lake devlin and pinedale glacial history front range colorado abstract glacial lake devlin was an ice margin lake that formed during pinedale time when the north boulder creek valley glacier blocked the mouth of a tributary near the downvalley limit of glaciation the lake was about 2 km long and as much as 60 m deep and was fed by meltwater from small valley glaciers it accumulated sediment apparently without interruption for about 10 000 yr the inception of lake devlin between 23 500 and 21 000 yr b p provides a limiting date for the maximum expansion of pinedale ice because the lake could not form until the north boulder creek glacier had advanced to within 2 km of its downvalley limit the maximum lowering of timberline which occurred about 19 000 yr b p is assumed to coincide with the pinedale glacial maximum apparent timberline elevations determined from picea pinus pollen ratios suggest that ice front positions during the time between 23 000 and 19 000 yr b p may have been only slightly less extensive than at the pinedale maximum lake devlin drained catastrophically about 13 000 yr b p which was after ice had receded from the outlet area pinedale glaciation in the front range began about or after 30 000 yr b p and final deglaciation occurred between 15 000 and 12 000 yr b p

stable isotope ratios of carbon in phytoliths as a quantitative method of monitoring vegetation and climate change the 13c 12c ratios of occluded carbon within opal phytoliths from the northern great plains show potential as a basis for paleoclimatic reconstruction a significant correlation exists between the carbon isotopic composition of a host plant and that of the organic matter in its phytoliths the 13c 12c ratios for phytoliths from surface layers of soils along climatic gradients reflect the current proportions of c3 and c4 plants variations in the 13c values of phytoliths with soil depth are caused by a variety of processes burial of soil surface by dust bioturbation and possible illuviation by percolating water also contributions of phytoliths by dust and roots have unknown isotopic effects the 13c values of phytoliths from soils increase with decreasing 14c age suggesting that the proportion of c4 plants in this region has increased during the holocene phytoliths of apparent mid holocene age suggest exclusive dominance by c4 plants which agrees with paleoclimatic interpretations of an arid middle holocene climate

a unified kinematic model for the evolution of detachment folds abstract detachment folds represent a major structural element in a number of fold belts they are common in the jura mountains the zagros fold belt the central appalachian fold belt the wyoming fold belt the brooks range the parry islands fold belt and parts of the subandean belt these structures form in stratigraphic packages with high competency contrasts among units the competent upper units exhibit parallel fold geometries whereas the weak lower unit displays disharmonic folding and significant penetrative deformation two distinct geometric types disharmonic detachment folds and lift off folds have been recognized however these structures commonly represent different stages in the progressive evolution of detachment folds the structures first form by symmetric or asymmetric folding with the fold wavelength controlled by the thickness of the dominant units volumetric constraints require sinking of units in the synclines and movement of the ductile unit from the synclines to the anticlines continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and hinge migration initially limb rotation occurs primarily by flexural slip folding but in the late stages of deformation the rotation may involve significant internal deformation of units between locked hinges the folds eventually assume tight isoclinal geometries resembling lift off folds variations in the geometry of detachment fold geometry such as fold asymmetry significant faulting and fold associated with multiple detachments are related to variations in the mechanical stratigraphy and pre existing structure

spatial and temporal patterns of late quaternary eolian deposition eastern colorado u s a abstract eolian sediment covers about 60 of colorado east of the rocky mountains about 30 of the sediment is sand and 70 is loess initially flood plains were the principal sources of eolian sediment but during the holocene dunes formed from older eolian sand and alluvium on uplands since latest pleistocene time dominant dune forming winds have been northwesterly in the northern part of the region and southwesterly in the southern part at present sand sheets and dunes mainly parabolic types are stable and covered with vegetation in dunes sand is commonly 20 30 m thick but elsewhere averages

large scale stabilized dunes on the high plains of colorado understanding the landscape response to holocene climates with the aid of images from space principal component analysis of landsat thematic mapper images from eastern colorado reveals stabilized dune fields that are composed of single and compound parabolic dunes some longer than 10 km these dunes have been reactivated at least four times in the past 10 000 yr at ca 9500 to 5500 yr b p 5500 to gt 4800 yr b p 4800 to gt 1000 yr b p and lt 1000 yr b p during droughts that probably exceeded the dry conditions during the 1930s dune orientations indicate that these landforms were molded by winds from the northwest which now dominate during the winter and spring with the passage of pacific air masses this study underscores the sensitivity of the landscape of the high plains to drought conditions which may become more prevalent because of the anticipated effects of greenhouse warming

