Dennis Dahms

Mineralogical evidence for eolian contribution to soils of late quaternary moraines, Wind River Mountains, Wyoming, USA

Abstract Volcanic heavy minerals are found in the A and B horizons of soils on moraines of the western Wind River Mountains of Wyoming. Volcanic minerals do not occur in the underlying moraine sediments derived from plutonic and metamorphic bedrock. Sources of the volcanics are the sedimentary rocks of the Green River Basin. The presence of volcanic minerals in the Green River Basin and their patterns of distribution in the moraine soils indicate eolian sedimentation is an important factor of soil genesis in this region. Moraines affected by eolian sediments include the Pinedale and Temple Lake/Indian Basin type localities. The presence of eolian sediments in both Pinedale and Holocene deposits suggests eolian addition is not simply a late glacial phenomenon, but continues throughout postglacial time. Relations between eolian sediments and soil development must be accounted for when comparing soil development on moraine chronosequences in this and possibly all alpine regions of western North America.

Spatial Patterns of Glacial Erosion at a Valley Scale Derived From Terrestrial Cosmogenic 10Be and 26Al Concentrations in Rock

Abstract The fundamentally geographic issue of the amounts and spatial patterns of erosion necessary to produce classic glacial landforms such as U-shaped valleys has been debated by scientists for over a century. Terrestrial cosmogenic nuclide (TCN) measurements in glacially abraded bedrock were used to determine patterns of glacial erosion and to quantify the amount of rock removed during the last glaciation along valley-side transects in Sinks Canyon, Wind River Range, Wyoming, and the South Yuba River, Sierra Nevada, California. Surface exposure ages from bedrock and erratic samples obtained during this study indicate last deglaciation between 13–18 ka in the South Yuba River and 15–17 ka in Sinks Canyon. These ages are in agreement with previously published glacial chronologies. In both areas, samples from valley cross sections revealed a pattern of erosion during the last glaciation that decreased toward the lateral limit of ice extent, as predicted by numerical models, while transects further upstream recorded >1.4 meters of bedrock removal throughout. The effects of varying interglacial erosion and surface exposure histories on modeled glacial erosion depths were tested, validating the methodology used. The results demonstrate that the TCN technique, applied at the valley scale, provides useful insight into the spatial pattern of glacial erosion. Extensive sampling in areas with limited erosional loss may provide detailed records of erosion patterns with which to test predictions generated by models of ice dynamics and erosion processes.

Glacial stratigraphy of Stough Creek Basin, Wind River Range, Wyoming

Abstract Multiparameter relative-age (RA) techniques identify four post-Pinedale morphostratigraphic units in each of three cirque valleys tributary to Stough Creek Basin, Wind River Range, WY. Soil development, lichenometry, boulder weathering characteristics, and the geomorphic relations among morphostratigraphic units indicate glacial deposits here correspond to the sequence previously described in the Temple Lake valley [Arct. Alp. Res. 6 (1974) 301]. Cirque deposits in Stough Creek Basin correspond to the Temple Lake, Alice Lake, Black Joe, and Gannett Peak alloformations [GSA Abs. Prog. 32 (2000) A-16]. 10 Be ages from moraine boulders and polished-striated bedrock [Assoc. Am. Geogr. Annu. Mtg. Abs. (2000) 155] support recent numeric age estimates from Temple Lake and Titcomb Basin that indicate the Temple Lake Alloformation corresponds to the Younger Dryas climate episode [Geogr. Phys. Quat. 41 (1987) 397; Geology 23 (1995) 877; Science 268 (1995) 1329; GSA Abs. Prog. 31 (1999) A-56]. Soils described from Pinedale recessional deposits here represent the first systematic description of Pinedale alpine deposits in the WRR.

