WHEN SEDIMENTARY GEOCHEMISTRY AND GEOCHRONOLOGY FAIL AS PROXIES IN PROVENANCE ANALYSIS: WELCOME TO THE “STEPLADDER EFFECT”

It is generally assumed that the ‘immobile’ element composition of clastic sedimentary rocks accurately reflects the composition of the sediment source. However, changes resulting from weathering, sediment transport, and aeolian contamination could impact the utility of determining sediment provenance by using geochemical techniques. A previous study of modern sediment derived from the Stepladder Mountains in SE California has shown that such processes can alter the trace-element composition of first-cycle sediments. This finding is striking given the characteristics of the area: 1) the area is arid thus bedrock has undergone minimal chemical alteration; 2) the Stepladder granodiorite is effectively a point-source for proximal sediment; 3) the granodiorite is compositionally homogeneous thus variation in sedimentary chemical composition is not caused by granitoid facies changes.

To further investigate the relationship of sediment immediately proximal to exposed bedrock we determined U-Pb ages of zircon from the same samples for which whole-rock geochemical data was previously collected. About 75% of the zircons in both bedrock and regolith have ages consistent with the emplacement age of the Stepladder intrusion (~ 74 Ma); the remaining ages are Proterozoic and are presumably xenocrysts. In contrast, 45% or less of the zircon from the sediment are ~ 74 Ma, the remainder are mostly Proterozoic. Results from Kolmogorov-Smirnov tests comparing ages among all 120 possible sample pairs suggest that it is not possible to reject the null hypothesis that the bedrock and regolith ages are derived from the same source population. Conversely, in most instances the hypothesis that sedimentary and granodiorite zircon were drawn from the same population would be rejected. The significant abundance of Precambrian ages in the granodiorite-derived weathering products may indicate a higher degree of xenocrysts, or it may reflect the contribution of grains from an additional (e.g. aeolian) source. The lack of correspondence between the granodiorite grain ages and those among its weathered products, both in whole-rock geochemistry and in U-Pb zircon detrital ages, shows that the “Stepladder Effect” may have a greater impact previously thought for sedimentary provenance studies.