GROUND WATER IN NORTH-CENTRAL TENNESSEE

62 GROUND WATER IN NORTH-CENTRAL TENNESSEE
GEOLOGIC STRUCTURE
NASHVILLE DOME
The major feature of geologic structure in central Tennessee is the
Nashville dome. This is a very broad elliptical flexure whose axis
trends N. 20°-30° E. and passes near Fosterville and Lascassas, in
eastern Rutherford County, and close to Norene, in southeastern
Wilson County. At the apex of the dome, which is approximately
at the center of the southern boundary of Rutherford County, about
2 miles south of Fosterville (pi. 4), the top of the Chattanooga shale
the most reliable horizon marker of the area is 1,300 feet above
sea level. From its apex the axis of the dome plunges northward and
southward between 5 and 10 feet to the mile. The transverse dips
are generally slightly steeper, however, and are about 15 feet to the
mile within 50 miles of the apex. This average dip is so slight that it
is likely to be undeterminable in any one outcrop.
The top of the Chattanooga shale or the projected original posi­
tion of that stratum prior to the erosion of the Nashville Basin is
between 1,100 and 1,300 feet above sea level in most of Rutherford
County, which occupies the highest part of the dome. On the east
flank of the dome this horizon marker is about 700 feet above sea
level at McMinnville and at Celina, which are approximately at the
respective centers of Warren and Clay Counties. On the west flank
of the dome on which lies the greater part of the region covered by
this report the Chattanooga shale slopes northwestward and is
about 900 feet above sea level at Franklin and Columbia, the principal
cities of Williamson and Maury Counties, respectively. It is between
350 and 400 feet above sea level at most places in the central part of
Dickson County and in the vicinity of Ashland, in Cheatham County.
Farther west, however, the average dip of the strata is either very
flat or else reverses in a shallow syncline, for the Chattanooga shale
crops out at several places along the Tennessee River (pi. 4) between
325 and 400 feet above sea level. These outcrops are the only known
exposures of the Chattanooga shale west of Williamson and Cheatham
Counties within the region covered by this report. Furthermore,
very few deep wells have been drilled to this shale in the intervening
area, and the thickness and stratigraphy of the overlying beds have not
been worked out in any detail. Consequently the data at hand are
inadequate to show accurately even the general features of the struc­
ture in that part of the region that lies west of the Highland Rim
escarpment.
The Nashville dome is the southerly one of two major structural
domes on the crest of the Cincinnati arch, a geanticline whose axis
appears from beneath Cretaceous and Tertiary rocks in northeastern
Mississsippi and northwestern Alabama and trends about N. 30° E.