Stephen Wells

New Mexico Tech, Office of President, Past President

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Environmental history recorded in eolian deposits under stone pavements, eastern Mojave Desert, USA

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NSHE's Nevada Renewable Energy Consortium

The 3rd Annual Renewable Energy Symposium took place on the UNLV campus August 11 & 12. The event... more

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Are the benches at Mormon Point, Death Valley, California, USA, scarps or strandlines?

Quaternary Research, 2004

The benches and risers at Mormon Point, Death Valley, USA, have long been interpreted as strandli... more The benches and risers at Mormon Point, Death Valley, USA, have long been interpreted as strandlines cut by still-stands of pluvial lakes correlative with oxygen isotope stage (OIS) 5e/6 (120,000-186,000 yr B.P.) and OIS-2 (10,000-35,000 yr B.P.). This study presents geologic mapping and geomorphic analyses (Gilbert's criteria, longitudinal profiles), which indicate that only the highest bench at Mormon Point (∼90 m above mean sea level (msl)) is a lake strandline. The other prominent benches on the north-descending slope immediately below this strandline are interpreted as fault scarps offsetting a lacustrine abrasion platform. The faults offsetting the abrasion platform most likely join downward into and slip sympathetically with the Mormon Point turtleback fault, implying late Quaternary slip on this low-angle normal fault. Our geomorphic reinterpretation implies that the OIS-5e/6 lake receded rapidly enough not to cut strandlines and was ∼90 m deep. Consistent with independent core studies of the salt pan, no evidence of OIS-2 lake strandlines was found at Mormon Point, which indicates that the maximum elevation of the OIS-2 lake surface was -30 m msl. Thus, as measured by pluvial lake depth, the OIS-2 effective precipitation was significantly less than during OIS-5e/6, a finding that is more consistent with other studies in the region. The changed geomorphic context indicates that previous surface exposure dates on fault scarps and benches at Mormon Point are uninterpretable with respect to lake history.

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Spatial and Temporal Deformation along the Northern San Jacinto Fault, Southern California: Implications for Slip Rates

Bulletin of the Seismological Society of America, 2002

The San Timoteo badlands is an area of uplift and erosional dissection that has formed as a resul... more The San Timoteo badlands is an area of uplift and erosional dissection that has formed as a result of late Quaternary uplift along a restraining bend in the San Jacinto fault, of the San Andreas fault system in southern California. This bend currently is located in a region where late Quaternary deposits and associated surfaces have formed in lower San Timoteo Canyon. We have used morphometric analysis of these surfaces, in conjunction with computer modeling of deformational patterns along the San Jacinto fault, to reconstruct spatial and temporal variations in uplift along the bend. Morphometric techniques used include envelope/subenvelope mapping, a gradient-length index along channels, and denudation values. Age control is determined using a combination of thermoluminescence (TL) and near infrared optical simulation luminescence dating (IROSL) and correlation of soil-development indices. These approaches are combined with an elastic half-space model used to determine the deformation associated with the fault bend. The region of modeled uplift has a similar distribution as that determined by morphometric techniques. Luminescence dates and soil-correlation age estimates generally agree. Based on soil development, surfaces within the study area were stabilized at approximately 300-700 ka for Q3, 43-67 ka for Q2, and 27.5-67 ka for Q1. Luminescence ages (both TL and IROSL) for the formation of the younger two surfaces are 58 to 94 ka for Q2 and 37 to 62 ka for Q1 (ages reported to 1r uncertainty). Periods of uplift were determined for the surfaces in the study area, resulting in approximate uplift rates of 0.34 to 0.84 m/ka for the past 100 ka and 0.13 to 1.00 m/ka for the past 66 ka. Comparison of these rates of uplift to those generated by the model support a higher rate of lateral slip along the San Jacinto fault than commonly assumed (greater than 20 mm/yr, as compared to 8-12 mm/yr commonly cited). This higher slip rate supports the proposal that a greater amount of slip has transferred from the San Andreas fault to the San Jacinto fault than generally held. The San Jacinto fault may have accommodated a significant portion of the plate boundary slip during the past 100 ka.

