Diatoms in Historical Tsunami Deposits, Northern California, USA

A fundamental challenge in using microfossils to differentiate paleotsunami deposits from those of other sources (storms, floods) is to identify characteristics that favor one mode of deposition over the other. The silt- to sand-size siliceous hard parts (valves) of diatoms are commonly found as transported particles in tsunami deposits, but logically, may also be found in other types of coastal deposits of the same grain size. To date, observations on diatom preservation and provenance have been invoked as supporting evidence for paleotsunami deposits. These observations can be tested and refined by detailed observations of diatom assemblages in recent, well-documented tsunami deposits. As a component of the U.S. Geological Survey Science Application for Risk Reduction (SAFRR) project, diatoms were examined in two historical tsunami deposits on the central and northern California coast: the 1946 deposit on the north end of Half Moon Bay (37.5°N) and the 1964 deposit about 10 km south of Crescent City (41.7°N). Both tsunamis were the result of distant-source events across the Pacific Ocean from California: the M 8.1 Eastern Aleutians Islands earthquake (1946) and the M 9.2 Alaska earthquake (1964). At both localities tsunami inundation was documented by eyewitness accounts. The deposits are now preserved in the shallow subsurface as ~1-10 cm thick layers of silt and sand intercalated in peaty marsh or clay-rich lagoon deposits. These historical tsunami deposits are particularly useful for documenting characteristics of entrained diatom assemblages for comparison to paleotsunami deposits. First, the deposits consist of mostly fine sand and silt, and therefore are an appropriate particle size for containing diatoms. Second, although they are recent enough to have been documented by eyewitness accounts, they are also old enough to have been altered by natural geological processes (e.g., burial, compaction, taphonomic affects on diatom valves) as would be found in paleotsunami deposits. Finally, although they are both the result of deposition by a distant-source tsunami at a coastal location not affected by coincident earthquake deformation (i.e., non-subsiding coast), they accumulated and were preserved in different kinds of coastal environments, and therefore the diatom records for each are different. At Half Moon Bay, the 1946 tsunami swept across a shallow brackish lagoon and salt marsh, leaving a deposit of clean, very fine sand containing large, allochthonous epipsammic diatoms consistent with landward-directed transport from the adjacent shallow marine nearshore. Where the deposit settled out and was later buried by clay-rich lagoon deposits, both the tsunami sand layer and entrained diatoms are well preserved. In contrast to the open, preservation-conducive lagoon environment at Half Moon Bay, the 1964 tsunami south of Crescent City inundated a freshwater marsh with thick stands of vegetation. The resultant paleotsunami deposit is limited in lateral extent, and dominantly contains well-preserved freshwater diatoms that mask obvious evidence of marine inundation.