H22: LaVerkin Structure 02/22/06

When one climbs the volcanic cone within the Hurricane city limits, at the city park (where a trail leads off from 200 west, 800 north Streets going west to the summit), there is an excellent view of the entire geological display around the city of Hurricane. Scan the horizon above the Virgin River, and you can see various features which give clues to the geological past- especially of the movements of the earth in this area, throughout the last 100 million years.

Although there are mountains all around (Pine Valley to the north, Zion highlands to the east, the Hurricane cliffs or scarp to the NE and SE, three volcanic cones to the south and west, and the Beaver Dam far west of St. George), the immediate feature of interest is the large prominence to the north- almost bordering the Virgin River. This is called a monocline (one slope), since the multi-colored sedimentary rocks it displays plunge to the east and disappear into the earth. The monocline levels off to the west, where there is considerable erosion, and one cannot discern whether it dips down to the west. If it plunged down to the west, the beds would be hill-shaped, and then be classified as an anticline. Keep in mind that it was once buried, and has been unearthed (so to speak) by erosion. At least 2 miles of rock have been washed away by rivers, as indicated by Pine Valley mountains- which were completely buried by at least a kilometer of sediments when they first arose (since the granite-like rock never melted its way to the previous surface of the ground).

This monocline, which we will call Laverkin structure, is divergent from the other features in the general area. That is, it trends roughly N-S, as compared to the Virgin anticline- which trends NE-SW, as do the almost parallel PV mountains. The 3 volcanic cones are in no particular pattern, but hint that they rose in a westward progression. The only distant feature trending N-S is the Hurricane fault, a mile to the east, and it produces a scarp in the same direction.

When one looks broadly, the conclusion is reached that the Laverkin monocline doesn’t fit in- it is going in the “wrong” direction, compared to neighbors. Possibly it has more to do with the newly-arrived Hurricane cliffs than other features in the area. Since we are studying the great fault, to determine its age and reason for being, we will investigate the divergent feature close-up.

Wed, 22 Feb, 06, 5 stalwart souls took a hike crossing the Laverkin & Ash Creeks to the Mesozoic beds to the west of Laverkin. It is necessary to cross the creeks via downed tree trunks, and for the faint at heart, bring a pair of boots (if you are afraid of heights above babbling brooks). Enter via Center Street (which has a traffic light), drive to the dead end, and walk down to the Confluence Park.

There is a large monocline west of the creeks which trends NNW-SSE, and plunges to the south also, which we will investigate. These compressional features are generally associated with the Laramide orogeny (50 mybp), but in this case the orientation is not like any in the area, and I suspect that it has originated separeately from the uplift of the Pine Valley intrusion (21 mybp, post Laramide). If this is correct, the PV laccolith could have shoved the previous Laramide anticline south on its east end, causing a clockwise rotation of the structure. This would place a rotation on the nearby sedimentary rocks to the east, causing them to shear CCW, raising the Pk seen above Toquerville and Ash Creek, and causing the large fissures noticed above the Virgin River (near the diversion dam). We will test all of this, by:

1. Determining whether there is an anticline, instead of a monocline, by finding if the beds dip down to the west on the western side;

2. Finding whether this structure continues to the north, as is faintly indicated by binoculars;

3. Ascertaining whether the anticlinal axis of the nearby Virgin structure (Laramide) has been distorted on its east end (the PV intrusion would shove more on the east side of the anticline, compared to that near St. George);

4. Studying the faulting which accompanies all of the above; and

5. Finding whether the younger Cretaceous sediments are shoved up by this PV compression to the SE: they should be higher in elevation, if the PV has shoved them from underneath, as it rose.

1. The prominent monocline, which can be seen best from Hurricane, is best described as a N-S trending hogback- which means that it is a Laramide structure. The beds can be seen as arching upward to the west, and on the north side of the structure, there are vertical beds running 160 to 170 degrees from north, in the bottom of a wash. This part of the monocline continues farther to the north, but appears to die out near the town of Toquerville, where another trend orienting NE-SW is apparent. This fits in with the idea that the Laramide (Cretaceous) structures were thrusts from the west (or slightly WSW), and that the one nearest the PV Mountains has been rotated CW by the mushrooming of the laccolith to the SE, in later Miocene time;

2. The hogback structure seems to be independent of the influence of the PV mountains, and is independent of the Virgin anticline also (they trend in significantly different directions), hence it must not have been influenced by the mushrooming of the igneous intrusion. This does not preclude the possibility that the Toquerville and Ash Creek environs were sheared CW on the west side of the main Hurricane fault. Closer to the PV mountains, the region close to the fault seems to have been sheared to the SE, possibly causing disruption to the Hurricane fault all the way to Laverkin;

3. Further investigation is required along the fault east of Laverkin, since there is a definite anomaly there. The Kaibab cliffs Pk in the outcrops east of the fault can be seen to down drop to the north, starting at the fossil canyon reported in hike H18 and proceeding northward. This is a separate fault block trending E–W, which is about .5 km in N-S extent. This is finally terminated against another block which dips up to the north in red beds of the Mesozoic. The larger picture suggests that there is a major disruption to the stratigraphic column, both west and east of the H fault, adjacent to the town of Laverkin, which fits in with the observation that the Pk cliffs east of Toquerville are also exaggerated by the PV laccolith.

4. There is considerable faulting near the hogback, some running down dip of the structure, as indicated by saddles oriented N-S; some other faulting occurs east of the hogback, and this must be investigated all the way to the PV mountains. There are also fissures in a wash just east of the main hogback, which orient N-S (similarly to the hogback trend). These are parallel fissures, in unconsolidated earth, in a wash and have not been filled by recent runoff. Since there is loose earth all around, and the fissures run for hundreds of feet, they are recent openings. They are not on hillsides, but in the bottom of a wash, hence are not gravity slumps in the immediate sense. The hills that border them are at least 30 meters above the Ash Creek, however, so that one could conclude that there is a more general slumping of rock toward the creek. The more general observation is that they represent weaknesses due to the orientation of the hogback and its N-S oriented bedding, and that regardless of slumping, they represent N-S weaknesses, as does the Hurricane fault.

5. West of the hogback, there are outcrops atop the monocline- which has flattened out to the west- which are Jurassic and Cretaceous? These are atop Moenkopi redbeds, then Jn (Navajo) which overlie them, and finally tan to white siltstones and calcareous sandstones of Cretaceous sedimentary beds; no creek crosses the hogback, hence these softer beds have been preserved.

Altogether, the conclusion follows that the hogback is not congruent with the Virgin anticline- they run at 45 degrees to each other. These two compressive features must have formed at vastly different times (the hogback during the Laramide- 100 m.y.years ago until Eocene time- and the Virgin at a later time). The shoving of the Pk to the east must have happened along with the uplift of the laccolith, and the Hurricane fault must have happened later (approximately 1 m.y. ago). This might help explain why there are two major splays of the fault east of Toquerville.