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Factors and processes governing the 14C content of carbonate in desert soils

Earth and Planetary Science Letters, 1994

A model is presented describing the factors and processes which determine the measured 14C ages o... more A model is presented describing the factors and processes which determine the measured 14C ages of soil calcium carbonate. Pedogenic carbonate forms in isotopic equilium with soil CO 2. Carbon dioxide in soils is a mixture of CO 2 derived from two biological sources: respiration by living plant roots and respiration of microorganisms decomposing soil humus. The relative proportion of these two CO z sources can greatly affect the initial 14C content of pedogenic carbonate: the greater the contribution of humus-derived CO 2, the greater the initial 14C age of the • • • 1"4 carbonate mineral. For any gwen mixture of CO 2 sources, the steady-state CO 2 distribution vs. soil depth can be described by a production/diffusion model. As a soil ages, the 14C age of soil humus increases, as does the steady-state 14C age of soil CO 2 and the initial 14C age of any pedogenic carbonate which forms. The mean 14C age of a complete pedogenic carbonate coating or nodule will underestimate the true age of the soil carbonate. This discrepancy increases the older a soil becomes. Partial removal of outer (and younger) carbonate coatings greatly improves the relationship between measured 14C age and true age. Although the production/diffusion model qualitatively explains the 14C age of pedogenic carbonate vs. soil depth in many soils, other factors, such as climate change, may contribute to the observed trends, particularily in soils older than the Holocene.

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A soil chronosequence on Lake Mega-Frome beach ridges and its implications for late Quaternary pedogenesis and paleoenvironmental conditions in the drylands of southern Australia

Quaternary Research, 2015

The terminal lake systems of central Australia are key sites for the reconstruction of late Quate... more The terminal lake systems of central Australia are key sites for the reconstruction of late Quaternary paleoenvironments. Paleoshoreline deposits around these lakes reflect repeated lake filling episodes and such landforms have enabled the establishment of a luminescence-based chronology for filling events in previous studies. Here we present a detailed documentation of the morphology and chemistry of soils developed in four well-preserved beach ridges of late Pleistocene and mid-to-late Holocene age at Lake Callabonna to assess changes in dominant pedogenic processes. All soil profiles contain evidence for the incorporation of eolian-derived material, likely via the formation of desert pavements and vesicular horizons, and limited illuviation due to generally shallow wetting fronts. Even though soil properties in the four studied profiles also provide examples of parent material influence or site-specific processes related to the geomorphic setting, there is an overall trend of inc...

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Extracting Holocene paleohydrology and paleoclimatology information from modern extreme flood events: An example from southern California

Geomorphology, 1997

The extraction of paleohydrological and paleoclimatological information from a modem hydrological... more The extraction of paleohydrological and paleoclimatological information from a modem hydrological system, shown to represent unique and extreme hydroclimatological conditions, is illustrated by an example from the Mojave River drainage basin in southern California. The Mojave River allows only the most extreme floods to reach its terminal basin in the Silver Lake playa and to form ephemeral lakes. All the other floods are lost by transmission into the alluvial aquifer along its 200 km channel. This filtering out of regular floods by the river provides an essential tool in establishing a physical link between atmospheric and hydrologic conditions. We demonstrate such a link between anomalous, present-day atmospheric circulation patterns over the North Pacific Ocean, extreme storms in southern California that produced the heaviest precipitation on record, the largest floods of record in the Mojave River watershed, and ephemeral lakes in its terminal playa. This physical link determines the possible cause of the formations of perennial, short-duration, shallow lakes in Silver Lake playa during the late Holocene and dharacterizes the hydroclimatic conditions that prevailed during these lacustrine episodes. Hydrological simulations of this river and its filtering character demonstrate that these lakes could have formed only if the most extreme modem storms and floods were more frequent in at least an order of magnitude during specific time episodes. We conclude that such extn:me hydroclimatic conditions occurred more frequent in past episodes during which the Holocene lakes formed. In turn, this conclusion indicates that the cause of these storms and floods, i.e. the anomalous atmospheric circulation pattern, must have been more frequent. This research outlines a way to extract information on Holocene climates in hydrologic settings that demonstrate a unique cause and effect relationship. 0 1997 Elsevier Science B.V.

