ACTIVE FAULTING AND QUATERNARY PALEOHYDROLOGY OF THE

TRUCKEE FAULT ZONE NORTH OF TRUCKEE, CALIFORNIA

By

Aaron Dwight Melody

A Thesis

Presented to

The Faculty of Humboldt State University

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

In Environmental Systems: Geology

September, 2009

ACTIVE FAULTING AND QUATERNARY PALEOHYDROLOGY OF THE

TRUCKEE FAULT ZONE NORTH OF TRUCKEE, CALIFORNIA

By

Aaron Dwight Melody

Approved by the Master's Thesis Committee:

Dr. Mark Hemphill-Haley, Major Professor Date

Dr. Brandon Schwab, Committee Member Date

Thomas Sawyer, Committee Member Date

Dr. Chris Dugaw, Graduate Coordinator Date

Dr. John Lyon, Dean for Research and Graduate Studies Date

ABSTRACT

ACTIVE FAULTING AND QUATERNARY PALEOHYDROLOGY OF THE

TRUCKEE FAULT ZONE NORTH OF TRUCKEE, CALIFORNIA

By

Aaron Dwight Melody, Master of Science, Geology

Active faulting has been documented along the northwest-striking Mohawk Valley fault zone (2-3 normal-dextral rupture events in the Holocene with geologic slip rates of

~0.2mm/yr) and in the Lake Tahoe basin (mostly on north-striking normal faults with geologic slip rates of up to 0.4 mm/yr). Evidence for Holocene faulting in the region between these zones; for example along the Truckee fault zone, has been sparse to absent.

Soil cores and trenches were hand-dug in a meadow north of Truckee, California bound by a low (~1m) north-south trending, east-facing scarp. Radiocarbon age estimates of organic sediment indicate the meadow was a marsh during the Late Quaternary and was abruptly infilled and/or desiccated with the deposition of the ~7,000 yr B.P Tsoyowata tephra (Mt. Mazama). Both the tephra and the marsh sediment are offset ~50-100 cm across a vertical fault striking sub-parallel with the scarp. This study provides evidence for at least one surface-faulting event during the Holocene and possibly another in the late Pleistocene along the Truckee fault zone. These findings may aid in the identification of other seismic sources capable of significant ground rupture in the area.

iii

ACKNOWLEDGEMENTS

This project was supported by the U.S. Army Corps of Engineers (Ronn Rose) and the U.S. Bureau of Reclamation (Larry Anderson) - both of whom readily offered

support. Without their contributions, this investigation would have stopped at conjecture.

The Truckee Ranger Station granted access to U.S. Forest Service Lands. A huge thanks to Roland Rueber, Mike Foget, Pat Barsanti, Erik Nielsen, John Aveggio, Shane Beach,

Tom Stephens, Gary Simpson, Anson Call, Dave Bradley, Greg Williston, Joe

Aufdermauer, Dave Gonzales, and all the staff at SHN Consulting Engineers and

Geologists, Inc. in Eureka, CA. SHN provided respectable employment during the entire course of this endeavor. Guidance and encouragement are greatly appreciated from Dr.

Mark Hemphill-Haley, Dr. Brandon Schwab, Dr. Bud Burke, Dr. Sue Cashman, Dr. Ken

Aalto, and the entire HSU Geology Department staff. I would also like to thank the premier northern Sierran tectonic geologist Thomas Sawyer. He, more than anyone, encouraged me to keep on even in the early days of abundant auger refusal and volcanics everywhere. “Keep looking,” he said. A very heartfelt thanks to Beau Whitney,

Christopher Slack, Ronna Bowers, Martha Mitchell, and Tami Darden for help in trench digging during the inaugural pre-FOP (2006)! Thanks to Alan Ramelli, Rich Koehler,

Joanna Redwine, Peter Birkeland, Chris Henry, and Chad Pritchard for many thought- provoking conversations. Despite not understanding the attraction to digging in the dirt, I thank my parents, who supported me 100%. My wife Kathryn accompanied many field excursions to the meadow and I thank her for her faith in me and patience in this process.

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TABLE OF CONTENTS

Page

ABSTRACT ...... iii

ACKNOWLEDGEMENTS ...... iv

TABLE OF CONTENTS ...... v

LIST OF TABLES ...... viii

LIST OF FIGURES ...... ix

LIST OF APPENDICES ...... x

INTRODUCTION ...... 1

Tectonic Setting ...... 4

Cummulative Dextral Motion ...... 5

Truckee Fault zone ...... 6

Geodetics...... 8

Bedrock Geology ...... 9

Pre-Miocene Rocks ...... 10

Miocene-Pleistocene Volcanics ...... 10

Pliocene Deposits of Boca Basin and Verdi Range Uplift ...... 17

Prosser Creek Alluvium ...... 20

Pleistocene Glaciations ...... 21

Hobart Glaciation ...... 22

Donner Lake Glaciation ...... 23

Tahoe Glaciation ...... 23

v

Tioga Glaciation ...... 24

Outwash Terraces ...... 25

Sagehen Creek Terraces ...... 25

METHODS ...... 27

Air Photo and Topographic Analyses ...... 30

Lithostratigraphic Descriptions ...... 30

Textural Analysis ...... 30

Hand-Excavated Soil Coring ...... 31

Hand-Excavated Soil Pits and Trenches ...... 31

Tephra Analysis ...... 32

Radiocarbon Dating ...... 32

Correlation of Pre-Holocene Deposits ...... 33

STUDY AREA MEADOW STRATIGRAPHY ...... 34

Textural Analysis ...... 35

Tephra Glass Composition ...... 44

Radiocarbon Age Estimates ...... 44

Pit HM-Pit-...... 45

Pit HM-Pit-03...... 46

Pit HM-Pit02 ...... 47

Pit HM-Pit-02-North Wall ...... 48

Pit HM-Pit-02-South Wall ...... 50

Hand Auger Cores...... 52

DISCUSSION ...... 55

vi

Paleohydrology ...... 55

Most-Recent Rupture Event ...... 56

Penultimate Event ...... 58

CONCLUSION ...... 60

REFERENCES ...... 62

APPENDIX A TEXTURAL ANALYSIS RESULTS ...... 70

APPENDIX B TEPHRA ANALYSIS RESULTS ...... 71

vii

LIST OF TABLES

Table Page

1 Radiocarbon Age Estimates ...... 33

viii

LIST OF FIGURES

Figure Page

1 Simplified tectonic setting of the western United States...... 2

2 Map of Quaternary faults and/or lineaments (thick black lines), major drainages (colored lines), and selected features...... 3

3 Simplified geology and drainages of the study area...... 11

4 Simplified geologic map and drainage pattern at the Independence Creek-Little Truckee River confluence...... 12

5 Simplified geologic map of Prosser Creek where it crosses Highway 89...... 13

6 Simplified geologic map (modified from Sylvester et al., 2008) and major drainages in the headwaters of Sagehen Creek...... 14

7 Simplified geologic map and major drainages of Sagehen Creek at Highway 89...... 15

8 Location map and aerial photograph of the study area alpine meadow (shaded area) along Highway 89 between Prosser and Sagehen creeks...... 28

9 Annotated aerial photograph with approximate trench and core locations in the study area meadow...... 29

10 Trench log of HM-Pit-01 and hand boring GC-2...... 36

11 Trench and core log of the north wall of HM-Pit-03...... 37

12 Trench log of the north wall of HM-Pit-02...... 38

13 Trench and core log of the south wall of HM-Pit-02...... 39

14 Schematic cross section west of, and parallel to, the fault-scarp trend...... 40

15 Schematic cross section east of, and parallel to, the fault-scarp trend...... 41

16 Schematic cross section west of the fault-scarp trend...... 42

17 Schematic cross section orthogonal to fault (red lines) strike...... 43

ix

LIST OF APPENDICES

Appendix Page

A Textural Analysis Results ...... 70

B Tephra Analysis Results………………………………………………………………71

x

INTRODUCTION

The Walker Lane belt (Figure 1) essentially accommodates dextral motion of the

Sierra Nevada block relative to the Great Basin and marks an abrupt physiographic change, whereby the predominant north-northeast-trending topographic grain in the Great

Basin gives way westward to more heterogeneous terrain (Faulds et al., 2005). To the south, this system of faults merges with the Eastern California shear zone, which merges with the San Andreas fault system in southern California, while in the north it terminates in northeast California near the southern end of the Cascade arc (Figure 1). Holocene faulting has been documented in the Lake Tahoe basin (Brothers et al., 2009; Dingler et al., 2009; Kent et al., 2005; Schweikert et al., 2000) and Mohawk Valley (Sawyer and

Page, 1993; Sawyer et al., 1995) located in northeastern California (Figure 2), but evidence for Holocene ground-rupturing faults in the intervening region (Truckee basin) is sparse to absent. My study area includes much of the Truckee basin and is focused on a meadow on the west side of the Truckee basin, ~10 km north of Truckee, California

(Figure 2).

The primary purpose of my study is to document evidence of Holocene faulting in the Truckee basin. There were no mapped Quaternary faults within the study area meadow (Jennings, 1994), therefore, the data presented herein will greatly increase the accuracy of seismic hazard and risk analysis in the Truckee basin.

1 2

Figure 1. Simplified tectonic setting of the western United States.

Plate velocity vectors (arrows) are relative to stable North America (Bennett et al., 2003; Dixon et al., 2000; Hammond and Thatcher, 2004). Shaded area is the approximate extent of the Sierra Nevada microplate. CSZ-Cascadia subduction zone, ECSZ -Eastern California shear zone, LT- Lake Tahoe, MFZ- Mendocino fracture zone, MV-Mohawk Valley, SAF-San Andreas fault.

3

Figure 2. Map of Quaternary faults and/or lineaments (thick black lines), major drainages (colored lines), and selected features.

Faults/lineaments simplified from Olig et al., (2005b); Saucedo and Wagner, (1992); Grose, (2000a, 2000b); and USGS Quaternary Fault and Fold Database (March, 2009). All faults and lineaments mapped as solid lines for visual purposes only. Plate velocity vector from Hammond and Thatcher (2004, 2007). Blue lines: Truckee River drainage to Pyramid Lake, green lines: drainage to Honey Lake, orange lines: drainage to Sierra Valley. PH-Prosser Hill, DL-Donner Lake, AH-Alder Hill, BM-Bald Mountain, PCR- Prosser Creek Reservoir, LTR-Little Truckee River, IL-Independence Lake, SV-Sardine Valley, IC- Independence Creek, TR-Truckee River, BR-Boca Ridge, SH -Sagehen Hills, DV-Dog Valley, JF-Juniper Flat, HB-Hirschdale Basalt area. Small red triangle in center is approximate location of the meadow where the trenching study was conducted.

4

Tectonic Setting

Deformation across the western United States is attributed primarily to right- lateral transform motion that links plate boundary triple junctions near Cape Mendocino and the Gulf of California (e.g., Atwater, 1970, 1989; Bennett et al., 1999, 2003). The majority of this deformation is taken up by the northwest-striking San Andreas system with the remaining transform motion occurring on a similarly trending zone of faults that extend through the Mojave Desert and along the entire east side of the Sierra Nevada

(Sauber et al., 1986; Dokka and Travis, 1990b; Dixon et al., 2003). The zone of faults, all or part, has been previously referred to as the Walker Lane (Locke et al., 1940), the

Walker Line (Billingsley and Locke, 1941), the Walker Belt (Stewart, 1980), the Walker

Lane Belt (Carr, 1984), and in the south, as the Eastern California Shear Zone (Dokka and Travis, 1990b). The Walker Lane belt is marked by generally broad and discontinuous strike-slip faults and is a zone of transtension (Oldow, 2003; Unruh et al.,

2003; Wesnousky, 2005) along the east side of the uplifting Sierra Nevada (Wakabayashi and Sawyer, 2000, 2001).

The study area is in the northwest Walker Lane, a structural transition zone between the relatively stable Sierra Nevada microplate to the west and the extensional

Basin and Range to the east (Figure 1). Cenozoic, distributed, dextral-shear has been accommodated on northwest-striking dextral faults and north-striking normal faults, with possibly lower (or older) activity on northeast-trending sinistral faults (Olig et al., 2005a,

2005b).

5 The down-dropped region that includes Lake Tahoe, the Truckee basin, Sierra

Valley, Mohawk Valley, and areas north have been referred to as the Tahoe-Medicine

Lake trough (Page et al., 1995). It extends ~ 500 km along the western margin of the northern Walker Lane belt and overlaps the eastern margin of the Sierra Nevada (Page et al., 1995), and it is characterized by left-stepping, en e´chelon tectonic depressions that formed, presumably, as a result of dextral transtensional deformation and associated abundant volcanism (Figure 3) since the Miocene (Page et al., 1995). Most patterns of

Cenozoic deformation are consistent with the modern stress field (Unruh et al., 2003) but many rates of deformation are not in agreement (Briggs and Wesnousky, 2001; Adams et al., 2001).

Cummulative Dextral Motion

Up to ~1100-1500 km of transform motion has occurred since the inception of the

transform margin ~28-30 Ma (e.g., Stock and Molnar, 1988). Only about one-half to one- third of the estimated total transform motion is currently attributed to displacement on currently active strands of the San Andreas and Walker Lane fault systems (Powell et al.,

1993).

Estimates of cumulative dextral offset across the eastern California shear zone

and the Walker Lane since Miocene time are ~50-100 km in eastern California (e.g.,

Dokka and Travis, 1990), 48-75 km in west-central Nevada (Ekren and Byers, 1984;

Oldow, 1992), 20-30 km in the northern Walker Lane (Faulds et al., 2005) and essentially

zero at its northern terminus in northeastern California suggesting the northern Walker

Lane is one of the least developed, and possibly youngest, parts of the transform

6 boundary. The cumulative net right-slip across the southern Walker Lane since inception of the San Andreas appears to be no more than ~150 km and likely closer to 75-85 km

(Wesnousky, 2005).

Left-stepping dextral faults of the northern Walker Lane belt may be early development Reidel shears with no through-going strike-slip fault at depth (Faulds et al.,

2005). Northwest-oriented dextral faults of the northern Walker Lane belt terminate obliquely against northerly-striking normal faults in the northwestern Great Basin and along the eastern front of the Sierra Nevada. This suggests northwest-directed dextral shear is being accommodated by NW-SE extension (Faulds et al., 2005; Unruh et al.,

2003). The estimate of the long-term slip rate from the offset Oligocene paleovalleys (2-

10 mm/yr) is compatible with GPS geodetic observations of the current strain field

(Hammond and Thatcher, 2004).

Truckee Fault zone

Olig et al. (2005a, 2005b) mapped a broad zone of discontinuous north-to

northwest-striking faults and lineaments along the eastern escarpment of the Sierra

Nevada in the western Truckee basin that they have named the “Truckee fault zone”. The

Truckee fault zone (TFZ) forms the southwestern edge of the Tahoe-Medicine Lake

trough (Page et al., 1995) and accommodates overall down-to-the-east displacement

(Sylvester et al., 2008).

The TFZ is comprised generally of two zones: a western zone of linear northwest-

striking, left-stepping faults and lineaments extending from Martis Creek to

Independence Lake, and an eastern zone of north-striking faults and lineaments extending

7 from the east flank of Alder Hill north to Kyburz Flat (Figure 2). The eastern group passes directly through the study area meadow.

The presence of apparently offset drainage channels and the distribution of alluvial deposits suggest Quaternary faulting in the TFZ. Offset moraines of

Independence Lake suggest dextral motion with a down-to-the-east component of slip

(Olig et al., 2005b; Sylvester et al., 2008). The south-flowing reaches of Sagehen Creek and the Little Truckee River parallel the eastern zone for approximately 2 and 4.5 km, respectively, and suggest fluvial occupation of a fault zone (Figures 2 and 3).

Construction of Highway 89 undoubtedly obliterated much geomorphic evidence of recent faulting along the trend of the eastern zone. Preliminary mapping of the Sagehen

Creek drainage by Sylvester et al. (2008) indicates several northwest-striking faults in the upper reaches of Sagehen Creek exhibiting down-to-the-east motion.

Although a fault scarp along the east side of Alder and Prosser hills was previously recognized (Birkeland, 1963; Hawkins et al., 1986; Latham, 1985; Saucedo and Wagner, 1992; Olig et al., 2005a, 2005b), the length and continuity of the zone was not well defined. The northwest-striking faults and lineaments extend along the western margin of Martis Valley to Independence Lake where they are separated from the southern Mohawk Valley fault zone by the north-striking faults and lineaments extending from Truckee to Kyburz Flat (Figures 2 and 4). A 5-km wide left stepover at Kyburz Flat separates the eastern TFZ from the southern Mohawk Valley fault zone (Figure 2), however, considerable erosion has removed obvious evidence of an inter-connecting

8 Quaternary fault(s) and this stepover “gap” may reflect the lack of a through-going structural link between the TFZ and the Mohawk Valley fault zone.

The TFZ is an important transitional structural zone accommodating regional oblique strain with large gaps in geomorphic expression and a highly distributed nature, suggesting that it is an incipient fault zone lacking through-going structures. The TFZ may also be conjugate to the northeast-striking sinistral Dog Valley fault zone (Figure 2).

Geodetics

Geodetic studies of the modern stress field allow for the quantification of crustal deformation along the Pacific-North American plate boundary, and areas with a high modern stress field correspond well to areas of high seismicity. Analysis of GPS and plate kinematic data by DeMets and Dixon (1999) places the pole of Pacific-North

American plate rotational motion at 50.5°N, 248.2°E with angular velocity 0.776° My-1 for a derived Pacific-North American plate vector of 50 mm/year oriented 323° (Figure

1). Bennett et al. (2003) and Hearn and Humphreys (1998) described the distribution of the displacement field across the western United States and concluded, along with Dixon et al. (2000), that the Sierra Nevada is essentially a rigid microplate (block) that translates

~12 mm/year oriented 313° with respect to stable North America (Figure 1). GPS results also indicate ~10-11 mm/yr across a system of right-lateral strike-slip faults in the western Great Basin (Hammond and Thatcher, 2004, 2007). This zone, collectively known as the Walker Lane belt in its northern reaches (Locke et al., 1940; Stewart, 1988) and the eastern California Shear Zone (Dokka and Travis, 1990) in the south (Figure 1),

9 accommodates ~20-25% of dextral Pacific-North American plate motion (Bennett et al.,

2003; Dixon et al., 2000; Hammond and Thatcher, 2004; Oldow et al., 2001; Thatcher et al., 1999).

Bennett et al. (2003) further concluded that the 12 mm/year Sierra Nevada vector may be divided into components reflecting extension across the Basin and Range (~2.8 mm/year at 276°) and components of deformation within the Walker Lane and western

Great Basin (9.3 mm/year at 323°). The latter vector can be decomposed into parallel and perpendicular components to the average trend (330°) of the Walker Lane, demonstrating a small (~1mm/year) component of extension, oriented perpendicular to the main trend of the Walker Lane (Unruh et al., 2003; Wesnousky, 2005). Other geodetic studies indicate that 6 +/- 2 mm/year of northwest-directed shear is occurring in the northern Walker Lane at 39°N to 40°N latitude, resulting in dextral transtensional deformation (Dixon et al., 2000; Svarc et al., 2002; Thatcher et al., 1999; Unruh et al.,

2003). In summary, geodetic studies strongly suggest that the strike-slip fault system of the Walker Lane is transtensional.

Bedrock Geology

Detailed bedrock geology maps are not readily available for the study area and I did not map bedrock in detail during this study. Detailed descriptions of Tertiary volcanic rocks and sediment can be found in Birkeland (1961), Cousens et al. (2008),

Faulds et al. (2005), Grose (2000a, 2000b), Henry and Perkins (2001), Latham (1985),

Saucedo et al. (2005), and Saucedo and Wagner (1992). A simplified geologic map of

10 the Truckee basin (Figure 3) summarizes previous bedrock mapping pertinent to this study, while Figures 4 through 7 present more detailed geologic mapping in the study area.

Pre-Miocene Rocks

Pre-Miocene rocks of the Truckee basin include Triassic-Jurassic metamorphic

rocks (siliceous hornfels and greenschist) and medium-coarse-grained Mesozoic granitic

intrusions of the Sierra Nevada batholith (Saucedo and Wagner, 1992). Batholithic rocks

occur primarily northeast of the study area in the Verdi Range and extensively in the

Sierra Nevada crest to the west (Figure 3). These rocks are overlain by 31-23 mya ash-

flow tuffs (Faulds et al., 2005).

