143-16 Detrital Zircon Geochronology of the Mesoproterozoic Freda Sandstone: Northeastern Wisconsin and Upper Peninsula, Michigan, USA

Abstract:

The Freda Sandstone of Michigan's Upper Peninsula and northeastern Wisconsin is a unit of interbedded arkosic sandstones and siltstones that provide insight into sedimentation following the failure of the Superior Province's 1.1 Ga Midcontinent Rift System (MRS). Historical studies posit that the entirety of the Freda was deposited by fluvial channels carrying sediments from the south and east into the Superior Province. However, the Freda varies texturally in basal sections of the unit and in outcrops west of the Porcupine Mountains. These textural variations have prompted consideration of subdivision of the Freda due to possible differences in provenance and competence between these hypothetical members. To determine whether subdivision is warranted, samples of the Freda were collected from three locations: an outcrop on the Presque Isle River, an outcrop east of Redridge, MI, and the St. Amour borehole in Alger Co., MI. These samples were processed for detrital zircons to determine U/Pb ratios via LA-ICP-MS. Analysis of detrital zircon populations using detritalPy and DZMDS have yielded results that further characterize sedimentation in the Superior Province during the Mesoproterozoic. Probability density plots of the collected samples display distinct zircon age profiles for each sample, with some similarities. Zircon populations in all three samples are low in rift-sourced zircons, most prominently in the Presque Isle River and St. Amour borehole samples. These samples display their largest age peaks at 1450 Ma, with minor peaks at 1650 Ma, indicating sourcing from the Picuris and Mazatzal orogenies. The Redridge sample, however, lacks zircons of these ages, and is instead characterized by Grenville-aged peaks at 1325 and 1200 Ma. These data indicate that sedimentation after the failure of the MRS was dynamic and variable, was subject to MRS-generated topography (such as the Porcupine Mountains), and that sediment supply was dominated by distal sources, burying units generated from the failed MRS.  

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