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Response of alluvial fan systems to the late Pleistocene to Holocene climatic transition: contrasts between the margins of pluvial Lakes Lahontan and Mojave, Nevada and California, USA

CATENA, 1999

Dated shorelines of late Pleistocene pluvial Lakes Lahontan Great Basin Desert, northwest . Ž . N... more Dated shorelines of late Pleistocene pluvial Lakes Lahontan Great Basin Desert, northwest . Ž . Nevada and Mojave Mojave Desert, eastern California provide timelines for the assessment of alluvial fan sedimentation at the lake margins during the late Pleistocene to early Holocene. Two sets of alluvial fan systems have been mapped: the Stillwater fans, feeding Lake Lahontan; and the Zzyzx fans, feeding Lake Mojave. Their contrasting morphologies suggest different responses of the two fan systems to late Pleistocene to early Holocene climatic change. At the time the Stillwater fan systems underwent minimal sedimentation, with the catchment hillslopes apparently stable. The Zzyzx fans experienced major changes in water and sediment supply from the catchment hillslopes. There was a major phase of hillslope debris-flow activity, followed by fanhead trenching and distal fan progradation. Both areas were wetter and colder in the late Pleistocene than they are today, but during the transition to the Holocene the Zzyzx area was more likely to experience intense rains associated with the monsoonal penetration of warm moist tropical air into the Southwest. Vegetation reconstructions for the late Pleistocene to the early Holocene suggest that catchment hillslopes in the Mojave supported a desert shrub vegetation, but those in the Stillwaters supported juniper woodland and grasses at low elevations and pine at higher elevations. Contrasts in hillslope vegetation cover together with storm activity may account for the different responses of the alluvial fans to climatic change during the Pleistocene to

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Environmental history recorded in aeolian deposits under stone pavements, Mojave Desert, USA

Quaternary Research, 2016

Reconstructing the evolution of arid landscapes is challenged by limited availability of appropri... more Reconstructing the evolution of arid landscapes is challenged by limited availability of appropriate environmental archives. A widespread surface feature — stone pavement — traps aeolian fines and forms a special accretionary archive. Seven stone pavement-covered sections on basalt flows in the eastern Mojave Desert are condensed into a composite section, comprising five sedimentological units supported by an OSL-based chronology. Three of the units are of accretionary nature and each is covered by a stone pavement. They were deposited > 50.9–36.6 ka, < 36.6–14.2 ka and < 14.2 ka, and they are intimately coupled with the history of nearby Lake Mojave, which advances the current understanding of regional aeolian activity. End-member modeling analysis of grain-size distributions yielded seven sediment transport regimes. The accretionary system operates in two modes: A) episodic formation of a stone pavement by lateral processes once a vesicular horizon has formed on a barren ...

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

Paleoenvironments and paleohydrology of the Mojave and southern Great Basin deserts, 2003

Alluvial fan deposits along the Providence Mountains piedmont in the eastern Mojave Desert that (... more 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.

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Measuring the Age of the Lathrop Wells Volcanic Center at Yucca Mountain

Science, 1992

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The vesicular layer and carbonate collars of desert soils and pavements: formation, age and relation to climate change

Geomorphology, 1998

The vesicular, fine-grained A horizon Av is the widespread, ubiquitous surficial horizon of deser... more The vesicular, fine-grained A horizon Av is the widespread, ubiquitous surficial horizon of desert soils in diverse landforms and parent materials of varying ages. Now known to form mostly through accumulation of eolian dust, recent studies show that dust accumulation and concomitant soil development are genetically linked to stone pavement formation. Changes in the magnitude of eolian activity and effective leaching related to Quaternary climatic changes are also hypothesized to have influenced the evolution of the Av horizon. Numerical modeling, geochronologic, and fieldrlaboratory studies elucidate the nature of pedogenic processes controlling compositional evolution of Av, how the changing Av horizon increasingly influences soil infiltration and carbonate translocation and accumulation, and the control that clasts of the evolving pavement exert on pedogenic processes. Results of a model that determines soil bulk chemical composition based Ž . on mixing of estimated proportions of externally derived eolian material and parent materials imply that the evolution of the soil bulk composition is strongly influenced by Av horizon formation. The early development of a weakly to moderately developed Av horizon directly over gravelly parent material in late and middle Holocene soils moderately influences soil infiltration, but significant leaching of very soluble materials and some carbonate in dust are permitted. In older, Pleistocene soils, however, the texturally more mature Av and underlying, cumulic nongravelly horizons more strongly limit the rate and depth of leaching, and soil bulk composition therefore more closely approximates a simple mixture of dust and parent material. Other aspects of Av horizon development and its relations to the pavement are evaluated through studies of pavement clasts with coatings of soil carbonate, referred to as carbonate collars. Development of a numerical model that integrates soil hydrology, a CO production-diffusion model, calcite kinetics and thermodynamic considerations, composi-

