Groundwater silcrete linked to brine migration in a continental rift: An alternative to the near-surface model of silcrete

Abstract

Silcretes have long been recognized as modern and ancient duricrusts, but more recently also as silicified layers precipitated at groundwater tables, termed groundwater silcrete. However, the silica sources, transport mechanisms, and precipitation processes of groundwater silcrete are not well understood, and models are limited to the near-surface groundwater environment, where silica saturation is low. Here, an example of a groundwater silcrete from Upper Cambrian strata of the Potsdam Group is described and interpreted to be formed in a rift where Cambrian fault reactivation coincided with silcrete formation. Field relationships strongly support a connection between fault activity and silicification, including a systematic thickening and development of massive silcrete horizons above shear zones, brecciated silcrete near where faults intersect shear zones, and nodules along the margins of shear zones. Petrographic and cathodoluminescence microscopy of silcrete reveal early pre-compaction overgrowth cements with abundant primary fluid inclusions. Fluid inclusion microthermometry indicates that these fluids were high salinity (22.7–25.8 eq. wt% NaCl+CaCl2) brines with homogenization temperatures of ~120.2 °C–151.6 °C, which implies that silica precipitated from a hot, silica-saturated crustal brine from Grenville Province basement. A combination of weathering reactions and direct quartz dissolution explains the chemical evolution of the source fluid, which likely originated as infiltrated meteoric water that had chemically equilibrated with Grenville crust at depth. Later, this brine was mobilized upward along reactivated faults during the Late Cambrian, and ultimately to the water table, where a combination of reduced pH and temperature promoted quartz supersaturation and quartz overgrowths on detrital quartz. This case example, therefore, expands the definition of silcrete to include near-surface silicification from externally sourced crustal fluids, here termed brine silcrete, and provides a basis for interpreting silcrete as a feature of deformation and fluid migration along shear zones in fault-bounded continental basins.