Backbone Ridge
The Backbone Ridge graben is a major
structure in the eastern Llano region. The
graben is wedge shaped, widening to the
NE. The axis of the graben is aligned
northeasterly. The graben is positioned over
granite basement in the SW and Valley
Springs and Packsaddle metamorphic
terranes to the NE. In the SW region, the
graben is positioned over the inferred
boundary of two neighboring granite
plutons. Two large displacement faults
bound the graben. The Roaring Spring fault
is the northwest bounding fault and trends
N35E. The Bald Mountain fault is the
southeast bounding fault and trends N70E.
Throw on both bounding faults increases
progressively from SW to NE attaining
estimated maxima of 730 m on the Roaring
Spring fault and 560 m on the Bald
Mountain fault. In the SW portion of the
graben, bedding strikes northeasterly or
northwesterly with a easterly dip
component. Many smaller faults are
mapped within the graben and generally are
subparallel to one of the bounding faults.
The Hoover Point road cut is located within
the Backbone Ridge graben near its SW
terminus. The local geology in the vicinity
of the road cut was mapped by Barnes and
Warren in 1945 (in Cloud and Barnes, 1948,
Plate 11) prior to filling of Lake LBJ and construction of today's FM 1431. Four ENE-trending faults are mapped in the
area; the two bounding faults and two internal faults. Only one of these faults is exposed in the road cut; this is the internal
fault nearest the map label " Hoover Point". This fault trends N70E.
This is a 500 m long, drilled and pre-split road cut of arcuate plan form with steep slopes (ca. 70°) up to 23 m high. The
orientation of the cut slope changes from about N-S at the northern end to N40W at the southern end. The strike of the
slope and its dip should be kept in mind when viewing the geology. The dip of faults and bedding is primarily an apparent
dip. For example, the dip of many faults is steeper than suggested by the outcrop pattern seen in the cut, because the strike
of many faults is distinctly oblique to the slope strike. In places, it is possible to orient oneself so as to look down the
strike of the faults and see their true dip and cross sectional form. Also, bedding dip may be significantly different than
suggested by the outcrop bedding traces
The road cut exposes the upper part of the Riley formation and the lower part of overlying Wilberns formation, which are
Upper Cambrian in age. The upper half of the Cap Mountain member of the Riley is exposed, primarily at the northern
and southern ends of the road cut. The Lion Mountain member of the Riley is fully exposed and is a prominent part of the
outcrop immediately opposite the overlook pullover. The Welge, the basal member of the Wilberns fm., is fully exposed
in the central region of the road cut, but it is too high in the slope for direct outcrop access. The basal portion of the
Morgan Creek member overlies the Welge in the slope opposite the pullover. Representative blocks of the inaccessible
units are located along the lake side of the pullover parking lot.
Morgan
Creek
Welge
Lion
Mountain
Cap
Mountain
This is the pinkish unit that comprises the upper third of the slope in the central region. The contact with the
Welge is placed at the top of the uppermost, glauconitic, recessive bed in the Welge. The outcrop has a
discernible pinkish color that is characteristic of basal Morgan Creek across the Llano region. This basal
zone of the Morgan Creek member is primarily a coarse grained limestone with quartz sand and scattered
glauconite grains.
The unit consists of 0.5 -1.0 m thick, lithified beds of coarse grained, cross-bedded quartzose sandstone with
thinner, recessive, glauconite-rich interbeds. The basal sandstone bed has a reddish/pinkish hue, but the
other sandstone beds have a yellowbrown color. This unit is 3.7 m thick.
The Lion Mountain is a prominent unit because of the large amount dark green glauconite as well as the
distinctive trilobite coquina lenses in one horizon. Close examination of glauconte-rich zones shows
abundant, rounded quartz sand grains. The unit is 10 m thick. For mapping purposes it is subdivided into
five subunits, based primarily of the amount of glauconite and the friability of the rock; three glauconiterich and friable subunits are separated by more lithified carbonate-rich zones
• Unit 1 is a distinctly bedded unit with alternating dark, glauconite-rich beds and lighter colored,
coarse grained, glauconitic limestone beds. This unit is 8.5 m thick.
• Unit 2 is primarily a gray, coarse grained limestone with prominent, closely spaced, glauconite-rich
stylolite seams. Bedding is discernible as a consequence of color changes, but beds are relatively
thick. This unit is 8.8 m thick.
