Stephen G. Wells

Influences of eolian and pedogenic processes on the origin and evolution of desert pavements

Well-developed desert pavements are present above eolian deposits that mantle flows of the Cima volcanic field, located in the Mojave Desert, California. Soil-stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements. Eolian dust (1) accelerates mechanical fragmentation of flow rock, providing the source material for pavements, and (2) accumulates slowly below basaltic colluvium in flow depressions, eventually promoting development of cumulate soils below the evolving stone pavement. An increase in dust flux during the Holocene has raised ancient Pleistocene pavements as much as 20 cm above the former land surface. The results of our studies demonstrate for the first time that most desert pavements do not form by deflation, by overland flow, or by upward migration of stones through a slowly formed, clayey argillic horizon. Desert pavements are born and maintained at the surface.

Influence of Late Quaternary Climatic Changes on Geomorphic and Pedogenic Processes on a Desert Piedmont, Eastern Mojave Desert, California

Radiocarbon dating of late Quaternary deposits and shorelines of Lake Mojave and cation-ratio numerical age dating of stone pavements (Dorn, 1984) on the adjacent Soda Mountains piedmont provide age constraints for alluvial and eolian deposits. These deposits are associated with climatically controlled stands of Lake Mojave during the past 15,000 yr. Six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces, as well as on soil-geomorphic data. All but one of these stratigraphic units were deposited in response to time-transgressive climatic changes beginning approximately 10,000 yr ago. Increased eolian flux rates occurred in response to the lowering of Lake Mojave and a consequent increase in fine-sediment availability. Increased rates of deposition of eolian fines and associated salts influenced pedogenesis, stone-pavement development, and runoff-infiltration relations by (1) enhancing mechanical weathering of fan surfaces and hillslopes and (2) forming clay- and silt-rich surface horizons which decrease infiltration. Changes in alluvial-fan source areas from hillslopes to piedmonts during the Holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less-permeable soils. Inferred increased in early to middle Holocene monsoonal activity resulted in high-magnitude paleo-sheetflood events on older fan pavements; this runoff triggered piedmont dissection which, in turn, caused increased sediment availability along channel walls. Thus, runoff-infiltration changes during the late Quaternary have occurred in response to eolian deposition of fines, pedogenesis, increased sheetflood activity in the Holocene, and vegetational changes which are related to many complicated linkages among climatic change, lake fluctuations, and eolian, hillslope, and alluvial-fan processes.

Sedimentologic and geomorphic variations in storm-generated alluvial fans, Howgill Fells, northwest England

In June 1982, a storm with a return period greater than 100 yr, but lasting less than 2.5 hr, destabilized hillslopes and produced a suite of geomorphologically and sedimentologically diverse alluvial fans. Thirteen major fans were deposited at the tributary junctions between small ( 2 ) catchments and two north-flowing; headwater streams of the River Lune, northwest England. Storm-generated fans spread over or became inset into older stable fans and produced localized vertical accretion of as much as 3 m and lateral accretion of as much as 100 m. Sedimentary processes operating during deposition involved debris flow, transitional flow, and streamflow. Six facies types are recognized on the basis of depositional topography, sedimentary texture and fabric, and matrix content: viscous debris flow (D1), dilute debris flow (D2), transitional flow (T1), fluvial bars and lobes (S1, S2), and fluvial sheet gravels (S3). Regionally, streamflow deposition prevails over debris-flow deposition, and type S3 facies has the greatest areal extent. Temporal and spatial variations in facies deposition during the storm, however, resulted from water:sediment ratio variations. Fan deposition involved an early phase of debris flow to transitional flow due to large inputs of sediment from hillslope failures. This was followed by a systematic change to more dilute conditions, resulting in streamflow deposition and eventually in channel incision. A significant amount of geomorphic work and complex variations in sedimentary processes during the storm resulted, in part, from extensive overland flow and hillslope destabilization. Discriminant analyses indicate that catchment size, channel gradient, and percentage of area eroded during the storm controlled whether debris-flow or streamflow facies dominated a fan sequence. Smaller, steeper catchments had a greater percentage of the area yielding sediment and are dominanted by debris flows, whereas larger catchments produced more runoff resulting in dilution and streamflow. The results indicate that the facies sequences and fan entrenchment in the Howgill Fells, which are typically considered products of longer term climatic change or tectonics in other localities, are here primarily affected by thresholds related to catchment geomorphology, by the type of sediment available, and by the position within the storm cell.

Late Cenozoic landscape evolution on lava flow surfaces of the Cima volcanic field, Mojave Desert, California

Landscape evolution in the eastern Mojave Desert is recorded by systematic changes in Pliocene to latest Pleistocene volcanic land-forms that show discrete periods of eolian deposition, surface stabilization, drainage-network expansion, and erosion on basaltic lava flows. These processes are documented by K-Ar dating in conjunction with morphometric, sedimentologic, pedologic, and geophysical studies. Lava-flow surfaces are composed of constructional bedrock highs and accretionary eolian mantles with overlying stone pavements. The stratigraphy of these mantles records episodic, climatically induced influxes of eolian fines derived from playa floors and distal piedmont regions. The relative proportions of mantle and exposed bedrock vary with flow age, and flows between 0.25 and 0.75 m.y. old support the most extensive eolian mantle and pavement reflecting landscape stability. Drainage networks evolve on flows by (1) rapid initial extension, (2) maximum extension and elaboration, and (3) abstraction of drainage. Increases in bedrock exposures and erosion of the eolian mantle on flows >0.70 m.y. old coincide with maximum drainage extension and significant changes in soil and hydrologic properties within this mantle. Increasing the content of pedogenic clay and CaCO 3 causes the accretionary mantle9s permeability to decrease; decreased mantle permeability promotes increased runoff, surface erosion, and drainage development. In the late Cenozoic landscape evolution of lava flows, four major stages reflect variations in landscape stability that are controlled by the impact of episodic influxes of eolian fines and increasing soil-profile development on infiltration-runoff properties of the flow surfaces.

Quaternary uplift astride the aseismic Cocos Ridge, Pacific coast, Costa Rica.

The Pacific coast of Costa Rica lies within the Central American forearc and magmatic-arc region that was created by northeastward subduction of the Cocos plate beneath the Caribbean plate at the Middle America Trench. From the Peninsula de Nicoya south-eastward toward the Peninsula de Osa and the Peninsula de Burica on the Panamanian border, the Middle America Trench loses its physiographic expression where it intersects the aseismic Cocos Ridge. Interaction between subduction of the buoyant, aseismic Cocos Ridge and the overriding Caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from Nicoya to Burica. The Pliocene and Pleistocene stratigraphic record and Holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the Peninsula de Osa, immediately landward of the aseismic Cocos Ridge. Crustal uplift rates decrease northwest toward the Peninsula de Nicoya, and to a lesser extent southwest toward the Peninsula de Burica. The late Quaternary stratigraphy on the Peninsula de Osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates. Crustal uplift rates calculated from these sequences systematically decrease from 6.5 to 2.1 m/ka north-east across the peninsula. Deformation of the peninsula is modeled as uplifted and down-to-the-northeast-tilted fault blocks with an angular rotation rate of 0.03° to 0.06° per thousand years. Although less well constrained, crustal uplift rates on the Peninsula de Nicoya, 200 km to the northwest of the Peninsula de Osa, vary from <1 m/ka for Pliocene and Pleistocene sediments to 2.5 m/ka for Holocene marine terraces. In the Quepos region, 100 km to the northwest of the Peninsula de Osa, calculated uplift rates derived from incision of late Quaternary fluvial terraces range from 0.5 to 3.0 m/ka. On the Peninsula de Burica, only 60 km to the southwest of the Peninsula de Osa, calculated uplift rates range from 4.7 m/ka for a late Holocene marine terrace to 1.2 m/ka for post-late Pliocene deep-sea sediments. The variations in calculated uplift rates on the Peninsula de Osa constrain a dynamic model for subduction of the Cocos Ridge and the resulting uplift of the overriding Caribbean plate. Deflection of the Caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted Cocos Ridge. Because the shape of the subducted end of the Cocos Ridge is unknown, two scenarios are evaluated: (1) a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and (2) a ridge where the subducted end was truncated by the Panama fracture zone. The best-fit model utilizes a truncated ridge that has been subducted during the past 0.5 m.y. ∼50 km beneath the overriding Caribbean plate, which has an effective elastic thickness of 5 km. The model predicts that the highest uplift rate should be ∼3.7 m/ka and occur on the southwest coast of the Peninsula de Osa. The rate of uplift slows considerably to the northeast and indicates that the Peninsula de Osa is tilting to the northeast, which agrees with observations in that region. The predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the Peninsula de Osa, resulting in little uplift that can be attributed to Cocos Ridge subduction in the northwestern portions of the Peninsula de Nicoya.

Regional variations in tectonic geomorphology along a segmented convergent plate boundary, Pacific coast of Costa Rica

Abstract Pacific coastal mountain/piedmont landforms of Costa Rica extend across the tectonic boundary between the forearc and magnetic arc region of an active convergent margin. This plate boundary became segmented circa 1 million years ago when the aseismic Cocos Ridge impinged upon the Middle America Trench offshore from the southernmost coastal area of Costa Rica. Morphometric analyses of 100 mountain fronts and numerous river long-profiles, radiometric dating, and field studies were conducted in two study areas located arcward from the plate boundary where oceanic lithosphere of the Cocos plate is being subducted beneath the Caribbean plate (region I) and the partially subducted aseismic ridge is uplifting the plate margin by isostatic and collisional processes (region II). Values of tectonic geomorphic parameters [mountain front sinuosity (S), percent dissected facets (Ffd), river concavity (K)] are not only different statistically in regions I and II but are also different in the areas experiencing isostatic and collisional responses to the subducting aseismic ridge. In the area experiencing collisional responses, mountain fronts, developed along NE-dipping imbricate thrust and high-angle reverse faults, step upward and inland from the coast; morphometric data along with the divergence of river-terrace profiles from the coast piedmont inland toward the mountains indicates higher uplift rates along interior-range mountain fronts. Isostatic uplift in the outer forearc area in region II produces a distinctly different morphologic and neotectonic style characterized by regional uplift distributed across a number of blocks bounded by normal faults. Geomorphic analyses indicate a general southward trend of increasing tectonic uplift from region I into region II where the highest frequency of mountain fronts with low values of S and Ffd, as well as rivers with the highest values of K, occur over the crest of the subducted ridge. Field and historical seismic data for these regional trends include: (1) fault scarps displacing late Quaternary fluvial terraces and colluvial soils in areas of collisional responses; (2) Holocene-latest Pleistocene marine sediments uplifted tens of meters along normal faults in areas of isostatic response of region II; and (3) more frequent shallow, high-magnitude earthquakes in region II. This study indicates that spatial variations in the plate tectonic framework can be detected by regional morphometric analyses using techniques applied in extensional and compressional terranes of arid and semiarid regions but not previously applied to forearc systems along convergent plate boundaries in tropical areas.

Use of multiparameter relative-age methods for age estimation and correlation of alluvial fan surfaces on a desert piedmont, eastern Mojave Desert, California

Numerical and calibrated age determinations of the late Quaternary alluvial fan deposits of the Soda Mountains piedmont in the Mojave Desert provide an opportunity to study the utility of the multiparameter relative-age (RA) method for distinguishing and mapping geomorphic surfaces on a desert piedmont. Most RA parameters could not discriminate between deposits of Holocene age, although pavements have formed over locally significant parts of surfaces as young as middle Holocene. Several parameters, including lithologic composition, particle size, soil development, and varnish cover, permit distinguishing between Holocene surfaces and late Pleistocene surfaces. Statistically significant differences in initial particle size and lithology of the deposits, inferred to be the result of complex interaction among hillslope, alluvial fan, and eolian processes and climatic change, create conditions unfavorable for use of most RA techniques. In contrast, soil-profile development and varnish cover data are successful in discrimination among deposits of Holocene and Pleistocene age. This is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan.

Response of Quaternary fluvial systems to differential epeirogenic uplift: Aguas and Feos river systems, southeast Spain

The geomorphic expression of two fluvial systems, the Rio de Aguas and the Rambla de los Feos, in southeast Spain reflects their adjustment to differential epeirogenic uplift and local tectonic activity during the Quaternary. Regional uplift of a depositional marine surface resulted in the development of a southward-flowing drainage network during late Pliocene–early Pleistocene time. Disruption of this consequent drainage began as the Alhamilla-Cabrera Sierras and the Sorbas basin were differentially uplifted relative to the surrounding basins. The ancestral Feos drainage responded by incision across the uplift through a combination of superimposition and antecedence. Differential uplift of the Sorbas basin enhanced headward erosion of strike-oriented drainages and promoted capture of the ancestral Feos drainage by the Rio de Aguas during the late Pleistocene. Stratigraphic relations and soil development indicate that epeirogenic uplift and local tectonics, together with climatic fluctuations, have influenced the development of the drainage systems throughout the Quaternary.

Regional response of alluvial fans to the Pleistocene-Holocene climatic transition, Mojave Desert, California

Alluvial fan deposits along the Providence Mountains piedmont in the eastern Mojave Desert that (1) are derived from diverse rock types, (2) are dated with luminescence techniques and soil-stratigraphic correlations to other relatively well dated fan, eolian, and lacustrine deposits, and (3) have some of the highest peaks in the Mojave Desert, provide a unique opportunity to study the influence of Pleistocene-Holocene climatic transition on regional fan deposition across diverse geomorphic settings. Geomorphic and age relations among alluvial and eolian units along the Providence Mountains and Soda Mountains piedmonts indicate that most of the late Quaternary eolian and alluvial fan units were deposited during similar time intervals and represent region-wide changes in geomorphic factors controlling sediment supply, storage, and transport. Deposition of alluvial fans in the desert southwestern United States during the latest Pleistocene has been largely attributed to (1) a more humid climate and greater channel discharge and (2) time-transgressive changes in climate and an increase in sediment yield. Stratigraphic and age relations among depositional units demonstrate that a regional period of major alluvial fan deposition occurred between ca. 9.4 and 14 ka, corresponding with the timing of the Pleistocene-Holocene climatic transition. This age range indicates that deposition of these fans is not simply a result of greater effective moisture and channel discharge during the last glacial maximum. Increases in sediment yield during the Pleistocene-Holocene transition have been largely attributed to a timetransgressive decrease in vegetative cover with an increase in hillslope erosion. Geomorphic relations along the Providence Mountains, however, suggest that that changes in vegetation cover during the Pleistocene-Holocene climatic transition may have had a limited impact on hillslope instability and sediment yield because of (1) the inherently high infiltration capacity of coarse-textured soils and colluvium, (2) possible strong spatial variations in soil cover across hillslopes, and (3) modern vegetation cover appears to provide enough stability for the buildup of soils and colluvium. An increase in sediment yield may instead be largely due to an increase in extreme storm events, possibly an increase in tropical cyclones. Extreme storms would provide the rainfall intensity and duration to mobilize permeable sediments from mountain catchments and into distal fan areas.

Short-Duration Holocene Lakes in the Mojave River Drainage Basin, Southern California

Stratigraphic, sedimentologic, and pedologic studies of beach ridge and lacustrine deposits indicate that up to five times during the Holocene, shallow lakes covered Silver Lake playa in southeastern California for periods of years to decades. The two youngest lacustrine events (at about 390 ± 90 yr B. P. and 3620 ± 70 yr B. P.) coincide with the early and late Neoglacial episodes of North America. Increasing evidence in recent years from other nonglaciated areas leads us to conclude that the effects of these climatic episodes were much more widespread than previously thought. The climate during these episodes was characterized by an increased frequency of winter storms in the southwestern United States, causing wetter conditions that affected diverse, hyperarid environments in the Mojave Desert and adjacent regions. We propose that this wide areal coverage was caused by large-scale, winter atmospheric circulation patterns, which are probably related to changes in sea-surface temperatures and oceanic circulation in the eastern North Pacific Ocean.