As part of a long term geologic and hydrologic study of several regional
groundwater flow systems in western Utah and eastern Nevada, the U.S.
Geological Survey was contracted by the Southern Nevada Water Authority
to provide geophysical data. The primary object of these data was to enable
construction of the geological framework of the flow systems. The main
new geophysical data gathered during the study were gravity observations,
and existing aeromagnetic data were also compiled. These data resulted in
regional maps of the isostatic gravity and aeromagnetic fields of the area.
The isostatic gravity map shows a north-south grain to most of the area,
which was imparted by post-20 Ma basin-range tectonism; whereas the
aeromagnetic map shows an east-west grain to the area, imparted by
Eocene to lower Miocene calc-alkaline calderas and source intrusions.
To de-emphasize surface and near-surface features and to gain greater
insight into contributions from deeper sources, the isostatic gravity
anomalies were upward continued by 3 km and the aeromagnetic data
were transformed to their magnetic potential ("pseudogravity").
Identification of maxima of the horizontal gradients in the gravity and
magnetic-potential data helped define deep-seated crustal blocks that are
characterized by major changes in density and magnetization. Maps
showing these maxima were useful in defining large faults, especially
range-bounding faults, and margins of igneous bodies and calderas. A
gravity inversion method was used to separate the isostatic residual anomaly
into pre-Cenozoic basement and young basin fill. Inasmuch as the primary
aquifer in the area is sedimentary basin fill, this method is especially valuable
for hydrogeologic analyses because it estimates the thickness of the fill.
As befits its name, the geology of the Confusion Range of Utah has been a
point of contention for many years, so we looked at it in greater detail in the
course of our regional study. The northern part of the range is underlain by a
large gravity high, which continues south through the Conger Range, Burbank
Hills, and northern Mountain Home Range. This is the "structural trough"
reported in the literature that was mapped as the axial part of a Sevier
synclinorium and contains the maximum thickness (7 km) of high-density
carbonates in the area, thus the largest high gravity anomaly.