Ancestral Cascade arc rocks
Ancestral Cascade arc rocks
Pliocene to Miocene
Consists of predominantly intermediate composition volcanic and volcaniclastic sedimentary deposits distributed extensively across the central and northern Sierra. Collectively, this unit represents arc volcanic products and derivative sediments, referred to as Mehrten Formation by Curtis (1954).
Volcanic component is calc-alkaline and predominantly andesite and basaltic andesite with lesser basalt and sparse dacite and rhyolite (e.g., Busby and others, 2018). Andesite is commonly porphyritic with plagioclase ± hornblende ± augite ± hypersthene ± biotite phenocrysts. Basalt is commonly finely porphyritic with olivine ± plagioclase ± clinopyroxene ± augite phenocrysts. Less commonly, alkalic compositions including trachyandesite, trachydacite, and trachybasalt are present (notably within map units sg, tml, and tkt). Volcanic rocks occur as lava flows and domes and varied pyroclastic deposits (most commonly tuff breccia ± pumiceous pyroclastic flow and fall deposits). Intrusive equivalents commonly occur as dikes, plugs, and (less commonly) sills. Most dated volcanic deposits range in age from ~15 to 3.4 Ma (e.g., Busby and others, 2008a, b, 2016, 2018; Cousens and others, 2008; John and others, 2015; Sylvester and others, 2012;). However, Saucedo and others (1992) reported K-Ar ages of up to ~19.7 Ma in the northern Sierra, and Cousens and others (2008) reported a small volume of 28 Ma pyroxene andesite lavas that erupted in the Carson Range east of Lake Tahoe.
Sedimentary component includes volcanic debris flow (lahar) deposits, volcanic lithic sandstone and conglomerate, lesser siltstone, and minor clay. Debris flow deposits are typically poorly sorted or chaotic, with angular to subrounded polylithic volcanic clasts of variable size supported in a tuffaceous sandy to silty to clayey matrix. Across all sedimentary deposit types, clasts and matrix are predominantly derived from volcanic material (described above), with lesser material incorporated from older Cenozoic deposits and granitic and metamorphic basement, especially in lower strata. Rare debris avalanche deposits are also present (e.g., Berkebile, 2003; Busby and others, 2016, 2018; Harwood and others, 2014).
Vent locations are along and east of the modern Sierran crest. Composition (especially alkalinity) varies somewhat north-south along the length of the arc (e.g., Putirka and others, 2012). Exhibits pronounced lateral facies changes: proximal (near arc) strata occur at higher elevations near and east of the modern range crest and consist predominantly of primary volcanic deposits (lavas, tuff breccias and ignimbrites); intermediate strata occur at mid elevations on the western slope and consist predominantly of debris flow and fluvial deposits interbedded with lesser, farther-travelled lava flows and (uncommonly) pyroclastic deposits; distal strata occur at lower elevations along the western Sierra Nevada foothills and consist predominantly of fluvial strata, lesser volcanic debris flow deposits, and rare interbedded lavas.
Relatively persistent and voluminous volcanic centers have been identified and include the 15-6 Ma Bodie Hills volcanic field (John and others, 2012); the 12-5 Ma Little Walker-Sierra Crest volcanic center and the 6-4.6 Ma Ebbetts Pass volcanic center in the central Sierra Nevada (Busby and others 2018); and the Squaw Peak, Martis Peak, and Mount Pluto (inferred) stratovolcanoes north of Lake Tahoe (Sylvester and others, 2012). Unit includes Mehrten, Kate Peak, Disaster Peak, and Relief Peak Formations.