Julie E. Laity

Sapping processes and the development of theater-headed valley networks on the Colorado Plateau

Ground-water sapping is an erosional process that produces landforms with unique characteristics. Sapped drainage systems differ in morphology, pattern, network spatial evolution, rate of erosion, and degree of structural control from their fluvial counterparts. Investigation of deeply entrenched theater-headed valleys in the Glen Canyon region of the Colorado Plateau indicates that ground-water sapping is the predominant mechanism of growth. The canyons occur in the Navajo Sandstone, a highly transmissive aquifer underlain by essentially impermeable rocks. Within this formation, two populations of valleys with markedly different features are identified. The first group exhibits theater heads: longitudinal profiles with high, step-like discontinuities and commonly asymmetric, structurally controlled patterns. The second group is characterized by tapered terminations; a relatively smooth, concave-up profile; and a more arborescent network. Because the valleys have developed under the same lithologic, stratigraphic, and climatic conditions, the differences in form are attributed primarily to structural constraints that determine the relative effectiveness of overland-flow and ground-water (sapping) processes. Of particular importance is the dip direction of the beds relative to that of valley growth, inasmuch as this relationship controls the occurrence and distribution of ground-water seepage at valley walls. Laterally flowing ground water also exploits fractures at depth, so that the drainage pattern of theater-headed valleys reflects that of the regional jointing pattern. Martian valleys exhibit numerous morphologic similarities to canyons formed in the Navajo Sandstone. These include theater-shaped heads, nearly constant width from source to outlet, high and steep sidewalls, hanging outlets, and a large degree of structural control. Although the constituent materials, scale, climate, structure, and ground-water conditions of Mars cannot be replicated in any Earth analog, the striking similarities in form suggest that the gross geomorphic processes may be similar and that sapping processes have operated to create the Martian valleys.

Landforms of Aeolian Erosion

Aeolian erosion develops through two principal processes: deflation (removal of loosened material and its transport as fine grains in atmospheric suspension) and abrasion (mechanical wear of coherent material). The relative significance of each of these processes appears to be a function of the properties of surface materials and the availability of abrasive particles. The landforms that result from aeolian erosion include ventifacts, ridge and swale systems, yardangs, desert depressions, and inverted relief.

TOPOGRAPHIC EFFECTS ON VENTIFACT DEVELOPMENT, MOJAVE DESERT, CALIFORNIA

I nvestigations into processes of ventifact formation in the east-central Mojave Desert, California confirm the importance of topography as a control in the location, orientation, and intensity of ventifact abrasion. Ventifacts in the region appear to be relict in nature and probably formed during a period that ended several thousand years ago. Comparison of groove orientations with available wind data shows that regional flow direction has not changed in the recent past. Although west to northwest winds are the most frequent and intense, and therefore dictate the regional erosion pattern, low to moderate southeasterly flow is recorded on ventifacts near the crests of hills owing to the effects of velocity acceleration. Two conditions that affect ventifact development are considered in this paper: (1) wind acceleration through topographic constrictions; and (2) wind acceleration up the windward flanks of hills. Constrictions in the Barstow-Bristol trough allowed velocity increases that resulted in ventifa...

Aeolian Destabilization along the Mojave River, Mojave Desert, California: Linkages among Fluvial, Groundwater, and Aeolian Systems

A notable aspect of ecosystem deterioration in arid regions is sand movement subsequent to the loss of plant cover. The Lower Mojave Valley in the Mojave Desert of southern California is undergoing rapid environmental change owing to groundwater drawdown, causing the death of phreatophytic vegetation and the reactivation of aeolian sand. Historical documentation shows that in the early twentieth century, sand derived from the open river channel accumulated to form large stabilized nebkhas, anchored by mesquite (Prosopis glandulosa), a plant that requires the water table to lie 9 m or less below the surface. The natural balance between winds, sand supply, and vegetation changed in the later twentieth century, as increasing demand for groundwater caused the water table to drop from a near-surface elevation to as much as 30 m below the surface. The mesquite that anchored the dunes declined in vigor or died. Riparian vegetation, including cottonwood and willow, was scoured from the channel during flood events, and did not recolonize as the low water table prevented the establishment of seedlings. Extensive new areas of the channel were laid open to wind erosion. Sand derived from the channel and cannibalized from the degrading nebkhas began to migrate downwind. Long sand stringers extended from west to east across the dunefield. In the 1990s, barchans developed atop the sand stringers, growing rapidly from incipient low-amplitude forms to well-developed barchans 60 m in width with sharply defined slipfaces 6 m in height. The dunefield has changed from a predominant mode of deposition to one of erosion and transportation, in which the water table and vegetation no long exert an influence. Dune encroachment from migrating sand streaks and dunes buried equipment, rendered buildings and homes unusable, blocked the entrances to property and destroyed pasture areas. Changes to the aeolian environment of the Lower Mojave Valley reflect complex interactions between the wind regime, river channel morphology and sediment, surface and subsurface hydrology, geology, vegetation, and human influence. This study demonstrates the sensitivity of aeolian systems to land-use changes, particularly in environments where wind is not a limiting factor, and illustrates the rapidity with which such changes can occur.

Wind Abrasion and Ventifact Formation in California

Ventifacts are found in several physical settings in California: in formerly glaciated areas, in periglacial areas above or beyond glacier limits, in presently semiarid areas, along the coast, and in true deserts. In several localities, both active and fossil forms are found. Ventifacts and abraded surfaces develop wherever strong winds, laden with abundant sediment, erode resistant boulders or bedrock. In California, as elsewhere, the abrasive agent is most commonly a fine- to medium-grained aeolian sand. In arid areas, ventifacts occur near to Pleistocene lake shorelines, downwind of alluvial rivers, near dune fields, or in corridors of regional sand transit. They formed principally during a drier middle Holocene period from 8 to 5 ka. Staining and discoloration of the abraded face, patchy granular disintegration and spalling of abraded surfaces, and stabilized aeolian sand provide evidence for current inactivity of wind erosion. Ventifacts found on moraines beyond receding ice fronts are much older, dating from Pleistocene cold stages.

DIAGENETIC CONTROLS ON GROUNDWATER SAPPING AND VALLEY FORMATION, COLORADO PLATEAU, REVEALED BY OPTICAL AND ELECTRON MICROSCOPY

Investigations into processes of valley formation on the Colorado Plateau have confirmed the important role of sapping in the Navajo Sandstone. The sapping process produces drainage systems that differ uniquely from fluvially eroded networks in their valley morphology, network pattern, spatial evolution, and degree of structural, lithologic, and stratigraphic control. The Navajo Sandstone is a highly transmissive aquifer. Sapping results from groundwater emergence above a permeability boundary formed by the underlying Kayenta Formation. This discharge undercuts cliff faces, and causes massive slab failures and vertical cliff recession. The principal agent for the physical weakening of the Navajo Sandstone at a site of seepage appears to be the mechanical separation of sand grains by the deposition of calcite from saturated waters. The control of porosity and permeability by textural and mineralogical features in the Navajo Sandstone and Kayenta Formation was studied using reflected light and cathodolumine...