Sedimentary Fill of 1,100 Ma Mid-Continent Rift System

Richard W. Ojakangas

In the Lake Superior region, four "sequences" of sedimentary rocks reflect the tectonic-sedimentary framework before, during, and after the magmatic event that resulted in 10,000 m of dominantly basaltic volcanic rocks and large layered gabbroic intrusions.

The oldest sequence includes four geographically separated, thin (100 m) pre-volcanic, white to pink, quartzose sandstone units that were deposited in braided alluvial plain-lacustrine environments within the shallow basin that was the initial manifestation of rifting.

The second sedimentary sequence consists of immature sediment, largely derived from the volcanic sequence and deposited in alluvial fan, fluvial, and lacustrine environments during intervals between extrusive episodes. These red silty to conglomeratic units range from a few centimeters to hundreds of meters in thickness.

The Oronto Group and the younger Bayfield Group and their equivalents are post-volcanic, dominantly red-bed sequences of siltstone, sandstone, and conglomerate, deposited in alluvial fan, fluvial, and lacustrine environments within the elongate basin. The Oronto Group (600 m) includes the Copper Harbor Conglomerate, the Nonesuch Shale (gray, carbonaceous, pyritiferous, and cupriferous argillaceous siltstone) and the Freda Sandstone.

The Bayfield Group (2,100 m) includes the Orienta Sandstone, the Devils Island Sandstone (100 m of orthoquartzite), and the Chequamegon Sandstone. Whereas volcanic detritus is dominant in most of the Oronto Group and the equivalent Solor Church Formation, extrabasinal granitic detritus dominates in the Bayfield Group and its equivalents (Fond du Lac Formation, Hinckley Sandstone, and Jacobsville Sandstone).

Paleocurrent data indicate a general basinward transport of sediment during deposition of the four sequences, 1,100 Ma(?) to 950 Ma(?).

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.