The onshore tsunami record in southern Chile - historical and beyond: Sedimentology, geochemistry, stratigraphy and regional correlation
Final Report Abstract
The Chilean coast is highly vulnerable to the impact of earthquakes and tsunamis. Therefore it is crucial to understand the complex tsunami history of Chile which can be deciphered by studying the geological record of former tsunamis in the form of paleotsunami deposits. Such a study permits to establish the recurrence times of tsunami and to improve risk assessments of coastlines. However, study sites have to picked carefully as the subduction zone is subdivided into several segments with different earthquake records. Regional comparisons and correlations have to bear this in mind. We performed reconnaissance studies at a number of localities in southern central Chile between Mehuin (39° 26' 55.3'' S, 73° 12' 21.5'' W) to the S of Puerto Saavedra in the southern segment of the subduction zone and Pelluhue (35°48'31.00"S, 72°33'49.43"W) in the northern segment. Our working area therefore stradles the subduction zone segment boundary located approximately at the latitude of the city of Concepción (36°49'12.35"S, 73° 2'39.70"W). Detailed studies of the coastal plain successions containing the paleotsunami deposits were undertaken at Tirúa (38°20'44.05"S, 73°29'17.19"W) in the S and Pelluhue in the N. We performed a multi-method study including the determination of OSL ages, the analysis of the sedimentology, grain size distributions, petrography, quartz mineral surfaces, geochemistry, and diatom assemblages. Such a comprehensive effort is required to identify and characterise paleotsunami deposits with confidence. At Tirúa, we encountered six tsunami intercalations in the uppermost 2 m of the river floodplain at distances of 1.2 to 2 km from the coast. The youngest was deposited by the 2010 Maule tsunami. The OSL ages of the five older tsunami layers are interpreted to reflect the AD 1960 Valdivia tsunami, the AD 1730 Valparaiso or AD 1751 Concepción tsunamis, one tsunami at c. AD 1390, one between AD 1250 and AD 830, and another one between AD 530 and AD 490. The three oldest tsunami events extend the record to pre-Columbian time with the oldest one dated at ca. AD 500. The tsunami sand layers are characterised by erosional bases, landward thinning and fining and consist of well sorted, unimodal sand with grain sizes commonly >3ϕ. The fluvial marsh sediments, in contrast, exhibit polymodal grain size distributions with a distinct fraction commonly <3ϕ. The diatom data indicate environmental changes due to neotectonic movements for the lowermost sand layers. All six tsunami deposits are enriched in Ca, Si, Sr, Ti, Fe and depleted in Al, K, Rb, and yield a low LOI resulting from heavy mineral accumulations and low CaCO3 in these layers. At Pelluhue in the northern segment, we observed five sand layers intercalated in the uppermost 2 m along the river bank at distances between ~ 200 and 1000 m from the shoreline. These sand layers show significant landward thinning and fining over these distances, which are common features of tsunami deposits. Sand intercalations (medium to coarse sand) have erosional lower contacts and yield grain sizes coarser than the surrounding river bank and river floodplain sediments (silty fine sand); the grain size differences of the intercalations and fluvial sediments range mostly from 0.5 to 1.0 ϕ suggesting deposition from different flow velocities. Geochemical analysis revealed relatively similar major element concentrations of both sediment types. The LOI values are slightly decreased within the sand intercalations of both localities indicating less organic matter and CaCO3, patterns which are considerd typical of tsunami deposits. The OSL-ages of Pelluhue class all sand intercalations as younger than 1950. The top sand layer was deposited by the 2010 Maule tsunami, whereas sand layers 2 and 3 relate to a smaller scale tsunami in 1985. Sand layer 3 yields younger ages than sand layer 2 indicating deposition of multiple-layers during a single tsunami event. Sand layers 4 and 5 yield the same age range from 1950 to 1980, which implies a deposition by the large-scale 1960 Valdivia tsunami, following the strongest earthquake recorded globally so far. For the southern site, our OSL age results appear to define a recurrence interval of 252 years if the data were taken at face value. This number is unrealistic in view of much shorter recurrence times in the historical period on the order of 100 years or less. Considering the relative continuity of the subduction zone activity, this emphasises the incompleteness of the paleotsunami record identified so far, the problem of the limited preservation potential of tsunami deposits, and thus the need for further study of both problems.
Publications
- (2014): Reconstructing the tsunami record in central Chile with OSL.- 14th Luminescence and Electron Spin Resonance Dating conference, abstracts
Nentwig, V., Tsukamoto, S., Frechen, M., Bahlburg, H.
(See online at https://doi.org/10.1016/j.quageo.2015.05.020) - (2014): The onshore tsunami record at Tirua, central Chile.- GeoFrankfurt 2014, abstracts, Schriftenreihe der Deutschen Geologischen Gesellschaft 85, 116
Nentwig, V., Bahlburg, H., Huber, B., Bellanova, P., Jungheim, V., Montecinos, M., Encinas, A.
- (2015): Multi-proxy analysis of tsunami deposits -the Tirua, Chile, example.- American Geophysical Union Fall Meeting 2015, San Francisco, abstract
Nentwig, V., Bahlburg, H., Tang, H., Weiss, R., Gorecka, E., Witkoski, A., Encinas, A., Huber, B., Bellanova, P., Montecinos, M.
- (2015): Reconstructing the tsunami record in Tirua, Central Chile, beyond the historical record with quartz-based SAR-OSL.-Quaternary Geochronology 30, 299-305
Nentwig, V., Tsukamoto, S., Frechen, M., Bahlburg, H.
(See online at https://doi.org/10.1016/j.quageo.2015.05.020) - (2015): Test of the microtextural analysis of quartz grains of tsunami and non-tsunami deposits in Tirua (Chile) - an unsuitable method for a valid tsunami identification.- American Geophysical Union Fall Meeting 2015, San Francisco, abstract
Bellanova, P., Nentwig, V., Bahlburg, H.
- (2016): Microtextural analysis of quartz grains of tsunami and non-tsunami deposits - a case study from Tirua (Chile).- Sedimentary Geology 343, 72-84
Bellanova, P., Bahlburg, H., Nentwig, V., Spiske, M.
(See online at https://doi.org/10.1016/j.sedgeo.2016.08.001) - (2017): Young vs. old onshore tsunami records in Central Chile.- GeoBremen 2017, DGGV Annual Meeting, abstracts
Nentwig, V., Bahlburg, H., Tsukamoto, S., Bellanova, P., Encinas, A.
