Buried bedrock valleys revealed in Michigan’s central Upper Peninsula using the horizontal-to-vertical spectral ratio passive seismic method

Abstract

A network of north–south linear troughs occupies the eastern portion of the Lake Superior basin. Morphology suggests these features are tunnel valleys that formed through erosion as subglacial conduits and conveyed meltwater to a former glacial margin. We demonstrate the southern continuation of these valleys onshore into the central Upper Peninsula of Michigan by mapping bedrock topography using the Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic method. This is a single location, noninvasive seismic technique that records ambient signal. HVSR data are used to estimate subsurface elevation of the sediment-bedrock contact at each site. Regional bedrock topography was estimated using HVSR data from 347 locations. We delineated, for the first time, eight buried (or partially buried) bedrock valleys with geometric characteristics (width, depth, orientation, and continuity) suggesting a southward extension of the Lake Superior tunnel valley network. The largest valleys terminate near the crest of the Munising/Grand Marais upland. This association suggests the upland was constructed by deposition of sediment-laden meltwater at the terminal ends of these tunnel valleys as fans (or heads of outwash). Episodic erosion of the largest valleys likely began during the Two Rivers phase (∼13 ka) and were again active during the Marquette phase (∼11 ka). The smallest valleys developed later during final regional deglaciation following the Marquette phase.