H16:  Hurricane, Utah, Volcanic Hill:

Radio Tower Hill (S27 41S 13W, 10/11/06)

 

Hike and Visible Features

 
The hike to Radio Tower hill (volcano) will aid a better understanding of the dynamics of the edge of the Colorado Plateau, particularly showing the larger trends and movements near the City of Hurricane, Utah.
 
 To access this volcanic crater by foot, drive onto 200 West Street near the Hurricane City Park, and turn west (left) at the northern park boundary (750 North Street).  Proceed a short distance to the end of the paved road.  At this location, a fenced corner will be present, which accesses an animal trail leading west toward an old cattle road.  Proceed through the fence northwest (NW) and upward to the crater rim overlooking the Virgin River.
 
 From this vantage point atop the crater rim, several large important features are easily visible and are characteristic of the Western United States (US).  These features include the:
 
1.  Volcanoes—at the edge of the Colorado Plateau (CP) to the east;
2.  Hurricane Fault—a north-south (N-S) normal fault, typical of those to the west in the Basin and Range (B&R) Province (but not typical as to incipience);
3.  Virgin Anticline—a northeast-southwest (NE-SW)-trending compressional buckle (Laramide event and later, Cretaceous-Oligocene) in the earth’s crust at least 30 km long;
4.  Pine Valley (PV) Mountains—an intrusion (granite-like melt) that occurred during the Miocene Epoch at 21 million years before present (mybp), which is almost parallel to the Virgin Anticline;
5.  Toquerville Peak, labeled Pk Peak by me—an outcrop of Permian Kaibab limestone that can be used to indicate direction because the northwest (NW) face, which is near one splay of the Hurricane Fault, is oriented NE-SW (also somewhat parallel to the anticlinal trend); and
6.  Sharp bends in the Virgin River—indicating the presence of several competing fracture systems prevalent in the region.  Throughout the Western US, most parallel fractures trend northwest-southeast (NW-SE), and a divergence from the norm indicates a local geologic anomaly.  Volcanoes, intrusions, local uplift, local rebound, sinkholes, and shear zones are anomalies- frequently identified by divergences from the regional fracture trends.
 
Other features visible from the crater rim include the:
 
A.  Faulting present in the backyard of a Laverkin resident adjacent to Laverkin Creek,
B.  N-S hogback and monocline which are causing the faulting, and
C.  Weak (shear?) zone trending SE-NW similar to a local intrusion near the Wet Sandy Creek in the PV range, proceeding to a major spring near Toquerville, to parallel faulting in the quarry east of the town of Laverkin, and to fissures identified on the north side of the Virgin River, west of the town of Virgin.
 
We will investigate all of these features during the 2006-2007 hiking season to confirm what I have posted on my website, and to expand our knowledge and understanding of the mechanical and geothermal dynamics of the region.
 

Discussion

 
Section 27 has a basalt extrusion (100 thousand, or 100K, years old) with a slight crater still remaining; this extrusion may be called Radio Tower (RT) hill or East Crater.  The objective of the hike was to overlook the Virgin River, to which more recent movements present shear cliffs of basalt and sedimentary rock. A great sweep of Laramide distortions, lava flows, and later faulting can be evaluated from a single vantage point.
 
At least two flows (of three in the area) of Quaternary basalts (Qb) are visible and indicate that they (1) flowed out onto a sedimentary surface, (2) flowed into river canyons, and (3) were cut by the latest Virgin River erosion (forming the most recent canyon, which is at variance with the older canyons).  On the north side of the Virgin River, an outcrop of Pleistocene (Ps) sediments (rubble) underlies a volcanic tuff.  Both deposits are conformable with each other, but not with the underlying Mesozoic and overlying rock.  These Ps sediments are now at a 20-degree unconformity with the underlying sedimentary rock (Mesozoic sandstone).  The deposits of tuff and basalt were extruded by the RT volcano (1) on the south side of the river, and (2) on the opposite side of RT volcano (dipping in the opposite direction).  This opposite directional dip is visible below Sky Mountain Golf Course on the south side of the Virgin River.  The tuff and basalt are tentatively dated as Quaternary volcanics (Qv) and are evidence that the Virgin River was not present at either location at the time of deposition.
 
