Volcanic Dam Hypothesis and Possible Sherwin Glaciation Age for the McGee Till, Long Valley Caldera, Sierra Nevada, California
David R Stockton
The boulder deposits upon the summit plateau of McGee Mt have long been recognized as glacial in origin, but the exact nature of their emplacement has lacked a satisfactory explanation. The nearest bedrock exposures of the Round Valley Peak granodiorite from which these boulders are derived, lie to the south, four kilometers up canyon, with the most likely cirques of origin situated some eight kilometers distant from McGee Mt. McGee Creek Canyon intervenes between the granodiorite boulder deposits and their source. The highest till deposits on McGee Mt sit some 800 meters above McGee Creek, which exits the range-front through a deep canyon along the southeast base of McGee Mt. Late Pleistocene, Tahoe and Tioga age glaciers followed the present course of McGee Creek onto the Long Valley floor, building massive moraines which extend beyond the canyon mouth. Tahoe/Tioga lateral moraines along the base of McGee Mt lie hundreds of meters below the McGee Till deposits.
These topographic relationships make it improbable that the McGee Till could have been emplaced upon the present landscape. In his 1931 treatise on eastern Sierra Nevada glaciation, Eliot Blackwelder hypothesized that the McGee Till was deposited upon an earlier topography of subdued relief, before incision of the present McGee Creek Canyon. This explanation gives the till great antiquity, and as a result, the McGee Mt deposits represent the oldest recognized Sierra Nevada glaciation (Fullerton 1986, D. Clark et al 2003, Gillespie and Zehfuss 2004). Dalrymple (1963) potassium-argon dated the tephra covered basalt, which underlies portions of the McGee Till, at 2.6 ma, constraining the maximum age for the deposits. But subsequent attempts by Sharp (1969), and Birkeland and Janda (1971), to date the McGee Till using weathering characteristics showed no significant differences between the McGee Till and nearby type Sherwin glacial deposits, which have a currently accepted mean age of 820 ka (D. Clark et al 2003, Gillespie and Zehfuss 2004, Kaufman et al 2004). What differentiates the McGee Till from the Sherwin is its unique elevated and isolated location upon McGee Mt (Bateman and Wharhaftig 1966, M.M. Clark 1967, Sharp 1969).
The McGee Mt block shows a smoothed and subdued relief as if having been overridden by ice. Compared to the surrounding jagged towering peaks, the relatively flat McGee Mt plateau is an anomaly. Noticeably rounded is Aggie-McGee Ridge which extends southwest of McGee Mt to Mt Aggie. To the west, across Aggie-McGee Ridge from McGee Creek Canyon, lies Aggie-Morrison Canyon which drains into Convict Canyon/Tobacco Flat. The upper shaded portion of this hanging valley is narrow and squeezed between Mt Aggie and Mt Morrison. But west of McGee Mt the Aggie-Morrison Canyon widens considerably, and the Tahoe/Tioga moraine complex here appears underfit, only occupying the left-hand portion of the lower canyon. Evidently this short drainage did not produce enough ice to fill the width of the lower canyon. These circumstances correlate with a McGee Creek Canyon glacier spilling across McGee Mt and Aggie-McGee Ridge, into the lower portion of Aggie-Morrison Canyon, significantly eroding and widening the canyon in the process.
McGee Mountain is located in the central eastern Sierra Nevada, California, overlooking Lake Crowley, between the towns of Bishop and Mammoth Lakes.
Figure 1
Continuity of Highest Deposits Defines a Possible Ice Surface
Glacial deposits along canyon sides can be correlated if they show continuity with a projected ice surface (M.M. Clark 1967). Three high boulder deposits along McGee Creek Canyon correlate with and define a possible McGee Creek Canyon glacier highstand. These three boulder fields are; granodiorites on an old upland surface between Esha Canyon and McGee Creek Canyon, granodiorites on the McGee Mt southeast rim, and quartzites on Mt Aggie’s north shoulder. All three are within 180 m elevation.
The Esha-McGee upland deposits, recognized and mapped by Rinehart and Ross (1964), are the furthest up canyon and at 3440 m are the highest in elevation. They are on the eastern canyon wall and so would be glacier right lateral. By all appearances they are a right lateral moraine deposited in an embayment where the ridge protrudes into the canyon. These boulders are perched above a steep chute, but uphill behind them is a gentle erosional surface. Had the ice once been higher than this location, granitic debris would be expected to extend further up slope. The elevation of these boulders can be interpreted as a maximum elevation for glacier ice at this location.
