MARINE INUNDATION OF A LATE MIOCENE FOREST: STRATIGRAPHY

AND TECTONIC EVOLUTION OF THE SAINT GEORGE FORMATION,

CRESCENT CITY, CALIFORNIA

by

Dane T. Robinson

A Thesis

Presented to

The Faculty of Humboldt State University

In Partial Fulfillment

Of the Requirements for the Degree

Masters of Science

In Environmental Systems: Geology

May, 2002 MARINE INUNDATION OF A LATE MIOCENE FOREST: STRATIGRAPHY

AND TECTONIC EVOLUTION OF THE SAINT GEORGE FORMATION,

CRESCENT CITY, CALIFORNIA

by

Dane T. Robinson

Approved by:

Ken Aalto, Major Professor Date

William Miller III, Committee Member Date

Angela Jayko, Committee Member Date

Roland H. Lamberson, Graduate Coordinator Date

Donna E. Schafer, Dean Research and Graduate Studies Date ABSTRACT

A paleosol developed on the Mesozoic Franciscan Complex is depositionally overlain by 48 m of late Miocene (diatom age ca. 6.0-6.4 Ma), mostly shallow-marine

Saint George Formation. At the base of this sequence is a buried paleoforest of rooted tree stumps, found at three separate sites. Above the rooted stumps is a sequence of wave-reworked colluvium containing woody debris interpreted as a tsunami deposit, succeeded by beach deposits, and bioturbated, mollusk-rich mudstone having occasional hummocky cross-stratified sandstone interbeds. Regional correlations of the Saint George Formation with the Wimer Formation, preserved on the highland

Klamath erosion surface immediately east of the study area (at elevations up to 680 m) and basal Pullen Formation at Scotia, California, 145 km to the south, suggest that these units are remnants of a single transgressive shelf sequence that blanketed northwestern California. These sediments have a likely Idaho batholith provenance and accumulated in response to rapid subsidence of the leading edge of North America during the late Miocene.

The late Miocene Saint George Formation [units 1-3] dips easterly along

Pebble Beach and is preserved in an open, northwest-trending syncline north of Point

Saint George. The newly-described 'Pebble Beach thrust fault' has an north- northwest strike and west-southwest vergence and runs onshore from mid-Pebble

iii Beach to north of Point Saint George, cross-cutting Pleistocene marine terraces and displacing a Holocene peat [radiocarbon date of 3000± 60 BP]. Saint George rocks are folded into an anticline-syncline pair disrupted by a smaller reverse fault. The folds are U-shaped, symmetrical, have widths of about 8 m and plunge north- northwest at about 10 degrees. They do not appear on the north side of Point Saint

George. The reverse fault extends to the north side, for in its line of projection are two

reverse faults that drop Pleistocene terrace deposits south side down against basal

Saint George Formation in a step-like manner. North of Point Saint George, dip

reversal across the lower plate (the Pebble Beach fault) syncline exposes a Franciscan

Complex paleosol-basal Saint George conglomerate contact to the south, and paleosol

with rooted stumps overlain by swash-cross-stratified sands to the north. Dips across

this fold do not exceed 20 degrees and flexural slip formed bedding-parallel mudstone

breccias. Fold and clustered fracture data record east-northeast-west-southwest crustal

shortening in the lower, and north-northwest-south-southest extension in the upper

plates. Offshore structures trend north-south and record east-west shortening.

Contrasting orientations of structures along the Pebble Beach Fault reflect movement

over an oblique thrust ramp and local thrust sheet rotation.

iv ACKNOWLEDGEMENTS

I thank Ken Aalto for being a great advisor. His advice and analysis of geologic problems has helped me in completing this thesis. Without his tireless patience and willingness to listen I would not have been able to even begin this project.

I thank my wife and daughter, who spent many hours listening to me try and explain what I was doing all this time. Without my daughter Cristina's help, many of the forest exposures would not have been found. Without my wife Liza's help, the final draft never would have been finished.

I want to dedicate this thesis to my good friend Richard Burkhart. I miss you

Richard and wish you could be here to enjoy this with me.

Special thanks go to John Barron, Sam Clarke, and Angela Jayko of the U. S.

Geological Survey, Diane Erwin of the University of California Museum of

Paleontology, William Miller, and Jay Patton of Humboldt State University for contributions toward our interpretations and assistance in figure preparation. Thanks

to William Miller for manuscript review and M. B. Underwood for vitrinite

reflectance analyses and their interpretation. We also thank SEPM and the University

of California Museum of Paleontology Annie M. Alexander Endowment Fund for

partial financial support of this study. TABLE OF CONTENTS

Page

ABSTRACT iii

ACKNOWLEDGEMENTS v

TABLE OF CONTENTS vi

LIST OF TABLES viii

LIST OF FIGURES ix

INTRODUCTION 1

METHODS 8

REGIONAL TECTONIC SETTING 10

STRATIGRAPHY 20

Introduction 20

Paleosol 23

Saint George Formation Unit 1 29

Saint George Formation Unit 2 32

Saint George Formation Unit 3 36

Provenance 40

PALEONTOLOGY 42

Holocene Exposures 46

DEPOSITIONAL HISTORY 49

vi TABLE OF CONTENTS (continued)

Page

STRUCTURAL GEOLOGY 58

Introduction 58

Faults and Folds 60

Structural Data 63

Tectonic Interpretation of Structures 67

CONCLUSION 83

REFERENCES 83

vii LIST OF TABLES

Table Page

1 Fransiscan Paleosol Sandstone Composition 24

2 Saint George Formation Sandstone Composition 25

3 Diatiom Paleoecology 35

viii TABLE OF CONTENTS (continued)

Page

STRUCTURAL GEOLOGY 58

Introduction 58

Faults and Folds 60

Structural Data 63

Tectonic Interpretation of Structures 67

CONCLUSION 83

REFERENCES 83

vii LIST OF TABLES

Table Page

1 Fransiscan Paleosol Sandstone Composition 24

2 Saint George Formation Sandstone Composition 25

3 Diatiom Paleoecology 35

viii LIST OF FIGURES

Figure Page

1 Geologic and location maps for the Crescent City area 2

2 Regional Geology in the vicinity of the study area 4

3 Crescent City Platform 5

4 South end of Pebble beach showing the Saint George Formation wave- cut platform 6

5 Mid Pebble Beach view northwest looking towards the south side of Point Saint George and unit 2 of the Saint George Formation 6

6 View northwest overlooking the Saint George Formation wavecut platform on the north side of Point Saint George 7

7 Map showing migration of the Mendocino Triple Junction (MTJ) over the last 10 million years 11

8 Detailed regional setting of the field area (inset map) 13

9 Regional fold and thrust belt map 18

10 Stratigraphy of South Point Saint George fold out back cover

11 Basal Saint George Formation paleosol resting on highly weathered Franciscan Complex saprolite. North side of Point Saint George 21

12 Rooted stump on highly weathered Franciscan Complex saprolite. South side of Point Saint George, north Pebble Beach. 22

13 Large logs in paleosol. North side of Point Saint George. 27

14 Large rooted stump in Franciscan Complex saprolitic paleosol. Scale on right is 10 cm. North side of Point Saint George. 27

15 Lignitic bog deposit, north Pebble Beach. Scale is 2 meters long 28 ix LIST OF FIGURES (continued) Figure Page

16 Saint George Formation unit 1- view northwest. Note the pulse-like layering of the bedding structures. North Pebble Beach. 30

17 Rhythmically bedded cycles within unit 1 of the Saint George Formation.... 31

18 Saint George Formation unit 2. Note lignitic log within the Saint George Formation in the lower left of the photo 33

19 Saint George Formation unit 3. Note the shell couplet layers visible along strike 37

20 Hummocky cross stratification within unit 3 of the Saint George Formation. North Pebble Beach 38

21 Late Miocene pinecones found within Saint George Formation unit 1 sediments 45

22 Close up of a bioturbation structure within unit 2 of the Saint George Formation 45

23 Pinecones found adjacent to the Pebble Beach Fault within the Holocene bog exposure 47

24 3000 year old (± 60 years) bog exposure, mid-Pebble Beach. Note the Pebble Beach Fault runs northwest under the riprap and across Point Saint George 47

25 Model depicting a coastal forest being inundated by tsunami (modified from Carver, 1996) 52

26 Air Photo of Point Saint George. Pebble Beach runs from just south of Point Saint George to the lower center of the photo 59

27 Structures within the Saint George Formation found on the south side of Point Saint George 61

28 Idealized lineation map drawn from color air photo's of Point Saint George 62 LIST OF FIGURES (continued)

Figure Page

29 Rose diagram from fracture data collected on both side of Point Saint George (a.), poles to fault planes on the (b.) 64

30 Stereonet of bedding planes measured from within the Saint George Formation 64

31 Beginning of the broad syncline with an internal shear zone on the north side of Point Saint George 65

32 Fault plane with striations found within broad syncline. North side of Point Saint George 66

33 "Taffy" like appearance of Saint George Formation unit 2 and 3 found in the shear zone within the broad syncline on the north side of Point Saint George. 66

34 Idealized lineament map drawn from the USGS 7.5 minute quadrangle for Crescent City and Sisters Rocks. 68

35 Tectonic setting of the Cascadia subduction zone. Juan de fuca plate is subducting is subducting beneath North America at an oblique angle 70

xi INTRODUCTION

The purpose of this paper is to describe remarkable exposures of a late

Miocene forest that appears to have been rapidly buried by littoral sediments of the

Saint George Formation that are preserved on the wave cut platform of Point Saint

George, near Crescent City, California (Figure 1). These remarkable outcrops, characterized by rooted stumps and large logs, were covered by a mantle of Holocene beach sand until the beaches were degraded during the 1997 El Nino event. This study will also attempt to develop a structural model that explains the evolution of the

Crescent City coastal plain and the relationships between the identifiable units exposed on the coastal plain in the vicinity of Point Saint George. Inundation of the forest appears to have been sudden and may relate to rapid co-seismic subsidence engendered by large-scale slumping of the Crescent City area and inundation by tsunami. Correlation of basal Saint George sediments with those of the marine facies of the Wimer Formation preserved some 10-15 km inland of Crescent City and Pullen

Formation 140 km to the south suggests that initiation of rapid subsidence in this region marks the beginning of regional down-dropping of the leading edge of the

North America plate during the late Miocene. Thus the record of initiation of subsidence at Crescent City has regional tectonic implications.

Point Saint George is located near the southwest tip of the Crescent City coastal plain, also known as the Smith River plain, at approximately 41° 40' to 42 °

00' north latitude, and 124° 00' and 124° 15' west longitude. The Crescent City

1 2

Figure 1. Geologic and location maps for the Crescent City area. Kjfc: Jurassic- Cretaceous Franciscan Complex, Msg: Miocene Saint George Formation, Qpml: terrace sediments overlying the 125 or 200ky bedrock platform, Qpm2: terrace sediments overlying the 105ky bedrock platform, Qpm3: terrace sediments overlying the 80ky bedrock platform. Sites of Miocene rooted stumps and/or abundant logs in basal Saint George Formation are indicated with an F on the map. Site of Holocene peat with radiocarbon- dated pinecone is indicated with H. Inset map shows structural detail of the north end of Pebble Beach (boxed area on main map, mapping by Aalto). PBF is the Pebble Beach Fault. Geology is based on Aalto (1989a), Polenz and Kelsey (1999) and this study. Key map CSZ is the Cascadia subduction zone. 3 coastal plain is a low-lying surface in which late Quaternary sediment overlies

Mesozoic and Tertiary sandstone basement. The coastal plain is situated in the

Cascadia forearc, between the Cascadia deformation front offshore and the Siskiyou

Mountains to the east (Figure 2). The coastal plain spans a 26-km stretch of coastline, extends up to 10 km inland (Polenz, 1999). The coastal plain is level in most places,

with the total relief about 25 meters (Back, 1957). Exposures of bedrock and

unconsolidated sediments are rare. The coastal plain is bordered on the east and north

by the range front of the Siskiyou Mountains. To the south and west, the coastal plain

wedges out between the Siskiyou Mountains and the ocean, with Point Saint George

the southwestern most point of the coastal plan (Figure 3).

