Distinguishing tectonic joints from cooling joints in the Bandelier Tuff (Pleistocene), Pajarito Plateau, Los Alamos County, New Mexico
Margaret Anne Rogers, Karin E. Budding, and V. L. Christie

Abstract:

Most of the Bandelier Tuff's joints were produced by cooling after an ash flow or a series of ash flows was emplaced. The Los Alamos National Laboratory's (LANL's) waste management operations are primarily conducted on or in the fractured Bandelier Tuff, which crops out on the Pajarito Plateau near the western boundary of the Rio Grande rift. Cooling joints, in a semi-arid climate, may be of limited concern as potential contaminant migration pathways because of their restricted horizontal and vertical extent. Tectonic joints, however, could represent pathways for contaminants to reach surface water and/or ground water. In the Bandelier Tuff, almost all joints (both cooling and tectonic) are vertical or near-vertical. Joint data from pits at Materials Disposal Area G, TA-54, have azimuths that appear to have random distribution. If tectonic joints have preferred orientation, then cooling joints must behave as if they have random orientation. A statistical method was used to differentiate between joints with random orientation and joints with preferred orientation. It is inferred that the joints with preferred orientation are tectonic. No evidence was seen that cooling joints (at any given location or plateau-wide) have preferred orientation. Over 3000 joint measurements were made at 18 locations. Composite data show statistically- identified trends of N 20°-29° E, N 1°-10° W, and N 71°-80° W. This joint study was part of LANL's Program A415, Waste Disposal Sites Studies, a program to satisfy research needs for waste management operations and to establish site-specific monitoring systems.


Citation:

  1. Rogers, Margaret Anne; Budding, Karin E.; Christie, V. L., 1996, Distinguishing tectonic joints from cooling joints in the Bandelier Tuff (Pleistocene), Pajarito Plateau, Los Alamos County, New Mexico, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 293-301.

More information...