graehl - thesis defense 5-2-12 ver 1

Late Holocene paleoseismicity, tsunamis, and relative sea-level changes in Yaquina estuary, central coastal Oregon

Thesis defense – HSU M.S Environmental Systems in Geology2 May 2012

Nicholas A. [email protected] OFF-TILT

Acknowledgements

I would like to thank the following people:

Rob Witter Lori DenglerHarvey Kelsey EileenHemphill-Haley

Acknowledgements

Laura Stimely Morgan & Jody Kyle French Heath Sawyer

I would like to thank the field crew:

Pacific Northwest Paleoseismology 101-Seismic Sources

CSZ Megathrust (Interface) CSZ Wadati-Benioff Zone (Intraslab) Crustal Faults Volcanic Sources

Source: Oregon Historical Society

Pacific Northwest Paleoseismology 101-CSZ megathrust earthquakes

Plate deformations during a great Cascadia earthquake cycle

Source: Atwater et al., 2005


Records of great earthquakes in intertidal wetlands


Classic three-layer-cake geology

Source: At Risk

Holy Graehl’s Path to Enlightenment-Overall approach

Document the record of subduction zone earthquakes on the central Oregon coast.

Modified Goldfinger, 2008

Holy Graehl’s Path to Enlightenment-What’s to come...

Introduction Research objectives Results Stratigraphy Age control Diatoms Conclusion Questions

IntroductionCascadia subduction

zoneStudy area

Introduction-Map of Yaquina Bay & River

3 Main Research Objectives

1. Evaluate stratigraphic evidence for tectonic subsidence and tsunami with great CSZ eq’s

2. Evaluate whether subduction zone earthquake recorded at SS are in part or wholly consistent with the reported chronology of subduction zone earthquakes at other coastal sites along the CSZ

3. Evaluate whether or not the central Cascadia margin may have ruptured independently

Results A 4,500 year long record of interbedded peat, sandy

silt, and mud deposits. The sharp contacts between peat and sandy silt/mud

mark the burial of 12 former marsh or forest soils. Buried soils and overlying sandy-silt and mud deposits

record evidence of coseismic subsidence and tsunami deposition.

Chronology of earthquakes recorded at Yaquina Bay is consistent with paleoseismological investigations along other parts of the CSZ.

A north-central segment of the CSZ (Yaquina Bay to Cannon Beach) may have ruptured between ~500-700 yrs B.P.

Average earthquake recurrence interval of 380-400 years.

Research Approach

1. Detailed descriptions of subsurface stratigraphy gathered from closely spaced cores.

2. Radiometric dating mud-over-peat contacts to assess the time of soil burial and aid in correlation of buried soils in adjacent cores.

3. Analyses of fossil diatom assemblages to assess paleoenvironmental changes across peat-mud and peat-sandy-silt contacts.

Google Map of Yaquina BayOblique view looking westward

Google Earth, 2012 – vertical exaggeration x3

Map of Sallys Slough marshLocations of all gouge and Russian peat auger cores

sites

Map of Sallys Slough marshLiDAR bare-earth imagery

Sallys Slough marsh conditions

SS-3

SS-11

SS-5

Stratigraphy of Sallys SloughCoring Campaign

Stratigraphy of Sallys SloughCoring campaign and detailed lithologic

descriptions

Stratigraphy of Sallys SloughSurvey core locations relative to tidal datum

(MLLW)

Map of Sallys Slough marshLocations of all gouge and Russian peat auger cores

sites

Stratigraphy of Sallys Slough


533-649 yrs BP

1536-1708 yrs BP

2777-2957 yrs BP3246-3413 yrs BP

3840-4082 yrs BP3987-4162 yrs BP

2454-2793yrs BP

3717-3903yrs BP

4417-4588 yrs BP4417-4588 yrs BP

Buried soil IIA

Modified from Atwater et al., 2005

Classic three-layer-cake geology?


SS-11 Buried soil IIA

Radiometric dating

Radiometric dating‘Plucking out’ all of the delicate detrital

material

Radiometric datingExamples of detrital material used for

14C

Sedimentation Rate for Sallys SloughBased on calibrated radiocarbon age determinations

for specific sample depths in core SS-8

Fossil Diatom AnalysesSampling for fossil diatoms

Fossil Diatom Analyses

Fossil Diatom Analyses50 angstroms = 5.0 × 10-9 meters 50 µm – strand of human

hair

Source: nsf.gov

Fossil Diatom AnalysesHow do fossil diatoms help infer

paleoenvironments?

Environmental preferences of certain benthic taxa Inferred paleoenvironment is assigned to 1 of 4 zones:

1. Tidal flat

2. Low marsh

3. High marsh

4. Upland/freshwater zone

Fossil Diatom AnalysesDiatom associations with intertidal zones

Fossil Diatom Analyses - In the zoneSoil IIA -relative percent of total diatom assemblage

Fossil Diatom Analyses - In the zoneSoil V -relative percent of total diatom assemblage

Fossil Diatom AnalysesInferred paleoenvironments

UplandHigh marsh

Low marsh

Tidal flatUpland

Upland

Low marshTidal flatUpland

Upland

Low marsh

High marsh

Sandy silt deposit- Tidal vs. Tsunami2 Interpretations for the Mode of Deposition

ConclusionRepeated Widespread Abrupt Submergence Events:

Episodes of tectonic subsidence accompanied by tsunami

BUT it is unclear whether there is a stratigraphic signature of the tsunami at Sallys Slough

Conclusion - How many and how often?

12 earthquakes have occurred at Sallys Slough since 4417-4588 yr BP

Average earthquake recurrence interval of 380-400 years. Interval of time separating earthquakes is as short as 280

years and as long as 540 years.

Comparison of Sallys Slough Earthquake Record to Cascadia Subduction Zone Earthquake Chronology

Conclusion – Compare Chronology (14C ages)

Conclusion - Chronology of CSZ Earthquakes

Summary Buried soils were developed in high to upland

freshwater marsh environments and were coseismically subsided 12 times since 4417-4588 yrs BP.

Soils subsided to low marsh or tidal flat elevations where they were buried by deposition due to tidal inundation.

Tsunami accompanying the subduction zone earthquake additionally may have left a stratigraphic signature.

Average earthquake recurrence interval of 380-400 years.

Chronology of earthquakes recorded at Yaquina Bay is consistent with paleoseismological investigations along other parts of the CSZ.

A north-central segment of the CSZ (Yaquina Bay to Cannon Beach) may have ruptured between ~500-700 yrs B.P.

Q: How does the chronology at Yaquina compare with the offshore turbidite chronology?

Onshore vs. Offshore Records

Q: How do previous core descriptions taken at Sallys Slough compare with my descriptions?

Old vs. New Interpretations

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graehl - thesis defense 5-2-12 ver 1

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