Glaciations of the Sierra Nevada, California, USA

Transcription

1 Chapter 34 Glaciations of the Sierra Nevada, California, USA Alan R. Gillespie 1, * and Douglas H. Clark 2 1 Quaternary Research Center, University of Washington, Seattle, Washington , USA 2 Department of Geology, Western Washington University, Bellingham, Washington 98225, USA *Correspondence and requests for materials should be addressed to Alan R. Gillespie. arg3@uw.edu INTRODUCTION The Sierra Nevada is a major north south mountain range in California that separates the internally drained basins and desert ranges to the east from the Central Valley to the west. Although it runs from the Tehachapi Mountains on the northern edge of the Mojave Desert to near Mount Lassen ( N), the highest part of the Sierra Nevada (crest elevations of m) lies between 36 N and 38 N, the approximate latitudes of the towns of Olancha (south) and Bridgeport (north). This entire reach was heavily glaciated during the Pleistocene, and some small glaciers still occupy sheltered cirques high in the mountains. Enhanced accumulation and shading from adjacent peaks allow these modern glaciers to exist well below the regional climatic snowline (estimated at 4500 m elevation at 37 N; Flint, 1957, p. 47). Although the glaciers occur at progressively lower altitudes to the north, the topographic crest north of Bridgeport plunges below the average snowline, or equilibrium-line altitude (ELA), for the modern glaciers. During the major Pleistocene glaciations, however, the ELA was 820 m lower than for modern glaciers (Warhaftig and Birman, 1965; Gillespie, 1991), and then large glaciers occurred in the northern Sierra Nevada also (Fig. 34.1). Remarkably, the terminal moraines of maximum advances of different ages cluster closely together (Fig. 34.2), such that there appears to be a rough maximum limit to the size of Sierran glaciers over a half million years or more. In the Sierra Nevada, the advance and retreat of glaciers are especially sensitive to changes in winter precipitation and summer temperature. At present, the Sierra Nevada receives moisture mainly from winter low-pressure systems from the Pacific Ocean guided by the jet stream. Mean annual precipitation on the eastern slope of the Sierra Nevada at 37 N ranges from 100 cm a 1 at the crest to 25 cm a 1 at the range front (Danskin, 1998); east of the crest it is controlled by a strong rainshadow. During Pleistocene glaciations, the jet stream shifted south so that precipitation increased and temperatures were reduced, although not necessarily in phase. There are two main sources of precipitation: northerly, from the Gulf of Alaska; and southerly (the so-called Pineapple Express ), from tropical latitudes in the Pacific driven by a southern branch of the jet stream. Therefore, precipitation in the Sierra Nevada and the Cascade Mountains to the north is not necessarily in phase on an annual or even a decadal scale. Antevs (1938, 1948) first proposed this modern view of the ice-age climate, with Sierra Nevada precipitation controlled by intensified winter Pacific storms, driven farther south than today due to the influence of the Cordilleran and Laurentide ice sheets. However, modelling by Rupper et al. (2009) has shown that except for arid environments with precipitation less than 150 mm a 1, there is generally enough snowfall to grow glaciers and summertime temperatures have a strong control on their advance and retreat. The Sierra Nevada consists of a batholith of Mesozoic plutons of intermediate composition, commonly granodiorite, intruded metamorphic marine sedimentary rocks and island arc volcanic rocks, both now preserved as roof pendants. The range is a large crustal block that has been tilted to the west and broken by extensional basin-and-range faulting on the east. The timing of these tectonic events is controversial, but tilted Miocene lava flows west of the crest, and faulted Pliocene basalt flows in the ranges east of the Sierra Nevada indicate that much of the early tectonic activity started before the Quaternary Period, although faulting continues today. Developments in Quaternary Science. Vol. 15, doi: /B ISSN: , # 2011 Elsevier B.V. All rights reserved. 447

2 448 Quaternary Glaciations - Extent and Chronology FIGURE 34.1 Extent of Quaternary glaciers in the Sierra Nevada, modified from Warhaftig and Birman (1965) and Clark (1995). Late Pleistocene (Tioga) and older glacial deposits are, in general, so similar in extent that they are difficult to distinguish at this scale; see Figs and Base map: USGS NED dataset (3-s resolution, 90 m) Background California in the nineteenth century was a long way from the European centres where geological theory was first developed. Nevertheless, the Sierra Nevada received a surprising amount of attention, and the understanding of mountain glaciation was advanced in no small part there. For example, Josiah Whitney, the California State Geologist, and John Muir, prominent naturalist, debated famously the relative roles of glacial, and tectonic processes in sculpting the spires, ridges, and deep valleys of the Sierra Nevada, with special attention given to Yosemite Valley. LeConte (1873) modified the positions taken earlier and derived an essentially modern view that glaciers had excavated the highlands and widened the upper reaches of river valleys, but that the basic landscape was nevertheless formed largely by fluvial activity. Matthes (1930, 1965) undertook decades of study of the glaciated troughs of the western

