GEOPHYSICAL INVESTIGATION OF LATE QUATERNARY GEOLOGY, LAKE VATNSDALSVATN, ICELAND

New seismic surveys of the Vatnsdalsvatn basin, in Iceland, provide additional constraint on the late Quaternary 10 kHz seismic facies previously interpreted by Strom et al. (2012). Here, we: 1) incorporate 3.5 kHz sub-bottom data, taken in 2013, to better constrain acoustic basement, and 2) quantitatively correlate seismic-facies with litho-facies in lake sediment cores from 1979, 2002, 2012. Reflection coefficients were calculated from GEOTEK impedance data on the 2002 cores, and were used to correlate core data, including radiocarbon dates, with the seismic-facies. Unit 1, the acoustic basement, is identified as Miocene basalt overlain by till. Unit 2, defined by multiple continuous internal reflectors and hyperbolic reflectors positioned non-systematically within the unit, is indicative of dropstones or erratics. Core data show Unit 2 as dense (1.9 ± 0.5 g/cm³) sediment with high magnetic susceptibility (MS) values (959.9 ± 426.3 SI). We interpret this facies as proglacial lake deposit. A prominent reflector forms an erosional truncation of Unit 2 that likely represents the draining of a proglacial lake. This reflector, which crops out in deeper water, is ground-truthed as gravelly sand (d_max = 1cm). Unit 3 overlies that sand, with thickness greatest at the lake basin margin, pinching out towards the lake axis. Internal reflectors exhibit baselap, suggesting that this unit has progradational fan deposits sourced from east and west basin margins. Magnetic correlation of the 2002 core with the well-dated 1979 core (Thompson and Turner, 1985) constrain the base of Unit 3, and hence the commencement of post-glacial lake sedimentation, as > 9,300 ± 300 cal BP. This interpretation is consistent with independent reconstructions of the Gláma ice cap retreating from the region after 10,100 cal BP (Ingolfsson et al., 2010). Core data show Unit 3 as having lower density (1.4 ± 0.1 g/cm³) and lower MS (245.2 ± 43.1 SI) as compared to Unit 2. A prominent continuous reflector is observed within the upper section of Unit 3. Preliminary radiocarbon dates from this section of the 2002 core reveal accelerated sedimentation rates and ages consistent with Icelandic Landnám, the initial human settlement of Iceland.