Point Saint George is made of 2 distinct pre-Quaternary formations, the

Mesozoic Franciscan Complex and the late Miocene Saint George Formation (Back,

1957) Figures 4, 5, and 6. The Franciscan Complex makes up the central, less

erodable portion of Point Saint George, while the more easily erodable nearshore

marine Saint George Formation makes up most of the wave-cut platform on the

northern and southern sides of Point Saint George, as well as the cliffs running south

along Pebble Beach on the southern side of Point Saint George.

The Saint George Formation rests uncomformably on top of the Mesozoic

Franciscan Complex. This formation is exposed at low tides as far as 3 km north of

Point Saint George and makes up the wave-cut platform currently subjected to erosion

by the Pacific Ocean. The Saint George Formation extends south of Point Saint

George along Pebble Beach for approximately 2 km. Outcrops of the Saint George 4

Figure 2. Regional Geology in the vicinity of the study area (copied from Stone, 1993, Figure 4) 5

Figure 3. Crescent City Platform (copied from Stone, 1993, Figure 5). 6

Figure 4. South end of Pebble beach showing the Saint George Formation wave-cut platform. Note the Franciscan saprolite/paleosol contact along the center of photo.

Figure 5. MidIvebble Beach view northwest looking towards the south side of Point Saint George and unit 2 of the Saint George Formation. Note the tilted grassy Holocene surface in the background and the tilted bedding of the Saint George Formation in the foreground. 7

Figure 6. View northwest overlooking the Saint George Formation wavecut platform on the north side of Point Saint George.

Formation have also been reported as far south as Crescent Beach, approximately 5-7 km (Diller, 1902), but those exposures were not visible during the course of this study.

Citing seismic reflection data from his field work and several well logs, Back (1957) contends that the Saint George Formation is buried in several locations throughout the southern half of the Crescent City coastal plain, and reaches its maximum thickness at about 120 m. Thus the Saint George Formation may be widespread beneath the surface of the Crescent City coastal plain. The Saint George Formation is overlain by the late Pleistocene nearshore marine Battery Formation (Back, 1957). METHODS

Samples were collected at regular intervals during measurement of stratigraphic sections for petrologic and paleontologic analysis. All thin sections of sandstones were stained for plagioclase and potassium feldspar and point counted at

300 grains per thin section for determination of standard detrital modes (cf. Dickinson,

1984). In addition, samples were collected at 10 cm intervals downwards within weathered Franciscan Complex sandstone beneath the Saint George Formation at the north side Point St. George unconformity and point counted for percent apparent matrix determination to assess the effects of diagenesis in the paleosol (Tables 1 and

2). Sample locations for thin sections are shown in Figure 26. Bulk X-ray diffraction analysis was undertaken of paleosol clays and Saint George Formation mudstones.

Stratigraphy was measured using a 2.5 meter staff with 10 cm increments.

Diatom samples were collected from within the paleoforest beginning on the south side of Point Saint George and continuing southeast at lm intervals up section within the Saint George Formation as shown in Figure 26. Diatom analysis and relative age dating was conducted by Dr. John Barron of the United States Geological

Survey (Tables 3). Paleobotanical data was collected from within the forest exposures and interpreted by Dr. Diane Erwin of the University of California at

Berkeley Museum of Paleontology.

Structural data was measured using a standard Brunton compass and plastic plate. Some structural data was acquired from Dr. Angela Jayko of the United States

8 9 Geological Survey. All petrographic data for the Franciscan paleosol and petrographic data for selected Saint George Formation sandstones collected from upper unit one and units two and three is provided by K. R. Aalto. Two radiocarbon-14 samples were collected from in place exposures and analyzed by Beta Analytic, Coral Gables

Florida. Degree of bioturbation was estimated qualitatively using the bedding plane bioturbation index card of Miller and Smail (1997). REGIONAL TECTONIC SETTING

During the Neogene much of northern California was covered by a continental slope and shelf sediment cover that may have extended as far south as the San

Francisco Bay region and into the southern San Joaquin Valley. As the Mendocino triple junction migrated northward during the late Cenozoic, the southern margin of the shelf sediment cover sequence was uplifted, sediments were stripped off by erosion, and the locus of sedimentation shifted progressively northward through time.

Preserved but isolated fragments of this sediment cover provide a partial record of the original paleogeography and tectonic framework of northern California during the late

Cenozoic (Nilsen and Clarke, 1989).

The plate tectonic setting of the region was dominated by subduction of the

Farallon plate along the continental margin until about 30 Ma, when interaction of the

North American and Pacific plates initiated northward migration of the Mendocino triple junction from an original position in southern California to its present location near Cape Mendocino (Figure 7). As a result of the northward migration of the triple junction, right lateral strike-slip related tectonics replaced the previous subduction related tectonics south of the triple junction, with subduction continuing to present day north of Cape Mendocino (Nilsen and Clarke, 1989).

The general paleotectonic transition from Miocene through Pliocene time up to the present is shown by comparison of Figure 7 (A, B, C). Northward migration of the

10 11

Figure 7. Map showing migration of the Mendocino Triple Junction (MTJ) over the last 10 million years. A represents the present location of the MTJ, B represents the location of the MTJ 6 million years ago, and C represents the MTJ location at 10 million years ago (copied from Nilsen and Clarke, 1989, Figure 6). 12 Mendocino triple junction during late Cenozoic time is broadly matched by (1) northward migration of the southern erosional edge of the contiguous Eel River forearc basin, (2) northward migration of the northern limit of strike-slip faulting associated with the San Andreas fault system, and (3) northward migration of the southern limit of active magmatic volcanism (Nilsen and Clarke, 1989).

Plate tectonic reconstructions for the Coast Ranges suggest a change in motion during the past 5 Ma between the Pacific and North American plates, to a slightly more transpressional regime (Nilsen and Clarke, 1989). This slight change may have caused the development of extensive Pliocene and Pleistocene uplift, folding, and thrusting in northern and central California. The geographical and temporal separation of compressional and extensional features in northern and central California from the

late Miocene to present strongly supports the concept of a change in relative plate motions between the North American and Pacific plates (Nilsen and Clarke, 1989).

A model developed by Kelsey et al., 1996 using previous paleomagnetic studies, has the leading edge of the North American plate along the Cascadia subduction zone deforming and rotating clockwise as a consequence of both the

underthrusting and collision of the Gorda — Juan de Fuca plate with the North

American plate (Figure 8). Upper plate structures in central coastal Oregon located

approximately 200 km north of Point Saint George are active, and deform as part of a

broad deformation zone that bounds the western margin of the North American plate

(Kelsey et al., 1996). Using the degree of soil development as a means of correlating

wave cut platforms along the southern Oregon coast some 25 to 30 km north of Point 13

Figure 8. Detailed regional setting of the field area (inset map) (from Polenz, 1996; Polenz and Kelsey, 1999, Figure 1). 14 Saint George, Kelsey and Bockheim (1994) documented that surface uplift is variable in a shore parallel sense, with variability being a function of differential vertical displacement of crustal blocks along faults in the upper plate of the Cascadia subduction zone. Uplift rates range from moderate (.7 to .9 m/ky) to low (.05 to .2 m/ky). At Point Saint George uplift rates were calculated using the same degree of soil development method and averaged .01 to .05 m/ky (Polenz and Kelsey, 1999).

Late Miocene Saint George Formation sediments located in the vicinity of

Point Saint George near Crescent City are believed to have been part of the Eel River basin shelf sediment blanket (Nilsen and Clarke, 1989; Aalto et al., 1995). A total of more than 3,600 meters of marine and nonmarine Miocene to Holocene sediments have accumulated in the onshore part of the Eel River basin during two main phases of deposition (Figure 7). The first phase occurred during the early and middle Miocene, and the second during the late Miocene to middle Pleistocene. The Pliocene and

Pleistocene deposition in the Eel River basin (Figure 7B) appears to record a gradual basin infilling without significant tectonism until the middle Pleistocene, when major deformation affected the entire region (Nilsen and Clarke, 1989).

The Eel River basin (Figure 7) developed as an active forearc basin during the

Miocene in response to subduction of the Gorda (Farallon) plate beneath North

America. Units of the Wildcat Group (Aalto et al., 1995) that formed within the southern flank of the Eel River basin show, in general, a basal eastward transgression of the shoreline during the late Miocene and early Pliocene (the Pullen Formation, which is believed to be coeval with the Saint George Formation, Aalto et al., 1995), 15 rapid deepening of the basin to lower bathyl - abyssal depths (about 2000m) with basin - plain and submarine fan, and basin - slope deposition during Pliocene and early

Pleistocene (Pullen, Eel River, and Rio Dell Formations). Westward regression of the shoreline followed into the middle Pleistocene (Clarke, 1992).

Ca. 5 Ma the northwest Klamath Mountains were at sea level, as indicated by the presence of the Wimer marine sediments on the present day elevated erosion surface 10 to 15 km east of Crescent City (The "Klamath peneplain" of Diller, 1902).

Poorly dated, course fluvial conglomerates of the eastern Wimer facies may reflect local uplift and warping of the surface about a northwest — trending axis (Stone, 1993).

Sediment progradation and shoaling over a wide coastal shelf, onset of deposition of locally derived fluvial sediments in the Coast Ranges, and regional uplift during the middle Pleistocene may relate to the decreasing age of the subducting oceanic plate,

lessening angle of subduction, and migration of the Blanco fault northward along the continental margin of northwestern most California. Uplift of the Klamath peneplain

to elevations as high as 1,220 m may largely have occurred during the late Pleistocene

and Holocene (Aalto et al., 1995).

This thick lateritic weathering profile of the Klamath saprolite is thought to

represent a relic Tertiary surface of uniformly low relief and prolonged tectonic

stability (the Klamath peneplain of Diller, 1902). This saprolitic surface shows no

evidence of faulting (Stone, 1993), unlike the region 10 to 15 km to the west near

Point Saint George. Faulting is suggested by the apparent north — northwestward

coincidence in trend of a northeast dipping thrust fault about 2.5 km offshore south of 16 Crescent City with the back (northeastward) rotated Franciscan basement high that forms sea cliffs for about 5 km northeast of Crescent City to Point Saint George.

Also, a faulted anticline located about 10 km northwest of Point Saint George extends shoreward along Saint George Reef (The Saint George thrust, see Figure 8). Similar faulting of the basement surface is suggested by a north — northwest trending, down- to-the-south step in the Franciscan basement surface along which the Smith River flows to the sea 15 km north of Crescent City (Clarke, 1992). This postulated fault is aligned in trend with a northwest trending thrust fault in the southern Oregon offshore.

