Hydrogeologic control on zeolitic diagenesis of the White River sequence

Smectite, opal-CT, and clinoptilolite are common alteration products in the White River sequence, a mid-Tertiary unit in the Great Plains and northern Rocky Mountains. The White River sequence is largely made up of fine-grained tuffaceous material that is rich in unaltered rhyolitic glass. Smectite is ubiquitous, and it or a precursor phase was present during White River soil formation but also formed after burial. Smectite Fe (from 5% to 7% Fe₂O₃) and Ti (from 0.4% to 0.8% TiO₂) concentrations in mud rocks suggest formation from dacitic to latitic glass. Rhyotitic glass, however, was the primary source for opal-CT and clinoptilolite. Opal-CT and clinoptilolite have a highly variable distribution over macroscopic to regional scales of observation. Stable isotopic compositions of smectite (δ¹⁸O_(SMOW) = 11.8‰ to 13.3‰), opal-CT (δ¹⁸O_(SMOW) = 19.9‰ to 24.3‰), and late diagenetic calcite (δ¹⁸O_(SMOW) = 10.3 ± 2.0‰) suggest that burial diagenetic reactions took place at temperatures ranging from 27 to 55 °C. Clinoptilolite apparently formed in areas with high contact times between paleoground water and vitroclastic material. In White River rocks of Wyoming, clinoptilolite occurs only in localities that are interpreted as lying within paleo-ground-water discharge zones. In the Great Plains, clinoptilolite is rare in eastern Colorado and southern Nebraska but progressively increases in abundance to the north, reflecting a northward decrease in permeability.

Zeolitic alteration near Douglas, Wyoming, changed the hydrologic role of tuffs from aquifers to aquitards. Tuffs were originally aquifers because of their high Permeability (>30 md) relative to interbedded mud rocks (10^-4 to 1 md), Signigicant thicknesses (0.5 to 2 m presentday), and lateral continuity. Tuff alteration, however, decreased permeability by ∼4 orders of magnitude relative to glass-rich portions. Similar diagenetic effects on mud-rock permeabilities are likely.

Mass flux into altered tuffs at Douglas is indicated by an increase in bulk rock density from ∼1.3 g/cm³ for glass-rich tuff to ∼1.7 and ∼2.0 g/cm³, respectively, for zeolitic and siliceous counterparts. Local silica redistribution accouts for the bulk of the density increases. A systematic increase in Ba content of clinoptilolite over a 150-m vertical section (from 0.1 to 6.6 mol% of the exchangeable cations) suggests that it was derived from underlying units.