Navajo-Sand Hollow, S18,19 T42S R14W 3/9/05 A. The volcanic flows on the east side of the road (basalt, on the 1 my. time scale, all seem to terminate along a ragged line running N-S. Since these involve several flows of basalt, one on top of the other, how is it that they all terminate just short of the road? B. Since you immediately suspected an old river valley- in which direction did the currrent flow (there is no imbrication, unless we can find some conglomerates)? C. If you guessed southward, by looking at the rounded shape of the dam (a dead give- away for a wide river valley), why do the present ephemeral streams run northward? D.If you looked at the cobbles and pebbles in the present stream bed to the west, they have more than basalt and Navajo clasts. But in the immediate area, along the road there are no other outcrops, except basalts and Navajo (Jurasic ss.). Where did these exotics originate? E.Try to determine within 45 degrees, the orientation of fractures or faulting in the pink hills as we drive along. Is it N-S. NE-SW, or NW-SE? F. If the old river flowed southward, against the present gradient, could the land have tilted upward on the south side- reversing the stream course? G. If this is the old course of the ancestral Virgin, what could reverse the stream course, to cause flow northward into the present Virgin River? Note that since the vulcanism, there could have been a river course at about the basalt level, which came from the east along the present Hurricane Cliffs- Jerry is going to check this out, by walking to the south of the irrigated fields south of the airport, to see whether a south-moving stream could have turned west and emerged along the basalt cliffs south of Sullivan's Knoll. The sequence, for the development of this Hurricane area would be as follows: 1. Compression from the Pacific plate moving eastward in Cretaceous times, created anticlines, monoclines and folds oriented N-S here (as with the Kaibab plateau, as seen along the East Kaibab Monocline to Lee's ferry (starting near Cameron, AZ); 2. These stresses died away in Oligocene times, allowing the area to become extensional (after the time of the dog-leg in the Hawaiian-Emporer seamount chain, of 41 my. time)- the reason for this appears to be that the spreading center which is now apparent offshore Mexico (not west of California) has moved under the continental US (because of the westward movement of the Continental plate) creating spreading under the West and fissures which allowed basalt to penetrate the crust in: a. basaltic flows, and b. cones, later, of more silicic lava- which are more explosive; 3. Wrenching in Oligocene times was toward the NW- creating the omnipresent NW-SE fractures over most of the West US; this was because of the strike slip along faulting created by the spreading center, now dying out under the continent; 4. Some 2 my.ago, the original N-S weaknesses (created by the east-stressing Sevier and Laramide plate movements) were rejuvenated by extension (this is seen in the Verde ls. in Verde Valley- which is younger than 5 my, and in N-S rivers such as Rio Grande, Verde, and south Colorado Rivers.); 5. Vulcanism started- first the basaltic type, as seen in our area as a clean contact of basalt above Mesozoic with hardly any ash or gravels in the contact. The most recent extrusion has been of the ash fall type. In Hawaii, this is noticed as a rejuvenation phase, some 1-2 my., after the individual islands were isolated by eruptions further to the SE. The rejuvenated alkalic lavas tend to be on the SE side of islands (such as Diamond and Koko Heads); 6. The vulcanism has moved westward, in the transition zone- the youngest noticed is off hiway 18 near Snow Canyon. In AZ there has been a westward movement of extrusives also, but in this case over a 20 my. time span, and well off the C.P. into the basin and range. The transition zone appears to be the extent of the spreading center, in Utah- looking east to west. This entire movement from east to west is strange, since the continental plate- moving westward- should make the apparent movement of vulcanism from the previous spreading center, to go eastward with time; 7. Uplift along the Huricane fault continues, and the fracture indications in Zion Park appear to have been re-oriented from NE-SW (as with the P.V. Mtns) to 160-340 degrees from north. One can see major canyons orienting in these two linear arrangements, while the older drainage tends toward the NE-SW (with the younger tending to be more N-S, as with Coal Pits Creek). Complications: a.Do the younger volcanic cones orient toward the N-S, in our area? b. Do recent earthquakes align along a N-S axis here, as contrasted to NW-SE in the Grand Canyon area? If so, we have isolated our system here from that further south, where the C.P. exhibits behavior for its SW side, as opposed to our being on the west side where fracture behavior would be expected to tend N-S. c. We still have to explain the NE-SW orientation of the P.V. Mtns. and Virgin anaticline, which should have been oriented N-S- if only under the onus of the east-moving Pacific plate? The author hiked westward of the large mesa at the Honeymoon canyon, running 160- 340 degrees along the Hurricane cliffs, to determine whether the beds dipping up to the west formed an anticline which had been breached and eroded at its central portion. This is mostly buried, but there is an exposure of Moenkopi to Chinle, dipping upward to the west still, at a dip angle of 45 degrees terminating in the air- about 1 mile to the west of the Hurricane cliffs. To the south one can see a slight dip of beds down to the SW, and this may be a separate feature- this must be investigated further. The author revisited the Tertiary