Event Stratigraphy Based on Geochemical Anomalies within a Mixed-Sediment Backbarrier Sequence, Southern New Jersey.

摘要

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 sequences. In tide-dominated environments, 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. 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 Sewell Point, Cape May, New Jersey. In these 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 and a substantial increase in MS values (> 200 microSI). 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, and MS are positively cross correlated due to the relatively high content of sand-sized heavy minerals such as magnetite, rutile, ilmenite and biotite, as well as phlogopite and muscovite mica. These minerals were also identified using a Rigaku Dmax/B X-ray diffraction device. 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 4.5 mm/yr, which is consistent with its proximity to a historically active tidal inlet. Lithological anomalies at Sewell Point are interpreted as the signatures of episodic large-magnitude sediment fluxes into the paleo-lagoon. The calibrated ages of organic remains (mollusks and rhizomes) constrain chronology and allow interpretation of these stratigraphic layers as event horizons with historical storms of 1594, 1743 and 1821, along with pre-historic storms in the 11th-13th centuries. 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.