GROUNDWATER SILCRETE IN THE POTSDAM GROUP, OTTAWA GRABEN: A CASE EXAMPLE OF SHALLOW FAULT-CONTROLLED SILICIFICATION AND DESILICIFICATION IN A TECTONICALLY ACTIVE BASIN

Silcrete forms by silicification of sediment or bedrock at the Earth’s surface due to soil-forming processes (pedogenic silcrete), or in the shallow subsurface (< 100 m) along the water table (groundwater silcrete). Unlike pedogenic silcretes, the origin of groundwater silcretes are poorly understood with existing case studies highlighting near-surface silica flux, whereas extrabasinal sources and/or migration of silica-bearing fluids along faults have not been demonstrated. Based on spatial and textural characteristics of a groundwater silcrete from the Cambrian-Ordovician Potsdam Group in the Ottawa Graben, however, a close association between the migration of silica-rich fluids and multiple northeast-trending faults is suggested. This ≤ 150 cm thick silcrete horizon underlies an unconformity that caps a succession of fluvial quartz arenite along the flanks of fault-bounded ridges of Grenville basement. The silcrete horizon thickens toward these faults and shows a systematic change in morphology from nodular to massive to brecciated. Cathodoluminescence reveals an early luminescent zoned cement (C1) associated with silcrete formation, and a later massive non-luminescing cement (C2) that occurs as overgrows filling remaining porosity in these and adjacent Potsdam strata. Fracturing and autoclastic brecciation of silcrete occurs within tens of meters of faults and is characterized by jigsaw and collapse breccia made up of silcrete clasts in a massive sand, kaolinite, and Fe-oxide matrix. Here it is speculated that silica-rich fluids moved along faults and advected into adjacent Potsdam sediment where the effects of pH change and evaporation promoted C1 precipitation. Later fracturing and migration of high pH fluids near faults led to local C1 dissolution and autoclastic brecciation. Finally, C2 cements formed throughout Potsdam strata following burial and pressure solution of non-silcretized arenite. Although the source of silica and fluid reservoirs for C1 remains uncertain, the scarcity of remnant detrital feldspar or unstable silicates in Potsdam strata suggest that the Grenville basement was the most likely source. Ongoing fluid inclusion analyses of C1 cements aims to resolve fluid temperature and salinity conditions, and a meteoric versus hydrothermal origin.