eolian landscape evolution and soil formation in the chaco dune field southern colorado plateau new mexico abstract eolian deposits of the chaco dune field cover a broad expanse of the southeastern colorado plateau in new mexico and comprise a large part of the late quaternary sedimentary record in this region the primary eolian landforms include sand sheets parabolic and modified parabolic dunes barchan barchanoid dune complexes and ridge dunes many of the eolian landforms in the chaco dune field have remained stable over a sufficiently long time to permit soil development weakly to moderately developed calcic soils which formed in eolian deposits of mid holocene and latest pleistocene age possess clay enriched weak argillic b horizons these soils have been buried progressively by younger eolian sediments forming regionally correlative bounding surfaces over the chaco dune field and recording relict latest pleistocene and middle holocene eolian landscapes soil geomorphic relations and radiocarbon dated material within the eolian deposits have been useful in 1 delineating three regionally correlative soil stratigraphic units and their associated landforms and 2 studying the types and rates of landscape evolution and climatic changes within this portion of the colorado plateau the most significant period of eolian landform construction is inferred to have occurred during the latest pleistocene glacial to interglacial climatic change forming widespread sand sheets and modified parabolic dunes eolian deposition during the middle and late holocene were dramatically influenced by the presence of previously developed latest pleistocene to early holocene buried soils which may have enhanced sand sheet and parabolic dune development due to the binding effect of the soils middle holocene eolian deposition may be related primarily to neoglacial climatic changes which enhanced ground surface moisture and promoted sand sheet and parabolic dune formation the youngest period of wolian deposition occurred after 1 5 ka and resulted in the formation of the most diverse suite of eolian landforms and deposition of volumetrically more eolian sediments than the mid holocene event in contrast to earlier interpretations the classic mid holocene or altithermal event apparently did not play a major role in the formation of eolian landscapes in the chaco dune field

environmental controls morphodynamic processes and ecogeomorphic interactions of barchan to parabolic dune transformations the transformation of barchans into parabolic dunes has been observed in various dune systems around the world precise details of how environmental controls influence the dune transformation and stabilisation mechanism however remain poorly understood a horns anchoring mechanism and a nebkhas initiation mechanism have previously been proposed and selected environmental controls on the transformation have been explored by some modelling efforts but the morphodynamic processes and eco geomorphic interactions involved are unclear and comparison between different dune systems is challenging this study extends a cellular automaton model informed by empirical data from fieldwork and remote sensing to fully explore how vegetation characteristics boundary conditions and wind regime influence the transformation process and the resulting dune morphologies a dynamic growth function is introduced for clump like perennials to differentiate between growing and non growing seasons and to simulate the development of young plants into mature plants over multiple years modelling results show that environmental parameters interact with each other in a complex manner to impact the transformation process the study finds a fundamental power law relation between a non dimensional parameter group so called the dune stabilising index s and the normalised migration distance of the transforming dune which can be used to reconstruct paleo environmental conditions and monitor the impacts of changes in climate or land use on a dune system four basic eco geomorphic interaction zones are identified which bear different functionality in the barchan to parabolic dune transformation the roles of different environmental controls in changing the eco geomorphic interaction zones transforming processes and resulting dune morphologies are also clarified

stratigraphic evidence for late quaternary dune activity near hudson on the piedmont of northern colorado stabilized dune fields are common features near hudson on the piedmont of northern colorado exposures in dune and interdune sites expose a sequence of eolian sediments and paleosols that record a complex history of eolian activity during the late quaternary radiocarbon and thermoluminescence age estimates on a horizons buried by eolian sand indicate that dunes were reactivated sometime between 7 and 9 ka on the basis of morphology of surface soils the dunes were most recently stabilized 3 ka at present there are no data to indicate if there were multiple periods of dune movement and stabilization during the holocene the penultimate pre holocene dune forming episode probably terminated ca 13 ka and may be coincident with the pinedale glaciation the stratigraphy at interdune sites shows at least two eolian depositional events prior to the penultimate event they were separated by periods of pedogenesis one of which may have exceeded 40 ka this analysis indicates that dunes in northern colorado were active during both cold arid and warm arid periods in the late quaternary

