Ralph R. Shroba

Quaternary soils and dust deposition in southern Nevada and California

Eoliandustconstitutesmuchofthepedogenic material in late Pleistocene and Holocene soils of many arid regions of the world.Comparisonofthecompositionsand influx rates of modern dust with the eolian component of dated soils at 24 sites in southern Nevada and California yields informationon(1)thecompositionandinflux rate of dust in late Pleistocene and Holocene soils, (2) paleoclimate and its effects on the genesis of aridic soils, especially with regard to dustfall events, (3) the timing and relative contribution of dust from playa sources versus alluvial sources, and (4) the effects of accumulation of dust in soil horizons. The<2mmfractionsofAandBhorizons of soils formed on gravelly alluvial-fan deposits in the study area are similar to moderndustingrainsize,contentofCaCO3and salt,majoroxides,andclaymineralogy;thus, they are interpreted to consist largely of dust. The major-oxide compositions of the shallow soil horizons are nearly identical to that of the modern dust, but the compositions of progressively deeper horizons approachthatoftheparentmaterial.Theclay mineralogyofmoderndustatagivensiteis similar to that of the Av horizons of nearby Holocene soils but is commonly different from the mineralogies of deeper soil horizonsandoftheAvhorizonsofnearbyPleistocenesoils.Theseresultsareinterpretedto indicate that dust both accumulates and is transformed in Av horizons with time. Changes in soil-accumulation rates provide insights into the interplay of paleoclimate,dustsupply,andsoil-formingprocesses. Modern dust-deposition rates are more than large enough to account for middle and late Holocene soil-accumulation rates at nearly all sites. However, the early Holocene soil-accumulation rates in areas near late Pleistocene pluvial lakes are much higher than modern rates and clearly indicate a dust-deflation and -deposition event that caused rapid formation offine-grained shallow soil horizons on uppermost Pleistocene and lower Holocene deposits. We interpret late Pleistocene soil-accumulation rates to indicate that dust-deposition rates were low during this period but that increased effective moisture during the late Wisconsinan favored translocation of clay andCaCO3fromnearthesurfacetodeeper inthesoilprofile.Pre‐latePleistocenerates are very low in most areas, mainly due to a pedogenic threshold that was crossed when accumulations of silt, clay, and CaCO3 began to inhibit the downward transport of eolian material, but in part due to erosion.

Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, Western United States

A substantially modified history of the last two cycles of Lake Bonneville is proposed. The Bonneville lake cycle began prior to 26,000 yr B.P.; the lake reached the Bonneville shoreline about 16,000 yr B.P. Poor dating control limits our knowledge of the timing of subsequent events. Lake level was maintained at the Bonneville shoreline until about 15,000 yr B.P., or somewhat later, when catastrophic downcutting of the outlet caused a rapid drop of 100 m. The Provo shoreline was formed as rates of isostatic uplift due to this unloading slowed. By 13,000 yr B.P., the lake had fallen below the Provo level and reached one close to that of Great Salt Lake by 11,000 yr B.P. Deposits of the Little Valley lake cycle are identified by their position below a marked unconformity and by amino acid ratios of their fossil gastropods. The maximum level of the Little Valley lake was well below the Bonneville shoreline. Based on degree of soil development and other evidence, the Little Valley lake cycle may be equivalent in age to marine oxygenisotope stage 6. The proposed lake history has climatic implications for the region. First, because the fluctuations of Lake Bonneville and Lake Lahontan during the last cycle of each were apparently out of phase, there may have been significant local differences in the timing and character of late Pleistocene climate changes in the Great Basin. Second, although the Bonneville and Little Valley lake cycles were broadly synchronous with maximum episodes of glaciation, environmental conditions necessary to generate large lakes did not exist during early Wisconsin time.

Nomenclature of alpine glacial deposits, or, What's in a name?

