H21: Confluence, Laverkin side: S23,26 T41S R13W (2/8/06)
Layman's Arena
This hike is an easy one- it allows limited parking on the cliff above the Laverkin Creek, at the end of Center Street (turn west at the traffic light in Laverkin town and park where it does not interfere with private driveways). The hike will be for a mile or so to the east into the Virgin gorge, where can be seen some sedimentary beds interspersed with Pleistocene basalts. Proceed down the hill at the fence, and you are now in the Tortoise Preserve- which is public lands. There is a good view of the Virgin and Laverkin, Ash Creek confluence from the top of the cliff, where you can orient yourself before trekking down into the gorge. The Confluence area of the town parks also may be accessed by driving to the end of 200W roadway, and hiking down from the Hurricane side of the river. This road is steep, and one may walk the banks of the Virgin in either direction on the Hurricane (Town) side of the river.
The Virgin canyon has cut into basalt flows from the nearby dormant volcanoes, and the cliffs are very shear and resistant. One can immediately see that a dominant basalt flow has filled an old channel of the old waterways- creating 30-50 meter sheer walls. Upon descending the roadway leading to the river, there are a few Mesozoic beds near an old abandoned power plant, which are colorful with their red and tan sandstones. Initially, down the gravel road, there are almost level sandstones, covered by a relatively young (< 1 mybp- million years before present) conglomerate. These will be studied in the technical report to show how these gravels tell a story about the historical progression of the river drainage.
At the base of the road, one can proceed east, to walk along the present Virgin or one can walk west to the present Laverkin, crossing to the Ash Creek when water depth is low. The three creeks combine- hence the name Confluence- and mix water from the north with the turbid waters of the Virgin (which has just incorporated hot spring waters from Pah Tempe). Old pathways to the power plant and irrigated farms along the creek banks can be used to walk east along the roaring Virgin. Mostly the riverbank will present basalt rubble, but one may have a lunch break at the old power plant at a manifestation of the Hurricane fault and vertical sedimentary beds beside it. Mesozoic sandstone makes a bulwark for the sometimes flooding river and protects the plant. Sit in the sun, and enjoy the flow of the slightly fragrant water (from sulfurous fumes of the middle-earth).
Geological assessment
Our immediate target is to find the oldest sandstone or conglomerates Pc, associated with the previous drainage of the Ash, Laverkin, & Virgin waterways, before the vulcanism commenced in this area. We have found that there was a SW drainage (as indicated by SW-wardly oriented gravels) near the town of Virgin, and below basalt flows at Coal Pits Creek, North Creek by the Kolob hiway, and at the edge of the west flow from Sullivan's Knoll (off Flora-Tech road). The few Pc remaining are protected by overlying basalts, which are stronger than sedimentary rocks, hence they are "Saved) for our analysis. These have been found at 3823 feet- east of the Hurricane scarp- and at 3120, 3200 and 3000 feet elevations- going westward. The outcrop at elevation of 3120, west of the Virgin bridge, would indicate that the latest uplift, in possibly three cycles, was approximately 700 feet (relative to Pc of 3823 on the east side).
The few times we have found the Pc which is highest in the stratigraphic column, and underneath the first basalt flows, the conglomerates have been significantly higher in elevation than the present Virgin River. This is expected, since erosion has proceeded about a foot/500 years, since the last major uplift cycle of the Hurricane cliffs and vulcanism commenced. For 1000 feet of erosion, as at North Creek valley north of Virgin Town, this would calculate a time change of 1/2 million years since the basalt there was emplaced. This is only approximate, but radioactive dating could produce an accurate age; we cannot measure it, but it might be in the literature. Using a million years, and 700 feet= 213,500 mm, yields an uplift rate of .2 mm/year. We need to improve the accuracy of this technique (this is a low approximation, since the age of the gravels at 3823 is probably closer to 1/3 m.y.), which can be done with accurate dating of the first basalt flows and more careful elevation measurements on both sides of the Hurricane scarp. After we improve the rate of uplift calculations, we can then try to analyze a previous uplift (there are at least 3 visible cycles in the scarp east of Hurricane, indicated by level terraces or benches above the city).
