Latest Pleistocene to Holocene River Terrace Deformation

LATEST PLEISTOCENE TO HOLOCENE RIVER TERRACE DEFORMATION

WITHIN THE SOUTHERNMOST EXTENT OF THE LITTLE SALMON FAULT

ZONE; GEOMORPHIC INSIGHTS TO FAULT TERMINATION AND RUPTURE

HISTORY, VAN DUZEN RIVER, NORTHERN CALIFORNIA

Sylvia R. Nicovich

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

Committee Membership

Dr. Mark Hemphill-Haley, Committee Chair

Dr. Andre Lehre, Committee Member

Thomas Leroy, M.Sc., Committee Member

Dr. Melanie Michalak, Committee Member

Dr. Christopher Dugaw, Graduate Coordinator

July 2015

ABSTRACT

LATEST PLEISTOCENE TO HOLOCENE RIVER TERRACE DEFORMATION WITHIN THE SOUTHERNMOST EXTENT OF THE LITTLE SALMON FAULT ZONE; GEOMORPHIC INSIGHTS TO FAULT TERMINATION AND RUPTURE HISTORY, VAN DUZEN RIVER, NORTHERN CALIFORNIA

Sylvia R. Nicovich

The southern Cascadia subduction zone (CSZ) of northwestern California exhibits northeast-directed contraction, transitioning to north-northwest directed translation within the broad San Andreas fault (SAF) transform margin to the south. The Little Salmon fault

(LSF) is one of the southern-most, active thrust faults within the onshore fold and thrust belt of the CSZ, and lies proximal to the transition from compressional to dextral stress across the Mendocino triple junction (MTJ). Thus, it is an ideal location to characterize strain associated with this complex region of transitional stress regimes. High precision topographic data (LiDAR) enabled detailed mapping of geomorphic features otherwise obscured by dense vegetation of the area. The Van Duzen fault (VDF), a northwest trending mole track scarp, sub-parallel and south of the main splay of the LSF is observed on LiDAR imagery. This fault exhibits up-to-the-northeast offset and traverses several

Van Duzen River terrace risers and treads that range from Pleistocene to potentially

Holocene in age. A shallow, exploratory trench was hand-excavated across the VDF. The shallow, roughly 1.5 m-deep, 16 m-long trench exposed imbricated gravels that dip into the base of the trench in the upper end. Coring within the lower end of the trench mapped

the southern extent of the unconsolidated, clast-supported gravel deposit revealing vertical separation of 2.5 m, displaying an up-to-the-northeast step. The linear map expression of the VDF across river terraces of varying elevation and age suggests that the fault may be relatively steeply dipping. Exposed offset bedrock display reverse offset along the VDF, but with no stratigraphic constraint to measure offset. River terraces are some of the youngest geomorphic features within the study area. By constraining a sequence of relative ages for terraces associated with the Van Duzen River, in combination with regional uplift assumed to be equivalent to incision rates, rough terrace ages have been determined. Slip rates calculated from estimated net slip along the VDF and inferred terraces ages range from ~ 0.05 to 0.5 mm/yr. The west-northwest orientation of the compressional faults and folds within the study area suggest SAF- parallel compression, possibly near the transition from transform to compressional tectonic regimes.

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ACKNOWLEDGEMENTS

First and foremost, I would like to thank all of the faculty and staff at Humboldt

State University for making my experience at this fine university transformative, positive, eye opening, and insanely fun. Huge thanks to Dr. Mark Hemphill-Haley for being by geologic sensei through thick and thin, trusting my knowledge, treating me with respect and as a colleague. Infinite thanks to Tom Leroy for formulating the objective of the project and supporting me enthusiastically along the way. Thank you to Humboldt

Redwood Company, specifically Shane Beach, Mike Miles and Jason Butcher, for allowing me the use of their LiDAR data and granting me land access to HRC land.

Thanks to Dr. Melanie Michalak for consistently mentoring me through my research endeavors, my geological thought process and lending treasured life advise. Thanks to

Steve Tillinghast for training me to be the best field geologist I could possibly be in times of steep talus piles and lost bottle openers. Thanks to Laurie Marx for always ridding me of any troubles. Thank you to Dr. Andre Lehre for participating as a member of my graduate committee. Thanks to Dr. Bud Burke for always being available for discussion, impromptu geomorphology lessons, and inspiring me to look at coffee under a new light.

Thanks to Dr. Brandon Schwab for kick-starting my experience with research as a senior undergraduate, for beating the bucket drums in our field band, and for gifting me the best piano of all time, enabling me to learn to play ragtime. Thank you to the digging crew;

Casey “The Mule” Loofbourrow, Michelle “Dusty Damsel” Robinson, Jason “Montana

Sharp Shooter” Padget, Heath “Soils” Sawyer, Maddy “Moo” Schriver, Steven “ Silent

Blisters” Medina, Vanessa “Momma” Davis and Toby “The Fish” Haskett, for lending your weekend, muscles, and sacrificing your cleanliness to my thesis effort. Thank you to

SHN Consulting, specifically Ansen Call, Paul Sundberg, and Jason Buck for alerting me when discovering a bedrock exposure of the VDF and helping around the trench. Thanks to the bore-hole and survey team; Kelsey Conger, Ian Pierce and Lianna Winkler-Prins.

Thanks to the Spring 2014 Neotectonics class for imaging terraces, profiling fault scarps, and enjoying the Van Duzen River with me. Thank you to Jim Falls of the California

Geological Survey for long discussions of the LSF zone and use of the unpublished Owl

Creek Quadrangle. Super thanks to my lovely roommates, Hannah, Jesse, Maia, Maddy,

Lukas and Sloane, that have put up with me through the late nights of work, dirty front door threshold, and ambitious laundry loads aiming to eradicate poison oak. Thanks to my oldest brother, Adam, for always answering the phone with the inquiry, “Are you writing your thesis?” Thanks to all my friends and family, and especially wonderful cat,

Beaverton. Finally, thanks to my stallion, the Black Mamba, as she sacrificed her life to carry me many long miles to the field and back.

TABLE OF CONTENTS

ABSTRACT ...... ii

ACKNOWLEDGEMENTS ...... iv

TABLE OF CONTENTS ...... vi

LIST OF TABLES ...... ix

LIST OF FIGURES ...... x

INTRODUCTION ...... 1

GEOLOGIC SETTING ...... 4

Regional Tectonics ...... 4

Little Salmon Fault ...... 4

San Andreas Fault ...... 6

Stratigraphic, Geomorphic, and Structural Setting ...... 9

Structure and Geomorphology ...... 11

METHODS ...... 13

Geospatial Analysis ...... 13

Faults ...... 13

Bedrock mapping ...... 13

Terrace Mapping ...... 16

Incision and Uplift Rates ...... 21

Field Analysis ...... 21

Van Duzen fault ...... 21

Bedrock Mapping ...... 25

RESULTS ...... 26

Faults within the Study Area ...... 26

Little Salmon fault ...... 26

Van Duzen fault ...... 26

Trench exploration ...... 35

Net slip along Van Duzen fault ...... 37

Terraces ...... 39

Terrace Grouping ...... 39

Tilt of Van Duzen River terraces ...... 42

Geology and Channel Morphology of the Van Duzen River ...... 42

Sinuosity, knickpoints and knickzones ...... 43

DISCUSSION ...... 47

Transform Strain ...... 47

Van Duzen River Terraces ...... 49

Terrace relative ages based on position and soil properties ...... 49

Terrace ages estimated from incision rate and maximum clay percent ...... 50

Tilt of terraces ...... 53

Little Salmon Fault ...... 53

Van Duzen Fault ...... 54

Geomorphic Sequence ...... 56

CONCLUSION ...... 59

REFERENCES ...... 61

APPENDICES ...... 67

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Appendix A ...... 67

Appendix B ...... 76

Appendix C ...... 77

Appendix D ...... 78

Appendix E ...... 84

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LIST OF TABLES

Table 1. Calculated net slip estimates along Van Duzen fault based on measured vertical separation and estimated and measured dip of Little Salmon fault ...... 38 Table 2. Relative ages of river terraces mapped within study area based on topographic relation...... 39 Table 3. Quantitative sinuosity measurements for the Van Duzen River within the confines of the study area...... 45 Table 4. Relative ages of river terraces mapped within study area compared to those mapped by Benner (1983) ...... 49 Table 5. Relative ages and soil properties of terraces mapped within study area. Soil data collected from Benner (1983)...... 50 Table 6. Terrace ages inferred from uplift/incision rates and maximum clay composition...... 51 Table 7. Approximates of incision rates based on max clay % ages...... 53 Table 8. Estimated slip rate range for the Van Duzen fault. This range includes the assumption of scenario 1 (multiple events) for minimum rates and scenario 2 (single event) for maximum rate...... 55

