GROUND WATER IN NORTH-CENTRAL TENNESSEE

OCCURRENCE OP GROUND WATER IN LIMESTONE 60,
GROUND WATER
OCCURRENCE OF GROUND WATER IN LIMESTONE
TYPES AND ORIOIN OF WATER-BEARINQ OPENINGS
As Meinzer 39 has pointed out, no rock differs more radically with
Tespect to yield of water than limestone. In some regions limestone
ranks among the most productive water-bearing rocks; in other
regions it is as unproductive as shale. These differences are related
in part to differences in the mode of origin of limestone and in part to
differences in the history of the rocks after they were laid down as
calcareous sediment.
The pore space of any rock may be termed continuous if the voids
are connected with one another or discontinuous if the voids are not
connected.40 Continuous pore space is that which renders the rock
permeable to water or to other fluids; discontinuous pore space,.
although it may aggregate a considerable part of the total volume
of the rock, does not impart permeability. Both types of porosity
are common in limestone.
The pore space of a rock may also be termed primary if it existed
in the sediment from which the rock was formed by consolidation,
or secondary if it has been formed after the consolidation of the rock.
Newly deposited calcareous sediment may contain many interstices
and have considerable aggregate porosity. However, during com­
paction and lithification of the sediment, calcite generally crystallizes
between many of the grains, so that the original interstices tend to
become filled. Hence such primary pore space as remains in the
consolidated rock may be largely discontinuous and may not render
the rock permeable. The pure thick-bedded limestones of the older
rock systems, such as those of Paleozoic age in central Tennessee, are
generally very dense and contain no primary pore space visible to the
eye other than minute openings in bedding planes. According to
Howard 41 the pure calcareous rocks that have appreciable primary
porosity are chalk, oolitic limestone, primary crystalline limestone
and dolomite, and coral limestone. Such rocks are not common in
north-central Tennessee. The earthy limestones and those that are
interbedded with shale are also commonly without continuous
primary pore space that might store and transmit water. The
sandy limestones and calcareous sandstones, on the other hand, may
have considerable primary porosity if the original interstices between
the sand grains are not entirely filled with calcareous cement.
!» Meinzer, O. E., The occurrence of ground water in the United States: IT. S. Qeol.
Paper 489, pp. 131-137, 1923.
* Murray, A. N., Limestone oil reservoirs of the northeastern United States and of Ontario,. Canada^
Econ. Geology, vol. 25, No. 5, p. 453, 1930.
« Howard, W. V., A classification of limestone reservoirs: Am. Assoc. Petroleum Geologists Bull,,, vol.
12, p. 1155, 1928.