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

GALLATIN COUhTTY 8 1
of the Harrisburg (No. 5) coal bed but with
less relief oa all curves. The third resistivity
curve is usually a re-entrant since
the coal bed is rarely thick enough to cause a
peak pointing to the right. Generally only
one curve stands out as representative of a
coal bed (pls. 6 and 7; and control wells
Nos. 133, 123, and 342). If either of the
beds is split or one is absent, it is difficult
to arrive at the correct interpretation (pl.
7).
Most of the Tradewater and Caseyville
strata below the Davis coal bed follow
Weller's designation^.^' The chart (pl. 6)
shows the general nature of the succession
encountered in drill holes in Gallatin
County as now kno'wn. The use of definite
names, such as Davis, Curlew, IVIurray
Bluff, Delwood, Grindstaff, Pounds,
and Battery Rock, for seven fairly thick and
persistent sandstone members of the Tradewater
and Caseyville groups, and the designation
05 intervening coal, shale, and
limestone beds as stonefort, Bald Knob,
Ice House, Willis, and Battery Rock
members, probably oversimplifies conditions
as they actually exist. This implies more
simplicity in the identification and correlation
of individual members than is actually
encountered even employing accurate and
detailed well logs. The beds are lenticular
and the sandstones
Often two or
more sandstones coalesce to form a single
massive unit, and recognition of the original
beds, as such, is impossible.
No diagnostic petrographic, lithologic,
or paleontologic characteristics have been
found which differentiate the beds, except
possibly the occasional quartz pebbles in the
sandstones of the Caseyville group. Such
pebbles have not been fomd in beds of
Tradewater age. However, the presence of
such pebble conglomerate in the lower
Pennsylvanian sandstones penetrated in
drill holes in the Illinois basin is rarely reported.
This may be because bro,ken fragments
in the cuttings are seldom recognizable
as pebbles. Pebbles have been reported
from drill holes as far north as Clay
County.'
lGWeller. 1. Marvin, Geology and oil pos~ibilities nf
extreme sout!~ern Illinois: Illinois Geol. Survey Rcpt. Tnv.
71. OD. 36-42, 1940.
Found by the writer in cuttings from drill holes in the
Bible Grove pool in Clay County.
The thickness of the Tradewater and
Caseyville groups in Gallatin County ranges
from 830 to 920 feet in the control drill
holes logged to the base of the Pennsylvanian
system (Nos. 119, 200, 342).
STRUCTURE
Structure of the Pennsylvanian strata
in Gallatin County is based on the variations
in altitude of No. 6 coal bed. This
bed is present in outcrop in the southern
part of the county and has been explored
with rod drills where it lies at shallosv
depths and with diamond drills at various
places iarther north. Within recent years
many rotary-drill holes have penetrated the
bed; of these, 11 are classified as control
drill holes, having been logged through part
or all of the Pennsylvanian succession by
Survey field parties. Control wells were so
located as to extend stratigraphic information
into parts of the county where such
data were not previously known, where
electric logs constituted the only information
available, or where interpretation of
structural conditions required additional
information. The various datum points
upon which the structure map is based are
fairly well distributed. Diamond-drill and
control wells, with reliable lithologic records,
provide adequate control of coal beds
in drill holes at intervening locations for
which only electric logs are available (fig.
17).
The present map (pl. 8) is a revision
of an earlier one which included also part
or all of Hamilton, White, and Saline counties.ls
No. 6 coal bed was selected as a
structure datum because of its widespread
economic importance in southern and southwestern
Illinois, although it is somewhat
less important than No. 5 bed in Saline
and Gallatin counties. No. 6 coal is identifiable
with relative ease where encountered
in drill holes because of its exceptional
thickness compared to other Illinois coal
beds and its asscciation with a fairly thick
characteristic limestone caprock.
1s Cady. G. 13.. Structure of Herrin (No. 6) coal bed
in Hamilton, Saline. and Gallatin counties, Illlno~s, north
of Shawneetown Fault: Illinoi.; Geol. Survey Cir. 42 (structure
map), 1939.