Stanislaus Group
Stanislaus Group
late Miocene
Consists of an extensive sequence of dominantly high-K, trachyandesite to basaltic trachyandesite and minor trachydacite lavas and trachydacite ignimbrites with lesser interstratified volcaniclastic sediments and lower-K (subalkaline) mafic to intermediate lava flows, distributed across the central Sierra Nevada in a strip approximately 60 km wide (north to south) and 150 km long (east to west). Divided into three formations or members, described below in descending stratigraphic order:
The Dardanelle Formation is the uppermost formation of the Stanislaus Group, and consists of lava that has been confused with lavas of the stratigraphically lower Table Mountain Latite, but where its stratigraphic position is well established, consists of a single aphyric shoshonite lava with an Ar-Ar age of 9.137 ± 0.017 Ma (Busby and others, 2018).
The Eureka Valley Tuff (EVT) is the middle formation of the Stanislaus Group and consists of the following, in ascending order (Busby and others, 2018)): (1) Basal Lava Flow Member, trachydacite lavas and hypabyssal intrusions with an Ar-Ar age of 9.94 ± 0.03; (2) Tollhouse Flat Member trachydacite welded ignimbrite with an Ar-Ar age of 9.54 ± 0.04 Ma; (3) Lava Flow Member, with trachydacite, trachyandesite, trachybasaltic andesite and basalt lavas; (4) By-Day Member trachydacite welded ignimbrite with an Ar-Ar age of 9.4 ± 0.3 Ma; and (5) Upper Member trachydacite nonwelded ignimbrite with an Ar-Ar age of 9.34 ± 0.04 Ma.
The Tollhouse Flat member is the most widespread of EVT members, and the upper member is the least widespread. The Tollhouse Flat Member welded ignimbrite has more biotite than the By-Day Member welded ignimbrite, and the Upper Member is easily distinguished by its non-welded character (Busby and others, 2016, 2018). Anisotropy of magnetic susceptibility in the Eureka Valley Tuff Member indicates that ignimbrites flowed radially away from the Little Walker caldera (King and others, 2007).
The Table Mountain Latite is the lowest formation of the Stanislaus Group and consists largely of trachyandesite to basaltic trachyandesite lavas with lesser basalts, basaltic andesites and andesites, with Ar-Ar ages of 10.41 ± 0.08 Ma to 10.36 ± 0.06 Ma (Busby and others, 2018). It erupted from fissure vents up to 10 km long as well as point sources sited on faults, mapped along the range crest between Sonora Pass and Ebbetts Pass (Busby and others, 2018). It has been proposed that additional Table Mountain Latite vents may lie in the Little Walker caldera southeast of Sonora Pass (Noble and others, 1974; Priest, 1979) but this has not been confirmed. The Table Mountain Latite is easily recognized in most places by its large (up to 13 mm), abundant (up to 40%) flow-aligned sieve-textured plagioclase and smaller black pyroxene. Table Mountain Latite lavas are voluminous and flowed 140 km down the Cataract paleochannel, from vents at the Sierra Crest to Knights Ferry in the foothills (Gorny and others, 2009; Busby and others, 2016). Where map scale allows, this unit is broken out as map unit tml.
Volcanic rocks of the Stanislaus Group are interpreted to be a part of the ancestral Cascade arc (map unit aca; e.g. Busby and others, 2008b) but are broken out here due to regional extent, distinctive high-K composition, and because they serve as an important stratigraphic marker within the ancestral Cascade arc sequence. Where present, Stanislaus Group strata separate volcanic and volcaniclastic rocks of the underlying Relief Peak Formation from those of the overlying Disaster Peak Formation, which are both subalkaline.