The most useful and objective classification for alpine glacial deposits, as well as for many other Quaternary deposits, appears to be one based on parameters which vary with age as a result of postdepositional modifications. In areas and units for which radiometrically datable materials are rare, age information is gained primarily from these postdepositional modifications by what are here called relative-dating (RD) methods. In order for alpine glacial deposits to be subdivided and formally named, sufficient field data for consistent recognition and mapping need to be collected. These data include measurements of age-dependent parameters such as soil properties, rock-weathering characteristics, and landform changes. Subdivision and naming of glacial deposits should be no more detailed than the resolution of the RD methods, which generally decreases with time. Only when deposits have been objectively characterized by these relative-dating methods can correlations with deposits in other areas or with other types of records be substantiated.

Conical sandstone landforms cored with clastic pipes in Glen Canyon National Recreation Area, southeastern Utah

Abstract Clusters of conical sandstone landforms, many with summit weathering pits, have developed on barren outcrops of the Jurassic Entrada Sandstone in Glen Canyon National Recreation Area, southeastern Utah. The conical landforms have developed on cylindrical bodies of fluidized sandstone (clastic pipes) that typically have near-vertical contacts with the enclosing cross-bedded, eolian sandstone. These landforms vary in size and shape due chiefly to differential erosion of the clastic pipe relative to the enclosing sandstone. The greater resistance to weathering of the clastic pipes is due in part to their higher content of calcite cement. Conical, pipe-cored landforms develop progressively from low domes to cones as high as 70 m. Some of the clastic pipes have relatively soft cores and resistant contacts, leading to the development of conical landforms with summit weathering pits. With time, the size of these pits increases as does the relief of the conical landform. The summit pits are as deep as 16 m and have width–depth ratios as low as 1.5. The resistant rims of these pits are due in part to calcite-enriched pipe contacts. Sandy pit-floor sediment is removed principally by strong wind rotors and vortices. Intense eolian activity in and near the landforms is indicated by abrasional features and pit-floor sand dunes. Factors that promote the development of these conical landforms include (i) the presence of clastic pipes, some with relatively soft cores; (ii) porous, friable, fine-grained pipe and host sandstones; (iii) aridity; (iv) strong winds; and (v) virtually sediment-free, unvegetated bedrock outcrops.

Soils Developed in the Glacial Deposits of the Type Areas of the Pinedale and Bull Lake Glaciations, Wind River Range, Wyoming, U.S.A.

The degree of soil development in glacial deposits in the Fremont Lake area (FLA) and Bull Lake type area (BLTA) on opposite sides of the Wind River Range of western Wyoming is chiefly influenced by the ages of the parent materials although other soil-forming factors are important. Soil morphology, clay content, and calcium carbonate content are useful in distinguishing moraines of the Bull Lake glaciation (about 140 to 150 ka) from those of the Pinedale glaciation (about 14 to 35 ka) in these areas. In the FLA, soils in Bull Lake deposits have an average Profile Development Index (PDI) of 39 index-cm and average 15% clay and 7% calcium carbonate (CaCO3), and soils in Pinedale deposits have an average PDI of 25 index-cm and average 6% clay and 1% CaCO3. In the BLTA, soils in Bull

Soil Relative Dating of Moraine and Outwash-terrace Sequences in the Northern Part of the Upper Arkansas Valley, Central Colorado, U.S.A.

Profile development indices for soils developed in moraines and outwash near Twin Lakes and in outwash near Leadville support the correlation of moraines with subdued morphology and two high outwash terraces with the Bull Lake glaciation (ca. 130-160 ka) and the correlation of hummocky moraines and two low outwash terraces with the Pinedale glaciation (ca. 14-47 ka). Elsewhere in the northern part of the upper Arkansas Valley, glacial sequences are correlated by mapping outwash terraces near the mouths of major tributaries of the Arkansas River. Near Twin Lakes, indices for soils on low, outer lateral moraines suggest that the older Pinedale glaciers extended beyond the margin of high, younger Pinedale lateral moraines with hummocky topography. A few subdued moraines near Twin Lakes and Leadville probably record one or more glaciations significantly older than the Bull Lake. The downvalley extent of Pinedale glaciers in the Mosquito Range on the east side of the Arkansas Valley is uncertain: most likely, Pinedale glaciers were almost as extensive as Bull Lake glaciers but built no prominent terminal moraines at their maximum positions.