WEATHERING-RELATED CLAST SIZE AND SHAPE EVOLUTION IN ARID AND SEMI-ARID SITES IN CALIFORNIA THROUGHOUT THE QUATERNARY (Invited Presentation)

Episodic, geologically instantaneous deposition provides a natural laboratory for studying weathering processes over geologic time by applying a space-for-time (chronofunction) approach. On abandoned alluvial deposits the size and shape of surface clasts inevitably evolves as weathering ensues and the mechanical properties, size, and shape of the clasts change. These changes dictate surface erodibility and sediment production within the alluvial system. However, the rates and processes of such evolution on exposed natural rock are scantly constrained (e.g., Johnstone et al., 2018; Mushkin et al., 2014).

Here, we used modified Wolman pebble count data to provide chronosequences of clast size and shape evolution over the past 117 kyr in arid and semi-arid California (Mojave Desert and Owens Valley). Combining surface clast-size data with thousands of manually measured crack lengths and morphologies from 1663 boulders we determined that: 1) cracking rates decrease significantly over the first ~6 kyr of rock exposure from 0.4 – 1.8 cm growth every thousand years (cm/kyr) to ≤ 0.1 cm/kyr after ~20 kyr of exposure; 2) crack morphologies broadly predict the shape of sediments produced, with surface-parallel cracking peaking just before elongated clasts are produced; and 3) different rock types experience different cracking rates and produce different clast sizes and shapes. For example, granitoid rocks typically weather to grus while fine-grained volcanics weather to larger, more elongated shards. Predictably, overall mean clast sizes generally decrease over time, and by 30 kyr exposure the median clast size of all lithologies at both sites stabilizes to between 1.6 – 3.0 cm long axis length.

These data can help inform the size evolution of surficial clasts over geologic time and provide further constraint on understanding fresh rock surface area exposure, which may inform our understanding of chemical processes and nutrient availability. We hope these data can also be incorporated into sedimentological and paleoclimate interpretations of abandoned deposited and buried sediments by informing how the clast sizes may have been modified post-abandonment.