Orientation
and spacing of joints in deformation band shear zones east of
Mount Hillers, Utah
Hoefert,
Catherine M. and Tindall, Sarah E.,
2004, Orientation
and
spacing
of
joints
in deformation band shear zones east of Mount Hillers, Utah
[abs]: Northeastern Section (39th Annual) and Southeastern Section
(53rd Annual) Joint Meeting (March 25–27, 2004), Geological Society
of America Abstracts with Programs, Vol. 36, No. 2, p. 102.
Deformation
bands
are planar shear zones that accommodate fault offset in porous
sandstones. They form by porosity collapse and cataclasis of sand
grains, resulting in low porosity and permeability across the shear
zone. Deformation bands form pervasively throughout deformed
sandstone bodies and vary in thickness from 1mm to tens of meters,
so they present significant barriers to fluid flow. In some cases,
however, deformation bands host closely spaced joints that may serve
as conduits for fluid flow across these impermeable shear zones. The
relationships among deformation band orientation, joint orientation,
deformation band thickness, and joint spacing have the potential to
affect fluid flow through a porous sandstone reservoir, and are the
subject of our study.
We examined deformation bands in Jurassic Entrada Sandstone east of
Mt. Hillers, one of the Henry Mountains laccoliths in southern Utah.
Deformation bands formed in Entrada Sandstone in response to
Oligocene – early Miocene laccolith emplacement. The shear zones
occupy two dominant orientations in the study area: a north-striking
set and an east-striking set, both dipping nearly vertically. In
deformation bands that contain joints, the relationship between
deformation band thickness and joint spacing is roughly linear;
narrow deformation bands develop closely-spaced joints, and joints
in wider deformation bands are more widely spaced. East-striking
deformation bands are consistently jointed throughout the study
area, and display two perpendicular joint sets. In horizontal
outcrops, these deformation bands display north-striking, vertical
joints, and a set of horizontal joints is evident in east-striking
deformation bands exposed on steep outcrop faces. However, most
north-striking deformation bands are not jointed. Our data imply
that the development of joints within deformation bands is dependent
on deformation band orientation relative to the direction of stress
responsible for jointing.
Future work will trace progressive changes in orientations of
deformation bands and associated joints around the Henry Mountains,
and will relate orientation changes to joint spacing variations
within deformation bands.