17 Science at the Intersection of Soils, Weathering, Geomorphology, Climate, Ecology and Wildfire—A Tribute to the Careers of Les McFadden and Grant Meyer

Wednesday, 16 March 2022
8:00 AM - 12:00 PM
CBC-A 108

ROCKSLIDE ORIGIN OF THE BUTTERMILK BOULDERS, EASTERN CALIFORNIA

Paper number: 17-2
Program: Symposia Sessions
Time: 8:20 AM – 8:40 AM

Authors

Abstract

The main Buttermilk climbing area, located about 15 km west of Bishop, California, hosts some of the best bouldering in the world. Individual boulders over 10 m across are common, and the Peabody boulders, before separating along a joint, formed an ellipsoidal giant 15 m tall, 20 m wide, and 27 m long. Although these boulders are typically described as erratics, several lines of evidence demonstrate that they are locally derived and formed by rolling and sliding down from bedrock above, perhaps during large earthquakes.

The boulders, underlying bedrock, and cliffs above are all composed of weakly porphyritic Mesozoic granite. There are no boulders of the abundant mafic and metamorphic rocks that would have been among those brought in by glaciers from the west. In-place bedrock uphill is strongly jointed along a dominant steep, NNE-striking set that forms tall fins, and top surfaces of bedrock joint blocks are riddled with shallow weathering pans up to 1-2 m across and with width:depth ratios of ~4. Boulders typically also have weathering pans on top, but some have planar surfaces with weathering pans on sides or undersides, and a few lack any pans on top. Boulders typically have smooth, curving undersides and limited contact area with underlying rock, which is typically deeply weathered, pulverized, cut by gently dipping fractures, and microbrecciated.

We interpret these observations to show that the climbing boulders rolled and slid from joint fins upslope. Smooth undersides and crushed rock beneath rule out a core stone origin. Side or bottom surfaces with weathering pans were likely top surfaces of in-place blocks, and smooth upper surfaces of some blocks indicate that they fell too recently to have developed weathering pans. In this area the 1872 Lone Pine earthquake produced ground shaking estimated at modified Mercalli intensity VIII with peak ground acceleration of ~0.5g. It is possible that the Buttermilk boulders record a history of this and earlier great earthquakes.

Geological Society of America Abstracts with Programs. Vol. 54, No. 2

doi: 10.1130/abs/2022CD-373732

© Copyright 2022 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.