The Tassajara Formation is a geologic unit within the

Livermore Valley The Livermore Valley, historically known as the Valle de San José (Valley of San José), is a valley in Alameda County, California, located in the East Bay region. The city of Livermore is located in the valley. Geography The valley is bound ...

of Northern California, United States. The formation surfaces only in the northern upland parts of the Livermore Valley and underlie the central part of the valley floor at a depth ranging from to . The Tassajara Formation consists of sediments ranging from brown to gray mudstone,

andesitic Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomin ...

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

, conglomerate, and minor

bentonitic Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-mon ...

and

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular v ...

ous

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

.C.Michael Hogan and Marc Papineau, ''Phase I Environmental Site Assessment, Vicinity of Deerwood Drive and Bollinger Canyon Road, San Ramon, California'', Earth Metrics Inc.File ref 7815, San Mateo, Ca. (1989) In the northern San Ramon area, the Tassajara Formation underlies Quaternary valley fill material. The

Bishop Subbasin The Bishop Subbasin is an aquifer that resides between two subsurface structures of the Tassajara Formation in the northern extremity of the Amador Valley, California. This aquifer is a sub-unit of the Livermore-Amador Groundwater Basin. The Bis ...

is an

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteris ...

that resides between two subsurface ridge formations of the Tassajara Formation in the northern extremity of the

Amador Valley Amador Valley is a valley in eastern Alameda County, California and is the location of the cities of Dublin and Pleasanton. Part of Tri-Valley, the valley is bounded by the foothills of the Diablo Range on the north and south, Pleasanton Ridge t ...

References

Geologic formations of California Geology of Alameda County, California Neogene California Sandstone formations of the United States Tuff formations Diablo Range Livermore Valley {{AlamedaCountyCA-geo-stub

See also

References