THE ELSING GREEN, A NEW PLEISTOCENE ALLOFORMATION IN THE VIRGINIA COASTAL PLAIN

Recent STATEMAP geologic mapping in the Tunstall 7.5-minute quadrangle and Williamsburg 30- by 60-minute quadrangle revealed a morphologically distinct estuarine terrace overlooked in previous mapping. As with other coastal plain terraces, the underlying sediments were deposited during a marine transgression; what remains are flats that range between 28 and 35 feet in elevation and with a scarp toe (sea level high stand) at approximately 38 feet. These features are clearly evident at the 17^th-century plantation, Elsing Green, on the Pamunkey River. There, auger drilling identified predominantly sandy sediments inset against Shirley and Chuckatuck alloformation sediments, and in turn are incised by Tabb-Sedgefield sediments. On the east side of Hog Island on the James River, Elsing Green morphology and distinctive sediments are also pronounced. On most existing geologic maps, the Elsing Green has been incorporated into the Shirley Alloformation. Based on morphology, the Elsing Green correlates with the Ten Mile Hill Beds in South Carolina.

At Holland Point on the Piankatank River (Wilton 7.5-minute quadrangle), Crassostrea and Mercenaria within the Elsing Green were studied by Kris Kusnerik in his 2013 William and Mary Geology senior thesis; amino acid ages on several of these shells suggested an age of marine oxygen isotope stage (MIS) 5e (125 ka). In addition, we determined the concentration of ¹⁰Be in samples of quartz sand from three different depths along a single depth profile at Elsing Green Plantation. This depth profile produces a modal age for the deposit of 132 ka [+117/-58 ka] when analyzed using the Hidy et. al. (2010) Monte Carlo simulation model to determine the age and inheritance pairs that best fit the measured ¹⁰Be profile. We note that the model does not account for possible bioturbation of the sediments. The dates from the two independent procedures are in agreement with one another and indicate the Elsing Green was likely deposited in a marine transgression associated with MIS 5e.