regional aridity in north america during the middle holocene increased aridity throughout the great plains and rocky mountain region during the middle holo cene has been documented from pollen records aeolian proxy variables in lake cores and active sand dune migration varve calibration provided by a continuously varved record of the holocene from a core from elk lake northwestern minnesota shows that the influx of aeolian clastic material increased beginning about 8 ka and ended about 3 8 ka with peak aeolian activity at about 6 ka if aeolian influx to elk lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in minnesota dune fields then the varve calibration in elk lake provides precise time calibration of periods of peak aeolian activity in minnesota palaeowind studies from the minnesota dune fields show that the dominant wind direction when the dunes were active was from the northwest the same as the dominant wind direction in dune fields throughout the great plains and rocky mountains if the mid holocene aeolian act

lacustrine evidence for moisture changes in the nebraska sand hills during marine isotope stage 3 abstract in the central great plains of north america loess stratigraphy suggests that climate during the late pleistocene was cold and dry however this record is discontinuous and there are few other records of late pleistocene conditions cobb basin located on the northern edge of the nebraska sand hills contains lacustrine sediments deposited during marine isotope stage 3 beginning approximately 45 000 cal yr bp and continuing for at least 10 000 yr the lake was formed by a dune dam blockage on the ancient niobrara river and its deposits contain a diatom record that indicates changes through time in lake depth driven by changes in effective moisture during the earliest stages of lake formation the climate was arid enough to mobilize dunes and emplace dune sand into a blocking position within the niobrara streambed diatom assemblages suggest that lake level was shallow at formation increased substantially during a wet interval and then became shallow again as arid conditions resumed by about 27 000 cal yr bp the lake was filled and a shallow ephemeral river occupied the basin

origin of glossic horizons in cryalfs of the eastern rocky mountains usa abstract cryalfs are the predominant suborder along the eastern front of the rocky mountains from northern nm through mt and into alberta approximately 50 of the cryalfs in the western usa contain a glossic horizon defined as a degraded argillic horizon containing properties of both an e and a bt horizon using the nrcs database we examined 257 cryalf soil series 79 of which contain a glossic horizon the glossic horizon occurs at an average depth of 32 cm and averages 24 cm thick the glossic horizon is most commonly an e bt with textures similar to the e above and bt below waxy or glossy argillans are common as well as clay enriched nodules and seams laboratory data are available for only 10 cryalf pedons containing a glossic horizon based on these data the glossic horizon contains an average of 24 clay 1 0 clay cec7 0 6 organic c a cation exchange capacity of 19 cmolc kg soil 85 base saturation and a ph of 5 6 several theories for the origin of the glossic horizon were considered from a weight of evidence approach the glossic horizon in cryalfs of the rocky mountains may be attributed to an input of eolian silt which creates a lithologic discontinuity and a mid holocene 5000 yr bp to present lowering of treeline containing the strongly podzolizing species picea engelmannii and abies lasiocarpa although glossic horizons are common in upper montane soils now containing lodgepole pine pinus contorta they are uncommon in lower montane soils supporting lodgepole and ponderosa pinus ponderosa pines

new geochemical evidence for the origin of north america s largest dune field the nebraska sand hills central great plains usa abstract the nebraska sand hills region is the largest dune field in north america and has diverse aeolian landforms it has been active during both the late pleistocene and late holocene despite decades of study the source of sediment for this large sand sea is still controversial here we report new trace element compositions of aeolian sand that are compared to four hypothesized sediment sources tertiary rocks of the arikaree group and ogallala group unconsolidated sands of pliocene age and platte river system sands all four potential sources have a mineralogy that is similar to the nebraska sand hills k rb k ba sc th la eu eu lan ybn as sb and fe sc values show however that pliocene sediments and sands from the platte river system are not likely sources the arikaree group could be a minor contributor but sands from the ogallala group appear to have the best compositional fit to the nebraska sand hills although past studies have proposed the ogallala group as an important sand source the hypothesis has been questioned because the unit is well cemented by calcrete in its upper part however examination of the landscape upwind of the nebraska sand hills shows that the ogallala group where it occurs at the land surface is highly dissected in much of this region which makes sand sized particles available for aeolian entrainment whenever drought conditions diminish a protective vegetation cover

