Saturday, April 19, 2008
The Laverkin Sink
USGS Photo shows the quarry under investigation, which is also shown below. This Quarry, or Epicenter, lies above the sink, which is now dropping more slowly, since most of the heat has been released from the underlying magma chamber via springs in late Pleistocene. Laverkin Creek has now cut through the conglomerates dumped into the sink in Post-volcanic flow times.
Above is a Google Photo which shows the Area of Investigation. Notice the parallel Ash and Laverkin Creeks in the bottom center, above which there is more meandering of the Ash north of the parallel side-by-side creeks. The Ash turns NW-SE just south and a km west of the position of the conjectured sink (which would follow the older fracture system of early Tertiary).
Above is a view to the East, of what I am naming the Laverkin Sink. This has little to do with the present Laverkin Creek, since the creek has eroded down through the original down-dropped sediments. To be a Sink, there has to be dipping beds from all directions into the conjectured Sink (I have found these on all sides except the north, where I am presently investigating).
This north-looking view is along Hf, and shows the Conjectured Laverkin Sink to the left, and fractures or small-displacement faulting just west of the Main Fault (pointing toward the Saddle to the north).
This Report is in Progress, and will be finished in April
I have conjectured that the Laverkin Monocline and Hogback to the west, the Slickensided Graben to the south, the multi-faulted Quarry to the east, and the flows of lava and Pleistocene or quaternary gravels to the north and NE are all dropping near the Hurricane Fault, Hf, to form a sink under the quarry. I am developing this evaluation with photos and conclusions in the following remarks. The next photo shows the young gravels and sediments which are temporarily residing in the young sink, and are not necessarilly related to the erosion of the present Laverkin Creek. This creek may have taken its path as a result of the downdrop, however.
There is a larger block of tilting sedimentary rock to the north, which will impinge on the movements under the Quarry, and this will be incorporated into the report. This is the large tilted block of Mesozoic near Toquerville, at the incipience of Ash Creek. This larger feature influences the movement of the Laverkin Sink, but does not necessarily create it. I conjecture that the Sink is the result of thermal cooling and sinking after the eruption of the Pleistocene vulcanism nearby (3 + volcanoes in Hurricane and some to the north and west of Laverkin).
Looking North, by Telephoto, the dip is still down to the north and west of Mesozoic rocks. The first photo shows the Big Picture of the Sink as it is seen above the quarry in the city limits of Laverkin. The sink is so large that it is difficult to grasp in one photo, but this shot allows the boundaries in two directions to be evaluated- the Hogback and the tilt of some of the sedimentary rock and gravels.
Topographical Conditions for a Sink:
A sink is defined as an elevationally-depressed area on the ground surface or in the subsurface, where fluids or sediments stagnate (coming from all directions). It will have sediments being dumped there, since the flowing water and the general terrain will introduce these deposits when stream current drops to zero. The Sink will eventually fill with sediments, and the fluids may then drain or evaporate. In the desert, where the evaporation is greater then the supply of water, there may be a continuous influx of water and sediments until the depression is completely filled. After that point, an outlet is formed, and subsequent drainage may remove one of the surrounding barriers or sink walls. A sink will have closed contours, when measuring the elevation of surrounding landscape. Ιn the subsurface, such as a sinkhole with a covering, the depression or cave will be surrounded by walls which are impenetrable to sediments.
Laverkin Sink must have the following features:
1. A local deposit of sediments thicker than that in the uphill streambed;
2. Canyon walls or hills surrounding the deposit- in the Laverkin case, the streams may have eroded the southern barrier or wall;
3. Older sedimentary deposits, if present, will slope downward toward the sink from all directions (this basin configuration initially forms the depression); and,
4. A bed or layer of gravel, sand, silt or clay will have been dumped into the depression to form horizontal strata. This may be presently eroding by through-going streams, but the original bedding would have been flat and level at one time.
Notice the Stratigraphic Column, which has Pre-basalt gravels(almost flat and level, indicating flat terrain, but the cobbles indicate a swift stream) at the bottom, with overlying oldest basalt for the Quarry area. This could be an ancient meander in a shallow canyon, but the flatness indicates a bevelling of the oldest sedimentary beds before lava flows.
(5) Photos show that the sedimentary column above the present Laverkin Creek has basalt flows covering a flat and almost level Pleistocene conglomerate. This indicates that, pre-vulcanism, the land surface was gentle with few canyons.
(6) Trail Ridge is a new development, lying between Laverkin and Toquerville, on the south side of Laverkin Creek (where Pre-Basalt conglomerates had little dip).
A Basin is much larger than a sink, and may have an outlet so that the water does not evaporate to form an evaporitic basin (which would be an over-sized sink). Above is my stylized cross-section of a basin. showing how uplift, sinking, stratification of water zones, and compaction of sediments occurs due to these factors and to the thickness of overburden- which causes change of water quality with depth.
A Photo below shows the large deposit of gravel and other sediments which are being quarried just below the Hf graben (SE), multi-faulting (to the east), and Monocline to the west, in the town of Laverkin.
Notice that the sand is almost falt and level, while the overlying gravels have a greater dip (this is an unconformity)
A Meander is somewhat similar to a Sink, allowing only a minimal flow of water through it. The bedding will again be essentially flat and level, there will be deposits such as bars and spits, and on a small scale it may be impossible to distinguish them. Cross-bedding may occur in both cases, but for meanders there was never lack of drainage (no water-stagnancy). Meanders will typically have large horseshoe bends in the creek flow, and the gradient of flow is almost zero, but there is drainage through the downstream exit. This meandering occurs to the west in the Virgin River, where a volcanic dam has temporarily impeded the flow in the past, and where a 50 foot thick conglomerate now resides above the level of the river. The Pleistocene conglomerate, Plc, has great strength- having been cemented by saturated carbonate-containing waters from Pah Tempe and other hot springs upstream (at Hf).
