Pinedale Moraines Research Articles

Cosmogenic 10Be exposure dating of Bull Lake and Pinedale moraine sequences in the upper Arkansas River valley, Colorado Rocky Mountains, USA

AbstractMany formerly glaciated valleys in the western United States preserve detailed glacial features that span the penultimate glaciation through the last deglaciation; however, numerical age control is limited in many of these systems. We report 35 new cosmogenic 10Be surface exposure ages of moraine boulders in the Sawatch Range, Colorado. Eight ages suggest Bull Lake moraines in Lake Creek (range: 132–120 ka, n = 4) and Clear Creek (range: 187–133 ka, n = 4) valleys may correlate with Marine Isotope Stage 6. In Lake Creek valley, 22 10Be ages from Pinedale end moraines average 20.6 ± 0.6 ka, and 5 10Be ages from a recessional moraine average 15.6 ± 0.7 ka, indicating that glaciers occupied two extended positions at ~21–20 and ~16 ka. The glacial extent dated to ~16 ka was nearly as great as that of the earlier glacial phase, suggesting that climate conditions in the Colorado Rocky Mountains at this time were similar to those of the last glacial maximum. Combining these moraine ages with seven previously published 10Be ages from cirque and valley-bottom bedrock reveals that the Lake Creek paleoglacier lost 82% of its full glacial length in ~1.5 ka and was completely deglaciated by ~14 ka.

Moraine pebbles and boulders yield indistinguishable 10Be ages: A case study from Colorado, USA

Cosmogenic exposure dating of moraines during the last two decades has vastly improved knowledge on the timing of glaciation worldwide. Due to a variety of geologic complications, such as moraine degradation, snow cover, bedrock erosion and isotopic inheritance, samples from multiple large boulders (>1–2 m) often lead to the most accurate moraine age assignments. However, in many cases, large boulders are not available on moraines of interest. Here, I test the suitability of pebble collections from moraine crest surfaces as a sample type for exposure dating. Twenty-two 10Be ages from two Pleistocene lateral moraine crests in Pine Creek valley in the upper Arkansas River basin, Colorado, were calculated from both pebble and boulder samples. Ten 10Be ages from a single-crested Bull Lake lateral moraine range between 3 and 72 ka, with no statistical difference between pebble ( n = 5) and boulder ( n = 5) ages. The lack of a cluster of 10Be ages suggests that moraine degradation has led to anomalously young exposure ages. Twelve 10Be ages from a single-crested Pinedale lateral moraine have a bimodal age distribution; one mode is 22.0 ± 1.4 ka (three boulders, two pebble collections), the other is 15.2 ± 0.9 ka (two boulders, five pebble collections). The interpretation of the two age modes is that two glacier maxima of similar extent were attained during the late Pleistocene. Regardless of moraine age interpretations, that 10Be ages from pebble collections and boulders are indistinguishable on moraines of two different ages, and in two different age modes of the Pinedale moraine, suggests that pebble collections from moraine crests may serve as a suitable sample type in some settings.

Quantifying glacial moraine age, denudation, and soil mixing with cosmogenic nuclide depth profiles

Glacial boulders and soils on moraines are often dated to quantify the timing of glaciations and/or rates of chemical weathering in moraine chronosequences. A common assumption is that moraine crest erosion and soil mixing are unimportant. However, several studies suggest moraine denudation may be substantial. We evaluate the magnitude of moraine denudation and soil mixing in the Pinedale (∼21 ka) and Bull Lake (∼140 ka) moraines at Fremont Lake (Wyoming, United States) using cosmogenic nuclide depth profiles and a numerical model of moraine erosion and nuclide production and decay. Depth profiles indicate mixing of the surface layer from 40 to 60 cm depth. Age constraints from depth profile dating of cosmogenic nuclides result in ages of 17–24 ka for the Pinedale moraine and 70–127.5 ka for the Bull Lake moraine. These ages are comparable to ages based on the Lal and Chen approach which uses a sample from the mixed layer and the unmixed layer. However, the Lal and Chen approach is difficult to apply in real environments as the mixing depth needs to be determined independently. We find our best fit model based ages for the Bull Lake moraine are younger than independent age constraints from boulder exposure dating due to incomplete mixing. Exposure age constraints from boulders are more straightforward for moraines than ages based on depth profile dating. Finally, we find that moraine crests do significantly erode and are mixed, suggesting that previous weathering and dust accumulation rate studies on moraines provide minimum estimates for these processes.

