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

SUBSURFACE GEOLOGY OF EDWARDS COUNTY
BY
HENRY L. SMITH AND GILBERT H. CADY
1x1s REPORT describes the position, distribution,
and character of certain key
beds and other prominent members of the
Pennsylvanian system in Edwards County,
presents structure maps of the Herrin (No.
6) coal bed and the West Franklin limestone
(pls. 4 and 5), and discusses the
structural features and evaluates the resources
of workable coal beds. The use of
the structure maps in the exploration of the
oil resources is considered briefly.
The sources of information upon which
the study is based are three logs of cabletool
drill holes, 317 electric logs of rotarydrill
holes, including among them the logs
of 11 control drill holes (figs. 8, 9, 10) ,'
the drilling of which through part or all
of the Pennsylvanian strata, as the case
might be, was observed and timed by Survey
field parties. There has been no diamonddrill
exploration of the coal beds in this
county.
The names of the individuals composing
the field parties that observed the drilling
of the various holes during the years 1942,
1913, 1944, and 1945, and the names of
those persons who studied the cuttings may
be found in the table accompanying the introductory
paper of this series. (See table
2, page 17.)
The tabulated data for this county will
be found in the Appendix.
The graphic geological logs shown in these charts were
compiled from a study of cuttings, with depth controls determined
by reference to the drilling-time logs that were compiled
as the holes were being drilled. The electric log was
not available as the geological log was being compiled. At
many points therefore the two logs may not appear to be
in agreement. To obtain such an agreement the geological
log must be shifted up at some places and down at others,
which would result in uncertain accuracy. For this reaqon
it seems preferable to show both logs as originally recorded,
with the correlation lines run to points on the electric log.
Corresponding points on the geologic log are usually obvious
and suggest correlations between the two types of records
without making necessary adjustments in thickness on the
geological log, the positions of which would be difficult to
deterrn~ne.
PENNSYLVANIAN KEY BEL)S2
The key beds which can be differentiated
most accurately in the logs of drill holes in
Edwards County are the West Franklin
and Herrin limestones and the "No. 7," No.
6, and No. 5 coal beds. A sedimentary zone
about 350 feet thick near the middle of the
Pennsylvanian system in this county includes
all these beds. The two limestones
and "No. 7" coal bed are found in the
lower part of the McLeansboro group,
which here is about 1100 feet thick ; No. 6
and No. 5 coal beds lie in the upper part of
the Carbondale group, which in this county
is about 300 feet thick, with the Palzo
sandstone as the basal member. About
1000 feet of Tradewater and Caseyville
beds of the Pennsylvanian system underlie
the Carbondale group.
Other beds, some at higher and others
at lower ~ositions in the succession, may
eventually acquire importance as key beds
when information about them in this and
adjoining counties becomes available. Undoubtedly
they will be coal and limestone
beds, because of their greater continuity
and the usually greater ease of identification,
but some sandstone members of the
Carbondale and McLeansboro groups appear
to be widely present at fairly definitely
established positions.
Only about 200 feet of the upper Mc-
Leansboro beds are in the eroded zone
and outcrops are uncommon, discontinuous,
and lithologically monotonous, and hence
are difficult to correlate. For the remaining
some 2000 feet of beds, knowledge depends
on rotary drilling and upon information
supplied by inference from observation in
counties at some distance where these lower
beds may outcrop.
See p. 9 for explanation of key beds,