The Tassajara Formation is a geologic unit within the Livermore Valley of Northern California, United States.Livermore Valley Groundwater Basin: State of California Groundwater bulletin 118. 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 sandstone, conglomerate, and minor bentonitic and pumiceous tuff.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 is an aquifer that resides between two subsurface ridge formations of the Tassajara Formation in the northern extremity of the Amador Valley.
See also Arroyo de la Laguna Arroyo Valle Arroyo Mocho Mocho Subbasin South San Ramon Creek
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