120-13 Origin of Devonian-Mississippian Critical Mineral-Enriched Metalliferous Black Shales in the U.S. Midcontinent: Intersection of Acadian Far-Field Tectonics and Global Biogeochemical Events

Abstract:

The successful application of the marine chemocline mineral system model for identifying potential critical minerals resources in the U.S. midcontinent is highly dependent on the accuracy of regional basin reconstructions. Several competing models were previously proposed for genesis of Devonian-Mississippian metalliferous black shales in this region. However, few incorporate robust regional chronostratigraphic and paleogeographic constraints. New findings challenge traditional midcontinent paleogeographic reconstructions and pinpoint the timing of regional basin restructuring via far-field Acadian tectonism and the variable manifestation of global biogeochemical events in adjacent basins.

Devonian core and outcrop samples collected across the midcontinent (Iowa to Ohio, Oklahoma to Tennessee) were analyzed to map facies changes, unconformities, critical minerals anomalies, and sediment provenance. Sections were logged; whole-rock elemental profiles were generated using EDXRF, WDXRF, and ICP-MS; carbon isotope samples were analyzed from associated carbonates; weakly cemented shales and carbonates and were disaggregated and dissolved for conodont biostratigraphy; and zircon separates from sandstones were analyzed via LA-ICP-MS. The resulting chronocorrelation framework shows significant regional thickness changes reflecting truncation and onlap patterns that document areas of uplift and provide minimum displacement estimates. Detrital zircon results reinforce these finding through evidence for reworking older lower Paleozoic strata and unroofing of crystalline basement.

The midcontinent basin was tectonically partitioned during the later Givetian Age into northern and southern components, adjacent to the Acadian foreland basin. Facies analysis reveals the asynchronous timing of black shale deposition across these basins. The newly partitioned northern midcontinent basin (IA, IL, WI, MI) hosted initial carbonate-evaporite deposition that abruptly shifted into long-lived black shale deposition with the upper Kellwasser Event. Black shale deposition persisted through much of the Famennian Stage and ended with onset of the Hangenberg Event. By contrast, the southern midcontinent basin (IL, IN, KY, TN) initiated with black shales deposited on karst remnants of Devonian carbonate platform strata. Black shale deposition was interrupted by widespread phosphorite deposition associated with the Kellwasser Event(s) and critical mineral-enriched metalliferous black shales proliferated during the Hangenberg Event. These findings emphasize the ability of regional basin conditions to modulate global biogeochemical events and provide important considerations in the application of marine chemocline mineral systems models.

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