H19: Gravel Pit, LaVerkin, Utah

To access the foothills below Hurricane Mesa, drive north on highway 17 from LaVerkin, Utah, and turn east onto an asphalt road exit, just short (south) of the LaVerkin Creek bridge. Proceed toward the active gravel pit, passing the weighing scales, and park on the east side of the road at the 2^nd turnoff on the right. Hike on the south side of the gravel operation in a dry canyon to the east to investigate the Pleistocene sandstone (Ps) in the arroyo walls of the dry wash with the largest creek bed.

Discussion

The sandstone on the south side of the arroyo is hardened almost to Pliocene strength (as demonstrated by punch test), and dips 17 degrees up to the east in the first distinguishable block faulting. This dip is continuous for ¼ km, even though it is overlain by nearly level Recent alluvium. This sequence suggests that the Ps is from a previous cycle of deposition. It contains no basalt rubble, and contains minor to no cobbles (except sporadic, thin layers of rounded pebbles). The constant dip, relatively high angle of dip, and near absence of cobbles suggests the Ps was uplifted on the east side as a block, causing the unusual hardening (the area must have been fairly flat at the time of deposition). In addition, the presence of the minor cobbles is not consistent with the dip angle- flow at such a high angle should have carried boulders). Furthermore, Triassic material containing abundant conglomerate is present in the scarp above the Ps to the east, indicating deposition of the Ps occurred prior to the formation of any significant scarp of the Hurricane Fault. A 100-meter scarp is now present up to the east. These factors, in conjunction with the level alluvium present above the Ps, indicate that Ps was deposited during a cycle previous to that occurring now.

The fact that level beds (or down-to-the-east sediments containing large boulders) overlie the Ps, indicates that the entire column was tilted down to the east sometime after the block faulting occurred toward the west (with dip down to the west). Such tilting is opposite to the original movement. The imbrication[1] present shows the stream flowed westward in the anomalous down-to-the-east layer, similarly to that now occurring.

A question to ask is “Why isn’t the emplacement of young sediments considered Pliocene (or older)?” This question can be answered by comparison to a nearby fault scarp formed less than 1 mybp. Scarps farther east should be progressively older to the east. We must retain this question for future solution, separating the three (?) scarps to the east, which uplift progressively higher in elevation, as one hikes eastward. Perhaps a period of peneplanation occurred between uplifts. The important entity here is that this is the only location in the Hurricane area where the scarp is multi-faulted with definite weakened strata, indicating an interference with the main H fault.

The Ps has vertical beds at its eastern exposure, almost 200 meters west of the present Hurricane Fault scarp, indicating the Triassic Moenkopi (Trm) cliffs have “walked” significantly eastward since the last uplift. At the scarp, the outcrop is Moenkopi, indicating that the whole feature is a block that has rotated (1) up to the east and (2) later down to the east (to yield the flat and level rubble layer visible west of the vertical beds. The block must have been raised normally with the uplift to the east, and then rotated back a lesser amount, resulting in flat, level beds containing cobbles and rubble. This later event would be a “rollover” fault action, opposite to the initial motion. Such a sequence of events would happen in a time of crustal extension.

To better understand these opposing movements of young strata, and to delineate the varying stresses throughout the Tertiary to Recent, refer to the website on Mantle Plumes (www.mantleplumes.org/Hawaii.html). This website may help to explain the structural activity west of us in the Basin and Range (B&R) Province. The Pacific plate movements have had a profound influence on the Colorado Plateau (CP) and the B&R to the west.

In my opinion, The CP is still being sheared at the edges by the NW-SE stress that has been occurring since the Oligocene. Furthermore, the Transition Zone (our current study area as far west as the Beaver Dam Mountains in the B&R) results from two stress systems. The oldest system was oriented NW-SE, typical with most of the Western US in the Miocene and later; and the youngest system is oriented N-S as with the Hurricane Fault (near 1 mybp and later) and older B&R faults. Consequently, two systems interfere to create two drainages: the older is NW-SE (or its orthogonal system SW-NE as in Zion National Park), and the younger is N-S along the Hurricane Fault as with Ash and LaVerkin Creeks. Subsequently, we see evidence of both stress systems at Coal Pits Wash, where the obviously younger, southerly portion runs N-S after the <1-mybp breach of the volcanic dam. The older system (set in stone, so to speak) is in the Navajo (Jn) Sandstone in the main part of Zion Park and is dated Miocene and earlier.

Hikers did find indications of both stress systems, as shown by parallel fractures. Young parallel N-S fractures were more dominant, probably created by the Hurricane Fault system; however, some parallel fractures were oriented NW-SE, a similar direction to that for the conjectured NW-SE anomaly that projects from the Pine Valley Mountains (PVM) near the Wet Sandy Creek. The local NW-SE alignment of saddles visible in the topography east of the pit also points toward the southern end of the basalt flow between the towns of LaVerkin and Toquerville.

Overall, hikers found only tentative evidence that an anomalously stressed zone proceeds from the PVM down the Wet Sandy creek toward the slump zone near the gravel pit. Further investigation may help determine whether the Toquerville spring, the toe of the basalt flow to the north, the pit slump zone, and the large fissures on the north side of the Virgin River are all related to a common stress anomaly. The presence of such fissures on the south side of the river is not noticed as obviously.

2. When the diversion dam was emplaced in the Virgin River bed, the water disappeared suddenly into the rock bed, re-appearing in the Pah Tempe spring. This event indicates that the fissures were already present in the subsurface, and not just near the cliff walls above the Virgin River (as would be expected if the large fissures were caused by a simple gravity slump.

4. The flow of the large perennial spring at Toquerville indicates that it probably drains a large area: either from the north or from the PVM. If a large NW-SE fracture is present and proceeds along the Wet Sandy path, such a fracture could be a conduit for the water. Additional study is needed to determine the origin of the large spring upstream in the Wet Sandy Creek to the NW.

To best consider the above discussion, it is important to understand crustal plate movements and the resultant compression and extension in the Western US. The author suggests that hikers read the previously referenced website article that discusses plate movements and their relation to the Hawaiian Islands. In so doing, it may be easier to understand the development of the B&R Province as influenced by Tertiary movements of the Pacific Plate.

No conclusions are made in the article, but several maps and diagrams are presented, and may help form an understanding of extension tectonics as they relate to the B&R and possibly as far east as our study area, the western edge of the CP (where extension occurs also).

[1] Imbrication: A sedimentary fabric characterized by disk-shaped or elongate fragments dipping in a preferred direction at an angle to the bedding. It is commonly displayed by pebbles in a streambed, where flowing water tilts the pebbles so that their flat surfaces dip downward looking upstream.

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Discussion