This Laverkin Creek canyon wall shows the conditions prevailing Pre-basalt. Emplacement of basalt shows the wide shallow valley which prevailed- since basalt flows similarly to water (albeit much slower, but preserving the shape and orientation of the valley due to the erosion-resisting nature of the tough rock). The ancestral creek could have been meandering over a wide range, but nevertheless with significant gradient- since the cobbles shown in this photo are fairly uniform in size.
This view from the north, east of Toquerville, shows that the N boundary of Laverkin Sink is a topographical drop- not a stratigraphical dipping toward it.
Below shows the same S shot, using Telephoto, so that the Hiway 9 Graben can be seen (with the water tank down in the gully).
This view is also from the Toquerville Hill, looking more to the SE.
Toquerville sits below a Mesozoic block (dipping down to E), which diverts Hf to the west, and which creates a blockage locally. This causes the water from the NW to emerge as the town water supply in Ash Creek spring (this is suspected to arise along the NW-SE-trending Wet Sandy Creek)
A massive shift of Hf occurs just north of Toquerville, moving the fault almost NE for a segment- the terrain rising another 300 meters- and causing a diversion of Hf into two splays (Ash and Laverkin Creeks are divided by the block tilting upward to the west, then downward to the west just a km to the South). Notice that Radio Tower hill sedimentary beds dip up to the west, while the large block nearer Toquerville dips downward to the west.
A Basalt flow, which poured over a portion of the Toquerville uplift to the north along Hf, has been split by Hf, showing a 500 meter uplift over 500 kiloyears (approximate), along the fault to yield almost 1 mm/year uplift. Notice the domal character of the flow which moved westward when the Terrain was much flatter. Nowadays, the connection may only be surmised by noticing the terrace-like domal spread, which is some 500 meters lower than its source.
(Laverkin Town is subjected to another type of sink, as of this writing. Budgetary problems, caused partly by purchases of land, have required major layoff of personnel. Even the mayor’s hike- which I attempted to follow on the 3rd Saturday- was deferred. This weight-losing endeavor, which allows locals to view the natural life of the area, showing off the myriad colors of Spring, hopefully will not be a caricature of Man’s dilemma during this time of required sacrifice.)
Virgin river Meandering is discussed below, so that one can compare it with the sink deposits found at Laverkin.
This meander may be seen from Sky Mt. Golf course- looking down into the Virgin Canyon. It is distiniguished from most of the bends in the river path by the roundness of the path, as opposed to angular 90 degree turns for those turns which are fracture-dominated.
This meandering occurs to the west in the Virgin River, where a volcanic dam has temporarily impeded the flow in the past, and where a 40 foot thick conglomerate now resides above the level of the river. The Pleistocene conglomerate, Plc, has great strength- having been cemented by saturated carbonate-containing waters from Pah Tempe and other hot springs upstream (at Hf).
Plc was temporarily blocked by a down-stream lava flow, which has been eroded. The meander has formed after the blockage was eroded and has by-passed most of the conglomerate.
There are some erosional channels through Plc, but these have been by-passed also (after the meander formed).
A well-cemented Pliestocene Conglomerate Plc, has been cemented by the saturated carbonated waters of Pah Tempe and other hot springs, when a volcanic dam blocked the path of the river-borne rocks.
Objections to the Laverkin Sink conjectures:
1. The stratigraphic column on the Toquerville side of the Laverkin Creek (Trail Ridge photo) indicates that the meandering cobblestones underlie the lava flow. Doesn’t this refute the idea that the sinking occurred after the lava flow, due to cooling-shrinking of the magma chamber, from which the basalts originated, since this rock lies above the sink and north of the quarry (which didn’t sink, with cooling- since it lies above the Creek)?
Answer to 1: This lava flow came from the north (flowing downstream along the ancestral Laverkin), and preceded the magma chamber contributing to the flows at the Virgin River to the south. Unless this flow from the north was from the same magma chamber, it was independent of the later flows of about 100-200kyears.
2. Looking at the USGS black and white photo, it appears that the quarry lies in an old meander. Would this be the case for an old-age stream, not a deposit which later sinks with cooling?
Answer to 2: This is a possible conclusion, and it was noted that a meander is similar to an enclosed sink, except for current velocity (which is zero for a sink, larger for a meander). The sink is conjectured based on beds dipping from 3 directions (toward the quarry) in sedimentary deposits surrounding the quarry location.
3. How can it be that the Laverkin Creek now flows below the conjectured sink- shouldn’t the creek be sinking at about the same rate as the magma is sinking?
Answer to 3: The way out of this dilemma is determined by the uplift of the Colorado Plateau relative to the sinking of the Basin and Range, B&R. When the CP raises quickly enough, the gradient of the stream- which determines erosion rate- may cause faster canyon erosion.
The KEY to all of this conjecture lies with the time of uplift of the Laverkin Hogback and Monocline. If it occurred before the Pliocene, this weakens the case for a sink. If it is currently rising, this strengthens the notion of a sinking below the Laverkin Quarry. The other dipping entities- graben and multi-faulting- are fairly straightforward.
I will be looking at views such as this one of Mt. Baker, so missives for the next 6 months will have to do with the San Juan de Fuca plate, rather than the Pacific plate subduction under the western USA. Comments on Spreading centers moving eastward under the West US to create B&R faulting and extension will still be accepted. Don't expect that Hf and its independant movement- separate from the Laramide and later extensional events- will be forgotten but will still be uppermost.