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Abstract

Meteoric diagenesis represents alteration that occurs at or near the earth’s surface in strata influenced or pervaded by waters of recent atmospheric origin. The meteoric environment is typically divided into unsaturated (vadose) and saturated (phreatic) zones divided by a water table (see top diagram, facing page). The interfaces between surficial meteoric fluids and strata filled with other pore fluids (seawater or basinal waters) are “mixing zones” that can have special diagenetic characteristics.

Many, perhaps most, shallow marine carbonate deposits undergo meteoric diagenesis, either as a consequence of buildup of sediments above sea level, or through drops in sea level that expose platform carbonates. In addition, meteoric water can circulate well below the land surface to alter carbonate deposits far older than the exposure interval. Meteoric processes commonly act over time periods of hundreds to millions of years.

Meteoric diagenetic patterns typically are complex and variable for the following reasons: 1. regional and temporal variations in starting material; 2. variations in rainfall and water throughput rates (in part, related to permeability variations); 3. variations in water chemistry (from locality to locality or vertically through the water column at any one site, especially at mixing interfaces); 4. variations in the duration of exposure or alteration during multiple episodes of exposure; and 5. the effects of plants and plant-derived acids that vary regionally and also changed through geologic time as a consequence of evolution of different plant groups.

The vadose zone is characterized by extensive dissolution of unstable carbonate minerals (aragonite and high-Mg calcite), often with reprecipitation of more stable carbonate (low-Mg calcite). As a consequence, primary porosity commonly is filled during meteoric diagenesis, and secondary porosity is created.

Unless there is a thinning or collapse of the rock section, meteoric diagenesis is relatively porosity neutral, at least at the scale of grains, with dissolution at one site supplying solutes for reprecipitation elsewhere. Meteoric diagenesis does, however, have a strong effect on permeability (e.g., permeability reductions through cementation of interconnected primary pores or permeability increases through solution enlargement of fractures).

Many vadose cements have fabrics reflecting the selective distribution of water in that environment — pendant (microstalactitic or gravitational) cements hanging from undersides of grains and meniscus cements concentrated at grain contacts. Whisker crystals (also termed needle-fiber cements), calcified filaments, blackened pebbles, root structures (rhizoliths), microspar, and Microcodium also are common features.

Phreatic zone cements are typically isopachous rims or complete pore fillings of equant calcite.

Freshwater meteoric calcites are depleted in Sr2+, Mg2+, δ18O, and δ13C, relative to their marine precursors. Most, but not all, meteoric settings are oxidizing, resulting in typically low Fe2+ and Mn2+ contents in meteoric cements (reflected in non-ferroan staining and no cathodoluminescence response).

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