Soil-water-rock interactions I: The pediment problem

You may have not initially appreciated that piedmonts (landscapes between steep mountain masses and depositional basins) are not all covered by alluvial fans. In fact, piedmonts, along with adjoining low-relief mountain passes and sediment-covered domes in the arid southwest are often erosional landforms. They are smooth, low-sloping bedrock surfaces mantled by a thin, uniform sediment blanket--a pediment (Figure 1). Cosmogenic radionuclide measurements suggest that the bedrock and sediment surfaces of pediments are lowering through time in near unison through active weathering and erosion. The transition between pediments and adjoining rocky mountain masses is often abrupt, forming a sharp slope discontinuity, the 'piedmont junction', despite the absence of faulting or changes in rock type. Despite the ubiquity of these landforms in arid and semiarid environments, they embody a century-old riddle to geologists.

The crux of the 'pediment problem' is two-fold. 1) We know that erosion on pediments is predominantly caused by stream flow. Stream erosion typically roughens and corrugates a surface (at a variety of spatial scales) and forms incised drainage networks because flows concentrate downstream and because sediment transport is a nonlinear function of flow concentration. But many pediments are smooth, being only weakly incised by immature drainage networks characterized by subtle channels that are often observed to fade within tens of meters without connecting to a larger drainage network or contacting bedrock. So, how is it that erosion by stream flow on pediments creates a smooth surface rather than a rough, channelized one? 2) If the pediment is erosional, than one might expect the sediment blanket to erode down to the underlying bedrock, leaving a hard, rocky landform. But what we see in the field is a relatively smooth, uniformly thick sediment mantle over an analogously smooth bedrock floor. But how does the bedrock surface lower and keep pace with the eroding sediment surface above?

A plausible and likely explanation for 1) above is firstly that channels on pediments are laterally unstable because of sparse vegetation and non-cohesive sediment. These conditions allow flows to bifurcate (split apart) and effectively widen downslope rather than deepen and incise (as you tend to see in more humid landscapes). Secondly, in many desert environments, coarse sediment provides high infiltration rates which are especially effective at reducing runoff magnitude and flow distance, reducing downstream flow concentration. Thirdly, typical localized storms exhibit footprints smaller than the piedmont landscape, creating localized areas of stream flow concentration bounded downslope by areas of flow dissipation and hence sediment deposition. And finally, the presence of non-cohesive soils promotes sediment transport driven by rainsplash, which tends to fill in rills and incipient channels in many arid/semiarid environments.

An experimentally-supported mechanism that addresses 2) above and that produces the thin and persistent sediment blanket that characterizes pediments is illustrated in Figure 3. The figure plots an hypothesized weathering rate (which produces sediment), W(h) [m/m.y.], as a function of the thickness h [m] of the sediment blanket, together with alluvial surface erosion rates (labeled e and e' [m/m.y.]). Here, the development of a persistent equilibrium sediment blanket of thickness h_eq on pediments can be explored through examination of perturbations about h_eq. For example, when h>h_eq, sediment surface erosion rates are greater than bedrock weathering rates, and thus the sediment blanket will thin over time. This thinning causes bedrock weathering to accelerate, resulting in an equilibrium sediment blanket thickness (h-->h_eq, from right). When h<h_eq, the sediment blanket thickens over time, causing bedrock weathering rates to decrease; the thickness of the sediment blanket ultimately becomes stable (h-->h_eq, from left). Another stable, equilibrium sediment blanket thickness at h_eq = 0 m derives from the same mechanism, operating to the left of the hump in the weathering curve.

By incorporating these processes and feedbacks in a model, geologists have been able to explore the development of pediments (and piedmont junctions) numerically with a simplified initial geometry--a steep, inclined, planar bedrock surface (Fig. 4A). Weathering of the initially bare range-front produces debris (Fig. 4B). For steep bedrock slopes (approx. >5°), runoff will tend to remove this debris faster than weathering produces it. Meanwhile, the slope of the bare-bedrock mountain mass remains constant as it weathers and retreats. The fate of the debris and evolution of the pediment are determined by downslope boundary conditions. For a hydrologically-open basin with a fixed base level (Fig. 4A-D), weathered material accumulates at, and upslope of, the fixed base level, creating an alluvial surface whose low slope is graded to transport the material delivered to it from the retreating bedrock mass (Fig. 4B). As the mountain mass dwindles and the rate of sediment delivery decreases, the slope and therefore elevation of the alluvial surface decreases, and a pediment forms (Fig. 4C-D). Because the retreating range-front remains steep, an abrupt piedmont junction forms where the upslope-propagating pediment intersects the range-front.

A hydrologically-closed (depositional) basin (Fig. 4A, E-H) leads to a rising local base level, whereby material weathered from the upslope bare-bedrock mass accumulates downslope in an alluvial fan that aggrades at decreasing rates as the weathering mountain mass is consumed (Fig. 4E-F). Sediment continues moving downslope following the mountain mass' total disintegration, at which point, the sediment surface begins to erode upslope while aggrading downslope. This transition, in which the upslope portion of a previously depositional surface becomes erosional, allows bedrock weathering and sediment erosion rates to equilibrate, producing a pediment upslope (Fig. 4G). If the upslope portion of the sediment surface begins to erode before the mountain mass vanishes, a piedmont junction will again develop as in the open-basin simulation (Fig. 4H).

In both open and closed basins, pediments will ultimately extend to the topographic crest as the mountain mass is removed. This long-term state corresponds to a pediment pass (if the pediment extends upslope to a divide between mountain mass remnants) or dome (in the absence of any remnants), both of which are common in the deserts of the southwestern United States. Prominent examples include Cima Dome, California, pediment passes in the Sacaton Mountains, Arizona, and pediment passes and domes in Joshua Tree National Park, California.

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