Monday, March 12, 2007

Geology Report with Fissures, cont'd

9. Fissures in Kaibab Limestone (Adjacent to Virgin town and Virgin River), 1/11/06

The Kaibab limestone at S29 & 30 T41S R12W has fissures some 200 meters from the Virgin Canyon, which at this location is over 100 meters deep. The fissures run parallel to the canyon in places, but some run at varying angles. At first glance they might look like sinkholes. The interesting features are:
a. The fractures trace N-S parallel to the Hurricane fault, but do not when there are perpendicular turns of the river.
b. The large fissures have N-S fractures in the blocks bordering them at slopes of up to 20 degrees from vertical, and they also have NW-SE fractures as well- some of these are 5 meters deep.
c. Gravity appears to have forced large blocks of limestone to rotate slowly toward the canyon and also has opened up the fissures wider at the surface of the ground near the Virgin. These blocks are not parallel to the fractures that cross them.
d. Sinkholes are usually rounded, as seen at the ground surface, but some of the fissures and fractures near the Virgin are at the top of mounds.
e. Sinkholes created by the dissolution of carbonate rocks (dolomite and limestone) under the surface are not linear in shape.
f. The Virgin River makes perpendicular turns, tracing either N-S or E-W in this Section 29, but further east it makes NW-SE or NE-SW channels.
g. The Kaibab limestone away from the river looks distorted and wrenched. As far as a half kilometers away from the canyon, the surface yields rectangular blocks of limestone at the surface which are separate from the others and which have soil between them.