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Controls on Porosity and Permeability in the Karstic Knox Group, Alabama Foldand-Thrust Belt
The Knox Group, a thick sequence of Cambro-Ordovician carbonates, chert, and minor
sandstone, underlies a large area of the Southeastern US including parts of Alabama. Since
the Knox Group consists primarily of soluble carbonates (dolomite and limestone), and has
experienced at least 440 million years of geologic history, it contains karstic features (solution
cavities, caves, sinkholes, etc.). Regionally the Knox is an important water-supply aquifer, as
well as a host for oil, natural gas, and Mississippi Valley-type ore deposits. As a substrate for
diverse land uses, the Knox has received environmental pollutants, and has created numerous
foundation problems for roads, buildings, dams, and other types of engineered structures.
In this study, controls on porosity and permeability in the lower Knox Group were determined
from a large and diverse data base. Components of the study included analysis of aerial
photography and remotely-sensed imagery; geologic mapping; logging of greater than 9100 ft.
of core and stratigraphic correlation; analysis of dye (tracer) tests, grouting records, and
piezometric data; groundwater and surface water geochemistry; and statistical analysis. Both
stratigraphic and structural controls on porosity and permeability were identified, and a
hydrogeological conceptual model was developed for the lower Knox in the study area.
In the study area, the lower Knox contains major solution features in dolomite, including deep
vertical fissures (greater than 600 feet deep) as well as sinkholes. Solution cavity development
is often closely associated with, and mimics the geometry of, structural features such as faults
and zones of fracture concentration. Thrust faults and normal faults produce different depths
and geometries of solution cavity development. It is proposed that extensional forces,
associated with rifting and opening of the Atlantic in the Mesozoic, created the initial deepseated secondary porosity and permeability which was subsequently enhanced by the
dissolution of the carbonate rocks.
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