Characterization of Unusual Ground Fissuring in a Dry Lakebed—Broadwell Basin, San Bernardino County, California

Ground fissuring at playa lakes is a common geohazard where differential settlement of aquifer sediments caused by groundwater extraction is considered to be the mechanism for fissure development. Although this mechanism may be a common problem, fissures may also originate from natural causes, including desiccation or tectonic movements. Broadwell Basin near the town of Ludlow, in San Bernardino County, California contains a narrow playa lake approximately four miles long and a mile wide that was considered as a site for possible industrial development. The location is isolated and not subject to groundwater extraction, but the lake bed contains numerous fissures that could be as wide as about 1.8 meters (six feet) across at the surface. The characterization of the Broadwell Basin fissures was conducted on the basis of several approaches. High-resolution S-wave seismic reflection surveying mapped the overall tectonic structure of the basin. The fissures were trenched to as deep as 9 meters (30 feet) to understand their physical characteristics as a function of depth. The timeframe of fissure activity was addressed by means of age dating of carbonized wood from the trench with the C¹⁴ technique. A comprehensive program of laboratory testing of disturbed and undisturbed samples was conducted with emphasis placed on understanding the mineralogy of the soils, the distribution of natural moisture content, and the nature of shrinkage of these soils when they are desiccated. The basic conclusion of these studies is that the fissures are not tectonic. They exhibit neither vertical nor horizontal displacement. The fissures currently visible formed hundreds and probably thousands of years ago. Laboratory testing confirms the slow rate of their formation and indicates that the lake sediments are currently at a moisture content where the formation of additional fissures would not be expected to develop under current climatic conditions.