LIST OF FIGURES

Figure 1. Regional tectonic map of northern California showing major faults and folds. Sources of data: Ogle, 1953; Kelsey and Carver, 1988; Aalto et al., 1992; Koehler, 1999; Kelsey, 2001. GBS, Gold Bluff syncline; FHA, Fickle Hill anticline; FS, Freshwater syncline; HHA, Humboldt Hill anticline; THA, Tompkins Hill anticline; ERS, Eel River syncline; GBA, Grizzly Bluffs anticline; AA, Alton anticline; WS, Wolverton syncline; CKSZ, Cooskie shear zone; WGF, Whale Gulch fault. Orange shade represents Neogene sediments. Red box indicates study area...... 7 Figure 2. Plate and block model for region surrounding the Mendocino deformation zone (MDZ) modified from Williams et al (2006). Plate and block boundaries: Wells et al. (1998); Wang et al. (2003). Klamath province from Aalto et al. (1992). Arrows depict direction of block motion with respect to North America. Circular arrow is rotational pole between Oregon Coastal block and Sierra Nevada-Great Valley block (Wang et al., 2003). Star represents study area. HB-Humboldt Bay, CM- Cape Mendocino ...... 8 Figure 3. Stratigraphic column of geology within the study area. Sources of data: Ogle (1953) and McLaughlin (2000)...... 10 Figure 4. Van Duzen River valley morphology indicating meandering morphology to the west and a straight channel to the east...... 12 Figure 5. Study area reference map for method figures. Red squares highlight locations of Figures 6, 7, 8, 9, and 11...... 14 Figure 6. Black arrows point to trace of Van Duzen fault. A) Aspect map derived from DEM clipped to extent of river terraces. Colors represent orientation of pixels within LiDAR data. Pixels are categorized into one of four colors representing aspect of slope; northeast- orange, southeast- yellow, southwest- green, northwest- blue. B) Hillshade of Van Duzen fault. PG-Pamplin Grove, RPR- Riverside Park Road...... 15 Figure 7. Strike and dip measurement process; a) selected points for elevation and geographic constraints used to calculate three-point problem, b) generated measurable plane within these points—a TIN (Triangulated Irregular Network). Color within triangles represent elevation, c) plotted strike and dip measurements...... 16 Figure 8. River terrace mapping methods; a & b) terrace surface selection, c) 4.6 m buffer and ≤2° slope, d) 1.5 m DEM applied to modified terrace surfaces, e) aggregated DEM to 30.5 m f) points representing cell information...... 18 Figure 9. Terrace data points. Each point contains elevation and the distance of the most proximal valley center point. The valley center is marked by a dense series of points (appears as black line above)...... 19

Figure 10. Plotted valley center distance versus elevation data for interpretation of terrace flights...... 20 Figure 11. Trench location along the Van Duzen fault. a) World Imagery, source: ESRI b) hillshade. Trench location in red...... 22 Figure 12. Trench site, process, and workers. From top down; Mark Hemphill-Haley, Steve Medina, Michelle Robinson and Casey Loofbourrow begin to dig, Sylvia Nicovich, Casey Loofbourrow and Maddy Schriver hard at work, Michelle Robinson cleaning tools by finished trench...... 23 Figure 13. Section of mosaic image of northwest wall of trench. Grid is 0.5 meter high and 1 meter long...... 24 Figure 14. Geology of study area...... 27 Figure 15. Cross section from A to A’ of geologic map (Figure 14)...... 28 Figure 16. Study area reference map for results figures. Yellow boxes indicate area of Figures 17, 22 and 28...... 29 Figure 17. Location of profile transects 1, 2 and 3 and confluence of Heley Creek and the Van Duzen River, where bedrock deformation is exposed. Here, the Van Duzen fault scarp profiles are aligned at 0 m (elevation) to display relative vertical separation. Dotted lines project to relative elevation (measured vertical separation). Note the VDF is has different amounts of displacement as it traverses terraces of apparent different age, greater displacement on relatively older terraces, suggesting potential long-term displacement history. Figure 16 for location...... 30 Figure 18. Profile 1; topographic profile across Van Duzen fault along terrace T3...... 31 Figure 19. Profile 2; topographic profile across Van Duzen fault along terrace T4...... 32 Figure 20. Heavy vegetation atop terrace surface T8 conceals any sign of Van Duzen fault scarp. Figure 17 for VDF across T8...... 32 Figure 21. Profile 3; topographic profile across Van Duzen fault along terrace T8...... 33 Figure 22. Outcrop map and geologic map of the Van Duzen fault exposure at the confluence of Heley Creek and the Van Duzen River. Star on geologic map represent fault exposure. Figure 16 for location...... 34 Figure 23. Stitched photograph (a) and interpretation (b) are of the western trench wall. The trench was hand excavated along the lower deformed terraces (T4) of the Van Duzen fault scarp (location in methods). The subsurface contact between the lower gravel unit (Qg) and the overlying sandy silt (Qss1) was mapped using boreholes...... 36 Figure 24. Plot of terrace elevation data against valley centerline. Colors represent interpreted terrace flights that correspond to terrace map (Figure 25)...... 40 Figure 25. Map of interpreted terrace flights T1-T10. T10 is first to form (oldest) and T1 last to form (youngest). Tu (undifferentiated terraces) represents terraces that cannot be correlated to flights downstream...... 41 Figure 26. Stereonet plot of Van Duzen River associated terrace surface poles to planes. The stereonet plot displays all terraces are essentially flat lying...... 42

Figure 27. Stereonet plot of bedding orientation poles to planes of Wildcat Group sediments within study area. Bedding represents limbs of Wolverton syncline. Axis of fold is 12N63W. See Appendix E for data...... 43 Figure 28. Map of Chalk Mountain Landslide Complex. Numbers indicate terrace relative age; refer to Figures 23 and 24. For geologic key, refer to Figure 14. Figure 16 for location...... 44 Figure 29. Long profile of Van Duzen River plotted against lithologic contacts, faults, and sinuosity values. Chalk Mountain Landslide Complex depicted in yellow. .. 45 Figure 30. Long profiles of Van Duzen River and associated tributaries. Stars indicate intersection of mapped LSF along tributaries...... 46 Figure 31. Simplified block diagram modified from Leroy (2012) of the Mendocino triple junction area displaying the change in orientation of features subject to San Andreas fault-parallel compression from north-northwest to west-northwest. CSZ- Cascadia subduction zone, LSF-Little Salmon fault, VDF-Van Duzen fault, WSC- Wolverton syncline, RCf- faults of Root Creek (Oswald et al., 2006), SAF- San Andreas fault, MAF- Maacama fault, BSFZ- Barttlet Springs fault zone...... 48 Figure 32. Simplified terrace block diagram of scenarios 1) multiple events along VDF between formation of apparently different age terraces and 2) a single event along the VDF, where deposition accounts for increase in scarp height with elevation and along relatively older terraces...... 56

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INTRODUCTION

The Little Salmon fault (LSF) is a northwest trending, northeast dipping thrust fault located at the southern extent of the on-shore fold and thrust belt associated with the

Cascadia subduction zone (CSZ) (Woodward-Clyde Consultants, 1980; Kelsey and

Carver, 1988; Clarke and Carver, 1992; Carver and Burke, 1992; Hemphill-Haley and

Witter, 2006). The San Andreas fault (SAF) zone meets the southern extent of the CSZ at the Mendocino triple junction (MTJ) (Figure 1). Previous studies (Kelsey and Carver,

1988; Williams, 2006; Oswald et al., 2006; Leroy, 2007; Leroy, 2012) suggest the presence of San Andreas type deformation migrating north-northeast past the northern most mapped extent of the SAF zone.

The transition between these two tectonic regimes remains enigmatic (Kelsey and

Carver, 1988; Williams et al., 2006). However, accommodation of northward translational deformation associated with the SAF (Figure 1) has been recognized to include relatively east-west tending folds in the Eel River Valley and oblique slip on the

LSF and Mad River fault zones (Williams et al., 2006). This deformation also includes distributed right lateral shear strain to the east and northeast via the Garberville and

Maacama fault zone of the Bartlett Springs/ Lake Mountain fault zone (Figure 1)

(Williams et al., 2006).

