SIGNIFICANCE OF KENT ISLAND FORMATION TO GEOMORPHIC HISTORY OF THE MID-ATLANTIC REGION

Kent Island Formation (Qk) estuarine sands and silts were deposited as part of a transgressive-regressive sequence in the Chesapeake Bay region, over an area roughly twice that of the present bay. The age, elevation and stratigraphic relations of the Qk to units with which it is in contact are enigmatic. Optically stimulated luminescence (OSL) dating of coastal units by Scott (2006) and Burdette (2006) show that one or more transgressive sequences were deposited on the mid-Atlantic shelf during oxygen isotope stage (OIS) 3. Recent U/Th dating of corals by Wehmiller (2004) indicates an earlier sea level highstand during OIS 5a, and confirms the absence of exposed OIS 5e sediments in this region. A new optically stimulated luminescence (OSL) age of 37+/- 6.1 ka has been measured in Kent Island Formation (Qk) estuarine sands from Stump Neck on the Potomac River near the mouth of Mattawoman Creek. This age suggests that the Qk beds at this location, about 170 km west of the easternmost exposed Qk beds on Delmarva, are synchronous with the OIS 3a eustatic sea level peak. The upper age of estuarine deposition of Qk beds on the Delmarva peninsula is constrained by a number of radiocarbon dates on overlying peat of >28 ka, and a Kent Island surface radiocarbon date of 37 ka +/-1000 (USGS W2845) from a tree trunk (Owens and Denny, 1978). We hypothesize that the OSL age for the Kent Island is correlative with the relatively long interstadial (IS8) from ~36 ka to ~38 ka (GISP2 timescale), and that eustatic sea level drop following IS8 led to the formation of forested landscape on the exposed Qk surfaces, the earliest of which date to ~37 ka. Since the IS8 eustatic level was >30m below present sea level, present exposure of the Qk beds 1 to 10m above present sea level requires as much as 40m of uplift since ~36 ka. We agree with Scott (2006) that the most likely explanation for uplift in this tectonically quiescent region is the glacial forebulge created by loading of the Laurentide ice during OIS 2. The dimensions and rate of forebulge uplift, and subsequent subsidence, are critical to understanding the geomorphic evolution of this region. For example, uplift during the last glacial maximum provides a partial explanation for the rapid bedrock gorge cutting documented by Reusser et al. (2003) on the Potomac and Susquehanna Rivers that began during late OIS 3 at 36 ka.