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The Eltanin Impact and Its Tsunami Along the Coast of South America: Insights for Potential Deposits

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The Eltanin Impact and Its Tsunami Along the Coast of South America: Insights for Potential Deposits Earth and Planetary Science Letters 409 (2015) 175–181 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl The Eltanin impact and its tsunami along the coast of South America: Insights for potential deposits Robert Weiss a, Patrick Lynett b, Kai Wünnemann c a Department of Geosciences, Virginia Tech, VA 24061, USA b Sonny Astani Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, CA 90089-2531, USA c Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43, 10115 Berlin, Germany a r t i c l e i n f o a b s t r a c t Article history: The Eltanin impact occurred 2.15 million years ago in the Bellinghausen Sea in the southern Pacific. Received 21 June 2014 While a crater was not formed, evidence was left behind at the impact site to prove the impact origin. Received in revised form 10 October 2014 Previous studies suggest that a large tsunami formed, and sedimentary successions along the coast of Accepted 19 October 2014 South America have been attributed to the Eltanin impact tsunami. They are characterized by large clasts, Available online xxxx often several meters in diameter. Our state-of-the-art numerical modeling of the impact process and its Editor: J. Lynch-Stieglitz coupling with non-linear wave simulations allows for quantifying the initial wave characteristic and the Keywords: propagation of tsunami-like waves over large distances. We find that the tsunami attenuates quickly with −1.2 Eltanin impact η(r) ∝ r resulting in maximum wave heights similar to those observed during the 2004 Sumatra tsunamis and 2011 Tohoku-oki tsunamis. We compute a transport competence of the coastal flow and conclude tsunami modeling that for the northernmost alleged tsunami deposits, especially for those in Hornitos, Chile, the transport impact cratering competence is about two orders of magnitude too small to generate the observed deposits. numerical simulations © 2014 Elsevier B.V. All rights reserved. 1. Introduction The Eltanin impact occurred about 2.15 million years ago in the Bellinghausen Sea, Antarctica. The black circle in Fig. 1 annotated with “E” marks the location of the Eltanin impact site. The Eltanin The Eltanin impact structure belongs to the group of very few impact did not produce a typical bowl-shaped impact crater. It only impacts that are proven to have hit the marine environment. The reworked the sea-bed substrate and produced chaotic layering of reason for this low number of known marine impact structures is the sediment, which was first recognized by Kyte et al. (1981). multifold. Wünnemann and Lange (2002) calculated a 50% chance Gersonde et al. (1997) also found spherules and the well-known for an impactor to hit the deep sea. For an impactor to leave a Iridium anomaly. The latter two are evidence of the impact, and trace at the ocean floor, the impactor also must have a certain the fact that a crater was not formed was used by Mader (1998), diameter or larger. The Eltanin impact itself can be taken as a Wünnemann and Lange (2002) and Shuvalov and Trubetskaya threshold diameter because the impact did not form a crater, but (2007) to constrain an impactor diameter. These three independent impact-cratering processes and the water flow created by water modeling efforts estimated the Eltanin impactor to be between displacement during the impact process generated indications to 0.7 km and 1km in diameter, assuming a Stony meteorite (ρi ≈ link this structure to an impact. However, the unequivocal evidence −3 2700 kg m ). However, in a more recent study (Shuvalov and Ger- for the impact origin is given by the finding of remnants of the im- sonde, 2014)estimated the projectile diameter to be between 1.5 pactor in drill cores. For much smaller impactors than the Eltanin ◦ and 2km for a vertical and a 45 impact angle, respectively. impactor, water waves would have of course been formed as well. On a large scale, the study published by Ward and Asphaug However, it remains to be seen if the water flow alone is violent (2002) is the only publication that considers the propagation and enough to leave behind specific indications to be recognized as an coastal effects of the tsunami generated by the Eltanin impact. impact; if the shock wave does not reach the sea floor, none of the The theoretical means that the authors employ to calculate the classical impact evidence, such as shock metamorphosed rocks or wave spreading forces the waves to attenuate proportionally to 1/r, mineral phases, will be generated. where r is the radial distance from the impact location. Classical shallow-water wave theory, utilized for describing the propagation of earthquake-generated tsunami waves, only includes geometri- E-mail address: [email protected] (R. Weiss). cal spreading, which means that the amplitude of the propagating http://dx.doi.org/10.1016/j.epsl.2014.10.050 0012-821X/© 2014 Elsevier B.V. All rights reserved. 