CTD_DATABASE - Cascadia tsunami deposit database

Identification of CSZ Tsunami Deposits
The studies cited in the database use a variety of sedimentary features to identify tsunami deposits in outcrops and cores. Preliminary identification of tsunami deposits is often based on the recognition of anomalous layers of sand in environments where the deposition of sand layers is unusual, such as coastal marshes or lakes (Reinhart and Bourgeois, 1987). Tsunamis can transport sand, cobbles, boulders and debris from offshore and from beaches and deposit it over coastal lowlands (Dawson, 1994). However, other energetic processes, such as river flooding or storm surges, may leave sandy deposits in otherwise low-energy environments (Nelson and others, 1996). Key sedimentary characteristics of the deposit are often cited to confirm a tsunami origin for the sand layer. While no single characteristic may universally be used to distinguish a tsunami deposit, the combination of several characteristics are often used to rule out other processes and leave little or no doubt as to a tsunami origin for the deposit. Tsunami deposits may be distinguished from river deposits by distinct biological markers, spatial distribution, sediment characteristics and geochemistry. The presence of marine or brackish water macro- and microfossils is used to infer a marine rather than river source for the deposit (Hemphill- Haley, 1995). Thinning and fining of the deposit landward are often used to suggest a marine surge rather than a river source for the deposit (Atwater, 1987; Benson and others, 1997). Flop-overs, which consist of the leaves and stems of herbaceous plants bent over by the flow, preserve information about flow direction and may indicate a landward-directed flow, suggesting a marine source (Atwater and Hemphill-Haley, 1997). The composition and texture of the sand grains has been used to differentiate between a coastal or upriver source (Darienzo and Peterson, 1990; Peterson and Darienzo, 1996). Geochemical indicators, such as bromine enrichment, have also been used to indicate a marine source for the deposit (Schlichting, 2000). Storm surge deposits are more difficult to distinguish from tsunami deposits because, similar to tsunami deposits, they also contain marine or brackish water macro- and microfossils, have saltwater chemistry, and thin and fine landward (Nelson and others, 1996). Some studies use distance inland to indicate a tsunami source, arguing that it is not likely that storm waves or a storm surge could deposit sand inland to the extent of the deposit (Clague and others, 2000). Reinhart (1991) argues that in protected tidal channels, storms are unlikely to suspend the volume of sediment necessary to produce the deposits observed. When layers are present, their number and thickness are sometimes used to differentiate between a tsunami and storm deposit (Williams and Hutchinson, 2000, Witter, in press). Tsunami deposits tend to have several relatively thick normally graded beds, suggesting deposition from suspension by successive waves in the tsunami wave train, while storm deposits may be expected to have thinner and more numerous laminations from higher frequency but lower energy storm waves (Nelson and others, 1996b). Abramson (1998) uses the presence of rip-up clasts in the deposit to indicate the higher energy deposition of a tsunami. Association of the sand layer with paleoseismicity is often used to link a sand layer to a tsunami source. Coseismic coastal subsidence may accompany great subduction zone earthquakes (Atwater, 1996). Association of the sand layer with evidence of coseismic subsidence is often key to the identification of the sand layer as a tsunami deposit (Atwater, 1987, Darienzo and Peterson, 1990). This is usually seen as a buried soil or layer of peat, representing a marsh surface that formed above the reach of high tide, abruptly overlain by a layer of sand (the tsunami deposit), which is then overlain by tidal flat mud. Linking a sand layer to liquifaction features also may tie a potential tsunami deposit to a seismic event (Atwater, 1992). The presence of stems of herbaceous plants in growth position within the deposit is used to indicate approximate coincidence of deposition of the sand layer with submergence of the marsh surface (Atwater, 1992). Deposit geometry and the number of sand layers present in a core or outcrop are sometimes used to differentiate between a deposit produced by a tsunami from a great earthquake on the CSZ and one left by a tsunami with a distant source. Carver and others (1996) proposed using deposit extent and thickness to differentiate CSZ tsunamis from distant tsunamis by comparing them to deposits left by historic distant tsunamis. Witter (2001) uses the estimated 500-540 year average recurrence interval for great CSZ earthquakes (Atwater and Hemphill Haley, 1997) to suggest that at least two out of four sand layers in a 600 year interval were deposited by distant tsunamis or storm surges. In lake deposits, the source of the tsunami (local or distant) may not be evident in the sediments (Clague, 1997).

Edits were made to the metadata to bring it up to current standards and syntax. Keywords were refined and added to assist in data discovery to comply with current USGS open-access policies. No data were changed.

Edited metadata to add keywords section with USGS persistent identifier as theme keyword. No data were changed.