middle and late holocene dune reactivation in the nebraska sand hills usa a combination of optical dating of aeolian sand and radiocarbon dating of palaeosols and interdune sediments has allowed some refinements to the chronostratigraphy of the nebraska sand hills our results directly confirm previous inferences that multiple phases of aeolian deposition including megabarchan dune reactivation occurred within the sand hills during the middle and late holocene optical dates on stratified aeolian barchan dune sediments from two localities provide ages ranging from c 6 ka to 300 years ago weakly developed palaeosols intercalated within the dune sediments and other forms of post depositional modification indicate periods of relative stability and reduced dune mobilization optical dates on the upper 12 15 m of two dune exposures indicate repeated possibly regionally correlative reactivations which occurred between c 400 500 and 200 300 years ago comparison of late holocene aeolian activity with local and regional palaeoclimatic proxy records suggests that dune reacti

late quaternary eolian sand depositional record for southwestern kansas landscape sensitivity to droughts abstract eolian deposits and landforms are ubiquitous in western kansas particularly south of the arkansas river with the presence of a stabilized dune field stratigraphic studies and associated optical dating reveal a complex depositional history for this dune field spanning the late quaternary the oldest eolian deposits from ca 16 to 12 ka completely or partially bury high terraces with ages of ca 16 and 33 ka it is unknown whether these eolian deposits reflect regional aridity and or a change in sediment availability with the transition of the arkansas river from a single channel to a braided system there is pervasive evidence for episodic eolian erosion and deposition ca 9 8 to 6 3 ka generally coincident with loess deposition on upland surfaces olson c g nettleton w d porter d a brasher b r 1997 middle holocene aeolian activity on the high plains of west central kansas holocene 7 3 255 261 and thus it is inferred to reflect regional aridity sites within the dune field show a sequence of eolian sands and weak buried soils reflecting either dune migration or sand sheet accretion at ca 1490 430 380 320 180 and 70 yr ago which correspond well with continental scale droughts in the tree ring record eolian sand in the arkansas river dune field may be derived principally with variability in fluvial activity and in climate during the late pleistocene to the middle holocene with substantial reactivation of eolian systems during decadal scale drought variability in the past 2000 yr

holocene eolian activity in the minot dune field north dakota stabilized eolian sand is common over much of the great plains region of the united states and canada including a subhumid area of 1500 km2 near minot north dakota eolian landforms consist of sand sheets and northwest trending parabolic dunes dunes and sand sheets in the minot field are presently stabilized by a cover of prairie grasses or oak woodland stratigraphic studies and accelerator mass spectrometry radiocarbon dating of paleosols indicate at least two periods of eolian sand movement in the late holocene pedologic data suggest that all of the dune field has experienced late holocene dune activity though not all parts of the dune field may have been active simultaneously similar immobile element ti zr la ce concentrations support the interpretation that eolian sands are derived from local glaciofluvial and glaciolacustrine sediments however glaciolacustrine and glaciofluvial source sediments have high ca concentrations from carbonate minerals whereas dune sands are depleted in ca

stratigraphy of the younger dryas chronozone and paleoenvironmental implications central and southern great plains abstract the great plains of the united states was the setting for some of the earliest research in north america into patterns and changes in the character of late pleistocene environments and their effects on contemporary human populations many localities in the region have well stratified records of terminal pleistocene and early holocene human paleoindian activity and past environments these have proven important in debates over the character of the younger dryas chronozone ydc 11 000 10 000 14c bp 12 900 11 700 cal bp in the continental interior this paper reviews the lithostratigraphic record of the ydc on the central and southern great plains and summarizes paleobiological records largely isotopic the goal is to determine if there is any uniformity in the timing character direction and or magnitude of changes in depositional environments or broader geomorphic systems before during or after the ydc in order to address the question of the character of environments through this time the stratigraphic records of the late pleistocene to early holocene transition and in particular the stratigraphic records of the ydc vary through time and space the data clearly show that a host of geomorphic processes produced the terminal pleistocene and early holocene stratigraphic records of the great plains moreover the ydc is not necessarily manifest as a distinct lithostratigraphic or biostratigraphic entity in these different types of deposits and soils the various geomorphic systems of the great plains did not behave synchronously in response to any common climate driver these stratigraphic records reflect local environmental conditions and probably a complex response to the reorganization of mid latitude climates in the terminal pleistocene and early holocene

