Vertical crustal movements in the Charleston, South Carolina-Savannah, Georgia area

Abstract

First-order vertical level surveys (National Geodetic Survey) repeated between 1955 and 1975 suggest that modern vertical crustal movements have taken place in the Atlantic Coastal Plain between Charleston, South Carolina and Savannah, Georgia. The relative sense of these movements correlates with the sense of displacement of Tertiary strata on known geologic structures. Whereas regional dip of strata in most of the Atlantic Coastal Plain is southeasterly, the regional dip of Tertiary strata in this part of the Coastal Plain averages 2 m/km to the south or southwest. Positive structural features disturb this regional dip along a poorly defined zone, about 25 km wide, parallel to the coast between Savannah and Charleston. Structural relief on these features is as much as 20 m. Repeated level lines that cross the Atlantic Coastal Plain elsewhere generally show an increase in modern relative subsidence from west to east. However, in the Charleston—Savannah area, the amount of relative subsidence remains fairly constant or decreases from west to east across the structural highs. At two localities near Charleston, where Tertiary beds are offset by faults roughly on strike with one another, an abrupt break in a repeated level line occurs where the level line crosses the probable extensions of these faults. The average modern rates of relative uplift and subsidence (assuming they are constant) are compatible with rates noted throughout the Coastal Plain. Long-term extrapolation of modern rates appears unreasonable; episodic or oscillatory movements are much more likely.