Combining cosmogenic nuclides and major elements from moraine soil profiles to improve weathering rate estimates

Previous studies of chemical weathering rates for soil developed on glacial moraines generally assumed little or no physical erosion of the soil surface. In this study, we investigate the influence of physical erosion on soil profile weathering rate calculations. The calculation of chemical weathering rates is based on the assumption that soil profiles represent the integrated amount of weathering since the time of moraine deposition. The weathering rate of a surface subjected to denudation is the sum of the weathering loss from the existing soil profile added to the weathering loss in the material removed by denudation, divided by the deposition age. In this study, the amount of weathered material removed since moraine deposition is calculated using the denudation rate estimated from cosmogenic nuclide data and the deposition age of the moraine. Weathering rates accounting for denudation since moraine deposition are compared to weathering rates based on the assumption of no physical erosion and on the assumption of steady-state denudation for the Type Pinedale moraine (∼ 21 ka) and the Bull Lake-age moraine (∼ 140 ka) in the Fremont Lake Area (Wind River Mountains, Wyoming, USA). The total weathering rates accounting for denudation are 8.15 ± 1.05 g (oxide) m − 2 y − 1 for the Type Pinedale moraine and 4.78 ± 0.89 g (oxide) m − 2 y − 1 for the Bull Lake-age moraine, which are ∼ 2 to 4 times higher, respectively, than weathering rates based on the assumption of no physical erosion. The weathering rates based on denudation since moraine deposition are comparable or smaller than weathering rates assuming steady-state denudation. We find the assumption of steady-state denudation is not valid in depositional landscapes with young deposition ages or slow denudation rates. The decrease in weathering rates over time between the Type Pinedale and Bull Lake-age soils that is observed in the case of no physical erosion is decreased when the influence of denudation on the total weathering rates is taken into account. Fresh unweathered material with high reactive mineral surface area is continuously provided to the surface layer by denudation diminishing the effect of decreasing weathering rate over time.

Soils in a Geoecological Landscape Chronosequence on Glacial Drift in Southeastern Idaho

At the southwest corner of Yellowstone National Park, one geologist has mapped glacial moraines of three ages and later another mapped moraines of only two ages. Soils, including topographic differences, indicate glacial drift of four ages. These appear to be related to late Pinedale, early Pinedale, Bull Lake, and pre-Bull Lake drift that have been dated in northwestern Wyoming. The limits of late and early Pinedale moraines appear to correspond closely with ecological map unit boundaries and have specific soils that reflect greater weathering and soil development on early than on late Pinedale moraines. The weathering and soil development trend continues onto Bull Lake moraines, reaching a maximum there, with thick fine-loamy (or loamy-skeletal) argillic horizons. Soil development is no greater on the older pre-Bull Lake moraines, presumably because of development retardation by cryoturbation and mass wasting or solifluction. Ecological map units based on soil mapping do not correspond to limits of pre-Bull Lake (or Sacagewa) till mapped by a geologist. If geologists and pedologists had collaborated when mapping surficial geology and soils at the southwestern corner of the Park, both geology and soils (or ecological unit) maps and reports might be better representations of the landscapes.

Cosmogenic 36Cl and 10Be ages of Quaternary glacial and fluvial deposits of the Wind River Range, Wyoming

We measured cosmogenic 36 Cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the Wind River Range, Wyoming. We also measured 10 Be in 10 of the same samples. Most of the 10 Be ages were in good agreement with the 36 Cl ages, indicating that rock-surface erosion rates were very low. The oldest moraine investigated, the type Sacagewea Ridge site, yielded only a limiting minimum age of >232 ka. The oldest moraines in the type Bull Lake complex also could be constrained only to >130 ka. The main sequence of type Bull Lake moraines yielded age distributions indicating deposition within the intervals 130 to 100 ka and 120 to 100 ka; the best estimates are closer to the upper limits of these ranges, and associated uncertainties are in the range of 10% to 15%. These uncertainties could permit deposition in either marine isotope stage 6 or stage 5d. We found no evidence of glacial deposits dating to marine isotope stage 4. Both Bull Lake–age moraines from Fremont Lake, on the opposite side of the Wind River Range, and boulders on a fluvial terrace above the Wind River, gave age distributions very similar to that of the second oldest Bull Lake advance (ca. 130 to 100 ka). The distribution of boulder ages for Pinedale moraines at Bull Lake indicated deposition between 23 and 16 ka, nearly identical to the distribution of 10 Be ages previously reported for the type Pinedale moraines at Fremont Lake.