Miocene-Pleistocene Volcanics

Widespread volcanism occurred throughout the greater Lake Tahoe region from

28 Ma to 1 Ma, with major pulses at 16 Ma, 12 Ma, 10 Ma, 8-6 Ma, 5-3 Ma, and 2.6-1

Ma (Morton et al., 1977; Saucedo and Wagner, 1992; Henry and Sloan, 2003). Cousens

et al. (2008) proposed that the 28-3 Ma ancestral Cascades arc volcanic rocks of the

western Basin and Range and within the Sierra Nevada define a “Sierran Province” for

they all include a dominant lithospheric mantle component not evident in surrounding

volcanic suites.

11

Figure 3. Simplified geology and drainages of the study area.

Compiled from Birkeland (1963, 1964), Grose (2000a, 2000b), Henry and Perkins (2001), Latham (1985), Saucedo and Wagner (1992), and Sylvester et al. (2008). Recent alluvium not shown. Small red triangle denotes approximate location of trenches and cores. Notations as Figure 2.

12

Figure 4. Simplified geologic map and drainage pattern at the Independence Creek-Little Truckee River confluence.

Compiled from Grose (2000a, 2000b), Sylvester et al. (2008), and Saucedo and Wagner (1992). Unmapped areas (green topographic base) are inferred to be Tertiary volcanics and/or glaciofluvial deposits. Ice extent of Hobart glaciers is not obvious, however, broad areas of low relief present between ~6320-6600' elevation may be older (Donner or Hobart) outwash, till, or glaciated bedrock. Stream diversion to Sierra Valley is incised into thinly bedded, moderately indurated, well sorted, fine sand and silt at the drainage divide.

13

Figure 5. Simplified geologic map of Prosser Creek where it crosses Highway 89.

All units from Birkeland (1963) except undivided colluvium and drift of the Sagehen Hills (Sylvester et al., 2008). Unmapped areas (green topographic base) are inferred to be Tertiary volcanic rocks and/or Quaternary colluvium. Prosser Creek Reservoir not shown. Study area meadow is located just off the figure to the north at an elevation of ~6200' (Figure 7).

14

Figure 6. Simplified geologic map (modified from Sylvester et al., 2008) and major drainages in the headwaters of Sagehen Creek.

15

Figure 7. Simplified geologic map and major drainages of Sagehen Creek at Highway 89.

Figure modified from Birkeland, 1963 and Sylvester et al., 2008. Unmapped areas (green topographic base) are inferred to be Tertiary andesitic rocks and/or Quaternary glaciofluvial deposits.

16

Many volcanic centers in the Lake Tahoe area were so extensively glaciated that

delineation is difficult as only remnants remain (Lindgren, 1911). Although Tertiary

volcanism in the Sierra Nevada is dominated by volcaniclastic breccias and reworked

volcanic rocks (Hudson, 1951; Saucedo and Wagner, 1992; Wagner et al., 2000), lava

flow-dominated volcanic complexes are common in the Lake Tahoe / Reno region

(Cousens et al., 2008) and are the dominant rock type in the study area (Figure 3).

Evidence of volcanic activity between 28 and 16 Ma is rare. Eruptions at 12 Ma

were particularly voluminous and extensive with major andesitic stratovolcano development around the north end of the Carson Range (Henry and Perkins, 2001), and additional andesite erupted to the west and northwest (Cousens et al., 2008; Grose,

2000a, 2000b). This episode was coeval with a major episode of extension recognized along much of the eastern margin of the Sierra Nevada (Stewart, 1992; Stockli et al.,

2000; Henry and Perkins, 2001).

Numerous basaltic andesite lavas erupted shortly before 10 Ma are found from the

Interstate 80 corridor to Sierra Valley and in the Verdi Range (Henry and Perkins, 2001;

Prytulak et al., 2002). In the south, these rocks are collectively termed the I-80 suite

(Cousens et al., 2008), but may also include the Loyalton and Antelope Valley volcanic centers of southern Sierra Valley (Figure 2). Volcanism between 8 and 6 Ma included a distinct andesite stratovolcano centered near Martis Peak (Cousens et al., 2008).

Dating efforts (Chris Henry, 2009, personal communication) of basalt, basaltic andesites, and debris-flow breccias along Sagehen Creek have yielded 40Ar/39Ar dates

17 ranging from ~4.0-6.5 Ma. The basaltic Boca Hill vent and lava flows give 40Ar/39Ar ages of 4.41 +/- 0.03 Ma, 4.12 +/- 0.40 Ma, and 3.98 +/- 0.30 Ma (Cousens et al., 2008).

Late Pliocene – mid Quaternary volcanism is characterized by a volumetrically and geographically restricted, east-west array of Pleistocene mafic volcanic vents extending from the north shore of Lake Tahoe east to the Carson Sink in Nevada

(Dalrymple, 1964; Saucedo and Wagner, 1992; Cousens et al., 2008) and possibly west to

Sutter Buttes in central California (Hausback et al., 1990; Swisher et al., 2000). This group includes volcanic rocks of Bald Mountain, Alder Hill, Dry Lake Volcanic Field,

Hirschdale Basalt, and Boca Ridge. Lindgren (1897) mapped Pleistocene basalt flows in the Truckee basin and thought they were extruded from numerous vents over topography similar to the present one. Confirmation of this hypothesis was demonstrated by

Birkeland (1961) who further described the age, chemistry, and distribution of many of the flows, and went on to suggest a faulting and warping event during or shortly after the eruptions.

Pliocene Deposits of Boca Basin and Verdi Range Uplift

Late Pliocene depositional response to tectonic and volcanic events in the

Neogene Boca basin suggests a major tectonic re-organization of the Truckee basin ~ 3

Ma (Henry and Perkins, 2001, Mass et al., 2009). The current stress field may be inherited from this re-organization.

Boca Reservoir occupies a portion of the Neogene Boca basin, a Mio-Pliocene depocenter containing at least 500 m of sediment (Mass et al., 2009) west of the Verdi

Range (Figures 2 and 3). Fluviolacustrine deposits of yellowish andesitic sands with

18 equally typical beds of diatomaceous ash and lenses of conglomerate containing andesite boulders began to accumulate in low areas ca. 12 Ma and continued to ca. 3 Ma (Henry and Perkins, 2001; Mass et al., 2009).

The Carson Range is directly south of the Verdi Range and is a west-tilted fault block bounded on the east by the active Genoa fault and similar, east-dipping normal faults of the Carson Range fault zone (Ramelli et al., 1999). U-Th/He data indicate uplift of the Carson Range at ~3 Ma (Surpless et al., 2000). Although the Genoa fault was considered the eastern margin of the Sierra Nevada microplate at this latitude

(Thatcher et al., 1999), Cenozoic faulting demonstrably occurs in the Verdi Range farther west, near Donner Pass, and around western Lake Tahoe (Hudson, 1951; Birkeland,

1963; Schweikert et al., 2000; Henry and Perkins, 2001; Brothers et al., 2009; Dingler et al., 2009). Olig et al. (2005b) consider the faults of west Lake Tahoe and the Truckee fault zone to be the eastern boundary of the Sierra Nevada microplate. Given this boundary, the western boundary of the Boca Basin is likely near Donner Pass (Figure 2).

However, the full extent of the Boca basin is unknown due to the overlying strata concealing outcrops and to the incipient deformation ca. 3 Ma that disrupted western

Nevada and the eastern Sierra Nevada (Cousens et al., 2008).

The Verdi Range separates the Neogene Boca basin from the Miocene-Pliocene

Verdi basin to the east. It has been proposed that these basins were connected prior to uplift of the Verdi Range and that the Truckee River maintained an antecedent course across the range during the uplift, maintaining hydrologic connection between Lake

Tahoe and the Great Basin (Henry and Perkins, 2001). Conformable relations between

19 andesite and the Neogene Boca basin basal sedimentary strata demonstrates that minimal tilting (if any) of the Verdi Range occurred prior to ~3 Ma and that deformation was intense and occurred throughout the greater Tahoe region (Henry and Perkins, 2001;

Mass et al., 2009). Donner Pass faults, perhaps the westernmost expression of the ca. 3

Ma deformation, were likely active prior to 8 Ma, for a 13 Ma andesite is displaced ~800 m, whereas an 8 Ma basalt is displaced ~200-300 m (Hudson, 1951; Saucedo and

Wagner, 1992). Quaternary displacement has been demonstrated along the southern continuation of the Donner Pass faults west of Lake Tahoe (Schweikert et al., 2000).

East-dipping faults east of the Verdi Range have dropped westward-dipping Verdi basin sediment down against Mesozoic rocks with cumulative offset of at least 2 km and suggest east-west extension with little if any lateral displacement (Henry and Perkins,

2001). Basal conglomerate near Boca Reservoir and a 10 Ma basaltic andesite capping the Verdi Range dip ~20° west but stratigraphically higher diatomite from the western edge of the basin dips ~5° west (Henry and Perkins, 2001). This indicates fanning dips up section or the progressive westward decrease in uplift of the Verdi Range. The implied hinge point for the Verdi Range uplift is in the western Truckee basin, west of the known distribution of Neogene Boca basin sediment (Henry and Perkins, 2001).

Of the 20° westward dip observed in Neogene Boca basin sediment, about 4° have occurred since the 2.61 +/- 0.03 Ma basaltic andesite was emplaced onto the west- dipping (15-20°) sedimentary rocks of Boca Reservoir (Henry and Perkins, 2001). Given the youngest ash bed is dated at 3.1 Ma, ~15° of tilt of the Verdi Range occurred between

3.1 and 2.61 Ma (Henry and Perkins, 2001).

20 In summary, ca. 3 Ma, sedimentation patterns changed drastically in the Boca

Basin and major deformation on regional faults accommodated westward-tilting block rotations of the Verdi and Carson ranges establishing the modern northwest structural grain. This deformation is expressed as volcanism that continued until about 1 Ma and includes flows of Bald Mountain, Alder Hill, the Dry Lake volcanic field, the Hirschdale

Basalt, Boca Ridge (Figure 3), and Pliocene-Pleistocene flows near the outlet of Lake

Tahoe (Birkeland, 1964).

Prosser Creek Alluvium

Prosser Creek alluvium is composed of fluviolacustrine sediment that

accumulated in the Truckee basin in response to volcanic damming of the Truckee River by the mid-Quaternary Hirschdale basalt flows just south of Boca Reservoir (Figure 3).

