SEDIMENTARY FILL OF THE 1100 MA MIDCONTINENT RIFT SYSTEM (MRS) IN THE LAKE SUPERIOR REGION

Four sedimentary rock "sequences" reflect the tectonic-sedimentary framework just before, during, and after the tectonic-magmatic event that resulted in the MRS.

The oldest sequence consists of thin, pre-volcanic quartz sandstones. Some were unlithified when the first flows (pillowed) were extruded into the same bodies of water in which the sands accumulated. Gentle downwarping presaged volcanism.

The second sequence consists of numerous, generally thin, interflow sedimentary units. This and the final two sequences are continental redbed deposits.

Cessation of volcanism was followed by deposition in Michigan and Wisconsin of the third sequence, the Oronto Group (Copper Harbor Conglomerate, Nonesuch Formation, Freda Sandstone). The Copper Harbor consists of conglomerate and lithofeldspathic sandstone largely composed of intrabasinal volcanogenic detritus deposited on alluvial fans and braided alluvial plains; it is as thick as 1340 m and includes basalts that represent the last volcanism. The Nonesuch (200 m), with copper mineralization and oil seeps in the closed White Pine Mine, is largely composed of black argillite and siltstone deposited in a lacustrine (?) basin in which organic material accumulated in a reducing environment. The Freda (1525 m) is a more quartzose lithofeldspathic sandstone deposited in a braided fluvial system.

The fourth sequence, the 2100-m-thick Bayfield Group (Orienta Formation, Devils Island Sandstone, Chequamegon Formation) is in Wisconsin. The Orienta and the equivalent Fond du Lac Formation in Minnesota are feldspatholithic sandstone-siltstone-shale units with fining-upward deposits typical of meandering river systems. The thin (100 m) Devils Island quartz sandstone and its Minnesota equivalent (Hinckley Sandstone) are products of the maturation of feldspatholithic sand by eolian and lacustrine processes on alluvial plains. The Chequamegon is feldspatholithic. The Jacobsville Sandstone in Michigan and Ontario is correlated with the Bayfield.

Paleocurrents indicate transport toward the basin axis during deposition of the last three sequences. There is an upward increase in maturity through the Oronto and Bayfield Groups; intrabasinal detritus dominates in the Oronto and extrabasinal detritus dominates in the Bayfield.