Demarcation of Early Pennsylvanian paleovalleys in depozones of the Appalachian foreland-basin system based on detrital-zircon U-Pb and Hf analysis
Authors: Allred Isaac , Blum Mike 
Published: October 2022

Detrital-zircon (DZ) U-Pb data show that Appalachian-affiliated sediment was transported to western Laurentia by the Carboniferous, yet additional DZ U-Pb data from the eastern United States suggest that sediment-routing systems were oriented south toward the Ouachita deepwater sink. Within this context, this study presents DZ U-Pb ages from the Lower Pennsylvanian Caseyville Formation of Illinois, and U-Pb ages and εHf values from the coeval Pottsville Formation of Alabama as well as sandstone petrographic data from the Caseyville Formation, the Pottsville Formation, and the Jackfork Group of the Ouachita Basin to document provenance, delineate drainage divides in the Appalachian foreland-basin system, and comment on the unlikelihood of transcontinental sediment routing from the eastern United States to western United States at this time.

Two DZ U-Pb age distributions from quartz arenite sandstones of the Caseyville Formation display prominent ca. 1250–950 Ma, 1550–1300 Ma, 1800–1600 Ma, and 3500–3000 Ma ages, consistent with ultimate derivation from Grenville, Midcontinent granite–rhyolite, Yavapai–Mazatzal, and Superior provinces, as well as minor contributions from ca. 500–400 Ma and 2000–1800 Ma grains. Two DZ U-Pb age distributions from sublitharenite sandstones of the Pottsville Formation display prominent ca. 500–400 Ma, 1250–950 Ma, 1550–1300 Ma, and 1800–1600 Ma ages, consistent with ultimate derivation from Appalachian, Grenville, Midcontinent granite–rhyolite, and Yavapai–Mazatzal provinces, as well as minor contributions from ca. 2000–1800 Ma and 3500–3000 Ma grains. The Pottsville Formation samples demonstrate a greater percentage of Appalachian and Grenville ages relative to the Caseyville Formation samples, whereas the Caseyville Formation samples have elevated Yavapai–Mazatzal and Superior percentages relative to the Pottsville. We interpret these differences to suggest parallel fluvial systems in the foredeep and back-bulge depozones of the Appalachian foreland-basin system.

Like DZ studies of modern deep-sea fans that demonstrate an affinity to feeder fluvial systems, this study demonstrates fidelity between endmember segments of ancient fluvial-to-deepwater systems. Multidimensional scaling (MDS) analysis shows that DZ samples from the Pottsville and Caseyville formations cluster with deepwater Jackfork Group samples, and we infer a source-to-sink relationship from these two distinct source areas to the Ouachita terminal sink. One example of large-scale inclined strata thickness from the Caseyville Formation also suggests a drainage basin area of &gt; 105 km2. Contextualized with these observations, we suggest that the foredeep and backbulge depozones of the Appalachian foreland-basin system steered distinct Early Pennsylvanian rivers across emergent continental shelves during periods of low sea-level, which discharged to distinct slope canyons and sourced &gt; 100-km-long deep-sea fans. Clearly circumscribed, southward- or southwestward-oriented paleodrainage areas provide a template of the Appalachian foreland-basin system, and as such the central and southern Appalachians were an unlikely source for the Appalachian signature observed in the western United States at this time.