Relative and Numeric Age Data for Pleistocene Glacial Deposits and Diamictons in and near Sinks Canyon, Wind River Range, Wyoming, U.S.A

Abstract Glacial deposits are identified from within and near Sinks Canyon, southwest of Lander, Wyoming, representing 6 first-order Pleistocene glaciations. Relative-age analyses of the deposits (including moraine morphology and soil development characteristics) indicate that they correspond to 4 glaciations previously identified from the Wind River Range (Pinedale, Early Wisconsin, Bull Lake, and Sacagawea Ridge) and to 2 older glacial events (Younger and Older Pre–Sacagawea Ridge). 10Be and 26Al exposure ages associated with recessional Pinedale deposits are similar to those associated with recessional Pinedale deposits elsewhere in the range. 10Be and 26Al exposure ages also support the identification of Early Wisconsin deposits here. The Early Wisconsin deposits represent the second locality where O-isotope stage 4 glacial deposits are described from the Wind River Range. Preliminary analysis of 10Be exposure data from the Older Pre–Sacagawea Ridge deposit suggests that a glacial advance occurred here before O-isotope stage 18 (>800 ka). If true, then Sinks Canyon contains the most complete record to date of glaciation in the Wind River Range and the only reported record of Pleistocene glaciation prior to O-isotope stage 18 in the Rocky Mountains.

Magnetic analyses of soils from the Wind River Range, Wyoming, constrain rates and pathways of magnetic enhancement for soils from semiarid climates

In order to constrain the rate of magnetic enhancement in soils, we investigated modern soils from five fluvial terraces in the eastern Wind River Range, Wyoming. Profiles up to 1.2 m deep were sampled in 5-cm intervals from hand-dug pits or natural riverbank exposures. Soils formed in fluvial terraces correlated to the Sacajawea Ridge (730–610 ka BP), Bull Lake (130–100 ka BP) and Pinedale-age (∼20 ka BP) glacial advances. One soil profile formed in Holocene-age sediment. Abundance, mineralogy, and grain size of magnetic minerals were estimated through magnetic measurements. Magnetic enhancement of the A-horizon as well as an increase in fine-grained magnetic minerals occurred mostly in Bull Lake profiles but was absent from the older profile. Such low rates of magnetic enhancement may limit the temporal resolution of paleosol-based paleoclimate reconstructions in semiarid regions even where high sedimentation rates result in multiple paleosols. A loss of ferrimagnetic and an increase in antiferromagnetic minerals occurred with age. Our findings suggest either the conversion of ferrimagnetic minerals to weakly magnetic hematite with progressing soil age, or the presence of ferrimagnetic minerals as an intermediate product of pedogenesis. Absolute and relative hematite abundance increase with age, making both useful proxies for soil age and the dating of regional glacial deposits. All coercivity proxies are consistent with each other, which suggests that observed changes in HIRM and S-ratio are representative of real changes in hematite abundance rather than shifts in coercivity distributions, even though the modified L-ratio varies widely.

Soil catenas of calcareous tills, Whiskey Basin, Wyoming, USA

Abstract We describe catenas developed on calcareous moraines of Pinedale (∼21–15 ka) and Bull Lake (>130–100 ka) ages at Whiskey Basin on the eastern flank of the Wind River Range, Wyoming, USA. We sampled one catena of each age from each of two separate moraine fields: the Jakeys Fork and Torrey Creek valleys. Soils of the Bull Lake catena at Jakeys Fork are more developed than those of the corresponding Pinedale catena: they have thicker sola, more horizons, more pedogenic clay, and more pedogenic carbonate presumably, because of their greater age. The Weighted Mean Profile Development Index does not distinguish between the Torrey Creek catenas. Pedogenic carbonate content is greater in the Torrey Creek Pinedale catena than the corresponding Bull Lake catena, and pedogenic clay is greater in the Bull Lake catena. Catenas on the Jakeys Fork moraines are generally more developed than corresponding catenas on the Torrey Creek moraines. We suggest that differences in catena development mostly result from the differences in slope length between the Jakeys Fork and Torrey Creek catenas. The Jakeys Fork catenas are shorter than those of the Torrey Creek, which appears to lessen the role that topographic position plays in catena development.

