GROUND WATER IN NORTH-CENTRAL TENNESSEE

40 GROUND WATER IN NORTH-CENTRAL TENNESSEE
or "black slate." This formation is a black or dark-brown fissile
carbonaceous shale that contains thin seams of bituminous matter
and disseminated small crystals of pyrite.
Generally the carbonaceous shale is between 20 and 35 feet thick,
but it attains a maximum thickness of 45 feet, as on Bledsoe Creek
3 miles north of Bransford, Sumner County, 69 and is entirely absent
at a few localities, as in the vicinity of Dog Creek, 3 miles northwest
of Kingston Springs, Cheatham County. 70 Northeast of the region
under investigation, on Flynn Creek, 5 miles south of Gainesboro,
Jackson County, 71 the Chattanooga shale thickens greatly in an area
about 2 miles in diameter and attains a maximum thickness of 149
feet, apparently having been deposited in one or more pre-Chatta-
'nooga sink holes. Similar features may exist elsewhere, although none
have been found in north-central Tennessee. In the south half of the
Nashville Basin the carbonaceous shale is underlain by a phosphatic
sandstone, the Hardin sandstone member, which attains a maximum
thickness of 15 feet in Wayne County. 72 In the north half of the
basin, however, this sandstone is generally only a few inches thick.
The Hardin sandstone has usually been considered to be the basal
member of the Chattanooga shale, although it is not unlikely that at
many localities in the region here described the Hardin has been con­
fused with the sandstone member at the base of the underlying Pegram
limestone. (See p. 41.)
The Chattanooga shale in Tennessee has long been considered to be
of late Devonian or early Mississippian age or possibly to represent a
transition between these two periods; but recently Swartz ra has con­
cluded that in central and western Tennessee it is wholly of earliest
Mississippian age. In view of the doubt that still exists regarding its
age, it is classified by the United States Geological Survey as Devonian
or Carboniferous. Because of its characteristic lithology, the shale is
a convenient datum plane for tracing the geologic structure and for
delimiting the major stratigraphic groups. However, it has some
limitations for these purposes, inasmuch as it lies unconformably upon
strata that range from Upper Ordovician to late Middle Devonian in
age, the whole of the Devonian and Silurian systems being locally
unrepresented.
Many seepage springs issue from the uppermost part of the Chat­
tanooga shale wherever it crops out on the steeper slopes. These
springs are the source of most of the so-called chalybeate and sulphur
water. The water from them carries a moderate quantity of iron
" Mather, K. F., op. cit., pp. 19-20.
» Jillson, W. R., Unique Devonic sandbar: Pan Am. Geologist, vol. 40, No. 5, pp. 333-337, 1923.
" Lusk, R. O., A pre-Chattanooga sink hole: Science, new ser., vol. 66, pp. 579-580,1927.
» Miser, H. D., Mineral resources of the Waynesboro quadrangle, Term.: Tennessee Qeol. Survey
Bull. 26, p. 23,1921.
** Swartz, J. H., op. cit., p. 28.