Consideration of marine sediment provenance suggests that the Klamath

Mountains were low during the Eocene, elevated during the late Oligocene (possibly by clockwise rotation and structural doming associated with detachment faulting), generally low from medial Miocene through early Pleistocene, and rapidly elevated during the late Pleistocene. Rapid down — dropping of the Eel River basin, followed by shoaling and fluvial input, is enigmatic, but most likely relates to large scale plate interactions (Aalto et al., 1995). The late Miocene eastward transgression of the shoreline and rapid deepening of the Eel River basin, along with the burial of the late

Miocene forest by the Saint George Formation at Point Saint George may be related to a regional tectonic episode of "subduction erosion". This episode of "subduction erosion" may occur when the sediments of the bottom portion of the North American plate are eroded away by the shallowly dipping, subducting Gorda plate. This creates a need for isostatic adjustment, and thus large regions of northern California would drop rapidly in response to an episode of subduction erosion. The eroded sediments 17 would then be transported underneath the Klamath Mountains, and aid later in the uplift of the region (Aalto et al., 1996).

Onshore and offshore parts of the basin are presently undergoing strong northeast-southwest directed contraction along northeast dipping thrust faults and northwest trending folds (Nilsen and Clarke, 1989). Faults in the northeastern part of the offshore Eel river basin commonly verge downward and westward with shallow northeast-dipping zones of deformation appearing in the acoustic reflection data

(Nilsen and Clarke, 1989). These faults have apparent dips of 7 to 12 degrees. A subduction zone dip of 10 to 11 degrees, determined from seismicity data (Nilsen and

Clarke, 1989), places the top of the subducted Gorda plate at a depth of 18 to 19.5 km beneath this region. Acoustic reflection studies from the area indicate that faulting, predominantly thrusting, in the northeastern offshore Eel River basin can be extended downward to about 4 km to the Gorda — North American plate interface, strongly suggesting this deformation reflects Gorda — North American plate convergence

(Clarke, 1992). In map view (Figure 9), these thrust systems are oriented approximately orthogonal to the direction of Gorda — North American plate convergence, and the structural blocks they bound form a series of overlapping

"shingles" that follow the western margin of the Klamath Mountains (Clake, 1992).

These fault systems are interpreted as a series of seaward vergent, imbricate thrust fans formed as a consequence of Gorda — North American plate convergence, with strong interplate couplings within the subduction zone. The Klamath Mountains act as 18

Figure 9. Regional fold and thrust belt map (modified from Clarke, 1992, Figure 2). 19 a buttress or backstop against which this part of the Eel River basin is being deformed

(Clarke, 1992).

Relatively youthful strata in the western portion of the Eel River basin paralleling the subduction front apparently lack the strength to transmit compressional stress and are decoupled or only weakly coupled to the subducting Gorda plate

(Clarke, 1992). The en echelon nature of the faults and folds east of the structural discontinuity that forms the western boundary for the domain of en echelon structures

(which is also the approximate seaward limit of shallow North American plate seismicity in the region), suggests that the discontinuity may also accommodate a component of right shear related to oblique convergence (Clarke, 1992).

The faults in Figure 9 show evidence of late Quarternary shortening of the

North American margin. These structures are interpreted as reflecting Gorda — North

American plate convergence (Clarke, 1992). They indicate that subduction is active and that the two plates are coupled at depth. This interpretation is in accord with studies of the Mad River fault zone and Little Salmon fault that show histories of large, episodic, late Quarternary thrust offsets. Any minor component of strike slip present is attributed to oblique convergence between the Gorda and North American plates (Clarke, 1992). STRATIGRAPHY

Introduction

The Saint George Formation (Back, 1957) rests in angular unconformity on overturned and tightly folded, thick-bedded sandstone of the late Mesozoic eastern

belt (Yolla Bolly terrane) Franciscan Complex (Aalto 1989a, 1989b). A late Miocene

age and marine depositional setting for this formation is confirmed by paleontologic evidence [discussed ahead]. It is in turn overlain by a complex series of Pleistocene

marine terrace deposits in angular unconformity (Polenz and Kelsey 1999). Figure 10

(see back envelope for fold out stratigraphic column) depicts the most complete Saint

George stratigraphic sections, divided into three units which are found both to the

north and south of Point Saint George. In both areas, Saint George Formation basal

conglomerates depositionally overlie a weathered surface (paleosol) developed on

Franciscan Complex sandstones (Figures 11 and 12). The entire Saint George

Formation exhibits an overall fining upwards trend.

Offshore acoustic-reflection data suggest a correlation of the Saint George

Formation at Crescent City with the submarine unit separating basement rocks

equivalent to the Franciscan Complex from deltaic sediments of the Klamath River

(Clarke, 1992). Based on water well litholog data and projection inland of the

northeasterly dip of bedding, the Saint George Formation appears to underlie much of

the Crescent City platform (Back 1957). From this same well log and other seismic

data, Back (1957) estimates the total thickness of the Saint George Formation to be

20 21

Figure 11. Basal Saint George Formation paleosol resting on highly weathered Franciscan Complex saprolite. North side of Point Saint George. 22

Figure 12. Rooted stump on highly weathered Franciscan Complex saprolite. South side of Point Saint George, north Pebble Beach. 23 approximately 120 meters. Diller (1902) describes an outcrop matching the description of both basal Saint George Formation units 1 and 2 exposed near the location of the Crescent City pier east of Battery Point, approximately 2 km south of

Point Saint George. Further to the south another 3-5 km, midway along Crescent

Beach, similar Saint George Formation deposits are described in Back (1957).

However, today the Saint George Formation is not presently exposed at these sites due to beach sand cover.

Paleosol

The base of the Saint George Formation rests uncomformably on a highly weathered saprolitic layer, inferred to be a "paleosol" (Kraus, 1999), radiocarbon dated at greater than 50,000 tears BP (Jayko, written communication, 1998), formed on the Mesozoic Franciscan Complex. In contrast to the medium to dark gray color and high degree of induration of unweathered Franciscan rocks, the paleosol is light gray to light tan or green, clay-rich and soft. The degree of lightening increasing and induration lessening gradationally upwards within a 40 cm zone of weathering (Figure

11).

Tables 1 and 2 (unpublished data of Aalto, 1999) presents data for samples collected at 10 cm intervals from the surface downwards within weathered Franciscan

Complex sandstone at the north side Point Saint George unconformity. Within the upper 30-40 cm of weathered Franciscan sandstone that constitutes the paleosol, matrix increases from 36 to 75%, QtFL%Qt increases from 38 to 87%, while QtFL%L Table 1. Fransiscan Paleosol sandstone composition with increasing depth based upon point counts of 300 grains per thin section. From Aalto 1999, written communication.

Depth (cn Qt* F L Qm F Lt Qp Lvm Lsm %matrix 0 87 8 5 48 8 44 69 31 0 75 10 54 16 30 30 16 54 45 44 11 48 20 47 33 20 27 33 40 50 20 30 53 30 38 47 15 18 47 35 57 37 6 36 40 34 43 23 21 43 36 36 53 11 40 50 35 42 23 19 42 39 41 46 13 47 60 39 46 15 22 46 32 53 34 13 40 70 41 29 30 21 29 50 40 36 24 45 80 35 47 18 17 47 36 50 44 6 37 100 37 41 22 17 41 42 48 33 19 37 Mean 45 35 20 24 35 41 49 38 13 46 StDev 16 14 7 9 14 7 10 9 9 12 *Qt: total quartzose (with chert), F: total feldspar, L: total lithics, Qm: monocrystalline quartz, Lt: total lithics (with chert) Qp: polycrystalline quartz, Lvm: volcanic and metavolcanic metamorphic lithics, Lsm: mudrock and metasedimentary lit 24 Table 2. Saint George Formation sandstone composition based upon point counts of 300 grains per thin section From Aalto, 1999 written communication

Sample # Qt* F L Qm F Lt Qp Lvm Lsm Qm P K DM Gr sz** 6A 72 21 7 63 21 16 56 19 25 75 19 6 0 Tr m 16 31 31 38 24 31 45 16 73 11 44 49 7 15 Tr f 17 30 27 43 21 27 52 17 75 8 44 52 4 39 Tr f 24 84 12 4 67 12 21 81 19 Tr 84 8 8 2 Tr m/c 35 58 37 5 40 37 23 78 22 0 52 41 7 0 Tr vf/f 36 61 23 16 42 23 35 54 34 12 64 28 8 0 Tr vf/f Mean 56 25 19 43 25 32 50 40 11 61 33 7 9 StDev 22 9 17 19 9 14 28 27 9 17 17 2 16

*Qt: total quartzose (with chert), F: total feldspar, L: lithics, Qm: monocrystalline quartz, Lt: total lithic (with chert), Qp: polycrystalline quartz, Lvm: volcanic and metavolcanic metamorphic lithics, Lsm: mudrock and metasedimentary lithics, P: plagioclase, K: potassium feldspar, D: dense minerals, M: detrital micas **Gr sz: grain size, vf: very fine, f: fine, m: medium, c: course, vc: very course 25 26 decreases from 20 to 5% and QtFL%F from 47 to 8%. This zone also contains clustered petal-like calcite ellipsoids (Microcodium) interpreted as calcified plant roots

(Figure 11; Klappa 1978).

Large fossil logs (Figure 13) are present along the unconformity at five sites

(marked by "F" in Figure 1). Stumps that range up to 2 meters in diameter with roots radiating outwards and downwards into the paleosol (Figure 14) are present at the northern and central exposures of the paleosol, at Pebble Beach and on the far north side of Point Saint George.

Many of the largest logs preserved within basal Saint George Formation strata lie with their long dimensions oriented generally north-northwest to north-northeast.

Soil appears compacted yet fresh at 4 exposures, with peds up to 2 cm across. The inferred paleosol is rich in plant fragments and in one exposure appears as a lignitic bog like deposit (Figure 15), with leaf traces as well as small sticks and twigs visible.

X-ray diffraction analysis reveals the presence of quartz+albite+white mica+chlorite, but no expandable clays.

Woody fragments, logs, and stumps in this layer have a shiny blackened lignitic appearance on the outside, with a fresher light brown to medium brown appearance on the inside. Tree rings are visible within many root and stump exposures. The shiny black surfaces of the logs, stumps, and woody debris suggest a certain amount of coalification. A mean vitrinite reflectance of 0.26-0.28% for four samples indicates that the wood is essentially unaltered thermally and was buried no greater than a few 100 m (M. B. Underwood, personal communication 2000). 27

Figure 13. Large logs in paleosol. North side of Point Saint George.

Figure 14. Large rooted stump in Franciscan Complex saprolitic paleosol. Scale on right is 10 cm. North side of Point Saint George. 28

Figure 15. Lignitic bog deposit, north Pebble Beach. Scale is 2 meters long.

