EVOLUTION OF SEDIMENTATION PATTERNS AND LOCAL RATE OF HOLOCENE SUBSIDENCE AT CAPE HENRY (VIRGINIA) REVEALED FROM LUMINESCENCE AGES OF BEACH RIDGES

A large beach ridge complex at Cape Henry (Virginia) aggraded into the mouth of the Chesapeake Bay during the past 5000 years. Seven recently collected luminescence ages combined with detailed Lidar mapping permit reevaluation of previous studies into the stratigraphy and geomorphic evolution of the complex. Near the scarp bordering the southern edge of the complex OSL analyses of the oldest ridge sediments indicate an age of 4999+290 years; the OSL age on sediment from the youngest dated ridge, 800 m from the present (accreting) shore, is 1030+70 years. Analyses based on all seven ridge ages and varying ridge morphologies across the complex suggest the area grew relatively steadily with episodes of localized shoreline stability; periods of shoreline retreat appear to be rarely substantial or extensive. Ridge patterns also suggest that the gradual northeastern-to-northern growth of the cape slowed markedly and veered to the northwest approximately 2000 years ago, perhaps as an hypothesized sediment-source headland to the southeast diminished. Prior studies noted that relatively coarse foreshore deposits lie at depth across the study area. Analyses using the basal elevation of that facies near the dated ridges suggest that the local average long-term subsidence rate due to glacio-isostatic adjustments was approximately 2 mm/year during the mid-late Holocene.