Glacial limits in the middle and southern Rocky mountains, U.S.A., south of the Yellowstone ice cap

Publisher Summary This chapter includes the mapped limits for glacial deposits of the Middle and Southern Rocky Mountains of the USA except Yellowstone National Park and the immediately adjacent ranges in Wyoming, Montana, and Idaho. Map units include: (1) the maximum limits of Pleistocene glaciation, (2) the limits of Pinedale glaciation, (3) the limits of Bull Lake glaciation, and (4) the limits of deposits that are presently thought to correlate to the European Younger Dryas climate event. Few map units that are identified in this region unequivocally correspond to the early-middle Wisconsin period. Numbers discussed in superscript refer to localities that are important to the Quaternary interpretations of this region and the location of each is indicated on the accompanying map. The chapter also discusses that the model for the glacial succession in Rocky mountain region was established. Early mapping recognized deposits that corresponded to two alpine glaciations in many ranges. Later maps delineated deposits that corresponded to three glaciations. It reviews that each of these glaciations subsequently has been subdivided, with subdivisions based on relative age and numeric age-data. It identified deposits corresponding to three Pleistocene glaciations in the Wind River Range: the Pinedale, Bull Lake, and Buffalo drifts.

Soils and Geomorphology of the Lower Little Cedar River Valley, Northeast Iowa

Few studies exist of the alluvial stratigraphy and geomorphology of the Iowan Surface landform region. We investigated the alluvial stratigraphy, soils, and geomorphology along a section of the lower Little Cedar River to aid in understanding the processes forming this landform region. We used terrace- and floodplain-surface height above the Little Cedar River, soil development, sedimentological characteristics, and stone lines to distinguish alluvial units and landforms. Three Pleistocene surfaces were mapped in the valley, including two terraces and an erosion footslope. The soils and sedimentary characteristics of these surfaces are extremely variable and are used to interpret geomorphic forces acting on the landscape. Holocene alluvial units identified include three terraces and the modern floodplain. Elsewhere in the Midwest, these units have been described as the DeForest Formation, and include the Camp Creek, Roberts Creek, and Gunder Members. The most strongly developed Holocene soils are found within the Gunder Member. Profiles exhibit either A/Bt/C or A/Bw/C horizonation. Soils in Roberts Creek alluvium are not as developed as those in the Gunder Member. These profiles are predominately A/Bw/C. Soil development within Camp Creek alluvium is minimal with A/C profiles.

Prediction of Soil Formation as a Function of Age Using the Percolation Theory Approach

Recent modeling and comparison with field results showed that soil formation by chemical weathering, from bedrock or unconsolidated material, is limited largely by solute transport. Chemical weathering rates are proportional to solute velocities. Nonreactive solute transport described by non-Gaussian transport theory appears compatible with soil formation rates. This change in understanding opens new possibilities for predicting soil production and depth across orders of magnitude of time scales. Percolation theory for modeling the evolution of soil depth and production was applied to new and published data for alpine and Mediterranean soils. The first goal was to check whether the empirical data conform to the theory. Secondly we analyzed discrepancies between theory and observation to find out if the theory is incomplete, if modifications of existing experimental procedures are needed and what parameters might be estimated improperly. Not all input parameters required for current theoretical formulations (particle size, erosion and infiltration rates) are collected routinely in the field; thus, theory must address how to find these quantities from existing climate and soil data repositories, which implicitly introduces some uncertainties. Existing results for soil texture, typically reported at relevant field sites, had to be transformed to results for a median particle size, d50, a specific theoretical input parameter. The modeling tracked reasonably well the evolution of the alpine and Mediterranean soils. For the Alpine sites we found, however, that we consistently overestimated soil depths by approximately 45%. Particularly during early soil formation, chemical weathering is more severely limited by reaction kinetics than by solute transport. The kinetic limitation of mineral weathering can affect the system until 1kyr to a maximum of 10kyr of soil evolution. Thereafter, solute transport seems dominant. The trend and scatter of soil depth evolution is well captured, particularly for Mediterranean soils. We assume that some neglected processes, such as bioturbation, tree throw, and land use change contributed to local reorganization of the soil and thus to some differences to the model. Nonetheless, the model is able to generate soil depth and confirms decreasing production rates with age. A steady state for soils is not reached before about 100 kyr.