The study area encompasses the southeastern terminus of the mapped LSF (Ogle,

1953; Irwin, 1960; Kelsey and Carver, 1988; McLaughlin, 2000; Kelsey, 2001). Here, the

strike of the LSF shifts from northwest to west-northwest (Figure 1). Oswald et al.

(2006), interpret features near the confluence of the Van Duzen River and Root Creek, within the study area, to represent dextral transpression. Oswald et al. (2006) and Leroy

(2007) proposed that these faults define a portion of a broad zone of deformation, the

Mendocino Deformation Zone (MDZ) of Williams et al. (2006) at the southeastern end of the LSF linking CSZ related contraction to the northerly migrating dextral shear of the

SAF zone (Figure 2).

The study area lies within the Mendocino Deformation Zone as defined by

Williams et al. (2006), where the orientation of the LSF zone expresses contraction parallel to the trend of the San Andreas transform margin (Williams et al., 2006) as opposed to north of the Eel River valley, which expresses contraction orthogonal to CSZ

(Figures 1 and 2). Previous studies (Oswald et al., 2006; Leroy, 2007; Leroy et al., 2012) have observed possible lateral shear within the study site and speculate SAF zone influence.

An apparent vertically dipping fault, deforming a series of young, possibly

Holocene, river terraces south of the main splay of the LSF, was identified using high- resolution topographic data. A trench was excavated across the scarp of the Van Duzen fault (informally named herein), in order to test whether this displacement was associated with compressional, transform or transpressional strain. The underpinning theme of my research is to map an area defined to be within the zone of tectonic transition from a regime of compressional strain to that of transform strain. How does transform strain transition to a zone of compression? How can the landscape illuminate the mechanics of

this transition? To achieve this prevailing goal and answer these fundamental questions, the main objectives in this study are 1) to refine mapping of southern extent of the LSF,

2) to map and classify characteristics of newly identified faults within the study area and,

3) to map in detail, the Tertiary Wildcat Group sediments.

GEOLOGIC SETTING

Regional Tectonics

Late Neogene and Quaternary crustal deformation of the Pacific Northwest can be simplified into two distinct stress regimes (Kelsey and Carver, 1988). Forearc compression characterized by thrust faults and folds associated with the subducting

Gorda and Juan de Fuca oceanic plates beneath the North American plate along the

Cascadia subduction zone, and dextral shear due to the northward migration of the Pacific plate along the transform margin of the San Andreas fault (SAF) zone (Kelsey and

Carver, 1988; Clark and Carver, 1992; Williams et al., 2006). Although the SAF terminates at the Mendocino triple junction (MTJ) (Carver, 1985; Carver, 1987; Kelsey and Carver, 1988; Carver and Burke, 1992), associated strain propagates tens of kilometers northward into the forearc, accommodated by right-lateral strike-slip faults east of the SAF, such as the Garberville/Maacama fault zones and the Lake

Mountain/Bartlett Springs fault zones (Kelsey and Carver, 1988; Williams et al., 2006).

Little Salmon Fault

The Little Salmon fault (LSF) zone is the major feature accommodating northeast-southwest shortening in the southern fold and thrust belt (Ogle, 1953; Carver and Burke, 1987; Carver and Burke, 1988; Kelsey and Carver, 1988; Carver and Burke,

1992). The LSF is a northwest trending, northeast dipping thrust fault with a dip of approximately 20-30° (Woodward-Clyde Consultants, 1980; Carver and Burke, 1992,

Vadurro et a., 2006). The LSF trends offshore, paralleling the CSZ (Clarke, 1992; Carver and Burke, 1992), extends to southern Humboldt Bay and continues inland ~ 40 km to the study area west of the town of Bridgeville (Carver and Burke, 1992) (Figure 1).

Onshore, the LSF zone comprises a series of reverse faults and folds that include the Table Bluff fault, Goose Lake fault, the Yager fault, the VDF, Humboldt Hill anticline, Table Bluff anticline, Grizzly Bluffs anticline, Alton anticline, Eel River syncline, and the Wolverine syncline (Figure 1) (Ogle, 1953; Woodward-Clyde

Consultants, 1980; Kelsey and Carver, 1988; Carver and Burke, 1992; Witter and Patton,

2006).

Holocene slip rates along the LSF vary geographically, dividing the fault zone in to east and west traces (Clarke and Carver, 1992). During the last 1700 years, the LSF zone has produced 3 major surface displacements with a dip slip of 3.6 to 4.5 meters per event on the west trace and about 1 to 2 meters per event on the east trace (Carver and

Burke, 1988; Clarke and Carver 1992). Holocene slip rates range from 6 to 12 mm/yr

(approximately 7mm/yr along west race and 2 to 3 mm/yr along east trace) and are suggested to be similar to long term averages since the deposition of upper Wildcat

Group sediments and late Pleistocene marine terraces (Clarke and Carver, 1992; Carver and Burke, 1992). Through trenching investigations from Woodward-Clyde Consultants

(1980), cumulative Quaternary vertical displacement along the LSF zone has been characterized to be approximately 2 km with a minimum net displacement of 4.3 km.

San Andreas Fault

Although the SAF terminates just south of the Mendocino triple junction (Carver,

1985; Carver, 1987; Kelsey and Carver, 1988; Carver and Burke, 1992), GPS derived velocity data presented by Williams et al. (2006) indicates lateral shear strain north of the latitude of Cape Mendocino that propagates as far as north as Humboldt Bay (Figure 2).

Consequently, the northern extent of San Andreas-related shear strain may extend beyond its offshore fault expression as distributed shear (Williams et al., 2006). The accommodation of this strain may be evident within the southeastern extent of the LSF zone, specifically the Van Duzen fault, or other west-northwest oriented contractional features near the study area.

Figure 2. Plate and block model for region surrounding the Mendocino deformation zone (MDZ) modified from Williams et al (2006). Plate and block boundaries: Wells et al. (1998); Wang et al. (2003). Klamath province from Aalto et al. (1992). Arrows depict direction of block motion with respect to North America. Circular arrow is rotational pole between Oregon Coastal block and Sierra Nevada-Great Valley block (Wang et al., 2003). Star represents study area. HB-Humboldt Bay, CM-Cape Mendocino 9

Stratigraphic, Geomorphic, and Structural Setting

The regional stratigraphy of the study area consists of marine and non-marine sediments of the Paleocene to late Eocene Yager terrane, draped by and thrust over the

Neogene Wildcat Group (Figure 3). The oldest bedrock unit in the study area is the Yager terrane of early Paleogene (Paleocene to Eocene) age (McLaughlin et al., 1997;

McLaughlin, 2000). The Yager terrane is a pervasively faulted and folded trench-slope deposit, principally composed of turbidites made up of argilliceous sandstone and conglomerate (McLaughlin, 2000). Neogene Wildcat Group sediments of the Eel River

Basin are in depositional and fault contact with the Yager terrane along the Yager fault of the LSF zone (McLaughlin, 2000, Ogle, 1953). Wildcat Group sediments consist of marine and non-marine, late Miocene to early Pleistocene strata, predominantly composed of mudstone, siltstone, sandstone, claystone, and conglomerate (Ogle, 1953).

The Wildcat group is composed of five formations as defined by Ogle (1953), only two of which are differentiated in the study site; the Plio-Pleistocene Carlotta Formation and

Scotia Bluffs Sandstone, and three undifferentiated units; the mid-late Pliocene Rio Dell, early Pliocene Eel River, and Mio-Pliocene Pullen formations, which remain mapped as undifferentiated Wildcat Group (Figure 3). 10

Figure 3. Stratigraphic column of geology within the study area. Sources of data: Ogle (1953) and McLaughlin (2000). 11

Late Quaternary river terraces flank the Van Duzen River within the study area.

Terraces of similar morphology with fault scarps up to 7 meters have been mapped along the Mad River and Jacoby Creek by Carver (1985) and age estimates of Holocene to

Pleistocene were made using soil development. Koehler (1999) mapped flights of Van

Duzen River terraces along reaches approximately 40 km southeast of the study site and produced similar ages based on uplift, incision, and soil morphology. These terraces consist of bedrock strath overlain by channel deposits of cobbles in a sandy and gravel matrix, topped with overbank deposits with laminated layers of sand with gravel

(Koehler, 1999).

Structure and Geomorphology

Compressional, transform, or transpressional strain may be evident in features beyond fault scarps (McCaplin, 1996, McCrory, 2000). Orientation of folds, faults, topographic lineaments, landslides and stream morphology might also lend insight to which type of strain is influencing the landscape.