3 Chapter 34 Glaciations of the Sierra Nevada, California, USA 449 FIGURE 34.2 Six glaciated drainages of the eastern Sierra Nevada, showing close proximity of preserved terminal moraines of different glaciations. The ELAs are also close (Table 34.1). Index map of California shows location of drainages (A F). (A) Green Creek. (B) Walker Creek/Bloody Canyon/Sawmill Canyon (Mono County). (C) Convict Creek. (D) Sawmill Canyon (Inyo County). (E) South Fork Oak Creek. (F) Independence Creek. The greater dispersion of moraines in (F) arises because the accumulation area there comprises three drainages versus one for the other examples; therefore, minor differences in ELA result in major extension of the ablation area. DELA (accumulation-area ratio method) is 60 m or less for the moraines shown. Moraines are identified by Roman numerals and colour coding. Name assignments are: pre-tahoe moraines (I, brown); Mono Basin moraines (II) and Tahoe moraines (III), blue; Tenaya (IV) moraines and Tioga (V) moraines, yellow and pink and latest Tioga moraines (VI, green). Elevation contours are in m a.s.l. Except for Convict Creek (Gillespie, unpublished map, 2009), mapping follows Gillespie (1982). Base maps are contoured and shaded NASA/ASTER 30-m digital elevation models (DEMs). slopes of the Sierra Nevada, and Blackwelder (1931) similarly analysed the glacial evidence east of the crest. Blackwelder, in particular, pioneered relative dating in an effort to correlate moraines from valley to valley, and to the glacial stratigraphy developed along the southern margin of the Laurentide Ice Sheet. Matthes and Blackwelder made an early attempt to reconcile their glacial sequences, but in the absence of numerical age control, correlation was

4 450 Quaternary Glaciations - Extent and Chronology difficult, as successive generations of glacial geologists have confirmed while refining the glacial stratigraphy. By the 1970s, the modern framework of the Sierra Nevada glaciations was well developed (e.g. Warhaftig and Birman, 1965; Fullerton, 1986), but numerical age control was weak or missing entirely. Recent investigations have improved this glacial history. In particular, a more thorough accounting has been developed for glaciations pre-dating marine oxygen isotope stage (MIS) 6 (> 186 ka), and the MIS 6 2 ( ka) and Holocene glacial history has been refined by numerical dating. Most of the research has taken place in the eastern Sierra Nevada, although recently new studies have been made west of the crest also (James et al., 2002). The improvements to the glacial history are based on continued exploration and mapping, improved and new methods of numerical and relative dating and extraction and analysis of sediment cores from lakes and bogs. Many advances in the past few decades have resulted from the development and application of rigorous dating techniques to glacial drift and landforms. Early studies included K/Ar dating of lava flows interbedded with till (Dalrymple, 1963, 1964) and 14 C dating of latest Pleistocene sediments in bogs that could be related stratigraphically to glacial deposits (Adam, 1966, 1967). K/Ar dating, however, was problematic and opportunistic, because the glacial deposits and landforms themselves could not be dated. For example, Gillespie et al. (1984) obtained high-precision 40 Ar 39 Ar dates for basalt flows at Sawmill Canyon (Inyo Country), but in the end could only conclude that the Hogsback moraine was less than 1193 ka and must postdate MIS 6 ( ka), a conclusion already reached by Burke and Birkeland (1979) on the basis of soil development. Beginning in the 1980s, advances in accelerator mass spectrometry (AMS) led to the measurement of trace concentrations of exotic isotopes created by cosmic-ray bombardment of rocks exposed at the Earth s surface, and the calculation of exposure ages of these rocks (e.g. Nishiizumi et al., 1989; Gosse and Phillips, 2001). From these dates, landform ages could be inferred if the erosion rate of the dated surfaces could be estimated. The glacial chronology worldwide was a logical and early target of this new technology, and convenient samples for analysis were found in boulders exposed on the crests of moraines. Evidence from the Sierra Nevada and other glaciated mountain ranges is incomplete, due to the overriding of earlier deposits by later glaciers and the high potential for erosion in the steep canyons. Constrained by the limited availability of local numerical ages, correlation to oxygen isotope data from marine sediment cores and/or to ice cores from Greenland or Antarctica has often been used as a proxy chronology for the local glacial history. In this review, we use MISs as a convenient chronological timescale, recognising that improved numerical dating may someday compel revision, especially for the past 50 ka for which soil erosion and boulder exhumation and erosion do not present such major complications as for dating older rocks GLACIAL ADVANCES Blackwelder (1931) recognised four main glaciations in the eastern Sierra Nevada: McGee, Sherwin, Tahoe and Tioga. Subsequently, the glacial history has been refined and revised. The recognised glaciations of the Sierra Nevada are summarised by Fullerton (1986, Chart 1, Table 34.1) and those discussed in this review are listed in Table Discussion draws on Kaufman et al. (2004) and Clark et al. (2003). The glacial advances are grouped for discussion below into Pliocene and early Pleistocene (MIS 22 6, ka; MIS 6, ka), late Pleistocene (MIS 5 latest 2, ka) and Post-Tioga glaciations and advances (latest MIS 2 and MIS 1, 14 ka to present). ELAs summarised in Table 34.1 were calculated using the accumulation/ablation area ratio (AAR, with a ratio of 0.65 giving the ELA) and the highest-moraine techniques for palaeoglaciers in 70 eastern Sierra Nevada valleys between 36.5 and 39.5 N(Gillespie, 1991). The modern ELA was taken to be 3860 at 37 N(Burbank, 1991), but the true value may be m higher (e.g. Meierding, 1982) or even 640 m higher (Flint, 1957) since anomalously low cirque glaciers can occur in sheltered locations Pliocene and Early Pleistocene Glaciations The earliest glaciations are represented by deeply weathered erratic boulders and diamictons on highland surfaces. These deposits occur in a wide range of topographic settings: mountaintops, arêtes, beheaded valleys hundreds of metres above modern canyons and benches on the eastern escarpment (Gillespie, 1982). Not all of the diamictons are necessarily glacial drift, but some appear to be (e.g. Brocklehurst et al., 2002). Given the range of sites and elevations of these deposits, it is likely that a number of unnamed glaciations are represented. However, our understanding of glacial chronology, extents and ELA depressions for this time is poor. The oldest identified till, the McGee till, is among the deposits in this category. The McGee till crops out near the summit of McGee Mountain, a large, broad peak south of the town of Mammoth Lakes ( N, E). Metamorphic rocks and the eroded remnant of a 2.7-Ma basalt flow (Dalrymple, 1963, 1964) here are covered by large, exotic granitic boulders from a source area located on the other side of McGee Canyon. Evidently, the time of transport was long ago (1.5 Ma; Huber, 1981) because McGee

5 Chapter 34 Glaciations of the Sierra Nevada, California, USA 451 TABLE 34.1 Recognised Glaciations of the Sierra Nevada, Their Ages and East-Side ELAs Relative to Tioga and Interpolated to 37 N Glaciation Mean age (ka) a DELA (m) b Age references Matthes (Little Ice Age) c Wood (1977), Stine (1994) Recess Peak c Clark (1997) Phillips et al. (2009) Tioga (retreat) James et al. (2002), Clark and Gillespie (1997) Tioga (start) c Clark et al. (2003) <25 c Bursik and Gillespie (1993) Tioga ( Tioga 2 4 ) Phillips et al. (1996) d Phillips et al. (2009) Tenaya ( Tioga 1 ) Phillips et al. (1996) d, Benson et al. (1996) 32 c Bursik and Gillespie (1993) Phillips et al. (2009) Tahoe II {{ Phillips et al. (1996) Tahoe I {{? 100(?) Phillips et al. (2009) Casa Diablo ? Bailey et al. (1976) pre-tahoe (Bloody ? Phillips et al. (1990), discussed in Canyon) e Clark et al. (2003) Mono Basin Phillips et al. (1990) d Walker Creek e 550? Clark (1968), A. M. Sarna-Wojcicki Sherwin Sharp (1968), Birkeland et al. (1980), Nishiizumi et al. (1989) Lower Rock Creek 920? Sharp (1968), Birkeland et al. (1980) McGee ? Huber (1981), Dalrymple (1963, 1964) a Ages are shown as ranges or approximate values. Uncertainties may be found in text and/or references. b Elevation differences are relative to the lowest Tioga ELA ( m) and interpolated to 37 N(Gillespie (1991). Uncertainties are 1 s and are random, and do not include the systematic uncertainties of 150 m. Accuracy depends critically on assignment of moraines to the correct glaciation. Regression was done on 70 glaciated drainages. c cal.ka BP. d Revised (see discussions in James et al., 2002 and Phillips et al., 2001). e Nomenclature from Clark et al. (2003). Creek has been incised 800 m since access to the summit of McGee Mountain for the erratic boulders was last possible (e.g. Putnam, 1962; Gillespie et al., 1999). The next youngest drift deposits occur in the vicinity of the late Pleistocene moraines, suggesting that deposition occurred when the landscape had much its present appearance. Moraines from this period have been largely eroded, but Sharp (1972) identified a degraded Sherwin moraine in the Bridgeport Basin. Two tills have been described: the old red till on Lower Rock Creek and the Sherwin till (Sharp, 1968). Sharp (1968) observed both tills exposed in a road cutting next to Little Rock Creek, just south of Long Valley. The lower till has deeply weathered, disintegrating granitic boulders and a red palaeosol. Birkeland et al. (1980) estimated from relative dating that the palaeosol on this buried

6 452 Quaternary Glaciations - Extent and Chronology till represents 100 ka of development. The till is overlain unconformably by a second deeply weathered till that also contains grusy boulders but lacks the distinctive red palaeosol. Sharp (1968) traced this second till 125 m up the canyon wall to the type locality of the Sherwin till, the Big Pumice Cut on U.S. Highway 395. Another till with a distinctive red palaeosol underlies till mapped as Sherwin by Sharp (R. P. Sharp, unpublished data, 1979) near the end of the left-lateral moraine of Big Pine Creek, 50 km to the south. Sharp (1968) demonstrated that the Sherwin till underlies and therefore pre-dates the Bishop Tuff at the Big Pumice Cut, near the southern end of Long Valley. Sarna-Wojcicki et al. (2000) dated sanidine crystals in the Bishop Tuff to 7592 ka( 40 Ar/ 39 Ar). R. P. Sharp (personal communication, 1976) estimated that upon burial, the till was weathered about as much as Tahoe till is today, requiring ka. Other studies support this estimate. From the development of the palaeosol on the buried till, Birkeland et al. (1980) estimated an age at burial of 50 ka, and Nishiizumi et al. (1989) analysed cosmogenic nuclides to yield an estimate of ka. Thus, the Sherwin glaciation probably occurred at 820 ka (i.e. late Early Pleistocene). It follows that Sharp s (1968) old red till at Lower Rock Creek is 920 ka. Figure 34.3 shows that the Sherwin glaciers of the central Sierra Nevada were more extensive than their successors, extending farther onto the low-angle floors of Bridgeport Basin and down the slopes of the Sherwin Grade at the Big Pumice Cut. Because the topographic gradients are relatively low here (<5 ), a minor additional depression of ELA could account for the larger length of the Sherwin glaciers. In the Bridgeport Basin, an additional ELA depression of as little as 100 m from the Tahoe ELAs might explain the difference in extent (Clark et al., 2003) MIS 22 6 Glaciations The period between 820 and 186 ka, the beginning of MIS 6, is sparsely populated with glacial evidence. However, two glacial or glaciofluvial deposits, one near the Sonora Junction and the other in Mohawk Valley, are dated to this interval by tephras. The deposit near Sonora Junction is from the West Walker River (Blackwelder, 1931; Clark, 1967, 1968). A roadcut near U.S. highway 395 exposes fluvial gravel of Wheeler Flats that contains a tephra identified as Rockland Ash-Tuff by Sarna-Wojcicki et al. (1985), now dated at 550 ka (A. M. Sarna-Wojcicki, quoted in Clark et al., 2003). If the gravel is glaciofluvial, as seems likely, the tephra may date an unnamed, pre-tahoe glaciation. Mathieson and Sarna-Wojciki (1982) found the Rockland ash in a similar stratigraphic relation in the Mohawk Valley in the northern Sierra Nevada. Several other tills appear to date from the MIS 8 6 ( ka) interval, but the age control is not compelling. Fullerton (1986) discussed a till from Reds Meadow, Devils Postpile National Monument. This till overlies the Bishop Tuff and underlies an andesite, the age of which has been very loosely constrained by K/Ar to ka. Curry (1971) and Sharp (1972) presented evidence for other tills of this same general period from Rock Creek and the Bridgeport Basin. Gillespie (1982) described two moraines outside the Mono Basin moraines at Bloody Canyon (QpMB I and QpMB II in Table 34.1) that appear to postdate the Sherwin till. Phillips et al. (1996) presented 36 Cl exposure ages from an older Tahoe moraine at Walker Creek (Bloody Canyon) in the 200 ka range, but other dates from the same moraine were 130 and 150 ka. The MIS 22 6 interval appears to have been marked by a number of glacier advances, despite the scarcity of widespread evidence. In the absence of accurate and precise numerical dates, correlation of tills and elucidation of the glacial history from this interval remains problematical MIS 6 Glaciations The record of Sierra Nevada glaciations becomes more detailed with MIS 6, but remains incomplete compared to the generalised history deduced from lake-sediment cores (e.g. Smith et al., 1991; Bischoff et al., 1997). Tills that probably date from MIS 6 include the Mono Basin, pre- Tahoe and Tahoe I tills of Bloody Canyon (Fig. 34.4), the Casa Diablo till of Mammoth Lakes and the pre-hogsback till of Sawmill Canyon in Inyo County. Moraines from this period are eroded and commonly broad-crested, with few exposed boulders. Boulders that are exposed may be disintegrated, heavily pitted and split. The type locality for the Mono Basin moraines is at Bloody Canyon, near Mono Lake (Sharp and Birman, 1963; Figs and 34.5). The degree of weathering and erosion is similar to many Tahoe moraines (Burke and Birkeland, 1979), and it is uncertain how many moraines from the two glaciations may have been misclassified. At the same distance from the range front, the Mono Basin moraines at Bloody Canyon are lower in elevation than the Tahoe moraines, possibly as a result of range-front faulting between glaciations (Clark, 1972). They were preserved because the subsequent Tahoe glaciers extended along a more northerly course. Phillips et al. (1990) measured cosmic-ray exposure ages averaging 103 ka for eight boulders from these moraines (Fig. 34.6). The dates were revised downwards to ka as new estimates for production rates of 36 Cl were made (Phillips et al., 2001; revised as discussed in James et al., 2002). Even these ages are minima, as soil and boulder erosion on the moraines (e. g. Birkeland and Burke, 1988; Hallet and Putkonen, 1994) reduce the exposure ages estimated by this technique.

