DEPOSITION OF THE 18.8 MA PEACH SPRING TUFF ON A THICK LAVA FLOW IN THE MARBLE MOUNTAINS, CALIFORNIA

In the southern Marble Mountains, California, the 18.8 Ma Peach Springs Tuff (PST) caps the Miocene section and was deposited on a 30-60 m thick porphyritic andesite (field name) lava flow with no intervening pre-PST sedimentary rocks. The southern part of the range is a structural block with 5-12° dips along 055-065° azimuths, and erosion into dip slopes resulted in elongate “finger” mesas of PST with tops that slope 5-8° along 044-078° azimuths. About 4-8 m below the base of the andesite flow, tuffaceous sandstone and tuff were deposited in a distal fluvial or lacustrine environment that represented a low relief depocenter. This sequence dips ~6° along a 065° azimuth. The andesite flow has phenocrysts of 15-20% plagioclase up to 2x4 mm, and 1-5% olivine <1 mm. There are 5-10% multi-grain crystal clusters up to 2x4 mm (a few are 3x8 mm), and most are grains of plagioclase with a few of plagioclase and olivine. The flow has a thick core with gray to dark gray groundmass and ~5% vesicles. The 5-10 m thick carapace has very dark gray to black groundmass and 20-35% vesicles, and the carapace was broken during flow into <1 m blocks. Locally, <2 m wide “squeeze ups” of sparsely vesicular lava intruded the rubbly carapace. The upper surface of the flow has non-systematic flow-induced ridges and troughs, but have peak-to-peak lengths of 25-225 m and peak-to-trough relief of 2-25 m. The andesite flow was a single eruptive event. PST was deposited across the andesite flow. Thickness of the PST varies from 7-25 m, and the stratigraphy is represented by lateral continuity and thickness variations in the zones of compaction (partial to dense) and crystallization (vitric or crystallized). All zones thin across ridges in the lava flow and thicken in troughs; however, the moderately and densely compacted and crystallized subzones vary in thickness only slightly, and the partially compacted and vitric subzone vary more. Simple erosion into dip slopes can result in elongate ridges, but the “finger” mesas are along variably oriented troughs in the andesite flow. The andesite lava flow formed a local highland above (presumed) surrounding alluvial environments, and the PST parental density current overtopped the highland with deposition of the ignimbrite influenced by lava flow morphology. Lava morphology affected the subsequent erosion and final landform.