TECTONIC ORIGIN FOR STRIATED SURFACE ON TOP OF THE MICHIGAMME FORMATION AT L’ANSE MICHIGAN: IMPLICATIONS TO THE JACOBSVILLE FORMATION

The Jacobsville Formation in the Lake Superior region is a predominantly fluvial sandstone that is poorly constrained in age, deposited between ca. 1.1 Ga volcanic rocks of the Midcontinent Rift and Cambrian sandstone of the Munising Formation. The Jacobsville overlies rocks of the Mesoproterozoic Midcontinent Rift and older Paleoproterozoic lithologies in angular unconformity. One such unconformity with slate of the ca. 1.8 Ga Michigamme Formation has been interpreted to be an erosional unconformity that has evidence of glaciation due to the presence of interpreted glacial striations. This interpretation of a glaciated surface, combined with age constraints from detrital zircons, has been used to argue that the Jacobsville was deposited either after the Sturtian or the Marinoan Snowball Earth glaciation. Such interpretations are integral to investigating the role of glacial erosion in the formation of the Great Unconformity and are critical for understanding the intra-cratonic evolution of Laurentia.

Our investigation focuses on the striated surfaces at the unconformable contact between the Jacobsville and the underlying ca. 1.8 Ga Michigamme Slate at L’Anse Michigan that has been interpreted to be glacial in origin. Here, we report detailed structural observations including thrust faults within the Michigamme slate as well as along the unconformable contact that penetrate into the Jacobsville. This faulting was accompanied by the development of slickensides, fault gouge, folds, and fault breccia. Structural measurements on slickensides and thrust planes indicate local NE-ENE directed shortening. This shortening also resulted in local fault repetition of the contact. Overall these findings demonstrate, that rather than being the result of Neoproterozoic glacial erosion, the contact is an erosional unconformity without evidence for glaciation that has been variably modified by series of thrust faults.