The aggradation and ponding upstream of the basalt dam resulted in what Hubbs and

Miller (1948) referred to as Lake Truckee. According to Daley and Poole (1949) basalt

intrusions in conjunction with faulting formed the conditions for Prosser Creek alluvium

deposition. Birkeland (1961) confirmed the non-tectonic basalt dam hypothesis of

Lindgren (1897) and added that Lake Truckee was pre-glacial, as it is overlain by glacial

till and outwash. This damming caused widespread aggradation and ponding in all

Truckee basin drainages. The basalt brackets the onset of Prosser Creek alluvium

deposition; however, the cessation of Prosser Creek alluvium (onset of incision) can only

be inferred to be pre-Hobart in age.

Prosser Creek alluvium is composed of lake and lake-margin fluviatile sediment

(Lindgren, 1897) deposited on Miocene andesitic bedrock in the western Truckee basin

21 and upon Pliocene volcanics near the area of the Hirschdale Basalt dam (Birkeland,

1961). The maximum known thickness is ~45 m east of Truckee, California (it is assumed to be thicker farther east in the Truckee basin) where it occurs at stream level with no base exposed (Birkeland, 1961). Gravel and boulders occur where major streams enter the basin and grade laterally into sand and silt farther into the basin (Birkeland,

1961). The alluvium is indurated enough to hold a steep face in cuts and is locally, very deeply weathered with cobbles that can be cut with a shovel.

High degree of sorting and even stratification of these sand and silt deposits indicate lacustrine deposition while cut-and-fill and cross-bedding structures indicate mostly fluvial deposition for the coarse fraction (Birkeland, 1961). The pre-Prosser

Creek alluvium basin floor was at or below the present stream level and the maximum elevation at which the alluvium is found corresponds to the elevation of the top of the

Hirschdale Basalt covering Juniper Flat (Birkeland, 1961). Figure 5 presents detailed mapping of Prosser Creek alluvium distribution at the junction of highway 89 and Prosser

Creek.

Pleistocene Glaciations

Pleistocene mountain glaciation has played a role in sculpting the terrain within my study area. Advance and retreat of these glaciers over primarily andesite bedrock obliterated much of the pre-existing geomorphology and make it difficult to identify short-lived geomorphic features indicative of active faulting. The following is a brief summary of the northeastern Sierra Nevada glacial history.

22 The four major glaciations distinguished by Birkeland (1963) north of Lake

Tahoe include from oldest to youngest: Hobart (originally mapped as Sherwin), Donner

Lake, Tahoe, and Tioga; with Frog Lake, the youngest mapped glaciation, being either a

Tioga readvance or a separate advance. Tioga and Tahoe glaciations were correlative to

Blackwelder (1931) but Birkeland (1964) created a separate chronology (Donner Lake and Hobart) specific to the Truckee basin for the older glaciations. Figure 3 depicts the general distribution of glacial deposits in the Truckee basin, while Figures 4 through 7 present more detailed mapping of glacial deposits in the study area.

Hobart Glaciation

The Hobart glaciation followed a period of fluvial downcutting into Pleistocene

volcanic rocks and fluvial/lacustrine sediment of Prosser Creek alluvium.

Hobart glaciers were quite extensive and till is highly weathered with oxidation at all

depths, disintegrated granite boulders, well developed weathering rinds on volcanic

rocks, and high clay content (Birkeland, 1961). The high degree of weathering, much

more than Donner Lake age deposits, suggests the Hobart-Donner Lake interglacial was

the longest of the interglacials (Birkeland, 1961). Hobart till does not exhibit morainal

form and very few exposures are mapped (Birkeland, 1964). Although, Birkeland (1964)

noted that many areas mapped as Donner Lake may be Hobart, most are Donner Lake. A

small Hobart till deposit was mapped by Birkeland (1964) along Prosser Creek at

Highway 89 (Figure 5).

23 Donner Lake Glaciation

Donner Lake glaciers were probably at least as extensive as Hobart glaciers

(Birkeland, 1964). The till is characterized by a 1-2 m soil with a distinct A-B-C profile

in which boulders are less weathered than Hobart till and much less weathered than

Prosser Creek alluvium (Birkeland, 1964). Figure 6 shows Donner Lake ice of North

Fork Prosser Creek that spilled northward as a small ice tongue into the headwaters of

Sagehen Creek (Birkeland, 1964; Sylvester et al., 2008). The time required for the

fluviolacustrine accumulation of Prosser Creek alluvium upstream of the Hirschdale

basalt dam, ~40 m of stream incision during lacustrine drainage, and the degree of soil

development on Prosser Creek alluvium and Hobart glacial deposits, suggests the onset

of the Donner Lake glaciation north of Lake Tahoe to be ~400-600 ka (Birkeland, 1964).

Donner Lake glacial deposits are mapped on Figures 4 through 7.

Tahoe Glaciation

A relatively long interglacial separates the Tahoe from the Donner Lake

glaciation (Birkeland, 1961). Tahoe glaciers were shorter than those of Donner Lake age but occupied all tributary valleys between Sierraville and Lake Tahoe (Birkeland, 1961).

Tahoe glaciers of Prosser Creek extended almost to highway 89 (Figure 5) while Tahoe glaciers of Sagehen Creek were much smaller and confined to the upper reaches of

Sagehen Creek (Figure 6). Tahoe till has subdued moraine topography with an A-C soil profile, spheroidally weathered granitic boulders, and fresh volcanic boulders within the weathering zone (Birkeland, 1963). Age estimates for the Tahoe glaciation in the

24 Truckee basin are not precise; however, estimates from the eastern Sierra Nevada suggest a glacial advance ~60-200 ka (Clark et al., 2003).

A small tongue of ice off the left-lateral moraine of the North Fork of Prosser

Creek spilled northward as Donner Lake glaciers, but did not extend as far into the

Sagehen Creek drainage (Figure 6). An interglacial period of relatively short duration separates the Tahoe glaciation from the Tioga (Birkeland, 1961).

Tioga Glaciation

Tioga glaciers were slightly shorter and likely thinner than those of Tahoe age and till

exhibits well preserved moraine topography, an A-C soil profile, and fresh boulders

within the soil (Birkeland, 1964). The Tioga glaciation was marked by several standstills

and recessionals followed shortly by a slight readvance (Frog Lake) from the cirques

(Birkeland, 1964). Eastern Sierra Nevada glacial chronologies suggest the age of the

Tioga glaciation to be ~15-30 ka (Clark et al., 2003). Along the central Sierra Nevada

crest, radiocarbon age estimates of lake sediment deposited behind moraines correlative

to Recess Peak, a post-Tioga glacial interval, demonstrate that Recess Peak glaciers

retreated before 13,100 cal yr BP (11,190 +/- 70 14C yr BP) (Clark, 1995). If the Recess

Peak glaciation of the central Sierra Nevada is correlative with the Frog Lake glaciation

in the northern Sierra Nevada, then Frog Lake glaciers likely retreated before ~13,000 cal

yr BP (11,900 +/- 70 14C yr BP).

25 Outwash Terraces

Three sets of terraces underlain by bouldery stream deposits are present along the

Truckee River, Donner Creek, and Prosser Creek (Birkeland, 1961). Originally mapped by Lindgren (1897), Blackwelder (1931) was the first to describe the terraces as outwash deposits of the Sierran glaciations. Figure 5 depicts these terraces along Prosser Creek, just south of the study area. Each terrace is separated by a period of erosion in which streams incised to at least their present levels. The level of channel aggradation has decreased with time producing a nested-fill terrace sequence with the highest terrace being the oldest. Each terrace can be traced upstream and correlated to a corresponding till (for example, Tioga till grades laterally to Tioga outwash along Donner Creek) where proglacial streams flowing from the ice front transported material downstream as outwash. During glacial periods sediment load, caliber of load, and discharge all increase, but the first two factors increase much more than the discharge resulting in stream aggradation. Interglacials display a reduction of all three factors, with the first two being reduced the most, resulting in erosion (incision) by the sustained discharge and surface weathering associated with soil development.

Sagehen Creek Terraces

In addition to the modern alluvial floodplain, Sylvester et al. (2008) map two

terrace units along Sagehen Creek (Figure 6). The older, higher terrace “t1-2” grades to,

but not beyond, Donner Lake till. The younger terrace “t-3” is incised into terrace t1-2,

as well as Donner Lake till and grades to Tahoe till (just off the map to the south on

26 Figure 6). Terraces t1-2 and t-3 both grade to till in a tributary of Sagehen Creek (Figure

6) and are not sourced from the Sagehen Creek cirque (Sylvester et al., 2008).

The terraces occur as a nested sequence, suggesting a similar aggradation/erosion history as those mapped regionally by Lindgren (1897), Blackwelder (1931), and

Birkeland (1963) in nearby major drainages. Both terraces pinch out ~500 m upstream of

Highway 89 at an elevation of ~6,200 feet (Figure 7) and appear to cross-cut deposits mapped as undivided colluvium and glacial drift (Sylvester et al., 2008). The left bank of

Sagehen Creek is mapped as colluvium and glacial drift between highway 89 and where the terraces pinch out whereas the right bank is mapped as younger alluvium (Figure 7).

Detailed lithologic and stratigraphic descriptions of the terraces were not available.

27 METHODS

This study is focused on an alpine (~6200 feet elevation) meadow that is a depocenter for fine-grained sediment (Figure 8). Young deposits with material suitable for dating (organic debris, ashes) are ideal in determining recent fault activity. Analyses of topographic maps and interpretation of aerial photographs of the region were used to identify geomorphic and stratigraphic units and potential fault-related features. After selecting the alpine meadow (Figure 8) for further study from the air-photo and topographic analyses, cores were dug to confirm the presence of material suitable for dating (Figure 9). Offset inferred from core stratigraphy is not always tectonic in origin, but may be due to sloping depositional contacts, or mass-wasting processes, and only open trenches that expose the stratigraphy in cross section can confirm tectonic offset of units. Trenches were excavated to confirm the stratigraphy and fault offset inferred from the cores (Figure 9). Detailed trench logs were created, and horizons were sampled for laboratory analysis (Figures 10 through 13). The age of the youngest offset unit

(Tsoyowata tephra) was determined by glass shard analysis (Foit, 2007) and corroborated with radiocarbon dating of the underlying organic sediment. Piercing points that may provide additional information on the lateral sense of motion were sought by constructing cross sections (Figures 14 through 17) to laterally define the boundaries of offset unit.

28

Figure 8. Location map and aerial photograph of the study area alpine meadow (shaded area) along Highway 89 between Prosser and Sagehen creeks.

