Monday, March 12, 2007

Slickensides and Grabens

Although the Hurricane scarp is well-developed near the town of Hurricane and further south, exhibiting sheer walls which cannot be climbed by automobiles, this is not the case east of the town of Laverkin. In this area, Hiway 9 easily climbs the faulted area, on the way to Zion Park, and allows hikers to examine the differing geology which exhibits this gradual ascent. And although the rim generally has sedimentary beds which dip up to the west, here the opposite occurs. These beds are ascended via switchbacks, which occur near or cross a graben and monoclines. A Graben, which is the German word for grave or ditch, is a normal-faulted drop of a block of rock which occurs whenever the earth is pulled apart (extension), allowing the graben block to fall into the earth along steeply-dipping slices in the earth. In this case the dip is about 70-80 degrees from the horizontal, making a mirror image on either side of the dropping block. For this case, the angle of drop perpendicular to the block can be measured by noting the striations made by the falling block (parallel scrape marks), as it drops into a pre-existing fracture in the earth. Further, the dropping is sufficiently slow, so that water enters the sliding plane- leaving a trail of hard shining surfaces of silicate coating in an otherwise carbonate rock (CaCO3 limestone). The silicate is brought in by warm water and precipitated in the cooler rock.
See these features by studying the photos shown further down in the manuscript, or by clicking on the links shown on the right side of this Blog (as titles or as Items in the Archives).

The graben can be seen to originate just uphill of hiway 9 on the south side, and it becomes deeper in vertical extent as it proceeds to the north (it tilts downward, underneath the hiway). This shows that the graben is a local feature, and not one connected with the fault mechanism in a more general sense (that is, it is created locally; it is only noted to occur within a kilometer distance). Further to the north, the graben is truncated by the appearance of a monocline. At this point, the graben which occurred in Pk at the surface, Permian Kaibab limestone, gives way to a monocline which outcrops entirely in the Mesozoic. Obviously, there is a crossing of the otherwise N-S Hurricane fault by an additional stress feature to create the abutment. It is conjectured that there was a previous NW-SE fracture system, which has been crossed by the N-S Hurricane fault. The NW-SE feature is believed to have originated in Miocene times (Pine Valley Mountains- which is incised by the fracture- originated at 21 mybp, as an intrusion dated by radioactivity); the Hurricane fault was rejuvenated in Pliocene to present time. This NW-SE feature shows up well in the nearby PVM, as a fracture system which has allowed both the headwaters of the Santa Clara and the Wet Sandy Creeks to align along a common NW-SE axis (even though the two creeks are on opposite sides of the mountain chain). The anomaly also expresses itself in a large spring at Toquerville, in a line of saddles heading towards Laverkin, and with a series of en echelon faults near the Laverkin rock quarry.
Insert a photo of saddles and slumping earth at the quarry: