Geomorphology in Arid Regions by Doehring Donald O.;

Author:Doehring, Donald O.; [Doehring, Donald O.]
Language: eng
Format: epub
Publisher: Taylor & Francis Group
Published: 2020-03-21T00:00:00+00:00

QUATERNARY TECTONIC ACTIVITY OF THE MOUNTAIN FRONTS

The mountain fronts of the study area were assigned to appropriate relative tectonic activity classes (see Table 1) on the basis of, (1) the mountain-front sinuosities and valley floor width-valley height ratios, and (2) other features (such as entrenched or unentrenched alluvial fans) which were observed during trips to the study area and on flights over the area. All of the mountain fronts are considered to be class 3 except in the northern subarea where four class 1 and seven class 2 fronts are present.

A typical class 1 landscape is shown in Figure 4. The rugged, narrow drainage basins have V-shaped cross-valley profiles and the valley floors are the same width as the floodplains. The canyons are being downcut into the Paleozoic sediments and are notched into an eroded fault scarp that comprises an unembayed mountain front. Thick alluvial fans are actively accumulating on the piedmont immediately downstream from the escarpment. This is an example of the type of landscape sketched in Figure 3.

An example of a class 2 terrain in granitic rocks is shown in Figure 5. Although rugged, the cross-valley profile of Wilson Canyon is more U-shaped than V-shaped. An embayment extends upstream from the topographic escarpment that marks the mountain front, and the terraces upstream from the embayment are clear evidence that the floodplain is much narrower than the valley floor. A fault scarp is not apparent and the embayments have created a more sinuous mountain front than probably was formerly present.

The unentrenched alluvial fan downslope from the mountain front shown in Figure 5 should not be considered as evidence for a class 1 front, particularly in view of the types of landscape elements noted above. The unentrenched nature of the fan is the result of the impact of the Pleistocene-Holocene climatic change on a rock type that is sensitive to climatic perturbations (Bull, 1976). Late Pleistocene climates in the Argus range were more conducive to denser plant growth on the hillslopes and for weathering of the hillslope materials than Holocene climates. The postulated decrease in vegetative density during the early Holocene resulted in rapid erosion of the unprotected materials on the slopes. Granitic colluvium responds more rapidly to such a perturbation because lower stresses are needed to move grus-size particles than blocks of rock that would be weathered from most metamorphic rocks. The resulting increase in sediment yield completely backfilled any entrenched stream channels in the valley or on the alluvial fan. In Homewood Canyon, 9 km to the north of Wilson Canyon the same sequence of events has occurred. The stratigraphy exposed in a streambank in the valley of Homewood Canyon consists of well-sorted, reddish-brown, clayey grus of apparent Pleistocene age that is overlain by 2 m of gray bouldery grus of apparent Holocene age. With continuing decreases in the amounts of colluvium left on the hillslopes during the Holocene, the stream systems in both drainage basins have changed their mode of operation and now are downcutting through the alluvium that accumulated on the valley floors.

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