Eddy Flux Tower Bedrock Geology
All of the flux towers lie within the Sierra Nevada Batholith, a collection of geochemically and texturally
heterogeneous igneous plutons (intrusions) that were emplaced in the Mesozoic. The bedrock geology is
well mapped at each of the sites; each site lies on a separate 15’ quadrangle mapped by the USGS in the
1960s-1980s. Primary unit descriptions below are taken from the individual maps, with additional details
and naming modifications taken from a more recent USGS Professional Paper (Bateman 1992).
The two lower elevation sites, SJER and Soaproot Saddle, lie exclusively within one rock type. The two
higher elevation sites, P301 and Shorthair Creek, have multiple rock types exposed near the towers. The
highest elevation site, Shorthair Creek, was glaciated in the Pleistocene.
SJER – 400 m elevation
15' Quad: Millerton Lake; 7.5' Quad: Millerton Lake West
Ward Mountain Trondhjemite (Kw) (formerly Leucotonalite of Ward Mountain)
“Medium- to fine-grained biotite leucotonalite (trondhjemite). Rock appears gneissic in outcrop and
deformed and recrystallized in thin section... Biotite, generally in tiny plates, is the sole primary mafic
mineral. Secondary muscovite in plates as much as 2 mm across is abundant locally... U-Pb age of … 115
m.y” (Bateman and Busacca 1982). “Biotite averages about 10 percent of the rock… most samples
contain less than 5 percent alkali feldspar… Deformed trondhjemite, which constitutes the bulk of the
rock, is fine grained and gneissic, and when viewed in thin section it can be seen to have a granoblastic
texture… The Ward Mountain Trondhjemite intrudes the Bass Lake Tonalite” (Bateman 1992).
Soaproot Saddle, 1100 m elevation
15' Quad: Shaver Lake; 7.5' Quad: Shaver Lake
Bass Lake Tonalite (Kbl, formerly Tonalite of Blue Canyon)
“Medium-grained biotite-hornblende tonalite facies characterized by large euhedral hornblende prisms”
(Lockwood and Bateman 1976). “Typical Bass Lake Tonalite is medium-gray, medium-grained,
equigranular tonalite with a conspicuous foliation that is shown both by the preferred orientation of
minerals, chiefly hornblended and biotite, and by crudely lens-shaped mafic inclusions… The color index
… for most samples is 10 to 30” (Bateman 1992).
P301, 2000 m elevation
15' Quad: Huntington Mountain; 7.5' Quad: Dinkey Creek
Dinkey Creek Granodiorite (Kdc, formerly Granodiorite of Dinkey Creek)
“Medium-grained, generally strongly foliated. Contains abundant disc-shaped mafic inclusions and mafic
clots composed of hornblende, biotite, sphene, plagioclase, and opaque minerals. Intrudes the quartz
diorite of Blue Canyon. Age by Rb-Sr whole-rock isochron 110 +/- 11 m.y.” (Bateman and Wones 1972).
The mafic inclusions are “as much as 30 cm long and 5 cm thick. The mafic minerals are intergrown, and
individual grains are anhedral” (Bateman 1992).
This unit is intruded locally around the tower by the Bald Mountain Leucogranite (Kbm, Formerly Quartz
monzonite of Bald Mountain), a medium-grained, equigranular, felsic granite with biotite as the exclusive
mafic mineral (Bateman 1992).
Short Hair Creek, 2700 m elevation
15' Quad: Blackcap Mountain
This site was glaciated in the Pleistocene (Gillespie and Zehfuss 2004) and is situated on glacial till. The
bedrock immediately below the tower is of unknown composition. However, two different bedrock units
are exposed within 500 m of the tower (Bateman 1965).
Aplite and felsic quartz monzonite dikes (Kap)
Granodiorite of Dinkey Creek (Kdc, see description for P301)
References
Bateman, P.C. 1965. “Geologic Map of the Blackcap Mountain Quadrangle, Fresno County, California”.
Washington D.C.: Department of the Interior, United States Geological Survey.
———. 1992. “Plutonism in the Central Part of the Sierra Nevada Batholith, California.” US Geol.
Survey Professional Paper; 1483.
Bateman, P.C., and Alan J Busacca. 1982. “Geologic Map of the Millerton Lake Quadrangle, Westcentral Sierra Nevada, California”. Reston, Va.: Department of the Interior, United States
Geological Survey.
Bateman, P.C., and D.R. Wones. 1972. “Geologic Map of the Huntington Lake Quadrangle, Central
Sierra Nevada, California”. Department of the Interior, United States Geological Survey.
Gillespie, Alan R., and Paul H. Zehfuss. 2004. “Glaciations of the Sierra Nevada, California, USA.” In
Developments in Quaternary Sciences, edited by J. Ehlers and P.L. Gibbard, Volume 2, Part
B:51–62. Quaternary Glaciations-Extent and Chronology Part II: North America. Elsevier.
http://www.sciencedirect.com/science/article/pii/S1571086604801854.
Lockwood, John P, and P.C. Bateman. 1976. “Geologic Map of the Shaver Lake Quadrangle, Central
Sierra Nevada, California”. Reston, Va.: Department of the Interior, United States Geological
Survey.