Grain size of granite and derived grus, Enchanted Rock pluton, Texas

Abstract

Twenty five samples of grus from non-vegetated weather pits in granite were sieved and found to be mostly pebble gravel. Average content of gravel, sand and mud was 76, 16 and 18%, respectively, by weight. Primary grain-size modes were between 2.8–23.0 mm, and most grus was composed of granite rock fragments. Twelve granite cores drilled in the same weather pits from which grus was collected were studied in thin section, with 856 grains identified mineralogically and measured for size. These measurements were recast as sieve data. Grain-size comparisons for each granite-grus pair showed that the grain-size mode of grus did not follow the crystal size of parent granite. Granite with average crystal sizes of 23.0 and 4.0 mm formed grus with a primary grain-size mode of 5.7 mm. For two different sample pairs, granite with crystal sizes of 4.0 and 1.0 mm yielded grus with primary grain-size modes of 5.7 and 11.6 mm. Modal size measurements were pooled for the twelve granite-grus pairs, and the average crystal size of parent granite was found to be 3.4 mm and the average grain-size model of derived grus was 5.7 mm. Folk and Patton (1982) have documented micro-sheet fracturing of granite as the earliest stage of weathering which explains the lack of correspondence between granite texture and grain size of grus at Enchanted Rock.