Physical and compositional stratigraphy in the 5.6 Ma Bicycle Lake basalt, southeastern Fort Irwin, California

Abstract:

The 5.6 Ma Bicycle Lake basalt (BLb) in southeastern Fort Irwin, California, is being studied to understand the development of the field, development of the geologic and hydrogeologic framework for Fort Irwin, and deformation in the Eastern California Shear Zone. BLb is exposed on a mesa near Bicycle Lake (BLM), and ridges west and south of Bitter Spring (BSW, BSS, respectively). Lateral extent of the BLb volcanic field is not well constrained because the margins were eroded or buried, and BLM, BSW, and BSS are separated by the east-striking Bicycle Lake and Coyote Lake faults and subsidiary faults. Where exposed, the BLb was deposited on fine-gained sedimentary deposits in playa, groundwater discharge, or distal alluvial fan environments. The total thickness of the field is not easily determined. Lava flows are well exposed, but sections are not deeply incised, and on BSW the maximum incision is <16 m. Along a probable edge of the field, there is a single ~1m thick flow, but projections through BSW indicate thickness might be 20-55 m thick. Many lava flows are 1-2 m thick, a few are locally 3-6 m thick, and all are interstratified with 0.5-3.0 m thick cosets of 0.1-1.0 m thick individual flows. In BSW, 11-16 m sections have 5-8 flows 1-6 m thick. Flow morphology implies low-viscosity (high-temperature?) lava filling low-relief topography developed on previous flows. BLb is basaltic andesite, a few flows are high-SiO₂ basalt. Bivariate plots of major and minor oxide concentrations, trace elements, and elemental ratios show the BLb differs from other Miocene and Pliocene basalts and basaltic andesites in the central Mojave Desert area. Although the range is not large, there are compositional variations locally and throughout the sections. For example, the typical Al₂O₃/TiO₂ ratio (TR) is 6.1-6.6, but a high ratio (HR) is 6.6-7.2 and a low ratio (LR) is 5.1-6.1. In south BSW, low in the section are only TR lavas. In south BSW and BSS, HR lavas are interstratified with TR lavas. In northern BSW, TR (and fringe HR) lavas are interstratified with LR lavas. In BLM, 14-18 km west-northwest across the Bicycle Lake fault from BSW, there are TR and HR lavas. Neither vents nor dikes have been identified (eroded or covered?), but distribution of compositions suggests minor (but not complete) sector partitioning of flow paths during the growth of the field.