In 2011 and 2012, the sedimentary basins in the Fort Irwin National Training Center, California, were evaluated for groundwater resources using a variety of techniques, including drilling of boreholes. This study summarizes lithostratigraphic features and deposits in 8 of 10 boreholes drilled in 2 basins located in the western part of Fort Irwin. The western part of Fort Irwin straddles the eastern edge of the Miocene Eagle Crags volcanic field; therefore, many of the rocks penetrated in the boreholes are primary volcanic deposits (lava flow, pyroclastic flow, and fallout tephra deposits) and tuffaceous or lithic-rich sedimentary rocks (siltstone to cobble conglomerates) deposited in alluvial, fluvial, lacustrine, and possibly groundwater discharge environments. Boreholes were drilled with mud-rotary techniques that result in cuttings samples, and only two to four cores ranging in length from 3 to 5 feet (ft) were collected in each borehole. Of the boreholes studied, maximum depths range from 684 to 903 ft. Borehole GOLD1 penetrated tuffaceous and lithic-rich sandstone, partially welded tuff, avalanche breccia, and basalt lava flows. In addition to cuttings fragments of these rocks, coatings (typically finegrained or smaller sandstone or tuff) attached to clasts can be used to approximate the type of matrix in the drilled rock. Boreholes NELT1 to NELT7, located in Nelson Lake basin, have cuttings fragments of mostly crystallized volcanic lithic clasts with minor amounts of tuff and granitic clasts. Similar to borehole GOLD1, coatings on lithic clasts are used to infer the lithicrich or tuffaceous matrix of the drilled deposits. Most rocks in these boreholes are sandstone to conglomerate that disaggregated (or were broken) during drilling.
Correlation of lithostratigraphic features to borehole geophysical logs (especially gamma and resistivity) helps to establish properties of the rock and enables identification of depositional sequences of similar rock types. Lithostratigraphic features and resistivity in boreholes compare reasonably well to nearby time-domain electromagnetic sounding (resistivity) model results.
There is no direct age control on the rocks penetrated in the boreholes. The abundance of tuffaceous material as primary or slightly redeposited matrix is used to identify rocks deposited during the activity of the Eagle Crags volcanic field in the Miocene. In contrast, sedimentary rocks composed of detrital and epiclastic grains (only a few of which are tuffaceous rocks as clasts) are inferred to have been deposited during the Quaternary or Pliocene(?). The lithostratigraphic-based estimates of relative age indicate the typical thickness of the Quaternary or Pliocene(?) deposits is 70–170 ft, and that several water-bearing horizons are probable in the Miocene(?) section.
Additional publication details
|Publication Subtype||USGS Numbered Series|
|Title||Lithostratigraphic framework in boreholes from Goldstone Lake and Nelson Lake Basins, Fort Irwin, California|
|Series title||Open-File Report|
|Publisher||U.S. Geological Survey|
|Publisher location||Reston, VA|
|Contributing office(s)||Geology, Minerals, Energy, and Geophysics Science Center|
|Description||vi, 133 p.|
|Larger Work Type||Report|
|Larger Work Subtype||USGS Numbered Series|
|Larger Work Title||Geology and geophysics applied to groundwater hydrology at Fort Irwin, California|
|Other Geospatial||Fort Irwin|