Sediment size on talus slopes correlates with fracture spacing on bedrock cliffs: implications for predicting initial sediment size distributions on hillslopes

Abstract

Abstract. The detachment of rock fragments from fractured bedrock on hillslopes creates sediment with an initial size distribution that sets the upper limits on particle size for all subsequent stages in the evolution of sediment in landscapes. We hypothesize that the initial size distribution should depend on the size distribution of latent sediment (i.e., fracture-bound blocks in unweathered bedrock) and weathering of blocks both before and during detachment (e.g., disintegration along crystal grain boundaries). However, the initial size distribution is difficult to measure because the interface across which sediment is produced is often shielded from view by overlying soil. Here we overcome this limitation by comparing fracture spacings measured from exposed bedrock on cliff faces with particle size distributions in adjacent talus deposits at 15 talus–cliff pairs spanning a wide range of climates and lithologies in California. Median fracture spacing and particle size vary by more than 10-fold and correlate strongly with lithology. Fracture spacing and talus size distributions are also closely correlated in central tendency, spread, and shape, with b-axis diameters showing the closest correspondence with fracture spacing at most sites. This suggests that weathering has not modified latent sediment either before or during detachment from the cliff face. In addition, talus at our sites has not undergone much weathering after deposition and is slightly coarser than the latent sizes because it contains unexploited fractures inherited from bedrock. We introduce a new conceptual framework for understanding the relative importance of latent size and weathering in setting initial sediment size distributions in mountain landscapes. In this framework, hillslopes exist on a spectrum defined by the ratio of two characteristic timescales: the residence time in saprolite and weathered bedrock and the time required to detach a particle of a characteristic size. At one end of the spectrum, where weathering residence times are negligible, the latent size distribution can be used to predict the initial size distribution. At the other end of the spectrum, where weathering residence times are long, the latent size distribution can be erased by weathering in the critical zone.

Sand, Gravel, Cobbles, and Boulders: Detrital Thermochronology Shows that One Size Does Not Tell All

“…Bimodality in granitic sediment has commonly been attributed to granular disintegration caused by biotite weathering (Buss et al., 2008; Shen et al., 2019; Wahrhaftig, 1965), which would tend to be enhanced at lower elevations in the catchment, where temperatures are warmer and vegetation is denser. Conversely, at higher elevations, where slopes are steeper and frost cracking is more common (Riebe et al., 2015), a diversity of processes produce a range of sizes, including abundant gravel and cobbles (Sklar et al., 2020), from a latent size distribution that is set by the fracture spacing of the underlying bedrock (Verdian et al., 2021). This sediment is then abraded and fractured as it travels downstream to the outlet, and thus the source elevations inferred from ages in stream sediment fail to match the observations of sediment size from hillslopes at higher elevations.…”

“…Direct measurements of sediment size distributions from other mountain landscapes confirm that such spatial variations in sediment production may be common (Sklar et al., 2017) and thus should be expected in applications of detrital thermochronology. For example, size distributions of sediment produced on hillslopes have been shown to correlate with variability in lithology (Lai et al., 2021; Roda‐Boluda et al., 2018), erosion rate (Riebe et al., 2015; Whittaker et al., 2010), topography (Attal et al., 2015), fracture spacing (Neely & DiBiase, 2020; Verdian et al., 2021), climate (Marshall & Sklar, 2012; Terweh et al., 2021), and geomorphic processes (Roda‐Boluda et al., 2018; Sklar et al., 2017, 2020). In mountain landscapes, where many of these factors often vary across individual catchments, different sediment sizes at the outlet may reflect sediment production from different parts of the catchment, where lithologic, climatic, and topographic conditions favor different sizes.…”

Rock properties and sediment caliber govern bedrock river morphology across the Taiwan Central Range

“…Previous work has demonstrated that the grain size distribution of coarse sediment delivered from steep hillslopes to channels can be dependent on bedrock fracture spacing ( 15 , 16 ), bedrock tensile strength ( 14 ), and grain size reduction during mass wasting ( 39 ). In the Taiwan Central Range, there is limited variation in bedrock compressive strength among major units ( 40 ), and we interpret the boulder size signal as corresponding to a decrease in the effective fracture density with increasing metamorphic grade.…”

“…River morphology can adjust through changes in both slope and width ( 10 ), and the partitioning between the two depends on sediment cover ( 11 ) and the erodibility of in-channel bedrock ( 12 ). Similarly, rock material strength sets the initial size distribution of sediment from hillslopes ( 13 ) through variations in lithology ( 14 ), fracture spacing ( 15 , 16 ), and the style of sediment delivery from hillslopes to channels ( 17 ). The role of boulders has been highlighted as a key linkage between bedrock fracture spacing, which sets boulder size ( 15 ), and channel width and slope, which are thought to be sensitive to boulder cover and size ( 18 ).…”

Size, shape and orientation matter: fast and semi-automatic measurement of grain geometries from 3D point clouds

“…Grains form initially by fragmentation or chemical weathering, transforming a cohesive rock mass into a granular material. The initial size or shape distributions are controlled by fragmentation, weathering processes and structure of the rock mass (e.g., fracture density and orientation, mineral size) (e.g., Molnar et al, 2007;Garzanti et al, 2008;Sklar et al, 2017;DiBiase et al, 2018;Neely and DiBiase, 2020;Verdian et al, 2021). These initial distributions then evolve due to the action of geomorphological processes, including attrition, chipping, abrasion, fragmentation, chemical weathering and transport of grains by wind, river flow, avalanches along hillslopes, and sea waves and currents (e.g., Attal and Lavé, 2006;Domokos et al, 2014;Miller et al, 2014;Várkonyi et al, 2016;Novák-Szabó et al, 2018;Marc et al, 2021).…”