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4.0 RESOURCES
suggest an aquifer grouping.
Aquifer Location
Figure 4-9 shows
where the indicated aquifer
group is present at the
surface. This is where the
aquifer is shallowest and
has the best quality, due to
the proximity to recharge.
As shown schematically to
the right, formations that
outcrop around the basin
margins commonly dip
beneath younger formations
present in the basin centers.
Thus, groundwater is
Cross-Section of Aquifer Types
commonly available from
productive basin-margin aquifers for some miles basinward of the outcrops depicted on Figure 4-9, albeit
at increasing depth.
Where a productive aquifer is overlain by less permeable strata, artesian conditions are set up
which may produce flowing wells where the less permeable strata are punctured. Wyoming examples
include prolific flowing wells on the flanks of the Black Hills and Bighorn Mountains. While the basinbounding
aquifers are generally present beneath the entire basin, their great depth, the absence of the
basin-bounding fractures that enhance productivity, and the commonly deteriorated water quality make
them poor development targets in the central basins.
For all but the alluvial aquifers, geologic structure (folds and faults) is as important as rock type
in providing useful supplies of groundwater. The map to the right shows the major geologic faults in
Wyoming, which provide a generalized picture of where fracturing may be an important component of
groundwater productivity. In the Thrust Belt of western Wyoming and bordering the major mountain
ranges, large-scale faulting is a controlling factor in nearly any bedrock aquifer. Across most of eastern
Wyoming and in the basin interiors, in contrast, large-scale fracturing is much less important, and the
grain size and cementing of aquifer materials play the key role.
Finally, in the carbonate aquifers (limestone and
dolomite), the rocks themselves are slightly soluble, and
groundwater circulation can increase the size and extent of
fractures through dissolution. This is dramatically expressed in
the creation of cave systems, but can vastly enhance
groundwater production capacity at much finer scales. Faults,
folds, and solution enhancement of permeability are most
commonly associated with the basin margins, where
deformation of the formations has accompanied mountain
building.
Faults
4-8