In the northeast corner of the meadow, an east-facing scarp and associated closed depression are visible as a thin white line (trending N-S) and an ovate, light colored area east of the scarp (standing water) rimmed with green (grasses). Air photo ID: 07-09-00, USDA-F, 16, Code 615170, roll number 500, photo number 136. Red line is approximate extent of an alluvial fan derived from the east.

29

Figure 9. Annotated aerial photograph with approximate trench and core locations in the study area meadow.

Filled circles denote locations of hand-dug soil boreholes. Filled rectangles denote hand-dug trench locations. Top of east-facing scarp is ~1m higher than HM-Pit-01 (lowest point in closed depression). See Figures 14-17 for cross sections. Air photo ID: 07-09-00, USDA-F, 16, Code 615170, roll number 500, photo number 136.

30

Air Photo and Topographic Analyses

I used black-and-white vertical air photos (U.S. Department of Agriculture, USDA, 2000) to identify open areas suitable for coring that corresponded to features indicative of late

Quaternary faulting such as topographic or vegetation/tonal lineaments. Topographic analysis was conducted using USGS 7.5 minute topographic quadrangles and shaded relief maps produced from 30 m digital elevation models (DEMs). The elongate alpine meadow was chosen for further study because of its strong linear nature and orientation sub-parallel to Sierra Nevada motion (relative to North America) and the presence of tonal lineaments and a low scarp-bounded closed depression. Additionally, the meadow is not a floodplain of an actively eroding drainage; rather it is a depocenter for alluvial fan runoff and is subject to less erosion and loss of stratigraphic record than typical floodplains along nearby streams.

Lithostratigraphic Descriptions

In general, stratigraphy from a variety of sampling methods is described based on texture, color, consistency, and any distinguishing features. Contact abruptness and thickness of units were noted either on core logs (inferred contacts) or from trench exposures (measured contacts).

Textural Analysis

In order to confirm field descriptions, textural analysis was conducted on select units. All analyses were conducted in the Materials Testing Laboratory of SHN Consulting

31 Engineers and Geologists, Inc., Eureka, California. Samples were processed under the supervision of the laboratory manager according to ASTM Method No. D422.

Hand-Excavated Soil Coring

All coring was done by hand using a 2.5 cm diameter gouge core in tandem with an 8 cm diameter hand-auger bucket. The gouge core was inserted to refusal (< 100 cm), rotated

two times, and removed from the ground. In order to minimize slough and error in

measuring depths, the total depth of the gouge core boring was noted along with length of

recovered core. The boring was then reamed with the 8 cm diameter hand auger to the

depth of the gouge core refusal. Auger bucket cuttings were used to supplement

descriptions from the smaller diameter gouge core. The gouge core was placed into the

reamed boring and advanced beyond the depth of the reamed boring into undisturbed

material, rotated two times and removed. This process was carried out until refusal of the

gouge core or attainment of target depth or horizon.

Hand-Excavated Soil Pits and Trenches

All pits were excavated by hand with a shovel. Exploratory soil pit HM-Pit-01 (Figure 9)

was excavated before the gouge cores GC-3 through GC-8 were excavated. Based on

offset observed in the cross sections derived from the series of gouge cores that followed

the excavation of HM-Pit-01, trench HM-Pit-02 was dug from the top of the scarp down

the east-facing slope (Figure 9). Trench HM-Pit-02 exposed a fault zone. Confirmation

32 trench HM-Pit-03 was dug across the west slope along the east side of the closed depression to confirm/deny offset (Figure 9).

Tephra Analysis

The age estimate of the youngest offset unit (Tsoyowata tephra) in the meadow was used to bracket the timing of the most recent rupture event on the fault exposed in pit

HM-Pit-02. Glass shards were analyzed by Dr. Franklin Foit at the Washington State

University Microbeam Laboratory in Pullman, Washington to determine the composition of the tephra and calculate a similarity coefficient to volcanic eruptions of known age

(Foit, 2007).

Radiocarbon Dating

I used radiocarbon age analyses of two organic sediment samples to constrain the timing of the last surface-rupture event on the fault exposed in trench HM-Pit-02. A third radiocarbon age estimate of a buried tree along the left bank of Sagehen Creek (Figure 7) was used to bracket the age of the sediments along that reach of the drainage. The samples were sent to BETA Analytical Laboratories, Inc. for radiocarbon age determination by accelerator mass spectrometry. Table 1 presents the results of the radiocarbon analyses.

33

Table 1. Radiocarbon Age Estimates

Sample Lab Lab reported Sample Sample Location* identification identification 14C yr BP age*** material number number** (cal yr BP)

Trench HM-Pit-02 9200 +/- 50 Organic HMP2OC4FT1 235863 (unit-5, black clay) (10260-10480) sediment

Trench HM-Pit-02 7720 +/- 50 Organic HMP2OS4FT1 235864 (unti-4, brown silt) (8430-8550) sediment Outermost Sagehen Creek 2430 +/- 40 rings of buried SHCCB02BUT 239025 (left cutbank ) (2360-2680) tree in growth position * Locations shown in Figures 7 and 13 ** Beta Analytical Laboratories Inc. *** Conventional radiocarbon age estimate +/- one standard deviation

Correlation of Pre-Holocene Deposits

In an attempt to address the possibility of fluvial input from Sagehen or Prosser creek into the study area meadow, I compared recent mapping of Quaternary glacial deposits in the Sagehen Creek drainage by Sylvester et al. (2008) and older mapping along Prosser Creek by Birkeland (1963) to the lithostratigraphic descriptions of the study area meadow. Soil descriptions, relative stratigraphic position, and topographic position of terraces were compared in an attempt to correlate the deepest stratigraphy of the study area with one of the following: Prosser Creek alluvium (Birkeland, 1961), weathered glacial outwash (Donner Lake or Hobart) of Prosser Creek (Birkeland, 1963), older terraces (t1-2 of Sylvester, 2008) of Sagehen Creek, or weathered alluvial fan of the

modern meadow drainage to the east.

STUDY AREA MEADOW STRATIGRAPHY

The study area meadow is between Sagehen and Prosser creeks ~10 km north of

Truckee, California within the north-striking, eastern zone of the TFZ (Figure 2). The

meadow is a long (~500 m), relatively narrow (< 100m wide), linear (oriented ~340°),

slightly south-sloping area of perennial grasses surrounded by Lodgepole and Jeffrey

pines (Figure 8). A low (~1m), north-trending, short (~50 m in length), east-facing scarp

is visible in the northeastern part of the meadow as a thin white line on the aerial photo in

Figure 8. The scarp forms the western boundary of a small (~400 m2) kidney-shaped

closed depression which appears white in the aerial photo due to the reflection of

standing water in the depression (Figure 8). An alluvial fan defines the eastern and

southern boundaries of the closed depression (Figure 8) and a steep slope of andesite

scree constrains the northeastern boundary (Figure 8).

I describe the subsurface stratigraphy using lithostratigraphic descriptions of

trench exposures and core samples (Figures 10 through 13) in the vicinity of the scarp, and then more generally across the entire meadow with schematic cross sections constructed from widely-spaced core logs (Figures 14 through 17). I will also present textural data, tephra chemistry, and radiocarbon age estimates of the lithostratigraphic

units.

In general, the deepest stratigraphy explored in the meadow area consists of

unweathered greenish-grey sand and silt (unit-8a and unit-8b) overlain by highly

weathered yellow-red clayey sand (unit-7) that in places is up to 250 cm thick. Up to 60

34 35 cm of highly prismatic, plastic grey clay (unit-6) overlies the weathered clayey sand.

Within the closed depression, the lower contact of unit-6 is mixed and gradational with unit-7, while the contact is very abrupt west of the scarp. Within unit-6 is an organic layer (unit 6a) that was only observed in core GC-2. Up to 30 cm of black silty clay

(unit-5) abruptly overlie unit-6. Brown clayey silt (unit-4), up to 75 cm thick, overlies the black clay along a fairly abrupt contact and is very abruptly overlain by a tephra couplet (unit-3c). This couplet consists of two distinct layers of fine white sand, each ~1 cm thick, separated by ~ 1 cm of the overlying sandy silt (unit-3b). This sandy silt is up to ~50 cm thick and is overlain by clayey silt (unit-3a) which also is up to 50 cm thick.

The modern meadow soil (unit-1) is up to 30 cm thick and overlies the clayey silt along a diffuse, undulatory contact that roughly coincides with rooting depth of perennial meadow grasses.

Textural Analysis

Texture was a major factor in defining unit boundaries and correlating units between trench exposure and core sample and textural field descriptions were confirmed by laboratory analysis of select horizons. Table A-1 in Appendix A presents the complete results of the textural analysis. Maximum textural values of up to 64% clay

(unit-6), 61% silt (unit-3b), 60% sand (unit-7), and 30% gravel (unit-2) were determined.

36

Figure 10. Trench log of HM-Pit-01 and hand boring GC-2.

Pit was located at the lowest topographic point of the closed depression. Descriptions below pit are from GC-2 auger bucket cuttings. Tephra couplet and fissure features of unit 3b are schematic. "Root casts" of unit 6a actually may be fractures filled with black organic clay. Given their clay content, units 5, 6, 6a, and 7 are interpreted to represent a significant aquitard across the entire closed depression. Tephra analysis conducted for this study by Foit (2007).

Figure 11. Trench and core log of the north wall of HM-Pit-03.

See Figure 9 for location. Contacts are the same as those used for HM-Pit-01 (Figure 10). Red dashed lines are fractures. Units 2 and 3a terminateagainst the boulder. Gouge core (GC) borings were installed before trench HM-Pit-03 was dug. Tephra couplet and fissure fills of unit 3b are schematic. Fissure fill texture consists of unit 3a mixed with units adjacent to the fissure. 37

Figure 12. Trench log of the north wall of HM-Pit-02.

See Figure 9 for location. Contacts are the same as those of HM-Pit-01 trench log (Figure 10). Gradational zone (unit 6/7) between unit 6 and 7 occurs only on downthrown side of fault. Blocks of unit 4 are completely surrounded by sediment of unit 3b. These intact blocks are not found in unit 3a; the colluvial / lacustrine unit and are inferred to be broken thrust sheets forced into saturated sediment of unit 3b. Black and grey clay "fault gouge" (not shown at this scale) is present along all fault traces (red lines). Gouge is up to 1 cm thick, with no sense of shear. Tephra couplet is schematic. 38

Figure 13. Trench and core log of the south wall of HM-Pit-02.

See Figure 9 for location. Contacts the same as those of trench log HM-Pit-01 (Figure 10). Gradational zone (unit 6/7) occurs only on downthrown side of fault. Blocks of unit 4 exhibit a "floating" nature and are aligned with the projection of fault traces (red lines). The deformed block of unit 4 present at the base of unit 3a is inferred to be a scarp-face blockfall or proximal facies of a paleoseismic colluvial wedge buried by unit 3a. Black and grey clay "fault gouge" (not shown at this scale) is present along all fault traces (red thick lines) up to 1 cm thick, with no sense of shear. Tephra couplet and fissure fill are schematic. 39

40

Figure 14. Schematic cross section west of, and parallel to, the fault-scarp trend.

See Figure 9 for location. All elevations and contacts are approximate. Except where noted, units are as shown in HM-Pit-02 trench log (Figure 12). Solid black fills represent black clay or highly organic clayey silt. Blue area denotes distribution of the ~7,000 yr BP Tsoyowata tephra (Foit, 2007). Auger refusal occurred only in core HA-23 (in sand/gravel with tephra).

41

Figure 15. Schematic cross section east of, and parallel to, the fault-scarp trend.

See Figure 9 for location. All elevations and contacts are approximate. Except where noted above, units are as shown in HM-Pit-02 trench log (Figure 12). Solid black fills represent black clay. Blue area shows distribution of ~7,000 yr BP Tsoyowata tephra (Foit, 2007). Auger refusal occurred only in cores HA-27 (on cobbles) and HA-31 (~2 cm into greyish brown silty sand with gravel).

Figure 16. Schematic cross section west of the fault-scarp trend.

See Figure 9 for location. All elevations and contacts are approximate. Units are the same as shown in cross section C-C' (Figure 14), except where noted. Units 4b and 4c are low organic content silt syndepositional with unit 4 and possibly unit 5. Unit 4c shows prominent oxidation features whereas unit 4b is reduced. Solid black fills represent black clay or highly organic clayey silt. Auger refusal in all cores (on gravel).

42

Figure 17. Schematic cross section orthogonal to fault (red lines) strike.

See Figure 9 for location. All elevations and contacts are approximate. Units as cross sections C-C' and D-D' (Figures 14 and 15), except as noted. Solid black fills represent black clay or highly organic clayey silt. Blue areas are the ~7,000 yr B.P. Tsoyawata tephra (Foit, 2007). Grey shaded areas are an andesite boulder and cobble. Auger refusal in all cores except HA-01, HA-03, GC-2, and GC-6. 43

44

Tephra Glass Composition

Glass compositional analysis for major oxides correlates the tephra encountered in trenches and borings with a precursor eruption of Mount Mazama that produced the

~7,000 yr BP Tsoyowata tephra (similarity coefficient of 0.97) (Foit, 2007). Laboratory results of the tephra analysis and the 15 best similarity coefficients to known tephra are presented in Tables B-1 and B-2, respectively, in Appendix B. Thus the Tsoyowata tephra provides a key stratigraphic timeline constraining Holocene activity on the newly discovered fault.

Radiocarbon Age Estimates

Based on radiocarbon age estimates, marsh conditions were established by at least the late Quaternary (deposition of unit-6) and persisted into at least the early

Holocene (Table 1). Radiocarbon age estimates confirm Holocene surface faulting along the east side of the alpine meadow. Age estimates for the buried tree along the left bank of Sagehen Creek indicate that the overlying sediment of the left bank of

Sagehen Creek mapped by Sylvester et al. (2008) as undifferentiated colluvium and drift is late Holocene in age.

45

Pit HM-Pit-01

Pit HM-Pit-01 was excavated to approximately 150 cm depth in the lowest topographic point of the closed depression at the location of gouge core GC-2 (Figure

9). This was the first pit excavated to confirm core stratigraphy and the deepest cored location (~250 cm) within the depression. Figure 10 represents the log for pit HM-Pit-

01 and core GC-2.

Two unweathered units (8a and 8b), were encountered below highly oxidized clayey sands and silt (unit-7). The lower unit (8b) consists of medium grey, loose, well- sorted fine sand with silt that was completely saturated and is overlain by greenish grey, loose, wet, fine-medium sand of unit-8a. Both units appear to have undergone very little weathering and are inferred to be fluviolacustrine. The unweathered sands were not encountered in any other location during this study and the lower contact of unit-7 was only inferred from auger-bucket cuttings.

The grey clay (unit-6) in this location is somewhat different than elsewhere within the meadow, as it includes an organic rich horizon (unit-6a, Figure 10). This unit was fairly distinct in auger-bucket cuttings so it was broken out from unit-6. The prominent black silty clay (unit-5) overlying the basal grey clay is close to 25 cm thick, which is approximately the maximum thickness of this unit encountered in the study.

The overlying brown clayey silt (unit-4) is ~50 cm thick and is abruptly truncated by the tephra (unit-3c) couplet (each ~ 1 cm thick) which is interfingerred with silt of unit-3b.

Unit-3b is sandy silt inferred to be deposited in a nutrient poor lacustrine environment,

46 whereas unit-3a is silty sand and appears to be colluvial / nearshore lacustrine.

Fissure fills of unit-3a were observed in unit-3b, and less so in other units. Modern meadow soil is thickest (up to ~25 cm) in the closed depression.

Pit HM-Pit-03

Pit HM-Pit-03 was excavated across the central part of the east edge of the closed depression (Figure 9) to investigate previous auger refusal and topographic slope. The excavation was approximately 3.5 m long, 0.75 m wide, and ~1.5 m at its deepest point. Figure 11 represents the log for pit HM-Pit-03 and cores GC-7 and GC-

8.

Pit HM-Pit-03 exposed stratigraphy similar to pit HM-Pit-01 with the exception of a jumbled light grey to white gravelly, sandy silt (unit-2) impounded behind a large, angular, andesitic boulder (Figure 11). Unit-2 thickens to the east and represents a large textural change from the closed depression sediment and is interpreted as a wedge of colluvium derived from the east. The marsh units (units-6, 5, and 4), tephra, and unit-

3b do not pinch out to the east and may be present at depth further east beneath the wedge of colluvium.

Also of note is a rounded andesitic cobble that is draped with the tephra couplet

(Figure 11). This cobble is texturally enigmatic but the proximity to hillside processes east and north of the closed depression suggest it may have rolled downhill into the marsh immediately before tephra deposition.

47 Fractures in unit-3b below and near the west edge of the boulder suggest this block of andesite struck the sediment of unit-3b from the east with enough energy to deform unit-3b as wells as unit-3c and unit-4. Fractures project through unit-3b but not into unit-3a, suggesting that most of unit-3b was already deposited when boulder impact occurred. The lack of fractures in unit-3a suggests it had not yet been deposited or did not behave in a brittle fashion during deposition of the boulder.

The relation of the colluvium to unit-3a is not completely clear, but it can be assumed that they are nearly syndepositional. Given that a boulder-sized clast was being supplied to the depocenter, it seems likely that silty gravels could also be transported in the same event or by the same slope processes. Unit-2 “engulfs” the east edge of the boulder along a very abrupt contact but does not occur beyond the boulder.

This abrupt vertical contact implies a lack of competing sedimentation with the colluvium following deposition of the boulder. It appears there is very little time between the deposition of the boulder and the accumulation of the wedge of colluvium.

Therefore, unit-2 is interpreted to be slightly older than, but mostly syndepositional with, unit-3a.

Pit HM-Pit02

Pit HM-Pit-02 was excavated across the scarp in the central part of the west edge of the closed depression to investigate offset inferred from logs of cores GC-3,

GC-4, and GC-5 (Figure 9). The excavation was approximately 3 m long, 0.75 m wide,

48 and ~2.25 m at its deepest point. Figures 12 and 13 present the logs for pits HM-Pit-

02-North Wall and HM-Pit-02-South Wall, respectively.

Pit HM-Pit-02-North Wall

Down-to-the-east offset of a Holocene tephra and marsh stratigraphy was observed in the north wall of pit HM-Pit-02 along a north-striking, near vertical, fault zone oriented sub-parallel to the scarp (Figure 12). The zone is up to ~50 cm wide, extends to within ~20 cm of the modern ground surface, and offsets at least 6 exposed units. Lithostratigraphy is similar to pits HM-Pit-01 and HM-Pit-03 but the contact between unit-6 and unit-7 is a mixed and gradational zone of both units (unit-6/7) on the east side of the fault only. All of the marsh units (and tephra) are thinner on the upthrown (west) side of the fault zone, particularly unit-3a and unit-3b. Unit-6/7 is not present on the west side of the fault, where the upper contact of unit-7 is abrupt and often resulted in auger refusal.

The north wall exposed a fault zone with ~3 anastomosing splays emanating from a common vertical fault (Figure 12). All splays contained very soft, black clay gouge which had no discernable internal fabric. The main trace is near-vertical, offsets

6 units, and appears to terminate at the base of unit-3a.

The westernmost splay appears to terminate at the base of unit-6 west of the main trace with no apparent vertical offset of the base of unit-6 whereas the easternmost splay dips to the west forming an apparent thrust wedge, that places unit-4 over unit-3c and lower part of 3b and terminates upward within the middle part of unit-3b. This

49 wedge contains deformed stratigraphy of 4 offset units and an east-dipping, sub-splay that soles back to the main trace.

Apparent reverse faulting is indicated along the easternmost splay which has accommodated vertical motion of a thrust wedge of unit-4 capped with the tephra couplet of unit-3c (referred to collectively as “unit-4/3c”). The orientation of this splay suggests more complicated motion than pure normal, down-to-the-east motion. Rather, this splay accommodates the eastward apparent thrusting of the wedge of unit-4/3c.

The structure, apparent reverse offset, as well as abrupt thinning of units across the fault are consistent with oblique-slip faulting. Other smaller intact blocks of unit-4, present east of the main trace, are surrounded by unit-3b but lack the tephra couplet cap. The abrupt contact of these blocks with the surrounding unit-3b and the suspended nature of their distribution, suggests unit-3b was fluid enough to facilitate the addition of the unit-

4 blocks, but dense enough to hold their position and preserve the abrupt contact.