The weathered Franciscan surface is interpreted as a late Miocene saprolitic paleosol with coastal forest that developed on the eastern belt Mesozoic Franciscan

Complex. Petrographically the clay rich texture of the inferred paleosol that grades down into eastern belt Franciscan graywacke appears to reflect decomposition of unstable lithic components and increasing concentrations of hydrated iron oxides at the expense of these grains and chlorite. Thus an episode of pre-late Miocene (pre-

Saint George deposition) soil development at Crescent City is in agreement with the classic view of Diller (1902) that a regional erosion surface (peneplain) formed in northwestern California at this time. This interpretation is supported by the similarity in age of the Saint George Formation and marine late Miocene Wimer Formation preserved in patches on elevated portions of this erosion surface 10 to 15 km east of

Crescent City (Stone 1993; Aalto et al., 1995; Irwin 1997). The regional extent of 29 this surface and existence of a thick paleosol indicates that erosion was insignificant and sedimentation relatively continuous.

Saint George Formation Unit 1

The basal member (unit 1) of the Saint George Formation unconformably

overlies the inferred paleosol and fines upward from poorly sorted basal conglomerate

to interstratified conglomerate and sandstone. Basal conglomerates are matrix-

supported diamictites, containing very poorly sorted (silts to gravels up to 5 cm),

chiefly angular to subangular chert, vein quartz, greenstone and sandstone pebbles and

cobbles of local derivation dispersed in muddy sandstone. Large wood fragments (5

cm up to 1 m) are abundant. Overlying this are up to 15 beds of stratified

conglomerates that consist of a gray to greenish brown, interbedded pebble

conglomerate, parallel laminated or cross-bedded, poorly sorted muddy sandstone and

sandy mudstone, arranged in fining-upwards cycles each several decimeters in

thickness (Figure 10 stratigraphic column; Figures 16 and 17). The uppermost muddy

portions of some cycles contain matted mixes of woody debris, needles and cones.

Upward within this unit clast rounding and sorting increase. The degree of

bioturbation is approximately 70%. Burrows are chiefly vertical, up to 2 cm across

and penetrate up to 15 cm of section. Clast size and the abundance of woody debris

diminish up section. 30

Figure 16. Saint George Formation unit 1- view northwest. Note the pulse-like layering of the bedding structures. North Pebble Beach. 31

Figure 17. Rhythmically bedded cycles within unit 1 of the Saint George Formation. View looking east on the north side of Point Saint George. Bedding strikes northwest 25 degrees and dips northeast about 19 degrees. Scale is 2 meters long.

By 3.5 m up-section the rhythmically bedded cycles are no longer evident.

Sorting improves and fine to medium grain, massive gray to brown sands with minor scattered pebbles, occasional claystone ripup clasts and rounded wood fragments are prevalent. At 4 m a distinct 20-40 cm-thick woody debris layer is present. Logs and plant fragment-rich mud dominate this layer. Immediately above this wood layer the unit again consists of fine to medium grain massive sands, with an absence of distinctive bedding structures. Towards the top of the unit at approximately 5 m up- section, the sandstone becomes highly bioturbated, defining the base of unit 2. 32 The presence of the diatom-rich rhythmically bedded cycles, woody debris and generally poor sorting in basal unit 1 suggests sediment influx in a series of pulses by high-energy currents, followed by relatively still (slack water?) conditions. The presence of late Miocene diatoms [discussed ahead] among these basal sediments confirm a marginal marine depositional setting (Barron, 1999, personal communication). These cycles perhaps reflect inundation of a coastal forest by a series of storm or tsunami waves, which would account for the preferential orientation of many large logs. Rapid submergence and sedimentation can also explain the excellent preservation of the stumps, logs and paleosol. Moreover, the absence of fossils or bioturbation in the basal portion of this unit of the Saint George Formation suggests a rapid sedimentation rate engendered by tsunami deposition. The course nature of the basal sediments suggests derivation from a mix of wave-transported course beach gravels and reworked terrestrial (forest floor) sediments. The presence of the woody layer at 4 m may reflect the settling out of the remains of the destroyed paleo-forest following inundation by a series of tsunami waves, or possibly some other type of catastrophic event such as a debris flow or El nino winter.

Saint George Formation Unit 2

Unit 2 consists of 10 m of interbedded medium gray muddy sandstone and

sandy mudstone containing minor blackened woody debris, pebbles, occasional

mollusk fossils and extensive bioturbation (Figure 18). Primary structures are absent.

Grain size fluctuates from very fine grain sands and silty clays up to gravel's less than 33 2 cm in diameter. From hand lens examination grains appear to consist of feldspar

and quartz, along with some disseminated fragments of black carbonaceous material.

Pebble clasts include vein quartz, chert, and serpentinite. Bedding is not visible in

most of the unit, but distinctive repetitious layering is present near the base of the

contact with unit 1. Such layers have sharply defined scoured bases marked by a

sudden coarsening, broken mollusk shell lag deposits and grade normally upwards into

zones of marked bioturbation.

The first mollusk concretion is present near 12 m up section, with an entire

layer 5-10 cm thick appearing just above the first concretion. The concretion layer

present on both sides of Point Saint George is orientated parallel to strike, and sits in

approximately the same stratigraphic position.

Figure 18. Saint George Formation unit 2. Note lignitic log within the Saint George Formation in the lower left of the photo. Scale is 2.5 meters long. North Pebble Beach. 34 Up-section, mollusk shells are both articulated and dispersed in mudstone, often preserved in concretions, and disarticulated and concentrated in 8-30 cm-thick layers. These layers occur at irregular intervals within the unit and reflect an increase in shell concentration within otherwise massive sandy mudstone. Such shell concentrations without associated scour may simply represent slackening in sedimentation rate (and thus more fossil accumulation). Unit 2 exhibits an overall fining upward trend and within finer bioturbated mudstones, the degree of bioturbation

[bioturbation index] decreases from 70% to 40%. Woody debris becomes smaller and less abundant higher up within unit 2. The contact with unit 3 is placed at the first appearance of current-reworked shelly debris at approximately 15.5 m up-section from the base of Unit 1.

Diatom paleoecology and sedimentary features suggest deposition in either an embayment with access to open marine conditions, or the nearshore upper shelf below normal wave base (Table 3). The overwhelming abundance of bivalves and absence of gastropods among the molluscan fauna, as well as the presence of brackish water diatoms, favors a muddy embayment and perhaps somewhat brackish water (Watkins

1974). The increase of bioturbation, as well as the overall fining-upwards trend throughout the unit suggests a lower energy, deeper water environment than that of unit 1. Table 3. Diatom Paleoecology. From Barron, 1999, written communication. 3 5 36

Saint George Formation Unit 3

At Pebble Beach, unit 3 is 33 m thick. The basal contact of unit 3 is marked by the beginning of a series of concentrated shell basal lag (couplet) layers (Figure 19) that are continuous along strike from south to north of Point Saint George. Just above the first set of concentrated shell couplet layers, unit 3 consists of medium gray sandy mudstone containing abundant bivalves that are either scattered randomly as articulated shells within the mudstone, or concentrated in 15-40 cm-thick shell-couplet layers commonly spaced from 20-30 cm up to 80-150 cm apart (Figure 19). These shell-rich layers commonly contain a more sandy, sometimes laminated basal zone of densely packed, articulated and disarticulated shells overlying minor scour surfaces which grades normally upwards into a zone of less sandy mudstone having dispersed, articulated shells. The highly fossiliferous portions of unit 3 have a mottled, crumbly appearance imparted by iron oxide staining and differential weathering along fossil molds. Less fossiliferous portions are massive in appearance. Some areas of unit 3 show a stepped appearance possibly related to normal faults within the Saint George

Formation as well as differential erosion of beds by wave action. The sandstone is texturally immature with more than 5 percent matrix and consists of angular to subangular grains.

At Pebble Beach, approximately 33 m up section at the 17.5 m level of unit 3, a normally graded, 180 cm-thick, hummocky cross-stratified sandstone (cf. Bourgeois

1980) overlies a scour surface channeled into fossiliferous mudstone (Figure 20). One 37

Figure 19. Saint George Formation unit 3. Note the shell couplet layers visible along strike. North Pebble Beach. Cliff height is about 10 meters. synformal hummock is approximately 2 m in length and 0.3 m in amplitude. The hummocky horizon is slightly more resistant to erosion than the two bounding bioturbated units. Small bits of coalified wood and disarticulated shells are present just above the basal erosional contact. Coarse, poorly sorted sandstone, above the basal scour surface, grades upwards within the bed to a very fine grain sandy mudstone. By 40 cm above the erosional unconformity grain size again begins to noticeably decrease to fine grain and very fine grain sandy mudstone. It should be noted that the hummocky cross stratification in the Saint George Formation is not repeated within unit 3 as it is at other similar formation outcrops along the coast in southern Oregon. The upper third of this sandstone is extensively bioturbated with vertical burrows that contain articulated mollusk shells. 38 Both north and south of Point Saint George the entire Saint George Formation exhibits an overall fining upwards and uppermost beds of unit 3 contain little sand, a decrease in bioturbation, and virtually no wood. The external exposures, of the many surfaces of unit three, exhibit an odd spheroidal weathering that may be related to the decreasing grain size up section in unit 3. The weathering first appears approximately

24 m up section, and appears to follow the fining upwards trends within the unit. A decrease in fossils is also noted in these finer grain spheriodally weathered portions of unit 3.

As with unit 2, concentrations of disarticulated shelly debris above scour surfaces and the concentration of shelly debris within the shell couplets very likely

Figure 20. Hummocky cross stratification within unit 3 of the Saint George Formation. North Pebble Beach. Scale is 2.5 meters long. 39 reflects current reworking, perhaps by large waves. The concentrated shell couplet layers may represent opportunistic communities that were established during brief periods of improved benthic oxygenation and increased substrate stability immediately prior to storm-related disturbance or burial (W. Miller, personal communication 1999).

The bivalve fauna is dominated by suspension feeders (Back, 1957), suggesting that the bottom waters were not very turbid. The diversity of fauna in the shell couplet layers is pretty low, suggesting the bivalve communities were opportunistic (W.

Miller, personal communication, 1999). This is common in ecologically stressed environments where certain organisms experience a population bloom during brief periods of favorable conditions (C. J. Tsujita, written communication 1998).

Although fossils are consistent with deposition in either a bay environment, or open marine conditions (Stone, 1993), the appearance of hummocky cross-stratified sandstone suggests deposition site was open to the ocean to permit incursion of large storm and/or tsunami waves (Dott and Bourgeois 1982; Leithold and Bourgeois 1984).

The absence of the hummocky cross stratification within unit 3 on the north side of point Saint George may be due to the presence of a shear zone disrupting that portion of unit 3, or possibly the layer was missed due to the different exposure angles and perspectives that exist north of Point Saint George.

The abundance of bioturbation and paucity of physical sedimentary structures in this portion of unit 3 suggests that biological processes overwhelmed the imprint of even the highest-energy physical processes. Bioturbated sandy-mudstone of inner- shelf subfacies probably represents deposition under relatively quiet water conditions. 40 Sediments that are deposited seaward of the surf zone on modern shelves are typically highly bioturbated (De Celles, 1987). Since few studies have documented the differences between proximal and distal storm deposits, unit 3 is interpreted as a low energy, proximal open shelf nearshore environment beyond normal wave base. The greatest preservation potential for hummocky cross stratification appears to be between fair weather wave base and storm wave base, typically between nearshore and outer-shelf at a water depth of a few to several tens of meters (Stone, 1993). Water depths for unit 3 are estimated to be greater than 10 m. The overall decrease in grain size, energy level decrease up section, and decreasing bioturbation within unit 3 suggest the possible continuation of regional subsidence is recorded in the Saint

George Formation. Islands may have been present in the region accounting for some of the difficulty in paleoenvironmental interpretation.