Chronology of Pleistocene glacial advances in the central Rocky Mountains

The distribution and ages of moraines in the Wyoming-type localities for pre-Pinedale Rocky Mountain glacial deposits are not well known. Through morphostratigraphic mapping, river-terrace longitudinal-profile analysis, and tephrochronology, we place the age of Sacagawea Ridge moraines in the Dinwoody Lakes type area at about 660 ka. Outwash and river terrace deposits in the Bull Lake type area, previously considered to be Sacagawea Ridge, are actually older by one or two glacial cycles. In the Bull Lake type locality, 15 individual moraines are divided into 4 groups on the basis of relative age criteria. Using 36Cl/10Be cosmogenic dating of boulders, the two older groups must be >130 ka; the younger groups are given age ranges of 130 to 100 ka and 120 to 95 ka. A terrace along Wind River can be morphostratigraphically traced to the youngest of these moraine groups; 36Cl dating of boulders on the terrace provides an age range of 125 to 100 ka. The combined relative age and cosmogenic nuclide chronology is consistent with previously published evidence from Yellowstone National Park demonstrating that mountain glaciers advanced not only prior to and during the later portion of marine isotope stage 6 but also during marine isotope stage 5d. In contrast with some other areas of the Rocky Mountains, we found no evidence for advances during marine isotope stage 4. At Bull Lake, Pinedale moraines that have a 36Cl/10Be age range of 23 to 16 ka suggest that glacial advances were limited to the later part of marine isotope stage 2. A river terrace that can be traced to these moraines has an age range of 23 to 16 ka. A possible glacial advance during early marine isotope stage 2 is indicated by a river terrace whose age range is estimated by incision-rate modeling at 46 to 28 ka, but no moraines were identified. Along Wind River, there are eight terrace levels capped with outwash that are older than Sacagawea Ridge; although the terraces are the record of early to middle Pleistocene climatic cycles, there are no correlative moraines preserved at the land surface.

Mid-Holocene Erosion of Soil Catenas on Moraines near the Type Pinedale Till, Wind River Range, Wyoming

Buried soils are described from the floors of four kettles on Pinedale piedmont moraines of the southwestern Wind River Range, Wyoming, near the type locality of the Pinedale Till. The buried soils indicate that a previously unreported episode of slope erosion has occurred along adjacent catenas on some of the moraines in this region. Radiocarbon ages of the buried soils indicate that slope erosion occurred during the middle Holocene from 8540 ± 190 to 4800 ± 60 14C yr B.P. The presence of buried soils in moraine kettles indicates that profiles on the crests and backslopes of some of the moraines in this region are not the products of continuous post-Pinedale soil formation. Rather, the crest and backslope soils on Pinedale moraines result from two periods of soil development separated by an interval of erosion. Thus, soils on crests and backslopes are considered to be welded profiles. Relative-age studies that use soil data to estimate the age of moraines in this region must take into account the possibility that a mid-Holocene erosion episode affected the genesis and morphology of soils on the moraines. This study corroborates results of previous work that suggests that data from complete catenas are more useful for estimating the characteristic soil development on Quaternary moraines than are data derived from crests alone.

Quaternary evolution of Cedar Creek alluvial fan, montana

Cedar Creek alluvial fan is a textbook example of an alluvial fan because of its fan shape with smooth, concentric contours and excellent symmetry. Similar planimetric shapes have been used to infer uniform fan deposition; however, Cedar Creek alluvial fan is composed of four fan deposits of Quaternary age, Qf1 (oldest) to Qf4 (youngest), indicating that fan deposition was nonuniform in both time and space. Field studies indicate that deposition of Cedar Creek alluvial fan is related to glaciofluvial outwash activity during the Pleistocene and upper-fan entrenchment and lower-fan deposition during the Holocene. Qf1 and Qf2 deposits are sub-horizontally bedded, clast-supported sandy gravels uniformly imbricated upfan. Comparison of soil profiles developed in these deposits to radiogenically-dated chronosequences within the region indicates that Qf1 and Qf2 are correlative with Bull Lake and Pinedale-age deposits, respectively. These relationships are substantiated by physical correlation of Qf1 and Qf2 with Bull Lake and Pinedale moraines, respectively, in the Cedar Creek drainage basin. The sedimentology and timing of Qf1 and Qf2 indicate deposition in high-energy, proglacial, braided streams. Furthermore, the present morphology of Cedar Creek alluvial fan was established largely during aggradation of Qf1 and Qf2 when sediment supply to the fan was sufficient to activate 60% to greater than 90% of the total fan area. During Bull Lake glaciation, the apex of Qf1 deposition formed the apex of Cedar Creek alluvial fan as Qf1 covered more than 90% of the present fan area. During Pinedale glaciation, Qf2 deposition shifted downfan; Qf2 is inset into Qf1 above the intersection point, but below the intersection point it eroded and/or buried Qf1 as it activated as much as 60% of the fan area. Qf3 and Qf4, comprising 21% of the fan area, are inset into Qf2 in the lower fan area. Soil development in Qf3 and Qf4 deposits indicate episodic deposition and entrenchment beginning in early Holocene and continuing to present. A post-glacial decrease in sediment supply to Cedar Creek alluvial fan is indicated by sediment storage within the Cedar Creek drainage basin. Decreased sediment supply to the fan resulted in upper-fan entrenchment of Qf2 and deposition of Qf3 and Qf4 in the lower-fan area.