The Wolverton syncline and the Alton anticline are the principal fold structures associated with the Little Salmon fault zone previously mapped to the west of the study area (Figure 1) (Ogle, 1953).

The morphology of Van Duzen River channel changes from a narrow, linear canyon in the eastern portion of the study area to a meandering stream in the western portion of the valley (Figure 4). This change in channel morphology coincides with a change in lithology, from the more resistant Yager terrane bedrock in the east to erosive

Wildcat Group sediments to the west in addition to the impingement of the Chalk 12

Mountain landslide complex (Figure 4) (Oswald, 2003). The Chalk Mountain landslide complex is a roughly 2.5 km², inactive, deep-seated, translational/rotational rockslide complex within the Yager terrane (Oswald, 2003; Oswald et al., 2006).

Figure 4. Van Duzen River valley morphology indicating meandering morphology to the west and a straight channel to the east.

METHODS

Geospatial Analysis

Airborne LiDAR (Light Detection and Ranging) data encompassing the study area were provided by Humboldt Redwood Company for this investigation. The Digital

Elevation Model (DEM) data has centimeter-scale precision for every 1.5-square-meter- cell. The DEM is derived from the last return of the light reflected off the ground surface, removing vegetation and housing and producing bare earth terrain data.

Faults

Preliminary topographic lineaments were mapped by geomorphic interpretation of

Geographic Information Systems (GIS) generated hillshades, slope maps, and topographic profiles from the LiDAR data (Figures 5 and 6). For lineaments to be considered faults, they must meet the following criteria; 1) accompany presence of a scarp, 2) offset geologic units or structure, 3) display vertical or lateral separation (Kelsey and Carver, 1988).

Bedrock mapping

I created an automated routine, using ArcGIS model builder, to estimate bedding attitudes of folded Neogene Wildcat Group sediments within the study area. Details of this routine are listed in Appendix A. The model uses the techniques of the classic three-

point problem (e.g., Vacher, 2000; Hasbargen, 2012). Three points on the same bedding plane of the sediments allowed me to create a planar datum to be measured within

ArcGIS (Figures 5, 7a). The computer then interpolates a plane from the selected spatial data, calculates the strike and dip of the plane, and plots the appropriate symbol and dip angle on the map (Figures 7b and 7c). A large fold of the study area, the Wolverton syncline, is plotted based on this routine.

Figure 5. Study area reference map for method figures. Red squares highlight locations of Figures 6, 7, 8, 9, and 11. 15

Figure 6. Black arrows point to trace of Van Duzen fault. A) Aspect map derived from DEM clipped to extent of river terraces. Colors represent orientation of pixels within LiDAR data. Pixels are categorized into one of four colors representing aspect of slope; northeast- orange, southeast- yellow, southwest- green, northwest- blue. B) Hillshade of Van Duzen fault. PG-Pamplin Grove, RPR- Riverside Park Road.

Figure 7. Strike and dip measurement process; a) selected points for elevation and geographic constraints used to calculate three-point problem, b) generated measurable plane within these points—a TIN (Triangulated Irregular Network). Color within triangles represent elevation, c) plotted strike and dip measurements.

Terrace Mapping

River terraces were mapped using LiDAR-derived hillshades, slope maps, and topographic profiles to delineate surfaces. Initially, flat surfaces (Figure 5, 8a and 8b) 17

were identified by bounding terrace risers (Jones et al., 2007). These surfaces were mapped as polygons from geospatial analysis and selectively field verified.

In order to group terrace surfaces in to flights of relative ages, the elevation data was extrapolated and plotted versus valley center length for interpretation. In order to obtain representative data, a 4.6-meter buffer was first applied to each of the mapped terrace polygons to ensure that eroded edges were not considered for elevation calculations. A slope map was generated to areas with a slope greater than 2°. This selection made it easier to identify nearly flat terrace surfaces. The product of these steps is a set of polygons that represent areas of potential river terraces, 4.6 meters from the edges, with a 2° slope or less (Figure 8c).

Next, the DEM was clipped to the extent of the modified polygons representing river terrace surfaces sufficient for measuring elevation. The resolution of the DEM was reduced by a factor of 20—from 1.5 meters to 30.5 meters, reducing 400 pixels to 1. Each pixel was then translated into a series of points containing spatial data (one point representing each cell). From this extrapolation, 7,626 points were derived from 240 river terrace surfaces (Figure 8d, 8e, and 8f). 18

Figure 8. River terrace mapping methods; a & b) terrace surface selection, c) 4.6 m buffer and ≤2° slope, d) 1.5 m DEM applied to modified terrace surfaces, e) aggregated DEM to 30.5 m f) points representing cell information.

The interpreted midpoint between oldest terraces, valley centerline, was assigned

10,242 points, one every 1.5 meters (Figures 5 and 9). A proximity analysis was run to determine the closest distance along the valley centerline for each of the terrace points

(Figure 10).

Figure 9. Terrace data points. Each point contains elevation and the distance of the most proximal valley center point. The valley center is marked by a dense series of points (appears as black line above).

The three-point problem previously described was used to measure the orientation and angle of the mapped terrace surfaces in order to analyze any tilt. 20

Figure 10. Plotted valley center distance versus elevation data for interpretation of terrace flights. 21

Three points were selected on each of the interpreted terrace surfaces and applied to the model.

Incision and Uplift Rates

Elevation data above the current river channel were collected for select terraces to calculate incision and uplift rates. Seven sites were used for analysis on three separate terrace heights (Appendix B). Terraces were measured using topographic profiles and elevation point data derived from the 1.5 meter DEM.

To measure average terrace flight elevation above the current river channel, the data derived to plot terraces was normalized to the thalweg. For each flight, the elevation of the Van Duzen River thalweg most proximal the valley center line was subtracted from corresponding terrace data most proximal to the valley centerline.

Field Analysis

Van Duzen fault

The Van Duzen fault was trenched to investigate its sense of surface deformation.

The location of the trench was picked perpendicular to the fault scarp in the most accessible area with least vegetation (Figures 5 and 11).

Figure 11. Trench location along the Van Duzen fault. a) World Imagery, source: ESRI b) hillshade. Trench location in red. Because of limited access and protected vegetation, heavy equipment use was not permitted. A roughly 1.5 m-deep, 16 m-long trench was excavated by hand (Figure 12).

To map sedimentological contacts that exceeded the depth of 1.5 m, holes were bored every meter until the contact was reached. Where contacts changed in vertical position, bore hole were excavated every 0.5 m to achieve a more accurate idea of their expression. 23

Figure 12. Trench site, process, and workers. From top down; Mark Hemphill-Haley, Steve Medina, Michelle Robinson and Casey Loofbourrow begin to dig, Sylvia Nicovich, Casey Loofbourrow and Maddy Schriver hard at work, Michelle Robinson cleaning tools by finished trench. 24

The trench was gridded; vertically every half meter and horizontally every meter.

The trench was then photographed in over one thousand segments. The photographs were

stitched together for logging using photogrammetric techniques (Figure 13). The units of

the west wall of the trench were primarily mapped out using nails to mark stratigraphy

and secondarily logged onto the stitched photograph.

Figure 13. Section of mosaic image of northwest wall of trench. Grid is 0.5 meter high and 1 meter long. 25

Bedrock Mapping

Bedding attitudes derived from the geospatial model were confirmed where exposed bedrock displayed suitable bedding planes, mainly on the rivers edge. Fluvial exposures were also used to confirm geologic mapping, along with logging road cuts and contacts along Highway 36. 26

RESULTS

Faults within the Study Area

Little Salmon fault

The Little Salmon fault (LSF) is expressed within the study area has been mapped as a series of northwest-trending splays that span an apparent width of about 6 kilometers

(Figures 14 and 15). The northern-most splay of the LSF zone within the study area is the

Yager fault of Ogle (1953). It defines the northern contact between the Paleogene Yager terrane and the Neogene Wildcat Group sediments (Figures 14 and 15). Approximately 6 km to the southwest of the Yager splay is the Van Duzen fault, marking the most southwestern surface expression of LSF zone. The VDF and the faults of Root Creek as mapped by Oswald et al. (2006) bound the southern extent of the LSF zone within the study area.

Van Duzen fault

The Van Duzen fault is a northwest trending, northeast dipping reverse fault associated with the Little Salmon fault zone (Figures 14 and 15).

A’

Figure 14. Geology of study area.

Figure 15. Cross section from A to A’ of geologic map (Figure 14).

Figure 16. Study area reference map for results figures. Yellow boxes indicate area of Figures 17, 22 and 28.

