Frenchman Flat and Yucca Flat, located along the eastern part of the Nevada Test Site, are major Cenozoic basins formed by Basin-and-Range tectonism.  Six wells were drilled to investigate stratigraphic and hydrostratigraphic units, alteration, and structural features in these basins.  Location, lithology, stratigraphy, and thickness information are tabulated below.

 

Wells/Location, SN	Alluvium (m)	Tuffs (m)	Granite (m)	Paleozoic Rocks (m)
ER-5-4 #2, FF, C	1100	1000
ER-6-1 #2,YF, S	100	450		420
ER-7-1, YF, E	67	448		247
ER-2-1, YF. NC	425	367
ER-12-2, YF, NW	146	45		1888
ER-8-1, YF, N	45	300	300	200

 

FF=Frenchman Flat, YF=Yucca Flat, S=south, NC=north central, NW=northwest,

C=central, E=east, N=north, and SN=south to north.

 

Alluvium and volcanic units are thicker in Frenchman Flat compared with Yucca Flat to the north. Correlative volcanic units are present in the upper sections of the three southern wells in Yucca Flat and the Frenchman Flat well. Volcanic units in the lower half of both basins are different.  The alluvium contains abundant detrital smectite, zeolites, calcite, and silica and insignificant post-depositional alteration.  In situ low-temperature alteration is pervasive in volcanic units, which contain zeolites, smectite, opal-CT, cristobalite, and calcite. Illite, minor chlorite, and pyrite are present in altered granite. Paleozoic rocks are dominated by limestone and dolomite; however, ER-12-2 contains argillite and sandstone with abundant illite, minor smectite, chlorite, kaolinite, calcite, dolomite/ankerite, siderite, and pyrite.

Despite limited information from Frenchman Flat, it is evident that thick alluvium and volcanic deposits occur along the axis of both basins compared with areas close to the margins. Thickness variation differences between wells ER-2-1 and ER-7-1, located on the west and east sides of Yucca Fault, respectively, are consistent with deep axial basin profiles created by faulting.