Geologic Evidence of Historic and Prehistoric Tsunami Inundation at Seaside, Oregon.

by

BROOKE K. FIEDOROWICZ

A thesis submitted in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE
in
GEOLOGY

Department of Geology
Portland State University

June 5, 1997

Table of Contents

Abstract
Acknowledgments
List of Figures
List of Tables
Introduction
Background
Tsunami Generation in Subduction Zones
Cascadia Margin Neotectonics
Historic Records of Subduction Zone Generated Tsunami
Pacific Northwest Impacts
1964 Alaskan Tsunami Impacts in Pacific Northwestern Region
Study Area
Methods
Results
Necanicum River (SSN)
Location
1964 Event
Prehistoric Events
Neacoxie Creek (SSX)
Location
1964 Event
Prehistoric Events?
Drainage to the East of Neacoxie Creek (SSXa)
Location
1964 Event
1700 AD Event
Stanley Lake (SSS)
Location
1964 Event
1700 AD Event
Neawanna Creek (SSW)
Location
1964 Event
Prehistoric Events
Seaside Paleogeography
Cobble Ridge Mapping
Ground Penetrating Radar
Summary
Discussion
1964 Event
Historic 1964 Tsunami Surges in Seaside Tidal Creeks
Historic 1964 Tsunami Deposits in Seaside Tidal Creek Wetlands
Comparisons of the 1964 Tsunami Impacts in Seaside with other Pacific Northwest Localities
Correspondence between 1964 Tsunami Runup and Tsunami Depositional Record
1964 Runup
Paleogeography of the Seaside Wetlands
Paleosubsidence Horizons in Seaside
1700 Subsidence Horizon
Older Subsidence Horizons
Discrimination of Potential Paleotsunami Deposits from Liquefied Intruded Sand Features
1700 AD Paleotsunami Inundation
Test of Barrier Overtopping
Minimum Overland Inundation
Comparisons with other studies
Historic Events
Prehistoric Events
Implications for Evacuation
Conclusions
1964 Event
1700 AD Event
References
Appendix A - 1964 OBSERVATIONS
Appendix B - GOUGE CORE LOG LOCATIONS AND DESCRIPTIONS
Appendix C - GOUGE CORE LOGS
Appendix D - LIST OF RADIOCARBON DATED SAMPLES
Appendix E - FIELD DATA GRID CORE LOCATIONS
Appendix F - FIELD DATA GRID CORE LOGS

