Wednesday, April 25, 2007
Anomaly to SE, enlargement
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: