Six seismic reflection profiles were collected in the vicinity
of the Jornada Horst between Goat Mountain and Tortugas Mountain
(northeast and east of Las Cruces, New Mexico) to delineate more
precisely the geometry of the horst and to determine whether
large, buried channels have been incised into the top of the
horst. The Jornada fault zone separates the southern Jornada del
Muerto ground-water basin from the Mesilla ground-water basin in
the Mesilla drainage basin.
The upper part of the Jornada Horst is composed of Tertiary
volcanic and volcaniclastic rocks; these rocks overlie Permian
sedimentary rocks. The horst, in turn, is overlain by unconsolidated
sediments of the upper Santa Fe Group. Some test holes indicate
that little or no ground water flows from the Jornada del Muerto
ground-water basin to the Mesilla ground-water basin over some
portions of the horst. However, some ground water flows through
the upper Santa Fe Group deposits above some portions of the
horst. Ground-water flow immediately east of the horst near U.S.
Highway 70 is deflected northward in the southern Jornada del
Muerto ground-water basin presumably because of the change from
higher hydraulic-conductivity values of aquifer materials in the
southern basin to lower hydraulic-conductivity values of materials
in the horst. Incised, buried channels, if present on the horst,
could be filled with alluvial material with higher hydraulic-
conductivity values than those of the material in the horst.
Incised, buried channels would allow ground water to readily
move from the Jornada del Muerto ground-water basin to the
Mesilla ground-water basin.
The gross geometry of the horst--eastern extent, constraints
on the western extent, and general altitude of the top--was
discerned by interpretations of the seismic profiles. The presence
or absence of large channels incised into the top of the horst
could not be confirmed by these interpretations. However, the
seismic interpretations suggest that the water table is above
the top of the horst for most of its extent between U.S. Highway
70 and Tortugas Mountain and that the top of the horst is above
the water table and acts as a subsurface flow barrier north of
U.S. Highway 70.