Detrital zircon evidence for the unroofing of the northern Appalachians in Early-Middle Pennsylvanian sandstones of North America

Charismatic outcrops of thickly bedded-sandstone are a common geologic feature in the river valleys of Iowa and Illinois. These ancient sedimentary rocks consist of detritus that was deposited during Pennsylvanian time (~300 million years ago). Mystery remains as to where the sediment that forms these rocks was originally sourced from. The goal of this project was to use a geochronological technique known as age-matching to address this problem, and to combine our results with detailed analysis of the sedimentary patterns and structures in order to better understand the environmental conditions under which strata were deposited. We extracted over 3,000 zircon mineral grains from Pennsylvanian-age rocks and determined the time at which the minerals crystalized by measuring uranium isotopes on a laser-ablation mass spectrometer. The relative abundances of different crystallization ages in each sandstone provided a “fingerprint” that was then used to match the sandstone unit with a potential source consisting of similar ages.

Our sample suite included rocks from two sedimentary basins, the Forest City Basin (southwestern Iowa) and the Illinois Basin (easternmost Iowa and Illinois). Ages of approximately 66% of zircons matched ages that were fingerprinted to the Appalachian region on the east coast of North America. Subordinate populations of grains consisted of ages matching more local sources, both in the midcontinent of the United States and from the Canadian Shield to the north. Proportions of grains matching Appalachian sources increased in the Illinois Basin from ~46% to ~79% between our stratigraphically lowest and highest samples, respectively. The Forest City Basin exhibited a similar upsection increase in Appalachian derived grains, which increased from ~52% in our stratigraphically lowest sample to ~70% in our stratigraphically highest sample. Overall, our data show that the rising Appalachian Mountains were likely responsible for increasing amounts of sediment that was deposited in present-day Iowa over the course of Pennsylvanian time. This conclusion supports lithological observations that evoke large, northeast to southwest trending river systems, active during this time period. These rivers likely delivered sediment from the northeast Appalachian margin across the continental interior. River systems likely drained a Himalayan-sized mountain belt on the present-day east coast of the United States that formed as a result of the continental collision between Africa and North America.