EFFECT OF CHEMICAL WEATHERING ON SEDIMENT RESISTANCE TO MECHANICAL WEAR IN STREAMS

Bedload sediments are reduced in size by mechanical wear during downstream transport through river networks. The rate of size reduction varies with rock type, and has been shown experimentally to scale with the inverse square of rock tensile strength. Chemical weathering reduces rock tensile strength, so that for a given rock type, strength, and thus rock resistance to mechanical wear, should depend on the degree of chemical weathering. Here we test this hypothesis using granitic rocks from the Santa Cruz Mountains, which have undergone various degrees of chemical weathering. We use a laboratory barrel tumbler to simulate downstream transport of gravel, and quantify the fractional mass loss to silt as a function of circumferential distance traveled. We combine these measurements with tests of rock tensile strength and indices of chemical weathering. With these data, we can determine whether the strength reduction due to chemical weathering in a uniform rock type has the same effect on particle wear rates as strength variation among different rock types. These results will be useful in understanding the interactions between chemical weathering, sediment durability, and the dynamics of river incision into bedrock.