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

80 ILLINOIS EASIN COAL RESOURCES
resistivity curve often shows a re-entrant
opposite the normal curve, and the potential
curve may have up to 20 millivolts of
negative potential.
The coal bed designated "No. 2," thought
to represent the LaSalle (No. 2) bed, commonly
overlies the Palzo sandstone. It is
usually thin, less than 3 feet, and is accompanied
by several feet of black "slate"
without a limestone. In electric logs the
position of this coal bed is indicated by an
abrupt increase of 10 to 20 ohm-meters on
the normal and third resistivity curves, and
5 to 20 millivolts of negative potential.
Sandstones immediately above and below
'(No. 2" coal bed occasionally have shows
of oil in this area (pl. 6).
The Palzo sandstone, basal member of
the Carbondale group as recently defined,''
is a conspicuous but lenticular sandstone.
In southeastern Williamson and southern
Saline counties it outcrops and forms
a line of prominent, although discontinuous,
hills extending from Palzo eastward to the
vicinity of Mitchellsville, a small hamlet
about 8 miles south of Harrisburg on State
Route No. 34. Although usually present
the sandstone is actually lenticular and may
be absent locally. Usually the underclay
of "No. 2" coal rests upon this sandstone.
Sometimes it rests on shale or siltstone,
probably the local equivalent of the sandstone.
The Palzo member is generally 40
to 50 feet rhick, but greater thicknesses
have been observed (County Nos. 200 and
229). The upper part of the Palzo sandstone
locally contains small shows of oil
(County No. 200).
The designation of the base of the Palzo
sandstone as the base of the Carbondale
group does not establish the position of the
top of the Tradewater group, and this position
has not been defined. However, in
many places the beds within 25 to 30 feet
be lo^ the Palm sandstone include one or
two beds of coal. When both are present,
the upper one is regarded as the Dekoven
b-d 2nd is rarely more than 3 feet thick
lVVellcr, 1. Marvin, Geology and oil posqibilities of
extreme eouthern Illinois: Illinois Geol. Sulvey Rept. Inv.
51, p. 36 (footnote 13). 1940.
and usually less. The lower one is the
Davis bed, with a maximum thickness of
about 4 feet. In western Kentucky the beds
have been called respectively the "threefoot"
and "four-foot" beds as well as Dekoven
and Davis.14 The thicknesses of the
beds are characteristic along the outcrops
of the beds in Williamson and Saline counties,
but are variable a few miles northward.
The interval between the beds varies
from about 10 to about 30 feet or more."
The lower surface of the Palzo sandstone
is very uneven. In several localities it
directly overlies the Dekoven coal bed,
which is considered the top of the Tradewater
group at such places. At some other
places it apparently rests on strata below
the Dekoven bed and may extend down to
the top of the Davis coal bed. Wherever
this is the case, the Davis coal bed is locally
the uppermost bed of the Tradewater
group. However, the base of the sandstone
may be some distance above even the top of
the Dekoven coal bed, and where this is
the case, little is known of the character
of the intervening beds. It cannot be definitely
stated that one or more thin coal
beds may or may not be included in these
strata. In most places the Dekoven coal
bed lies near the top of the Tradewater.
In subsurface the Davis and Dekoven coal
beds often split, making a total of three coal
beds. In some places diamond-drill cores
reveal the presence of only two underclays,
one beneath the middle of three beds and
one below the lower bed. The middle
bench therefore appears to be a split from
the upper bench. There is inadequate evidence
to prove whether this is the case in
I-otasy-drill holes showing three coal beds.
The Davis bed is the most persistent and
the thickest of the two beds; the Dekoven
bed is locally cut out by the Palzo sandstones.
Identification is difficult in electric
logs, however, and this may account for
the wide range of interval between No. 6
md the bed identified as Davis (table 8).
The Dekoven and Davis coal beds produce
an electrical log pattern similar to that
'"Lee, Wallace. Geology of Shawneetown quadransle in
1ieni~irk1-: TCentuckv Geol. Survey, pp. 28 and 30. 1916.
1T~ndy. G. H.. Coal resources of District 57 (Saline and
Gallatin counties): Illinois Geol. Survey Min. Tnv. Bull.
19. p. 10 and plate VI, 1919.