HR: 1340h
AN: V23A-0620    [Abstracts]
TI: Intrabasin Variability of Volcanic Ash Stratigraphy in a Small Kettle Lake; Lorraine Lake, Anchorage, Alaska
AU: * Kathan, K M
EM: kmk238@dana.ucc.nau.edu
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86011
AU: Werner, A
EM: awerner@mtholyoke.edu
AF: Department of Earth and Environment, Mount Holyoke College, South Hadley, MA 01075
AU: Kaufman, D S
EM: Darrell.Kaufman@nau.edu
AF: Department of Geology, Northern Arizona University, Flagstaff, AZ 86011
AU: Waythomas, C F
EM: chris@usgs.gov
AF: U.S. Geological Survey, Alaska Volcano Observatory, 4230 University Drive, Suite 201, Anchorage, AK 99508
AU: Wallace, K L
EM: kwallace@usgs.gov
AF: U.S. Geological Survey, Alaska Volcano Observatory, 4230 University Drive, Suite 201, Anchorage, AK 99508
AB: Lorraine Lake is a small (0.53 km$^{2}$) shallow (ca. 8 m) kettle located on the Elmendorf Moraine (Pleistocene age) 11 km northwest of Anchorage, Alaska. Situated in an area of low relief (49 m), the basin has a small drainage basin (1.3 km$^{2}$), no inflow and remains ice covered for approximately six months of the year. This study was initiated to resolve the volcanic ash-fall record preserved in the Holocene lake sediments from this basin, and to evaluate intrabasinal variability of ash stratigraphies. It was hypothesized that tephra deposition varies spatially across the lake and that some locations exhibit a more complete record of ash fall than others. This variation may possibly be due to tephra being redistributed by wind on the frozen or open-water surface, carried by currents once it sinks, or mixed by bioturbation following deposition. Six sediment cores between 3.2 and 5.8 m long were recovered from the north, south, east, and west parts of the lake, which is divided into two (north and south) sub-basins. A total of 21 AMS $^{14}$C ages were obtained on terrestrial macrofossils and basal ages from three cores are greater than 14,500 cal yr. BP, confirming that the cores contain the entire postglacial sedimentary record. Eleven tephra deposits, ranging from invisible to several centimeters in thickness, were correlated among the cores based on their relative depths, spacing, color, texture, thickness, high magnetic susceptibility (MS), low loss-on-ignition, X-ray gray scale value, and abundance of magnetic minerals. Although other diffuse tephra units occur, these 11 clearly defined units are used to compare tephra deposition within the lake. Several physical characteristics were compared to evaluate possible intrabasin variability including stratigraphic thickness, and X-ray density stratigraphy. A numerical classification scheme was developed ranking visual and stratigraphic prominence based on thickness, purity of ash and nature (sharpness and continuity) of stratigraphic contacts. Despite variability in sedimentation rates (ranging from 0.69 to 0.29 mm/yr) the physical characteristics (thickness, MS, and purity) of the tephra units display minimal variation. No consistent pattern of variability was recognized when comparing cores recovered from different depths and areas within the basin. The stratigraphic prominence of the tephra layers and their similarity between core sites implies that probable depositional complexities (e.g., aeolian reworking, wave action, and lake ice) and post-depositional processes (e.g., bioturbation, bathymetric focusing) have a minimal impact on the deposition and preservation of tephra units in small kettle lakes similar to Lorraine Lake.
DE: 8404 Ash deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting