High-resolution chemostratigraphy in carbonate mudstones: Salina A-1 carbonate (Silurian), Michigan Basin, USA

Portable Energy-Dispersive X-Ray Fluorescence spectrometry (PXRF) is routinely used to identify stratigraphic variations in the elemental chemistry of fine-grained siliciclastics. The application of this approach in carbonates is limited, however. In this study, over 9000 PXRF analyses from nine drill cores are used to document cm-scale variations in the carbonate mudstones of the A-1 Carbonate (Silurian) in the Michigan Basin. High-resolution elemental data are integrated with petrographic and mineralogical observations to evaluate the extent to which: (i) basin-center mudstones in the A-1 Carbonate (A-1C) have spatially correlative elemental signatures (i. e. chemofacies), and (ii) elemental variations reflect temporal changes in relative sea-level and sedimentation. Spatially correlative variations in concentrations less than 5 wt% of Si, Al, K, and S are observed throughout the A-1C. Higher concentrations of Si, Al, and K are associated with more abundant siliciclastic minerals as determined petrographically and with x-ray diffraction, and are thus interpreted to reflect intensification of siliciclastic sedimentation caused by relative sea-level fall. Associated S increases reflect a higher abundance of anhydrite, which is interpreted as diagenetically altered gypsum deposited during periods of low relative sea-level. Chemofacies successions for siliciclastic and evaporite proxy elements indicate four transgressive-regressive cycles. Collectively, these data show that variations in the minor, non-carbonate sediment fraction within the A-1C are regionally correlative suggesting that siliciclastic and evaporite deposition responded in tandem to basin-wide controls on sedimentation. The observed relationship between siliciclastics and evaporites further exemplifies the broader utility of quantifying the elemental signatures and non-carbonate mineral compositions in carbonate-dominated strata to constrain sequence stratigraphic interpretations. These findings add to a growing body of work that demonstrates the utility of PXRF to generate reliable geochemical data sets and highlights the importance of integrating elemental and mineral data in sedimentological interpretations.

Automated summary

Portable Energy-Dispersive X-Ray Fluorescence spectrometry (PXRF) is used to identify elemental variations in fine-grained siliciclastics. In this study, over 9000 PXRF analyses from nine drill cores are used to document variations in the carbonate mudstones of the Silurian A-1 Carbonate in the Michigan Basin. The data show that variations in the minor, non-carbonate sediment fraction within the A-1 Carbonate are regionally correlative, suggesting that siliciclastic and evaporite deposition responded in tandem to basin-wide controls on sedimentation.