Title

Analysis of Depositional Processes After the Columbia River Basalt Flows, Early Ellensburg Formation, Bettas Road, Ellensburg WA

Document Type

Oral Presentation

Location

SURC Ballroom B/C/D

Start Date

21-5-2015

End Date

21-5-2015

Keywords

Columbia River Basalt, Ellensburg Formation, Environmental Evolution

Abstract

The Columbia River Basalt (CRB) flows ~15 million years ago flattened the topography of central Washington and altered the environments of existing river systems. The goal of our project is to understand how the local fluvial system responded to the CRB flows and eventually evolved into the environment we live in today by analyzing the laterally extensive outcrop at Bettas Road, which is an excellent example of sediment deposition immediately after a flood basalt event. Via hand sample and thin section analysis as well as published CRB flow maps, we confirmed that the base layer of the outcrop was of the Grande Ronde CRB flow, which indicates that the sediments deposited directly atop the basalt are the earliest of the Ellensburg Formation. We hypothesized that the first sediments deposited after the basalt would be fine-grained due to the unchannelized nature of the flattened topography, and that volcanic debris flows would dominantly be fine-grained and hyper-concentrated. We further predicted that, as rivers developed channels, the grain size would become larger and the volcanic debris flows would be dominantly coarser-grained lahars. To test these hypotheses, we mapped the 930 meter long outcrop, identified sedimentary facies, and performed grain-size analyses using a point-count method as well as sieves and a Mastersizer. Our results indicate that the grain size of both the fluvial and volcanic sediments generally coarsened upwards, as the fluvial system was not able to transport coarse-grained sediment immediately after the CRB flows but gradually evolved to transport larger material via mature channels.

Poster Number

7

Faculty Mentor(s)

Breanyn MacInnes

Department/Program

Geological Sciences

Additional Mentoring Department

Geological Sciences

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May 21st, 8:30 AM May 21st, 11:00 AM

Analysis of Depositional Processes After the Columbia River Basalt Flows, Early Ellensburg Formation, Bettas Road, Ellensburg WA

SURC Ballroom B/C/D

The Columbia River Basalt (CRB) flows ~15 million years ago flattened the topography of central Washington and altered the environments of existing river systems. The goal of our project is to understand how the local fluvial system responded to the CRB flows and eventually evolved into the environment we live in today by analyzing the laterally extensive outcrop at Bettas Road, which is an excellent example of sediment deposition immediately after a flood basalt event. Via hand sample and thin section analysis as well as published CRB flow maps, we confirmed that the base layer of the outcrop was of the Grande Ronde CRB flow, which indicates that the sediments deposited directly atop the basalt are the earliest of the Ellensburg Formation. We hypothesized that the first sediments deposited after the basalt would be fine-grained due to the unchannelized nature of the flattened topography, and that volcanic debris flows would dominantly be fine-grained and hyper-concentrated. We further predicted that, as rivers developed channels, the grain size would become larger and the volcanic debris flows would be dominantly coarser-grained lahars. To test these hypotheses, we mapped the 930 meter long outcrop, identified sedimentary facies, and performed grain-size analyses using a point-count method as well as sieves and a Mastersizer. Our results indicate that the grain size of both the fluvial and volcanic sediments generally coarsened upwards, as the fluvial system was not able to transport coarse-grained sediment immediately after the CRB flows but gradually evolved to transport larger material via mature channels.