SEDIMENTATION IN THE MESOPROTEROZOIC MIDCONTINENT RIFT, LAKE SUPERIOR REGION

Four “sequences” of sedimentary rocks reflect the tectonic-sedimentary framework immediately before, during, and after the 1100 Ma magmatic event that resulted in 20,000 m of dominantly basaltic, bimodal, subaerial, flood lavas and large gabbroic intrusions.

The oldest “sequence” includes four geographically separated, thin (~100 m), pre-volcanic white to pink quartzose sandstone units deposited upon Paleoproterozoic turbidites: the Nopeming Formation near Duluth, the Puckwunge on northeasternmost MN, the Bessemer Quartzite in WI and MI, and the Osler on the North Shore of Lake Superior in ON. All are interpreted to be alluvial plain (and lacustrine?) deposits derived from Paleoproterozoic and Archean sources.. The Nopeming and the Bessemer show soft-sediment deformation beneath the lowest lava flows. Apparently a gentle downwarping presaged the rifting and volcanism.

The second sedimentary “sequence” consists of numerous red, interflow sediments deposited in alluvial fan, fluvial, and lacustrine environments. Most have a provenance within the rift.

Volcanism was followed by deposition of the Oronto Grp: Copper Harbor Cgl (1340 m), Nonesuch Fm (200 m), Freda SS (1525 m). Deposition of the red Copper Harbor and Freda was on alluvial fans and in braided streams. The gray Nonesuch, with chalcocite and petroleum, was deposited in organic-rich lakes.

The 2100-m-thick red Bayfield Grp in WI unconformably (?) overlies the Oronto Grp: the feldspatholithic Orienta Fm and Chequamegon Fm are separated by the quartzose Devils Island SS. Correlative units to the west in MN are the feldspatholithic Fond du Lac Fm and the quartzose Hinckley SS. The Jacobsville SS of MI and adjacent ON may be correlative. Fining-upward sequences indicate meandering stream environments. The quartzose units are the result of mineralogical and textural maturation in lacustrine and eolian environments.

There is an upward increase in textural and mineralogical maturity of the sandstones of the Oronto and Bayfield Grps and their equivalents. The provenance of the Oronto Grp was predominantly the volcanic rocks of the rift, whereas the Bayfield Grp had dominant extrabasinal sources.

Paleocurrent plots in the rift are dominantly toward the center of the Lake Superior Syncline from both sides of the rift.