7 Chapter 34 Glaciations of the Sierra Nevada, California, USA 453 FIGURE 34.3 Extent of the 820 ka Sherwin drift relative to 20 ka Tioga drift in the central Sierra Nevada. Contour intervals are 305 m (1000 ft). Modified from Warhaftig and Birman (1965), Kistler (1966), Gillespie (1982) and Huber et al. (1989). The Tahoe glaciation is one of the four major Sierra Nevada glaciations recognised by Blackwelder (1931). Gillespie (1982) showed that the Tahoe moraine of Sharp and Birman (1963) at Bloody Canyon was composite, with a young till comprising the crest and an older till the right-lateral flank (Figs and 34.5). These two tills were called Tahoe II and Tahoe I, respectively. However, they are probably from different glaciations. Phillips et al. (1990) obtained cosmogenic ages for the Tahoe II moraine. These ages averaged 60 ka, later revised to ka. No age estimate for the Tahoe I moraine was obtained. Phillips et al. (1990) did measure 36 Cl dates for five boulders from the older Tahoe moraines, discussed above, that protrude from the right-lateral composite moraine. The stratigraphic relation between this older Tahoe moraine and the Mono Basin moraines is unclear, although both are buried by Tahoe I till (see Clark et al., 2003; Kaufman et al., 2004). The 36 Cl dates for the older Tahoe till, called pre-tahoe I, post-mono Basin till in Fig. 34.5, are ka (unrevised). Although these older Tahoe values have not been adjusted downward in accordance with new production

8 454 Quaternary Glaciations - Extent and Chronology FIGURE 34.4 View west from pre-tahoe end moraines of Bloody Canyon (Fig. 34.2B), showing important among the younger Tahoe and Mono Basin moraines. The left-lateral Tahoe II moraine comprises the high ridge to the right (north). The older Tahoe I moraine crops out part way down the flank of the moraine. Both Tahoe moraines bury the older Mono Basin left-lateral moraine, seen emerging from the composite Tahoe moraine in left centre. Photograph by D. H. Clark. FIGURE 34.5 Map of the Bloody Canyon moraines (after Gillespie, 1982). Jl, Ka and Kja are plutonic rocks; Qal is Quaternary alluvium. Qsh is Sherwin till of Sharp and Birman (1963). QpMB I and QpMB II are pre-mono Basin moraines; QMB is Mono Basin moraines. QpTa is the oldest set of moraines (pre- Tahoe) along Walker Creek ( older Tahoe of Phillips et al., 1990); QTa I and QTa II are the Tahoe I and II moraines (Gillespie, 1982). QTe (shaded for clarity) is the Tenaya moraine, and QTi are the undifferentiated Tioga moraines. Topographic contour interval is 24 m (80 ft).

9 Chapter 34 Glaciations of the Sierra Nevada, California, USA 455 FIGURE Cl dates (Phillips et al., 1990, 1996) for Bloody Canyon moraines with the following independent age control (courtesy of R. M. Burke): (1) cal. abp(yount et al., 1982; Clark and Gillespie, 1997; Konrad and Clark, 1998). (2) cal.ka BP ( C ka BP; Clark, 1997). (3) 30 cal.ka BP (Bursik and Gillespie, 1993). (4) >25 ka on 10 Be and 26 Al (Gillespie, unpublished data). See text for discussion of "older Tahoe". rates, it is clear that they are greater than the 36 Cl ages for the Mono Basin moraines 1 km away. Phillips et al. (1990) regarded the older Tahoe as pre-dating the Mono Basin glaciation. These findings appear to contradict field relations among the moraines. It is noteworthy that only a small number of boulders were dated, and scatter among dates for each till is large. The dating studies have illuminated the conflicts between stratigraphic and chronologic analyses, and the current strengths and deficiencies in each. The pre-tahoe Casa Diablo till, near the town of Mammoth Lakes, is weathered to a similar degree as nearby Tahoe till (Burke and Birkeland, 1979; Birkeland et al., 1980). Given the degree of soil development, the Tahoe till was taken to date from MIS 6 by Burke and Birkeland (1979). Although the Casa Diablo till is interbedded with basalt flows that should afford a good dating opportunity, K/Ar analyses by Curry (1971) and Bailey et al. (1976) disagree, constraining the till to either or ka, respectively. On the basis of relative dating techniques, Burke and Birkeland (1979) suggested that the Casa Diablo till was correlated with the older Tahoe and/or Mono Basin tills to the north. Fullerton (1986) pointed out that the absence of 185-ka quartz latite boulders in the Casa Diablo till, and their presence in the nearby Tahoe till, suggests that the Casa Diablo till may predate MIS 6. However, recent field work shows that the quartz latite boulders do occur in the Casa Diablo till, and the dates of Bailey et al. (1976) are now generally accepted as the age range for the till (R. M. Burke, personal communication, 2003). Another till from the same general age range occurs in the oldest moraine in Sawmill Canyon, Inyo County. It is also interbedded with basalt flows, but dates for the flows only loosely constrain the moraine to ka (Gillespie et al., 1984). On Bishop Creek, an unusually complete sequence of moraines has been preserved. Phillips et al. (2009) measured 36 Cl exposure ages for Tahoe moraines of ka there (Fig. 34.7). ELAs for the MIS 6 glaciers were depressed about m below the levels for the MIS 2 Tioga glaciers of the last glacial maximum (LGM; Table 34.1) Late Pleistocene Glaciations Blackwelder (1931) recognised that there typically were two Tioga moraines in Sierra Nevada valleys. Sharp and Birman (1963) and Birman (1964) recognised a third, Tenaya moraine between Tioga and Tahoe in relative age. Lake-core evidence (e.g. Benson et al., 1998b; Fig. 34.7) suggests that a dozen or more short-lived advances occurred between 50 and 14 ka. Only moraines from the largest advances appear to have been preserved near Mono Lake, and at Bishop Creek, Phillips et al. (2009) found a gap in the record until 26 ka (Fig. 34.7). The Tahoe II and Tenaya advances at Mono Lake seem to have occurred during earlier and later parts of MIS 3, respectively. MIS 2 Tioga advances culminated around ka and ended by 15 ka with retreat to the Sierra Nevada crest, or even complete disappearance for a brief period. ELAs for maximum Tioga glaciers were m (Table 34.1), depressed about 820 m relative to modern glaciers and perhaps m below the climatic ELA, depending on how it is estimated. The climatic ELA is above the crest of the Sierra Nevada, and cirque glaciers exist only in sheltered localities.