• Unit 3 is primarily a gray, finely laminated, silty limestone. A prominent 1 m thick zone of several
beds of reddish-orange, coarse grained limestone occurs within this unit. This zone is important
because it allows estimation of throw across an important fault in the southern segment of the
roadcut. This unit is 3.1 m thick.
• Unit 4 is a yellowish-brown, laminated silty limestone. Its thickness is unknown, but it is a
prominent unit in the southern most part of the roadcut
The road cut exposes a large number of faults that formed in conjunction with development of the graben and the larger
faults. Characterization and analysis of these faults may assist inference of the tectonic conditions under which faulting
occurred. Study of these well-exposed faults also provides insight into the process of fault formation, which is key to
developing mechanical models of fault formation. The Hoover Point road cut provides an opportunity to study the effect
of different lithologic factors on fault development. In addition to the faults, deformation is reflected in spatial variations
of bedding attitude.
Multiple fault orientations are
seen at any given section of the
road cut, but the number of fault
sets and their attitudes varies
with position along the roadcut .
Four categories of faults are
discriminated using the
magnitude of fault dip and type
of dip slip component:
1) high angle with a normal slip
component,
2) high angle with a reverse slip
component,
3) bed parallel or subparallel,
(found only in subunit E of the
Lion Mountain) and
4) low angle, reverse slip. Using
cross cutting relations, the first three categories are observed to be essentially coeval.
The fourth category, the low angle reverse faults, appears to be younger. The vast majority of faults are high angle faults
with a normal slip component. There are two significant and a few small high angle faults with reverse slip components.
Aside from a few exceptions, the strike of all high angle faults lies in the NE quadrant. Striae are visible on slip surfaces
of some faults, especially for faults in Unit 2 of the Cap Mountain at the northern end of the road cut, and typically exhibit
a rake of about 40° to the SW. Thus NW dipping faults exhibit a left-lateral sense of strike slip and SE dipping faults
exhibit a right-lateral sense of strike slip. The existence of a significant strike slip component also is seen in the cross
sectional geometry of most high angle faults. Viewed down strike, fault slip surfaces or narrow zones of cataclasis often
exhibit a pronounced sinuosity or corrugation. This marked roughness complicates measurement of fault attitude.
Typically one must visually "smooth" the fault surface to determine the strike line and dip angle. Consequently, attitudes
of faults have a larger range of variability.
The road cut exposes five faults
with throws greater than 2 m.
Two high angle, normal slip
faults of 4.2 m and 6.6 m throw
occur in the northern region and
offset different units of the Cap
Mountain. A high angle, reverse
slip fault with at least 8 m of
throw occurs in the southern
region a short distance southward
of the overlook parking lot. Unit
2 of Cap Mountain is juxtaposed
against Unit 3 of Cap Mountain
across this fault. This is the fault
near the map label "Hoover
Point". The two other larger
faults are located about midway
along the road cut approximately
opposite the northerly entrance to
the overlook parking lot. The two faults define a horst bounded on the south by a fault zone with 7.5 m of throw and a
high angle, reverse fault with 2.5 m of throw on the north.
The structure of a high angle fault depends in part upon the lithology within which the fault occurs. Faults in Unit 2 of the
Cap Mountain exhibit very well defined slip surfaces with little or no evidence for an associated cataclasis gouge zone
even when the fault exhibits a pronounced corrugation. Dissolution processes are hypothesized to account for these
observations. Excellent examples are seen in the northern part of the road cut. In contrast, faults in the Lion Mountain
usually possess cataclasis zones of finite width in which the iron in the glauconite has been reconstituted into an ironoxide mineral, as evidence by the red coloration. Faults in the Lion Mountain appear to exhibit less of a corrugated
character except where the fault cuts the more lithified subunits (e.g. subunit D) (see Figure 9). All faults exhibit some
degree of segmentation, which is observed to be a fundamental attribute of faults. The segmentation is inferred to result in
part from linkage of early-formed segments. Units with beds of contrasting mechanical properties, such as the Welge and
Unit 1 of the Cap Mountain (Figure 11) exhibit segmentation associated with bedding. Faults in the Welge can be
markedly segmented as seen by en echelon segments in the lithified beds. Excellent examples can be seen in the cliff
across from the overlook parking lot.
Faults often form networks with members usually associated with two fault sets. Commonly a fault from one set
terminates against or initiates from a member of the other set. Cross cutting of one fault by another occurs, but the
occurrence of an abutment is more common. On occasion, one will see two conjugate faults joining at a common point or
emanating from a point. This "point" sometimes is associated with a bedding surface or other weak interface.