There is probably some cyclicity to the uplift- a surging of uplift, followed by a quiescent period, which allows the terraces to erode into the rock to the east. During this time soils would develop, creeks would develop new drainages, and weathering of scarp rocks would proceed. The determination of cyclicity can give us some idea of the heating or expansion mechanism of the Colorado Plateau at this western edge (or beginning of the transition zone, before the Basin and Range, B&R, commences). We will try to define a cycle versus an uplift- we can see three cycles in the limestone cliffs east of Laverkin; are they part of the same Pleistsocene uplift, or is there sufficient time between cycles for each to be called an uplift?
There have been at least two major uplifts in this area:
1. The Laramide uplift was brought about by the subduction of the Pacific plate underneath the whole West USA, creating a wedge to the NE (all the way to the Rocky Mountains).
2. After the Laramide orogeny subsided in the early Tertiary, a wrenching to the NW relieved the compressional stresses, creating extension (this re-orientation date is probably the same as that of the dogleg in the Hawaiian- Emporer Seamount chain);
3. In early Miocene time, 21 mybp, the Pine Valley laccolith created local uplift, raising the region at least as far south as the Virgin River;
4. Radioactive decay of U, Th. K and other isotopes caused a Thermal Expansion of the wedge underlying the Colorado Plateau; and finally,
5. Compaction (density increase) of the wedge created re-crystallization of the minerals to release heat of crystallization- causing expansion.
6. A complication to all this is the recent finding that the hemisphere is being compacted by shrinking of its equatorial bulge, as the earth slows (allowing centrifugal force to be reduced) and cools (due to a significant number of radioactive half lives of dominant isotopes having occurred). U, Th, and K all have half lives greater than a billion years, and at least three half lives have transpired for K40, indicating that a lot of heating is behind us (so to speak). The mechanical shrinking has resulted in the expression of large scale faulting to occur in N-S and E-W lineaments, and the shear from the resolution of the two creates NW-SE and orthogonal fractures and transforms (as in California).
On 2/8/06 we found that the several outcrops of Pc above the present Virgin (near the power plant) have strength appropriate to Pliocene, which is the similar situation downstream below the Sky Mountain golf course. This was found to be due to the cementation created by the influx of Pah Tempe waters- high in calcareous cementation. This contrasts with the low shear strength Pc found above the present Laverkin Creek, at the start of our trek, before descending the Virgin canyon. This high shear strength indicates that the conglomerates in the present Virgin canyon are influenced by Pah Tempe cementing agents and are therefore not the ones found above the Laverkin canyon. Further, the ones at elevation of 3190 feet in Virgin canyon (found on this latest hike) have an imbrication indicating a west or NW flow, not likely to be from the ancestral Laverkin creek. Another factor of interest to the Pc assessment is that of associated conformable sandstones. In the case of the south Virgin bank, the sand was above the Pc at 3120 feet, whereas at 3190 feet, further east above the Virgin on the north side, the sand was below Pc. This indicates that the river was meandering, with sand sometimes covering Pc and at other times causing cobbles to cover the sand. The meandering feature is important, since this gives a sense of the river flow. Any sandstone would indicate a slight gradient to the river flow, whereas cobblestones would tell that the current has picked up (as in a flood or storm). Overall, the conclusion is that the Virgin at the time of first vulcanism was already established as coming from the east (even though further west, below the Sky Mtn. Golf course the pre-basalt Virgin canyon was non-existent). Whether it was the dominant stream, compared to Laverkin or Ash Creeks, has not yet been determined. However, the fact that all creeks further to the west proceeded SW, in outcrops just above the Mesozoic sedimentary rocks, indicates that the Ash and Laverkin creeks were prominent since the Pine Valley Mountains became the highest feature in the area. On the west side of the Hurricane fault, the presence of PV intrusive rocks (granite-like) in Pc is to be expected, since they merely rolled down hill. However, whenever they occur in outcrops east of the fault, they relate that there was no scarp in their path. This feature was the case noticed for the gravels located just below the basalt outcrop near the North Creek, north of the town of Virgin. The basalt there was dated by others to be > 1 mybp age, hence the scarp of the Hurricane fault was not in the pathway to obstruct SE-wardly drainage at the million year age. Although there might have been a scarp further to the south, we will assume that this was not the case. This may be a poor assumption, since there is a local anomaly (shearing by rotation) indicated by opening of large fissures, underneath and near the Virgin River. The focus of the uplift of the present fault cycle may have been initiated near the present river; the hot spring at