structural geology, propagation mechanics and - Stanford School of ...

Figure 1.1 (opposite page) Location and general geology of the study area in the Aztec
sandstone at the Valley of Fire State Park, southeastern Nevada. The Aztec crops out as a
1,400-m-thick sequence of gently folded, northeasterly dipping (~25°) æolian deposits
consisting of ~800 m of red-stained sandstone (lower Aztec) and ~600 m of bleached and
variegated sandstone (upper Aztec). The Aztec is immediately, though unconformably
overlain by up to 1,300 m of Cretaceous deposits (principally the quartz-rich Baseline
sandstone in the air photo). The Aztec has been extensively affected by Basin and Range
extension, principally by north-northeasterly trending left-lateral strike slip-faults and
subsidiary northwesterly trending right-lateral strike-slip faults (apparent in the many
offsets of the red/buff alteration front). The major left-lateral bounding faults for the
immediate study area (Waterpocket fault and Baseline fault) are indicated (after
Bohannon 1983), as are the Overton Syncline (after Carpenter and Carpenter, 1994) and
two representative bedding orientations in the lower Cretaceous units (after Bohannon,
1983). Less obvious are remnants of the east-vergent Willow Tank thrust (upper right
corner, teeth denote hanging wall), marking the easternmost extent of compressional
tectonism related to the Cretaceous Sevier ororogeny. The aerial photo base is a mosaic
constructed of images downloaded from Google Earth.
Within the general vicinity of the Valley of Fire (Figure 1.1), the 1,400-m-thick Aztec
unconformably overlies terrigenous clastic, evaporitic and limestone deposits of the
Triassic and Permian, which themselves unconformably overlie miogeoclinal carbonates
of Pennsylvanian age and older. The Aztec in turn is unconformably overlain by up to
1,300 m of upward-coarsening Cretaceous clastics and, following a depositional hiatus
through the Paleogene, another 300 m of Neogene limestones and clastics (Bohannon,
1983; Bohannon et al., 1993; Eichhubl et al., 2004). Figure 1.2 presents a generalized
stratigraphic section of the region from latest Paleozoic time through the present day, as
modified from Bohannon (1977).
The Aztec sandstone itself is a subarkose containing up to 8% orthoclase and minor
amounts of lithic fragments (Eichhubl et al., 2004). It is composed of moderately rounded,
fine to medium-sized detrital grains (0.1 to 0.5 mm, 0.25 mm average) arranged in a
large-scale wedge and tabular-planar cross stratified sedimentary architecture
characteristic of æolian deposition (Marzolf, 1983). The Aztec is generally quite friable,
with porosity averaging about 20-25% (Sternlof et al., 2005) and permeability ranging
from about 100 to 2,500 mD (Flodin et al., 2005; Chapter 5 this thesis). Clay, chiefly
kaolinite, comprises up to 6% of the rock by volume and forms the predominant grain-
bridging, pore-filling cement.
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