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

92 ILLINOIS BASIN COAL RESOURCES
graphic level. In some drill holes many
feet of igneous rock have been peneti-ated
(fig. 18). It has not been demonstrated
that the Omaha uplift was caused by the
arching effect od igneous intrusion in the
form of a thick sill or laccolith, although
deeper drilling may prove this to be the
case. The relatively shallow occurrences
appear to be dikes rather than sills. It seems
probable that such dikes are not restricted
to the area of the uplift, but have been reported
there mainly because of the more
closely spaced drilling, and because of the
alertness of company geologists in correctly
interpreting the character of drill cuttings
early in the exploration of the Omaha structure.
Although dikes are relatively common
in Saline County, examination of many
drillers' records, both cable-tool and diamond-drill,
reveals few that record igneous
rock. It is doubtful that Illinois drillers,
who are usually unfamiliar with this type
of lithology, would recognize and report
i~neous rocks.
In the Omaha dome area igneous rocks
occur at various stratigraphic levels. They
are found as deep as 1800 feet where they
occur in Chester beds, and as close to the
surface as 250 feet in Pennsylvanian beds.
The thickness penetrated ranges from a few
to 50 or 60 feet (fig. IS). Contact metamorphism
of a mild character, sufficient to
coke an adjacent coal bed for several feet
on either side, but with only very mild alteration
on adjacent sedimentary rock, has
been observed in mines in Saline C~unty.'~
Similar mild contact metamorphism probably
attended intrusion in the Omaha dome.
The igneous bodies are represented in
electric logs by as much as 400 ohm-meters
on the sesistivity curves, whereas the potential
curve is usually either a straight line
Gr sho,~vs a slight positive potential. A more
complete description of the igneous rock of
the Omaha dome is given by English and
G~ogan.~'
2i Ref. 23.
2s English, R. M.. and Grogan, R. M., The Omaha pool
and mica-peridotite intrusives. Gnllntin County, Illinois:
Structure of typical American. oil fields. vol. 111, A.A.P.G.,
pp. 189-212. April 1948; Illino~s Geol. Survey Rept. Inv.
130, Allgust 1948.
GOAL RESOURCES
Coal resources in the No. 5 and No. 6
coal beds in Gallatin County north of the
Shawneeto~vn fault have been estimated at
1971 million tons (1919). This estimate
was based on an average thickness of 4 feet
for each coal bed, 225 square miles underlain
by No. 5 and 210 square miles underlain
by No. 6.2The calculations were
made on the basis of 1,132,800 tons per
square-mile-foot (1770 tons per acre footsp.
gr. 1.30). Present studies require some
revision of the estimate. The accompanying
map indicates about 221 square miles
underlain by No. 6 coal bed and about 232
square miles by No. 5, giving a total of
1000 million tons o,f No. 6 and 1053 million
tons of No. 5 or a total of 2053 million tons,
assuming an average thickness of 4 feet for
each bed.
More recently (1944) the amount of
coal in No. 5 and NO. 6 beds north of the
§hawneetown fault was estimated as approximately
2000 million tons and the resources
present in the Davis and Dekoven
beds as about 1000 million tons.30
The present studies have thrown additional
light on distribution and thickness
of the Davis and Dekoven beds in Gallatin
County. The Davis bed underlies approximately
250 square miles of the county
and represents about 990 million tons of
coal with an average thickness of 3.5 feet.
The combined thickness of the two beds is
in the order of 6s feet (p. 78), but the
Davis bed appears to be the only one that
maintains a thickness averaging more than
3 feet. Thus the estimated total amount
of coal in the No. 6, No. 5, and Davis beds
is in the order of 3000 million tons. Of
this quantity less than 1 percent has been
mined or rendered unrecoverable by mining
mocesses. However, structural irregularities
are probably sufficiently important along
the fp-ult lines to prevent the recovery of
2Tady, G. H.. Coal resources of District V (Saline and
Gallatin counties) : Illinois Geol. Survey Min. Inv. Bull.
19. table 15. p. 106, 1919.
30 Cady, G. H.. Coal resources based on information obtained
from rotary drilling: Illinois Geol. Survey Rept. Inv.
93, pp. 37-38. 1944.