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

SUBSURFACE GEOLOGY OF HAMILTON COUNTY
BY
MARY BARNES ROLLEY
HE STRUCTURAL FEATURES of the
Herrin (No. 6) coal bed in Hamilton
County, as delineated in an accompanying
structure contour map, and the more important
features of the Pennsylvanian stratigraphy
of the county are described in this
report.
Hamilton County, an area of about 435
square miles,' is in the southern part of the
Illinois basin (fig. 1) and at the northern
margin of the main southern Illinois coal
field of Franklin, Williamson, and Saline
counties. No coal mines are present in the
county, but in the southern part a large
volume of workable coal is present in Hersin
(No. 6) and Harrisburg (No. 5) coal
beds according to the records of diamonddrill
holes.
TABULATED DATA
The data used in compiling the structure
map appear in tabulated form in the Appendix.
Coal thickness is most reliably recorded
in records of diamond-drill holes,
and next in the logs of control ~vells, the
drilling of which was observed by a Survey
field party. In general thicknesses cannot
be accurately determined from electric logs.
Table 10, which follows, gives the interval
between the top of No. 6 and the top
of No. 5 coal beds, and the top of No. 6
coal bed and the top of the Little Menard
limestone, the lo~~ermost limestone member
of the Menard formation of the Chester
Series, for a number of drill holes fairly
evenly spaced over the area of the county.
KEY BEDS
Certain Pennsylvanian beds are traceable
from drill hole to drill hole over considerable
areas of Hamilton County and are
1 Estimn~ed by D. H. Swaun from measurements based on
~opopraphic mapc.
therefore regarded as key beds in the identification
of stratigraphic positions in the
Pennsylvanian.
The key beds for Hamilton County are
rnainlp within the IMcLeansboro group2 but
also include three beds within the Carbondale
group.3 The records of the control
wells (figs. 19, 20) are insufficiently definite
in regard to the stratigraphic units
of the underlying Tradewater and Caseyville
groups to identify definitely individual
beds in these predominately sandy and shaly
strata.
Within the McLeansboro group the
following limestone and coal beds are fairly
conspicuous and widespread: Shoal Creek
limestone, Cutler coal bed, Banksto,n Fork
limestone, and Herrin limestone, the caprock
of the Herrin (No. 6) coal bed. Within
the Carbondale group three coal beds
are commonly recognized in the records of
drill holes of sufficient depth. These are
Herrin (No. 6) and Harrisburg (No. 5)
coal beds, recorded in most of the diamonddrill
holes in the county, and the co,al bed
called "No. 4" lying about 75 to 95 feet
below No. 5 coal bed.
No diamond-drill
hole in the county extended more than a few
feet below the No. 5 coal bed, but the
"No. 1" bed was picked up in logging seven
of the control wells. The presence of "No.
4" in many other rotary wells is indicated
bj~ the characteristic pattern of the electric
logs at the appropriate position.
Slzoal Creek li7nestone.-This limestone
is believed to be continuous into Wayne
County" and to be represented by the New
Haven limestone outcropping at New
DcWolf. F. W., Studie.; of Illinois coal-Introduction:
Tllinois Geol. Survey Bull. 16, p. 181, 1910.
3 Cad>-, G. II., Analysis of Illinois coals: U. S. Bur. Mines
Tech. Paper 641, p. 9, 1912.
"ims, P. K., et al., Pennsylvanian key beds in Wayne
County and the structure of the Shoal Creek limestone and
the I-Ierlin (No. 6) coal bed: Illinois Geol. Survey Rept.
Inv. 93. p. 28, 1944.