Provenance

Basal conglomerate clasts reflect local derivation, consisting of common

Franciscan Complex lithologies such as sandstone, chert, greenstone, and vein quartz.

Saint George Formation sandstones of upper unit one and units two and three exhibit detrital modes, N=5 (Table 2): %Qt-F-L=56-25-19, %Qm-F-Lt=43-25-32, %Qm-P-

K=60-33-7 (Tables 1 and 2). Sandstone grains are principally derived from lithologies and minerals consistent with those of the Franciscan Complex, Galice Formation, and or the Josephine ophialite (Stone, 1993). Significant potassium feldspar (K-spar) content suggests possible sediment input from a source(s) other than the Klamath 41 Mountains or Coast Ranges since these provide comparatively K-spar poor sands

(Underwood and Bachman 1986; Aalto 1989, 1992). Among late Miocene-early

Pleistocene sandstones within the Wildcat Group, exposed in the Coast Ranges some

145 km to the south, Aalto et al., (1995, 1998) recognized sediment contribution from the Idaho batholith based upon sandstone petrology (specifically the abundance of K- spar) and 40Ar / 39Ar dating of detrital micas. Given the fact that the Saint George

Formation is coeval with transgressive marine Wildcat Group basal sandstones based on diatom analysis, it is reasonable to consider an Idaho batholith derivation for some of the K-spar despite obvious Klamath Mountains basement contributions to basal conglomerates of both the Saint George and coeval Wimer Formations. PALEONTOLOGY

The molluscan fauna is dominated by bivalves and in need of further study

(William Miller, personal communication 1999). Marine fossils found in Saint

George Formation float on Fort Dick Beach and identified by Hertlein (in Back, 1957, p.21) include: Cardium corbis Martyn, ?Cryptomya sp., Macoma nasuta Conrad,

Protothaca cf. P. staleyi Gabb (juvenile), ?Saxidomus sp, Solen cf. S. sicarius

Gould, ?Ocenebra sp. (cast), Panomya ampla Da11, Siliqua patula nuttalii Conrad,

Yoldia strigata Dall, and Polinices sp., as well as an extinct species of sand dollar,

Anorthoscutum cf. A. oregonense quaylei Grant and Hertlein.. This assemblage of fossils is similar to others occurring during the Pliocene in northern California (Back,

1957).

Diatoms have proven to be the most reliable means of dating and correlating the Monterey Formation and overlying siliceous units such as the Capistrano, the

Sisquoc, and the Purisma Formations (Barron, 1986). Stratigraphic ranges of diatoms have been documented in numerous sections, both in onshore and offshore areas of

California (Barron, 1986).

The most widely used diatom zonation in California has been the northeastern

Pacific diatom zonation of Barron, 1981 (Barron, 1986). The Barron, 1981 zonation utilizes mostly temperate to cool water planktic diatom species, was synthesized from offshore DSDP Sites 173, 469, 470, and 472 as well as from an onshore section at

Newport Beach (Barron, 1986). DSDP 173 was located just offshore and south of

42 43 Cape Mendocino, approximately 200 km southwest of the study area. It was the

Barron (1986) diatom zonation that was used to obtain the relative age dates for the basal Saint George Formation and give an estimated age as to when the paleoforest/paleosol was buried.

It should be noted that the data used by the correlation model of Barron (1986) points out that within the entire Humboldt Basin the time interval between 7.8 and 5.3

Ma is missing. A portion of this record, 6.4-5.38 Ma (latest Miocene), has been found and is continuous in the Saint George Formation along the cliffs at Pebble Beach

(Barron, written communication, 1998) and is described below.

Latest Miocene diatoms and silicoflagellates of the Saint George Formation include Delphineis sachalinensis (late Miocene to Pliocene), Lithodesmium minusculum (latest Miocene to earliest Pliocene), Thalassiosira antiqua (latest

Miocene to Pliocene), Thalassiosira temperei (last occurrence at 5.4 Ma in the North

Pacific), and Thalassiosira miocenica (limited to the interval between 6.4 and 6.0 Ma)

(Yanagisawa and Akiba, 1998), and Dictyocha aspera clinata (the silicoflagellate)

(Table 3). The absence of Thalassiosira oestrupii (first occurrence at 5.5 Ma) is supportive of the age range of 6.4-5.38 Ma assigned to the Saint George Formation within the study area by Barron (1998, written communication). Both open ocean, intertidal, and brackish water diatoms were found in samples taken from within the

Saint George Formation. This diatom assemblage is distinctive and typical of biosiliceous sediments that directly and often uncomformably overly the Monterey

Formation (Purissima Formation of Santa Cruz; Sisquoc Formation of Santa Maria 44 and Santa Barbara basins), and in the basal part of equivalent terrigenous-enriched, biosiliceous sediments at DSDP 173 (Barron, 1998 written communication).

Bioturbation structures are common in unit 2 of the Saint George Formation, with several different sizes and types present. An example of unit 2 bioturbation is seen in

Figure 22.

An examination of palynormorphs by Jacobson and Consultants from (Stone,

1993) revealed mainly woody debris, with rare palynomorphs including marine dinoflagellate cysts, plus spores and pollen. The dinocysts include ?Pentadinium sp.,

Selenopemphix spp., Spiniferites frigidus, ?Thalassiphora sp., and Xandarodinium variable. They indicate an age range from late Miocene-Pliocene-early Pleistocene, based on the northern Pacific ranges of S. freigidus (late Miocene to recent) and X. variable (middle Miocene-early Pleistocene). The terrestrial palynomorphs include fern spores (Cyathidites sp., Laevigatosporites ovatus, Polpodiisporites sp.), conifer pollen and angiosperm pollen (including Carya sp., and Myriophyllum sp.) (Stone,

1993).

The plant debris, which consists of ovulate cones, wood, and palynomorphs, suggests the Point Saint George forest was composed primarily of conifers with ferns in the understory. Cones 1.8 - 3 cm long resembling those of the genus Tsuga

(hemlock) are preserved three-dimensionally in a small sandstone block collected from the North Point Saint George site (Figure 21). Their small size is consistent with

Tsuga heterophylla (western hemlock), the lower elevation species commonly found today along the northern Pacific coast. Identity of the wood from the North Point 45

Figure 21. Late Miocene pinecones found within Saint George Formation unit 1 sediments. Sample was found as float on the north side of Point Saint George. Scale is 3 cm. Analysis done by Diane Erwin of the University of California at Berkeley.

Figure 22. Close up of a bioturbation structure within unit 2 of the Saint George Formation. Exposure is located at the south end of Pebble Beach. Camera case is 6 inches long. 46 Saint George site is less certain but available characters match Larix (larch),

Pinus (pine), and Pseudotsuga (Douglas fir). Possible presence of early wood tracheids with spiral thickenings in the fossil wood would eliminate Larix and Pinus, but their presence needs to be confirmed. Pinus is, however, recorded in the pollen record.

Holocene Exposures

At Pebble Beach, an entirely organic Holocene peat containing abundant woody debris was exposed at very low tides (February-March, 2000), near the thrust

(Figure 24). Deposition of the peat, which unconformably overlies unit 2, of the Saint

George Formation, possibly pre-dates motion on this fault. If so, it has been elevated by faulting to a higher topographic position and juxtaposed against unit 3 Saint George

Formation mudstone. A conventional radiocarbon date of 3000± 60 BP was obtained from two pine cones (Figure 23) from these younger sediments (the 2 sigma calibrated range is Cal BP 3355 to 2980, Beta Analytic, Inc., Miami, FL) (Erwin, 2000, written communication).

Small to medium size fossil logs (Figures 23 and 24) are present at one site along mid- Pebble Beach (marked by "H" in Figure 1). Stumps that range up to .75 meters in diameter with roots radiating outwards and downwards into unit 2 form a second younger peat/bog like deposit. Many of the logs and wood fragments preserved within this deposit lie with their long dimensions oriented generally north-northwest to north-northeast. Material from this exposure is not lignitic and has none of the 47

Figure 23. Pinecones found adjacent to the Pebble Beach Fault within the Holocene bog exposure. Cones were radiocarbon dated as 3000 years b.p.± 60

Figure 24. 3000 year old (± 60 years) bog exposure, mid-Pebble Beach. Note the Pebble Beach Fault runs northwest under the riprap and across Point Saint George. Unit 1 and 2 Saint George Formation sediments are visible in the background. 48 qualities associated with the late Miocene exposures. This location is related to more recent activity in the study area and is not related to the much older late Miocene stumps and paleosol described earlier. DEPOSITIONAL HISTORY

Saint George Formation stratigraphy suggests that rapid subsidence of the leading edge of North America occurred during the late Miocene. The relative depth changes indicated in the Saint George Formation stratigraphic sequence probably reflects changes in the balance between sediment supply, basin subsidence, and eustatic sea-level changes (Leithold and Bourgeois, 1984). Deposition and reworking by chaotic seas associated with local storms and by long period swell associated with distant storms may he indicated by the presence of hummocky cross stratified beds in unit 3 of the Saint George Formation (Leithold and Bourgeois, 1984). Biogenic structures are an important clue to the rates and frequencies of deposition within the

Saint George Formation (Leithold and Bourgeios, 1984). The absence of biogenic structures in the basal Saint George Formation sediments indicates rapid sedimentation and burial of the late Miocene forest. The increase in biogenic structures within unit 2 of the Saint George Formation indicates a slowing of sedimentation rates. The variation in biogenic structures throughout unit 3 of the Saint

George Formation may be a function of both eustatic and local sea-level changes in the late Miocene. The basin in which the Saint George Formation was being deposited in throughout the latest Miocene continued to subside relative to sea level until at least

5.38 m.y.

The existence of the paleosol indicates that at least some period of landscape stability and tectonic quiescence is recorded in the late Miocene in an otherwise

49 50 tectonically active continental margin (Kraus, 1999). Paleosols are also helpful in stratigraphic studies, they can be used for studies at both the local and regional level.

The formation of uncomformities is controlled by allogenic factors such as sea level

fluctuations, global or regional climate change, and regional tectonics, processes that

influence geomorphic systems over time intervals from hundreds of thousands to tens

of millions of years. Uncomformities are commonly regional in scale, and they can be

highly irregular surfaces along which the amount of missing time varies considerably.

Consequently, the paleosol associated with an uncomformity can show lateral changes

on a regional scale, and these changes can be used to interpret variations in topography

and missing time along an ancient landscape (Kraus, 1999). Thus the development of

the paleosol on Franciscan Formation turbidites at Point Saint George may be regional

in scale. The presence of rooted stumps, large logs, soil pedogenesis, and

Microcardium also support this interpretation.

Based on vitronite reflectance data the late Miocene forest under the Saint

George Formation was buried no greater than a few hundred meters. The excellent

preservation of the rooted tree stumps, woody debris, and paleosol (Figures 11, 12, 13,

14 and 15) at the base of the Saint George Formation suggests the late Miocene

paleoforest found at Point Saint George underwent rapid burial, followed by continued

subsidence of the region. Bioturbation structures in units 2 and 3 suggest a decrease in

sedimentation rates throughout the late Miocene. The coeval Wimer Formation to the

east of Point Saint George suggests as much as a 10 km eastward shift in the shoreline

in late Miocene time (Stone, 1993). 51 A late Miocene coastal forest may have been inundated by a tsunami and the forest floor down-dropped at least several meters to allow deposition of littoral marine sediments over basal conglomerates. The marine sands that overlie the late Miocene forest are from 1/4 m to 2 m thick. If the source of the marine sand depositionally overlying the paleoforest is from a tsunami, then it is reasonable to assume the event that generated the tsunami was of extremely large magnitude and relatively close to the region surrounding Point Saint George in the late Miocene, possibly along the inferred Del Norte fault of previous studies. Where Holocene event coseismic subsidence submerges the coast, the event horizon is commonly preserved as a contact between underlying terrestrial or upper intertidal wetland peats or soils, and the overlying intertidal mud or sand (Figure 25) (Carver, 1996).