least was present- as indicated by unusual conglomerate cementation- and it flowed into the river at a time when the river was 700 feet higher than now. Obviously the spring is artesian, and has helped to create the large fissures still spreading east of the cliffs. We will attempt to determine whether the anomaly is created by unusual hydraulic pressure from a magma at great depth. Already, we have seen that there is excessive Na+ in the spring water, indicating that some other salt besides NaCl is responsible for the high mineralization. K is too low and Na+ is too high, indicating that if the weathering of Feldspars is the culprit (hot chemical dissolution of silicate rock), then the feldspars are likely plagioclase-albite, rather than orthoclase- K being the usual anomalous ion emerging in active faulting above geothermal anomalies. K travels easier as an ion, because of its smaller hydrated size, compared to Na+ which hydrates more so, (relate this to the case of heart patients, Na+ who must use K salt, rather than Na salt, because of the difficulty of water retention- large hydrated ionic size- with ordinary NaCl salt). The excessive sodium ion must pass through some intermediate stage and this might be trona, or some sodium salt combining with the excessive HCO3 from the carbonates in the shallow water. We will look at the minor constituents in the spring water, to see if there is a clue as to the original derivation of the hot water. Unusual temperature of the spring water would not be the culprit for creation of fissures nearby, since solubility of carbonates decreases as T increases, but the greater acidity would cause dissolution of the limestones. Increase of pressure (artesian) would also dissolve limestones and dolomites, so that the combined acid water and higher pressure would offset the effect of higher temperature, to create solution channels.
I visualize the sequence of events causing the Virgin River to exit the bluff west of the town of Virgin to be as follows:
1. The previous formations and topography dipped up to the west, creating a barrier to drainage to the west, when the land surface was much higher than now;
2. Unusual hot acid waters near the present Virgin crossing of the fault (shown by Pah Tempe spring) causes expansion and fissures to open west of the town of Virgin;
3. Acid water and artesian pressure create cavities in the subsurface in carbonates below the ground surface. These cavities align with the regional fractures (NW-SE and the orthogonal direction) to present weaknesses in the rock all the way from great depths to the old ground surface;
4. Precipitation, which is also slightly acidic, find the fractures and creates local gullies. These are then captured by the ancestral Virgin and allow the river to exit what now is a hill. Keep in mind that the ground surface was at 3823 feet and higher, when the river changed course;
5. The spreading of fissures and solution cavities continues and allows the river to erode its bed much faster than normal.
The above analysis is partially confirmed by an event, happening in 1985 and reported by Geologist Ben Everitt, when the present diversion dam and pipeline were installed in the Virgin bed east of the hot spring:
A. Within months, river water below the new dam entered new bedrock fissures and emerged via Pah Tempe springs, increasing spring flow- eventually cooling and diluting it. The extra saline & cooler water from the spring was noticed by those who had a pecuniary interest in the previous spring water (irrigators and wildlife monitors downstream);
B. The slug of river water acted as a piston under ground, driving the heavier salt water in front of it, and temporarily increased spring flow with higher salinity.
C. Eventually, the spring flow normalized to that which had preceded the slug event, and the original T, salinity, and turbidity returned.
This yields the following conclusions, relative to the uplift phenomenon:
a. The fissures were incipient in the river bed, but were exaggerated by man's activities of adding a head of water above the dam to cause static stresses to be augmented;
b. The above conclusion is fairly certain, since the river bed took the water immediately and allowed it to be moved underground toward the spring;
c. Fissures can be seen above the Virgin cliffs, as far away as 100-200 meters, and are rapidly spreading (as indicated by inability of soil to fill them as fast as they appear);
d. Although some fissures are parallel to the present river bed, and are undoubtedly due to gravity spalling, others are parallel to regional NW-SE and orthogonal stresses; and
e. There exists a local shearing (as evidenced by isolation of Kaibab blocks of limestone at the surface, which have been sheared in at least two directions), greater than the regional NW-SE and NE-SW fracturing, which is isolated within an area between Toquerville and Gould's wash, bounded by the Hurricane fault splays to the west.
These conclusions seem to be confirmed by the presence of anomalous folds, faults oriented reverse to that expected for normal faults dragging upward in a down-dropping environment, and unusual concentration of volcanic cones in the Hurricane- Laverkin area. Further, Pah Tempe seems to be the only anomalous spring along the Hurricane fault in this area- this area is anomalous, geologically, compared to the region.