10 456 Quaternary Glaciations - Extent and Chronology

11 Chapter 34 Glaciations of the Sierra Nevada, California, USA 457 From 36 Cl cosmogenic exposure ages at Bloody Canyon and other canyons, Phillips et al. (1996) inferred four separate Tioga stades ranging in age from 25 to 14 ka, revised for the changes in production rates as discussed above. Their dates failed to resolve the Tenaya as a separate glaciation at Bloody Canyon. James et al. (2002) measured 10 Be and 26 Al cosmogenic exposure ages in the South Fork of the Yuba River, suggesting that the maximum extent of the Tioga glaciation there occurred ka. Likewise, the record at Bishop Creek gave moraines ranging from 26 to 15.5 ka in age (Phillips et al., 2009), or latest MIS 3 2 (Fig. 34.7). In Humphreys Basin, above the headwaters of Bishop Creek, Phillips et al. (2009) reported 36 Cl cosmicray exposure ages of ka that suggested to them that even the crest of the Sierra Nevada was nearly free of ice by then. It follows that retreat of glaciers from their maximum Tioga extents was rapid, taking only 1000 or 2000 years. Based on 14 C ages of ostracodes in Mono Lake sediments interbedded with basaltic ash, Bursik and Gillespie (1993) inferred at nearby June Lake an age of < cal.ka BP for the maximum Tioga advance, which overrode a cinder cone. They inferred an age of 31.7 cal. ka BP or more for the Tenaya moraine, through which an eruption may have occurred while ice was present. The two cinder cones are the only sources that have been discovered for the ash in the lake sediments. Bursik and Gillespie (1993) regarded the Tenaya as a separate advance. Other age control for the Tioga glaciation is available from sediment cores collected from bogs and lakes. D. H. Clark cored Grass Lake Bog, south of Lake Tahoe, and dated a sharp transition from Tioga glacial to overlying non-glacial sediments at cal. ka BP. This date may mark the onset of retreat of the Tioga maximum advance, and is consistent with the 36 Cl dates from Bishop Creek (Phillips et al., 2009; Fig. 34.7). Basal lake sediment from the west slope of the Sierra Nevada yielded a minimum date for the beginning of Tioga retreat of C ka BP ( cal.ka BP; Wagner et al., 1982, cited in Fullerton, 1986). The basal age from the Greenstone Lake cores instead demonstrates that the area near Tioga Pass was deglaciated by CkaBP ( 15.5 cal. ka BP; Clark, 1997). James et al. s (2002) cosmogenic data show that the Tioga glaciers retreated rapidly from the middle elevations of the Yuba River 15,000 14,000 years ago. Clark and Gillespie (1997) agreed that the Tioga glaciers vanished entirely or were restricted to cirques during this interval Post-Tioga Advances The Hilgard advance, proposed by Birman (1964) as a separate post-tioga glaciation, was considered to be a very late Tioga stade by Birkeland et al. (1976), and a recessional standstill by M. Clark (personal communication, 1988). Thus, the Hilgard glaciation is no longer regarded as a separate stade. The Recess Peak glaciation is the first post-tioga advance in the Sierra Nevada for which evidence has been discovered. As first described by Birman (1964), Recess Peak moraines are restricted to the vicinity of Pleistocene cirques. Because of their fresh character, most early workers concluded that the Recess Peak moraines were Neoglacial, constructed within the past years (Birman, 1964; Curry, 1969; Scuderi, 1987). However, soil work by Yount et al. (1982) suggests that Recess Peak deposits are early Holocene or older. Firm numerical constraints on the moraines from sediment coring of nearby lakes demonstrates that the Recess Peak advance began by 14.2 cal.ka BP and ended before 13.1 cal.ka BP (Clark, 1997; Phillips et al., 2009). Plummer s (2002) 36 Cl cosmogenic ages of ka (production rate uncertainties included) overlap Clark s (1997) age range. ELA estimates for Recess Peak glaciers were only 340 m higher than that for the maximum Tioga glaciers (Table 34.1). This value may probably overestimate the climatic severity during the Recess Peak advance because cirque glaciers can occur at anomalously low elevations. Curry (1971) recognised two Holocene Neoglacial advances having lichenometric ages equivalent to 1100 and C a BP (1015 and 920 cal.a BP). Coring of the Conness Lakes indicates that Neoglaciation began there by C ka BP (3.4 cal.ka BP, Konrad and Clark, 1998; 3.2 cal.ka BP, Bowerman and Clark, 2011). Curry (1971) dated the Matthes (Little Ice Age) advances at C a BP ( cal.a BP). The absence of a C a BP (630 cal.a BP) tephra blanketing Matthes moraines coupled with evidence from dendrochronology (Wood, 1977) indicates that Matthes glaciers reached their maximum positions after that eruption (~180 cal.a BP: Bowerman and Clark, 2011). Stine (1994) identified two droughts at about AD and , FIGURE 34.7 (A) d 18 O record (left) and total inorganic carbon (TIC) from Mono Lake (right), for ka. Low stands L1 L4 have been labelled and shaded. Middle curve shows the d 18 O record from the GISP2 ice core, Greenland, with Heinrich events H1 H4 and Dansgaard Oeschger events D2 D8 labelled for reference. From Benson et al. (1998b). 10 Be cosmic-ray exposure ages for Bishop Creek compared to three nearby drainages, Walker Creek (Fig. 34.2B), Little McGee Creek and the South Fork of the Yuba River, on the west side of the Sierra Nevada are shown on the far right. Error bars are 1s and the integer by each indicates the number of samples dated. (B) Dates for the same drainages over an expanded time range (250 0 ka) compared to the SPEC- MAP marine d 18 Orecord(Imbrie et al., 1984; data from Imbrie and McIntyre, 2006). Marine oxygen isotope stages are labelled and shaded for reference. Dating references: a Phillips et al. (2009); b Phillips et al. (1996); c James et al. (2002); d Porter and Swanson (2008); e Bierman et al. (1995); f Dünhnforth et al. (2007).

