Calibrated, late Quaternary age indices using clast rubification and soil development on alluvial surfaces in Pilot Knob Valley, Mojave Desert, southeastern California

The orange coating (varnish) that forms on the undersides (ventral sides) of clasts in desert pavements constitutes a potential relative-age indicator. Using Munsell color notation, we semiquantified the color of the orange, ventral varnish on the undersides of clasts from 15 different alluvial fan and terrace surfaces of various ages ranging from less than 500 to about 25,000 yr. All of the surfaces studied are located along the central portion of the left-lateral Garlock fault, in the Mojave Desert of southern California. The amount of left-lateral offset may be used to determine the relative ages of the surfaces. The previously published slip rate of the fault may also be used to estimate the absolute age of each surface. The color of the ventral varnish is strongly correlated with surface age and appears to be a more reliable age-indicator than the percentage coverage of dorsal varnish. Soil development indices also were not as strongly correlated with age, as were the colors of the ventral varnish. In particular, rubification appears to be more useful than soils for distinguishing relative ages among Holocene surfaces. Humidity sensors indicated that the undersides of clasts condensed moisture nightly for a period of several days to over a week after each rain. These frequent wet-dry cycles may be responsible for the rapid development of clast rubification on Holocene surfaces.     

Evidence from Xenoliths for a Dynamic Lower Crust, Eastern Mojave Desert, California

Garnet-rich xenoliths in a Tertiary dike in the eastern MojaveDesert, California, preserve information about the nature andhistory of the lower crust. These xenoliths record pressuresof 10–12 kbar and temperatures of 750–800C. Approximately25% have mafic compositions and bear hornblende + plagioclase+ clinopyroxene + quartz in addition to garnet. The remainder,all of which contain quartz, include quartzose, quartzofeldspathic,and aluminous (kyanitesillimanite-bearing) varieties. Mostxenoliths have identifiable protoliths—mafic from intermediateor mafic igneous rocks, quartzose from quartz-rich sedimentaryrocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathicfrom feldspathic sediments and/or intermediate to felsic igneousrocks. However, many have unusual chemical compositions characterizedby high FeO(t), FeO(t)/MgO, Al₂O₃, and Al₂O₃/CaO, which correspondto high garnet abundance. The mineralogy and major-and trace-elementcompositions are consistent with the interpretation that thexenoliths are the garnet-rich residues of high-pressure crustalmelting, from which granitic melt was extracted. High ⁸⁷Sr/⁸⁶Srand low ¹⁴³Nd/¹⁴⁴Nd, together with highly discordant zirconsfrom a single sample with Pb/Pb ages of 1.7 Ga, demonstratethat the crustal material represented by the xenoliths is atleast as old as Early Proterozoic. This supracrustal-bearinglithologic assemblage may have been emplaced in the lower crustduring either Proterozoic or Mesozoic orogenesis, but Sr andNd model ages> 4 Ga require late Phanerozoic modificationof parent/daughter ratios, presumably during the anatectic event.Pressures of equilibration indicate that peak metamorphism andmelting occurred before the Mojave crust had thinned to itscurrent thickness of <30 km. The compositions of the xenolithssuggest that the lower crust here is grossly similar to estimatedworld-wide lower-crustal compositions in terms of silica andmafic content; however, it is considerably more peraluminous,has a lower mg-number, and is distinctive in some trace elementconcentrations, reflecting its strong metasedimentary and restiticheritage. ^* Author to whom correspondence should be addressed. Present address: Rensselaer Polytechnic Institute, Department of Earth and Environmental Sciences, Troy, New York 12180, USA. Fax: 518–276–8627; email: hanchj{at}rpi.edu.     

Manganese Accumulation in Rock Varnish on a Desert Piedmont, Mojave Desert, California, and Application to Evaluating Varnish Development