The VDF can be traced using LiDAR imagery and partially on foot from its northwestern extent—west of Riverside Park Road—to its southeastern extent, approximately at the entrance of Pamplin Grove County Park (Figure 6). The VDF deforms terraces of varying ages and elevations (Figure 17).

Figure 17. Location of profile transects 1, 2 and 3 and confluence of Heley Creek and the Van Duzen River, where bedrock deformation is exposed. Here, the Van Duzen fault scarp profiles are aligned at 0 m (elevation) to display relative vertical separation. Dotted lines project to relative elevation (measured vertical separation). Note the VDF is has different amounts of displacement as it traverses terraces of apparent different age, greater displacement on relatively older terraces, suggesting potential long-term displacement history. Figure 16 for location.

Where present on deformed fluvial terraces, the VDF displays linear mole track features.

The VDF offsets Quaternary Carlotta formation of the Wildcat Group sediments, visible

in an exposure located at the confluence of Heley Creek and the Van Duzen River (A.

Call and P. Sundberg, personal communication, April 2015) (Figure 17). At this

exposure of the VDF, alluvium atop of the faulted bedrock is not deformed.

The VDF expresses a mole track feature at the logging road off of Riverside Park

Road that vertically separates T3 with a sharp scarp, approximately 1.75 m at an elevation of roughly 65 m above mean sea level (Figures 6, 17 and 18).

Figure 18. Profile 1; topographic profile across Van Duzen fault along terrace T3.

Traversing the VDF to the southeast, a mole track scarp with roughly 2 meters of vertical separation deforms T4 at the trench site—just east of highway 36 and Riverside

Park Road (Figures 6, 17 and 19). Further to the southeast near Pamplin Grove County

Park (Figure 6), the VDF is visible only on geospatial data due to immense vegetation

(Figure 20). Here, T8 displays vertical separation of 2.75 meters (Figures 17 and 21). The broad scarp across T8 displays the most slope diffusion of the three measured topographic profiles. 32

Figure 19. Profile 2; topographic profile across Van Duzen fault along terrace T4.

Figure 20. Heavy vegetation atop terrace surface T8 conceals any sign of Van Duzen fault scarp. Figure 17 for VDF across T8.

Figure 21. Profile 3; topographic profile across Van Duzen fault along terrace T8.

All three topographic profiles display a depression in elevation on the backside (northeast side) of the fault scarp.

The exposure of the VDF at the confluence of Heley Creek and the Van Duzen

River is a roughly 3*4.5 meter outcrop of deformed Carlotta FormationThe conglomerate member of the Carlotta Formation (Ogle, 1953) is in fault contact with the muddy sandstone member below (Figure 22). 34

Figure 22. Outcrop map and geologic map of the Van Duzen fault exposure at the confluence of Heley Creek and the Van Duzen River. Star on geologic map represent fault exposure. Figure 16 for location. 35

The VDF exposed at Heley Creek expresses a set of two faults of different offsets.

The prevailing fault with major offset is oriented N86W with a dip of 36° to the northeast

(Figure 22). There are no stratigraphic markers to measure the amount of slip on this fault. The subsidiary fault cuts the immediate footwall at an apparent dip of 51° to the northeast and measurable apparent dip slip of 5 centimeters (Figure 22). There is no surficial fault expression in this area and no folded young sediments atop the deformed strath exposure (Figure 6).

Trench exploration

The trench exposes a series of fine-grained sediments interpreted to be overbank deposits associated with the Van Duzen River (Figure 23b, Units Qss3 and Qss2). These densely compacted sandy silts overlie a section of very dense sandy silt atop poorly sorted, clast-supported, sub- to well-rounded coarse alluvial gravels (Figure 23b, Units

Qss1 and Qg).

Figure 23. Stitched photograph (a) and interpretation (b) are of the western trench wall. The trench was hand excavated along the lower deformed terraces (T4) of the Van Duzen fault scarp (location in methods). The subsurface contact between the lower gravel unit (Qg) and the overlying sandy silt (Qss1) was mapped using boreholes.

In the northern side of the trench, the coarse gravels and dense sandy silt (Qss1 and Qg) are notably deformed, folding toward the southwest where the gravel deposit dives into the base of the trench at an angle of approximately 50° over a distance of approximately 2.5 meters (Figure 23b). The vertical separation of the Qg-Qss1 contact is approximately 2.5 meters. The overlying overbank deposits (Qss3 and Qss2) onlap the folded sediments.

Net slip along Van Duzen fault

Net slip of the Van Duzen fault was determined using estimated and measured dip of the LSF (Woodward-Clyde Consultants, 1980; Carver and Burke, 1988; Hemphill-

Haley and Witter, 2006) and measured dip of the VDF from the deformed strath exposure at Heley Creek (Figure 22). Using LSF dip to calculate VDF slip is based on the reasonable assumption that the VDF manifests a similar dip to the LSF. Net slip estimates derived from measured vertical separation of Van Duzen River terrace and displacement of river sediments in combination with assumed dips are shown in Table 1.

The maximum slip along the VDF is estimated to be 5.5 meters, using an estimated dip of 30° and maximum vertical separation of 2.75 meters (Table 1). The minimum dip slip is calculated to be approximately 2.0 meters using a dip of 60° and a minimum vertical separation of 1.75 meters. The most reasonable range of assumed dip for the VDF is from 30° to 60° based on; 1) the most proximal measurement of the LSF dip is 45° and 60° from Hemphill-Haley and Witter (2006); and 2) measured dip of 36° at 38

Heley Creek strath exposure. Using this dip range, it is reasonable to infer that the net slip along the VDF is within the range of 2 to 5.5 meters.

Table 1. Calculated net slip estimates along Van Duzen fault based on measured vertical separation and estimated and measured dip of Little Salmon fault

Vertical separation of river terrace surfaces and Estimated dip of fault Calculated cumulative dip displacement of river (degrees)β slip on fault (m) sediments (m)α

1.75 60 2.0 1.75 45 2.5 1.75 30 3.5

2 60 2.3 2 45 2.8 2 30 4.0

2.5 60 2.9 2.5 45 3.5 2.5 30 5.0

2.75 60 3.2 2.75 45 3.9 2.75 30 5.5

α Vertical separation measured from topographic profiles. Displacement of river deposits (2.5 m) measured from trench exposure.

β Dip measurements and estimates from Woodward-Clyde Consultants (1980), Carver and Burke (1988) and Hemphill-Haley and Witter (2006). 39

Terraces

Previous mapping of terraces associated with the Van Duzen River have not been extensively differentiated in detailed flights of relative age within the study area (Ogle,

1953; Woodward-Clyde Consultants, 1980; Benner, 1983, O’Dea, 1992). In this study,

Quaternary terraces were mapped in detail by topographic relation. These differentiated terraces were then used as a basis for constraining rupture history and determining local deformation (Figures 24 and 25).

Terrace Grouping

Terraces were grouped based on their topographic relation in longitudinal valley profile

(Figure 24). Terrace ages were assigned relatively by topographic position. The Van

Duzen River terraces were split into ten groups of terrace flights, increasing in relative age with elevation. T1 was assigned to the youngest terraces and T10 to the oldest terraces (Table 2). Some terraces (Tu) could not be differentiated (Figures 24 and 25).

Table 2. Relative ages of river terraces mapped within study area based on topographic relation.

Relative Ages Nicovich Terrace

Younger T1 T2 T3 T4 T4 T5 T5 T6 T7 Tu (?) T8 T9 Older T10

274.5

244

213.5

183

) 152.5

(

o i

t 122

e l E 91.5

30.5

0 0 3048 6096 9144 12192 15240 Distance (m)

Figure 24. Plot of terrace elevation data against valley centerline. Colors represent interpreted terrace flights that correspond to terrace map (Figure 25).

Figure 25. Map of interpreted terrace flights T1-T10. T10 is first to form (oldest) and T1 last to form (youngest). Tu (undifferentiated terraces) represents terraces that cannot be correlated to flights downstream.

Tilt of Van Duzen River terraces

The terraces associated with the Van Duzen River (Figure 25) are the youngest measurable meso-scale feature that encompasses the study area. Measurements of the orientation of terrace surfaces are plotted in Figure 26. Terraces do not display tilt greater than 2°.

Figure 26. Stereonet plot of Van Duzen River associated terrace surface poles to planes. The stereonet plot displays all terraces are essentially flat lying.

Geology and Channel Morphology of the Van Duzen River

The primary structure of the study area is the Wolverton syncline of Ogle (1953).