Crescent City California was the site of a destructive tsunami following the

1964 Alaskan earthquake. Vibracoring in back beach freshwater marshes in the

Crescent City area, reveals the presence of multiple (up to 13) tsunami derived sands interstratified with peat horizons (Garrison-Laney, 1998). These contain marine diatoms and extend at least 500 m landward of the present beach (Aalto et al., 1999).

While the 1964 tsunami deposited less than a centimeter of sand in one marsh south of

Crescent City, older tsunami derived sands in lower portions of the cores are as much as 15 cm thick (Aalto et al., 1999). The sand appears clean, fine to medium grained, and has some fine organic debris scattered throughout (Aalto et al., 1999). The sand contacts are sharply defined, and abruptly overlying muddy reddish brown peat (Aalto et al.., 1999). All of this evidence is remarkably similar to what is seen at the base of 52

Figure 25. Model depicting a coastal forest being inundated by tsunami (modified from Carver, 1996). This is what may have happened to the late Miocene forest located at Point Saint George, with the region eventually getting buried under a few hundred meters of sediment. Uplift of the late Miocene forest and overlying Saint George Formation most likely occurred during the late Pleistocene. the late M iocene Saint George Formation. The only difference is the total thickness of the marine sands.

In Carver (1996) five general field criteria for recognizing abrupt coseismic subsidence of the coast are listed: (1) suddenness of submergence, (2) amount and permanence of submergence, (3) spatial extent of submergence, (4) coincidence of tsunami or liquifaction sands at the event horizon, and (5) synchroneity of submergence. All 5 of these field criteria are present within the Saint George

Formation in the Crescent City area. This supports the model that the late Miocene paleoforest was rapidly buried by a tsunami following a large magnitude seismic event occurring offshore along the Cascadia subduction zone.

The suddenness of submergence is reflected by the abrupt contact between sediments containing evidence of distinctly different elevations relative to sea level

(Carver, 1996). In many bays and estuaries, sudden submergence places vegetated 53 coastal wetlands into newly formed, highly active subaqueous depositional environments where quick burial by intertidal mud and sand is possible (Carver,

1996). In such settings the above ground stems and leaves of herbs and sedges are entombed in the mud and preserved in growth position (Carver, 1996). The stumps of

trees growing along the pre-earthquake shoreline and in brackish coastal wetlands may

also be buried and preserved in intertidal mud following large subsidence events

(Carver, 1996). Other indicators of sudden submergence are an abrupt change from

subaerial to intertidal pollen, diatoms, or foraminifera at the peat-mud contact (Carver,

1996).

Few nontectonic mechanisms can quickly cause a relative sea level change of

greater than 0.5 m, so the amount and permanence of sea level change is a critical

criterion (Carver, 1996). Field evidence for greater than 0.5 m change in relative sea

level includes lithologic or biostratigraphic successions that bypass one or more

intermediate facies or faunal zones, which would have been present if subsidence had

been gradual (Carver, 1996). If the event horizon indicates a greater than 0.5 m

relative sea level change, and the change in relative sea level is permanent, a

coseismic origin is indicated (Carver, 1996).

Changes in relative sea level due to earthquakes should have a much wider

spatial extent than vertical changes of similar magnitude that may result from

nontectonic mechanisms such as channel bar migration or river flooding which should

be restricted to individual estuaries (Carver, 1996). In the Pacific Northwest, event

horizons that can be correlated between core holes over an entire estuary (hundreds of 54 meters to a few kilometers laterally) or between estuaries are often considered to be coseismic (Carver, 1996). The coincidence of tsunami sands or liquifaction features with peat-mud contacts is a strong indicator that the contact is coseismic (Carver,

1996). This is what is scene in the field at Point Saint George with the late Miocene paleoforest exposures, as well as with the possible Holocene tsunami derived sands discussed earlier.

Tsunamis produced by regional seafloor deformation during subduction earthquakes typically include trains of waves with periods of several tens of minutes

(Carver, 1996). Successive wave crests arrive along the coast for several hours and create repeated landward surges, followed by seaward return flows of the marine water

(Carver, 1996). From a paleoseismological standpoint, the tendency for a tsunami to

transport and deposit sand and silt in the coastal zone is most important (Carver,

1996). Thin, often discontinuous, sand sheets have been observed in areas inundated

by several modern tsunamis including Chili 1960, Alaska 1964, Nicaragua 1992,

Indonesia1992, and Japan 1993 (Carver, 1996). Where environments are favorable for

deposition and preservation, the waves are mainly recorded as thin discontinuous sand

sheets (Carver, 1996). These sites are located where abundant sand is present seaward

of a coastal lagoon, marsh, or other low-lying depositional site (Carver, 1996).

At most sites the 1964 Alaska tsunami is represented by thin (less than 10 cm),

well sorted, massive sand lying sharply on the pre-earthquake surface (Carver, 1996).

At some locations the sand is normally graded and locally contains rip up peat clasts,

scattered pebbles and cobbles, and woody debris (Carver, 1996). As many as 4 55 upward fining sequences of well sorted tsunami derived sand sheets are recorded at

Kalsin and Middle Bay on Kodiak Island (Carver, 1996). Figures 16 and 17, taken at the basal Saint George Formation/paleosol contact on both the north and south side of

Point Saint George show what appears to be stratigraphic evidence of these tsunami generated wave trains inundating the paleoforest and burying it under several meters of thin marine sand and gravel sheet like deposits. These thin sheet like layers are composed of several fining upwards sequences interpreted to represent successive wave train pulses as described in Carver (1996).

The late Miocene buried paleoforest found at Point Saint George (see Figures

11 through 15) satisfies all five of the criteria listed above. All five of the field criteria for recognizing abrupt coseismic subsidence of coasts are met at all exposures of the paleoforest covering an area of several kilometers. In Carver (1996) it states that it is unlikely that any nonseismic contact could satisfy more than two of these criteria.

This supports the model that the area around Point Saint George underwent significant subsidence following a large magnitude Cascadia subduction zone event in the late

Miocene. The general paleotectonic transition from Miocene and Pliocene time to present is shown by comparison in Figure 7 (Nilsen and Clarke, 1989). Northward migration of the Mendocino triple junction (MTJ) during late Cenozoic time is broadly matched by (1) northward migration of the southern erosional edge of the contiguous

Eel River forearc basin, (2) northern migration of the northern strike-slip faulting associated with the San Andreas fault system, and (3) northern migration of the southern limit of active magmatic arc volcanism (Nilsen and Clarke, 1989). 56 Local subsidence may relate to seaward slumping of the coastal plane, but rapid regional subsidence is suggested by the record of a rapid transition from shallow to deep marine sedimentation within the coeval Pullen Formation to the south (Aalto et al. 1995, 1996), as well as an approximately 10-km eastward shift of the shoreline recorded in the coeval basal Wimer Formation (Stone 1993). Differences in water depth between the Saint George Formation and the Pullen formation to the south may be related to the progressive younging of the subducting plate to the south. The younger subducting plate to the south may have resulted in differential subsidence along the Cascadia subduction zone. The younger subducting plate could have been responsible for a greater volume of material to be eroded away from underneath the

North American plate during the late Miocene subduction erosion event (Aalto 2000, written communication). Although this may coincide with an episode of eustatic sea level rise (Abreu and Anderson 1998), the rise is insufficient to account for the episode of deepening recorded in the Pullen Formation.

Aalto et al. (1996, 1998) suggest that coastal subsidence was related to an episode of subduction erosion and subsequent extension of the forearc, and that subducted sediments were eventually transferred to a position beneath the Klamath

Mountains block. This ultimately resulted in the Pleistocene uplift of the Klamath erosion surface and erosional stripping of a Wildcat Group-Saint George-Wimer

Formation shelf sediment blanket.

Uncomformities in the late middle Miocene (ca. 10.7 to 9.0 Ma) and latest

Miocene (ca. 7.0 to 6.0 Ma) are common in sections of the Monterey Formation and 57 its equivalents found offshore of the California coast (Barron, 1986a,b). These

uncomformities in the late middle Miocene and latest Miocene appear to be associated

with major tectonic events (Barron, 1986a,b). Barron (1986a, b) notes that

unconformities are commonplace in California at the top of the Monterey Formation,

where they are typically centered at about 6.8 Ma (age updated to Berggren et al.,

1995 time scale). In many places these unconformities are characterized by angular

discordance between the Monterey Formation and the overlying more terrigenous-rich

diatomaceous rocks (e.g., the Sisquoc and Purisima Formations), supporting the

presence of a region-wide tectonic event. The stratigraphy scene in the Saint George

Formation at Point Saint George is inferred to result from coseismic subsidence

caused by slip on the late Miocene Cascadia subduction zone megathrust landward of

the downdip limit of rupture. The model proposed here uses the idea of subduction

erosion as the mechanism responsible for causing not only the 10 kilometer eastward

shift in the shoreline recorded in the coeval Wimer Formation, but also the regional

subsidence recorded both within the Saint George Formation at Crescent City and the

coeval Pullen Formation located over 100 kilometers to the south. STRUCTURAL GEOLOGY

Introduction

The Saint George Formation dips easterly along Pebble Beach and is preserved in an open, northwest-trending syncline north of Point Saint George (Figure 26).

North of Point Saint George, dip reversal across the syncline results in exposure of the

Franciscan paleosol-basal Saint George conglomerate contact to the south and paleosol with rooted stumps overlain by swash-cross-stratified beach sands to the north. South of Point Saint George the Saint George Formation is preserved along the cliffs at Pebble Beach for a distance of approximately 2 kilometers. Near mid-Pebble

Beach the Saint George Formation drops beneath the modern beach sands just to the southeast of the Pebble Beach thrust fault, then reappears approximately 100 meters to the southeast. Rooted lignitic stumps and paleosol, as well as a lignitic bog like deposit are present at 3 separate locations along north Pebble Beach, with the basal paleosol exposed on the wave cut platform at the southeast end of Pebble Beach

(Figure 1). Fresher, non-lignitic stumps and logs are also present at north Pebble

Beach and may be related to the Holocene peat deposit found approximately 1 kilometer to the south at mid-Pebble Beach (Figure 1) adjacent to the Pebble Beach thrust fault. These fresher, non-lignitic stumps and logs on the north end of Pebble

Beach show none of the properties associated with the lignitic late Miocene logs and stumps found at other locations both north and south of Point Saint George.

58 59

Sample location for thin section analysis.