12 458 Quaternary Glaciations - Extent and Chronology just before the onset of the Matthes advances in the Sierra Nevada. ELAs for Matthes glaciers were 480 m higher than ELAs for the maximum Tioga glaciers. The absence of moraines between the Recess Peak and Matthes moraines, and the absence of outwash deposits between 13.1 and 3.4 cal. ka BP, suggests that no significant glacier advances in the Sierra Nevada occurred during that time, including during the Younger Dryas interval (Clark and Gillespie, 1997). If Younger Dryas glaciers were present in the Sierra Nevada, they must have been smaller than both the Recess Peak and Matthes glaciers and restricted to cirques DISCUSSION Younger Dryas The absence of any moraines between the Recess Peak moraines and the Matthes moraines, as well as the absence of any periods of outwash between 13,100 and 3400 cal. a BP, indicates that no significant glacial advances in the Sierra Nevada occurred during that time, which includes during the Younger Dryas interval (Clark and Gillespie, 1997), although alluvial fans have experienced limited Holocene aggradation (e.g. Bierman et al., 1995; Zehfuss et al., 2001). This finding, combined with evidence favouring Younger Dryas advances in the Rocky Mountains and potentially the North Cascades (e.g. Reasoner et al., 1994; Kovanen and Easterbrook, 2001) suggest a complex regional climate during the late-glacial period in western North America. It also indicates that the climate during the Little Ice Age was the coldest and/or wettest (i.e. most glacial) in the Sierra Nevada of the past 13,000 years Comparison with the Cascade Range Porter and Swanson (2008) reported Cl ages for a moraine sequence from Icicle Creek, in the Cascade Range. The dates cluster at , , , 19.13, , and ka (Fig. 34.7). The younger, Tioga-age dates are in good agreement with those from the Sierra Nevada. However, the agreement is less clear for MIS 4 and 5 moraines. It is possible that this is due to incomplete preservation, or due to the noted problems with dating older, eroded deposits, but it is also possible that the glaciations themselves were more synchronous up and down the Pacific coast of North America at the MIS 2 LGM than before Lake Records Direct glacial deposits and landforms present an incomplete record of mountain glaciations. This record has been be fleshed out by analysis of lake-sediment cores (e.- g. Bischoff et al., 1997; Fig. 34.7). Core data can establish the duration of glaciations, information that is difficult to acquire from moraines and drift alone. The findings of Benson et al. (1996) at Mono Lake suggest that the average duration of glacial advance and retreat during MIS 3 2, period including the Tioga glaciation, was on the order of 3 ka. In fact, the fluctuation of Sierra Nevada glaciers inferred from Owens Lake (Bischoff et al., 1997; Bischoff and Cummins, 2001) and Mono Lake (Benson et al., 1996, 1998a,b) cores seems to show the same three scales of climatic oscillation as the marine/global system: Milankovitch, Heinrich and Dansgaard Oesger (Benson et al., 1996, 1998a,b). Benson et al. (1998a) interpreted the data to show that glacier activity in the Sierra Nevada was synchronous with cold periods in the North Atlantic Reliability of Older Cosmic-Ray Exposure Dates Figure 34.7 suggests that, especially for glacial deposits older than 50 ka, there may be geologic scatter in the dates in excess of analytic and systematic measurement errors and in excess of what might be expected from geologic mapping and relative dating, possibly due to unaccounted-for effects of soil and boulder burial and erosion. It is also possible that our ability to read the landscape and correlate landforms on the basis of relative weathering is less than we have thought and hoped; thus, preservation of moraines may have been more erratic than we have assumed. Nevertheless, the spatial richness of the record from glacial deposits and landforms supplies information difficult to glean from a limited number of cores alone, especially in the latest Pleistocene Dating Paraglacial Deposits For a time, it seemed that the record of glaciation might be better read from alluvial or outwash fans downvalley from the glaciers than from the moraines themselves. Gillespie (1982) suspected that paraglacial deposition accounted for much of the Sierran Bajada, and Whipple and Dunne (1992) elaborated a process-based explanation for this synchrony. Bierman et al. (1995) measured pairs of 10 Be and 26 Al of cosmic-ray exposure dates that seemed to confirm the hypothesis. Dünhnforth et al. (2007) added further dates for the glaciated Shepherd Creek fan, and the adjacent unglaciated Symmes Creek fan, finding more Holocene aggradation in the unglaciated drainage. Nevertheless, comparison of fan and moraine dates (Fig. 34.7) suggests that the glacial record is not much better preserved in the fans than in the moraines. It may be that fan-resurfacing floods that do occur occasionally beneath glaciated canyons

