DETERMINING THE LATE QUATERNARY GEOLOGIC AND RELATIVE SEA LEVEL HISTORY OF SALEM HARBOR USING DATED SEDIMENT CORES AND SUB-BOTTOM GEOPHYSICS

The focus of this study is to quantitatively define and groundtruth the Late Quaternary stratigraphy in Salem Harbor (MA) and to constrain the timing of relative sea level (RSL) transgression in the region. Previous geophysical work in Salem Sound suggests the stratigraphy contains a bedrock basement unit draped by a glaciomarine unit disconformably overlain by an estuarine unit (Boglione and Hubeny 2014). The current study utilized new geophysical data to redefine the glaciomarine unit. A dredged navigation channel with most of the upper estuarine unit removed was cored to obtain physical samples of the disconformity between the two sedimentary seismic units. A modified Kullenberg piston corer retrieved cores SS16 KC1 and SS16 KC2. SS16 KC1 had estuarine mud facies to 84cm with a mean magnetic susceptibility (MS) of 13.6 x 10^-5 SI and highest Loss on Ignition (LOI) percentages in core. A convoluted transitional facies of estuarine mud and clay continued to 111cm and had a mean MS, Wet Bulk Density (WBD), Dry Bulk Density (DBD), and LOI of 14.1 x 10^-5 SI, 1.58g/cm³, 0.98g/cm³, and 3.15%, respectively. A basal facies from 111cm-125cm consists of banded clay (0.5-1.5cm thick) terminating in a basal sand. Mean MS, WBD, DBD, and LOI of facies is 27.1 x 10^-5 SI, 1.82g/cm³, 1.31g/cm³, and 1.43%, respectively. SS16 KC2 had convoluted estuarine mud and clay with a mean MS of 17 x 10^-5 SI and was cored to refusal in a sandy unit immediately above which an oyster shell was dated 6890-6745 cal BP (5982 ± 27 ¹⁴C BP). An additional oyster shell and a scallop shell were collected from the core catcher of SS16 KC2 and radiocarbon dates are pending. The presence of marine organisms represents a minimum age of marine inundation for Salem Harbor. The glaciomarine unit had a maximum thickness of 37.6m with strong internal parallel reflectors interpreted as sandy interbeds by core refusals at the topmost reflector. Convoluted sediments in cores suggest dredging disturbance in some sediments overlying the disconformity as supported by the absence of bedding and variable MS values. Based on results, regional RSL was at -12m elevation at or before 6890-6745 cal BP. This temporal constraint is consistent with previous regional RSL reconstructions (i.e. Hein et al., 2012) and provides independent confirmation for the interpreted timing of regional transgression.