Rock varnish coatings tend to become thicker, darker, and more continuous over time, leading to the use of changes in overall varnish color and the percentage of clast surfaces covered by varnish as relative-age indicators. Manganese is the most characteristic element of subaerial rock varnishes, and the progressive development of varnish coats can be quantified by measuring the amount of Mn accumulated on a given area of rock surface. Manganese oxides were dissolved off varnished clasts collected from alluvial surfaces on the Soda Mountains piedmont in the Mojave Desert, California, and the amount of Mn was measured using inductively coupled plasma emission spectroscopy. On the distal piedmont, maximum varnish development increases from a mid- to late-Holocene surface, typically containing up to 0.15 mg/cm² of accumulated Mn, to an early- to mid-Holocene surface with up to 0.21 mg/cm². However, varnish is less developed on a nearby late Pleistocene surface, suggesting extensive abrasion of clasts on the Pleistocene desert pavements or disturbance of the clasts. Varnish is better developed on the proximal piedmont, typically containing up to 0.30 mg/cm² of Mn, although varnish from a Pleistocene surface is again no better developed than from a nearby early- to mid-Holocene surface. These data demonstrate that rock varnish can show significant spatial variation in degree of development on geomorphic surfaces of similar age, and imply that collecting varnish as old as a geomorphic surface may be difficult on surfaces as young as late Pleistocene.     

Geochronology and paleoenvironment of pluvial Harper Lake,Mojave Desert,California, USA

Accurate reconstruction of the paleo-Mojave River and pluvial lake (Harper, Manix, Cronese, and Mojave) system of southern California is critical to understanding paleoclimate and the North American polar jet stream position over the last 500 ka. Previous studies inferred a polar jet stream south of 35°N at 18 ka and at ~ 40°N at 17–14 ka. Highstand sediments of Harper Lake, the upstream-most pluvial lake along the Mojave River, have yielded uncalibrated radiocarbon ages ranging from 24,000 to > 30,000 ¹⁴C yr BP. Based on geologic mapping, radiocarbon and optically stimulated luminescence dating, we infer a ~ 45–40 ka age for the Harper Lake highstand sediments. Combining the Harper Lake highstand with other Great Basin pluvial lake/spring and marine climate records, we infer that the North American polar jet stream was south of 35°N about 45–40 ka, but shifted to 40°N by ~ 35 ka. Ostracodes (Limnocythere ceriotuberosa) from Harper Lake highstand sediments are consistent with an alkaline lake environment that received seasonal inflow from the Mojave River, thus confirming the lake was fed by the Mojave River. The ~ 45–40 ka highstand at Harper Lake coincides with a shallowing interval at downstream Lake Manix.     

Paleoenvironment of geomorphic surfaces of an alluvial fan in the eastern Isfahan,Iran, in the light of micromorphology and clay mineralogy

Alluvial fans are one of the most important landforms in geomorphological and paloenvironmental studies. The objective of this study was the application of clay mineral assemblages and micromorphological properties of the studied paleosols in the geomorphic surfaces of an alluvial fan in the eastern Isfahan as proxies for paleoenvironmental and paleoclimatic changes. Micromorphology, X-ray diffraction, and scanning electron microscopy approaches were used to study the representative pedons. The results indicated that the illuviation process in the calcareous soils of the arid regions of the eastern Isfahan was probably in response to Quaternary moist conditions. There was no significant difference between clay coating properties of the studied relict and buried paleosols. Clay mineralogical study suggested that kaolinite and illite were inherited from the parent materials, while smectite and palygorskite were formed in the soil environment. Paleoargillic horizon was characterized by smectite and calcic (especially the calcrete) horizons were dominated by palygorskite. Palygorskite was accumulated by both neoformation and illuviation processes. High clay content, high intensity of smectite peak, and activity of the illuviation process in paleoargillic horizon demonstrated the seasonality of climate (rainfall) even in the moist periods of Quaternary in Central Iran. Clay mineralogical assemblages suggested a trend of increasing environmental aridity in the study area. This study, therefore, highlighted the role of clay mineralogical investigations in arid lands’ geomorphological and paleoenvironmental researches.     

基于SAR影像的干旱区冲/洪积扇地貌面定量分期研究−以河西走廊西部沙漠区的疏勒河洪积扇为例

河流作用形成的洪积扇和河流阶地可以提供过去构造活动、气候变化和地貌演变过程的有效记录;而准确划分洪积扇地貌面的期次是开展环境变化及构造活动定量研究的基础。已有研究往往利用L波段数据SAR后向散射系数值作为地貌粗糙度替代参数,进行地貌面定量分期,但并未考虑不同时间数据源对分期结果的影响。以疏勒河洪积扇为研究对象,通过分析多时相L波段SAR数据后验统计指标以及大气评估条件,确定最佳数据源,并运用最大似然分类法对后向散射强度值进行分类,以实现地貌面的定量分期。结果表明：使用分期后验统计指标作为选取最佳时像影像数据的标准,可以获得更好的分期结果;L波段HH单极化数据可得到较好的分期结果,与C波段数据相比,对于不同年龄地貌面的划分更具优势,且数据更易获取,具备自动化分期潜力;SAR影像质量以及分期结果与成像时大气条件密切相关,而与季节相关性不大,因此建议优先选择成像时地表含水量较低的影像,例如,高蒸发强度的夏季。文章提出的这套对遥感数据质量分析并进行地貌面分期的方法可用于完成干旱地区大尺度冲/洪积扇的快速定量分期,为构造和气候的研究提供有价值的信息。     

