Uranium-Series Ages of Faulted Alluvial Fan Deposits, Mesquite Regional Landfill, Southeastern California

Abstract

A multifaceted investigation of the western portion of the Mesquite Regional Landfill (MRL) site and vicinity (study area), located in the desert of southeastern Imperial County, California, included evaluation of slip geometry and activity of a fault located within the boundaries of the proposed landfill. Municipal waste will be placed on the southwest margin of the Chocolate Mountains on a gently southwest-sloping bajada consisting of seven laterally coalescing Quaternary alluvial fan units capped with variably developed desert pavement surfaces. Q1, the oldest, is estimated to be of early to middle Pleistocene age; Q7, the youngest, consists of historical and Holocene channel and flood deposits. Numerical ages of two intermediate units were calibrated using uranium-series (230Th/U) dating of pedogenic carbonate coatings on gravel clasts. The dates indicate minimum depositional ages of 106.2 ± 3.3 ka for Unit Q3 and 142.9 ± 6.1 ka for Unit Q2. A trench located over a buried step in crystalline basement exposed a near vertical, northwest-trending dextral fault with small (< 1 m) apparent vertical and horizontal separations cutting unit Q2. An extensive varnished desert pavement surface developed across overlying unit Q3 is undeformed across the projected trace of the fault. This observation and the uranium-series ages demonstrate that the minor fault was not active during the Holocene, a regulatory requirement for municipal landfills. The geomorphic expression, pedogenic development and numerical ages indicate that fan deposits and surfaces in the study area resemble alluvial fan sequences found elsewhere in the arid southwestern United States.