outcrop-Tc, which lies at 3823 feet elevation, above the Virgin River (S30 T41S R12W), which contains clasts of Mesozoic sandstone and a few Tertiary basalts. This was to find whether a generality exists, as to orientation of beds west and east of the Hurricane cliffs- all, so far, dipping upward to the west. The Tc outcrop is oriented along a hill- mostly loose cobblestones- in an NE-SW orientation, pointing to another hill to the SW, where there are many loose clasts of simillar cobbles, some 1/2 km distant; a fossil river course, which is now abandoned, lies along a path which would have intercepted a tributary of the pre-historic Virgin (now a deep dry gorge)- this is evidenced by loose cobbles all along the path to the SW. The elevation of both hills is the same- 3823 feet- and it seems that the old channel, which is now captured by the present Virgin was fairly level (suitable for cobblestones). The elevation of the present outcrops is almost equal to that of the Hurricane cliffs at the maximum- 3844 feet- this gives some idea of the gradient of the fossil river valley (not a canyon, but a valley or flood plain). The fractures in Tc are of two orientations: 70-250 degreees from north, and NE-SW, confirming that the topography was under the influence of whatever oriented the P.V. Mtns. and the Virgin anticline, at the time-but not the present N-S Hurricane fault; this also hints that there are some independant E-W fractures. Imbrication shows that the fossil valley ran toward the south (+ 45 degrees). The sparsity of basalt (much loose basalt- well rounded- but only a few vesicular basalts in the outcrop, still cemented), indicates that the fossil valley was flowing in Pliocene- Miocene times. Altogether, the strength, imbrication, weathering, fracture orientation, and type of clasts relate a Miocene age (5+ to 24 my. age). What is the significance of this low-gradient stream in Miocene time? a. The cobblestones, Tc, relate that there was a raised area to the east, of Permian to Cretaceous derivation, which emptied onto a low relief flood plain, which had no cliffs to the west (the almost constant elevation of the cobbles going south or westward shows that no canyon was being formed); b. The fact that only one cemented vesicular basalt was found iin the outcrop (not counting loose basalt clasts), says that vulcanism had just started to the east; c. If the basalt cobble were dated, this would give an upper limit of the time before significant Pleistocene Hurricane cliffs formation ( estimated 5+ m y. age); d. The fact that the stream ran south or SW-ward (both the mesa and combination of two hills with Tc run in the NE-SW orientation), tells that the NE-SW P.V. and Virgin anticlinal orientation was still dominant- this is not the case now, where both NW-SE and N-S fractures have superceded the stresses in effect at the Miocene time; and e. Tc sits atop of Trm (moenkopi), which is about the same elevation as Pk (at the rim to the west); this tells that the dip was up to the west at that time. This dip, probably created by the Laramide, is up to the west on both the east and west sides of the present Hurricane fault; f.Therea are meanders both east and west of the present Hurricane fault, in the Virgin River. Since the east ones sit atop Paleozoic, whereas the west ones sit atop Mesozoic, this determines that the peneplane at the time of renewed uplift of the Hurricane fault, covered the evidence of previous scarps for the Old Hurricane faulting. There may have been manyu surges of uplift of the fault, peneplaned, before further uplift of the present fault. Answer to A, B, and C: The author and hiking group hiked atop a range of hills, which are south of Sullivan's Hill- west of Sand Hollow Park, to determine whether the river channel on its east side was the ancestral Virgin- going back 1 my. There is no present outlet for the stream (going now northward) to the south or west. Sand Hollow reservoir sits atop the river valley (which is 1/2 km wide, or more), and if there were an ancestral stream going south, it could only have exited to the west across what is now uneroded ground (that is, no river valley runs west from S.H. Lake). This appears to eliminate the difficulty of having the ancestral Virgin flowing south, atop Navajo ss. contrasting to that near the town of Virgin flowing on top of Triassic Moenkopi, before the present Huricane fault accelerated its uplift. Younger formations to the west would have meant that the Hurricane fault was already present whenever the Sand Hollow channel was being used (the channel was on top of younger beds than for the location east of the fault- where it lies on older beds). We still haven't answered why the S.H. channel existed in the first place, but during the Pleistocene, much more water was available, to form the 5 mile long river valley. But this whole analysis shows that there was an ancestral Hurricane fault before Pleistocene, which had been peneplaned or converted to a minor scarp; Answer to D: Cherts, limestones, and iron, manganese filled sandstones and intrusives occurred in the wash just east of the S.H. hills (in addition to the expected basalt and Navajo clasts). The limestones tell us that flow has occurred from the east, bringing the Kaibab or Moenave clasts; Answer to E: Dominant fractures orient in the NW-SE or N-S, but rarely in the NE-SW directions, telling us that the P.V Mtns determine fractures as far away as 20 miles to the south, and that the newer N-S fractures are now being manifested (young by virtue of being less fissure-like). The immediately preceding NW-SE fracts are more subtle, indicating that a local anomaly is overwhelming them; and Answers to F and G. will have to come from other measurements. Harold L. Overton