Sample locations for diatom analysis. Samples taken North to South from Basal Saint George Unit 1 at (a) and at 1M intervals along cliff exposure up through St. George Formation Unit 3 at (b)

Figure 26. Air Photo of Point Saint George. Pebble Beach runs from just south of Point Saint George to the lower center of the photo. Note the linear trend of the topography and vegetation across the point (private photo provided by Aalto, 1998). 60 Faults and Folds

The Saint George thrust fault lies immediately offshore (Figure 8) and has the same north-northwest strike and southwest vergence as the newly-described thrust fault (the "Pebble Beach thrust") that runs onshore from mid-Pebble Beach to north of

Point Saint George (Figure 1). The Pebble Beach thrust at mid-Pebble Beach is exposed both on the wave cut platform fracturing units 1 and 2 of the Saint George

Formation, and is visible cutting across Point Saint George in air photo analysis

(Figure 26). The Pebble Beach thrust may also be responsible for the cliff failure at mid-Pebble Beach, but rip-rap put in place several years ago conceals evidence of this.

At the north end of Pebble Beach removal of sand for the first time in July

2000, exposed a plunging anticline-syncline pair disrupted by a small reverse fault

(Figure 27). The folds are U-shaped, symmetrical, have widths of about 8 m and plunge north-northwest at about 10 degrees. They do not appear on the north side of

Point Saint George. However, the small fault that disrupts the fold pair may extend to the north side, for in its line of projection are two subvertical reverse faults, the southernmost dipping 80 and the northernmost dipping 45 degrees northeast respectively. These latter reverse fault drops Pleistocene terrace deposits 2.5 meters south side downwards. The former fault shows at least this much apparent offset and possible strike slip striations. Both faults juxtapose young terrace deposits against basal Saint George Formation in a step-like manner. Holocene windblown sands show evidence of disturbance by the smaller northwest striking fault. The lineament map drawn from air photos (Figure 28) shows a fracture pattern that may reflect 61

Figure 27. Structures within the Saint George Formation found on the south side of Point Saint George. The U-shaped folds are symmetrical and plunge northwest about 10 degrees. The Pebble Beach Fault cuts across Point Saint George several hundred meters to the east (Aalto, 2000, written communication). For scale and orientation see Figure 1. 62

Figure 28. Idealized lineation map drawn from color air photo's of Point Saint George. Note how the drainage/linear pattern cuts across all of the wind blown Holocene sands that cover most Point Saint George. 63 deformation on this smaller fault. Small normal faults are also present within the Saint

George Formation along north Pebble Beach.

Numerous east-northeast, west-southwest trending normal faults are also present within the Saint George Formation in the cliffs along north Pebble Beach.

They commonly drop unit 2 Saint George Formation sediments down to the southeast in a step like manner. Wave cut benches cut into Unit 2 show normal fault offsets of up to several meters. These normal faults do not cut into the late Pleistocene terraces above the Saint George Formation unconformity along Pebble Beach.

Structural Data

Fault, fold, and clustered fracture data (Figures 29, and 30) record dominantly east-northeast west-southwest crustal shortening among lower plate (relative to the

Pebble Beach fault) rocks, and north-northwest south-southeast extension among both upper and lower plate rocks along Pebble Beach. Joint and fracture data (Figure 29a) taken north of Point Saint George on the wave cut platform within Saint George

Formation units 2 and 3 shows 3 distinct sets of fractures. One set oriented northwest, one set oriented northeast (the majority of fractures), and a smaller set oriented east- west. This fracture data is similar to fracture data taken on the wave cut platform and within the cliffs along Pebble Beach on the south side of Point Saint George in other studies (Jayko, unpublished data, written communication, 1998). 64

Figure 29. Rose diagram from fracture data collected on both side of Point Saint George (a.), poles to fault planes on the (b.)

Figure 30. Stereonet of bedding planes measured from within the Saint George Formation. Data is from both the north and side sides of Point Saint George. n=5 (a.) Stereonet of rake data collected from within the broad syncline on the north side of Point Saint George. n = 17. (b.) 65 Dips across the fold on the north side of Point Saint George do not exceed 20

degrees. The syncline on the north side of Point Saint George is a broad open fold,

with a vertical axial surface and gentle northwest plunge. Orientations of 17 small

faults measured for rake data within the syncline were varied, with only the most

easily recognizable striations being measured. Rake data was collected by breaking

off a large enough piece of the hanging wall (or upper plate) to be able to clearly

recognize fault striations and take accurate rake measurements. The fault rake data

taken from within the syncline (Figures 31, 32, and 33) north of Point Saint George

shows flexural slip within the fold and has resulted in the formation of near-bedding-

parallel mudstone breccias.

Figure 31. Beginning of the broad syncline with an internal shear zone on the north side of Point Saint George. This syncline is in line with the lineation drawn from the color air photos seen in Figure 28. 66

Figure 32. Fault plane with striations found within broad syncline. North side of Point Saint George.

Figure 33. "Taffy" like appearance of Saint George Formation unit 2 and 3 found in the shear zone within the broad syncline on the north side of Point Saint George. 67 Tectonic Interpretation of Structures

This structural data is compatible with fold and fault data from other onshore and offshore studies, reflecting Quaternary Gorda-North America plate interaction

(Clarke, 1992; Robinson et al., 2001). Previous structural geology studies of the

Crescent City area by Maxson (1933), Back (1957), Stone (1993), and Polenz (1997) have discussed possible structures and mechanisms for the development of the

Crescent City coastal plain. The inferred Del Norte fault (Figure 9) was originally invoked by Maxson (1933) to explain the steep linear front of the Siskiyou Mountains just east of Crescent City (Polenz, 1997). More recent studies have suggested a second, separate fault on the northwest trending segment north of the town of Smith

River, while other studies have advocated only the north-south trending segment

(Polenz, 1997). Still other studies have questioned the need for any fault (Polenz,

1997). The model this study develops attempts to explain the existence of this steep mountain front and uses it as the basis for the initial dropping of the Crescent City coastal plain in late Miocene time.

Uplift rates for the Crescent City coastal plain were calculated in Polenz (1996). The

area of the highest uplift rates was in the vicinity of Point Saint George and were

calculated to be approximately 0.2 mm/yr on the north side of point Saint George, and

0.1 mm/yr along Pebble Beach (Polenz, 1996). The zone of relatively high uplift rates

from Point Saint George to the southeast defines the southwestern flank of the Lake

Earl syncline and parallels the structural trend of the Saint George Reef scarp (Polenz,

1996). The reef scarp is an expression of the Saint George anticline, which has been

interpreted as the hanging wall anticline in the upper plate of 68

Figure 34. Idealized lineament map drawn from the USGS 7.5 minute quadrangle for Crescent City and Sisters Rocks. The dashed line running along the western edge of the Siskiyou Mountains represents the inferred "Del Norte" fault. Note the fracture pattern made by drawing in the drainage pattern over the entire Crescent City Platform. 69 the northwest trending, northeast dipping Saint George thrust fault (Figure 8). The trace the Saint George thrust fault is offshore just to the west of the reef scarp (Polenz,

1996). Cumulative offset on the Saint George fault has been estimated to exceed one kilometer (Clarke, 1992, unpublished seismic profile data, and personal communication 1998).

Movement along the Saint George thrust fault may explain the growth of the

Lake Earl syncline (Polenz, 1996). The northeast tilting of the Saint George

Formation along Pebble Beach and north of Point Saint George are both a product of recent activity on the Saint George thrust fault as well as possible back tilting resulting from activity on the Del Norte fault in the late Miocene. The idea of the northeasterly dip of the Saint George Formation being from downward movement of the Crescent

City block prior to deposition of the overlying late Pleistocene Battery Formation was first proposed by Back (1957). Back (1957) also suggests that movement along the

Del Norte fault occurred during one or more intervals before and after deposition of the late Miocene Saint George Formation, in agreement with the model proposed for the down dropping of the late Miocene forest and observations made in the field during this study.

Building on earlier paleomagnetic models, previous studies have linked rotation of the Cascadia fore arc to translation of the Sierra Nevada in an integrated model for Neogene deformation of the Cordillara (Wells et al., 1998). Assuming

Neogene coastal rotations are still occurring today and linking them to geodetic data for current motion of the Sierra Nevada, Euler poles of rotation for the Oregon fore arc 70

Figure 35. Tectonic setting of the Cascadia subduction zone. Juan de fuca plate is subducting is subducting beneath North America at an oblique angle. The Euler rotation poles relative to North America shown for the Oregon Coastal (OC) block and the Sierra Nevada (SN) block is located directly behind the Crescent City coastal plain (modified from Wells et al., 1998, Figure 1). block and its motion with respect to North America have been calculated (Wells et al.

1998). The Euler pole of rotation for the Oregon fore arc block is located approximately 100 km to the east of the Crescent City coastal plain in the Klamath

Mountains (Figure 35). During the Cenozoic the Oregon forearc block has been 71 rotating clockwise at about 1.5 degrees/m.y. (Wells et al., 1998). Changes in plate convergence vectors post Saint George Formation deposition and pre-100,000 yr terraces may be the result of the continued migration of the Mendocino Triple Junction

(MTJ). The block rotations described in Wells et al., (1998) is most likely related to the continued MTJ migration northward.

Oblique subduction of the Juan de Fuca plate northeastward beneath North

America has created a complex, seismically active convergent margin and volcanic arc in the Pacific Northwest (Figure 35) (Wells et al., 1998). This oblique subduction along the Cascadia subduction zone as well as continued northward migration of the

Mendocino Triple Junction may in part be responsible for some of the clockwise rotation of the region. Northward motion of the Coast Range averages 6 mm/yr, and as much as 17 mm/yr northward transport of the accretionary complex may be occurring offshore at the deformation front (Wells et al., 1998). The translation of the

Cascadia fore arc has been linked to motion of the Sierra Nevada bloc, which is translating northwest at 1cm/yr as a result of Pacific-North America dextral shear and

Basin and Range extension (Wells et al., 1998).

An active accretionary fold-and-thrust belt lies outboard of the Oregon block along the subduction zone (Wells et al., 1998). The Crescent City coastal plain and

Point Saint George lie at the back edge of this fold and thrust belt. At regional scales such fault systems tend to form long, arcuate belts containing generally parallel faults bounding elongate slices of rigid crust (Carver, 1996). In cross section imbricate thrust systems are commonly wedge or lens shaped (Carver, 1996). Large 72 accretionary fold and thrust systems along convergent plate margins are commonly modeled as the faults in internally deforming wedges, much like a pile of snow being pushed by a snow plow (Carver, 1996). Thrust faults are commonly interspersed with folds, particularly folds produced by the movement of thrust sheets over bends in underlying thrusts (fault bend folds or ramp folds) and formed at the propagating ends of thrusts (fault propagation folds) (Carver, 1996). Perhaps due to the complex interaction of faulting and folding, surface displacement on thrust faults tends to be irregular along strike, even more so than with other types of faults (Carver, 1996).

On a regional scale the Klamath Mountains may be acting as a backstop for the

Crescent City coastal plain as it is compressed within this fold-and-thrust belt and continues the clockwise rotation that has been ongoing throughout the Neogene. The model proposed in this study has the northward migration of the MTJ trying to force the Crescent City coastal plain around a corner, with the Klamath mountains acting as the backstop, and the Oregon coastal block continuing to rotate clockwise. This model fits the observed joint and fracture data seen in Figure 29, as well as the lineament pattern drawn from the USGS 7.5 minute quadrangle map (Figure 34). The forcing around a corner model explains the east-west oriented fractures measured within the

Saint George Formation as well as the east-west oriented drainage pattern drawn from the USGS 7.5 minute quadrangle map.

