QUARTZ AND FELDSPAR STABILITY, STEADY AND NON-STEADY-STATE WEATHERING, AND PETROGENESIS OF SILICICLASTIC SANDS AND MUDS

The kinetic and thermodynamic properties of quartz, plagioclase, and K -feldspars, which constitute 70 to 80% of the upper crust, provide a f ramework 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 zo nes, compositionally much different from the parent rock. Mass wasting of such profiles produces sediments that reflect the mineralogy of th e zones exposed to mechanical erosion, rather than the composition of fresh bedrock. The relative rates of chemical weathering and mechanica l erosion determine which mineralogical zones are exposed to mass wast ing, and therefore control the compositions of siliciclastic sediments . Stable rates of chemical weathering and erosion result in steady-sta te 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-st ate weathering occurs where climate and tectonism vary. Changing condi tions alter rates of chemical weathering and physical erosion, resulti ng in exposure of different, and sometimes all, weathering zones of pr ofiles, or exposure of bedrock. Such conditions result in production o f sediments with diverse mineralogy, reflecting incipiently-to-highly weathered zones of profiles. Steady- and non-steady-state weathering o f granitic rocks therefore can be assessed by study of variations in q uartz and feldspar contents of sands and variations in bulk compositio ns 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 an d tectonic conditions in source lands, as well as information about pr ovenance composition.