Abstract

Clastic dikes in slate in the Precambrian Michigamme and Siamo Formations of northern Michigan have been described as being parallel to slaty cleavage; this relationship was then used to support the hypothesis that the cleavage formed in response to tectonic dewatering. Careful measurement of cleavage and dike attitudes at the two previously studied localities demonstrates that the dikes are not parallel with cleavage. The dikes and other soft-sediment structures in these rocks are cut by the cleavage and were deformed by the strain that accompanied cleavage formation. In addition, it appears that deformed calcareous concretions in the Michigamme Formation were spheroids variably flattened in bedding when tectonic deformation began. As a result, the ab plane of many ellipsoidal concretions does not coincide with cleavage. Considering all of the data, there is no evidence in these rocks or anywhere else that tectonic dewatering has been a significant factor in the formation of regional slaty cleavage.

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