Quartzite boulders where the south end of Aggie-McGee ridge abuts the north shoulder of Mt Aggie lie across and slightly down canyon from the Esha-Mcgee upland. The Aggie North Shoulder deposit is at 3320 m, 120 m below the elevation of the Esha-McGee deposits. These scattered quartzite boulders rest upon the relatively horizontal bedrock surface of Aggie-McGee Ridge which is smoothed as if having been overrun by ice. This surface then was in contact with the bottom of the overriding ice mass. The upper ice surface would have been many meters higher in elevation. There is no apparent upper ice surface trimline along the adjacent north slopes of Mt Aggie, though a faint lower scour-line projects across from the surface of Aggie-McGee Ridge. These Mt Aggie north shoulder deposits are glacier left lateral.
The McGee Mt Southeast Rim granodiorite boulder deposit sits in a protected location above a bedrock outcrop at 3260 m. This small boulder field is an eastern outlier of the South Boulder Ridge deposits. The surrounding slopes are too steep to retain surficial deposits and another level protected site some twenty meters higher along the ridge towards the McGee Mt summit shows no evidence of glacial deposits. The Southeast Rim deposits are free of the metasedimentaries characteristic of McGee Creek Canyon glacier-left ice. If the entire ice flow was across the McGee-Aggie complex then this site would be right-lateral. If the glacier bifurcated, with some ice exiting the range through the present McGee Creek Canyon outlet, then the Southeast Rim boulders would be mid-glacier.
Esha-McGee Upland
Looking north down onto the Esha-McGee Upland. The right hand portion of this old erosional surface is hidden behind a ridge. Visible in the photo, slightly below center is the granodiorite boulder field. Mid-background is McGee Mt, SE Rim Deposit shows as a light patch on the right. Horizontal white band is granodiorite covered South Boulder Ridge.
View of the Esha-McGee Boulder Deposit looking to the north towards McGee Mt. This small granodiorite boulder field is perched at the top of a large chute which is undercutting the deposit. Boulders sit upon a colluvial surface covering metasedimentary bedrock. Visible in the background is the McGee-Aggie complex. On McGee-Aggie skyline from right to left; the SE Rim Deposit, South Boulder Ridge, SW Corner Hill, Aggie-McGee Ridge, Mt Aggie is a shadow underneath Mt Morrison. This photo shows the three high deposits which define the ice surface; Esha-McGee Upland, SE Rim, and Aggie North Shoulder. In between is McGee Creek Canyon
Closer view of Esha-McGee Upland boulders looking towards the southeast. Had these originated from a source on Mt Morgan (N), then it would be expected that granodiorite boulders and pebbles would be scattered across the slopes above. This boulder field appears to be remnants of a McGee Creek Canyon glacier right-lateral moraine. Boulders from this site are actively working their way down the chute to the right.
Boulders at the Esha-McGee Upland site show typical characteristics of the Round Valley Peak granodiorite found in the cirques up canyon; nonpegmatic, equigranular, with dark elliptical mafic inclusions.
The photo above left shows the Esha-McGee Upland. Esha Canyon is to the left below Mt Morgan (N).
Right hand photo shows Mt Aggie and Aggie-McGee Ridge extending to right. Both photos were taken from McGee Mt summit, looking south, up McGee Creek Canyon.
Left; Looking across the Southeast Rim boulder deposit towards the Esha-McGee Upland. These granodiorite boulders originated in the cirques visible up canyon.
Right; Looking across what is possibly an old eroded whaleback towards the Aggie North Shoulder. This linear sandstone form is aligned with what would have been the direction of ice flow. Light patch with trees is granodiorite on South Boulder Ridge. Note the horizontal scour line across the north face of Mt Aggie, level with the surface of Aggie-McGee Ridge.
Figure 2
Bedrock within the McGee Creek Canyon drainage.
Lithic Correlation of Deposits with Glaciation
Upper McGee Creek Canyon is underlain by both granitic and metasedimentary bedrock. The canyon drains from north to south. Cirques in the eastern headwaters which would feed into the right side of a McGee Creek Canyon glacier are situated on Round Valley Peak Granodiorite. Bedrock in the western headwaters of McGee Creek Canyon, which would feed into the left side of the glacier, is composed of both Round Valley Peak Granodiorite and Paleozoic metasedimentaries of the Morrison pendant. The two cirques lying between Mt Aggie and Mt Baldwin (Aggie-Baldwin Cirques N and S) have headwalls of Round Valley Peak granodiorite, but are underlain by the quartzite rich lower silicious member of the Convict Lake formation as mapped by Rinehart and Ross (1964). These two cirques would be the last to feed into the left side of a glacier before the ice flowed around Mt Aggie (which is composed of stratified brown hornfels) and spilled across Aggie-McGee Ridge and the McGee Mt plateau.
