TERTIARY FRAMEWORK, QUATERNARY PALEODRAINAGE AND SALTWATER INTRUSION AT THE BEAUFORT ARCH, GEORGIA – SOUTH CAROLINA INNER SHELF

The Savannah and Broad-Beaufort Rivers today enter onto a mixed-energy mesotidal coast undergoing Holocene marine transgression. Two pre-Holocene paleodrainage systems are partially preserved beneath the shoreface and inner shelf, having incised during regressive and lowstand events and backfilled during subsequent transgressions. As regional bedrock, the Oligocene-Eocene aged Upper Floridan aquifer (UFA) was sufficiently resistant to fluvial downcutting during Quaternary subaerial exposure of the shelf to have influenced the paths, gradients, and topographic relief of fluvial systems. Paleovalleys have less relief and gradient than counterparts on the Mid Atlantic Bight, due largely to localized “grounding” on UFA bedrock.

Oligocene strata are locally absent on the inner shelf due to erosional truncation on the Beaufort Arch. Miocene strata are a regional aquiclude and generally range in thickness from 0 m at paleochannel incision sites to almost 50 m off the Hilton Head High. Inferred Pliocene strata are generally thin (<10 m) but can attain thicknesses of just over 20 m landward of the Hilton Head High. The Quaternary section is also generally thin, averaging 4-5 m, but can be as thick as 25 m where modern ebb-tidal deltas overlie paleochannel thalwegs. Where paleochannels incise the UFA, valley fills are preserved in one of two styles: an oxygen isotope stage 2 system has passed through an estuary-mouth environment and is dominated by a relatively coarse-grained, post-tidal ravinement, estuary mouth - tidal delta depositional system. An inferred stage 6 fluvial system did not pass completely through an estuary-mouth environment and is dominated by a pre-tidal ravinement, fine-grained, mid-upper estuary depositional system. Because of long-term groundwater extraction, a 5800 km² cone of depression on the UFA's potentiometric surface allows the opportunity for seawater to migrate through paleochannel fills to recharge the aquifer at incision sites. Paleochannel fill style may thus influence potential recharge rates. At offshore incision sites, where the coarse-grained stage 2 paleochannel reoccupies its stage 6 precursor, salt water may be more likely to reach the UFA than at nearshore stage 6 paleochannel incision sites despite having lower hydraulic gradients.