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

included distributary channel or delta-front environments, or as
barrier islands and strand plains in the interdeltaic areas. These
massive sandstones, when originally deposited, formed extensive
aquifers with considerable lateral extent (Henry and Morton, 1982).
Growth faulting, with the attendant thicknesses of sediments, has
resulted in a structural setting in the Rio Grande Embayment which
includes major fault trends and minor associated faults, some of
which are parallel, and some of which are transverse to the main
trends (Figs. 2,3). Syndepositional units which thicken toward the
main faults form folds, or rollovers, which dip markedly into the
faults.
Rapid sedimentation also resulted in shale ridges, and
shale diapirs (Collins, 1983). Well No. 5 (Fig. 1 and 2)
penetrated a shale diapir which displaced the section vertically
upward (Table 1).
Wi thin the study area, the dominant growth fault is the
McAllen fault, which extends from south of the Rio Grande northward
as much as 150 miles (Collins, 1983) (Figs 1 and 3).
This fault
may be due to instability, or weakness, in the basement which
resulted in activity throughout the Oligocene-Miocene depositional
interval. The greatest movement, or acti vi ty, of the fault
occurred during deposition of the marine (lower and middle) Frio
sequence (Collins, 1983). To the east in the study area, the Donna
fault created a relatively stable area (Collins, 1983). The
movement on the Donna
fault was not as continuous, and the
displacement was not as great, as along the McAllen fault.
This
differential movement resulted in a flattening of the dip towards
the Donna fault. The relatively small Weslaco fault, farther to
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