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A case study of a precariously balanced rock, its partially exhumed corestone platform, and encasing saprock and soil

Craig M. Hall, Heather N. Webb, Gary H. Girty, Amir A. Allam, & Thomas K. Rockwell

Published January 2019, SCEC Contribution #8883

Lying between the Elsinore and San Jacinto faults, ~0.5 km SE of Roundtop, is a tonalitic corestone perched precariously on an exhumed corestone base. Such features are referred to as precariously balanced rocks (PBRs). Cosmogenic 10Be data listed in an online abstract suggest a tentative exhumation age of the PBR near Roundtop at ~35,000 years ago. The absence of PBRs adjacent to the Elsinore and San Jacinto faults implies that they were shaken free from their perches during ground shaking. Alternatively, their absence may reflect a more highly fractured and intensely weathered terrane. Because the weathering histories of PBRs have not been studied previously, we analyzed the texture, clay mineralogy, and geochemistry of the Roundtop PBR to answer several basic questions. Can the characteristics of the regolith that once encased the precariously balanced rock be reconstructed? What chemical processes operated in the regolith prior to exhumation, and are these processes still occurring? Following exhumation, did new fracture development modify the original regolith? To answer these questions, we extracted three ~1 m long cores from the PBR near Roundtop, and collected 23 saprock and 9 pebbly sandy loam samples from a trench excavated near the base of the corestone platform. Plagioclase and biotite alteration, along with the concomitant loss of Ca, Na, Sr, Ba, K, Mg, Mn, Nb, and P mass, are the primary evidence of weathering intensity within the saprock. Additionally, the presence of mostly kaolinite, and lesser quantities of mixed-layer biotite/smectite in the saprock, along with the above observations and data, point to significant fluid/rock interations during saprock development. Subsequent formation of pebbly sandy loam appears to have been accomplished primarily through pedoturbation within the uppermost portions of the underlying already weathered saprock, and produced little additional chemical weathering. Hence, our data support the general idea that Roundtop is an area of weak ground shaking, and that since current exhumation little weathering has affected this PBR locality.

Key Words
Precariously balanced rocks, San Jacinto fault, Elsinore fault, Regolith development, Geochemistry, Corestone

Hall, C. M., Webb, H. N., Girty, G. H., Allam, A. A., & Rockwell, T. K. (2019). A case study of a precariously balanced rock, its partially exhumed corestone platform, and encasing saprock and soil. CATENA, 172, 719-737. doi: 10.1016/j.catena.2018.09.029.