ANALYSIS OF THE MERDEN LAKE ESKER, STEARNS COUNTY, MINNESOTA: A NEW INTERPRETATION

A gravel pit in south central Stearns County, Minnesota exposes sands and gravels of the Merden Lake esker attributed to deposition by a sub glacial stream associated with the Wisconsin-aged Superior-lobe of the Laurentia ice sheet. An analysis of these sediments was conducted from samples collected through a vertical trench on an east-facing exposure of the gravel pit. A total of 12 distinct stratigraphic horizons were recognized. Each horizon was described in terms of overall color, grain size and lithology, carbonate content, and sedimentary structures. A total of 436 clasts (> 3 cm) were collected for lithology studies. Trench samples were collected across each horizon for grain size sieve analysis. Clasts were dominated by basalts (37.6%), granites (28.2%), gabbros (14.5%), quartzite (6.2%), diorites (3.5%), mica schists (3%), and andesites (1%). Minor components consisted of pisolitic claystones, shales, sandstones, limestones, dolostones, iron stones, bauxite, quartz, agate, and amethyst. The sedimentology provides an overall color of yellowish-orange to brown. Many of the large clasts (> 5 cm) contained a rind of calcareous-cemented coarse sand. Several, large (18-35 cm) armored clay balls were collected from the basal horizon. Grain size ranged from boulders (up to 37 cm in diameter) to clays. Average cobble size was 10.48 cm. The dominant grain size through the trench was 1-2 mm. Sedimentary structures included graded beds, minor cross bedding, and imbrication. Superior-lobe lithologies are dominated by red volcanics and sedimentary rocks from the Middle Proterozoic Keeweenawan Superior Group and have an overall brown to reddish-gray appearance. Rainy (Wadena)-lobe lithologies are dominated by Precambrian igneous and metamorphic rocks from southwest Ontario and northeast Minnesota and have an overall yellowish to yellow-brown appearance. Sedimentological analysis of the Merden Lake esker is more consistent with sub-glacial stream deposition within the Pierz-sublobe of the Rainy-lobe of the Laurentia ice sheet.