Sedimentary processes and facies of the waterlaid Anvil Spring Canyon alluvial fan, Death Valley, California

The Anvil Spring Canyon fan of the Panamint Range piedmont in central Death Valley was built entirely by water-flow processes, as revealed by an analysis of widespread 2- to 12-m-high stratigraphic cuts spanning the 9·7 km radial length of this 2·5–5·0° sloping fan. Two facies deposited from fan sheetfloods dominate the fan from apex to toe. The main one (60–95% of cuts) consists of sandy, granular, fine to medium pebble gravel that regularly and sharply alternates with cobbly coarse to very coarse pebble gravel in planar couplets 5–25 cm thick oriented parallel to the fan surface. The other facies (0–25% of cuts) comprises 10- to 60-cm-thick, wedge-planar and wedge-trough beds of pebbly sand and sandy pebble gravel in backsets sloping 3–28°. Both facies are interpreted as resulting from rare, sediment-charged flash floods from the catchment, and were deposited by supercritical standing waves of expanding sheetfloods on the fan. Standing waves were repeatedly initiated, enlarged, migrated, and then terminated either by gradually rejoining the flood or by more violent breakage and washout. The frequent autocyclic growth and destruction of standing waves during an individual sheetflood resulted in the deposition of multiple coarse and fine couplet and backset sequences 50–250 cm thick across the active depositional lobe of the fan. Erosional intensity during washout of the standing wave determined whether early-phase backset-bed deposits or washout-phase sheetflood couplet deposits were selectively preserved in a given cycle. Two minor facies are also found in the Anvil fan. Pebble–cobble gravel lags (0–20% of cuts) are present above erosional scours into the sheetflood couplet and backset deposits. They consist of coarse gravel concentrated through fine-fraction winnowing of the host sheetflood facies by sediment-deficient water flows. This reworking occurred during recessional flood stage or from non-catastrophic discharge during the long intervals between major flash floods. This facies is common at the surface, giving rise to a ‘braided-stream’ appearance. However, it is stratigraphically limited, present as thin, continuous to discontinuous beds or lenses that bound 50- to 250-cm-thick sheetflood sequences. The other minor facies of the Anvil fan consists of clast-supported and imbricated, thickly stratified, pebbly, cobbly, boulder gravel present in narrow, radially aligned ribbons nested within sheetflood deposits. This facies is interpreted as representing deposition in the incised channel of the fan, a subenvironment characterized by greater flow competence resulting from maintained depth from channel-wall confinement, and by more frequent water flows and winnowing events caused by its direct connection with the catchment feeder channel.     

The formation and evolution of a coastal alluvial fan in eastern Taiwan caused by rainfall-induced landslides

The formation and evolution of a coastal fan in eastern Taiwan associated with a sequence of rainfall-induced landslides during the 2009–2013 period are explored in this study. The evolution of these landslides is mainly attributed to the head-cutting process initiated by Typhoon Parma in October 2009. During the attack of Typhoon Megi in October, 2010, a subaerial coastal fan with a surface slope of 8.9° was formed after the mobilization of the rainfall-induced landslides. The geomorphic features both in the steep gully and on the coastal fan were categorized as the sequence of granular debris flows and sheet floods. Severe fan toe erosion occurred thereafter due to the wind-wave forcing. Even if the variations of both the cumulative rainfalls and the drainage areas are one or two orders of magnitude among devastating fan-forming landslides worldwide, the mean annual precipitation and the basin ruggedness index (Melton ratio) are effective indicators to normalize the rainfall threshold and to characterize the fan surface slope, respectively. Severe catastrophic landslides generally occur when the normalized cumulative rainfalls with respect to mean annual precipitation are greater than 0.1. The fan slope generally increases with the increasing Melton ratio for the catchment. Uchiogi’s empirical model is applicable for predicting the rainfall-induced area ratio of newly generated landslides. In this case study, the relationship of the fan area to the total landslide area follows a linear regressive curve when the ratio of landslide area with respect to the drainage area exceeds 0.0056.     

