(GP/GT) for Additional Water Supply in the Lower Rio Grande

the east, results in a "reversal" of the dip away from the McAllen
fault and toward the Gulf Basin.
The Shepherd fault (Figs. 1,2), which is transverse to the
main McAllen fault, was also active during the time of deposition
of Frio sediments. The same stratigraphic section is present on
both sides of the Shepherd fault, but the section is thicker on the
downthrown (north) side of the fault (Collins, 1983).
Sediment thickening toward the McAllen and Shepherd faults has
resulted in a structural axis which migrates upward in the section,
and geographically toward the northeast. This axis trends from the
southeast (Donna) toward the northwest (Edinburg) (Fig.2). Along
this axis, or flattening of the dip angle, faulting is less
persistent, which results in greater continuity of the aquifers
(Swanson and others, 1976). Swanson referred to this area as "a
promising area for the occurrence of continuous geopressured
reservoirs of broad areal extent ..• " (Swanson and others, 1976).
Throughout both potential reservoir areas, the approximate
depths to the top of the geopressured zone averages approximately
9,000 feet. The geopressured zone ranges from approximately 8,500
feet on the west side of Reservoir area C to 9,500 feet on the east
side of Reservoir area B, with relatively uniform depth throughout
the study area (Fig. 2).
The minimum depth of the 300°F isotherm
appears to center in an area which includes the northwest-southeast
trending axis between the cities of Edinburg and Donna.
Salinities of the connate water in Reservoir Area C ranges
from 9,000 to 15,000 ppm Cl (Swanson and others, 1976). Higher
temperatures occur at greater depths (approximately 12,000 feet) in
11-7