THE APPLICATION OF MICROPALEONTOLOGY TO RECOGNIZE COSEISMIC UPLIFT AND CHARACTERIZE TSUNAMI DEPOSITS ON THE CENTRAL CHILEAN COAST

Although metropolitan central Chile (32-35^oS) has a 470-year-long written seismic history, the country’s longest, no attempt has been made to match this history with the geological record. Doing so would extend back in time, perhaps in thousands of years, the seismic story of the Chile’s most populated area. Here, we use historical, sedimentary and biological evidence to reconstruct the paleoseismic record of central Chile.

A coastal lowland site north of Valparaíso contains geological and biological evidence for three instances of coseismic uplift, with accompanying paleotsunami deposits. Each earthquake is marked by a buried soil sharply overlain by a distinct, tabular, and laterally continuous sheet of pale sand. The inferred earthquakes and tsunami deposits are dated to 4,300, 3,800, 3,500 yrs BP from in situ plant remains. This record of paleoseismicity in central Chile is typical of a subduction zone coastline that has experienced falling relative sea-levels during the late Holocene. During the early to mid-Holocene, regional sea-level rise that culminates in a highstand creates the accommodation space necessary for the preservation of such coastal stratigraphy.

Analysis of the particle size and lithology indicate a strong similarity between the inferred paleotsunami deposits, which consist mainly of fine sand (150-225 mm) with micaceous particles, and modern beach and surf sand (175-225 mm). Samples collected from old dunes that surround the study area are composed of medium (>275 mm) reddish-tan sand, with no mica, ruling out aeolian deposition of the sand-sheets. Normal grading, which is characteristic of sediment settling out of suspension and is often observed in modern tsunami deposits, is present in two of the three sands.

Fossil diatom assemblages vertically spanning each buried soil also provide paleoecological evidence to support the oceanward source of the sand deposits and reveal evidence of coseismic uplift. Buried soils are dominated by low-marsh brackish species while overlying sands are composed of a mixture of marine species common on tidal flats, marine spores, low-marsh brackish species, and high-marsh freshwater species. Soils overlying each sand deposit display a clear paleoecological shift to fresher high-marsh taxa, indicating sudden and lasting coseismic uplift.