Wednesday, April 25, 2007
Oak Grove, PVM hike, cont'd
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: