Cenozoic/Mesozoic Volcanism of the Eastern Sierra Nevada
Version 2007

David R. Jessey
Cal Poly - Pomona

Note from the Author: This site was created for educational purposes. The images and many of the illustrations are the property of the author.  Those that are not, have been cited.  I receive many requests to utilize images and content from this web site. Unfortunately, time will not allow a response to all of those requests. As such, feel free to utilize the content from this site in any manner you wish (please acknowledge the source when possible).

Also note that some changes have been made in the guide. The location maps have been removed in favor of more accurate GPS Lat/Long locations. Stop numbers have been removed and replaced with place names. This was done to accommodate the ever-expanding nature of this guide without the necessity or renumbering stops. Finally, the table format was abandoned in favor of a "normal" page layout since IE 7.0 now formats print output for most printers with something that is a reasonable facsimile of what you see on screen. For those of you still using IE 6 or some other browser, I have created .pdf files for Day 1, Day2 and References. Click on the link below to retrieve those files. Be aware, however, these pdf files will no longer be updated and they may not represent the most recent iteration of this field guide.

Note: most images are hyperlinked, clicking on them will bring up a full screen version (suitable for framing!) with an explanation and credits.

Day 1 PDF

Day 2 PDF

References

DAY 1

0.0  -  Red Rock Canyon State Park Visitors Center (3522'24"N, 11759'24"W). From the Visitor's Center proceed south on Ricardo Road toward CA 14.

0.5 (0.5) STOP - Ricardo Volcanics (3522'11"N, 11759'10"W). The Miocene Ricardo volcanics consist of twenty-three basaltic to andesitic flows as well as rhyolitic pyroclastic flow and fall deposits. The pyroclastics have been used to age date the Ricardo volcanics from 8.2 to 10.1 Ma. The other units within the Ricardo Group are fluvial and lacustrine rocks whose compositions range from conglomerate to mudstone and siltstone. The field lies to the north of the left-slip Garlock fault with some researches suggesting the offset equivalent of the field can be found tens of kilometers to the east on the south side of the fault. The source vents for the volcanics have been removed by erosion, however, the lateral continuity of flows suggests the Ricardo basalts were sheets flows that may have traveled some distance from their source.

Anderson (2005) sampled and analyzed the basalts from the Ricardo volcanics. The figure to the right summarizes her results. Note that nearly all samples are tholeiitic basalts, in marked contrast to many of the basalt fields in the Owens Valley and Mojave Desert (see later discussion). Wang et. al., (2002) suggested that Neogene basalt composition across California and western Arizona varied as a function of depth of melting. Basalts to the west would be generated at relatively shallow depth. Takahashi and Kushiro, (1983) demonstrated that shallow partial melts lead to basalts of tholeiitic composition. Thus, the tholeiites of the Ricardo field represent shallow partial melts. They may represent the waning stages of volcanism from an underplated East Pacific Rise or be related to tectonic evolution of the Owens Valley (see discussion at the next stop). The basalt flow at this stop is typical of those within the Ricardo Group. If you examine a hand sample you will note the fine-grained matrix of plagioclase and pyroxene with an occasional phenocryst of pyroxene. The most interesting feature of this outcrop are the nearly-circular dark gray to black phenocrysts that are soft and easily removed from the matrix. The thin section to the left reveals that these are comprised largely of a calcium-rich variety of siderite (Photo a). It is believed these have formed from the alteration of preexisting grains of iddingsite (Photo b). At this locality much of the iddingsite has been replaced by siderite, but if you stop along CA 14 just north of the intersection with Ricardo Road (there is very little room to pull off so be careful) you will find much fresher samples in roadcuts consisting of iddingsite surrounding cores of olivine. Although siderite is most likely formed during weathering, iddingsite is more controversial. Baker and Haggerty (1967) stated that iddingsite can form both as a product of weathering and primary dueteric alteration at both high and moderate temperatures. This primary alteration may represent changes in oxygen fugacity of the crystallizing magma. They listed numerous criteria for the recognition of primary iddingsite, leading Darrow (1972) and Lusk (2007) to conclude the iddingsite of both the Big Pine and Darwin fields was of primary dueteric origin. It seems likely that this would also be the case for Ricardo. Speculation centers on just what have been responsible for the changes in oxygen fugacity. Perhaps it may be indicative of magmas that were ponded at shallow depth before being extruded? It could also reflect vertical changes in the chemistry of flows although none of our research, to date, supports this hypothesis.

0.8 (0.3) - Turn left on CA 14.

24.8 (20.0) - Junction with U.S. Highway 395, continue north.

31.9 (7.1) - Inyo County Line

44.7 (12.8) - Cinder Road, turn right to Fossil Falls.

46.9 (2.2) STOP - Fossil Falls Parking Area (3558'19"N, 11754'39.5"W). From the parking area follow the well marked trail south to Fossil Falls. Owens River draining from the lake that occupied Rose Valley to the north of Red Hill cinder cone carved Fossil Falls. The water flowed from Rose Lake into China and Searles Lakes and from there to the Panamint Lake. Drainage from Panamint Lake was to Lake Manley in Death Valley. The last overflow into China and Searles Lakes probably occurred within the last 5000 years.

The Coso volcanic field lies to the east of the Sierra Nevada Range at the western edge of the Basin and Range. It consists of Pliocene to Quaternary rhyolite domes and basaltic cinder cones covering 150 sq miles. The youngest eruptions are bimodal, with basaltic lava flows intruded by 38 rhyolite lava flows and domes. Volcanic activity in this field began in the Mid/Late Pliocene. has continued into the Holocene.

If you look to the south you can see a prominent valley cut by the Owens River between the Coso Range and the Sierra Nevada Range to the west. The basalt flows within the valley have been dated allowing a rough stratigraphy to be established. The oldest flows lie to the south. The Upper (prominent columnar jointing) and Lower Little Lake Ranch basalts are 130,000 and 400,000 years old respectively. Here at Fossil Falls, the basalt of Red Hill (click photo to the right for an enlarged view) overlies the Little Lake basalts. It has not been dated, but is younger than 130,000 years and older than 10,000 years. The most recent volcanic activity at Red Hill is dated at about 10,000 years although there is evidence for a more recent flank eruption from the small vent of the northwest flank of the cinder cone. No exhaustive petrographic study of the basalts here at Fossil Falls has been done, but they appear to be alkali basalts based upon the prominent phenocrysts of olivine. This is consistent with the Master's Thesis of Groves (1996) on the Coso field that reports a CIPW normative mineralogy typically including olivine and variable amounts of nepheline.

It is interesting to compare data for the basalts of the Coso (Groves, 1996), Ricardo (Anderson, 2005) and Darwin (Lusk, 2007) volcanic fields of the southern Owens Valley. The Table below summarizes the age dates for each field, as well as the nature of the volcanism (dominant rock type in CAPS). The Ricardo basalts are the oldest, while Coso is the youngest. Ricardo is a bimodal volcanic field, however, basalts are clearly more voluminous than felsic volcanics. Coso is also bimodal with abundant rhyolites, while Darwin volcanics are essentially all basalts. The Darwin and Coso fields appear to be structurally controlled, while the Ricardo field is enigmatic.

Basalt Field

Age

Rock Types

Structural Control

Coso

<2.5 Ma

rhyolite - basalt

transform faulting

Darwin

8-4 Ma

rhyolite-andesite-basalt

transform and high angle normal faulting

Ricardo

10-8 Ma

basalt

?


The basalt tetrahedron (shown to the right) for the southern Owens Valley depicts the geochemical differences between the three volcanic fields. As noted previously, Ricardo basalts (green) are dominantly tholeiites. In contrast, basalts from the Coso field (purple) are generally alkali basalt. Those of the Darwin field (brown) show a wide spread encompassing the entire spectrum of basalt composition. A plot of TiO2 vs Zr/P2O5 shows the field differences more dramatically. The black line on this diagram represents the thermal divide separating quartz normative rocks from those containing normative nepheline. Theoretically, evolving magmas cannot cross this divide at low to moderate pressures suggesting differing source regions for the Coso and Ricardo volcanics. However, Darwin basalts do cross the thermal divide. This indicates Darwin magmas either have multiple sources or were fractionated at very high pressures (~2Gpa).

To better understand the contrasting nature of the basalts one needs to look at the tectonic setting of the southern Owens Valley. The valley has undergone both extension creating the typical Basin and Range graben-half graben, as well as dextral shear along the Eastern California Shear Zone. The timing of these events is controversial, but recent motion is thought to be dominantly right-slip (Stockli, et. al., 2003). The transition between dip-slip and strike-slip is thought to have occurred sometime between 10Ma and 3Ma. If this is indeed the case, then Ricardo basalts were emplaced during a period dominated by dip-slip fault movement and Coso basalts during a period of strike-slip. The Darwin field would record a record of both events as it was emplaced during the transition from extension to dextral shear. As noted at the previous stop, Ricardo basalts are generally thought to be the product of a shallow partial melt. Perhaps Basin and Range extensional faults tapped shallower pools of magma resulting in tholeiites. Subsequent transform faulting (Coso) penetrated deeper levels of the crust and mantle, hence the more alkaline basalts. Darwin basalts would be a product of both environments. A word of caution: magmatic activity is not normally associated with transform faulting, however, accommodation zones associated with pull-apart basins (e.g. Death Valley) have been the locus of basaltic volcanism. Also, we cannot neglect the role of changing oxygen fugacity in the evolution of Owens Valley basalts. Basalts from the Ricardo field are dominated by iddingsite, those from Darwin contain both iddingsite and olivine and Coso only olivine. Can changes in oxygen fugacity account for the differences in basalt chemistry? If so, what factors are controlling the changes in fugacity? Do these factors relate to the shifting tectonic picture? Also, does crustal contamination play a role in the genesis of Owens Valley basalts? We will take up the latter topic during a stop in the Big Pine volcanic field.

Return to vehicles and US 395 and turn right (north).

89.3 (42.4) STOP - Alabama Hills (3633'15"N, 11802'57"W). The Alabama Hills and Diaz Lake lie to the west and north of U.S. 395 with the high Sierras in the distance. The Inyo Mountains lie to the east of the highway. For the next 45 miles we will be in an area affected by the "great" March 26, 1872 Owens Valley earthquake (M=8.3). Surface breaks and scarps were common and many have survived to the present day. The main zone of faulting is along the east side of the Alabama Hills. Lake Diaz (to the northwest) was created when a graben formed between the Owens Valley fault and a small fault east of the lake.

The Alabama Hills are a block of Triassic/Jurassic metavolcanics intruded by Mesozoic granite. The cartoon sketch to the right illustrates the structural relationship of the Alabama Hills to the Sierra Nevada. On the eastern side of the Alabama Hills geophysical surveys suggest that the depth to bedrock is approximately 9,000', about the same as the elevation difference between the floor of Owens Valley and the summit of Mt. Whitney (14,495'). The base of the graben block thus sits nearly four miles below the crest of the Sierras. The fact that the Alabama Hills outcrop at all indicates fault motion must have been a combination of uplift of the Sierra Nevada Mountains with concurrent down dropping of the Owens Valley. The valley that lies between the Alabama Hills and the Sierra Nevada escarpment has been a very popular movie location. So many westerns were filmed in this locale that you can walk into almost any older restaurant in Lone Pine and find an autographed picture of John Wayne. Either he ate in many of Lone Pine restaurants or one of the locals forged his signature and got numerous free meals.

92.6 (3.3) - Whitney Portal Road. Turn left (west).

93.2 (0.6) - Turn right on dirt road just beyond the aqueduct.

93.5 (0.3) STOP - 1857 Earthquake Scarp (3636'17"N, 11804'30"W). This impressive scarp, on the Lone Pine fault, preserves some of the clearest evidence of slip during the great earthquake of March 26,1872. Vertical offset is approximately 15 feet and right-slip 45 feet. While there is no general agreement on the magnitude of this earthquake many believe it to be the largest to strike California in the last 200 years. John Muir was living in a cabin in Yosemite Valley at the time of the quake. He told me that the quake frightened him to the extent that he spent three days outside sleeping in a tent. Twenty-seven residents of Lone Pine lost their lives in the disaster. The enterprising survivors buried the victims in a mass grave atop a pressure ridge uplifted during the earthquake. The cemetery, located just north of town remains today as a memorial.

Although historic accounts indicate the escarpment you are examining was created during the 1872 earthquake, recent exposure dating suggests that the vertical offset actually results from three separate earthquakes over the past 10,000 years. Slip from the 1872 quake was about four feet in the vertical plane and 18 feet in the horizontal plane. Examine the fault scarp carefully. Can you find any evidence to support three separate events?

Despite numerous studies there is no general agreement on Holocene slip rates for the Owens Valley fault. Estimates vary from a low of 1.5 mm/yr to a high of 5mm/yr. Recurrence interval is thought to be 3000-4000 years, but this is only for the strand of the fault broken in 1872.

Data suggests an east-west extensional regime resulted in uplift of the White Mountains beginning at approximately 12 Ma. A range-front normal fault system along the west side of the White Mountains was then reactivated at 3-5 Ma as dextral slip faults. Some extension occurred with this regime of right lateral, transtensional deformation (Stockli, et. al 2003). The reactivation of deformation seems to be coincident with increased slip rate along the San Andreas Fault (Bierman, et. al 1991). The San Andreas Fault Zone experiences right lateral motion at a rate of approximately 35 mm/yr (Fig). However, it does not accommodate all motion between the opposing Pacific and North American plates. Studies indicate that the Sierra Nevada Mountains and portions of the Central Valley act as a microplate between the two plates. Accommodation of motion occurs within a 30-60 mile wide zone known as the Eastern California Shear Zone (ECSZ). Shear creates a series of major, sub-parallel oblique-right slip faults such as; the Owens Valley Fault Zone, Panamint Valley Fault Zone, and Death Valley Fault Zone. It has also produced a number of northeast trending high angle normal faults resulting in pull-apart basins (Stockli, et. al 2003; Unruh, et. al 2003). It is thought that the ECSZ may accommodate up to 15 mm/yr of dextral shear (Unruh, et. al 2003). Earthquake foci suggest that local deformation in the eastern Sierras, associated with the ECSZ, has generated a transtensional environment in the Panamint Valley/Mountains resulting in the volcanism of the Darwin Plateau as well as the central Owens Valley (Big Pine volcanic field). Deformation in the Coso range (Coso volcanic field) results from extension due to a step over; transferring dextral shear between fault zones (Unruh, et. al 2003).

Return to US 395, turn left (north).

129.3 (35.8) - Tinemaha Road, turn left.

130.2 (0.9) - Birch Creek Road (dirt road) turn right.

130.8 (0.6) - Turn right just beyond power lines on Powerline Road.

131.5 (0.7) STOP - Big Pine Volcanic Field (3704'36"N, 11816'20"W). From the parking area, walk east toward the small knoll in the foreground (see image to the right). The Fish Springs cinder cone lies along the scarp of the Fish Springs fault, a splay of the Owens Valley fault zone. Note the igneous and metamorphic boulders as you are walking toward the crater. Can you see where these have come from in the Sierra Nevada mountains to the west? Do you see the contact between the light-colored intrusive and darker metavolcanics? The cinder cone we are standing on is one of 30 or more in the 400 sq mile Taboose-Big Pine volcanic field. All were emplaced during several episodes of volcanism spanning 2 million years. A fault scarp cuts the toe of this cinder cone, offsetting it more than 25 feet (in the vertical plane). You walked up the offset portion of the toe to get to this overlook. If you wish, you can climb down the fault scarp and up onto the cinder cone to look into its crater on the east side of the hill. (Note you can see the crater from Fish Springs Road). This cinder cone, as well as most in the Big Pine field, is comprised or red, scoraceous pumice. To the north the summit of Crater Mountain is visible just south of the town of Lone Pine. It's summit is 1500 feet above the valley floor, but the elevation is misleading since the basal portion of the mountain is a block of Sierra Nevada granite. A case can be made for a frontal fault system acting as feeders for the volcanics. Most cinder cones lie along the east or west side of the valley, near the mountain fronts. To the east, in the center of the valley is Tinemaha Reservoir completed in 1928 by the City of Los Angeles as part of the "grand plan" to steal all of California's water.


 

Our research (Varnell and Jessey, 2006) indicates the Big Pine basalts (yellow squares) vary in composition from tholeiitic to alkaline, a range that is very similar to that of the Darwin volcanics. The Darwin field lies approximately 60 miles to the southeast, on the east side of the Owens Valley fault system. Assuming a slip rate of 5mm/year only accounts for 5-10 miles of this geographic separation. Therefore, the two fields do not represent offset equivalents. Trace element and isotope studies of the volcanics from the Big Pine field suggest that crustal contamination played a role in genesis. A spider diagram (see below) reveals that the Big Pine field (yellow squares) is enriched in incompatible elements. While such enrichment might suggest a different source, it also can indicate crustal assimilation. Available isotope data (see below) indicates the Big Pine volcanic field plots off the mantle array and on a mixing line with the EMII reservoir of Zindler and Hart (1986). The EMII reservoir requires assimilation of continental crust to generate the high (>.720) strontium isotopic values. Note that the Coso field lies on the mantle array indicating little or no interaction with crustal rocks. But where does the Darwin field fit into this picture? On the spider diagram it plots closer to Coso and Ricardo and does not show the noticeable trace element enrichment of Big Pine. This suggests a limited interaction with continental crust. Unfortunately, isotopic data is unavailable for Darwin. Why then do the two fields have compositionally similar basalts? Perhaps the Big Pine field had a similar evolutionary history to that of Darwin. Darwin was emplaced during a period of transition, when both shallow and deep melts could have been tapped (see discussion at Fossil Falls Stop). Although the Big Pine field is younger (2-6 million years), it is underlain by significantly thicker lithospheric crust. Thus the volcanic trends evident to the south may have manifested themselves later in the Big Pine field 50-60 miles to the north. That field is experiencing the evolution from the Pliocene to Recent times that characterized the Darwin-Coso-Ricardo Fields during the late Miocene and Plio-Pleistocene.




Return to Highway 395 and turn left (north).

156.5 (31.0) - Bishop, intersection of 395 and Line Street. Turn left, proceed west, staying on the main road (Line Street).

161.4 (4.9) STOP - Knopf's Knob (3720'35.5"N, 11828'39.5"W). Stop at the isolated hill on the north side of the road. Use caution climbing this hill because the local's call it "Rattlesnake Hill" with good reason. Knopf's Knob was named for Adolph Knopf one of the first geologists to visit the Owens Valley. This hill is an unusual exposure of columnar-jointed, Jurassic (Bateman, 1992) Tungsten Hills, quartz-monzonite intruded by Pleistocene (?) basalt. (Note the ? mark for the age of the basalt.  The author has not been able to find a published age date for the basalt dike and assumes the Pleistocene age is most probably the result of "guilt by association" with the basalts of the Big Pine field 25 miles to the south.) Geochemical analyses by Varnell (2006) cast some doubt on this assumption.

Intrusion of the basalt produced glass at its contacts with the quartz monzonite. The basalt contains numerous xenoliths and xenocrysts (quartz), especially near the contacts. Columns occur in both the quartz monzonite and basalt, but are more striking in the former. They range in length from four to seven feet and average one foot in width (Lipshie, 2001). Many are vertical, but unusual horizontal columns can be seen near the base of the hill. The origin of the columns is uncertain but Lipshie (2001) suggests that intrusion of the basalts at temperatures in excess of 1000C caused partial melting of the quartz monzonite. Subsequent thermal stresses upon cooling formed the columnar jointing.

164.7 (3.3) STOP - Overview of Volcanic Tableland (3719'40.5"N, 11831'06"W). From the parking area we will climb the hill to the northeast for an overview of the town of Bishop and the Volcanic Tableland. Looking north we can see the Wheeler Crest, a sharp, fault-bounded escarpment between the Sierra Nevada Range and Round Valley. In the foreground, the low hills are the Tungsten Hills, named by Knopf (1913) for scheelite occurrences there. The Tungsten Hills consist predominantly of Jurassic quartz monzonite, but may also include slivers of Paleozoic roof pendant rocks similar to those of the Pine Creek pendant further to the north. If you look carefully you can see scattered tungsten prospect pits and mine workings. Bateman (1965) described the mines and prospects in detail. If you look through the notch between two of the higher Tungsten Hills, you can see Highway 395 as it climbs Sherwin Grade and the broad surface of the Volcanic Tableland comprised of Bishop Tuff deposited during the catastrophic caldera eruption 760,000 years ago. Just to the east of the Highway 395, the linear feature that is clearly visible is the Los Angeles aqueduct. The Volcanic Tableland (see map below) surface is arched about a north-south axis plunging to the south beneath the alluvium of Round Valley. The incised gorges of Rock Creek and the Owens River are hidden by the Tungsten Hills. Also note the steep frontal face of the tableland This is not a fault scarp, but rather an erosional feature created by the Owens River as it changes course from north-south to flow nearly east-west, before turning southward again at the mouth of the Chalfant Valley. Numerous north-south trending faults, west side down, can be seen cutting the tableland surface. Looking to the east-southeast we see the town of Bishop, home to Schat's Bakery. To the south is the gentle ramp-like Sierra front known as the Coyote Warp. Lipshie (2001) describes the Warp as a broad, open anticline dipping to the east toward the Owens Valley. Geophysical surveys suggest the structure extends beneath the valley to the front of the White Mountains where it is truncated by the White-Inyo fault. Bateman (1965) argues for a Pleistocene age for the warping. To the west is the recreational area know as Bishop Creek Canyon. There are numerous campgrounds and lakes in the upper reaches of the canyon.

Make a U-turn and return to U.S. 395 in Bishop.

173.1 (8.4) - Turn left (north) on US 395.

173.9 (0.8) - Bear right on U.S. 6 at the intersection with 395.

181.6 (7.7) - Turn left on Rudolph Road to the Chalfant Quarry.

182.5 (0.9) STOP - Chalfant Quarry (3728'04"N, 11822'00"W). Two main units of the Bishop Tuff deposit are visible here: The lower 15 feet of the section consists of the poorly-sorted airfall tuff that was deposited downwind from the eruption of the Long Valley caldera. The upper 15-20 feet of the section consists of the basal portion of the pyroclastic flow that comprises much of the Volcanic Tableland. At this location, it is remarkably well-sorted for a pyroclastic flow. The dark layers" just below the contact between the two units are manganese oxide stains resulting from groundwater circulation. The table below summarizes the average major element content of 32 samples taken from the two units exposed in the Chalfant Quarry and the Upper and Lower Bishop Tuff of the Owens River gorge (next stop). Statistically, the four sample locations/units are indistinguishable. This supports the theory that the Bishop Tuff was emplaced in a single event over a limited span of time.


 

 

 

SiO2

TiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

Chalfant Ash Flow Tuff

77.88

0.08

11.80

0.93

0.05

0.07

0.45

3.52

5.21

0.018

Chalfant Air Fall Tuff

78.07

0.08

11.80

0.99

0.04

0.08

0.56

3.36

5.03

0.016

Owens River Upper BT

78.22

0.07

11.60

0.86

0.03

0.05

0.44

3.65

5.08

0.013

Owens River Lower BT

78.24

0.07

11.83

0.92

0.03

0.05

0.50

3.42

4.95

0.015

Return to the junction with U.S. 395 in Bishop via Rudolph Road and U.S. 6.

191.1 (8.6) - Turn right on U.S. 395 (north).

202.9 (11.8) - Exit US 395 at Gorge Road, turn right (east).

203.6 (0.7) - Road ends at T intersection, turn left (north).

206.7 (3.1) - Veer right and drive to locked gate.

206.9 (0.2) STOP - Owens Valley Gorge This a very popular area for rock climbers and parking on weekends can be problematic. (For best viewing try to make the stop in the late afternoon.)

The Owens River has eroded downward 500 feet entirely through Bishop Tuff at this locality. The Tuff is comprised of two lithologies. The upper unit (UBt) is poorly indurated and has striking radial columnar jointing. Column diameters typically range between 3 and 5 feet (Gilbert, 1938). Most columns are oriented in a radial pattern. The lower Bishop Tuff (LBt) is a strongly-welded, massive tuff with irregularly developed vertical jointing.

The Upper Bishop Tuff consists of pale pink, poorly welded, vitric pumice ash. It readily darkens to gray on weathered surfaces. The UBt contains abundant pumice shards as well as phenocrysts of sanidine, quartz and plagioclase. The Lower Bishop Tuff is more strongly-welded with flattened and elongated pumice fragments common (See if you can locate the contact as you walk down the DWP access road; in 2003 it was marked by Chinese writing!). Unweathered LBt is pale red to gray and noticeably denser than the overlying UBt. In all respects the lower unit resembles a "textbook" ash flow welded tuff. Note from the table presented at the last stop the two units are chemically very similar.

The origin of the radial joint sets remains controversial. However, Mike Sheridan, while a graduate student at Stanford, undertook a computer simulation of gas flow in geothermal systems. He suggested (Sheridan, 1970) that each joint set represents the locus of fumarolic activity. The radial jointing is similar to the heat flow pattern developed during cooling around a gas vent (see figure below). The joints form normal to isothermal surfaces. If you look to the east, across the gorge you will see a hummocky terrain with numerous surface bumps. Each "bump" would be the location of an inactive fumarole.


Turn around and make the trek back to US 395.

210.9 (4.0) - Turn right (north) on U.S. 395

221.3 (10.4) STOP - Big Pumice Cut (3733'28"N, 11839'25"W). We are at the Junction of Highway 395 and Lower Rock Creek Road on the left; large road cut on the right. Park off the highway on the left and watch for crazy drivers!

This roadcut was created in 1957 when highway 395 was realigned. The Big Pumice Cut was thought to be important because it laid to rest a controversy regarding the relative ages of the Sherwin Till and Bishop Tuff. A student, reportedly from UCLA, convinced Cal Trans to preserve the road cut although it was to be graded out of existence. While some geologists hailed this decision, it has proven to be less fortuitous. Weathering has almost obscured the stratigraphic relationships and few geologists care about the Sherwin Till-Bishop Tuff controversy these days. So what we have is a roadcut that constantly sheds debris to U.S. 395 occasionally blocking the northbound lane and costing taxpayers money to clear!

About all that can be seen today is the contact between the boulder-laden Sherwin Till (.80 Ma) marked by vegetation growth and the overlying Bishop Tuff (.76 Ma). Boulders in the till are highly weathered and have largely decomposed. We must rely on a guidebook by Lipshie (1976) for a description of the Bishop Tuff at this locality; "the basal 15 feet of the tuff consists of air-fall ash and lapilli, whereas the overlying material is an ash-flow deposit that was emplaced as a nuee ardente. Layering in the tuff is parallel to the till surface for the air-fall unit and roughly horizontal for the ash-flow unit. If you look closely you can see the angular unconformity between the two tuff layers near the prominent clastic dikes. The tuff in this roadcut is only weakly indurated and as such, rather atypical of the Bishop Tuff." Several clastic dikes(?) can be seen cutting the Bishop tuff.  The dikes are comprised of the same Tahoe age boulder gravel that caps the hill. They were thought to originate as tension cracks when the Volcanic Tableland arched upward. Subsequently, gravel outwash slumped into the fissures to create the "dikes".

228.9 (7.6) - Turn left on McGee Creek Road.

230.7 (1.8) STOP - Hilton Creek Fault Scarp.  (3733'52"N, 11847'12"W). Park on the road just above the campground. (You will see a small portion of the road that has been repaired) On June 8th and July 14, 1998 two earthquakes of M=5.3 ruptured the Hilton Creek fault. The June 8th quake had its epicenter (right lateral strike-slip motion) here at the campground, the July 14th epicenter (normal movement) was 2 km to the west of the campground. If you look downslope from the parking area you will see a large rounded, granite boulder the size of a small car. During one of the two quakes (it is not certain which) the boulder rolled down the hill from where it had been perched on the scarp of the Hilton Creek fault to its current location. During its journey it left a trail of flattened vegetation and shattered pavement. Remarkably, as of 2003 the path of the boulder was still visible although the road has been patched and repaired. The boulder had originally set atop a 50 foot scarp of the Hilton Creek fault that cuts the McGee moraine. Its new home is now the McGee Creek campground. The story goes that two campers were asleep in their tent on the night of the earthquake and awoke in the morning to find the boulder four feet from their tent. If you examine the campsite, it is hard to imagine how a tent could have been pitched four feet from where the boulder came to rest unless the campers were accustomed to sleeping on boulders the size of cantaloupes. This appears to be a classic case of embellishment. If we revisit this story in a decade, the boulder will have come to rest four inches from the tent and in fifty years one of the campers will have awoken to find it lodged on his foot!

Just to the north of this location are the epicenters of the M=6.0 1980 earthquakes heralding the renewal of seismic activity along the Hilton Creek fault. Offset from those quakes since 1980 has been as much as a foot in the vertical plane. The triggering mechanism for the earthquakes remains controversial, but one intriguing theory suggests it is related to magma injection along fractures. This causes the fault blocks to be shoved aside. Seismic surveys suggest magma may be as near as 3-5 kilometers from the surface in this area.  Such a pool of magma could easily exert tensional stresses in the upper crust generating normal faulting. While this explains normal motion along the Hilton Creek fault, it is uncertain what role magma plays in strike-slip motion.

Turn around and return to U.S. 395.

232.5 (1.8) - Turn left (north) on U.S. 395.

236.6.(4.1) - Turn left on Convict Lake Road and stay on this road to parking area.

239.0 (2.4) STOP - Convict Lake Parking Area (3735'39"N, 11851'05"W). On weekends in the spring parking can be very limited.  Try to find any available spot. One of the most asked questions is always, "How did Convict Lake get its name?" Despite books on the subject there remains some inconsistencies in the details. What is know is that a large group of inmates escaped from the federal prison in Carson City, Nevada in 1871. A group of escapees subsequently robbed and murdered a pony express rider. Posses were organized and three of the escaped felons were discovered (apparently from the smoke of there campfire) in Monte Diablo Canyon. A fierce firefight ensued and one or two of the posse were killed (details are sketchy). The convicts escaped unhurt, but were later captured north of Bishop. Vigilantes hung two in Bishop while the fate of the third is uncertain. Monte Diablo Canyon is now known simply as Convict Lake in honor (?) of the events that occurred here. Mt. Morrison is actually named after the leader of the posse who was killed in the firefight.

The geologic relationships within the Mt. Morrison roof pendant are typical of those found in roof pendants throughout the eastern Sierra. What is atypical is the exposure, completeness of the section and detailed geologic mapping. A bird's-eye view of the geology can be seen by walking down to the east shore of Convict Lake where an interpretive plaque discusses the geology. However, the best way to "see" the geology is by making the 11/2 mile walk around the lake. The hike follows a relatively level path and can be made in as little as 20 minutes, although an hour is necessary to do justice to the geology. Utilizing a digital camera, and some cut and paste technology we have constructed a 180 panorama of the lake (if you click on either of the above images it will take you a much larger single image that is more appropriate to view the geologic relationships).The geology is from Greens and Stevens (2002)Geologic Map of Paleozoic Rocks in the Mount Morrison Pendant, Eastern Sierra Nevada, California. Any errors in interpretation of the geology are solely those of the author and not the excellent work of Greene and Stevens.

We will begin by walking around the north shore of the lake, since the ridge on the northwest side of the lake provides a good exposure of the lower Paleozoic section.  As we walk to the west, the steeply east-dipping rocks comprise the overturned limb of a syncline, so the units will become progressively younger. The first rock unit we encounter is the Cambrian Mount Agee Formation. Outcrops are sparse on this side of the lake, but float is abundant. The Mount Aggie is a dark gray siliceous argillite. Continuing to the west, the next unit to outcrop is the Ordovician Convict Lake Formation. It is somewhat lighter in color, and while it is often difficult to distinguish in weathered float the contact can be readily seen in the ridge face. To the west of the Convict Lake Formation lies, the Silurian/Devonian Aspen Meadows Formation. Aspen Meadows float is sparse and the distinctive green-banded hornfels is best seen by making the hike up to the ridge crest. The remainder of the ridge is comprised of the well-indurated quartzite of the Devonian Mt. Morrison Formation. This highly resistant rock forms many of the near vertical slopes of Sevehah Cliff. A large boulder of Mount Morrison can be seen in the field south of the ridge (see our image).

At the west end of Convict Lake the view of Sevehah Cliff is truly impressive. The calcareous quartzite of the Mount Morrison Formation is the most prominent unit. Bedding dips steeply to the northeast, more or less parallel to the slope of the cliff creating apparent structural complexity that is actually a function of the steep dip and subsequent erosion. The more gently sloping base of the cliff is caused by metasomatic alteration of the Mt. Morrison by underlying intrusives.  Quartz + calcite --> wollastonite, a softer, more easily eroded mineral. On the southern base of the cliff, Aspen Meadows and Convict Lake crop out. Above the Mt. Morrison, on the north end of the cliff, the rusty brown chert and argillite of the Squares Tunnel Formation is a striking marker horizon.  At the very top of the cliff, Mt. Morrison lies in fault contact with the underlying Squares Tunnel. At the base of the cliff, the Convict Thrust fault (not visible) repeats the section in the low hills to the southwest.  In the far distance Mississippian Bright Dot and Pennsylvanian Mount Baldwin Marble are visible across the main strand of the Laurel Convict fault.  On the south side of the lake Convict Lake and Mount Aggie Formations outcrop. Small scale folds in these units mimic the large scale structural complexity seen in Sevehah Cliff. Further to the south, Mount Morrison is the prominent peak on the horizon.  It lies to the southeast of the Mount Morrison fault. In our panorama only Devonian Mount Morrison Formation is clearly visible, but from the parking area, a fault (a splay of the Laurel-Convict fault?) can be seen to cut the face of the mountain juxtaposing Aspen Meadow and Convict Lake Formations. Since this an igneous/metamorphic field trip, we won't get deeply into the interpretation of the Paleozoic section. Suffice it to say, most of the units are siliciclastics indicative of deeper water shelf/slope transitional environments. Similar, unmetamorphosed, strata outcrop to the east in the White Mountains. The Mount Morrison Paleozoic section, however, is unlike the shallow water carbonate-dominated Paleozoic sections of the Inyo Mountains and Mojave to the south. This ends our stroll around the lake.  For a much better discussion of the geology see Stevens and Greene (2000).

Return to highway 395, turn left (north) to Highway 203 and the town of Mammoth Lakes.END of DAY 1. Day 2 will begin from the parking area for the Mammoth Ranger Station on the north side of Highway 203, approximately 1/4 mile east of the intersection (traffic light) with Old Mammoth Road.

Day 2

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