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

GALLA TIN COUNTY 77
accentuated electrically by its position above
predominately silty or sandy beds up to 25
feet or more thick separating this limestone
from the Jamestown and Herrin limestones.
In wide areas in southern Illinois, including
parts of Gallatin County, these
sandy beds are represented by the Anvil Rock
sandstone member, which may occupy the
entire interval of 15 to 25 feet between
the two limestones, and in some places may
even extend to, into, or through the No. 6
coal bed as it does in certain areas in Saline
The Bankston Fork limestone, about 5
feet and rarely more than 8 feet thick, is a
dense light gray limestone that weathers to
an ochre color. It is sparingly fossiliferous
and is the uppermost bed in which Fusulina
girtyi has been found. Its occurrence, however,
is infrequent in contrast with the common
occurrence of this fossil form in the
Herrin limestone. The Bankston Fork
limestone is somewhat unique among limestones
in the McLeansboro and Carbondale
groups because it is usually underlain by
siltstone or sandstone rather than by black
shale and a coal bed. There is, however,
often a thin coal bed a short distance above
the Bankston Fork limestone. The limestone
therefore has the position usually occupied
by the so-called "freshwater') or underclay
limestone of the typical Pennsylvanian
cyclical formati~n.~~ The Bankston
Fork limestone is definitely a marine limestone
carrying brachiopods and fusulinids.
In electric logs, at the position of the
Bankston Fork limestone, the normal resistivity
curve shows 10 to 50 ohm-meters
relief with the third resistivity curve
showing a re-entrant unless the bed is unusually
thick (pl. 6). The potential curve
usually shosvs a slight negative potential.
A similar pattern a few feet above the position
assigned to the Hel-rin limestone, on
the basis of the prominent peak characteristic
of that bed, map be ascribed to the effect
of the Jamestown limestone and coal bed.
However, this bed cannot be differentiated
l1 Cady, G. H.. Coal resources of District V (Saline and
Galiatin counties) : Illinois Geol. Survey Min. Inv. Bull. 19,
pp. 21-25.
l2 Cady. G. H., Areal geology of Saline County: Trans.
lllinois Acad. Sci., vol. 19. p. 261, 1926.
in most electric logs from the Herrin limestone.
A fairly prominent deflection to the
right of the normal and third resistivity
curves and a corresponding relatively high
negative potential generally indicate the
presence of the Anvil Rock sandstone.
Briar Hill (NO, 58) coal bed and the
Absher sandstone.-The thin No. 5A coal
bed, 1 to 2 feet thick in this area, lies 40
to 60 feet below the No. 6 bed. It is usually
accompanied by an overlying bed of black
shale 1 to 2 feet thick, but rarely by a limestone
above the shale. Between No. 5A and
No. 6 coal beds there is often a more o,r less
massive sandstone (pl. 6). Like other
Pennsylvanian sandsrones this one is not
continuous in a massive form. Sandstone at
this position is well exposed at the Absher
Post Office in southeastern Williamson
County. When beds other than the sandstone
are present, it cannot be assumed that
these beds are necessarily contemporaneous.
They may be considerably older.
No. 5A coal bed produces a srnall peak
on the normal curve usually with less than
10 ohm-meters of relief, whereas the third
resistivity curve shows a re-entrant. Occasionally
a small negative potential is indicated,
but more often this curve is smooth
When the sandstone between No. 6 and
No. 5A coal beds is present, a sandstone
pattern is present in both normal resistivity
and potential curves. This and the Palzo
sandstone described below are the two most
conspicuous sandstones of the Carbondale.
"No. 4" coal bed.-This thin coal bed
ranges from a few inches to 3 feet, but is
commonly 1 to 2 feet thick and lies 161 to
202 feet below No. 6 in this area. The
average interval is 185 feet. The interval
to No. 5 coal bed is from 70 to 90 feet.
The bed is usually overlain by black shale
or "slate"; a thin bed of limesto,ne is commonly
present below the coal bed.
These beds produce a fairly distinctkre
pattern on the electric log. The normal
curve usually has 10 to 20 ohm-meters of
relief with twin points on the highest resistivity
part of the curve. The third