Survey Rept. Inv. 93, pp. 16-19, 1944.
"ell, Alfred H., Ball. Clayton, and McCabe, Louis,
Geology of .the Pinckneyville and Jamestown areaq, Perry
Countv. Illmo~s: Illinois Geol. Survey Illinois Petroleum
19, p..'3, 1931.
10 Sims Paul I(.. Payne, J. Norman. and Cady, Gilbert
H., ~enni~lvanian key beds in Wayne County, etc.: Illinois
Geol. Survey Rept. Inv. 93, pp. 28-29, 1944.
commonly lies about 100 feet below No. 6
coal bed but it may be as near as 25 feet
below and as far as 125 feet below the
upper bed.
In the electric logs its ~osition
is usually marked by a fairly strong pattern
in the normal curve (AM = 18") consisting
of a single symmetrical peak (see footnote
8) extending to about the 50 ohm-meter
line.
Between No. 5 and No. 6 coal beds, the
thin Briar Hill (No. 5A) coal bed usually
occupies a position midway to two-thirds
of the distance below No. 6 bed. This is
not regarded as a typical key bed, however.
At the present time no beds other than
those described above can be regarded as
suitable widespread key beds in the Pennsyl-
vanian system. Outcrops of beds below
No. 5 coal bed in southern Illinois are relatively
poor down to the top of the Tradewater
formation. Below that position, although
there are numerous outcrops, the
structural relationships are difficult to decipher
so as to produce a clear picture of the
stratigraphic succession, a difficulty which
is enhanced ~ J J the lenticular nature of
most of the beds of the Tradewater and
Caseyville groups. There has been relatively
little core drilling through this part of
the Pennsylvanian succession. Further
studies of the cuttings obtained from drill
holes logged in the course of this investigation
will be necessary before it will be suitable
to use any of these beds as key horizons.
STRUCTURE MAPS
Construction of structure nznps.-In this
series of reports the structure maps are constructed
with close adherence to the engineering
limitations provided by irregularly
distributed datum points represented by
drill holes, Because the key beds do not
crop out, the delineation of structure is
based upon information supplied by drilling,
and it is assumed, except where faults are
known to exist, that the slope of the datum
bed is uniform between any two adjacent
drill holes. The contours are spaced on the
basis of this assumption.
The data upon v:hich the reports are
based consist of occasional limestone or coal