Sedimentation and extensional basin evolution in a Miocene metamorphic core complex setting, Alvord Mountain, central Mojave Desert, California, USA

The Lower Miocene Clews Formation at Alvord Mountain, Mojave Desert, California, comprises an upward coarsening sequence of synextensional continental deposits of variable thickness; this sequence pinches out to the west against the Alvord Mountain pre-Tertiary basement complex and thickens eastward to c.300 m over a distance of 7 km. Two stages of sedimentation are recognized in the formation. During the initial stage of sedimentation, the depocentre was delineated by a lacustrine system that was bounded to the west by small, sheetflood-dominated alluvial fans and to the north by a southerly flowing fluvial braidplain. Lacustrine mudstone, siltstone and carbonate suggest an initially closed basin. Conglomerate to the west represents small, sheetflood-dominated fans. Igneous clasts and a south-east palaeoflow direction suggest a proximal, low-relief source in the western Alvord Mountain area. Pebbly sandstone of the southward-prograding braidplain has a metasedimentary provenance in the Paradise Range to the north. The second stage of deposition was dominated by coarse conglomerate and breccia. West to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the Cronese Hills, 8 km to the east. These strata reflect rapid westward progradation of sheetflood- and debris flow-dominated alluvial fans that advanced across the axial braidplain and lacustrine system. Basin development is interpreted to have been controlled by regional NE-directed extension on a detachment fault associated with the central Mojave metamorphic core complex. 30 km to the west. The Alvord Mountain - Cronese Hills region was initially transported as a single hangingwall block on the E-dipping detachment. Initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall. Subsequent westward progradation of alluvial fans out of the Cronese Hills is believed to record the propagation of a NW-striking, SW-dipping normal fault antithetic to the low-angle E-dipping detachment fault.     

Chemical and physical weathering in a hot‐arid,tectonically active alluvial system of Anza Borrego Desert,California

Investigation of chemical and physical weathering of bedrock and alluvial sediment in the Anza Borrego Desert, California, sheds light on weathering processes in hot‐arid systems and clarifies interpretations of climate from alluvial sediment. All of the alluvial sediment in the study area emanates from Cretaceous tonalite of the Peninsular Range, enabling exploration of the effects of external variables – climate, transport distance and tectonics – on the physical and chemical properties of the sediment. Chemical weathering in this area is dominated by plagioclase alteration observed in both bedrock outcrops and sediment, evinced most clearly by changes in the Eu anomaly. Biotite chemical weathering, manifested by interlayer K⁺ loss, is not evident in bedrock, but clearly observed in the sediment. Despite the weak intensity of chemical weathering (Chemical Index of Alteration = 56 to 62), fine‐grained (<63 μm) sediment displays a clear weathering trend in A–CN–K space and contains up to 25% clay minerals. Physical abrasion and grain‐size reduction in biotite during transport predominates in the sediment, whereas physical (insolation) weathering affecting bedrock is inferred from estimates of differential thermal expansion of mineral phases in response to extreme temperature changes in the study area. Chemical alteration and Brunauer–Emmett–Teller surface area both increase within the active Elsinore fault zone at the distal end of the depositional transect, reflecting tectonic‐induced fracturing and associated accelerated weathering. Extensive fracturing, together with a more humid Pleistocene climate, probably facilitated in situ bedrock weathering, preceding arid alluvial deposition in the Holocene. This study demonstrates that both climate and tectonic processes can affect chemical and physical weathering, resulting in alteration of plagioclase, leaching of K⁺ from biotite in the sediment and formation of clay minerals, even in hot, arid systems.     

