IDENTIFYING TSUNAMI DEPOSITS USING SHELL TAPHONOMY

The Gulf of Oman is bordered to the north and east by the tectonically active Makran accretionary prism. On November 28th, 1945 a 7.8 magnitude earthquake with its epicenter approximately 100 km south of Karachi in the Northern Arabian Sea generated a large tsunami. Over 4000 deaths resulted from the earthquake and tsunami along the Makran coast of Pakistan. Most of these deaths were caused by the tsunami. The tsunami reportedly caused significant damage in Muscat Oman, however no other tsunami information was reported along the Omani coast.

Sur lagoon is shallow, microtidal and partially encircled by the town of Sur on the south eastern promontory of Oman. The lagoon has a long history as a harbor and ship anchorage likely dating back to the Bronze Age. Field work conducted during 2006 revealed a coarse shell-rich horizon (5-25 cm thick) found 20-50 cm below the sandy surface of the lagoon that extended over a ~18 km² area in all subenvironments of the lagoon. The shell horizon is rich in articulated shells of the clam Tellina (Quidnipagus) palatam (Iredale, 1929) which is an infaunal species living outside of the lagoon in subtidal sands to a water depth of 20 meters. The inclusion of numerous articulated specimens in the shell horizon provides direct evidence of bottom scouring and shoreward transport of sediment from the shallow shelf. The presence of articulated adult specimens (2-4) in 7 short cores, and test pits dug in the lagoon (4+ specimens/pit) further support the model of live transport, rapid burial, and minimal reworking. In addition, taphonomic data on fragmentation ratios of shells indicates transport under highly turbulent conditions and burial without subsequent reworking. This evidence, coupled with sub-recent cultural material recovered from the sand overlying the shell unit, indicates that this shell deposit represents the 1945 tsunami event. Further research on shell taphonomy is required to make it a useful indicator of tsunamigenic events vs. large storms in the geological past, but shell taphonomy does appear to be a useful tool.