Monday, October 15, 2007

Young Meanders in an actively-uplifting Area- Hurricane, UT

The large View of the Sky Mountain Golf Rim and its nearby Meander, above the Virgin River

Closeup View of Meander and Conglomerates

How can the Virgin River have a “youthful” Meander, in an area (Zion Highlands and Hurricane Fault) which has been uplifting for millions of years?

Meanders are created in streams which are old (for the sections of rivers where they occur). A meander requires that there be almost no stream gradient, that is, the velocity of the water is low enough to prevent active downcutting of the river bed- rather, the stream works sluggishly and laterally as in the Mississippi River, seen in lower Louisiana (almost at sea level elevation).

The Hogback northwest of the Virgin shows the severity of the slopes due to compression (uplift and Buckling)

An area which is uplifting will develop a high energy stream- with a rapid current. This will not allow a meander to develop. However, accompanying photos show not only a meander below Sky Mountain Golf course in Hurricane, but also flat and level conglomerates or sandstones which would be typical for a place where the sediments were dumped (the current was reduced to near zero, and its sediment load was placed in the river bed). The Virgin River is known to become violent sometime during each year, either when the snow melt washes down, or during the summer monsoon when violent storms develop. How then, did the large meander and its flat and level conglomerates form (in this area of ancient beds which are rapidly changing in elevation, with obvious drastic slopes and cliffs)?

Cooling Cracks create Hexagonal Columns, as the lava shrinks (Similar to Mud Cracks forming multi-sided blocks- but due to shrinkage by drying rather than cooling)
The answer is found by looking at the vulcanism which is also presented in the photos. Melted Basalt flows similarly to river water- running into the low elevation locations and displacing water (in the city limits of Hurricane, there are at least 3 young volcanic craters with age less than 200,000 years). This creates a damming of the previous river and develops a lake in its place. A lake has no noticeable current, and any sediment flowing down from nearby rivers would drop to the bottom- creating a sedimentary bed. A conglomerate is one type of deposit, indicating that indeed the streams feeding the lake had high energy (enough to bring in large boulders), but that the lake had a sluggish velocity. This low energy stream would create a meander, and the surrounding cliffs would be subject to wave action of the lake and to softening due to ice and annual weather- allowing the river loop or oxbow to enlarge sideways. This is a Meander and it would be restricted by the surrounding pre-existing cliffs and volcanic deposits. Incidentally, a meander is named after the original feature in Turkey seen by me, in the Menderes (a family name, made famous by a recent Turkish Premier) River- over which the ancient town of Ephesus presides.

Conglomerates (Cobbles and Boulders) are flat and almost level on both sides of the Meander

Eventually the water in the lake raises enough to broach the volcanic dam downstream, after which the increasing current would not only downcut the previous conglomerates but straighten the river. The first noticeable remains of this action is one of several canyons cutting the beds younger than (on top of) the conglomerates, where the river is low enough in elevation, to allow the youngest sediments to remain for us to see.
Look through the series of photos, to determine whether you can see the sequence of events, occurring over the last 100,000 years:
1. Sudden damming of the Virgin River by a basalt flow;

Volcanic Basalts flows are heavy and dark, filling the low spots where the Virgin resided- creating a dam for a Meander

2. Creation of a conglomerate bed, which is almost flat and level, at an almost constant elevation over the length of the meander;

View of Plc conglomerates on south side of Meander

View of the Plc Conglomerates on north side of Meander

3. Breaching of the volcanic dam, allowing the river to downcut previous beds and basalts;

4. Formation of small canyons in the beds in the center of the meander, as the rapidly-flowing water eroded the lake bed and surroundings;

5. Creation of side canyons, still continuing today, an example of which is Gould’s Wash flowing from the Hurricane Fault on the east side of town; and
6. Downcutting of the whole area by a fast-moving Virgin River, creating deep canyons and steep banks in the area of the volcanic basalts.

Before the vulcanism, there were flat and almost level sandstones of about 200,000 years age- which are seen on both sides of the Virgin River

Gould's Wash (A Canyon in this location) has eroded rapidly into the Virgin- showing how fast the fractured basalt shears

When first breached, the river level was higher- cutting small washes into the youngest sediments