The main trace of the fault zone vertically offsets at least the lower half of unit-

3b and appears to terminate at the base of unit-3a. The apparent reverse fault terminates upwards in the middle of unit-3b, which has accumulated to this point before the most- recent rupture event. Since the block of unit-4/3c is intact and surrounded by unit-3b, I interpret the minimum amount of deposition of unit-3b to be near the top of the block of unit-4/3c before rupture. The small blocks of unit-4 are found in close proximity to unit-4/3c, as well as vertically along the fault trace, all within unit-3b. To imply different origins for some of the small blocks of unit-4 would be much more

50 complicated than assigning them all as blockfall or “ejecta” into unit-3b during or shortly after the rupture event.

The low permeability silty clay of unit-6 inhibits the draining of the closed depression resulting in a perched groundwater table that is expressed as standing water in the closed depression. Unit-3b was likely saturated prior to the introduction of blocks of unit-4, thus making them susceptible to mixing (shaking/flowing) during rupture events and able to accommodate and preserve stiff blocks of clayey silt (unit-4) and preserve an abrupt upper contact. Therefore, unit-3b was apparently saturated (or nearly so) during the rupture event.

Pit HM-Pit-02-South Wall

The south wall of trench HM-Pit-02 displays some of the more compelling structure and stratigraphic evidence for the nature and timing of Holocene faulting

(Figure 13). The major difference from the north wall, even though it is only <1 m from the south wall, is how the fault zone is distributed along more traces (~7). As in the north wall of the excavation, six units are offset and “floating” blocks of unit-4 and unit-4/3c are present within unit-3b with one deformed block of unit-4 at the boundary of units 3a and 3b (Figure 13). All of the marsh units (and tephra) are thinner west of the fault zone, and unit-6/7 is not present on the west side of the fault, where the upper contact of unit-7 is abrupt and often caused auger refusal.

The fault trace has a half “flower structure” geometry (Sylvester, 1988) that has created overall, a down-to-the-east, stair-step configuration with thrust blocks (or sheets) oriented sub-parallel to the fault plane (Figure 13). Two main vertical fault

51 traces were observed in the bottom of the south wall with ~4 subsplays off the westernmost of the traces. The easternmost trace is near-vertical, offsets six units, and terminates within unit-3b. The more complicated western trace is near vertical near the lower third of the trench but branches out upward to define a half flower structure ~ 50 cm wide. Within this structure, units are displaced along steeply west-dipping splays that sole downward to the westernmost splay which is near-vertical, and accommodates down-to-the-east motion (Figure 13).

Unit-4, unit-5, and unit-6/7 are sheared between the easternmost splay and the easternmost subsplay, which align with the “floating” blocks of unit-4. A vertical fissure fill within unit-4 extends to unit-5 via sub-horizontally-oriented fissures that suggest soil parting. Black, and much lesser amounts of grey clay gouge are present along all fault traces except those stratigraphically higher than the tephra and are thickest in unit 7.

The deformed block at the contact of unit-3a and unit-3b (Figure 13) is silty, and incohesive, but its identification as derived from unit-4 is not in doubt. The block appears to be a post-rupture blockfall or collapse of a paleo-freeface onto the surface of unit-3b and subsequently buried by unit-3a. The deformed nature and lack of preservation of the initial cohesiveness of unit-4 suggests it was not buried rapidly enough to be protected from surface weathering. This contrasts with the intact blocks of unit-4 that were completely buried by unit-3b and have retained structural and textural features of unit-4. One particularly square in cross section, free-floating block of unit-4/3c observed in the south wall appears to have been apparently thrust up along

52 the projection of the westernmost trace (s) of the fault zone (Figure 13). Two smaller

“floating” blocks of unit-4 and displaced blocks containing unit-4 and unit-5 are distributed along this trace as well (Figure 13).

Hand Auger Cores

I hand-augured cores along three transects (A-A’, C-C’, and D-D’) parallel to the scarp trend and along one transect perpendicular (B-B’) to the scarp trend (Figure 9). These excavations enabled me to construct cross sections in order to define the lateral continuity of the marsh deposits (Figures 14 through 17). These schematic sections demonstrate the continuity of the sequence of marsh sediment throughout the northern portion of the meadow and suggest that standing water conditions occupied the full extent of the northern part of the meadow during the late Quaternary to early Holocene.

Along transect B-B’ (Figure 17) a highly plastic, brownish grey unit of equal parts silt and clay (unit-8c) was encountered below unit-7 in core HA-03 only. The unweathered, well-sorted sand and silt (unit-8a and unit-8b) encountered below unit-7 in core GC-2 suggest lacustrine deposition with possible minor fluvial input. The higher degree of weathering observed in unit-8c is likely the result of differential weathering associated with shallower burial and more oxidizing conditions than the sand and silt of unit-8a and unit-8b; which have virtually no evidence of weathering.

Units 8a, 8b, and 8c were not well defined in this study, but suggest unweathered or slightly weathered stratigraphy below unit-7 that may be the parent material on which unit-7 (as a soil) formed.

53 The thickness of unit-7 could only be determined from cores GC-2, HA-01, and HA-03 (Figure 17) and it ranged from ~50 to 150 cm thick. This thickness would be a compatible thickness for soils developed on Prosser Creek alluvium (Birkeland,

1961).

The basal grey clay (unit-6) and organic marsh sediments (unit-4 and unit-5) pinch out to the north and south at locations roughly equivalent to the margin of the modern closed depression and to the west roughly at the west edge of the modern meadow (Figures 14 through 17). These thinning geometries are in contrast to the east margin where marsh deposits do not pinch out or thin appreciably (Figures 11 and 17).

Unit-4 appears to grade laterally into loose, silty sand of units 4a, 4b, and 4c along the southern margin of the closed depression. These sandy units apparently represent distal alluvial-fan deposits encroaching into, and possibly interfingerring with, the quiet-water, organic, marsh-environment sediment. Unit-4c, which is not found in excavations within the closed depression, exhibits oxidation features. It does not appear to be associated with the reducing marsh environment and represents more of a gradation between the highly weathered unit-7 and facies related to unit-4. Units 4a and

4b are not oxidized to any considerable degree and are similar in color to the deposits of unit-4.

The tephra couplet was encountered within a ~30 m zone along both C-C’ and

D-D’ transects with greater preference of preservation and couplet thickness in the deeper water areas. The tephra encountered along transect C-C’ (Figure 14) is mixed

54 with gravels at the refusal depth in core HA-23 at the northern margin of the meadow. Refusal on similar tephra-containing gravel was observed in core HA-27

(Figure 15).

Unit-3b thins to the north of the closed depression but does not pinch out entirely. The unit is absent in the southernmost parts of the depression where it apparently pinches out coincident with the projection of the east-west vegetative/topographic lineament between HA-09 and HA13 (Figures 9 and 16) and the western boundary of the meadow. It is present beyond the margins of the closed depression to the east (Figure 11). Unit-3a, which is confined by the boulder on the east, abruptly pinches out to the north, south, and west of the scarp area, but is present in the northernmost parts of the meadow (HA-08, Figure 16). Unit-2 was inferred from refusal in hand cores GC-7 and GC-8 (Figure 11).

Slightly oxidized dark grey silt (unit-1a) occurs along the base of the modern topsoil (unit-1) as a discontinuous, variably thick silt horizon. In core HA-22 (Figure

14) a thin layer of highly organic silt similar to unit-5 was encountered within unit-1a.

These features may be associated with capillary fringe weathering and seasonal wetting depth areas.

DISCUSSION

Paleohydrology

The highly weathered appearance of unit-7 suggests an age much older than the overlying units. The age and origin of unit-7 would provide evidence for the pre-

Holocene depositional patterns and paleohydrology of the study area meadow. Potential sources considered for unit-7 include: weathered alluvial-fan deposits of the modern drainage to the east; weathered glacial outwash of Sagehen or Prosser creeks; and Prosser

Creek alluvium. Birkeland (1961) established several criteria for distinguishing glacial outwash from Prosser Creek alluvium in the Truckee basin. In general, Prosser Creek alluvium is well sorted, contains less granitic lithologies than does the outwash, and is indurated enough to stand in vertical cuts; conversely, the oldest glacial outwash is only moderately weathered. Birkeland (1961) further describes Prosser Creek alluvium as well sorted and stratified sand and silt deposited in a lake dammed by the Hirschdale

Basalt (Figure 3). Lithologic data for the alluvial-fan deposits was not obtained for this study. However, based on the textures of units 2, 4a, 4b, and 4c, it appears that younger deposits of the distal alluvial fan are only slightly oxidized, if at all.

If unit-7 (and the underlying units 8a, 8b, and 8c) is Prosser Creek alluvium and not glacial outwash, it can be inferred that the extent of the Prosser Creek alluvium depositional environment (fluviolacustrine) inundated the study area. Additionally, drainages may have transported material into the meadow and deposited well-sorted sand and silt, perhaps in a fluviodeltaic environment. 55 56 Given the similarity in texture, environment of deposition, and contact nature of unit-6 and unit-5 (radiocarbon age estimate of 9200 +/- 50 14C yr, ~10,370 cal yr BP) it

appears the two units are part of the same allostratigraphic unit (marsh) and close in age.

The grey marsh clay (unit-6) is inferred to have been deposited before the retreat of Frog

Lake glaciers (~11,190+/-70 14C yr BP, ~13,100 cal yr BP), with organic input (unit-5

and Unit-4) accompanying the changing climate of the waning Frog Lake glaciation. The

organic marsh conditions persisted in the study area meadow from ~10,370 to ~7,000 cal

yr BP and were abruptly terminated by tephra deposition.

Any discussions of fluvial flow into the study area meadow from Sagehen Creek

are tenuous and subject to future research. To fully understand and determine the age and

provenance of units 7, 8a, 8b, and 8c, a more detailed study of the soils and deposits

between the town of Hobart Mills and the meadow area needs to be conducted.

Most-Recent Rupture Event

The most-recent rupture event geomorphically enhanced a pre-existing scarp-

bound closed depression that had been partially infilled with late Quaternary-Holocene

marsh stratigraphy. The most-recent-rupture event occurred <7,000 yr BP, as evidenced

by the offset Tsoyowata tephra (Foit, 2007) observed in pit HM-Pit-02 (Figures 12 and

13). Tephra offset is in an apparent normal, down-to-the-east motion of ~50-100 cm on a

north-striking vertical fault zone <1 m wide. Offsets of the magnitude observed in trench

HM-Pit-02 have been associated with large magnitude (>M6) events (Wells and

Coppersmith, 1994). At least one unit overlying the tephra is offset, and at least four

57 units underlying the tephra are offset (Figures 12 and 13). Offset increases with depth with maximum offset of ~1.25 m (base of unit-6, Figures 12 and 13). In addition to the apparent normal sense of motion, apparent eastward-thrusted blocks within the fault zone and clay gouge along fault splays indicate compression, or given the half flower structure and marsh (sag pond), localized compression (Figure 12 and 13).