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Cosmogenic 3He surface-exposure dating of stone pavements: Implications for landscape evolution in deserts

Geology, 1995

The formation of stone pavements, a ubiquitous gravel armor mantling landforms in arid regions of... more The formation of stone pavements, a ubiquitous gravel armor mantling landforms in arid regions of the world, has been previously attributed to erosion by wind and water or alternating shrinking and swelling of soil horizons, implying that gravel is concentrated at the land surface in a time-transgressive manner. A newly proposed model for pavement evolution differs from these models in that pavement clasts are continuously maintained at the land surface in response to deposition and pedogenic modification of windblown dust. In-situ cosmogenic 3 He surface-exposure ages on volcanic and alluvial landforms in the Mojave Desert of California are used to understand pavement evolution over geologic time scales and to test this new model. These exposure ages are stratigraphically consistent, show internal consistency at each site, and, for stone pavements adjacent to pristine, continuously exposed volcanic bedrock, are indistinguishable at the 1 level. We conclude that stone pavements are born at the surface and that pavements may provide one of the longest-term records of geologic, hydrologic, and climatic processes operating on desert surfaces.

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

Geology, 1987

WeJt-developed desert plvements Ire present lbove eolian deposits that mantle noww 01 the CiItUl ... more WeJt-developed desert plvements Ire present lbove eolian deposits that mantle noww 01 the CiItUl volcanic rleld, localed in lhe Mojave Desert, California. Soil-straticnphic data ODd geochemical data demonstrate that eolian and pedogenic processes play rn¥x' rotes in the evolution of these pavements. Eolian dust (I) I=leroles mechanical mogmentatloa oIliow rock, providing the source materiaJ for pavements., and (2) accumulates slowly below basaIdc colluvium in Row depressions, eventually promoline development of cumulate loils below the evolving stone pavement. An Increase in dust nux during the Holocene has raised andeaC Pleistocene pavements as much as 20 •cm above the former land surface. The results 01 our studies demonstrate for the first time that most desert pavements do not ronn by deflation, by overland now, or by upw2rd migration of stones through I slowly formed, clayey arcillk horizon.. Desert pavements arc born and maintained at the surfac:e. INTRODUcnON Desen pavements. oonslstmg of • oneto two--panicle.thick layer of closely packed, angu• ' We thank Jobn Dobrenwend for userul disclt and comments regardina the aeologic history 0 Cima volcanic field; Brent Turrin for his efTorl.o ,I K•Ar dating or flows in the Cima vok:anic field, II well as his contributions to the undem.andinl 0' .~ geologic history of this area; and D. Weide O. RitlCT for constructive aiUtism.

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

Geological Society of America Bulletin, 1985

Landscape evolution in the eastern Mojave Desert is recorded by sys1ematic changes in Pliocene to... more Landscape evolution in the eastern Mojave Desert is recorded by sys1ematic 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 nows. These processes are documented by K•Ar dating in conjunction with morphometric, sedimentologic. pedologic, and geophysica1 studies. Lava-Dow surfaces are composed of constructional bedrock highs and accretionary eolian mantles with overlying stone pave-ments. The stratigrapby of these mantles records episodic, climaticaJJy induced influxes of eolian rmes derived from playa floors and distal piedmont regions. 1De relative proportions of mantle and exposed bedrock vary with flowage, and Dows between 0.25 and 0.7S m.y. old support the most extensive eolian mantle and pavement reflecting landscape stability. Drainage networks evolve on nows 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 Oows > 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 CaC0 3 causes the accretionary mantle's penneability to decr,ease; decreased mantle permeability promotes increased runoff, surface erosion, and drainage development. In the late Cenozoic. landscape evolution of lava nows, four major stages reneet variations in land• scape stability that are contTOlled by the impact of episodic influxes of eolian fmes and increasing soi1-profile development on infiltration• runoff properties of the Dow surfaces. The morphology of volcanic landforms is used as a basis for the classification of volcanic activity (IUttmann, 1962; lUtter, 1978) and for interpretations of the styles and rates of volcanic eruptions . Few studies, however, use volcanic landforms to interpret types, rates, and magnitudes of the surficial processes that modify volcanic forms. Basaltic lava Dows form rapidly, are datable by K•Ar analysis, and have original constructional swfactS. Rates at which long-term surficial processes modify these volcanic landforms can thus be measured by analyzing several lava Dows of similar origin and composition but of significantly different ages and by comparing surface morphologic differeDCeS between floM of G<:oIogicaJ Society of America Bulletin, v. 96, p. 151 8-t529, 12ligs., 4 tables, December 1985.

