SAND DEPOSITION MECHANISMS IN HOLOCENE MARSH STRATIGRAPHY FROM A TSUNAMI PRONE LOCALITY, CRESCENT CITY, CALIFORNIA, SOUTHERN CASCADIAN MARGIN

OBJECTIVESOBJECTIVESThe objectives of the current study were to describe the stratigraphy of South Crescent Marsh and to identify the candidate processes responsible for anomalous sand sheets in this coastal freshwater marsh.

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Sand Sheets

Storm-waves

Tsunami

OVERVIEWOVERVIEW

1.1. INTRODUCTIONINTRODUCTION

2.2. LOCATIONLOCATION

3.3. METHODSMETHODS

4.4. RESULTSRESULTS

5.5. CONCLUSIONCONCLUSION

1.INTRODUCTION1.INTRODUCTION

Tectonic Setting

1.INTRODUCTION1.INTRODUCTION

Tsunami Sites

1.INTRODUCTION1.INTRODUCTION

Magoon’s 1964 Tsunami MapSand Mine Marsh

1.INTRODUCTION1.INTRODUCTION

Several tsunamis have been recorded at Crescent City, yet these seismically induced sea-waves represent only one of several processes responsible for the deposition of anomalous sand sheets into coastal wetlands and marshes.

1.INTRODUCTION1.INTRODUCTION

(1) near-field tsunamis generated from the CSZ,

(2) far-field tsunamis generated from within the Pacific Rim, and

(3) storm-waves caused by extreme climatic conditions.

Three processes are considered as candidate depositional mechanisms in near sea level coastal marsh settings:

2.LOCATION2.LOCATION

Location Overview

2.LOCATION2.LOCATION

Oblique Aerial Photos

3.METHODS3.METHODSSelection of Study SiteSelection of Study Site

• Near & far-field tsunami deposits to the north in a similar setting

• Lies within the zone of inundation by the 1964 far-field tsunami

• Large drift logs located in and around the marsh area

• Low elevation & close proximity to the ocean

• Strandlines of woody debris overtopping the adjacent beach berm

3.METHODS3.METHODS

Field Study:Field Study:Survey Beach ProfileSurvey Beach Profile

3.METHODS3.METHODSField Study:Field Study:Gouge CoresGouge Cores

3.METHODS3.METHODSCatalog DepositsCatalog Deposits

FIELD DESCRIPTIONS INCLUDE

• Sediment lithology

• visual and tactile methods

• Lithologic modifiers

• Contacts between each layer

Lithology % Organics

Peat > 90%

Peaty-Mud > 50 - 90%

Muddy-Peat

< 50 - 5%

Mud < 5%

4.RESULTS4.RESULTS

Transect and Cross Section

4.RESULTS4.RESULTS

Core Logs

4.RESULTS4.RESULTS

Sand Sheet ISand Sheet I

Sand Sheet

Number of Cores†

Thickness of Sand (cm)‡

Depth in Core (cm)¥ Number of Basal Contacts¤ Eastward Extent of Sand

(m)§

Abrupt Sharp Gradational From MHHW

From Beach Berm

I 6 1.5-19 -14.5 to -32.5 4 2 - 95 80

• On the surface of the marsh stratigraphy

• Located adjacent to sand plume emplaced by a breach in the marsh beach berm.

• Interpreted as a high energy storm-wave deposit

Sand Sheet ISand Sheet I

Simplified storm-wave overwash

Sand Sheet IISand Sheet II

Sand Sheet

Number of Cores†

Thickness of Sand (cm)‡

Depth in Core (cm)¥ Number of Basal Contacts¤ Eastward Extent of Sand

(m)§

Abrupt Sharp Gradational From MHHW

From Beach Berm

II 12 0.5-15.5 -71.5 to -25.5 7 2 3 155 140

Sand Sheet IIISand Sheet III

Sand Sheet

Number of Cores†

Thickness of Sand (cm)‡

Depth in Core (cm)¥ Number of Basal Contacts¤ Eastward Extent of Sand

(m)§

Abrupt Sharp Gradational From MHHW

From Beach Berm

III 15 1.5-19 -95.5 to -35 11 3 1 185 170

Sand Sheets II & IIISand Sheets II & III• Preserved relatively shallow in the marsh stratigraphy suggesting recent deposition.

• Extend inland close to the edge of the mapped 1964 inundation by Magoon.

• Contain evidence of rip-up-clasts and have numerous abrupt basal contacts.

• Interpreted as the best candidate for a far-field tsunami deposit.

Sand Sheet IVSand Sheet IV

Sand Sheet

Number of Cores†

Thickness of Sand (cm)‡

Depth in Core (cm)¥ Number of Basal Contacts¤ Eastward Extent of Sand

(m)§

Abrupt Sharp Gradational From MHHW

From Beach Berm

IV 16 2-28.5 -129 to -35.5 13 2 - 212 197

Sand Sheet IVSand Sheet IVRepresents the best candidate for a near-field tsunami.

It exhibits a landward thinning of sand layers, contains normally graded sand layers, has abrupt basal contacts, and harbors evidence of rip-up-clasts.

This is the thickest of all sand sheets captured by the one meter long gouge corer.

Simplified tsunami inundation

4.RESULTS4.RESULTSSummary of Results

• Sand Sheets I-IV have similar characteristics, such as texture, mineralogy, lateral continuity, and display abrupt and sharp basal contacts.

• Only Sand Sheets II, III, and IV contain evidence of rip-up clasts.

• Only Sand Sheet IV exhibits normally graded sands and is also 9.5 centimeters thicker in size than the thickest deposits recorded for sand sheets I-III.

• All but Sand Sheet I rise in elevation as the distance inland increases across the marsh.

CONCLUSIONCONCLUSION• Marsh is a prime wash-over setting located at MHHW behind a relatively small berm, therefore elevated ocean levels from both tsunamis and storms have an equal chance of inundating this area.

• Documented four laterally continuous sand sheets within the stratigraphy of South Crescent Marsh.

(1) near-field tsunamis generated from the CSZ,

(2) far-field tsunamis generated from within the Pacific Rim, and

(3) storm-waves caused by extreme climatic conditions.

The candidate mechanisms responsible for the deposition of these four sand sheets are

Findings from this study cannot attribute with certainty which process caused the deposition of each of the sand sheets.