Identifying tsunami deposits using bivalve shell taphonomy

On 28 November 1945, in the Makran trench off Pakistan, a large earthquake (8.1 Mw) produced a tsunami that struck the coast of Oman and left a taphonomically distinct shell bed in Sur Lagoon. The shell bed was thick (5–25 cm) and laterally extensive, covering a >1 km2 area. The shell assemblage from the deposit contained a mean of 59% for articulated bivalves of allochthonous offshore and subtidal species (e.g., Tellina palatam) as well as a mean of 20% for lagoonal species, indicating large-scale erosion and transport. Taphonomic traits (e.g., articulation, rounding, fragment angularity) of all bivalve material >5.6 mm were quantified for eight sample horizons, and compared with a tsunamite from Caesarea, Israel. Some of the taphonomic characteristics between the shell beds from these two different depositional settings were similar, and three tsunamigenic specific traits were identified: (1) thickness and lateral extent of the shell deposit, (2) presence of allochthonous articulated bivalves out of life position, and (3) extensive angular fragmentation. These results show that tsunamis form shell accumulations and cannot be ignored when assessing shell bed origin for the geological record. When these three traits are collectively found, a tsunamigenic origin should be considered for the shell bed.