H10: Virgin Power Plant S26 T41S, R13W 10/26/05

 Proceeding from 200 West, to the most northerly part of this road in Hurricane, in an undeveloped city park, a gravel road leads to the Virgin in NE/4 S26 T41S, R13W. This road proceeds to a section of the river, at a power plant (north side of River), where the fractures controlling the river course change abruptly to the east, trending 30-210 from north, with 120-300 from N. orthogonals. The controlling fractures to the west are not as obvious, and display some meanders- east of a prehistoric lake caused by damming of the Virgin by a volcanic flow near Quail Creek. The power plant sits beside a vertical outcrop of Mesozoic sandstone which strikes northeast (almost parallel to the river courses to the east.

 The NE-SE strike labeled Mspp (power plant, at Mesozoic) outcrops on both of the river, vertical on the north side of the river, but dipping down 60 degrees to the NW on the south side. Since this is the point of fracture orientation change, it must be the locus of the original Hurricane fault whose scarp now lies some 4000 feet to the east (Pah Tempe hot spring). Should the scarp (in rock now eroded) have “walked” at a rate of 600 feet/foot, the last initiation of the uplift would have started 2.4 m.y.ago (late Pliocene to early Pleistocene time). Since fractures must have preceded the manifestation of a scarp, this yields an upper limit for the most recent uplift of the Colorado Plateau.

 The steeply-dipping Mspp rock south of the Virgin is overlain by an almost level conglomerate, whose base is flat- indicating a near-level fossil ground surface which has been faulted or tilted upward on its west exposure, and which has gravels at the base and sandstone at the top. The overlying basalt does not indicate a tilt, so was laid down post faulting (or tilting). The boulders in the Pc are as large as .5 meters size, indicating a strong current), but the overlying weakly cemented foot thick sandstone has no cobbles.

 While the orientation of the NE-SW trending vertical beds is not arranged parallel to the present average N-S Hurricane fault, it is similar in appearance to that at the Honeymoon Canyon, near the AZ border. There the scarp is seen to “walk” some 1300 feet from the vertical beds of Mesozoic from the vertical beds to the west. Using the 600 years/foot noticed in Permian sediments in the Mogollon Canyon, this calculates an age of .8 m.y. since latest initiation of the uplift. The present Hurricane scarp is expected to always exist east of the original Hurricane fault. There is maximum spalling and erosion near the present Virgin River, hence the present 4000 feet separation of fault and scarp is likely excessive (proceeding to walk at a rate <600 feet/year). 200 feet/year would yield a time of last uplift of 800,000 years- similar to that at Honeymoon Canyon. The three canyons investigated indicate that “walking” occurs faster whenever creek flow (erosion) is greater.

 The tilting or faulting of Pc on its west end near the power plant doesn’t fit any known stress orientation, but it is possible that the rise of the magma from Pleistocene volcanism could have initiated it (dictated from dikes nearby). The nearest cone lies just 2 km to the west, and several dike-like linears lie near the south side of the Virgin. These supposed dikes may be stabbings of basalt sheared and down dropped by simple gravity along vertical cliffs (some of which may be seen now, nearby).

 This geological presentation near the river is extremely complex, as observed by the following features which are nearby:

Conglomerates and Plateau Uplift

The Tertiary contains very little information in our area of the Transition Zone west of the Colorado Plateau; Only extrusives (volcanic ash-.2-2mm, lapilli- 2-64 mm, and bombs and basalt flows) and sediments such as sand and conglomerates are left for our analysis. These generally sit directly on top of the Mesozoic, showing a large erosional gap (of at least 100 m.y, called an unconformity).

 Consequently, to understand the time sequence of the uplift of the C.P, the basalts have to be radioactively dated and the relative sequence of the conglomerates must be dated in relation to the extrusives and intrusives- which cover them.

 We know that the Pine Valley Mountains have a date of 21 m.y. since intrusion. Since they were originally covered and now are exposed- some 2 km above the surrounding landscape- this represents an erosion of 3 km or so. The land has been continually eroding during this time, except for the extrusives and conglomerates formed in response to the damming of the landscape or with other obstructions to the various river flows. Sometimes thin layers of sand have covered the Mesozoic (as with soil cover), and these are rarely preserved, but these will tell a tale about the relative level of the landscape. Incidentally, the cover of intrusives (or laccoliths) may be estimated in the case of those shaped like a mushroom, since it is known that at 1 km depth, the force of uplift is the same as splitting vertically the rock (for rock which has zero shear strength), so that at 1 km, the melt takes a right turn (horizontally) rather than to continue to split the rock vertically. This assumes that Poisson’s ratio is ¼, and that the overburden is 1 psi/foot- at 1 km, the stress is 3300 pounds and the rock splits vertically at 3300 pounds also, so that the melt proceeds just as easily horizontally along bedding as it does vertically along fractures.

 The cliffs along the Hurricane Fault add a bit of information about relative dating- there was an ancestral scarp of about 50 meters which had been eroded significantly before the last scarp was created. The last scarp is more shear (less rounded off) than the previous one, and is larger in elevation difference, from top to bottom. The roundness of the earlier one suggests that there was sufficient time for the land to the west of the fault to become almost peneplaned (leveled off), before the final shear scarp was created.

 Putting these three features- basalts, scarps, and conglomerates- into a sequence allows us to date, relatively, the past uplift of the Colorado Plateau:

1. After the Pine Valley erosion was significant the land was sloping away from the mountains and was almost flat in large areas;
2. Thin sands were left in an almost constant thickness and elevation, in places where the Virgin River has exposed them (showing that the landscape was flat where the Virgin Canyon below the Sky Mtn. Golf course now exists);
3. Conglomerates moved down from the P.V. perpendicular to the mountain range, and formed valleys filled with clasts which can be identified by the granite-like rocks from the intrusion. When these have no basalts in them, this indicates a date of Pre-basalt time;
4. Vulcanism put impediments in the path of the previous rivers (such as the La Verkin and Ash Creeks), which tended to flow N-S or NE-SW along the earlier Hurricane scarp;
5. Lakes and soils were deposited behind these volcanic dams temporarily, causing the good soil to be developed to the south of Hurricane and towards Sand Mountain;
6. The basalts were subject to fractures by the new active stresses of uplift, and they were rapidly worn down to the west, with the increased gradients by elevation changes, creating the present west-ward flow of the Virgin;
7. All streams were either diverted or captured by the now dominant Virgin River, and erosion began to bring the previous soils and sands towards the new river.;

 All of these statements will be tested with field evidence; all conglomerates Qc or Pc (quaternary, or more vague Pliocene- Pleistocene), must be placed in a relative arrangement, with the intervening basalts- which can be dated- to make a sequence of events in measurable time.