Abstract

Widespread, porosity-occluding vadose cementation is restricted to hot arid and semi-arid regions where carbonate (caliche) is concentrated, and hot regions with wet-season, dry-season climates (savannah lands) where iron and aluminum hydroxides and oxides (laterites and bauxites) are concentrated. Rapid evaporation-triggered precipitation of carbonates, hydroxides, and oxides results in initial precipitation of finely crystalline grain-coating cement films, which separate grains, and later replacement of grains by cement to form increasingly enriched concentrations. Textures and structures of caliche, laterite, and bauxite are homologous and uniquely reflect vadose processes. The mechanism by which ions are concentrated as vadose cements is upward diffusion from the water table through moist soil, following periods of infiltration, and evaporation-triggered precipitation in hygroscopic films during drying intervals.

Cementation below the water table or in water-fiiled voids (aqueous cementation) in fresh water or sea water, occurs slowly and, consequently, the cement is coarsely crystalline, because large crystals have time to grow. Soon after burial silica is precipitated as "syntaxial" overgrowths on quartz grains at shallow to moderate depths under conditions of low temperature, slightly acid (?) pH. At greater depths silica precipitation is followed by calcite precipitation and replacement of quartz under higher temperature and pH. Silica mobilized at depth by replacement and solution of quartz, diffuses upward and carbonate diffuses downward where it precipitates as optically continuous or polycrystalline overgrowths on available calcite "seeds" deposited with the sands or with intercalated calcareous shales or limestones. Transportation of cementing materials by opposed diffusion gradients, through very slowly moving or static interstitial water, overcomes inadequacies inherent in the supposition that they were transported upward by abnormally large volumes of water, required to transport the cementing materials, expelled from compacting clays below depths where clays continue to undergo significant compaction.

In Upper Cretaceous sandstones of the Rocky Mountain Region faceted silica overgrowths were precipitated at shallow to moderate depths of burial on sands which accumulated as braided alluvial sheets (piedmont plains), extending outward from newly created uplifts, point bars deposited in channels of meandering rivers on swampy coastal plains, deltaic distributaries, and barrier islands, consisting of lagoonal, backshore beach, foreshore beach, surfzone, and infra-surfzone sands. Silica overgrowth molding and merging formed loosely cemented sandstones, but did not occlude porosity. Concentrations of oyster shells in lagoonal and backshore sands and concentrations of Inoceramus in distal extensions (infra-surfzones), where sands intermesh with calcareous offshore marine shales, provided calcite "seeds" upon which porosity-occluding calcite cement crystals were precipitated as "syntaxial" and polycrystalline overgrowths at considerable depths by downward diffusion of carbonate.