Wednesday, April 25, 2007
Pine Valley Mountain View to SE
Oak Grove Earth Science Hike along the Wilderness Area Boundary (S8, 9 T40S R14W): Apr 10/07, PVM
To access this border of the Pine Valley Mountains, SE side, exit I-15 on the north side of the freeway at the town of Leeds, Utah, then proceed northward on the Silver Reef road for two miles to a fork leading northward toward the Oak Grove Campground (graveled after crossing the creek). Just short of 8 miles from I-15, take a right fork to the trailhead to Columbine Spring, skirting the Wilderness area (this is a 100 meter dead end road, ending at the trailhead). Hiking north, there is a Tee at which good views to the east, of sedimentary outcrops, may be accessed. Follow this trail to the lowest point where you can then access the contact of igneous and sedimentary rocks by walking uphill cross-country in a dry wash.
This area has intrusive rocks, Tip, in contact with older Tertiary and Mesozoic outcrops; expect the following scenario:
1. The Miocene 21mybp intrusion (Pine Valley Mountains) rose to a kilometer or less depth (below ground surface) in Tertiary times, and then it moved outwardly and formed the shape of a mushroom on the SE side of the melt. I expect this since this is the situation with many salt domes (which are also hot plastic-flow rocks). At about this depth, it is easier to lift the overburden than to vertically fracture it, and the melt moves laterally- forming a sill or dikes. This problem has been solved by engineering, and the depth of the change in direction depends upon Poisson’s ratio and for the sediments to have essentially no strength for shear (the Tertiary Claron Tc or underlying Cretaceous most likely fits this requirement). The lateral hoop stress is about twice the normal stress (S: lateral=μ x Vertical, overburden stress, ~ .5 x depth x 1.2 psi/foot x 2 for a hoop, whereas Sv is ~ 1 psi/ft) for an intrusion pushing either outward or upward), and the rising melt cannot more easily vertically fracture the overlying rocks- rather the movement proceeds laterally.
Note: Tc is divided into at least two vertical sections by geologic mappers, and the lower is the pink or red found on this hike; it has a total thickness no greater than 2300 feet (about 700 meters) near Cedar City, Utah. If the melt reached the upper Tc, it would have been no more than 1000 feet below ground surface in Miocene time.
2. The melt (quartz monzonite) was an intrusion- never reaching the surface of the ground. Nevertheless, there it is exposed in the Pine Valley Mountains (PVM). Since large crystals and grains are portrayed in the igneous rocks (requiring a long slow cooling period), they definitely were covered while cooling- otherwise the rock would be glassy or aphanitic, without large crystals. The fact that the ground mass in the rock has fine crystals surrounding larger grains indicates that the large grains formed at depth with the slow cooling; the large grains would have formed before the hot mass reached a shallow depth where more rapid cooling of finer crystals occurred. To become exposed, the overlying Tertiary and Cretaceous rocks must have been eroded later, exposing some kilometer thickness of igneous rock (as seen now- from about 7,000 at the sedimentary contact to 10,000 feet at the highest mountain). This has happened in the 21 my time since emplacement. Since the soft sediments eroded rapidly, compared to the igneous rock, there was exposed to the air the PVM and its surrounding sills and dikes.
