LITHOLOGIC AND STRUCTURAL CONTROLS ON THE AVAILABILITY OF GROUND WATER IN CRYSTALLINE ROCKS OF THE LAWRENCEVILLE, GEORGIA, AREA

Lithology and structure exert considerable control on the availability of ground water in crystalline rocks underlying the Lawrenceville, Georgia area. The bedrock geology of this region comprises a gently-dipping and broadly folded layered sequence of biotite gneiss, granitic gneiss, mica schist, amphibolite, and quartzite. Focused study of the structural features in 32 wells indicates that many of the high-yielding wells penetrate and derive water, almost exclusively, from fracture openings formed parallel to compositional layering and foliation planes in the gneisses. These high-permeability “foliation fractures,” along with high-angle joints, seem to be the primary reason for the relatively high well yields in the area – ranging from 100 and 300 gallons per minute. In Lawrenceville, foliation fractures at contact zones between major rock types, and within layered rocks in favorable structural and topographic positions, are capable of supplying large quantities of water. The rocks in the Lawrenceville area are folded into a broad synform, resulting in subhorizontal zones of permeability at depth that can be intercepted by wells. Differential weathering of layered rocks is a significant factor controlling well yield. Zones of weathering in layered rocks probably are initiated from the outcrop area, perhaps aided by water infiltration along steeply dipping joints. Once a ground-water flow path is established, progressive differential weathering along foliation planes, layering, and lithologic contacts forms zones of increased bedrock permeability.

Because development of productive foliation-fracture systems is largely controlled by weathering characteristics of bedrock lithologies and regional tectonic structure, the most effective means of identifying areas having high ground-water-production potential in the Lawrenceville area is through detailed geologic mapping in combination with borehole-geophysical logging. These techniques can be applied to other areas in the greater Atlanta region with similar lithologic and structural controls.