From the distribution of rock types underlying the McGee Creek Canyon cirques, the expected lithological composition of mid-canyon glacial till is that glacier-right deposits will be derived from Round Valley Peak granodiorite. Debris deposited by glacier-left ice will contain a mixture of granodiorite and metasedimentary lithics. Far left lateral deposits should contain a significant quartzite component as well as prominently layered hornfels with some granitics.
Except for a quartzite boulder deposit on the McGee Mt North Rim, all the McGee Till deposits correspond to the expected pattern. On the east side of McGee Creek Canyon, the Esha-McGee Upland boulders are far right lateral and composed of granodiorite. The easternmost deposit on the McGee Mt block is the Southeast Rim boulder field which appears to be exclusively granodiorite. Working westward along the South Boulder Ridge, one finds an increasing metasedimentary component amongst the boulders (Putnam 1962). At the Southwest Corner Hill (the juncture of South Boulder Ridge, West Boulder Ridge, and Aggie-McGee Ridge) the number of quartzite boulders and other metasedimentaries is quite noticeable (then slowly decreases downstream along the West Boulder Ridge). Continuing across the glacial path along Aggie McGee Ridge are found some enormous granodiorite boulders amongst a scattering of metasedimentaries. Climbing up past the low saddle in Aggie-McGee Ridge there is a sharp transition from granodiorite to metasedimentary boulders. Rinehart and Ross (1964) mapped this as the southern termination of the McGee Mt deposits. Continuing south along Aggie-McGee Ridge the number of boulders decreases, but there is still a scattering of metasedimentaries, mostly quartzite, but completely absent of boulders of the typical Round Valley Peak granodiorite, all the way up to the shoulder of Mt Aggie. Putnam (1962) mapped these metasedimentary surficial deposits as McGee Till.
These deposits correlate with the expected pattern for a McGee Creek Canyon glacier spilling across the McGee-Mt Aggie complex into Aggie-McGee Canyon. Because Rinehart and Rosss (1964) mapped the Aggie-Baldwin Cirques as heading in a narrow band of Round Valley Peak granodiorite, it seems that this rock type should be found in the Aggie North Shoulder deposits. Along the southern Aggie-McGee Ridge are numerous granitic pebbles which could be from this narrow extension of the pluton. At Aggie Col the granodiorite as mapped by Rinehart and Ross, crosses the col as a dike and does not show the typical appearance of RVP granodiorite. Being a very narrow terminal extension of the pluton, it may have a slightly different composition and undoubtedly experienced a different cooling history from the central plutonic mass. Whether the granodiorite exposed in headwalls has a similar appearance is uncertain. Curry (1968) stated that in the Convict Lake region, it was to be expected that morainal lithologic mixtures should change between glacial advances as cirques eroded headward into differing rock types. In an earlier time the cirque headwalls would mostly be within the quartzite bearing lower Convict Lake formation, and granodiorite feeding into the glacier would be from the exterior portion of the pluton. Future work here needs to focus on comparing the composition of the McGee Till with differing lithologies in the Aggie-Baldwin cirques, particularly textural and compositional differences within the pluton. Why the granitic debris on south Aggie-McGee Ridge is only found as pebbles, is an interesting problem. These granitic pebbles are not shown on the glacial lithology maps in this manuscript.
North Rim Quartzites
The boulder deposit on the McGee Mt northern bench is an anomaly. Rinehart and Ross (1964) identified these quartzite erratics as originating from the lower silicious member of the Convict Lake formation. If derived from a McGee Creek Canyon glacier, these quartzite boulders would have originated within the Aggie-Baldwin Cirques (Curry 1968). In this case they should be glacier-left to any granodiorite boulders. But the North Rim Quartzites lie glacier-right to significant granodiorite boulder fields along Cinder Ridge, West Boulder Ridge, Aggie-McGee Ridge, and the western portion of South Boulder Ridge. If these quartzite boulders came from the west as suggested by Rinehart and Ross (1964) then it requires emplacement by a separate ice flow from that which carried the granodiorite from the headwalls of upper McGee Creek Canyon. Aggie-Morrison Canyon contains this quartzite but it is difficult to imagine how this small drainage could generate enough ice to flow up onto the McGee Mt North Bench, or why. Curry (1968) interpreted the quartzites as resting upon the basalt but possibly lying stratigraphically under or concurrent with the tephra deposits, and suggested a separate glacial advance for emplacement of this quartzite boulder field.
The presence of the quartzite boulders sitting on the North Bench contradicts Blackwelder’s hypothesis for emplacement of the McGee Till. If the McGee Mt deposits were derived from the upper McGee Creek Canyon drainage, and emplaced upon an earlier landscape in which the drainage flowed across the McGee Mt plateau, then there should be no significant granodiorite deposits to glacier-left of the quartzites.