Soil Catenas on pinedale and Bull Lake moraines, Willow Lake, wind river mountains, Wyoming

Soils in coarse-grained tills on Pinedale (about 20,000 years old) and Bull Lake (about 140,000 years old) moraines show significant variaton with slope position and age. Soils on the Pinedale moraine are mostly non-calcareous Typic Cryoborolls with A/Bw/C profiles that thicken considerably downslope. These soils are weakly developed due to their youth and to the unstable, steep (up to 24°), highly convexo-concave slopes. Soils on the Bull Lake moraine are mostly Argic Cryoborolls with A/Bt/C profiles. These soils are more strongly developed than soils at similar positions on the Pinedale moraine because the Bull Lake moraine is older and has less steep (up to 16°), more gently curved slopes. The soils on the Bull Lake moraine contain more clay, a higher proportion of pedogenic clay, more weathered stones, and more free sesquioxides. Erosion and dry conditions at the convex summit and shoulder sites on the Bull Lake moraine have resulted in thin soils with minimal argillic horizons; these soils are only slightly more strongly developed than soils at analogous sites on the much younger Pinedale moraine. In contrast, at the concave sites in downslope positions the soils on the Bull Lake moraine are much more strongly developed than analogous soils on the Pinedale moraine; slow deposition of weathered material, probably transported by creep, has acted concurrently with pedogenesis to form soils with thick, homogeneous argillic horizons. The relationship between hillslope curvature and soil properties on both moraines is shown by significant correlations between soil properties and the second derivative of the hillslope profile. This relationship is probably due primarily to the interaction of soil formation and soil creep.

Soil catenas on pinedale and bull lake moraines, willow lake, wind river mountains, Wyoming

Abstract Soils in coarse-grained tills on Pinedale (about 20,000 years old) and Bull Lake (about 140,000 years old) moraines show significant variaton with slope position and age. Soils on the Pinedale moraine are mostly non-calcareous Typic Cryoborolls with A/Bw/C profiles that thicken considerably downslope. These soils are weakly developed due to their youth and to the unstable, steep (up to 24°), highly convexo-concave slopes. Soils on the Bull Lake moraine are mostly Argic Cryoborolls with A/Bt/C profiles. These soils are more strongly developed than soils at similar positions on the Pinedale moraine because the Bull Lake moraine is older and has less steep (up to 16°), more gently curved slopes. The soils on the Bull Lake moraine contain more clay, a higher proportion of pedogenic clay, more weathered stones, and more free sesquioxides. Erosion and dry conditions at the convex summit and shoulder sites on the Bull Lake moraine have resulted in thin soils with minimal argillic horizons; these soils are only slightly more strongly developed than soils at analogous sites on the much younger Pinedale moraine. In contrast, at the concave sites in downslope positions the soils on the Bull Lake moraine are much more strongly developed than analogous soils on the Pinedale moraine; slow deposition of weathered material, probably transported by creep, has acted concurrently with pedogenesis to form soils with thick, homogeneous argillic horizons. The relationship between hillslope curvature and soil properties on both moraines is shown by significant correlations between soil properties and the second derivative of the hillslope profile. This relationship is probably due primarily to the interaction of soil formation and soil creep.

