Flow-path textures and mineralogy in tuffs of the unsaturated zone

By: , and 
Edited by: William C. HanebergPeter S. MozleyJ. Casey Moore, and Laurel B. Goodwin



The high concentration of chlorine-36 (36Cl) produced by above-ground nuclear tests (bomb-pulse) provides a fortuitous tracer for infiltration during the last 50 years, and is used to detect fast flow in the unsaturated zone at Yucca Mountain, Nevada, a thick deposit of welded and nonwelded tuffs. Evidence of fast flow as much as 300 m into the mountain has been found in several zones in a 7.7-km tunnel. Many zones are associated with faults that provide continuous fracture flow paths from the surface. In the Sundance fault zone, water with the bomb-pulse signature has moved into subsidiary fractures and breccia zones. We found no highly distinctive mineralogic associations of fault and fracture samples containing bomb-pulse 36Cl. Bomb-pulse sites are slightly more likely to have calcite deposits than are non-bomb-pulse sites. Most other mineralogic and textural associations of fast-flow paths reflect the structural processes leading to locally enhanced permeability rather than the effects of ground-water percolation. Water movement through the rock was investigated by isotopic analysis of paired samples representing breccia zones and fractured wall rock bounding the breccia zones. Where bomb-pulse 36Cl is present, the waters in bounding fractures and intergranular pores of the fast pathways are not in equilibrium with respect to the isotopic signal. In structural domains that have experienced extensional deformation, fluid flow within a breccia is equivalent to matrix flow in a particulate rock, whereas true fracture flow occurs along the boundaries of a breccia zone. Where shearing predominated over extension, the boundary between wall rock and breccia is rough and irregular with a tight wallrock/breccia contact. The absence of a gap between the breccia and the wall rock helps maintain fluid flow within the breccia instead of along the wallrock/breccia boundary, leading to higher 36Cl/Cl values in the breccia than in the wall rock.

Additional publication details

Publication type Book chapter
Publication Subtype Book Chapter
Title Flow-path textures and mineralogy in tuffs of the unsaturated zone
DOI 10.1029/GM113p0159
Volume 113
Year Published 1999
Language English
Publisher American Geophysical Union
Publisher location Washington, D.C.
Contributing office(s) Denver Federal Center
Description 26 p.
Larger Work Type Book
Larger Work Subtype Monograph
Larger Work Title Faults and subsurface fluid flow in the shallow crust
First page 159
Last page 184
Online Only (Y/N) N
Additional Online Files (Y/N) N