176 R. Weiss et al. / Earth and Planetary Science Letters 409 (2015) 175–181 South American continent are sufficient to move the grain sizes of these proposed tsunami deposits. Our analysis is based on impact modeling with iSALE (Wünnemann et al., 2006), wave modeling with different wave theories (Lynett and Liu, 2002), and simple es- timates of critical shear stresses to see which grain sizes could be transported. 2. Eltanin tsunami deposits along the Chilean coast The development of western South America has long been gov- erned by subduction and plate tectonics. The subduction and as- sociated processes created a very complicated geologic situation which is recorded in sediments in the coastal area. However, these processes work on different time and length scales at different parts of the subduction zone, which results in poorly constrained correlations of sediments in different areas along the western coast of South America. The discussion about deposits that might be- long to the Eltanin impact tsunami is an excellent example of this uncertainty. There are five prominent locations at which the Eltanin impact tsunami allegedly left evidence of its presence be- hind. However, this evidence is often circumstantial and tainted by poor stratigraphic constraints. Going from north to south, Horni- tos is located at the Mijeones Peninsula (H in Fig. 1). The tsunami origin of the seven to ten-meter thick conglomerate bed was first Fig. 1. The rectangle projected on the sphere represents the computational domain. put forward by Hartley et al. (2001) and reiterated by Goff et al. The location of the Eltanin impact size is marked with E. The sites of potential (2012). This bed contains large (approximately 5m in diameter) tsunami deposits formed by the Eltanin impact tsunami are: H – Hornitos, C/BI – angular to very angular boulders that are situated in a matrix of Caldera and Bahia Inglesa, N – Navidad, and CO – Concepcion. SD and SD refer to x y poorly sorted fine to very coarse sandstone. The basal part of this the computational subdomains used for the wave modeling. There are 18 longitudi- nal and 17 latitudinal subdomains resulting 306 subdomains in our state-of-the-art, conglomerate has an erosional contact with the underlying strata. high-performance numerical simulations of the wave dynamics. The second deposit is at Caldera and Bahia Inglesa (C/BI in Fig. 1). The deposits at Caldera consist of a debris-flow deposit with mega- clasts. Also associated with this deposit are well-preserved whale Table 1 √ Wave amplitude η proportional to 1/ r, 1/r and 1/r2 for 10 km, 50 km, 100 km, skeletons, which is seen as evidence of long-standing water af- = = and 200 km distance from the impact center and an initial amplitude of η(r 0) ter the tsunami inundation of the Eltanin impact tsunami (Goff 100 m. et al., 2012). The potential tsunami deposits at Bahia Inglesa con- Distance sists of a 42-m thick silt and fine-grained sand stone. Associated 10 50 100 200 with the deposits is a bonebed that contains boulders up to 6m (km) (km) (km) (km) in diameter. The deposit in Navidad consists of clast-supported √ η ∝ 1/ r 31.6200 m 14.1400 m 10.0000 m 7.0700 m conglomerate with angular to subrounded clasts. The fossil assem- 1/r 10.0000 m 2.0000 m 0.5000 m 0.5000 m blage within this deposit indicates a mix of marine and terrestrial 2 1/r 1.0000 m 0.0400 m 0.0100 m 0.0025 m origin. This basal conglomerate is overlain by interbedded sand and mudstones as well as conglomerates. The southernmost po- wave decreases due to the larger area that the wave engulfs. This tential tsunami deposits can be found near Concepcion, where a coarse beach sandstone contains rip-up clasts made up of mud- leads√ to an attenuation of the individual waves proportional to stone. Rip-up clasts are often found in tsunami deposits, such as 1/ r. Whether or not the √difference in wave amplitudes η(x, t) is the deposits left behind by the 1998 tsunami in Papua New Guinea η(x, t) ∝ 1/r or η(x, t) ∝ 1/ r is enormous. Table 1 gives√ an exam- (Gelfenbaum and Jaffe, 2003). This potential tsunami deposit also ple how wave amplitudes changes proportional to 1/ r, 1/r and contains melted quartz and glass particles, which was taken as po- 1/r2. tential evidence of the Eltanin impact by Goff et al. (2012). Does the Eltanin impact generate a tsunami that attenuates − with η(x, t) ∝ 1/r or other q’s in η(x, t) ∝ r q.
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