geothermal resource potential of the safford-san simon basin, arizona

Geology
Subsurface geology controls the distribution of the confined aquifers.
These confined aquifers are contained in permeable sediment, which is overlain
by an impermeable cap rock.
Drillers' logs of the San Simon area were
interpreted to delineate subsurface stratigraphy and locate the thickness and
lateral extent of the confining cap rock.
In the San Simon area, artesian wells
tap water contained in sand and gravels, which are below a blue clay.
This
blue clay is probably correlative with the green clay facies of Harbour (1966)
at Safford.
Fortunately, the blue clay is easily distinguished from other
basin-filling sediments; as a result, drillers' logs are highly useful in mapping
its subsurface extent and thickness. Figure 26 is a structure contour map
at the base of the blue clay.
The blue clay is deposited in a wedge-shaped
depositional basin bounded by steep structure contours oriented NNW
and WNW.
A structure contour map of the top of the blue clay (Figure Zn exhibits the
same geometry as that seen at the base of the blue clay except that the contours
delineating the margins of the deposition basin are not as steep.
The
closure over the lowest elevation at the top of the blue clay is displaced to
the south from the site of the lowest elevation at the base of the blue clay;
the closure is adjacent to steep contours oriented WNW.
The well showing the
lowest elevation for the top of the blue clay records a significant evaporite
occurrence. At this location, evaporites overlie the blue clay. The evaporite
sequence may represent deposition during the dessication of a lake in
closed basin conditions.
Structural and tectonic inferences are tentatively drawn from the structural
contour maps.
The steep structure contours of the blue clay may correlate
with faults that control the location of the depositional basin.
In
addition, faulting contemporaneous with dessication of a late Tertiary lake
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