Strandline records of high frequency, low magnitude drops in water level, glacial Lake Agassiz basin, Central Polk County, MN, USA

Abstract

Glacial Lake Agassiz strandlines expressed in LiDAR-based digital elevation models are detailed records of past water level change that reflect outlet incision and switching. Detailed 1:24,000 scale surficial geology mapping of a small section of coastline east of Crookston, MN, reveals strandlines of the Lockhart, Emerson, and Nipigon phases. Other than the Campbell and youngest Tintah Beach, the strandlines are not identified using the naming convention established near the southern outlet (Herman, Norcross, Upham, Tintah) because of the increasing number of strandlines northwards. The strandlines, for the most part, naturally exist in elevation clusters, and are assigned to 11 strandline groups (SG). SGs 1–9 are of Lockhart age at elevations higher than the Tintah Campbell Gap (TCGap). Elevation gaps between SGs are ~1–3 m, greater than the 0.5–2 m gap between successive strandlines within a SG. The many (n = 70) small, low-relief strandlines are interpreted to record nearly continuous incision of the southern outlet sill during Lockhart Phase. Beach ridges are estimated to have formed every ~20 years. The TCGap is ~4 km wide in the study area. The Campbell Beach of Emerson Phase age consists of several embayments, beach ridges and the large Melvin Spit. Based on morphological comparison with the modern Gull Point Spit in the Lake Erie basin, the Melvin Spit took ~500 years to form, during which water level dropped 8 m. The Nipigon Phase strandlines are small discontinuous beach ridges. North-south elevation plots of the most continuous strandline in each SG records southward slopes of 1.1 to 0.19 m/km explained by differential glacioisostatic adjustment active during strandline development that decreases through time at lower elevations. The three highest elevation OSL dates agree with published ages for the older strandlines (~14.4 ± 1.5 ka), but younger dated Tintah, Campbell, and sub-Campbell beaches are 2–5 ka older than published ages. Ground penetrating radar (GPR) datasets acquired at 11 sites at frequencies between 100 and 500 MHz, representing a cumulative distance of 11 km, form the basis for a simple beach ridge formation model augmented with hand auger holes and shallow hand-dug pits. Six radar facies (RF) were identified, dominated by RF2 which consists of lakeward-dipping reflections with a downlapping lower boundary recording a depositional regressive system, and RF4 which consists of landward-dipping reflections with a downlapping lower boundary recording overwash deposits from proposed storm events. Spit orientations and GPR data indicate littoral drift was southwards with offshore bars possibly nucleating successive beach ridges upon ~1-m drops in water level. Detailed analysis of strandlines from paleo lakes offer promise for high resolution reconstructions of past water level history.