Stratigraphy and age of a Neoglacial sedimentary succession of proglacial outwash and an alluvial fan in Langedalen,Veitastrond, western Norway

This study presents the sedimentary succession of an outwash plain and an alluvial fan located along the valley Langedalen at the south-eastern side of the Jostedalsbreen ice cap in inner Sogn, western Norway. A newly exposed ~2.8-m-high section along the southern riverbank of Langedøla river shows alternating layers of minerogenic sediments and peat layers with tree logs, identified as Salix sp. The section is situated in the distal part of an alluvial fan built out from the southern slope of Langedalen. Six AMS radiocarbon dates of tree fragments indicate that the accumulation of the fine-grained sediments in the lower part of the section was initiated earlier than the basal radiocarbon date of 914–976 calibrated years CE (1σ age range). These basal, fine-grained sediments are interpreted as proglacial outwash deposited in a floodplain depression or abandoned river channel in a low-energy glaciofluvial environment. Periods of low glacier cover, low river discharge or low-water stands over the floodplain allowed peat formation and the growth of trees and shrubs in the valley. The radiocarbon dates further indicate relatively rapid sediment accretion (~2.7–3 cm a⁻¹) between 190 and 125 cm below the sediment surface, equivalent to approximately 1220 to 1250 cal. a CE (1σ age range). At ~60 cm depth below the surface, dated to approximately 1590 to 1620 cal. a CE (1σ age range), a transition to more coarse-grained, sandy and gravelly sediments indicates increased sediment supply and distal expansion of the alluvial fan. This occurred most likely as a consequence of increased sediment yield from expanding glaciers along the southern valley side of Langedalen as a response to the initial Little Ice Age glacier growth. Based on these results, the accretion and progradation of glacier-fed alluvial fans mainly occur during periods of glacier advance rather than during glacier recession.     

Early Pleistocene glacial outwash deposits in the eastern San Joaquin Valley, California: a model for humid-region alluvial fans

ABSTRACT The early Pleistocene Laguna and Turlock Lake Formations and China Hat and Arroyo Seco Gravels along the east side of the San Joaquin Valley, California, were deposited in alluvial fans and marginal lakes. Upward-coarsening sequences of silt-sand-gravel record westward progradation of glacial outwash fans from the Sierra Nevada into proglacial lakes in the San Joaquin Valley. Distinctive sedimentary features delineate lacustrine, prodelta, and delta-front facies within fan-margin deposits and lower, middle, and upper-fan facies within alluvial-fan deposits. The lacustrine facies consists of a few metres of thinly and evenly bedded, rhythmically laminated claystone and clayey siltstone in varved couplets. Draped lamination, sinusoidal lamination, and load and pillar structures occur in some beds. Siltstone and claystone grade upward to slightly thicker wavy beds of siltstone and very fine-grained unconsolidated sand deposited in a prodelta setting. Convolute laminae within deformed steeply dipping foreset beds suggest slumping on the prodelta slope. The prodelta facies grades up to the delta-front facies, which consists of burrowed and bioturbated cross-bedded fine sand. Deltaic deposits are 5–6 m thick. The lower-fan facies forms the base of the fan sequence and consists of several metres of irregularly bedded, laminated, oxidized siltstone and fine sand. The middle-fan facies consists of cross-bedded, medium-grained to gravelly sand-filled channels cut into the lower-fan facies. Interbedded lens-shaped siltstone beds 2 m thick and several metres across were deposited in abandoned channels. The upper-fan facies consists of moderately to strongly weathered clayey gravel and sand containing pebble imbrication and crude stratification. Argillization during post-depositional soil formation has blurred the distinction between mud-supported debris-flow deposits and clast-supported channel deposits, but both are present in this facies. The deposits described here demonstrate the need for additional fan models in order to incorporate the variety of deposits developed in alluvial fan sequences deposited in humid climates. In previous models based on arctic fans, debris flows, abandoned channels, or widespread siltstone beds are not present in fan sequences, nor are marginal lacustrine and deltaic deposits well represented.     

Sediments and processes on a small stream-flow dominated,devonian alluvial fan,Shetland Islands

The main conglomerate type of a small Devonian alluvial fan in Shetland, northern Britain, is an inverse to normally graded framework-supported gravel. The sedimentological details of these beds and fundamental considerations of the mechanics of movement of highly concentrated flows suggests that neither debris-flow nor grain-flow were responsible for the deposition of these conglomerates. On the contrary, these inverse to normally graded conglomerates were deposited by water as thick gravel sheets with little topographic relief, but broadly analogous to longitudinal bars. They were deposited under high aggradation conditions first on the rising and then on the falling flood. The high concentration of material in transport on the rising and peak flood was responsible for the polymodal and unstratified nature of the conglomerates. As the flood waned, normal grading was developed and a preferred horizontal fabric was produced under low concentrations of sediment.Other conglomerate types, comparatively of lesser importance, were deposited from highly concentrated clast dispersions and are notably richer in matrix and locally possess inverse grading.The hydrological environment was one of flashy discharges of short duration but of high velocity. Flow was dissipated rapidly, perhaps due to extreme water loss.This example from the Devonian of Shetland provides an interesting alternative to the mass-transport dominated fan models currently in vogue.     

