Paper No. 25-15
Presentation Time: 9:00 AM-6:00 PM
STRATIGRAPHY AND PETROGRAPHY OF EARLY MIOCENE SYNEXTENSIONAL VOLCANIC ROCKS, WESTERN TEHACHAPI MTNS., CA
The 22.6±0.6Ma (Plescia et al., 1994) bimodal volcanic rocks (Tv) of the Tecuya Fm (Ttc) are exposed for 30km along the southern rim of the San Joaquin Valley 10km north of the San Andreas and Garlock Faults. New 1:6000 scale mapping has documented multiple northwest trending, east dipping, high-angle faults that contain 200-700m thick Tv deposits in the hanging wall. Tv is overlain in angular discordance by poorly exposed ≤17Ma coarse clastics. Tv can be divided into 3 subunits, each with multiple eruptive units: olivine basalt flows (Tv1), rhyolitic-dacitic tuff breccias (Tv2), and basaltic andesite, trachyandesite, and basalt flows, flow breccias, and shallow intrusives (Tv3). Tv1 (0-60m) is interbedded with channel-form fluvial conglomerates. Tv2 (0-40m) is thickest in the hanging wall of normal faults or near fissure vents. Tv3 is 100-600m thick and overlies all other units. Flows show trachytic to intergranular textures, high percentages of primary and secondary Fe-Ti oxides, and hb and px phenocrysts up to 4mm with skeletal growth patterns. Abundant amygdules >1cm are filled with chalcedony, opal, calcite, and zeolites. Tv3 contains interbedded channel-form fluvial conglomerates and volcaniclastic sandstones. Thin, laterally persistent laminated tuffs, and limestone with desiccation cracks are interpreted to have been deposited in small ephemeral lakes or ponds. Elongate vesicles were measured in the tops of well-exposed flow units in Tv1 and Tv3 and are interpreted to reflect NE-SW flow direction except were deflected by NE-facing erosional or fault-related paleoscarps. Interbedded fluvial conglomerates are NE flowing. Crossbeds in thin (<3m) pyroclastic flow deposits indicate flow was to the SW. Fissures scattered throughout the outcrop area contain peperites and other evidence of rhyodacitic phreatomagmatic vent-filling eruptions. Fissures have orientations ranging from 310 to 347. Shallow intrusives contain clastic quartz grains rimmed with glass, and xenoliths of Tertiary sedimentary rocks and partially resorbed Mesozoic tonalite. Peridotite xenoliths were found to the west in olivine basalt flows. Our new detailed volcanic stratigraphy and mapping will be used to better understand structure, paleogeography, and paleomagnetic results in the complexly deformed Tejon Embayment.