A 1538-m pre--Mt. Simon (Upper Cambrian) red bed sequence, encountered in the Gratiot County deep borehole, consists of undeformed recurring beds of submature and immature arkosic sandstone, 5--85 cm thick, that grade upward into mudstone units, 3 cm to 3.8 m thick. Similar facies are recorded by a red bed sequence penetrated in a well on Beaver Island, 260 km to the north, and both sequences occur within the mid-Michigan gravity high. We suggest penecontemporaneous deposition by sediment gravity flows, low-velocity clear water currents, and pelagic fallout on submarine fans forming in an elongate, broadly subsiding protoceanic basin which foundered isostatically following arrestment of Keweenawan rifting and production of oceanic lithosphere. We further suggest that protoceanic rifting in the Lake Superior region and that along the mid-Michigan gravity anomaly were approximately contemporaneous and that both areas underwent a similar sequence of thermotectonic volcanism, deformation, and sedimentation, followed by passive foundering and shallow marine and basinal turbidite sedimentation.