Monday, December 10, 2007
Rock Canyon, AZ (Arizona Strip)
The Canyon Entrance is simpler at Rock Canyon- Flexure-Created Intersecting Fractures?
One Way of finding how the Hurricane fault Hf, originates is to look at canyons which cut it, so that it may be viewed in three dimensions.
We have found that the Laverkin Quarry introduces a double anomaly- there is the Hf in several splays (parallel presentations), simultaneously showing interference with Hf by grabens, slickensides, and beds dipping down in two different directions- to the west and to the north. This is an additional geological anomaly, since mostly the stratigraphy dips up to the west for local manifestations of Hf. This feature is near Pleistocene vulcanism, which might help explain the additional anomaly, but more likely both vulcanism and Hf are caused by the same phenomenon. The basalt ascends the near-vertical fractures opened by whatever pushes up the highlands to the east. It is instructive to look at canyons which represent the normal Hf- without the added complexity of the second anomaly (it is difficult to climb Hf for most of its scarp, but at Laverkin one may drive up the scarp face because of the gradually sloping multi-faulted terrain & switchbacks which allow road-building to the Colorado Plateau, CP above). We have several simple canyons cutting Hf nearby, and Rock Canyon is one just south of the AZ border. We will assume that it is more easily understood. This canyon may be approached most easily from Hurricane town, taking the airport road south, crossing the border of AZ, and proceeding south another two miles (avoiding roads to the right which connect with roads from St. George), and turning west then south again where the obvious arroyo crossing is reached. This same unmarked road eventually connects with Black Rock Canyon road- a deteriorating road towards Mount Trumbull along the Hurricane Cliffs.
(700 W from Hurricane, UT, south to AZ, just past border, four miles past the diagonal road in UT- west to connect with BLM 59), S1, 2, 11, and 12 T41N R10W, AZ:
Rock canyon is a tributary of Fort Pierce Creek, and becomes Short Creek on the rim to the east above.
This canyon has Paleozoic Kaibab limestone, Pk, at the top and Toroweep, Pt, lower in elevation at the Hurricane cliffs. The dip of Pk is up to the west, at the present cliffs, but the red beds far to the west dip down to the west. Hike from the intersection of gravel road and arroyo, cross-country toward the canyon mouth- about one mile east.
The Kaibab and Toroweap Formations, occurring in the Grand Canyon, or upper Permian (Paleozoic) loom ahead of the canyon opening
The contact of Paleozoic vs. Mesozoic near Hf is covered with rubble over a 1/2 km zone E-W, so that the original fault plane cannot be seen. In this interval, some petrified wood can be spotted, so that one is certain that Mesozoic debris is at the ground surface, west of present cliffs.
Some small normal faults, falling down to the west, can be measured in the north wall of the canyon at the mouth in Pt or the underlying Queantoweap, Pq, with no more than 2 meters displacement.
Several small Normal Faults are easily measured at the Entrance of Rock Canyon
The fault planes are essentially vertical in all cases, parallel to N-S fracture surfaces in the creek bottom. Incidentally, N-S fractures in the creek bed cut older NW-SE oriented fractures and those at other angles, displaying horizontal displacement (shearing) of the older fractures. This demonstrates that the N-S shearing is younger, as would be expected for the N-S oriented Hurricane fault (and that some shear component is present, for the dominant normal faulting).
The conglomerates in the canyon walls appear to be young Pleistocene, Plc. The cementation is weak, and the boulders are mostly limestone. This Plc occurs in cross canyons west of the main cliffs, indicating very young N-S faults and associated washes.
Hike with Ben Everitt and Don Scholten, 12/5/07, eastward from the main gravel-Black Rock Canyon Road to the Rock Canyon Mouth:
Again, the fault trace is obscured by an outwash plain and alluvial fans, but splays on the east side of the main Hf can be measured as small displacement normal faults in the Pt or underlying Queantoweap, Pq (equivalent to Permian redbeds, or Supai).
