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

84 ILLINOIS BASIN COAL RESOURCES
Structure of the No. 6 coal bed.-From
its outcrop along the southern margin of
the area the bed declines from an altitude
of about 400 above to an altitude of 412 feet
below sea level, on the downthrow side of
the fault block in sec. 7, T. 8 S., R. 9 E.
(No. 165). The structure map (pl. 8)
shows a regional northward dip of 50 to
100 feet per mile. Interrupting the regional
northward dip are important irregularities,
including a conspicuous anticlinal
dome southwest of the town of
Omaha, anticlinal structures at the sites of
Inman, Inman West, Junction, and New
Haven oil pools, and several normal faults.
General parallelisnz of Pennsylvanian
beds and of Pennsylvanian and Chester
beds.-The structure of No. 6 coal bed (pl.
8) is closely parallel to other widespread
beds of the Pennsylvanian system and to the
Chester series as is shown by the interval
between No. 6 coal bed and the New Haven
and West Franklin limestones, the No. 5,
"No. 4," and Davis coal beds, and the lower
Kinkaid and Vienna limestones (table 8).
for the Vienna limestone. Variations in interval
between beds, in both the Pennsylvanian
system and between the Pennsylvanian
system and ,Chester series, are o,n the
order of 1 foot in 7 to I foot in 10.
Study structure maps of the Kinkaid and
Vienna limestones show anticlinal structures
close to the same positions as those
shown on the structure map of the No. 6
coal bed. The pattern of the contours delineating
the Omaha dome is essentially the
same for both Pennsylvanian and Cheater
strata.
The isopach map (fig. 16) shows the distribution
of variations in interval between
No. 6 coal bed and the base of the lower
Kinkaid limestone. The shorter interval
occurs near the center of the map with
longer intervals shown to the east and west.
The area where the shorter interval occurs
corresponds to the area of the Wabash
Valley (New Harmony) anticlinal graben.
It appears necessary, therefore, to postulate
either some folding and erosion of the
Kinkaid along this anticline before deposi-
The isopach map (fig. 15) shows the dis- tion of the Pennsylvanian beds or nontribution
of variations in interval between deposition during late Chester seas over
No. 6 and No. 5 coal beds. It reveals a high places or islands along the anticline.
slightly smaller interval between the coal After deposition of Pennsylvanian beds, subbeds
in the central part of the area as com- sequent faulting and further folding propared
with the interval between the beds to duced a graben along the axis of the prethe
east and west. A thickening of the Pennsylvanian anticline. Some drill holes
Absher sandstone, between No. 6 and No. failed to penetrate any Kinkaid, although
5A coal beds, generally accompanies an un- they reached beds at lower stratigraphic
usual increase in interval between these levels (Nos. 132, 369, 115, and 243 in T. 7
two coal beds.
and 5 S., R. 9 E.).
A co,mparison of the structure map of No. In the Kinkaid-No. 6 coal bed isopach
6 coal bed with study structure maps, made map (fig. 16), intervals for the drill holes
on Chester limestones, indicates that in adjacent to, and east of, the Inman East
general the interval increases toward the fault zone in the Inman East pool were diswest
(fig. 16) (T. 7 and 8 S., R. 8 E.). regarded because the holes cross the fault
In local areas the coal and the lower Min- plane. Thus, intervals between No. 6 coal
kaid limestone are nearly parallel, as for bed and lower Kinkaid do not represent true
example, across the Omaha dome.
stratigraphic intervals because of the omis-
Table 8 gives the variatioa of the average
interval between No. 6 coal bed and two
Chester limestones, by township : 21 0
feet (1272 to 1452 feet) for the 1o;c~~er
Kinkaid and 221 feet (1613 to 1534 feet)
sion of beds through faulting. Well No.
165, sec. 7, T. 8 S., K. 9 E., is cut by the
Ridgeway fault in at least two places. One
fault plane crosses the drill hole between
the Kinkaid limestone and No. 6 coal bed,