The Wolverton syncline deforms Neogene sediments of the Wildcat Group. The axis of the Wolverton syncline, plunges 12° to 64° west of north and has an overturned northern 43

limb in the northwestern section of the study area along the LSF (Figures 14, 15, and 27).

The geometry of the syncline is symmetrical with both limbs gently dipping towards the axial plane.

Sinuosity, knickpoints and knickzones

The sinuosity of the Van Duzen River changes from a straight, confined river with steep canyon walls in the northeastern reaches of the study area, to a sinuous channel with a broad valley to the southwest (downstream). This change in sinuosity is coincident with; 1) the toe of the Chalk Mountain Landslide complex (Figure 28), 2) a change in

lithology from Yager terrane to Wildcat Group, 3) faults of Root Creek and 4) a prominent knickzone in the long profile of the Van Duzen River (Table 3 and Figure 29).

Figure 28. Map of Chalk Mountain Landslide Complex. Numbers indicate terrace relative age; refer to Figures 23 and 24. For geologic key, refer to Figure 14. Figure 16 for location.

Knickpoints and knickzones along Van Duzen River tributaries, including those conterminous with the LSF, are shown in Figure 30.

Figure 29. Long profile of Van Duzen River plotted against lithologic contacts, faults, and sinuosity values. Chalk Mountain Landslide Complex depicted in yellow.

Table 3. Quantitative sinuosity measurements for the Van Duzen River within the confines of the study area.

Channel Valley Length (m) Sinuosity (CL/VL) Length (m)

Downstream 10180 20515 2.02 of LS

Upstream of 5429 6300 1.16 LS

Total 15609 26815 1.72 46

Figure 30. Long profiles of Van Duzen River and associated tributaries. Stars indicate intersection of mapped LSF along tributaries.

DISCUSSION

Transform Strain

Evidence for northward migration of the MTJ, through transform strain associated with San Andreas style deformation, is interpreted to reach as far north as Humboldt Bay

(Figure 2) (Kelsey and Carver, 1988; Williams, 2006). Oswald et al. (2006) propose that dextral strain is evident in faults that deform a strath formed in Wildcat Group sediments at the confluence of Root Creek and the Van Duzen River (Figure 14). Geological evidence for dextral transpression can explain strain unaccounted for in geodetic data

(Williams, 2006).

Geological evidence for dextral transpression along faults within the study area has not been observed beyond reported faults at Root Creek (Figure 14) mapped by

Oswald et al. (2006). However, evidence for SAF style deformation within the vicinity of the study area may be evident by SAF-parallel shortening expressed as contractional strain, accommodated by relatively east-west trending structures (McCrory, 2000;

Williams et al, 2006). These include; the comparatively east-west trend of the LSF at its southeastern extent, the VDF, and the axis of the Wolverton syncline. The west- northwest orientation of these compressional features deviate from the predominant north-northwest orientation of thrust faults and folds associated with the Cascadia subduction zone (Figures 1 and 14). I surmise that this is a product of the northward migration of the MTJ, and therefor the SAF system impinging upon the fold and thrust 48

belt. Although the features mapped within the study area stems from compressional stresses, their orientation gives evidence for possible SAF-parallel compression and not purely subduction related strain (Figure 31).

Figure 31. Simplified block diagram modified from Leroy (2012) of the Mendocino triple junction area displaying the change in orientation of features subject to San Andreas fault-parallel compression from north-northwest to west-northwest. CSZ-Cascadia subduction zone, LSF-Little Salmon fault, VDF-Van Duzen fault, WSC- Wolverton syncline, RCf- faults of Root Creek (Oswald et al., 2006), SAF- San Andreas fault, MAF- Maacama fault, BSFZ- Barttlet Springs fault zone. 49

Van Duzen River Terraces

River Terraces mapped by topographic relations (Figures 24 and 25) were used to constrain relative ages and infer numerical ages of tectonically influenced deformation, depositional features, and events of mass wasting within the study area.

Terrace relative ages based on position and soil properties

Terrace ages were assigned relatively and inferred numerically by topographic position and pedogenesis from properties described by Benner (1983). Four of the ten flights of river terraces, T2-T5 (Figures 22 and 23) were compared against data from

Benner (1983, unpublished senior thesis) who mapped and analyzed soil properties of this portion of the Van Duzen River and associated terraces within the study area

(Appendix C and D). The relative ages Benner (1983) concluded agree with relative ages herein however, are differentiated to greater detail (Table 4).

Table 4. Relative ages of river terraces mapped within study area compared to those mapped by Benner (1983)

Relative Ages Nicovich Terrace Benner (1983) Terrace

Younger T2 B-1, C-1, E-1

D-2

C-3

T3 B-3

T4 A-3

Older T5 E-4

Terrace ages estimated from incision rate and maximum clay percent

Data collected by Benner (1983) were used to determine inferred numerical ages

using maximum clay percent as a function of time (Merritts, 1991). For this method, it is

assumed that soil development begins after terrace abandonment and that the soils of the

Van Duzen River terrace deposits develop at similar rates to coastal northern California

soils (Table 5).

Table 5. Relative ages and soil properties of terraces mapped within study area. Soil data collected from Benner (1983).

Estimated Elevation Max caly Max Relative Nicovich Pit Elevation Depth Max age (ka) above % and dry ages Terrace locationα (m)β (cm)δ textureδ based on river (m)γ horizonδ colorδ clay %ε

23 - Younger T2 B-1 78 5 7.3 - C2 5Y 4/1 SL 102+ 4 C-1 76 6 8.4 - C1 0 - 25 5Y 5/2 SL 4 5Y E-1 70 9 13.7 - C1 0 - 18 SL 5.5/3 9

10YR D-2 74 7 18.5 - A 0 - 16 L 4.5/2 17 19.3 - 23 - 10YR C-3 79 9 L A/B 43 5/3 19

21.4 - 30 - 10YR T3 B-3 85 13 L A/B 41 5/3 26

27 - 2.5Y T4 A-3 93 17 17 - B21 L 43 5/4 14

43 - 10YR Older T5 E-4 96 34 18 - B22 L 77+ 7/4 16

α From Benner (1983). See Appendix B for pit location. ε Elevations measured from DEM. γ Elevations extrapolated from topographic profiles. δ From Benner (1983). See Appendix C for full soil data. β Using methods of Merritts et al. (1991), comparison of maximum clay % to time relation.

Terrace ages inferred based on implied incision rates are shown on Table 6. The amount of assumed incision was obtained by measuring the difference in elevation from the terrace surface to the thalweg perpendicular to the Van Duzen River. For incision rate estimates, it is assumed that incision and uplift rates are equivalent (Personius, 1993;

Merritts and Brustolon, 1994; Koehler, 1999).

Table 6. Terrace ages inferred from uplift/incision rates and maximum clay composition.

Incision Estimated age Estimated age Estimated age Estimated age Estimated Terraceψ from terrace (ka) assuming (ka) assuming (ka) assuming (ka) assuming age (ka) ϕ I=0.1 m/ky I=0.3 m/ky I=1m/ky and I=4m/ky and based on surface (m) and I=Uα and I=Uα I=Uβ I=Uγ clay %δ

T2/C-1 5 50 17 5 1 4 T2/D-2 7 70 23 7 2 17 T2/E-1 9 90 30 9 2 9

T3/B-3 13 134 45 13 3 26

T4/A-3 17 171 57 17 4 14

T5 34 340 113 34 9 16

T8 79 786 262 79 20

T10 188 1880 626 188 47

ψ Terraces grouped by interpreted relative ages, youngest on top, oldest on bottom. Terraces mapped in this study that correspond with Benner (1983) soil pits are dually named. See Appendix D for terrace location. ϕ Measured from topographic profiles as distance from terrace surface to thalweg of river. α Implied incision rates based on published uplift rates, Central Oregon Coast Range, Personius (1995). β Implied incision rates based on published uplift rates, Northern California, Kelsey and Carver (1988). γ Implied incision rates based on published uplift rates near the MTJ, Merritts and Vincent (1989). δ Using methods of Merritts et al. (1991), comparison of maximum clay % to time relation.

Incision rates were then applied to the measured amounts of incision to determine terrace ages (Table 6). Uplift rates used for these age estimates are from the costal ranges—from the vicinity of the Mendocino Triple Junction north to Central Oregon (Kelsey and

Carver, 1988; Merrits and Vincent, 1989; Personius, 1995).

