Quartz and Feldspar Stability, Steady and Non‐Steady‐State Weathering, and Petrogenesis of Siliciclastic Sands and Muds

Abstract

The kinetic and thermodynamic properties of quartz, plagioclase, and K‐feldspars, which constitute 70 to 80% of the upper crust, provide a framework for prediction of mineralogical and chemical changes involved in the production of siliciclastic sediments. Chemical weathering of bedrock may produce weathering profiles with distinct mineralogical zones, compositionally much different from the parent rock. Mass wasting of such profiles produces sediments that reflect the mineralogy of the zones exposed to mechanical erosion, rather than the composition of fresh bedrock. The relative rates of chemical weathering and mechanical erosion determine which mineralogical zones are exposed to mass wasting, and therefore control the compositions of siliciclastic sediments. Stable rates of chemical weathering and erosion result in steady‐state weathering, so that thickness and the mineralogical composition of eroded soil zones, and therefore the mineralogy of derived sediments, remain unchanged while steady‐state weathering prevails. Non‐steady‐state weathering occurs where climate and tectonism vary. Changing conditions alter rates of chemical weathering and physical erosion, resulting in exposure of different, and sometimes all, weathering zones of profiles, or exposure of bedrock. Such conditions result in production of sediments with diverse mineralogy, reflecting incipiently‐to‐highly weathered zones of profiles. Steady‐ and non‐steady‐state weathering of granitic rocks therefore can be assessed by study of variations in quartz and feldspar contents of sands and variations in bulk compositions of muds, as shown for sediments derived from the Sierra Nevada and Bega batholiths, and the Appalachian Piedmont. Compositional variation of sediments, or its absence, are controlled by the relative rates of chemical weathering and erosion and provides insight into climatic and tectonic conditions in source lands, as well as information about provenance composition.