The vertical orientation of the fault, anastomosing half flower structure and apparent reverse offset suggest a strike-slip component of displacement (Sylvester, 1988), however, no convincing piercing points (lateral) were delineated. The increase in unit thickness across the fault from west to east indicates deeper water to the east of the fault zone, but the change in unit thickness is too abrupt to be associated with pure normal offset. The pinching-out nature of small-closed depression marsh deposits would only exhibit such a drastic change in unit thickness if draped over an abrupt pre-existing scarp, or where thinner sections of the unit have been juxtaposed next to thicker sections of the same unit by lateral motion along a vertical fault striking through the marsh deposits.

Although no direct evidence for a sense of lateral motion was observed, the amount of apparent normal offset of “bowl-shaped” marsh stratigraphy was likely enhanced by the lateral juxtaposition of variably thick units during the most-recent rupture event on the newly-recognized Holocene fault crossing the meadow site.

58 Penultimate Event

Little direct evidence exists of a penultimate event. Given the necessity of pre- existing topography for deposition under marsh conditions, it can be inferred that an earlier tectonic event had created a scarp and closed depression facilitating standing water depth and sediment flux conditions consistent with marsh deposition. Differential offset across the fault zone, a terminating fault splay, and distribution of unit-7 suggest a penultimate-event preceding marsh deposition. Additionally, the presence of the mixed zone (unit-6/7) only on the down-dropped side of the fault suggests irregular deposition east of the Holocene fault trace during the initial stages of unit-6 deposition in the latest

Quaternary.

The base of unit-6 is offset ~1.25 m and the tephra is offset ~0.75 m, suggesting

~0.50 m of apparent normal offset prior to the deposition of unit-6. The westernmost splay exposed in the north wall of pit HM-Pit-02 (Figure 12) additionally suggests an event that preceded deposition of unit-6. Although there is no apparent offset of unit-7 along this splay, the gouge present along the splay suggests shearing.

The exact nature of the mixed-zone (unit-6/7) is unknown, but pre-existing fissures within unit-7 may have been filled with overlying grey marsh clay (unit-6).

Auger refusal was encountered in many locations on the upper contact of unit-7 west of the fault zone, suggesting erosional stripping of fine-grained material from unit-7 west of the fault. The mixed zone is inferred to represent colluvium of unit-7 that was shed eastward from the penultimate-event scarp as a colluvial wedge into the lower areas and subsequently draped by unit-6.

59 Changes in water depth and/or sediment flux during deposition of unit-6 are suggested by the organic horizon (unit-6a) within unit-6 as observed in core GC-2

(Figure 10). This horizon represents a change from low-organic, clay-dominant sedimentation to one with organic input. Subsequent increase in water depth or sediment flux could account for the return to clay-dominated sedimentation, but a tectonic origin for this return to clay deposition seems likely.

CONCLUSION

The mid-Quaternary Hirschdale Basalt dam along the Truckee River impounded water and caused upstream deposition of Prosser Creek alluvium in a fluviolacustrine environment that inundated the drainages of Sagehen and Prosser creeks, as well as the topographic pass between them that contains the study area meadow. Fluvial incision re- established drainage to the Great Basin and subaerial exposure of Prosser Creek alluvium fluviolacustrine deposits initiated soil formation.

Subsequent glacial and interglacial intervals deposited till and nested outwash terraces along Sagehen and Prosser creeks. However, the study area meadow remained beyond the limits of ice and evidence for glacial outwash was not observed. Following sufficient soil formation on Prosser Creek alluvium, an offset event (penultimate-event) created a scarp and closed depression in which early-Holocene organic silt/clay marsh deposits accumulated. Deposition of the ~7,000 yr BP Tsoyowata tephra (Mount

Mazama precursor eruption) in the closed depression abruptly terminated organic marsh deposition. Non-organic fine sand and silt were deposited abruptly after tephra deposition.

The fault zone exposed in this study has produced at least one surface-rupture event (most-recent event) in the Holocene, and at least one surface-rupture event

(penultimate-event) in the late Pleistocene. The most-recent event occurred along a near vertical, north-striking fault with overall down-to-the-east motion. However, apparent

60 61 eastward-thrusting of displaced blocks of Holocene silt was observed in the associated half “flower structures” and the fault zone likely has a strike-slip component. The penultimate-event likely occurred along similarly oriented structures. Additional stratigraphic studies of Quaternary age deposits in the drainages of Little Truckee River,

Sagehen Creek, Prosser Creek, and Alder Creek as well as Prosser, Stampede, and Boca reservoirs will greatly increase the ability to conduct neotectonic studies in the Truckee basin.

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70 APPENDIX A

TEXTURAL ANALYSIS RESULTS

Table A-1 summarizes the results of the textural analysis of select horizons in relative stratigraphic order.

Table-A-1. Textural Analysis of Select Horizons

Gravel Sand Silt Clay Lab # Location Depth* Notes Unit (%)** (%)** (%)** (%)** 7-049 HM-Pit-03 51-76 30 18 36 16 colluvium (lt grey) 2 7-056 GC-2 20-46 1 11 50 38 silt (dk grey) 3a 7-051 HM-Pit-03 51-71 0 28 61 11 silt (lt grey) 3b 7-046 HM-Pit-02 61 0 30 54 16 silt (lt grey) 3b 7-054 GC-2 46-61 0 38 50 12 silt (lt grey) 3b 7-058 HM-GC-3 20-43 0 22 50 28 silt (lt grey) 3b 7-059 GC-03 48-64 0 14 52 34 organic silt (brn) 4 7-060 GC-4 46-61 0 22 52 26 organic silt (brn) 4 7-044 HM-Pit-02 114 0 16 58 26 organic silt (dk brn) 4 7-050 HM-Pit-03 91-107 1 12 57 30 organic silt (brn) 4 7-043 HM-Pit-02 91 0 8 54 38 organic silt (brn) 4 7-053 GC-2 66-102 0 8 46 46 organic silt (dk brn) 4 7-062 GC-5 89-114 2 10 45 43 organic silt (dk brn) 4 7-061 GC-4 61-71 0 14 28 58 organic silt (brn-blk) 4 7-045 HM-Pit-02 127 0 8 38 54 silty clay (black) 5 7-055 GC-2 102-119 3 7 38 52 silty clay (black) 5 7-064 HA-08 46-69 0 8 28 64 clay (black) 5 7-052 HM-Pit-03 122-137 0 16 30 54 clay (grey) 6 7-063 GC-5 152-178 0 16 30 54 clay (grey) 6 7-057 GC-2 119-127 0 24 28 48 clay (grey) 6 7-048 HM-Pit-03 140-152 4 57 23 16 silty sand (YR) 7 7-047 HM-Pit-02 114-122 0 60 22 18 silty sand (YR) 7 7-065 HA-03 142-152 0 3 48 49 Silt / clay (brn-grey) 8c * Depth in centimeters below ground surface. ** Percentages by mass

71 APPENDIX B

TEPHRA ANALYSIS RESULTS

Tables B-1 and B-2 summarize the results of the study area tephra glass analysis.

Table B-1. Glass Composition of Study Area Tephra

Oxide Mass Percent*

SiO2 73.70 (1.08) Al2O3 14.46 (0.46) Fe2O3 1.97 (0.21) TiO2 0.35 (0.05) Na2O 4.79 (0.15) K2O 2.81 (0.10) MgO 0.39 (0.11) CaO 1.38 ( 0.30) Cl 0.15 (0.03) Total** 100 Number of shards analyzed 21 Probable Source / Age Tsoyawata (Mt. Mazama precursor eruption) / ~7,000 yr B.P Similarity Coefficient*** 0.97 * Foit, 2007, standard deviation in parentheses ** Analyses normalized to 100 weight percent *** Borchardt et al., 1972, Journal of Sedimentary Petrology, v. 42, p. 301-306

Table B-2. Similarity Coefficients to Study Area Tephra

Probable Correlation** SiO Al O Fe O TiO Na O K O MgO CaO SC* 2 2 3 2 3 2 2 2 (Age) 0.999 0.997 0.990 0.971 0.994 0.993 1.000 0.952 0.987 Tsoyawata (?) 0.999 0.992 0.985 1.000 0.969 0.983 0.974 0.958 0.981 Tsoyawata (?) 0.996 0.992 0.964 0.943 0.996 0.989 0.872 0.971 0.979 Tsoyawata (~7,000 BP?) 1.000 0.972 0.980 0.972 1.000 0.895 0.949 0.986 0.971 Bridge River (2670 BP?) 0.995 0.997 0.975 0.897 0.958 0.996 0.846 0.949 0.970 Tsoyawata bed (~7000 BP) 1.000 0.997 0.985 0.897 0.998 0.996 0.821 0.899 0.970 Tsoyowata tephra bed 0.993 0.997 0.952 0.972 0.939 0.961 0.846 0.993 0.968 Hidden Cave Tsoyawata 0.996 0.982 0.952 0.946 0.939 0.996 0.923 0.957 0.968 Llao Tephra (~7,000 BP) 0.999 0.984 0.949 0.886 0.978 0.983 0.872 0.949 0.966 Rio Dell ash bed (1.45 ma) 0.996 0.982 0.952 0.946 0.939 0.969 0.923 0.957 0.963 Mazama Lower Pumice Fall 0.993 0.949 0.970 0.972 0.969 0.895 0.974 1.000 0.963 Bridge River (2670 BP) 0.994 0.959 0.985 0.897 0.973 0.956 0.951 0.932 0.963 Bridge River (2350 BP?) 0.991 0.990 0.956 0.833 0.921 0.996 0.897 0.949 0.960 Tsoyowata Tephra Bed 0.996 0.962 0.995 0.921 0.950 0.906 0.951 0.939 0.956 Bridge River (2350 BP?) 0.987 0.943 0.919 0.972 0.958 0.962 0.974 0.958 0.956 Bridge River (2350 BP?) * SC: Similarity Coefficient (Borchardt et al., 1972, Journal of Sedimentary Petrology, 42, p. 301-306) **Determined from a total of 1716 records searched (Foit, 2007).