Assuming that incision is equal to uplift (I=U) (Personius, 1993; Merritts and

Brustolon, 1994; Koehler, 1999), applying an uplift rate 1 m/ky (Kelsey and Carver,

1988) agrees best with max clay ages of pit sites C-1 and E-1 (from Benner,1983) atop

Terrace 2 (T2). Pit site D-2 yields a maximum clay age estimate of 17 ka atop T2. D-2 is elevated 7 meters above the current thalweg of the river, equating to an incision (and therefore uplift) rate of approximately 0.4m/ka. Pits atop terraces 3 and 4 (T3 and T4) agree with uplift/incision rates that range from approximately 0.4-1 m/ka. (Table 7).

There are, however, discrepancies in the topographic relation and relative age based on max clay percent, but are overlooked due to a much more probably error with clay % analysis than with simply measured height of terraces above the thalweg. Despite these discrepancies, the most reasonable uplift rate to infer age through measured incision is

0.3- 1 m/ky. Using these rates, the oldest terraces within the study area were fomed ~

626-188 ka. 53

Table 7. Approximates of incision rates based on max clay % ages.

Nicovich Elevation above Estimated age (ka) Approximate Pit locationα Terrace river (m)γ based on clay %ε Incision Rate

T2 B-1 5 4 1 C-1 6 4 1 E-1 9 9 1 D-2 7 17 0.4 C-3 9 19 0.5

T3 B-3 13 26 0.5

T4 A-3 17 14 1

T5 E-4 34 16 2

α From Benner (1983). See Appendix B for pit location. γ Measured from topographic profiles as distance from terrace surface to thalweg of river. ε Using methods of Merritts et al. (1991), comparison of maximum clay % to time relation.

Tilt of terraces

Orientations of current and relict terrace surfaces were measured and plotted on a stereonet (Figure 26) to investigate a possible signal of regional strain inflicted upon these relatively younger surfaces. River terrace surfaces were not tilted beyond 2° (Figure

26) suggesting no regional deformation since the deposition of the Van Duzen River terraces, maximum 626 ka (Table 6) assuming the regional uplift rate to be 0.3-1 m/ky.

Little Salmon Fault

Geomorphic traits associated with thrust faults are subtle and hard to trace (Lettis and Kelson, 2000), hence, the LSF has not been mapped beyond fault scarp expression to the eastern extent of the study area. The Yager splay that bounds the northern limb of the

Wolverton syncline and defines the northern contact between the Neogene Wildcat 54

Group and the Yager terrane, and the Root Creek faults (Oswald et al., 2006), are the last prominent eastward splays of the LSF zone within the study area.

Although a fault scarp cannot be followed past the previously mentioned boundaries, other surficial features help to locate the trace of the LSF. The channel morphology of the Van Duzen River changes at the contact between Yager terrane and the Wildcat Group bedrock, accompanied by a knickzone evident in long profile (Figures

4, 14, 28, and 29). This could be entirely controlled by the coincident change in lithology, but may imply fault presence, presumably representing history of an abrupt change in base level due to tectonic lowering downstream relative to upstream. The knickpoint will migrate or broaden to form a knickzone until the stream power is no longer able to continue upstream migration (Foster and Kelsey, 2012)

Van Duzen Fault

Slip rate estimates for the Van Duzen fault, calculated from inferred net slip and estimated terrace ages to range from ~ 0.05 to 0.5 mm/yr (Table 8), are much lower than that of Quaternary rates along the LSF, documented to range from 1.3mm/yr to 10 mm/yr

(Kelsey and Carver, 1988; Carver and Burke, 1992)

A trench excavated across a splay of the VDF shows no evidence for surface rupture, only folded fluvial gravels. I am interpreting the scarp of the VDF to be formed by the steep limb of the fold (Stewart and Hancock, 1990; McCaplin, 1996; Lettis and

Kelson, 2000), based on folded strata where a scarp is present (Figure 23b). 55

The VDF displays different scarp heights on terraces of different apparent ages

(Figure 17). Two explanations are feasible; 1) multiple deformation events coeval with

terrace formation, 2) post-deformation deposition (Figure 32). Problems with the later of

these two scenarios is that the fault scarp would be subject to erosion given deposition of

enough sediment to account for differences in offset. However, it could be that energy

suspending the overlying deposited sediments was low enough to only have laminar flow

and not be able to erode the scarp.

Table 8. Estimated slip rate range for the Van Duzen fault. This range includes the assumption of scenario 1 (multiple events) for minimum rates and scenario 2 (single event) for maximum rate.

Estimated age (ka) Estimated Estimated Maximum Minimum Minimum assuming age (ka) age (ka) estimated Maximum slip Terrace estimated slip rate I=0.3 assuming based on dip slip rate (mm/yr)$ dip slip (m)π (mm/yr)¢ m/ky and I=1m/ky clay %δ (m)π I=Uα and I=Uβ

T2 17 5 4 Undeformed 23 7 17 30 9 9

T3 45 13 26 2 3.5 0.05 0.3

T4 57 17 14 2.3 5 0.05 0.4

T5 113 34 16

T8 262 79 3.2 5.5 0.07 0.5

α Implied incision rates based on published uplift rates, Central Oregon Coast Range, Personius (1995). β Implied incision rates based on published uplift rates, Northern California, Kelsey and Carver (1988). δ Using methods of Merritts et al. (1991), comparison of maximum clay % to time relation. Maximum clay % data from Benner (1983). π Dip slip calculated from fault angle estimates and vertical separation (Table 1). ¢ Minimum slip rate calculated from minimum estimated net slip/maximum estimated age of deformed terrace. $ Maximum slip rate calculated from maximum estimated net slip/minimum estimated age of deformed terrace. 56

Figure 32. Simplified terrace block diagram of scenarios 1) multiple events along VDF between formation of apparently different age terraces and 2) a single event along the VDF, where deposition accounts for increase in scarp height with elevation and along relatively older terraces.

Geomorphic Sequence

Tectonically influenced deformation, river terrace formation, stream incision, and

landslide failure within the study area can be constrained by a relative sequence of events

provided geomorphic evidence of superposition

The oldest time constraint is the deposition of Wildcat Group sediments in the

Pliocene, circa 5.3 to 2.6 Ma, from marine followed by non-marine sources (McCrory, 57

1989; McCrory, 2000). The Wildcat was subsequently folded, speculated by McCrory

(1989, 2000) at approximately 2 to 1.5 Ma. Thrust faulting began roughly 1 Ma, (Carver,

1992; McCrory 1996). Succeeding or coeval to uplift of these marine sediments via folding and faulting, the Van Duzen River formed its present drainage organization to the

Eel River approximately 2 Ma (Locke, 2006). Because I am using deposits sourced from the Van Duzen River, I can use this age of 2 Ma as the absolute oldest time constraint for terrace formation and correspondingly, deformation within the study area. This maximum age constraint agrees with a maximum calculated age of 626 ky for terrace T10.

Following the incision of the Van Duzen River, terraces T10 to T5 formed. After formation of terrace T8, deformation along the VDF occurred with a minimum of ~1-2 meters of net slip. This event is constrained by the fact that terrace T8 had to be formed in order to be tectonically disturbed. The estimated net slip involves the assumption of more than a single event along the Van Duzen fault, and is the difference in estimated net slip calculated from deformation across terraces T8 and T3.

Before the formation of terrace T4, failure of the Chalk Mountain Landslide complex must have occurred. This implication is based on the position of terrace T4 within the toe of the Chalk Mountain Landside. Oswald et al. (2006) have speculated the triggering mechanism for failure on this deep seated landslide to be that of intense ground shaking, possibly linked to the same deformation event that formed the scarp along the

T8 terrace, but not on terrace T4. 58

Based on the assumption of multiple events along the VDF, terrace T4 formed prior to an event with ~0.5 to 1.5 meter net slip. Later, terraces T3 formed and a subsequent 2 to 3.5 meters of net slip occurred along the VDF.

Before the formation of terraces T1 and T2, incision of Heley Creek took place, eroding potentially deformed young sediments atop terraces T8 to T4 in age, exposing underlying deformed bedrock. 59

CONCLUSION

Acquisition of high resolution LiDAR imagery along the heavily vegetated Van

Duzen River valley has enabled detailed mapping of strata, structure, and geomorphic features within the study area. This detailed mapping has elucidated characteristics of the

LSF zone and influences of transform strain within the compressional regime of the southern Cascadia fold and thrust belt that have not been previously recognized.

