Regional Hydrogeologic Concepts of Homoclinal Flanks

Abstract

A worldwide overview of structural geologic settings indicates that the homoclinal flank is one of the most widespread and common of hydrogeologic systems. The geomorphic conditions on the structural basin flanks correlate with distinctive ground‐water flow patterns. Relatively undisturbed coastal plain formations and all other sedimentary rocks that are arched or warped into marginal basins are flanked at the land surface by dynamic hydrologie systems. Their aquifers, on a local scale considered to be horizontal, extend into regional tilted and beveled systems. Dips of strata that are only slightly greater than the gross slope of the land surface, but in the same direction, are prominent and produce exposures of alternating permeable and less permeable beveled formations in elongated regional bands. Differential erosion has led to cuesta landscapes in varying degrees. On these step‐like plateaus, subsequent and consequent streams have distinctive streamflow characteristics. One common type of hydrologic system is that dominated by consequent streams flowing down the structural basin, generally through reentrants in scarps. These streams capture much ground‐water runoff at the lowest exposed point in the aquifer (at the apex of the V that the stream makes with an overlying aquitard). The discharging ground water forms a natural cone of depression in the aquifer, representing a chief core of the ground‐water circulation system. An example is the large cone of depression in a Cretaceous sand aquifer in the Savannah River valley 15 to 25 kilometers south of Augusta, Georgia. Graphic modeling of hydrogeologic systems in homoclinal flanks explains the uneven distribution of ground‐water discharge. Stretches of each stream can be classified readily according to the degree of ground‐water discharge on a homoclinal flank.

Simulation of ground-water flow in southeastern Coastal Plain clastic aquifers in Georgia and adjacent parts of Alabama and South Carolina

“…6). Potentiometric features characteristic of homoclinal flanks (LeGrand and Pettyjohn, 1981) were simulated by the model downstream of the outcrop area along the Chattahoochee and Savannah Rivers. In the central and eastern parts of the model layer, predevelopment downgradient flow is simulated to the southeast toward the Atlantic Ocean and toward an area of local discharge in southwestern South Carolina, which generally is delimited by the 100-ft contour.…”

Hydrology of the southeastern Coastal Plain aquifer system in South Carolina and parts of Georgia and North Carolina

“…indicates discharge from the aquifers to the rivers (Siple, 1960;LeGrand and Pettyjohn, 1981). Discharge to smaller streams has less effect on the potentiometric surface than does discharge to large streams.…”

Autobiography of Harry E. LeGrand

“…Ground Water , the most prestigious publication for most ground water scientists and engineers, has been strengthened through the years. I am proud to have had several papers published in Ground Water; besides those already mentioned, these include LeGrand (1968, 1980, 1982, 1987) and LeGrand and Pettyjohn (1981).…”