A GLACIAL FAN/DELTA DEPOSIT OF THE MACKINAC LOBE IN GLACIAL LAKE ROSCOMMON, IN NORTHERN LOWER MICHIGAN

We report on a glacial fan/delta deposit that formed in Glacial Lake Roscommon at ca. 19 ka. The fan/delta is exposed in a 23-m deep sand pit, located on the west side of the Lake City-Harrison Ridge in Missaukee County, MI. The eastern and southeastern faces of the pit expose the core of the fan/delta. Cobble gravels at the top of the exposure overlie a diamicton unit that varies from 1-4 m in thickness, which in turn overlies a fining downward series, from sand and gravel through to medium to fine sand, comprising the core of the fan/delta. The sandy fan/delta core contains plane bed, climbing ripples, and layers of draped clay over ripples that can be traced over long (>12 m) distances, and which also has occasional dropstones, water-escape, and ball-and-pillow structures. The deposit fines noticeably to the west, away from the presumed ice-margin, with an increase in plane-bed lamination, clay/sand rhythmites, and climbing ripples, indicating its lacustrine origin. Based on this stratigraphy, we interpret this pit to represent an exposure in a glaciodeltaic deposit which formed adjacent to an ice margin of the (advancing) Mackinac Lobe, which was located on the east side of the pit at that time. We believe that the diamicton unit in the upper part of the fan/delta represents slurry or debris flows off the ice margin, into the water at the head of the delta/lake, as evidenced by its alternation with sandy beds near the base of the diamicton, as well as the prevalence of soft-sediment deformation. Most of the sediment in this pit was deposited into a lake (Glacial Lake Roscommon), dammed between the Lake Michigan, Mackinac, and Saginaw lobes of the Laurentide Ice Sheet. We refer to this landform as the Myers fan/delta (for the pit owner). Sediment from one sample in the core of the delta sands yielded an OSL age of 17.3 ± 1.0 ka. This sample, along with four others from similar landforms in the Lake City-Harrison Ridge, yield an average age of 19.5 ± 1.7 ka. The Myers fan/delta is but one part of a vast series of coalesced deltas that, when combined, comprise much of the Lake City-Harrison Ridge. The morphology of the Lake City-Harrison Ridge in this part of the Basin resembles the water-lain delta moraines of southern Finland. Thus, we suggest that the Lake City-Harrison Ridge is largely a system of coalescing ice-contact deltas and subaqueous fans.