A new fault, interpreted to be a splay associated with the Little Salmon fault zone, has been mapped (Figures 6 and 14). This splay, the Van Duzen fault, is a northwest trending, northeast dipping reverse fault that offsets Pleistocene to potentially Holocene river terraces and underlying bedrock. The VDF does not daylight within the Pleistocene to Holocene fluvial sediments, but does display a mole track scarp along terraces of varying age and elevation (Figure 17). The VDF is observed to offset Carlotta Formation bedrock at the confluence of Heley Creek and the Van Duzen River (Figure 22).

Deformation associated with the VDF is similar in style to the LSF. West- northwest is the prominent orientation of the strike of the LSF within the study area,

Neogene Wildcat Group bedding, axial trend of the Wolverton syncline, and the strike of the VDF. Therefore, I interpret the VDF to be a part of the LSF zone.

A series of fluvial terraces occur within the Van Duzen River valley. These terraces have been mapped in detail and separated into ten flights based on topographic relation. Ages have been inferred based on topographic position and soil data (Benner,

1983) in order to constrain deformation rates along the VDF. These fluvial terraces range 60

from approximately 600 ka to 4 ka within the Late Pleistocene to Holocene, calculated using regional uplift rates of 0.3-1 m/ky. From these inferred terrace ages and estimated net lip of the VDF, a slip rate of ~ 0.05-0.5 m/ky has been calculated.

Bedding of underlying bedrock units within the study area, specifically the

Wildcat Group marine and not marine deposits, have been measured using an automated computer routine. From the results of this routine, the Wolverton syncline of Ogle (1953) has been mapped to project well into the study area, much further east than previously mapped, to the extent of the Wildcat Group sediments.

Northward encroaching transform strain associated with the San Andreas fault system is expressed within the west-northwest orientation of faults and folds mapped within the study area. Although the LSF zone, including the VDF and Yager fault, and

Wolverton syncline within the study area are compressional features, they deviate from the predominant north-northwest trend of thrust faults associated with Cascadia subduction zone compression. This is presumably due to San Andreas-parallel compression, surmising that the zone of tectonic transition from compressional to transform strain is within the vicinity of the southeastern extent of the LSF zone. 61

REFERENCES

Aalto, K. R., 1992, Late Cenozoic sediment provenance and tectonic evolution of the northernmost Coast Ranges, California: in G. A. Carver and K. R. Aalto (eds.), Field guide to the late Cenozoic subduction tectonics and sedimentation of northern coastal California: Pacifi c Section, American Association of Petroleum Geologists, Field Guide GB-71, p. 11 – 20.

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APPENDICES

Appendix A

MODEL CONCEPTS Three-point problems Where reasonable, geologists can assume certain geologic structures exist as a plane. Three point problems are used to measure bedding or other planar features when a fully measurable plane is not exposed. With topographic information from at least three points of a common outcrop, the entire plane can be projected and measured for strike and true dip. How can three-point problems be applied using GIS? In this example, the topographic expression of the geology within the mapping area, The Wildcat Group sediments, is that of distinct bedding planes including many dip slopes. Thus, when applying the three point problem technique to this specific site, points for plane projection can be located anywhere on the interpreted bedding plane. Because of high resolution topographic data, this method is a possibility for analysis and mapping. Otherwise, a detailed geological map would have to be constructed for the typical three point problem technique to be executed. All of the necessary steps of projecting and measuring the strike and dip of a plane are achievable using GIS. The conceptual steps are as follows

1. choose points on plane 68

2. Create functional plane from points. In Arc this is referred to as a TIN (triangular irregular network).

3. Gather and display strike and dip 69

PLOT STRIKE AND DIP MODEL Because of the tedious nature of GIS computations and the amount of steps necessary to complete the task at hand, models can be built to do all the legwork. Pictured is a screenshot of Model Builder in edit mode. Yellow Boxes represent tools found in the ArcToolbox and green ovals represent created/modified file. The P above select ovals represents the parameters users can select when using the tool.

What the model actually does 1. Identify area for analysis

2. Create point shape file. Select points for three point problems. Assign specific attributes to each group of points selected. 71

3. Use the add surface information tool to assign vertical data to points from the DEM. 72

4. Create TIN from points

5. Transform TIN into a polygon

6. Feature to point

7. Add and calculate fields for strike and dip

8. Plot strike an dip with proper symbology 75

9. The final product of the model turns up in ArcTools as a user friendly interface that runs aforementioned steps. The ‘Plot Strike and Dip ‘ tool provides selection menus for input files and folders where parameters were set.

Appendix B

Map of terrace locations used to measure height above thalweg for incision rates.

T3a

T3b

T4b

T4a T8a

T4c

500 Meters 77

Appendix C

Soil pit locations analyzed by Benner (1983, unpublished senior thesis)

Appendix D

Soil data from Benner (1983, unpublished senior thesis) compared to terraces map in this study.

Nicovic Terrace Horizo Depth Textural h Sand% Silt% %Clay Dry Color -Pit# n (cm) Class Terrace

N/A A-2 A12 6-28 22.3 54.7 21.1 silt loam 2.4Y 4/2 A/B 28-40 33.7 44.6 19.6 loam 10YR 4.5/3 B21 40-58 41 36.8 18.7 loam 5Y 5.5/3 B22 58-80 49.7 32.4 16.1 loam 5Y 5/3

loam/silt T4 A-3 A11 0-6 14 loam 10YR 4/3 A12 6-27 16 sandy loam 10YR 4.5/3 B21 27-43 17 loam 2.5Y 5/4 B22 43-62 8 loamy sand 10YR 5/4

T2 B-1a C1 0-23 81.5 9.7 6.2 loamy sand 5Y 5/1.5 C2 23-102 71.2 19.7 7.3 sandy loam 5Y 4/1

T3 B-3 A11 0-17 38.7 46.6 21 loam 10YR 4/2 A12 17-30 36.7 43.2 18.9 loam 10YR 4.5/2 A/B 30-41 40.3 36.6 21.4 loam 10YR 5/3 B 41-69 45.2 33.2 20.1 loam 4.5/3

N/A B-4 A 0-26 18 silt loam 10YR 5/3 A/B 26-37 17 silt loam 10YR 4.5/3 B 37-75 13 silt loam 2.5Y 6/4

T2 C-1 C1 0-25 63.7 30.1 8.4 sandy loam 5Y 5/2 C2 25-69 91.7 3.4 3.4 sand 5Y 3.5/1

T2 C-3 A 0-23 50.9 31.6 16.2 loam 10YR 4/2 A/B 23-43 44.5 34.4 19.3 loam 10YR 5/3 B2 43-75 39.1 41.2 17.6 loam 10YR 5.5/3 79

T2 D-2 A 0-16 47 31.3 18.5 loam 10YR 4.5/2 B 16-28 63.3 23.6 12.3 sandy loam 2.5Y 4.5/2

T2 E-1a C1 0-18 62.1 22 13.7 sandy loam 2.5Y 5/2 C2 18-32 72.2 17.8 7.9 sandy loam 5Y 4.5/2.5 C3 32-60 81.4 11.3 6.8 loamy sand 5Y 4/2

T2 E-1b C1 0-18 86 8.9 4.5 loamy sand 2.5Y 5.5/2 C2 18-28 75.3 17.1 6.2 loamy sand 5Y 5.5/3 C3 28-69 94.7 3 2.6 sand 5Y 5/2 IIB1b 69-91 85 11.6 3.3 loamy sand 2.5Y 5.5/3 IIB3b 91-112 83.3 10.6 5.4 loamy sand 2.5Y 5/5 112- IICb 64.2 26.2 7.4 sandy loam 127 5Y 5.5/3

N/A E-4 A11 0- 36.8 44.1 16 loam 10YR 4.5/3 A12 -33 40.5 41.3 15.7 loam 10YR 5.5/3 B21 33-43 42.1 40.2 15.6 loam 10YR 5/3 B22 43-77 48.2 33.7 18 loam 10YR 7/4

Clay percent versus depth

80 81

Appendix E

Strike and dip of Wildcat Group within study area (Azimuth). Strike Dip 148 45 157 34 141 44 131 76 147 26 144 30 133 26 250 20 139 46 255 25 273 24 265 27 268 19 121 36 119 39 100 29 309 60 292 54 159 30 278 21 252 18 281 24 272 18 274 18 299 38 259 13 270 16 264 9 259 3 160 21 147 32 262 21 287 14 279 14 271 11 85

288 40 303 44 291 52 288 53 284 43 272 30 274 40 271 32 274 44 282 50 286 78 298 71 121 45 275 48 124 56 277 45 127 48 301 11 288 48 275 40 273 48 267 51 269 43 282 55 262 45 257 52 261 42 235 22