Relationship between climate and vegetation and the stable carbon isotope chemistry of soils in the eastern Mojave Desert, Nevada

The relationship between the stable C-isotope composition of the soil environment and modern climate and vegetation was determined empirically along a present-day climatic transect in the eastern Mojave Desert. The δ¹³C of the soil CO₂ and carbonates decreased with increasing elevation and plant density, even though plant assemblages at all elevations were isotopically similar. Several factors, including differences in the ratios of pedogenic of limestone calcite and differences in past vegetation, were considered as explanations of this trend, However, it appears that in the sparsely vegetated Mojave Desert, the δ¹³C of pedogenic carbonate is controlled by differences in plant density and biological activity. This relationship may provide a tool for assessing past vegetational densities, as long as the vegetation is isotopically homogeneous.     

Gradual diversions of the Rio Pastaza in the Ecuadorian piedmont of the Andes from 1906 to 2008: role of tectonics, alluvial fan aggradation, and ENSO events

The successive courses of the Rio Pastaza in the upper Amazonian Puyo plateau (Ecuador) during the past century have been followed using historical maps, aerial photographs, satellite imagery, topographic and river long profiles, and field studies. The abrupt change in direction of the Rio Pastaza from transverse to longitudinal was a result of two avulsions occurred between 1906 and 1976 at the braided-meandering transition of the former alluvial plain. These avulsions are related to aggradation at the toe of a braided piedmont fan prograding on to a hinterland-dipping topographic slope formed by ongoing tectonic backtilting. The main avulsion proceeded by annexation of a south-dipping depression created in front of the cordillera by backtilting of the plateau. A partial and gradual avulsion having occurred upstream of the former site between 1976 and 2008 is marked by the progressive predominance of a newly formed inner branch. Tectonic backtilting enhanced aggradation upstream of the initial site while it offered the newly avulsed channel a still more favorable way along the cordillera by creating a westward lateral slope. The correlation between ENSO events and the occurrence of the 1976–2008 avulsions strongly suggests that the triggers of the avulsions were the floods caused by the high water and sediment discharges associated with ENSO (La Ni?a) events contrasting with the regular monthly discharge and the lack of actual ‘normal’ floods during the inter-ENSO periods.     

Timing and distribution of alluvial fan sedimentation in response to strengthening of late Holocene ENSO variability in the Sonoran Desert,southwestern Arizona,USA

The integration of geomorphic mapping, soil stratigraphy, and radiocarbon dating of alluvial deposits offers insight to the timing, magnitude, and paleoclimatic context of Holocene fan sedimentation near Yuma, Arizona. Mapping of 3400 km² indicates about 10% of the area aggraded in the late Holocene and formed regionally extensive alluvial fan and alluvial plain cut-and-fill terraces. Fan deposits have weakly developed gravelly soils and yielded a date of 3200–2950 cal yr BP from carbonized wood. Alluvial plain deposits have weakly developed buried sandy soils and provided a date of 2460–2300 cal yr BP from a terrestrial snail shell. Precipitation records were analyzed to form historical analogues to the late Holocene aggradation and to consider the role of climatic variability and extreme hydrologic events as drivers of the sedimentation. The historical precipitation record indicates numerous above-average events correlated to the Southern Oscillation Index (SOI) in the region, but lacks any significant reactivation of alluvial fan surfaces. The timing of aggradation from 3200 to 2300 cal yr BP correlates well with other paleoclimatic proxy records in the southwestern U.S. and eastern Pacific region, which indicate an intensification of the El Niño-Southern Oscillation (ENSO) climatic pattern and rapid climate change during this period.     

