Characterization of Unusual Ground Fissuring in a Dry Lakebed – BroadwellBasin, San Bernardino County, California

摘要

Ground fissuring at playa lakes is a common geohazard where differentialsettlement of aquifer sediments caused by groundwater extraction is considered to bethe mechanism for fissure development. Although this mechanism may be a commonproblem, fissures may also originate from natural causes, including desiccation ortectonic movements. Broadwell Basin near the town of Ludlow, in San BernardinoCounty, California contains a narrow playa lake approximately four miles long and amile wide that was considered as a site for possible industrial development. Thelocation is isolated and not subject to groundwater extraction, but the lake bedcontains numerous fissures that could be as wide as about 1.8 meters (six feet) acrossat the surface.The characterization of the Broadwell Basin fissures was conducted on thebasis of several approaches. High-resolution S-wave seismic reflection surveyingmapped the overall tectonic structure of the basin. The fissures were trenched to asdeep as 9 meters (30 feet) to understand their physical characteristics as a function ofdepth. The timeframe of fissure activity was addressed by means of age dating ofcarbonized wood from the trench with the C14 technique. A comprehensive programof laboratory testing of disturbed and undisturbed samples was conducted withemphasis placed on understanding the mineralogy of the soils, the distribution ofnatural moisture content, and the nature of shrinkage of these soils when they aredesiccated.The basic conclusion of these studies is that the fissures are not tectonic.They exhibit neither vertical nor horizontal displacement. The fissures currentlyvisible formed hundreds and probably thousands of years ago. Laboratory testingconfirms the slow rate of their formation and indicates that the lake sediments arecurrently at a moisture content where the formation of additional fissures would notbe expected to develop under current climatic conditions.