GSA Connects 2021 in Portland, Oregon

Paper No. 112-10
Presentation Time: 4:00 PM


GRIMM, Eric C., Dept Earth & Environmental Studies (Adjunct), University of Minnesota, Minneapolis, MN 55455, DONOVAN, Joseph, Department of Geology and Geography (Emeritus), West Virginia University, Morgantown, WV 26506, FRITZ, Sherilyn C., Earth and Atmospheric Science Department, University of Nebraska-Lincoln, 126 Bessey Hall, University of Nebraska-Lincoln, Lincoln, NE 68588-0340, BAKER, Paul, Division of Earth and Ocean Science, Duke University, PO Box 90228, Durham, NC 27708-0228, BROWN, Kendrick, Canadian Forest Service, Natural Resources Canada, Victoria, BC V8Z 1M5, Canada and MUELLER, Pietra, Springfield, IL 62704

Brush Lake, near the MT-ND and US-CAN borders, is ca. 130 ha. in area and 22 m at its deepest point. It lacks surface water inflow or outflow and has salinity 6 g/L, pH>10; its Holocene sediment contains both marl and tufa. Datasets to date include: water geochemistry, limnology, and mineralogy of tufa and lakebed marl (Donovan, 1989-92); five 16- to 17.5-m long cores from its deep hole (Grimm and Donovan, 1999, 2003); short cores from shallow water (Grimm, Mueller, and Fritz, 2005); analyses of marl in these cores for pollen, charcoal, mineralogy, LOI, Mg/Ca, and d18O (Grimm, Brown, Donovan, Baker, Mueller, Fritz); digital scans of split long cores (Grimm and Mueller, 2004); high-res seismic profiles (Baker and Fritz, 2007); and 14 AMS C14 dates from long cores. Like Kettle Lake 30 km to the east, Brush marl contains both detrital and endogenic (aragonite) fractions; the latter concentration is proportional to time-varying groundwater inflow and records wet and dry periods at sub-decadal scale. While the thickness of marl is similar between the two lakes, Brush has much more soft-sediment deformation in deeper sediments, including a large slump block up to 5 m thick whose base is >7000 BP. A composite core of the 5 long cores shows an ~7.5 m thick upper section (above the slump), a lower section ~3.7 m long (below it), and the slump itself. The basal block consists of sand/gravel and abundant wood; the overlying block has ~0.5 m of thick-bedded sediment grading upwards into finely laminated, probably varved sediment. The fault block itself is folded convolutedly. The date just below the slump is 7295±35 14C yr BP with about 600 years of sediment missing, similar to the estimated age of less extensive faulting in Kettle Lake. The core alternates between massive and laminated marl; we interpret the laminated layers were deposited during meromictic high-salinity phases and the massive layers during more humid ones. The Holocene fire (charcoal) record exhibits marked variability, similar to Kettle Lake. Perplexingly, the most recent ~2000 years of sediment is missing from the long cores, with a 14C date of 1880±35 14C yr BP at 10 cm depth; the top of each core lacks Salsola pollen (marker of NGP European settlement). We believe that Brush Lake contains a unique complex paleorecord of climate and lake sedimentary events that requires dissemination.