3. The surface geological map shows some granite-like igneous rocks (monzonite) with Tc on three sides in the lands to the SE of PVM, near Anderson Junction at I-15. These rocks are all too low in the section (compared to their original appearance higher up the PVM); consequently they must have moved since their emplacement. This is shown by the appearance of older rocks, such as Jn, Navajo sandstone, in a fairly normal circumstance nearby. For the case where Tc borders the intrusive rocks at an abnormally low elevation, one of the following must have occurred:
a. A dike or sill occurred below the top of Tc (at less than one km depth, in Miocene times), and after erosion of Tcu (upper) some 3000 or more feet, the whole mass moved down the mountain by creep. Keep in mind that under the overhang of igneous sills there would have occurred a normal section of sedimentary rock which was by-passed by the upwardly-buoyed laccolith nearby (to the NW). When Engineer M. King Hubbard proposed this mechanism 50 years ago for large-scale thrusting, it was derided by geologists as “solving geological problems, such as thrusting and generation of overpressure, by lubrication”. Nevertheless, after the dynamics of earth movements were investigated using physics, it was found that this is the likely mechanism for the movement of large blocks of rock associated with Geopressure- particularly over rock with a large component of fine grains (clay or shale). The overlying block of earth slides over a temporary cushion of over-pressured shale, allowing easy movement; or
b. The block of igneous rock on the SE side of PVM, surrounded by Tc was higher in elevation originally, but has since dropped with normal faulting. I think this likelihood is low, since the nearby columns of igneous rock as far away as five miles have performed similarly. Most igneous masses on the SE side of PVM do not have bordering Tc, so they could either have been original dikes off the main laccolith or could have detached separately. This detachment mechanism has been investigated extensively in AZ, and the underlying rock is usually highly metamorphosed. I find none of this in this location, and the likelihood is again low; or
c. The dikes are still in their original position, at least for those not having bordering Tc or Cretaceous rocks; they would not have moved (for these outcrops), but could have occurred because of the previously investigated Weak Zone on the SE side of PVM. It is interesting that there is no obvious Tip on the west side of PVM, and that extrusion occurs on the north side. Further, the cursory look at the west side seems to have sediments down-dipping against PVM (the opposite of what you would expect for a compressive intrusion); hence the melt incorporated the silicates to yield the characteristic quartz monzonite and did not shove north or westward. Extrusion (basalts and other flows) on the north side indicates that extension occurred there.
d. The fact that the obvious anomalies occur on the SE side yields additional suspicion that PVM has shoved toward the SE- either causing the weak zone or exploiting it. The present tilt of Pk peak near the Toquerville spring at an unusual angle (almost 45 degrees- greater than the Virgin anticline nearby) further indicates that PVM intrusion has compressed the SE region and not the region NW of PVM. In other words, the rise of PVM was toward the SE- pulling away from the northwest, and shoving toward the SE. This allowed extrusion to occur on the NW side where extension was occurring, simultaneously with compression of sediments on the SE side.
Observations made in the field, bearing on the structural history of PVM:
There are many outcrops of Mesozoic rocks in the Oak Grove area which generally indicate the following:
a. Sedimentary beds abut the intrusion without showing lateral distortion. Some may be nearly one hundred meters wide in a single outcrop;
b. The beds, however, are tilted generally down toward the intrusion while running parallel to it (SW-NE). This may be interpreted in at least 2 ways:
I. The beds in large blocks have slid sufficiently and at such a large sliding angle (nearby intrusive rocks rise upwardly at about 60-70 degrees), that they rotated into the monzonite as the underlying ground surface angle to the SE lessened; or
II. The sedimentary column has not necessarily slid from its original position, but has shrunk by thermal contraction near the border of PVM, as the melt contracted with cooling. This would make the beds closer to the laccolith appear to drop with cooling, while the more distant beds remained at a constant datum. One would then ask “Why would the individual beds not have shrunk by the same amount as they originally expanded by laccolith heating?” One answer can be: Sedimentary rock may expand upon heating in an almost permanent manner, due to heat strengthening and replacement of the original calcitic for silicate cement, but the overall porosity decrease must be greater with the heating and cooling cycle (for there to be an overall reduction of stratigraphic column thickness- as is postulated). Hot fluids dissolve more silica than cold, and the acidity of the hot water has changed. The reduction in porosity must be greater than permanent fractional matrix (cementation or solid) increase, for the final stratigraphic column to drop after cooling. The strengthened beds might then become somewhat contracted while the entire stratigraphic column dropped, while the solid intrusion was cooling and shrinking (the monzonite would have no porosity in its original unfractured condition, and some of the original quartz in the sediments would have dissolved in the melt). An ancillary question would be: “What happened to the beds far away from the melt, where they never expanded in the first place?” This cannot be answered without measuring the distant bed thickness, and the appropriate outcrops are not available for measurement. They would have been uninfluenced by both heating and cooling (to remain high relative to the beds closer to PVM), for dipping to occur near the PVM. This entire explanation depends upon a geochemical rearrangement, for shortening of the stratigraphic column next to the melt to occur.
