ANTECEDENT TOPOGRAPHIC CONTROLS ON QUATERNARY DEPOSITION IN A TRIBUTARY ESTUARINE SYSTEM, CROATAN SOUND, NORTHEASTERN NORTH CAROLINA

Drill hole data from 40 test borings, performed for bridge foundation conditions by the North Carolina Department of Transportation, were analyzed in concert with high-resolution seismic data. These data suggest that portions of depositional sequences representing as many as 13 Quaternary sea-level highstands are preserved within the upper 45 m of deposits along an eight km transect across Croatan Sound in northeastern North Carolina. Amino-acid racemization (AAR) age determination has constrained the age of at least nine of these depositional sequences to younger than 250 ka. Foraminiferal and molluscan fossil assemblages suggest that the marine facies of these sequences represent an open embayment depositional environment.

Sediment relationships observed in Croatan Sound, a drowned tributary of the Roanoke/Albemarle drainage system, may be representative of depositional patterns in similar drowned tributary streams. Where preserved, depositional sequences display a vertical succession of three major facies. 1) A basal lowstand facies characterized by soil development and sub-aerial erosion of uplands in concert with deposition of a spectrum of terrigenous sediments along stream valleys. 2) An intermediate estuarine facies produced by gradual flooding and broadening of stream valleys that results generally in deposition of mud in deeper channel areas and of shoreline erosion-derived sand on shallower non-channel areas. 3) An overlying fining-upward marine shelf facies that partially truncates the previously deposited lowstand and estuarine facies. Antecedent topography within the subsurface strongly influences depositional patterns in overlying sequences. Streams repeatedly occupy the same course during successive sea-level lowstands. Each depositional sequence alters or truncates portions of the preceding sequence or sequences. The result is a stacked series of similar marine shelf deposits that are locally separated by remnants of fluvial and/or estuarine channel deposits.