List of Figures

Figure 1. Cascadia subduction zone and plate configuration.
Figure 2. Study area location of Seaside, Oregon.
Figure 3. Seaside coastal wetlands.
Figure 4. Lewis and Clark sketch map of the Seaside area
Figure 5. Three native American midden sites
Figure 6. Coastal wetlands of Seaside studied for historic and prehistoric tsunami evidence..
Figure 7. Observation sites and core locations from the Necanicum River mouth to 12th Ave. bridge.
Figure 8. Observation sites and core locations for the Necanicum River from 12th Ave. to Broadway Dr.
Figure 9. Observation sites and core locations for the Necanicum River from Broadway Dr. to the islands (Ave. S).
Figure 10. Observation sites and core locations for the Necanicum River from Necanicum Islands (Ave. S) to the cove/golf course (Ave. U).
Figure 11. Northern Necanicum River cores SSN 871, 870, 863; showing a 1964 sand deposit.
Figure 12. Central Necanicum River cores SSN 759, 750, 747, 737; showing a 1964 deposit.
Figure 13. Southern Necanicum River cores SSN 731, 882, 884, and 877; showing the thinning of the 1964 deposit.
Figure 14. Northern Necanicum River trench 3A (core site SSN 864).
Figure 15. Central Necanicum River trench 3B (core site SSN 748).
Figure 16. Southern Necanicum River trench 3C (core site SSN 732).
Figure 17. Neacoxie Creek observation sites and core locations.
Figure 18. Neacoxie Creek east bank cores SSX 754 and 770.
Figure 19. Neacoxie Creek west bank cores SSX 771-774.
Figure 20. Neacoxie Creek grid core locations.
Figure 21. Neacoxie Creek east bank N-S grid cross-section (SSX III, VII, VIII, IX, X, XI, XXVII, and XXVIII)
Figure 22. Neacoxie Creek west bank grid cores (SSX XIII, XX, XXI, XXII, XXV, XXVI)
Figure 23. Neacoxie Creek E-W cross-sections, northern cores (SSX III and XVIII).
Figure 24. Neacoxie Creek E-W cross-section, central cores (SSX VI and XXI).
Figure 25. Neacoxie Creek E-W cross-section, southern cores (SSX X and XXVI).
Figure 26. Neacoxie Creek trench 2A (core site SSX 754).
Figure 27. Neacoxie Creek trench 2B (grid core site SSX X).
Figure 28. Drainage to east of Neacoxie Creek and Venice Park observation and core sites.
Figure 29. House located on 26th and Pine in Venice Park area.
Figure 30. Drainage to east of Neacoxie Creek, cores SSXa 853, 855-856.
Figure 31. Drainage to east of Neacoxie Creek, trench 4A (core site SSXa 850).
Figure 32. Drainage to east of Neacoxie Creek, prehistoric horizons?
Figure 33. Stanley Lake core locations.
Figure 34. Stanley Lake cores SSS 733 and 734.
Figure 35. Stanley Lake cores SSS 879-881.
Figure 36. Stanley Lake cores SSS 866-868.
Figure 37. Stanley Lake cores SSS 872-876.
Figure 38. Stanley Lake cores SSS 887-889.
Figure 39. Neawanna Creek observation and core locations between its mouth and 12th Ave.
Figure 40. Neawanna Creek observation sites and core locations for 12th Ave. to south of the Broadway Dr. bridge.
Figure 41. Neawanna Creek cores SSW 722, 723, 725, and 726.
Figure 42. Neawanna Creek cores SSW 718-720.
Figure 43. Neawanna Creek cores SSW 757, 756, 763.
Figure 44. Neawanna Creek cores SSW 803-808.
Figure 45. Neawanna Creek observation sites and core locations from south of Broadway Dr. to S Ave.
Figure 46. Neawanna Creek grid location map.
Figure 47. Northern SSW grid cores I-V, VIII, and X.
Figure 48. E-W cross-section of the central SSW grid cores XIII, XIV, XVI, and XVIII.
Figure 49. Southern SSW N-S grid cores XXVIII, XXVII, XXVI, and XXIX.
Figure 50. Southern SSW E-W grid cores XXVIII, XXII, and XXI.
Figure 51. Neawanna Creek trench 1A.
Figure 52. Neawanna Creek trench 1B.
Figure 53. Neawanna Creek cores SSW 720, 716, 717, 714, and 781.
Figure 54. Neawanna Creek cores SSW 712, 778, and 779.
Figure 55. Neawanna Creek cores SSW 847, 762, 766.
Figure 56. Neawanna Creek 808, 806, 803, and 768.
Figure 57. Neawanna Creek cores SSW 782 and 783.
Figure 58. Neawanna Creek cores SSW 823-825.
Figure 59. Neawanna Creek cores SSW 789-787, 784, and 818.
Figure 60. Neawanna Creek observation sites and core locations from S Ave. to the Mill Ponds.
Figure 61. Neawanna Creek cores SSW 735, 730, 828, 830, 831, 832.
Figure 62. Locations of cobble ridges extending throughout Seaside.
Figure 63. Areas in Seaside affected by wave amplification during the 1964 tsunami.
Figure 64. Seaside wetlands affected by tsunami attenuation during the 1964 event.
Figure 65. 1964 SSX grid isopach, determining thickness of the sand sheet in Seaside.
Figure 66. 1964 SSW grid isopach, determining thickness of the sand sheet in Seaside.
Figure 67. 1964 observation sites and gouge core deposit locations including 1964 inundation line.
Figure 68. Distribution of LIS related to the 1700 AD event liquefaction zones in the Seaside wetlands.
Figure 69. Arrows indicating propagation which possibly occurred during the 1700 AD event in Seaside.
Figure 70. Inundation line for the 1700 AD event in Seaside.

List of Tables

Table 1. Visual Estimation of Organic Material
Table 2. Radiocarbon Samples and Core Locations
Table 3. Diatom Samples Analyzed

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