Are Tsunamis
Always Sedimentologically Important Events? A Case Study of Sedimentological
and Geochemical Analyses of Coastal Lagoons,
Jackson, Kelly L.1, Eugene C.
Rankey1, Gregor P. Eberli1, Falk Amelung1,
Miriam S. Andres1, Larry C. Peterson1, Peter K. Swart1,
H.A.H. Jayasena2, K.V. Wilbert Kehelpannala3 (1)
University of Miami, RSMAS, CSL, Miami, FL (2) University of Peradeniya,
Peradeniya, Sri Lanka (3) Institute of Fundamental Studies, Kandy, Sri Lanka
Tsunamis are termed ‘waves of destruction.'
But are they always sedimentologically important events? Sediment cores from
Short cores from five lagoons feature
mud-dominated background sedimentation, interrupted by sand layers with sharp
erosive bases, and that fine and/or coarsen upwards, have variable thicknesses
up to 15 cm, and include grain sizes from fine granule to fine sand. These
layers are interpreted to represent paleotsunami deposits and at least 7 such
deposits are preserved within the upper 4 meters of sediment from Karagan
Lagoon, Hambantota. AMS C-14 dating of molluscs from these intervals suggest
ages from 4180 to 6120 radiocarbon ybp. These dates indicate that, the molluscs
within the lagoon are older than the sediment, that lagoon sediments have been
eroded, or that any tsunami deposits from the last 4000 years have not been
preserved.
Ongoing sedimentological and geochemical
analyses provide observations that differentiate paleotsunami deposits from
background lagoonal sedimentation including variations in grain size,
mineralogy, carbon and oxygen isotopes, and organic carbon. Although
paleotsunami deposits can be preserved, results show tsunamis are not always
pronounced sedimentological events that leave a widespread sedimentary record.
They also do not cause net alterations to the overall geomorphic and
hydrodynamic conditions.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California