Photo of Normal fault- buildup to Hf to the east
The North to South fractures and small faulting occur in a narrow band at the entrance to the canyon, but the main fault cannot be seen
Abrupt Terminations of westward-oriented outwash deposits may be seen in several outcrops to the north and south of the canyon, hinting that the youngest Hf is well west of the canyon mouth (but is obscured, except in one small outcrop of Pq at an abrupt outwash cutoff to the north). The 4 outwash terminations north and south of Rock Canyon do not align, and this can be expected if there is a discontinuity in the trend of Hf at the present scarp- canyon mouth. There seems to be a slight hinge across the Hf canyon, and this may have been the impetus for allowing Short Creek to exit at this weakness. An outstanding question concerning this feature is the presence of a few streams crossing Hf against the present topography (including this one- cutting through a hill), allowing creeks to exit across the Hf scarp when it dips up to the west. This present rise constitutes a barrier to stream flow, unless the water proceeded subsurface until it emerged as springs- which gradually increased in flow until a canyon was developed above them.
The hinge in the subsurface, allowing a flexure for fracture development, is rare near Ηf, and it has been investigated at the following locations: Laverkin quarry, Virgin River mouth, and Ash Creek. It is suspected that the exit of streams is dependent upon this geological anomaly- that hinges create subsurface fracture systems and this allows underground water to exit CP, when otherwise it would be blocked by a topographical barrier. With time, this subsurface flow creates channels (caves) and eventually creates canyons through the barrier. Laverkin quarry is the most prominent of these anomalies, while Rock Canyon is next- allowing the Short Creek to exit (but in a younger period of development).What needs to be confirmed is whether the NW-SE fracture systems were most necessary for this stream orientation (since the N-S fractures and their orthogonals do not show up in stream patterns (except for Rock Canyon). The Virgin River seems to follow NW-SE orientations and their orthogonals, allowing the river trace to zig-zag across the CP. Recall that I am testing the projection that the N-S fractures are younger than the NW-SE ones, and that they would have dominated drainage in the times since 2 mybp (Pleistocene).
The next projection is that these geological anomalies or weaknesses are accompanied by vulcanism- which allowed Mantle basalt to exit through the new fractures systems N-S and its orthogonal E-W weaknesses. This has happened near the Laverkin Quarry and the Ash Creek- Toquerville locations. This happens also at the Honeymoon Canyon, but it is not apparent at Rock Creek- which is a larger canyon carrying more water than Honeymoon. There is a vent SW 4 miles distant from the Rock Canyon mouth, but this seems too far for the magma to have been influenced by the suspected hinge. That Rock Canyon was created by a hinge is reinforced by the fact that Short Creek runs E-W, which would be on an orthogonal (perpendicular) to Hf fracturing.
We will proceed in this investigation, trying to determine whether:
1. Hf is initiated by a new stress system, which orients N-S, superseding the older NW-SE fracture system noticed over western outcrops from the CP into the B&R;
2. The new N-S system would be a normal faulting- dominated one, as contrasted with the older NW-SE shear stresses (from Pre-Pleistocene times);
3. Accompanying this new extensional system of normal faults would be vulcanism coming all the way from the Mantle. This would happen if the system is being initiated by equatorial bulge shrinkage, as the Earth slows (reducing the centrifugal force creating the initial bulge); and
4. A complication to all this conjecture about global shrinking is that of the type of stresses created by the reduction of centrifugal force as the Earth slows. The first result should be that of compression vertically, as the Crust shrinks vertically. Blocks of crust should move downward, as centrifugal force is replaced by gravitational attraction, and the relative movement would be determined by isostatic adjustment. Should blocks of crust be out of equilibrium with regional stresses (due to heating, subduction, erosion-rebound, or other slow stress-changing processes) this instability could be corrected simultaneously with the crustal shrinking. This would be similar to slinging a vial of mercury side-by-side with one of lower weight plastic hot rock altogether in a circular movement at the end of a string. As the circular swinging motion is reduced, the mercury would tend to drop back toward the center of swinging more than the lesser density rock. This would result in vertical shear between the two different materials, showing up as a normal fault. In a pure sense, there should be no lateral shear, but local density changes due to variations of stratigraphy could create shear. These effects should be minor and local- not regional.
This hike illustrates Hf for its simpler presentation, and no conclusions will be reached as to its incipience. Flexure of the crust, vulcanism correlation, updip-to-west stratigraphy, and broaching of Hf scarp by streams will be further cataloged.