Absolute pollen diagram of Redrock Lake, Boulder County, Colorado

Redrock Lake is situated at 3,095 m in the subalpine zone on the east flank of the Front Range, Boulder County, Colorado. The lake lies on Pinedale moraine, and it contains 170 cm of organic sediments that overlie 10 cm of silty clay. The oldest of seven 14C dates from the organic sediment is 9,490 ± 150 yr B.P., a minimum estimate of the time since Middle Pinedale ice receded from the lake basin. A percentage pollen diagram and an absolute pollen diagram providing pollen deposition rates in grains/cm 2/ 14C yr are compared and contrasted. Possible measurement errors are considered in detail, and for the first time confidence limits are assigned to all pollen data. A means of converting 14C years to calendar years is presented, and its effect on the diagram's deposition rates is discussed. The silty clay contains a peculiar Artemisia-dominated pollen assemblage with very low pollen influx rates, which suggests the lake was receiving large volumes of glacial meltwater during its early history. The overlying organic sediments are characterized by pollen deposition rates of 4,000–6,000 grains/cm 2/yr. With fluctuations, deposition rates of Pinus and total anemophilous pollen increase upward in the core; this evidence for a general upward shift in the region's vegetation zones during Postglacial time is supported by pollen trends from modern surface samples.

PLEISTOCENE GEOLOGY OF THE SOUTHWESTERN WIND RIVER MOUNTAINS, WYOMING

Patches of Buffalo till record the earliest glaciation in the southwestern Wind River Mountains. In places, these rest in youthful valleys cut in high gravel terrace. Two other younger and lower terraces are both topographically and stratigraphically associated with Buffalo till, which may indicate that the Buffalo advance was compound. The pattern of well-preserved moraines shows that during both Bull Lake and Pine-dale time the west-central portion of the range was covered by a mountain icecap from which piedmont glaciers flowed to the floor of the Bridger Basin. In the southern part of the range the glaciers were confined to valleys. Bull Lake stage is clearly double; two large, weathered, and modified moraines are present in several valleys. Each is associated with an extensive outwash plain and valley train. The Pinedale stage is recorded by massive, fresh, slightly modified moraines behind which are many recessional moraines. Extensive outwash aprons lead outward from the massive Pinedale moraines, and a small lower terrace can be traced through the main Pinedale moraine to the recessional loops. In several upper valleys well-formed slightly weathered small moraines of the Temple Lake stage occur within 2 miles of cirque head-walls. A terrace below the younger Pinedale outwash is correlated with the Temple Lake stage. In sheltered cirques, small very fresh moraines, probably from the Little Ice Age, occur upstream from the Temple Lake moraines and a few yards from existing small glaciers. The sequence of glacial deposits in this region is typical of many ranges in the Rocky Mountain region. Outwash terraces of the Bull Lake, Pinedale, and Temple stages have been traced down the Big Sandy valley nearly to the Green River and down the Sweetwater valley to the North Platte. Thus the Green-Colorado and Platte-Missouri drainages are linked across the Continental Divide by means of traceable outwash deposits. Eolian action was pronounced in the Eden valley during Bull Lake, Pinedale, and post-Pinedale time, and in the East Fork valley during Pinedale time. Frost action features record two phases of intensity in areas formerly covered by Pinedale glaciers: an earlier phase synchronous with the Temple Lake advance, a later during the Little Ice Age. Pollen profiles in the Eden valley show a climatic change during post-Pinedale time, notably a grass maximum which was probably contemporaneous with the Temple Lake advance and the occupation of the Finley site by Early Man.

Pleistocene Geology of the Sawatch Range, Near Leadville, Colo

Previous articleNext article FreePleistocene Geology of the Sawatch Range, Near Leadville, ColoS. R. Capps and E. D. K. LeffingwellS. R. Capps Search for more articles by this author and E. D. K. Leffingwell Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 12, Number 8Nov. - Dec., 1904 Article DOIhttps://doi.org/10.1086/621191 Views: 21Total views on this site Citations: 3Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Avriel D. Schweinsberg, Jason P. Briner, Joseph M. Licciardi, Ralph R. Shroba, Eric M. Leonard Cosmogenic 10 Be exposure dating of Bull Lake and Pinedale moraine sequences in the upper Arkansas River valley, Colorado Rocky Mountains, USA, Quaternary Research 45 (Apr 2020): 1–15.https://doi.org/10.1017/qua.2020.21Dennis E. Dahms Glacial limits in the middle and southern Rocky mountains, U.S.A., south of the Yellowstone ice cap, (Jan 2004): 275–288.https://doi.org/10.1016/S1571-0866(04)80203-3 W. W. Robbins Successions of Vegetation in Boulder Park, Colorado, Botanical Gazette 65, no.66 (Oct 2015): 493–525.https://doi.org/10.1086/332284