Deformation of evaporites near the Waste Isolation Pilot Plant (WIPP) site

Abstract

The Delaware Basin is a broad asymmetric sedimentary trough in southeastern New Mexico and west Texas. Basin subsidence occurred from the Pennsylvanian into the Triassic. The basin also underwent tilting since the early Cenozoic. Layered evaporite units of Ochoan age in the basin are 1000 m thick. These evaporites are divided into three stratigraphic units (listed in order of increasing age): the Rustler Formation, the Salado Formation, the Castile Formation. These units, especially the Castile, are deformed along portions of the margin of the Delaware Basin and in some areas internal to the basin. In the northern Delaware Basin adjacent to the WIPP site, the term "Disturbed Zone" (DZ) has been applied to an area in which deformation structures are found in boreholes and from which chaotic seismig reflection data were obtained. The origin and timing of this deformation is considered important for the determination of This docunlent is PUBLI&'L%7 RELEASABLE Authorizing Official Date: 81/31 / W O d

Draft Title 40 CFR 191 compliance certification application for the Waste Isolation Pilot Plant. Volume 5: Appendices D and D, DEF, FAC

“…The Environmental Evaluation Group (EEG) summarized various map limits to the DZ, including the area where the Castile dip begins to steepen (Neil1 et at., 1983). Borns et al (1983) included two separate areas south of the site as part of the DZ based on seismic character.…”

“…Borns et al (1983) present one diagram ( Figure 7.3-2) of the seismic time structure for the top of the Castile Formation which illustrates the variously defined boundaries as well as any diagram. The principal part of the disturbed zone is defined by a lobate area shown as an "area of complex structure" where the seismic data are considered "ambiguous".…”

“…Snyder (in Borns et al, 1983) does show an upthrown block (horst) through WIPP-11 on the top of the Bell Canyon Formation that is based on his projection of the thickness of the lower Castile; WIPP-11 did not penetrate the complete Castile section. Other areas, such as the Poker Lake structures, may display some relief on the top of the Delaware Mountain Group, but Borns and Shaffer (1985) do not attribute the relief to faulting.…”

“…Structure contour and isopach maps of the Salado and Rustler over areas of complicated Castile structure also show that the overlying units are successively less involved in the structure (e.g., Section 7.2; Borns and Shaffer, 1985;Borns et al, 1983;Holt and Powers, 1988). Lower units that are thicker and deformed are overlain by units that are thinner and less structurally involved in the deformation.…”

“…Borns (1 983, 1987;Borns et al, 1983) has extensively examined the macroscopic to microscopic features from cores taken within the structurally deformed areas. These studies follow earlier, broader studies of macroscopic features from the "state line outcrop" and ERDA-6 (Anderson and Powers, 1978).…”