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Environmental history recorded in aeolian deposits under stone pavements, Mojave Desert, USA

Quaternary Research, 2016

Reconstructing the evolution of arid landscapes is challenged by limited availability of appropri... more Reconstructing the evolution of arid landscapes is challenged by limited availability of appropriate environmental archives. A wide-spread surface feature -stone pavement -traps aeolian fines and forms a special accretionary archive. Seven stone pavement-covered sections on basalt flows in the eastern Mojave Desert are condensed to a composite section, comprising five sedimentological units, supported by an OSL-based chronology. Three of the units are of accretionary nature and each is covered by a stone pavement. They were deposited >50.9-36.6 ka, <36.6-14.2 ka and <14.2 ka and are intimately coupled with the history of nearby Lake Mojave, which advances the current picture of regional aeolian activity. End-member modelling analysis of grain-size distributions yielded seven sediment transport regimes. The accretionary system operates in two modes: A) episodic formation of a stone pavement by lateral processes once a vesicular horizon has formed on a barren surface and B) accretion of dust and eventually burial of the clast layer. These findings improve current concepts about stone pavement evolution and their environmental proxy function in arid landscapes. Stone pavement-covered accretionary deposits are a new key archive that allows quantifying the relative importance of dust accretion, slope processes, soil formation and vegetation cover.

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Fire and alluvial chronology in Yellowstone National Park: Climatic and intrinsic controls on Holocene geomorphic processes

Geological Society of America Bulletin, 1995

We employed a systemwide approach, a large and robust set of radiocarbon ages, and modern process... more We employed a systemwide approach, a large and robust set of radiocarbon ages, and modern process analogs to interpret the Holocene history of forest fire-related sedimentation and overall alluvial activity in northeastern Yellowstone National Park. Debris-flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire-related sedimentation within valley-side alluvial fans of Soda Butte Creek. Fire-related deposits make up approximately 30% of the late Holocene fan alluvium. Fifty 14 C ages on fire-related events cluster within the intervals of 3300 -2900, 2600 -2400, 2200 -1800, and 1400 -800 yr B.P. and suggest earlier episodes of large fires and fan aggradation around 7500, 5500, and 4600 -4000 yr B.P. A major pulse of fire-related debris-flow activity between 950 and 800 yr B.P. coincided with the height of the widely recognized Medieval Warm Period (ca. A.D. 1050 -1200). Instrumental climate records over the last ϳ100 yr in Yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods, enhancing the potential for severe short-term drought, major forest fires, and storm-generated fan deposition. Along lower Soda Butte Creek, fill-cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca. 8000 yr B.P. (terrace level T1a), 7000 -5600 (T1b), 3100 -2600 (T2), 2000 -1300 (T3), and post-800 yr B.P. (T4). These periods coincide with overbank sedimentation on Slough Creek and the Lamar River but alternate with intervals of fire-related fan deposition, implying a strong climatic control. Local paleoclimatic data suggest cooler, effectively wetter conditions during terrace tread formation. In warmer, drier intervals, reduced average runoff in axial streams results in meander-belt narrowing; concurrent channel incision may be caused by infrequent large floods. Greater resistance to downcutting, however, allowed fewer terraces to be formed along Slough Creek and the Lamar River. Alluvial systems in northeastern Yellowstone show a clear response to millennial-scale climatic cycles, wherein alluvial fans aggrade and prograde over flood plains in drier periods. Axial streams widen their flood plains and trim back the fans during wetter periods. ''Smallscale'' climatic fluctuations of the Holocene thus had substantial impact on postglacial landscapes in northeastern Yellowstone.

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Interpreting the Yellowstone Fires of 1988

BioScience, 1989

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

Geological Society of America Bulletin, 1987

Sedimentologie and geomorphic variations in storm-generated alluvial fans, Howgill Fells, northwe... more

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