Fold-and-thrust belts are comprised of faults that are linked in a three dimensional network of ramps and flats. Frontal, parallel, and oblique ramps strike perpendicular, parallel, and obliquely to the tectonic transport direction (Apotria, 73 1995). Oblique ramps are scale independent and have several structural associations.

Oblique ramps transfer displacement, accommodate differential displacement, or displacement termination (Apotria, 1995). Recent studies have indicated that thrust sheet rotation associated with oblique ramps, the sense of rotation depends on the strike of the oblique ramp with respect to the transport direction (Apotria, 1995).

Folds in both the hanging wall and footwall of the oblique thrust faults in Wyoming measured from one meter to several meters, with several of the folds exhibiting significant plunge (Apotria, 1995). These faults have been described in Apotria

(1995) as oblique thrust faults.

The structural data collected at the north end of Pebble Beach, where folds are

U-shaped, symmetric, about 8 meters across, and plunge approximately 10 degrees north-northwest, is similar to folds of the thrust faults described in Apotria (1995).

The folds of the thrust faults described in Apotria (1995) are located in the hanging wall near the frontal-oblique thrust fault ramp are not parallel to bedding, but at a low angle to the trace of the thrust. This is remarkably similar to the U-shaped folds on the north end of Pebble Beach (Figure 27). The syncline found along north Point Saint

George, as well as the fractures scene in air photos (Figure 26 and 28) at Point Saint

George are also similar to what Apotria (1995) describes from the oblique thrust faults in Wyoming.

Apotria (1995) suggests that outer arc extensional bending stresses at the lower hinge of a fault bend fold propagate strike parallel fractures. This pattern is also seen in the lineament map drawn from air photos of Point Saint George and from the 74 lineament map drawn from the USGS 7.5 minute quadrangle of the entire Crescent

City Coastal plain (Figures 28 and 34). A series of strike parallel (northwest trending) fractures cut through both Pleistocene marine terrace deposits as well as Holocene wind blown sand deposits starting at the north end of Pebble Beach, and continuing all the way across Point Saint George. It is possible that the fracture pattern seen in the lineament map (Figure 28) reflects a structural relief formed on the Saint George

Formation terrace uncomformity prior to deposition of the late Pleistocene marine terraces, with the resulting surface reflecting differential compaction over an uneven surface.

Apotria (1995) modeling of oblique ramps assumes that frictionless sliding of the hanging wall over the rigid footwall is occurring. Field mapping at Point Saint

George suggests that the true fault geometry and kinematics are more complex.

Apotria (1995) modeling also suggests that the hangingwall did not simply slide from a lower footwall flat, up the oblique ramp, and onto the upper footwall flat, as the models idealize. Instead, footwall folding and thrust sheet rotation due to differential displacements are two fundamental components of the deformation. With continued thrusting and uplift fractures may form associated with outer-arc bending stresses at the lower hinge of the ramp (Apotria, 1995). The model proposed in this study suggests the hinge of the offshore Saint George thrust ramp is located at the north end of Pebble Beach, where the Saint George Formation drops below the modern beach sands and the Holocene windblown sands show the fracture pattern outlined in Figure

28. 75 Previous studies have hypothesized that large magnitude earthquakes have occurred along the Cascadia subduction zone over the past several thousand years

(Carver, 1996). Seismic data indicates the youngest sediments offshore are cut by the

Saint George thrust fault (Clarke, written communication 1998). The presence of the

Holocene bog deposit just one kilometer to the south of Point Saint George, and possible Holocene tsunami deposits both in the marsh behind Pebble Beach (Aalto et al., 1999) are possible evidence of recent seismic events occurring along the Cascadia subduction zone offshore of Point Saint George. Interpreted Holocene tsunami deposits in Lagoon Creek some 27 kilometers to the south of Point Saint George

(Garrison-Laney, 1998), also support the idea that large magnitude seismic events have occurred along the Cascadia subduction zone during Holocene time. The model proposed in this study suggest it is more likely that the fractures seen in the air photos are due to Holocene activity on the offshore Saint George thrust fault as well as the recently discovered Pebble Beach thrust fault to the east.

The presence of the entirely organic Holocene peat deposit containing abundant woody debris, as well as faults cutting the youngest sediments offshore of

Point Saint George (Clarke, 1998 written communication) suggest both the Pebble beach thrust fault and the Saint George thrust fault offshore are active. Air photo analysis also supports the idea that the Pebble Beach thrust fault is active since both the late Pleistocene marine terrace deposits as well as the Holocene windblown sands are cut by what may be strike parallel fractures from the Apotria (1995) model. 76 Where late Holocene terraces are faulted, reverse or thrust displacement lifts the shoreline and raises the terrace on the upthrown side of the fault (Carver, 1996).

The surface expression of many thrust faults in thick unconsolidated sediments is not restricted to a narrow zone of faulting, but rather is commonly distributed across a broad zone on many small displacement faults and accommodated by broad warping and surface folding (Carver, 1996). This is the case with the mid-Pebble Beach fault where the Saint George Formation sediments (units 1 and 2) are highly fractured at the location of the Pebble Beach thrust fault. The surface of the Saint George Formation and overlying Holocene bog deposit is also visually and measurably warped at the location of the Pebble Beach thrust, with an elevation difference of nearly lm between the downthrown and upthrown side of the fault (Figure 1). The presence of the smaller fault at north Pebble Beach and plunging anticline-syncline pair also follows the Carver (1996) model.

It is widely hypothesized that the Cascadia subduction zone has had large megathrust events in the late Holocene. Within two major embayments along the central Oregon coast at Alsea Bay and Yaquina Bay, multiple buried soils are preserved, some in association with sand layers deposited directly on the buried soils.

These stratigraphic sequences have been interpreted as a late Holocene record of

repeated coseismic submergence events, with some events followed by deposition of

sand due to a tsunami (Kelsey et al., 1996). This is strikingly similar to what has been

documented at both the north and south sides of Point Saint George, with diatom dated

late Miocene marine sand layers deposited over lignitic rooted tree stumps and 77 paleosol (Figures 16 and 17). The similar orientation of large logs within the paleosol

(Figure 13), both at the base of the late Miocene Saint George Formation and in the

Holocene peat deposit at mid-Pebble Beach (Figure 24), suggest a tsunami was responsible for the burial of those surfaces following a large Cascadia subduction zone seismic event. The presence of buried peat-mud-sand sequences in the marsh directly behind the Pebble Beach thrust, as well as above the late Miocene lignitic stumps and paleosol also supports tsunami inundation following a large magnitude seismic event offshore. The tsunami inundation may have occurred both during Holocene time, as well as when the paleo-forest was initially buried in the late Miocene.

In Kelsey (1996) it is hypothesized that the submergence events are due to episodic megathrust earthquake events along the Cascadia subduction zone. Similar sequences of interbedded peat and mud have been described in estuaries in British

Columbia, southern Washington, northern Oregon, southern Oregon, and northern

California, opening the possibility that all or most of these marshes may have undergone repeated synchronous regional coseismic submergence (Kelsey, 1996).

For the model proposed in the case of the buried paleo-forest described in this study. A late Miocene seismic event (diatom dated at 6.0-6.4 m.y.) occurred along the

Cascadia Subduction zone, dropping the late Miocene paleo-forest below sea level,

followed by inundation of the coastal forest by the resulting tsunami. The Del Norte

fault of Back (1957) described earlier may be the normal fault responsible for

dropping the region around the late Miocene Crescent City coastal plain below sea

level (Figure 34), and not the thrust fault proposed in Clarke (1992). The Del Norte 78 fault may have stayed active for some time after dropping of the coastal plain, allowing deposition of the marine Saint George Formation as proposed in Back

(1957). The region continued to drop relative to sea level with the shallow marine

Saint George Formation sediments being deposited on top of the resulting uncomformity until at least 5.38 m.y. This later Miocene date of 5.38 m.y. is based on diatom analysis done at lm intervals from the base of the Saint George Formation through the 48m of exposed formation along Pebble Beach to the top of the Saint

George Formation uncomformity. The diatom analysis was conducted by Dr John

Barron of the United States Geological Survey for this study.

Sometime in the late Pleistocene the Crescent City coastal plain was uplifted above sea level, possibly from activity on the offshore Saint George thrust fault.

Throughout the late Pleistocene, there has been continued rotation of the Oregon forearc block and northward migration of the Mendocino Triple Junction (Wells et al.,

1998). This continuous rotation and migration has resulted in the Crescent City coastal plane being subjected to strong compressional tectonic forces resulting in the formation of the fold and thrust belt that lies mostly offshore of Point Saint George

(Wells et al., 1998). These same compressional forces have created the offshore Saint

George thrust fault, which is acting as a ramp, elevating the Saint George Formation to its current position slightly above sea level at Point Saint George. The Saint George thrust fault is also responsible for elevating the late Miocene forest and paleosol found at numerous locations in the vicinity of Point Saint George to its current position

located on the wave cut platform along Point Saint George. 79 The tectonic forces that effect the entire Oregon forearc block, have continued to effect Point Saint George throughout the Holocene, resulting in the formation of the

Pebble Beach thrust fault and smaller reverse fault found to the west along Pebble

Beach. The destruction of the 3000 (+ or -) year old bog at mid Pebble Beach may also be related to activity along either the offshore Saint George thrust or the Pebble

Beach thrust to the southeast. CONCLUSION

The presence of the late Miocene forest resting uncomformably beneath the late Miocene Saint George Formation in the area around Point Saint George suggests the area underwent rapid subsidence and inundation by a marine environment in late

Miocene time. Stratigraphic, sedimentologic, and paleontologic evidence indicates the burial of the late Miocene forest may have resulted from a tsunami following a large magnitude earthquake offshore along the Cascadia subduction zone. Evidence to the east of the Crescent City platform indicates the area may have been dropped along the

Del Norte fault following an episode of subduction erosion in the late Miocene. The

Klamath peneplain is draped by late Miocene Wimer Fm sediments [marine to west, fluvial to east] supporting a 10 km eastward shift in regional sea level in the late

Miocene.

The contrasting orientations of structures along the north-northwest-trending

Pebble Beach fault reflect movement over an oblique thrust ramp and local thrust sheet rotation. East of the Pebble Beach fault uplift of the Klamath Mountains block has occurred without extensive faulting. Regional sandstone provenance studies suggest that the ongoing uplift of the western Klamath Mountains and northernmost

California Coast Ranges is a Pleistocene to Recent event. The Klamath Mountains may be acting as a "back stop" for the Crescent City coastal plain as it is compressed within the fold and thrust belt and continues the clockwise rotation that has been

83 83 ongoing throughout the Neogene. The model proposed suggests that both the Saint

George thrust fault and the Pebble Beach thrust are very young structures.

A future study that may help in understanding the late Miocene event that inundated the coastal forest, and buried it under the Saint George Formation is a complete paleontological study. A paleobotanical analysis of both the late Miocene forest as well as the Holocene coastal bog would also be help in understanding the complex tectonics effecting the region.

A more detailed late Pleistocene through Holocene structural analysis would also be useful in helping to understand the active faulting in the region around Point

Saint George and the Crescent City area. REFERENCES

Aalto, K. R., 1989a. Franciscan Complex olistostrome at Crescent City, northern California: Sedimentology: v. 36, p. 471-495.

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