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Bridge Spring Ash Andesites

35.761061, -114.837952 Mafic lava flows below and above the Bridge Spring Ash fall of 15.2 Ma. The welded Bridge Spring Tuff examples shown were from the land of chaos 4 miles to the northwest of the same lithified ash here. Tdml Mafic lavas—Modified from Anderson (1977). Dark-gray and dark-grayish, red-purple andesite flows below rhyolite and rhyodacite unit (Tdrl) and above Tuff of Bridge Spring (Tb). Includes as many as 16 andesite flows with a total thickness of as much as 275 m. Distinctly porphyritic varieties contain about 25 percent phenocrysts of plagioclase, augite, and altered olivine with minor altered orthopyroxene as individual grains and as cores surrounded by augite. Principal accessory minerals are iron oxides and apatite. Pyroxene to olivine ratio is typically about 3:1. Includes some outcrops of Tdm of Anderson (1977). Ages range from 15.1 to 15.00 Ma (40Ar/39Ar; Gans and Bohrson, 1998) Tb Tuff of Bridge Spring (middle Miocene)—Modified from Anderson (1977). Welded to non-welded, gray to pale-red, quartz-free rhyolitic ash-flow tuff; purplish-gray andesitic lithic inclusions are common. Phenocrysts of sanidine, plagioclase, biotite, augite, and hornblende are present in varying amounts and proportions. Sphene is principal accessory mineral present in variable amounts. Where exposed in vicinity of highway 60, consists of two cooling units that total up to about 180 m thick. Also occurs in isolated outcrops south of Malpais Flattop Mesa and east of Indian Rapids (north of Willow Beach). Originally mapped and described by Anderson (1971), tuff is inferred to have erupted from a caldera in northern Eldorado Mountains (Gans and others, 1994). 40Ar/39Ar ages from outcrops near type section along highway 60 in Eldorado Mountains range from 15.03±0.06 to 15.34±0.06 Ma (Gans and Bohrson, 1998) Tpu Upper part—Modified from Anderson (1977, 1978). Dark-purplish-gray, massive, porphyritic pyroxene-olivine andesite and basaltic andesite flows locally interstratified with thin beds of whitish-gray tuffaceous sedimentary rocks. Includes at least one flow of hornblende-biotite rhyodacite near base; rhyodacite contains sparse quartz and sanidine and common accessory sphene. Exposed in southern part of map area along State Highway 60 and southwest of Malpais Flattop Mesa. Maximum thickness approximately 500 m Free Updated Geologic Map of the Black Canyon Area: https://pubs.usgs.gov/of/2013/1267/a/