Principles of Cementation and Porosity-Occlusion in Sandstones: ABSTRACT

Widespread, porosity-occluding vadose cementation is restricted to hot arid and semiarid regions where carbonate (caliche) is concentrated, and to 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 in 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 cement is upward diffusion from the ater table through moist soil following periods of infiltration, and evaporation-triggered precipitation in water films during drying intervals. Cementation below the water table or in water-filled voids (aqueous cementation) in fresh water or seawater, occurs slowly and, because large crystals have time to grow, the cement is coarsely crystalline. Soon after burial silica is precipitated as epitaxial overgrowths on quartz grains at shallow to moderate depths under conditions of low temperature and 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 sands or with intercalated calcareous shales or l mestones. 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 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 c lcite "seeds" upon which porosity-occluding calcite cement crystals were precipitated as epitaxial and polycrystalline overgrowths at considerable depths by downward diffusion of carbonate. End_of_Article - Last_Page 538------------