TEMPORAL CHANGES IN COMPOSITION AND DETRITAL ZIRCON U-PB AGES IN THE UPPER CAMBRIAN MOUNT SIMON SANDSTONE, MIDWEST UNITED STATES: IMPLICATIONS FOR FLUCTUATION IN SEDIMENTARY PROVENANCE

The lack of compositional variability in mature sediments associated with Cambrian cratonic sequences makes traditional petrographic discrimination of sedimentary provenance difficult. The Upper Cambrian Mount Simon Sandstone, in the Illinois Basin, is a mature quartz arenite (Q₉₃F₅L₂ and Qm₈₅F₅Lt₁₀) with poorly constrained sediment provenance. The Mount Simon is a target reservoir for CO₂ sequestration. Understanding how sedimentary provenance affects changes in composition and texture is necessary in understanding the Mount Simon Sandstone reservoir. This study identifies sedimentary provenance and demonstrates a flux in sediment origin throughout deposition of the Mount Simon. Petrographic analyses of 35 thin sections and LA-ICPMS analyses of 5 detrital zircon samples, collected from an 878’ core of the entire Mount Simon depositional interval in Stephenson Co., IL, yield new compositional data and U-Pb ages. Compositional heterogeneities, observed top-bottom along the core, such as a change in polycrystalline quartz (1%-22%) and feldspar (0%-19%), suggests potential flux in provenance during Mount Simon deposition. We observe 7 U-Pb age populations from 487 detrital zircon grains in the Mount Simon, which correspond with known ages of southern Laurentian Source terranes: Superior (3000-2800 Ma, 2780-2570 Ma), Penokean (1990-1800 Ma), Central Plains (1700-1620 Ma, 1790-1750 Ma), Granite-Rhyolite (1500-1300 Ma), and Grenville and Midcontinent rift (1280-1023 Ma). Vertical variation in detrital zircon age spectra and composition indicate temporal variability in provenance. Early in Mount Simon deposition, braided fluvial systems transported detritus from Archean and Mesoproterozoic sources. Coincident with an upsection shift to marine influenced deltas is an observed provenance change to predominantly Archean cratonic sources. We propose two models to account for this fluctuation: (1) Sea level rise during Mount Simon Sandstone deposition influenced headwater incision, driving erosion toward primarily Archean cratonic sources. (2) Subsidence of the Illinois Basin initiated during Mount Simon Sandstone deposition. The accommodation space allowed sediment to deposit over and obscure younger, Mesoproterozoic sources, leading to a predominantly Archean provenance record.