Thursday, November 29, 2007
Canyon Orientation is controlled by N-S Fracturing and Faulting in parts of the path
Comparison of Hf Scarp canyons to Laverkin Quarry Anomaly
One way of determining the anomalous character of the canyons leading eastward from Laverkin town is to compare them with other locations on the Hurricane fault, where there is only a simple erosional canyon (or wash). Almost all of Hf scarp consists of a massive wall, except near Laverkin, showing the upthrown older sedimentary rocks to the east, with the younger downthrown column buried beneath your feet. Only a wash leading from the Colorado Plateau to the east allows inspection of the rock, and normally there is no access by road or a simple ramp. Hiway 9 towards Zion NP is one of the rare exceptions, where switchbacks allow driving to the CP above.
Two of the canyons leading into CP are located within the town of Hurricane. Hikers may enter via Gould’s Wash (a significant drainage) and at the town airport- named Frog Hollow, a normally dry wash of only ½ km length. Access of these may be made via 700 West in Hurricane, where a left turn is made at 400 South to 160 East. Park at 400 S, since there is no parking on 160 E, for Gould’s Wash. To access the minor airport canyon, continue on 700 W to the 2300 South road at the airport entrance, turning left instead into a new housing area, and then right onto the last paved street. The Canyon mouth may be seen from the road, and the end of the pavement allows parking to walk the short distance to the canyon mouth.
Gould's Wash passes diagonally through Hurricane to empty into the Virgin River
Frog Hollow (below)exits at the local Airport, and rarely flows
Gould's Wash passes diagonally through Hurricane to empty into the Virgin River
Anomalous Behavior of Hf at Laverkin
Normally, Hf scarp dips up to the west- which prevents drainage from the east, but in both of the Hurricane canyons, slumping and faulting has occurred allowing flow during the rainy season. Hiking into the canyons over 200 meters shows that the rim has been disturbed by fractures and faulting- which allowed the drainage to capture the weaknesses of the rock and exit. This is not normally the case, and only a few streams or washes exit CP for the 50 km of scarp investigated.
Since slumping and faulting also have caused the anomaly at Laverkin Quarry, what is different about these Hurricane canyons to maintain the sharp scarp, and to prevent large erosion of the canyon mouth? Geologists mapping the stratigraphy would answer this simply, by saying that there are parallel faults for the quarry, while the stratigraphic change is entirely in competent limestone for the canyon mouths. This is insufficient for an understanding however, since something deep in the crust has created the multiple faulting- allowing the Mesozoic to slump down to the west for the quarry location. Furthermore, since there are indications from slickensides that there was lateral faulting, something is wrenching the CP at the quarry, but not at the limestone canyons. I have proposed that there is rotation of the CP at the quarry, and this is shown on both sides of a graben there- both sides showing that CP moved counter-clockwise relative to the Basin and Range, B&R.
Large active (unfilled by soil) Fissures exist on the north side of the Virgin, but not the south
After hiking in both canyons a few hundred meters, N-S fractures can be readily seen to have influenced the drainage from the east. Since the CP is to the east, this means that orthogonal (right-angle to the main openings) fractures are accessed also, for the drainage to proceed. This is easily seen for the Virgin River exiting between the towns of Virgin and Laverkin. In this case, the fracture pattern has changed from NW-SE to that of N-S (as with Hf) within the last two million years, 2 mybp, and the Virgin River found these new and old openings and used them to zig-zag westward. This may be seen by looking at the Google Earth geographic map below in an earlier Blog- focusing on the river path and its sharp turns.
Still again, What’s the difference?
The Virgin River shows that there are both NW-SE and N-S fractures operating on the topography, yielding openings for the river to work westward. There is the possibility that the older NW-SE fractures have extended from the large fissures seen in the ground surface above and to the west of the Virgin all the way to the Laverkin graben and quarry.
Large open unfilled Fissures exist near the north rim of Virgin River- some Spalling
Contrary to that in the limestone near the canyons to the south, the fractures near the north side of the Virgin are wide, obviously extending, and deepening (some are 2-3 meters deep, with hardly any soil in them). Some of this can be said to be due to gravity slumping into the nearby Virgin, but others are not parallel to the river trace, but at large angles to it. Most orient towards the splay of Hf at Black Ridge to the NW, which can be seen to align with them.
Another feature of the quarry arroyos is that they seem to have been mostly abandoned, after having deposited both boulders and fine sediments separately, compared to the Hurricane canyons. This hints that the lip up to the west of the CP near the scarp is a new feature, causing the streams which once were important to be abandoned now. Should the last rise of CP noticed at Hf have happened within the last 2 million years, this would have caused the up dip-to-the-west lip of CP to cut off most flow where erosion could not keep up with the dipping up to the west of the rim. In the case of the Hurricane canyons, there is no obvious dip-up-the-west, having been replaced by slumping and faulting- allowing the young streams to more easily exit.
CP, the Colorado Plateau, is bare, compared tp the green fields of Hurricane Valley
Why do the large fissures only occur on the north side of the Virgin River? Ben Everitt has proposed that there are evaporite beds (gypsum, in addition to soft limestone) below the Kaibab, allowing the river water to dissolve out caverns as it moved deeper into the earth. One of these sinkholes caused water to drop into the river bed when the new pipeline was installed (silt-free water would penetrate fractures more easily, after a dam was installed upstream). The weakness of this case is illustrated by the fact that the sinkhole taking the water was on the south side of the river, therefore the dissolution should have been strong on that side also. It is my opinion that both widening fractures and sinkholes were present before the river reached this deep elevation, and that a geological anomaly was doing its work before the river aggravated the erosion. The Virgin at the town of Virgin has no canyon, and has to eat through a 200 meter rise to exit the CP at the town of Laverkin. This rise must have occurred as the river was eroding rapidly, and fractures allowed for quick erosion by the river water.
Again, why does the Anomaly stop at the Virgin River?
Looking to the SE, a saddle occurs in the Mesa some 5 miles away, so that the anomaly continues, but dying out as indicated by the lesser erosion in that direction. The NW-SE fractures pattern is ubiquitous over the Colorado plateau, however, and does not terminate at the boundary of CP and B&R. I believe that the newer N-S fracture system is effective dominantly near the CP edge continuing on into the B&R, and is particularly noticeable near the normal faulting occurring west of CP (for example, in extensional basins across Nevada). The locations where I have investigated this occurrence are at Verde Valley (also in the transition zone), Coal Pits stream, and at Moreno Valley on the east side of CP.
Fractures above the compass (photo below) determine erosional path of Gould's Wash
N-S faint fractures are noted to control Stream Erosion and Orientation through the Hurricane Scarp