DOE 2000. - Waste Isolation Pilot Plant - U.S. Department of Energy

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 2
2.5.2 Geologic Structures and Tectonic Activity
A study of the WIPP facility region suggests a fundamental geologic and tectonic separation into two
significantly different subregions: (1) the Permian Basin and (2) the Basin and Range subregions. The
geologic structures and tectonism of the Permian Basin are dominantly associated with large-scale basin,
interbasin and basin margin subsidence or emergence that occurred during the Paleozoic era. Basin and
Range structures and tectonism to the west are those associated with Basin and Range topography. The
activity characteristic of this subregion began in middle to late Tertiary time and is probably still
occurring to some extent.
The Permian Basin subregion is defined as that part of the Permian Basin within the site region. The
WIPP facility is slightly more than 60 mi (97 km) from the western margin of the Permian Basin (Figure
2.5-19). The Permian Basin is a broad structural feature made up of a series of Paleozoic sedimentary
basins whose last episodes of large-scale subsidence during late Permian time were associated with a
thick accumulation of evaporites. This basin now exists as a subsurface structural feature extending
roughly from the Amarillo uplift on the north to the Marathon thrust belt on the south and some 300 mi
(483 km) eastward from the Diablo platform and Sacramento and Guadalupe Mountain areas into westcentral
Texas. 23
The development of the Permian Basin began with the formation of a broad sag (named the Tobosa
basin 24 ) following deposition of lower Ordovician strata. Prior to the late Mississippian, several periods
of minor folding, faulting and uplift with erosion occurred. Nevertheless, general structural stability
prevailed. 48,49,50 Subsequently, tectonic activity accelerated in the area climaxing in late Pennsylvanian
and was split into two rapidly subsiding basins (the Midland to the east and the Delaware to the west) by
the medial Central Basin Platform. 25 Structural development of the Permian Basin within this framework
continued until late Permian when broad-scale basement stabilization occurred concurrently with
evaporite deposition.
Thus, the major tectonic elements of the Permian Basin were completely formed before the deposition of
Permian salt-bearing rocks, and relative crustal stability of the region has been maintained since Permian
time. Since then, the Permian Basin has been characterized throughout the Mesozoic and Cenozoic eras
by erosional processes interrupted by only minor episodes of terrestrial and shallow water deposition.
Regionally, the Permian Basin has been tilted and warped, but deep-seated faults since Permian time are
rare except along the western margin of the basin outside the area of salt preservation. In areas where salt
is near the surface, such as southeastern New Mexico, there are no indication of younger deep-seated
faulting and only a few isolated igneous intrusives of post-Permian age. 25
The Basin and Range subregion is defined as that part of the Basin and Range physiographic province
within the site region. As shown in Figure 2.5 19, this subregion borders the western margin of the
Permian Basin subregion to the west and southwest of the site. The Basin and Range subregion is
characterized by fault block mountain ranges, many of which are bounded on the west by major
high-angle normal fault systems. Uplift along these fault systems has resulted in gentle eastward tilting
of the mountain blocks and the formation of intermontane or graben-like valleys. Major development of
these characteristic structural features occurred from late Tertiary into early Pleistocene time. 48,49,50
Continued tectonism in the Basin and Range subregion is suggested by widely scattered Quaternary fault
offsets on the order one to several meters. A number of fault offsets of this age along the western flanks
of the Guadalupe, Delaware, Sacramento and San Andres mountains are described in the
literature. 26,27,48,49,50 More recently, additional but similar fault systems have been found and described
within the Basin and Range physiographic province in Trans-Pecos, Texas. 28
2.5-6 January 24, 2003