Diagenesis of Upper Cambrian Mount Simon Sandstone in the Illinois basin: Microscale investigation of basinal fluid migration and mass transfer

Investigations were conducted to evaluate microscale evidence for basinal fluid migration in the Illinois basin during diagenesis of the Upper Cambrian Mount Simon Sandstone.; Two major generations of quartz overgrowths are observed based on cathodoluminescence (CL) and fluid inclusion (FI) studies. Generation 1 quartz overgrowths (Qog-1) have lower CL intensity and are associated with FI homogenization temperatures (Th) of 60–95°C. Generation 2 quartz (Qog-2) have higher CL intensity and related to Th of 100–145°C. Ion-microprobe analyses of authigenic quartz reveal that most Qog-1 have higher δ 18O values than Qog-2 (up to 6.8‰ difference). Constraints on fluid compositions using Th point-matched to δ 18O values of the quartz overgrowths containing these FI reveal two distinct diagenetic fluids corresponding to Qog-1 (T < 95°C) and Qog-2 (T > 95°C), each evolved from south to north. Total concentrations of alkaline and alkaline earth elements are generally lower in Qog-1 than in Qog-2.; South-to-north migrations of basinal brines (20–25 wt% NaCl equivalent salinity) are implied. There is a bi-modal, northward-decrease distribution of Th. δ18O values of authigenic minerals reveal a northward-increasing trend (14–24‰ for K-feldspar and 22–28‰ for quartz). δ18O values of evolved diagenetic fluids also suggest northward migrations of diagenetic fluids. Ion-microprobe trace-element studies reveal covariations among Sr-Ba-Pb-Rb, Na-Mg-Ca-Cl, and K-Al in authigenic cements. Estimated Fe, Ce, and Sr concentrations in diagenetic fluids show a tentatively northward increasing trend, consistent with northward migrations of basinal fluids.; This study demonstrates that in situ microscale analytical techniques have advantages over conventional techniques in obtaining geochemical data from small-size authigenic minerals. Our results support the hypothesis of basinal fluid migration in the Illinois basin.