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High Resolution Sequence Stratigraphy and Provenance of the Ordovician Ancell Group in Iowa: Insights into the Evolution of Mid-Continent Intracratonic Basins of North America


The Middle Ordovician Ancell Group, including the St. Peter Sandstone, Glenwood Shale and Starved Rock Formation, provides a record of intracontinental basin development during changes in eustatic sea level in Iowa and Illinois. The St. Peter Sandstone overlies the Prairie du Chien Group along an erosional unconformity that marks a major sequence boundary. The upper contact of the St. Peter Sandstone with the Glenwood Shale is a second sequence boundary. Data from 80 wells, selected well logs, and 20 core sections were examined to refine the high-resolution stratigraphy of the Ancell Group. Seven distinctive parasequences bounded by sequence boundaries are distinguished in the Ancell Group recording short term transgression-regression cycles. Facies variations define aggradational transgressive (TST), prograding highstand (HST) and down stepping falling stage (FSST) system tracts in both the St. Peter Sandstone and Glenwood Shale-Starved Rock Formation units. Thickness variations indicate that the St. Peter Sandstone thickens towards the northeast and thins to the northwest and southwest in Iowa. In contrast, the St. Peter Sandstone in Illinois thickens to the south likely recording a prolonged FSST or incised valley fill or the channel fill. Detrital zircon geochronology of 13 samples from the St. Peter Sandstone and Starved Rock Formation define common peaks at 1100–1500 Ma and 2500–2700 Ma with minor components at 1670–1750 Ma and 3000–3600 Ma. The detrital zircon signature is dominated by Archean and 1.0–1.3 Grenville and Mid-continent Rift ages. The near complete lack 1750 Ma Yavapai province and 1800 Ma Penokean province ages argues against their derivation from recycling of a large portion of the Midcontent Rift sedimentary section, suggesting these mature sandstones formed as first-cycle deposits.