CRYPTIC TEMPESTITES: GEOCHEMICAL AND CHRONOLOGIC EVIDENCE OF LARGE-MAGNITUDE EVENTS IN SOUTHERN NEW JERSEY WETLANDS

Detection of large-magnitude coastal events, especially in wave-dominated retrograding barrier settings, has traditionally relied upon lithological evidence, such as distinct overwash sand layers within the muddy backbarrier sections. In tide-dominated environments (Sea Breeze, Delaware Bay, NJ), unconformities in marsh stratigraphy have been interpreted as rapid drowning or erosion events caused by large storms. In transitional mixed-energy backbarrier environments, however, clearly identifiable event horizons are rare, due to unfavorable conditions for peat formation or to a lack of overwash (Sewell Point, Cape May, NJ). To address these challenges, the present study utilized X-ray fluorescence (XRF), magnetic susceptibility (MS), and X-ray diffraction (XRD) techniques to identify anomalies within 4-to-7-m-thick sequences recovered from the two sites. In the Cape May cores, at least five peaks were identified that exhibit up to three-to-four-fold increase in Fe (up to 6.2%) and Ti (up to 0.5%) concentrations, a doubling of K content (up to 2.5%), and a substantial increase in MS values (> 200 μSI). In Sewell Point cores, the sand fraction at these intervals exceeds 40% and is represented by moderately well-sorted, negatively-skewed, fine-grained sand (2.7 φ). Fe, Ti, K, Ca and S are positively cross correlated due to the relatively high content of sand-sized heavy minerals such as ilmenite, magnetite and chromite, as well as muscovite mica. Seven radiocarbon-dated in situ samples of intertidal gastropods and the few available rhizomes indicate that the Sewell Point sequence accumulated over the past 900-1,000 years, at an average sedimentation rate greater than 5 mm/yr, which is due to the proximity of a historically active tidal inlet. Lithological anomalies at Sewell Point are interpreted as the signatures of large-magnitude sediment fluxes into the paleo-lagoon. The calibrated ages of organic remains closely associated with these anomalies are consistent with regionally dated storm washovers from AD 1200-1300, as well as intense historical storms of 1495, 1540, 1596, 1691, and late 1780s. This research indicates that geochemically diagnostic intervals offer an effective tool for detection of event horizons and their regional correlation in mixed-sediment backbarrier settings.