Date

2019

Department or Program

Geosciences

Primary Wellesley Thesis Advisor

Katrin Monecke

Additional Advisor(s)

Kathleen Gilbert

Additional Advisor

Wes Watters

Abstract

Spatial variations of thickness and grain size of 2004 and prehistoric tsunami sand layers from Aceh province, Indonesia, are analyzed to better characterize sediment transport by tsunamis. Approximately 200 auger and gravity cores were collected along ~ 10 km of coastline over the past 10 years in an area that was severely impacted by the 2004 Sumatra-Andaman Earthquake and Indian Ocean tsunami. The sediment cores were described sedimentologically, and grain size distributions of selected sand layers were measured using laser diffraction techniques. The spatial distribution of sedimentary characteristics was analyzed using ArcGIS software. Radiocarbon dating and sediment analysis from onshore and offshore areas provide data on the timing and source areas of the different sand layers. The 2004 tsunami deposited sand up to 1.8 km inland with a maximum thickness of 71 cm and a minimum thickness of .80 cm, showing a trend of landward thinning. Cores collected in swales typically contain sand layers thicker than those located on adjacent beach ridges. The deposit is composed of very fine to coarse sand of mostly siliciclastic origin with some plant material. Grain size analysis provides evidence of normal grading and landward fining at several locations. Separated by layers of peat and organic rich mud, older sand layers of very fine to medium sand were also recovered in a number of cores between .65 and 1.8 km from the present day shoreline. These older, buried sand layers that were previously interpreted as paleotsunami deposits exhibit thicknesses of 1-17 cm and likewise show evidence of normal grading. Sediment cores taken closer to a river located on the northernmost section of the research area contain coarser sand layers with immature compositions of siliciclastics and heavy minerals more typical of fluvial deposition. Modeling suggests flow depths between .44 and 3.54 m and flow velocities between .36 and 1.24 m per s.

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