Reevaluation of multiparameter relative dating techniques and their application to the glacial sequence along the eastern escarpment of the Sierra Nevada,California

Four valleys, recently studied by other workers, were examined along the eastern Sierra Nevada to refine relative-dating techniques. A variety of weathering parameters and soil properties fail to delineate more than two major post-Sherwin Pleistocene glaciations. We correlate these two glaciations with the Tahoe and Tioga Glaciations. Type Mono Basin Till, usually considered to be pre-Tahoe, exhibits the following weathering similarities with Tahoe Till, if both are under sagebrush: (1) grusification of subsurface granitic boulders; (2) degree of pitting, mineral relief, and rind development on surface granitic boulders; and (3) very slight clay increase in the B horizon. Type Casa Diablo Till also has weathering characteristics similar to Tahoe Till, except a slightly more developed Bt horizon is present. Hence, dates on basalt of 0.126 ± 0.025 and 0.062 ± 0.013 my Casa Diablo Till also has weathering characteristics similar to Tahoe Till, except a slightly more developed Bt horizon is present. Hence, dates on basalt of 0.126 ± 0.025 and 0.062 ± 0.013 my (Bailey et al. 1976), which bracket type Casa Diablo, may provide age control on the Tahoe glaciation. In addition, we are unable to demonstrate that the Tenaya is a separate glaciation. In three of the four valleys studied our weathering data for Tenaya Till are equivalent with those for Tioga Till, but with those for Tahoe Till in the fourth valley. We were not satisfied with our ability to differentiate the Casa Diablo, Mono Basin, and Tenaya as separate glaciations even though data were collected in the type areas for two of these deposits. Reasons for suggesting a change back to a two-fold Tahoe-Tioga glacial sequence, rather than the present five-fold sequence, are that we have measured a greater number of parameters than has been done previously, soils were submitted to detailed laboratory analyses, and surface weathering features were studied under consistent present vegetation cover to avoid possible problems induced by ancient forest fires. Nevertheless our relative-dating scheme does not rule out the possibility of a more detailed glacial sequence.     

A framework of Holocene and Late Pleistocene environmental change in eastern Iran inferred from the dating of periods of alluvial fan abandonment,river terracing,and lake deposition

We review studies of the Holocene and Late Pleistocene stratigraphy of eastern Iran to infer past changes in the environment within this presently arid region. We build a scenario of widespread, and presumably climatically driven, evolution of the landscape through the Holocene. Six sites, covering a 10° range in latitude, indicate a regional abandonment of alluvial fan surfaces at ～10 ± 3 ka, with the younger (～9 ka) end of this age range supported by several of the best-constrained studies. Incision of rivers into the fan surfaces has occurred in discrete stages in the early to mid-Holocene (～9–7 ka) leading to the formation of flights of river terraces. Detailed records of lakebed deposition in the presently arid interior of Iran are rare, though the available data indicate lake highstand conditions at <7.8 ka at South Golbaf in SE Iran and at < 8.7 ± 1.1 ka at the Nimbluk plain in NE Iran. The major periods of Holocene landscape development hence correlate with a period of time where water was more abundant than at present, with incision of rivers into thick alluvial deposits possibly occurring due to a combination of decreased sediment supply and high levels of precipitation, and with the formation of inset river terraces possibly responding to century-scale fluctuations in precipitation. No major geomorphic changes are identified within the later part of the Holocene, from which we infer that increased aridity has slowed evolution of the landscape. A decrease in precipitation in the mid-Holocene may have had a detrimental effect on bronze age societies in eastern Iran as has been inferred elsewhere in the eastern Mediterranean region. The pre-Holocene environmental changes in eastern Iran are less well constrained, though there are suggestions of alluvial fan abandonment at 40–60 ka, at ～80 ka, and at ～120 ka.     

Comparison of three geostatistical methods for hydrofacies simulation: a test on alluvial sediments

The hydrodispersive properties of porous sediments are strongly influenced by the heterogeneity at fine scales, which can be modeled by geostatistical simulations. In order to improve the assessment of the properties of three different geostatistical simulation methods (Sequential indicator simulation, SISIM; Transition probability geostatistical simulation, T-PROGS; Multiple point simulation, MPS) a comparison test at different scales was performed for a well-exposed aquifer analogue. In the analysed volume (approximately 30,000?m³) four operative hydrofacies have been recognised: very fine sand and silt, sand, gravelly sand and open framework gravel. Several equiprobable realizations were computed with SISIM, MPS and T-PROGS for a test volume of approximately 400?m³ and for the entire volume, and the different outcomes were compared with visual inspection and connectivity analysis of the very or poorly permeable structures. The comparison of the different simulations shows that the geological model is best reproduced when the simulations are realised separately for each highest rank depositional element and subsequently merged. Moreover, the three methods yield different images of the volume; in particular MPS is efficient in mapping the geometries of the most represented hydrofacies, whereas SISIM and T-PROGS can account for the distribution of the less represented facies.