Description:
The northern Plomosa Mountains are a small metamorphic core complex in La Paz County, western
Arizona. The dominant structure of the range is the Plomosa detachment fault, a low-angle normal fault
that separates a single tilted footwall block from a highly extended hanging wall with diverse
stratigraphy and complex structure. The northern Plomosa Mountains were first mapped by
Scarborough and Meader (1983), with later detailed mapping of smaller areas by Stoneman (1985) and
Steinke (1997). The Arizona Geological Survey (AZGS) mapped the northern Plomosa Mountains at
1:24,000 scale (Spencer et al., 2014) with funding from the STATEMAP program, a component of the
National Geologic Mapping Act of 1992. This field trip is largely a showcase for geology mapped during
the AZGS mapping project.
On Day 2 we will be visiting several outcrops of the controversial Bouse Formation in the southern part
of the Blythe basin. Bouse deposits were first systematically investigated and formally defined as a
geologic formation as part of USGS geohydrology investigations along the lower Colorado River
(Metzger, 1968; Metzger et al., 1973; Metzger and Loeltz, 1973). These investigations described the
basic characteristics of the Bouse Formation, including: (1) basal carbonate deposits including limestone,
tufa (travertine) and marl; and (2) much thicker fine-grained siliciclastic deposits consisting of
interbedded clay, silt, and quartz-rich sand that are always stratigraphically above the basal carbonate.
Based primarily on interpretations that some of the microfauna found in southern Bouse deposits were
marine organisms (Smith, 1970), and to a lesser degree on interpretations of sedimentary environments
(e.g., Buising, 1990), the Bouse Formation was interpreted to be entirely marine/estuarine.