Reviewing, as the melt initially moved through the deepest part of Tc (by incorporation of the sedimentary rock) it forced the water contents of the rocks outwardly. This steam or hot water moved laterally through the surrounding porous rocks- reducing the porosity of the sediments by dissolution- and dumping the dissolved rock further away in the cooler rocks. Later, this increased distant mass remained relatively at its original elevation while the rock closer to the laccolith shrunk with cooling.
III. The original dip of the sedimentary column may be part of the present configuration, but the Mesozoic on the western side of the PVM indicates a similar dipping into the mountain, so that it is likely that we are looking at a general result of the intrusive process. The best conclusion from all of this is that sedimentary beds in contact with PVM indicate that there was no compression of the beds upward (of the North and West contacts) and that melting of sediments caused the incorporation of additional quartz into the melt (hence quartz monzonite), robbing the stratigraphic column of mass.
My inclination is to give option I the greatest likelihood, with the chance that we can tell the amount of sliding by the amount of dip toward the Laccolith. That is, the greater the dip angle toward the monzonite, the greater the distance that the sedimentary rock (or its associated dike-like rock) has slid down the mountain (because of the lesser slope of the ground surface further from the palisades). For sedimentary beds with dip angle essentially at zero, this would indicate no sliding- that is, the rock would have remained at its pre-laccolith orientation. This would be case for an original overhang of sill of the granite-like rock.
A comparison of the Tip outcrops standing alone on the SE side of PVM may yield some additional information about their incipience (ones having no surface contact with older beds of Claron Tc or sedimentary rocks). There are several of these dike-like igneous outcrops on the SE side of PVM, and a look at the grain and fracture patterns in these may be instructive.
The hiking group climbed cross-country to the contact of the Claron (pink fine-grained siltstone), collected samples of it and made photos of the contact zone. See these in the attachments.
While the Cretaceous sandstones below Tc are generally monotonous and flat and level (near the trail), they yield an observation about the possibility of detachment or sliding of the monzonite:
The sedimentary layers uphill from a dike-like deposit of monzonite (near Columbine spring and trail) on the kilometer scale in extent, are flat and level, undistorted- indicating that the dike could not have slid over them.
At the outcrop of Tc higher in the section, there is a major wash, which has many parallel fractures trending 150 degrees from north, in the adjacent monzonite laccolith. Further, above the fractures there is a saddle with scree- indicating a normal fault or active fractures. It appears that Tc has dropped down to the NE across the wash- all of these features exist on a NW-SE trend pointing to a saddle in the mesa near the town of Apple Valley. On this same trend there is a volcanic plug protruding from the skyline south of the hiway to Kanab (UT 59). All of this reinforces my projection that there is a major weakness extending from PVM toward the SE, through the hiway 9 switchbacks east of Laverkin toward the major fissures near the Virgin River.
This PVM and surrounding area should be statistically evaluated to confirm the following features:
1. Older (than Miocene) sedimentary beds abutting the laccolith have a dip into the igneous rock which depends upon the amount of sliding of the sedimentary column. Rock dropping and rotating will have the largest dip for the largest drop;
2. Tc, Claron, beds with no dip indicate no sliding, and nearby downhill igneous rock would be the location of an overhanging sill which has dropped into place (without distortion of the originally deeper sedimentary beds);
3. Sliding and dropping of Tc and associated monzonite occurs mainly on the SE side of the laccolith;
4. Extrusive rocks and undistorted sedimentary rocks are the normal circumstance for the NW side of PVM;
5. NW PVM is characterized by extension regionally, while SE of the mountains there is local compression (such as with the Pk peak north of Toquerville); and
6. Large-scale NW-SE fracturing and possibly lateral faulting has occurred since Miocene time in the region, affecting the topography all the way to the Virgin River.
This exercise using field analysis shows what can be done in the field using simple measurements and logic. There are 30 of these Earth Science hikes reported in the geohikes link shown on the right side of the Blog. Comments are invited from readers, to fortify or reject the various conclusions.
Attached are some photos, which show the general topography of the region near the Oak Grove Park, with its associated sedimentary and igneous rocks: