John C. Dohrenwend

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.

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.

Influences of quaternary climatic changes on processes of soil development on desert loess deposits of the Cima volcanic field, California

Summary Soils formed in loess are evidence of both relict and buried landscapes developed on Pliocene-to-latest Pleistocene basalt flows of the Cima volcanic field in the eastern Mojave Desert, California. The characteristics of these soils change systematically and as functions of the age and surface morphology of the lava flow. Four distinct phases of soil development are recognized: phase 1 - weakly developed soils on flows less than 0.18 M.y. old; phase 2 - strongly developed soils with thick argillic horizons on 0.18 – 0.7 M.y. old flows; phase 3 - strongly developed soils with truncated argillic horizons massively impregnated by carbonate on 0.7 to 1.1 M.y. old flows; and phase 4 - degraded soils with petrocalcic rubble on Pliocene flows. A critical aspect of the development of stage 1 soils is the evolution of a vesicular A horizon which profoundly affects the infiltration characteristics of the loess parent materials. Laboratory studies show that secondary gypsum and possibly other salt accumulation probably occurred during the period of phase 1 soil development. Slight reddening of the interiors of peds from vesicular-A horizons of phase 1 soils and presence of weakly developed B horizons indicates a slight degree of in situ chemical alteration. However, clay and Fe oxide contents of these soils show that these constituents, as well as carbonates and soluble salts, are incorporated as eolian dust. In contrast to phase 1 soils, chemical and mineralogical analysis of argillic horizons of phase 2 soils indicate proportionally greater degrees of in-situ chemical alteration. These data, the abundant clay films, and the strong reddening in the thick argillic horizons suggest that phase 2 and phase 3 soils formed during long periods of time and periodically were subjected to leaching regimes more intense than those that now exist. Flow-age data and soil-stratigraphic evidence also indicate that several major loess-deposition events occurred during the past ∼ 1.0 M.y. Loess events are attributed to past changes in climate, such as the Pleistocene-to-Holocene climatic change, that periodically caused regional desiccation of pluvial lakes, reduction of vegetational density, and exposure of loose, unconsolidated fine materials. During times of warmer interglacial climates, precipitation infiltrates to shallower depths than during glacial periods. Extensive, saline playas which developed in the Mojave Desert during the Holocene are a likely source of much of the carbonates and soluble salts that are accumulating at shallow depths both in phase 1 soils and in the formerly noncalcareous, nongypsiferous argillic horizons of phase 2 and 3 soils.

Degradation of Quaternary cinder cones in the Cima volcanic field, Mojave Desert, California

Basaltic cinder cones in the Cima volcanic field record a detailed history of progressive erosion in the arid environment of the eastern Mojave Desert. These cones range in age from ∼0.015 m.y. to 1.09±0.08 m.y., as dated by radiocarbon or K-Ar analyses of the youngest lava flows from each cone. Cone heights range from 50 to 155 m, and basal widths range from 400 to 915 m; on younger cones, height/width ratios average 0.17, and crater-width/cone-width ratios average 0.42. The degradational morphology of these cones displays several trends that are closely related to cone age. (1) Crater-width/cone-width ratios decrease from 0.48 on the youngest cone to 0.21 on the oldest cone with a preserved crater. (2) Mean maximum side slopes (Tan Sc) decrease from 0.575 on the youngest cone to an average of 0.41 on the oldest cones studied. (3) Debris-apron–height/cone-height ratios increase from <0.10 on the youngest cone to an average of 0.34 on the oldest cones. (4) Cone drainage evolves from irregularly spaced rills and gullies on the youngest cone to regularly spaced gullies on 0.20- to 0.35-m.y.-old cones to valleys as much as 110 m wide and 10 m deep on cones 0.59 m.y. old and older. These trends form the basis of an empirical model of cinder-cone degradation in arid environments. This model documents (1) an erosional loss of ∼15% of cone volume during the first million years; (2) progressive decline of cone slope (at an average rate of 0.006°/103 yr) and cone height (at an average rate of 2.25 cm/103 yr); (3) initial rapid stripping of the loose cinder mantle from upper-cone slopes accompanied by rapid debris-apron formation; and (4) a gradual transition, between 0.25 and 0.6 m.y., from relatively uniform stripping of upper slopes to localized fluvial dissection of both cone slopes and debris apron. This transition is apparently controlled by a concomitant change from diffuse subsurface drainage within the pervious cinder mantle to concentrated surface flow across the heterogeneous assemblage of agglutinate layers, dikes, and ponded flows of the cone interior.

K-Ar dating of the Cima volcanic field, eastern Mojave Desert, California: Late Cenozoic volcanic history and landscape evolution

Thirty-four K-Ar ages and supporting paleomagnetic measurements from flows of the Cima volcanic field provide a detailed volcanic history and a temporal basis for analysis of evolving landforms. The Cima field has undergone three periods of volcanic activity, spanning late Miocene through latest Pleistocene time: (1) 7.6 to 6.5, (2) 4.5 to 3.6, and (3) 1.0 to at least 0.015 m.y. ago. These precisely dated sequences afford a unique opportunity to quantify the long-term effects of pedogenesis and erosion on volcanic landforms in an arid environment. Morphologic and pedologtc data indicate that the most stable geomorphic surfaces in the field occur on flows between 0.25 and 0.75 m.y. old. Younger flows are dominated by eolian aggradation and outcrop rubbling, and older flows are dominated by surface runoff and fluvial dissection. A process-response model involving progressive pedogenesis and a subsequent shift from infiltration to surface runoff is proposed to explain this temporal variation in land-surface stability.

Cation-ratio and accelerator radiocarbon dating of rock varnish on Mojave artifacts and landforms

The first accelerator radiocarbon dates of rock varnishes are reported along with potassium/argon ages of lava flows and conventional radiocarbon dates of pluvial lake shorelines, in an empirical calibration of rock varnish K+ + Ca2+/Ti4+ ratios with age in the Mojave Desert, eastern California. This calibration was used to determine the cation-ratio dates of 167 artifacts. Although cation-ratio dating is an experimental method, some dates suggest human occupation of the Mojave Desert in the late Pleistocene.

Nivation landforms in the western Great Basin and their paleoclimatic significance

Abstract More than 10,000 nivation landforms occur in the higher mountain ranges of the western Great Basin. They range from small, subtle hollows with head scarps a few meters high and a few tens of meters long to broad, clearly defined terraces as much as 220 m wide bounded by bold, steeply sloping head scarps as much as 30 m high and 1600 m long. Distribution of these nivation hollows is strongly influenced by elevation, slope orientation, local relief, and substrate lithology. About 95% occur between 2200 and 3000 m elevation, and nearly 80% are situated on north-northwest-to east-northeast-facing slopes. They occur mainly in areas of moderately sloping terrain and moderate local relief, and they are preferentially developed on relatively incompetent substrates including terrigenous sedimentary deposits, volcanic and metavolcanic rocks of intermediate composition, and deeply weathered granitoid rocks. Nearly all of these nivation hollows are relict. They are most abundant near areas of late Pleistocene glaciation but rarely occur within such areas. Most are veneered with colluvium and are well vegetated, and many hollows in the Mono Basin area are veneered with volcanic ash at least 700 yr old. Distribution of nivation hollows suggests that (1) the full-glacial nivation threshold altitude (NTA) rose from north to south at 190 m per degree of latitude, subparallel to, and approximately 740 m lower than, the full-glacial equilibrium-line altitude (ELA) and about 1370 m lower than the estimated modern ELA; (2) the difference between the full-glacial and modern ELAs indicates an approximate 7°C full-glacial mean-annual-temperature depression throughout the Great Basin; and (3) the full-glacial mean annual temperature at the NTA is estimated to have been approximately 0° to 1°C, assuming little change in accumulation-season precipitation.

Relict sheetflood bed forms on late Quaternary alluvial-fan surfaces in the southwestern United States

Late Pleistocene and early Holocene alluvial-fan surfaces in southeastern California are locally mantled by narrow, widely spaced, transverse-to-slope bands of fine gravel and coarse sand. These features are interpreted to be relict bed forms of high-magnitude sheetfloods that inundated abandoned alluvial-fan surfaces. The majority of these sheetflood events apparently occurred in the latest Pleistocene to middle Holocene and produced meso–bed forms with wavelengths of 2–6 m. These flood events mobilized sediment with mean grain sizes of 2–8 mm on inactive fan surfaces under calculated velocities of 30–60 cm/s and may be responsible for increased dissection of upper piedmonts and widespread alluvial-fan deposition in the lower piedmont regions. An older sheetflood event produced macro-bed forms with wavelengths of 20–80 m on a pedogenic CaCO 3 -cemented piedmont in southwestern Arizona. Both scales of bed forms display characteristics of confined-flow fluvial bed forms which form under diverse flow regimes that cause either megaripples or transverse ribs.

Drainage development on basaltic lava flows, Cima volcanic field, southeast California, and Lunar Crater volcanic field, south-central Nevada

Drainage networks on the accretionary eolian mantles of 11 K-Ar-dated lava flows in the Cima volcanic field and 8 K-Ar-dated flows in the Lunar Crater volcanic field show several related and progressive changes with flow age. If it is assumed that these changes with flow age reflect changes through time, they provide a useful perspective on the early stages of drainage-network development. These drainage networks grow in a manner that is consistent with the qualitative model proposed by Glock. After accumulation of an eolian mantle (requiring 0.1 to 0.2 m.y.), master drainages extend to all parts of a flow. During this period of elongation, which lasts for ∼0.2 to 0.3 m.y., drainage density (D) and link frequency (F) increase rapidly whereas the value of Shreve9s kappa (κ) declines. After ∼0.4 m.y., elaboration replaces elongation as the principal mode of network extension and remains predominant throughout the remainder of the 1.1-m.y. period of record. Elaboration proceeds mainly by the formation of short tributaries along existing streams and by the creation of short streams along flow margins. During the early phases of elaboration, D and F attain a condition wherein the number and lengths of channel links are approximately adjusted to one another, and κ reaches a minimum value of ∼0.75. Thereafter, the continuing addition of short tributaries to existing streams causes D, F, and κ to increase slowly as the networks evolve toward maximum extension.

Systematic valley asymmetry in the central California Coast Ranges

Asymmetry of east-west-trending valleys is a prominent landscape feature in the Salinas Valley and Gabilan Mesa area of the central California Coast Ranges. North-facing side slopes in these valleys are significantly steeper, less dissected and more heavily vegetated than south-facing side slopes. This systematic asymmetry persists throughout terrains of widely varying lithology, structural style, and tectonic history, and it is most strongly developed in areas that are underlain by horizontal to very gently dipping semiconsolidated sedimentary rocks. Asymmetric distribution of fill terraces and beheaded streams and close correlation between valley side-slope angle and valley gradient demonstrate that preferential lateral stream erosion has played a dominant role in the development of this valley asymmetry. For most of the valleys studied, this preferential lateral erosion is not related to tectonic movement; it is caused mainly by the asymmetric operation of slope-wasting processes arising from microclimatic differences on opposing valley side slopes.