An uplift of the Mesozoic sediments is also present to the NE, which indicates a monoclinal trend northward toward the PV Mountains.  This uplift would not be a Laramide (100–50 mybp) structure, although the incipience of the anticline several miles to the north might have been a Laramide event.
 
It appears that the volcanism (1) created compressive stresses, which extruded basalt onto the ground surface, and (2) later incurred extensional stresses as the magma chamber cooled and shrunk.  This shrinkage is most pronounced along the Hurricane Fault (where heat could be readily released), and it caused the greatest downdrop of the ground surface.  The downdrop appears to be a monocline, with a hogback at its western side, trending N-S parallel to the Hurricane Fault. The monocline dips down to the east, and the rate of subsidence along the Hurricane Fault can be calculated using the dip angle to determine the total drop along the face.  The date of the volcanism is about 100K years ago, which yields a drop of about 1 km/100K years or 10 mm/year.  This drop rate is much larger than the drop rate along the Hurricane fault, which is caused by regional movements.
 
Several faults and fractures are visible in the cliff walls and on the surface Jurassic Navajo (Jn) sandstone on the north side of the Virgin River. These features have a general NW-SE trend, which is the normal strain pattern for the Western US over the preceding Pliocene Epoch and earlier.  The N-S patterns associated with the recent strain and the Hurricane Fault are visible only along the monocline, but otherwise are not as apparent in our subject RT location.  One mile west and beyond, river bed directions do have this N-S orientation.  In the RT location, however, the river orients either NW-SE or NE-SW, indicating a transition zone for the stress pattern just west of the Hurricane Fault. This zone probably resuled from the rising volcanism; some young-stream patterns on the east side of the RT crater orient N-S, indicating the age of the new stress system.  In my opinion, this stress change has occurred in the last 2–3 million years (m.y.), but might be offset by local anomalies such as volcanism.
 
It is generally believed that (1) the main Hurricane Fault trace is located one mile east of the RT volcano, as evidenced by vertical beds at the Laverkin Power plant on the Virgin River; and (2) the fault extends eastward in splays near the Pah Tempe Hot Springs (a 4000-feet separation).  The RT volcano is probably associated with the uplift of the Colorado Plateau at the Hurricane Fault, but statistics of volcano location depend more upon fracture systems than upon the plane of the fault; and at the ground surface, fracture systems might appear random.
 

Additional Study

 
The sweeping views visible atop the Hurricane volcanoes indicate the presence of several regional trends that must be considered in any additional study of the area near the Hurricane Fault.  These regional trends include the following:
 
1.  The PV Mountains, the Virgin Anticline, and the face of the Toquerville (Pk) peak all trend NE-SW, and this is probably caused by the influence of Precambrian weaknesses trending NE-SW.   This NE-SW trend of the anticline, which is almost unique for Colorado Plateau structures, suggests that the anticline that formed from Laramide compressive forces took the path of pre-existing weaknesses;
2.  The monocline and hogback, visible across the Virgin River to the northeast, trend N-S- parallel to the general trend of the Hurricane Fault (similarly to Ash and Laverkin creeks); and
3.  The low elevation zone just east of the town of Laverkin is not situated in a significant river bed, and is just a short drainage wash, but it contains several parallel faults.  This faulting, in conjunction with the tilting of sedimentary beds northward and southward, strongly suggests that a weakness has crossed the older Hurricane fault scarp.  This hypothesis will be subsequently investigated because other anomalies are present on the NW-SE trend, such as the Wet Sandy Creek, some parallel faults near the town of Toquerville, the Toquerville major spring, and large fissures near the north side of the Virgin River west of the town of Virgin.
 
The following photos illustrate these major trends:


   
   


The hike of 18 October 06 will visit the Laverkin quarry to investigate the weak zone trending NW-SE and its influence on local topography east of the town of Laverkin.  Several parallel faults have caused multiple slumps, and we will attempt to determine the time interval.  In addition, we will investigate the presence of possible NW-SE trends versus the N-S trend of the Hurricane Fault just below Hurricane Mesa.