origin of the late quaternary dune fields of northeastern colorado stabilized eolian deposits mostly parabolic dunes and sand sheets cover much of the landscape of northeastern colorado and adjacent parts of southwestern nebraska in four geographically distinct dune fields stratigraphic and soil geomorphic relations and accelerator radiocarbon dating indicate that at least three episodes of eolian sand movement occurred between 27 ka and 11 ka possibly between 11 ka and 4 ka and within the past 1 5 ka thus eolian sand deposition took place under both glacial and interglacial climatic conditions in the youngest episodes of eolian sand movement holocene parabolic dunes partially buried pleistocene sand sheet deposits late holocene sands in the fort morgan and wray dune fields to the south of the south platte river have trace element ratios that are indistinguishable from modern south platte river sands but different from ogallala formation bedrock which has previously been cited as the main source of dune sand on the great plains sands in the greeley dune field to the north of the south platte river have trace element concentrations that indicate a probable laramie formation source measurements of parabolic dunes indicate paleowinds from the northwest in all dune fields in good agreement with resultant drift directions calculated for nearby weather stations thus paleowinds were probably not significantly different from present day winds and are consistent with a south platte river source for the fort morgan and wray dune fields and a laramie formation source for the greeley dune field sand accumulated downwind of the south platte river to form the fort morgan duen field in addition sand was also transported farther downwind over the upland formed by the calcrete caprock of the ogallala formation and deposited in the lee of the upland on the southeast side because of high wind energy the upland itself served as a zone of sand transport but little or no sand accumulation took place on this surface these studies which demonstrate the importance of fluvial source sediments for dune fields in colorado may be applicable to other dune fields in north america because modern drift potentials in northeastern colorado are among the highest in the world the present stability of dunes in the region may be in part a function of the dunes being supply limited rather than solely transport limited extensive 7700 km2 late holocene dunes document that eolian sand in northeastern colorado is very sensitive to small changes in climate or fluvial source conditions

late quaternary loess in northeastern colorado part i age and paleoclimatic significance loess in eastern colorado covers an estimated 14 000 km 2 and is the westernmost part of the north american midcontinent loess province stratigraphic studies indicate there were two periods of loess deposition in eastern colorado during late quaternary time the first period spanned ca 20 000 to 12 000 14 c yr b p ca 20 14 ka and correlates reasonably well with the culmination and retreat of pinedale glaciers in the colorado front range during the last glacial maximum the second period of loess deposition occurred between ca 11000 and 9000 14 c yr b p this interval may be holocene or may correlate with a hypothesized younger dryas glacial advance in the colorado front range sedimentologic mineralogic and geochemical data indicate that as many as three sources could have supplied loess in eastern colorado these sources include glaciogenic silt derived from the colorado front range and two bedrock sources volcaniclastic silt from the white river group and clays from the pierre shale the sediment sources imply a generally westerly paleowind during the last glacial maximum new carbon isotope data combined with published faunal data indicate that the loess was probably deposited on a cool steppe implying a last glacial maximum july temperature depression relative to the present of at least 5 6 c overall loess deposition in eastern colorado occurred mostly toward the end of the last glacial maximum under cooler and drier conditions with generally westerly winds from more than one source

unprecedented long chain 1 chloroalkenes and 1 chloroalkanes in the holocene sediments of lake el junco galapagos islands abstract the occurrence of long chain chloroalkenes and chloroalkanes has not been previously reported in natural sediments here we report the presence of long chain c30 c36 n chloroalkanes and chloroalkenes in the recent sediments of the only permanent freshwater lake in the galapagos islands el junco both chloroalkanes and chloroalkenes have a chlorine atom at the terminal position the cis and trans alkenyl chlorides have double bonds near the middle of the hydrocarbon chain 15 16 17 alkenes in the lake sediments are typically c23 1 to c27 1 substantially shorter than the chloroalkenes and chloroalkanes the lipid chain lengths and the positions of functional groups imply a structural relationship between chloroalkenes and some alkenols diols and or hydroxy acids that most likely derive from algae