A tumbler experiment using the 0.0625- to 4-mm fraction of four granodioritic grus samples was performed to investigate the nature of fracturing in the production of siliciclastic sand. Petrographical modal analysis was performed on 240 subsamples representing four major lithological constituents (quartz, potassium feldspar, plagioclase, and polymineralic rock fragments), four grain-size fractions (0.50 to 0.70, 0.35 to 0.49, 0.25 to 0.34, and 0.177 to 0.24 mm), and four time periods (untumbled grus; 2, 4, and 8 days). Each tumbling day represents a maximum of 25 km of transport in water. Tests of sample means and analysis of variance suggest that all subsamples are statistically homogeneous with respect to the number of quartz, potassium feldspar, and plagioclase grains among these size fractions. There is a highly significant difference in the number of rock fragments in the 0.50- to 0.70-mm fraction as compared to finer grained fractions, but no such differences occur within the medium- and fine-grained sand fractions. These results are inconsistent with the concept that lithic composition of the detrital light-mineral fraction is strongly size dependent within the 0.177- to 0.50-mm range. Where such compositional differences do occur, they may reflect sedimentological processes other than the comminution of grains from a single protolith.
Grains from three grus samples examined for size changes displayed relatively rapid rates of disintegration during the first few days of tumbling, and then continued to fracture at reduced rates from days 4 through 8. From 8.9 to 16.2% of the relatively coarser grained fractions experienced comminution during 8 days (≃200 km) of transport. Of this amount, the sand fraction increased from 1.7 to 3.4%, whereas the silt plus clay fraction increased from 6.4 to 14.5%. For each sample, the weight percent of the fine- and/or medium-sand fraction remained nearly constant.
Shape-ratio values (length of short axis/length of long axis of the maximum projection grain outline) as well as amplitude-ratio values for harmonics 2 and 3, as measured by Fourier analysis, were used to examine the character of quartz-grain fracture as evidenced in the 0.25- to 0.50-mm size fraction. Although some across-grain fracture occurred during the first day of tumbling, an overall increase in the frequency of smaller shape-ratio values indicated the dominance of grain-parallel fracture in all three samples. Scanning electron microscopy indicated that abrasion was not obvious even after 8 days (≃200 km) of transport.
Average shape-ratio values were computed for at least 200 quartz grains in each of 49 samples from four high-gradient streams in southern California. Overall, the average shape-ratio values are remarkably consistent within each of the four streams, and closely resemble the value characteristic of the associated grus sample. Although significant decreases in average shape-ratio values occur along some local, very steep stream gradients due to high rates of grain-parallel fracture and/or shape sorting, the values reequilibrate after only 7 to 10 km of transport along lower gradient stream segments. Reequilibration probably reflects the dilution of more elongate by less elongate grains along lower stream gradients. No significant changes in the average shape-ratio values occur where small tributaries draining the same protolith enter the trunk streams, from sidewall erosion of the same source rock or across a contact with younger granitic source rock.
Simple shearing is known to produce self-similar fracturing. Such fracturing is suggested by the lithological analysis as well as highly elongate quartz grains as observed in this and other studies. Self-similar fracturing may be an important mechanism in explaining the compositional and textural aspects of nascent siliciclastic sand derived from plutons subjected to emplacement and/or postcrystallization tectonism.