13 Chapter 34 Glaciations of the Sierra Nevada, California, USA 459 (e.g. July 2007, Oak Creek) are sufficient to partially obliterate or at least add scatter to the recovered record, or again it may be that erosion on the fan surface is by itself sufficient to do so ELA Depression ELA depressions for the different Pleistocene Sierra Nevada glacier advances represented in the land record increase with age but show a remarkable consistency. Figure 34.8A shows the trend in Tioga ELAs from North to South for the Sierra Nevada. In Fig. 34.8B, the nearly parallel trend for the youngest Tahoe glaciers is shown. From the Tioga to the Sherwin glaciation, the ELA depression of the maximum advances was within a range of 200 m. That ELA depressions should be increasingly greater for older glaciations is expected because moraines for smaller, older glaciers were obliterated by younger glaciers. What is surprising is that the ELAs dropped time and again to within 10% or 20% of their MIS 2 values, even though the pattern of sea-level depression inferred from marine cores suggests that the high-latitude ice sheets were much larger during MIS 2 (Tioga) than, for example, during MIS 4 3 (Tenaya, Tahoe II; e.g. Martinson et al., 1987). FIGURE 34.8 Equilibrium-line altitudes (ELAs) along the eastern front of the Sierra Nevada. (A) Tioga glaciers. (B) DELA for Tioga (Ti) and late Tahoe (Ta) moraines (ELA Ti ELA Ta ). ELA Ti descends almost linearly from south to north. The cause of the deviation of up to 300 m above the regression line near 37.5 N is unknown. ELA Ta rose slightly higher to the north than ELA Ti ; the cause for this trend, if real, is also unknown. After Gillespie (1991).

14 460 Quaternary Glaciations - Extent and Chronology Gillespie and Molnar (1995) emphasised this discrepancy, but Shackleton (2000) pointed out that the sawtooth history of ice volume inferred from the marine cores was due more than previously suspected to effects of cold water. Consequently, James et al. (2002) suggested that the record of mountain and high-latitude glaciations was more similar than Gillespie and Molnar (1995) suspected. Nevertheless, there does appear to some fundamental limit to maximum ELA depressions, at least over the past 800 ka. This in turn implies a fundamental limit to climatic extremes on the western coast of North America SUMMARY The Sierra Nevada was repeatedly glaciated during the Quaternary Period. The glacial record on the eastern side of the range includes at least eight Pleistocene glaciations, and multiple stades are known for some of them. Lake-sediment core data suggest that this record is incomplete and that there have been many more advances than have been recognised, no doubt in part due to obliteration of evidence from smaller, older glaciers by larger, younger ones. The last Pleistocene glacier advance, the Recess Peak advance, pre-dated the Younger Dryas event in the Sierra Nevada, and any Younger Dryas glaciers there must have been restricted to the cirques. Because there is evidence elsewhere in western North America suggesting the presence of Younger Dryas glaciers, it appears that there may have been considerable local variability in the regional response to a global climatic event. Nevertheless, the available lake-core data have been interpreted to show a general synchronism of glacier advances in the Sierra Nevada to cold periods in the North Atlantic. ELA depressions of the largest glaciers were the same within 20%, independent of age. This suggests a remarkable consistency in the extreme climate in California over a time span of 800 ka or more. Recent efforts to date glacial deposits numerically have added detail to our understanding of Sierra Nevada glacial chronology, and have facilitated correlation with the lakecore records. However, except for results for the late Pleistocene advances, results may serve only as limits to the age of the glaciations, because of erosion and other geological complexities. Further numerical dating will probably be required in order to refine the glacial history, particularly for older glaciations for which great uncertainties remain. ACKNOWLEDGEMENTS We thank Laura Gilson, Harvey Greenberg and Paul Zehfuss for assistance in preparing this chapter. REFERENCES Adam, D.P., Osgood Swamp (C-14) date A-545. Radiocarbon 8, 10. Adam, D.P., Late Pleistocene and recent palynology in the central Sierra Nevada. In: Cushing, E.J., Wright, H.E., Jr. (Eds.), Quaternary Paleoecology. 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