PERMO-TRIASSIC LAURENTIAN ARC INITIATION AND RECORD IN TRIASSIC SEDIMENTARY STRATA IN THE INYO MOUNTAINS, EASTERN CALIFORNIA

The passive margin along the west coast of Laurentia developed into a convergent margin during Permo-Triassic time. Triassic sedimentary strata in the Inyo Mountains of eastern California were deposited during this transitional period and have yielded ~257 Ma detrital zircons that were likely derived from initial magmatism along this margin. In order to constrain the sedimentological response to arc formation, stratigraphic columns were measured in five locations across the Inyo Mountains.

The oldest Triassic unit, member C of the Conglomerate Mesa Formation, is a 41-52 m thick conglomerate unit with clasts of siltstone, limestone, sandstone, and chert. This unit fines upward into sandy to silty limestone in the lower member of the Union Wash Formation, which is 32-102 m thick and contains shells and ripple marks. The middle member, 248-729 m thick, is a turbidite sequence made up of mostly siltstone and shale with minor centimeter-scale limestone beds. Siltstone beds exhibiting Bouma layers C, D, and E were observed within the middle member. The upper member is ~332 m thick unit made up of limestone, siltstone, shale, and sandstone, and this unit contains ammonites and burrows in some areas.

Sedimentary facies in Member C and the lower member of the Union Wash Formation indicate deposition in relatively shallow water within the photic zone, while the middle member was deposited by distal turbidites in a basin plain environment. These interpretations suggest the units were deposited during rapid subsidence. The upper member was likely deposited in a shallow environment much like the lower member, providing evidence of Middle Triassic shoaling. Therefore, it is likely that the strata were deposited due to rapid deepening of the basin caused by the initial stages of retro-arc basin formation, followed by shoaling in response to the building of the volcanic arc.