Access and Purpose

To access this hike area, take Toquerville exit (State Highway 17) off Interstate 15.  Head west approximately 200 feet to the T-intersection and turn left (south) on the gravel frontage road.  Proceed <¼ mile to the first right turn (an old mine road) and park near the turnoff.  Near this location, an igneous intrusion is present in a group of 3 at a right angle to the uplift of the Pine Valley Mountains (PVM).

Discussion

Some puzzling anomalies are present at the study area near the town of Toquerville, continuing on to the town of Virgin.  These anomalies are listed below.

 

1.  Permian limestone (Pk) juts up to the NW at a pronounced angle, forming a cliff face parallel to the PVM; the elevation is higher than surrounding rocks, and the structure lies next to an outcrop of young basalt (Pb).

 

2.  South of this Pk outcrop, and south and east of Toquerville and LaVerkin, the rocks adjacent to the Hurricane Fault are faulted in separate blocks and many different directions:  down to the north, some to the south, and some to the east.

 

3.  The Hurricane Fault has two splays (or expressions), both leading generally south.  Ash Creek and LaVerkin Creek each follow a splay in this southward trend.

 

4.  Basalt in the area shows a sharp drop down (south) to the valleys, and clearly indicates southward flow prior to faulting.

 

5.  The valleys and hills trend N-S near Toquerville and on into the PVM, indicating that the N-S lineaments are younger than the mountain-building intrusions.

 

6.  Even though it is clear that the N-S lineament is related to the Hurricane Fault, the N-S orientation is continuous whereas the fault redirects to the NNE, north of Pk.

 

7.  Significant extrusive material is present in the area (with vents obscured), but rotated blocks of rock west of the fault are not visible. Such rotated blocks (if present) may be obscured by basalt; however, vents are visible near LaVerkin and Honeymoon Canyon (north of  the AZ border, crossing the gravel airport road to the south). A dike of Pv intrusive rock runs roughly SE from the PVM, and may explain the anomalies present near the Pk that juts above the town of Toquerville.  Previously, I have conjectured that the unusual dip of the Pk (jutting up to the NW) results from interference of PV rock with the pre-existing Laramide dip of Pk.  If compared to salt domes (my previous projects), the PV rock should have been shoved upward as the intrusive rocks domed in a mushroom shape as they reached the 1-km depth in the Miocene.  It is easier to lift zero-shear-strength rocks (primarily sedimentary sandstone) than to split them, as has occurred at greater depths.  By this process, an intrusion will split its way upward, overcoming hoop stress,[2] until it is easier to expand outwardly.  This process is partly determined by Poisson’s ratio,[3] which is near 0.25, and provides the rocks with extra shear resistance when subjected to hoop stress of expanding magma.  Consequently, the mushroom shape (right turn, so to speak, of ascending magma,) occurs at a depth of about 1 km, as it originally rose.

 

However, the visible dike allowed the hot intrusion to work to the SE.  Therefore, the anomalous Pk cliff can be said to be "laid off" onto the shove created by the dike in the Miocene.  Large boulders of Miocene intrusive rock (Mi, quartz monzonite or "granite-like rock") are present along the trail.  Monzonite is an intrusive igneous rock, which is equally composed of Plagioclase (Na and Ca aluminum-silicates) and Orthoclase (K spar) with minor quartz.[4]  The term "quartz monzonite" indicates that the original classification (monzonite) is violated and the modifier (quartz) is necessary.
 

In these intrusive rocks, large grains of minerals are visible but not recognizable as crystals.  This texture indicates that the intrusions were close to the surface, almost becoming extrusions.  Extrusions would have no visible mineral grains because of rapid cooling.  The larger grains indicate intermediate cooling, because the material was emplaced below (but close) to the earth’s surface as it cooled.  The boulders are rounded and large, resulting from breakage in large blocks and rounding as they rolled downhill.

These signature rocks are keys to events occurring near the Hurricane Fault, and I would suggest that hikers memorize the process to aid future identification.  The presence of these signature rocks is evidence that no scarp was present in their path (nothing to prevent them from rolling directly to the spot where you found them).  Such is the case for the boulders of quartz monzonite found near the town of Virgin.  Their occurrence there indicates no Hurricane scarp was present at the time they were deposited just below the basalt (<1mybp) north of the town.  This evidence helps to date the time of the last uplift of the Pk rocks just east of Toquerville.

 

There are two dominant trends of possible shear:

a.      A trend of fractures and grain manifestation orienting SE-NW, which is generally more eroded, but which orients the local wash. This grain may have been formed by stressing of the young intrusion by the regional shear, before the rock was completely rigid (still warm from below); and

b.     A trend perpendicular to the one above, but this trend has a dip to the grainy layers, down to the north on the north side of the intrusion, and sometimes down to the south at other locations. These layered sheets do not have the fractures that are so prominent in the SE-NW direction.  I conclude that the layering is due to a protrusion from below, and not from sliding from the PVM main laccolith.

Illustrations

   
   
   

 

Future Study

Further investigation is warranted to:

 

a.  Determine the lineation of the intrusives:  N-S as the UGS map indicates, or NW-SE as my orthogonal projection infers;

 

b.  Determine whether the trend of the Wet Sandy Creek is parallel to the intrusive outliers;

 

c.  Measure linear fracture patterns in sedimentary rocks along the creek bank;

 

d.  Note any difference in grain size or character in the intrusive outliers, compared to the main boulders that have rolled down, as in the town of Leeds, which are a few miles west of the study area.  (Larger grains would indicate slower cooling, which should be the case for lower elevation dikes, compared to PV outcrops in the mountains proper);

 

e.  Determine if the intrusives show any indication of having slid down the mountain.