SEDIMENTOLOGY, STRATIGRAPHY, AND PROVENANCE OF MESOPROTEROZOIC STRATA OF THE KEWEENAWAN SUPERGROUP WITHIN THE MIDCONTINENT RIFT SYSTEM, NORTHERN MICHIGAN

The Midcontinent Rift system is a ~2000 km long, Meso–Neoproterozoic, tectonic feature that has been geophysically imaged from southern Ontario to Kansas. Sedimentation and volcanism associated with rifting occurred between 1100–900 Ma. Outcrops of the Keweenawan Supergroup occur along the shorelines of Lake Superior in northern Michigan. Presented here are sedimentologic data, modal composition trends, and new detrital zircons ages from the Copper Harbor Conglomerate (CHC), Freda Sandstone, and Jacobsville Sandstone of the Keweenawan Supergroup.

Older rift-fill strata of the CHC show primary peak, U-Pb detrital zircon ages between 1100–1200 Ma and secondary peak ages of 1400, 1800, 2100, and 2700 Ma. Modal composition trends show pervasive occurrences of feldspar and lithic volcanic detritus. U-Pb ages from overlying rift-fill strata of the Freda Sandstone show primary peak ages of 1900 and 2700 Ma and secondary peak ages of 1200, 1500, and 2600 Ma. Modal composition trends show an abundance of monocrystalline quartz, moderate feldspar, and minor lithic volcanic detritus. U-Pb ages from overlying rift-flank strata of the Jacobsville Sandstone show primary peak ages between 1100–1600 Ma and secondary peak ages of 1700, 2000, 2700, and 2900 Ma. Modal composition trends show relative abundances of mono- and polycrystalline quartz with subsequent occurrences of feldspar and lithic volcanic detritus.

Provenance data from the CHC show a detrital age signature that correlates with Grenville/Keweenawan magmatic provinces. Detrital zircon ages from strata of the Freda Sandstone and Jacobsville Sandstone also show detrital age signatures that correlate with Grenville/Keweenawan magmatic provinces, in addition to age signatures of Granite/Rhyolite, Yavapai-Mazatzal/Central Plains/Labrador, and Penokean/Trans Hudson orogens. Thus, early- to middle-stage, rift-fill sedimentation may be characterized by isolated detrital contributions from local Grenville/Keweenawan sources. Exhumation and sediment dispersal during the later stages of rifting may have been much more widespread and characterized by detrital contributions from a number of regional magmatic provinces.