MAGNETIC FABRICS PRESERVED BY HOLOCENE SEDIMENT IN TWO NEW YORK FINGER LAKES REVEALED EVIDENCE FOR DEFORMATION DURING PISTON CORING AND CHANGES IN PALEOENVIRONMENTAL CONDITIONS

Anisotropy of magnetic susceptibility (AMS) is used widely by the oceanographic community for reconstructing paleoenvironmental conditions. These measurements are also applicable to lacustrine settings to infer primary and post-depositional conditions as well as sediment deformation during core collection and sampling. The combined use of AMS with geochemical and physical analyses permits differentiation of primary depositional conditions from sediment disturbance associated with post-depositional processes and core collection. Detailed analysis of piston cores from Seneca Lake and Owasco Lake (NY) revealed numerous instances of anomalous magnetic fabrics preserved in post-glacial sediment. Most samples, however, exhibited relatively weak anisotropies that resulted from normal lacustrine sedimentation. Anomalous magnetic fabrics resulted from core collection, subaqueous slides, and development of an unconformity associated with a mid-Holocene lowstand. Stratigraphic disruption (“flow-ins”) that formed during coring were typically recognized visually by vertically oriented laminae near the base of the cores or soupy sediment near the core tops and confirmed by magnetic fabric measurements. However, “flow-ins” throughout the uppermost 1-2 meters of three of the four cores could only be identified by a gradual shift in the dominant magnetic fabric. We infer that vertical strain in the sediment is responsible for the shift in magnetic fabric. Vertical strain is known to occur when the piston accelerates upwards instead of remaining at the sediment-water interface during coring. Overthickened sections that result from piston acceleration are not easily recognized visually because the oversampling may not destroy the stratigraphic integrity or the geochemical or physical properties of the sediments. Subaqueous slides were also recognized by abrupt changes in lithology, magnetic susceptibility, and magnetic fabrics. A mid- to late Holocene lowstand was recognized using a combination of the alignment of the magnetic ellipsoid with geochemical and physical properties. AMS proves to be a powerful tool to assess core integrity prior to paleoenvironmental reconstruction in lacustrine settings.