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Sedimentology and paleontology of a tsunami deposit accompanying the great Chilean earthquake of February 2010
Marine Micropaleontology 79 (2011) 132–138
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Marine Micropaleontology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r m i c r o
Research paper
Sedimentology and paleontology of a tsunami deposit accompanying the great Chilean earthquake of February 2010 B.P. Horton a,b,⁎, Y. Sawai c, A.D. Hawkes d, R.C. Witter e a
Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104, USA Earth Observatory of Singapore (EOS), Nanyang Technological University, Singapore 639798, Singapore Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Site C7 1-1-1 Higashi, Tsukuba 305-8567, Japan d Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA e Department of Geology & Mineral Industries, Coastal Field Office, Newport, OR 97365, USA b c
a r t i c l e
i n f o
Article history: Received 13 October 2010 Received in revised form 13 February 2011 Accepted 16 February 2011 Keywords: Micrfossils Tsunami 2010 Chile earthquake
a b s t r a c t At Pichilemu, in the northern third of the rupture area of the moment magnitude scale (Mw) 8.8 2010 Chile earthquake, deposits of the tsunami accompanying the earthquake consist of a lower layer of medium to fine sand (mean grain size of 200 μm) containing rock clasts, overlain by a thin, silty, very fine sand (mean grain size of 125 μm) layer. Based on a sedimentological model, most (90%) of the deposit is finer than 401–408 μm suggesting tsunami flow velocities were between 7 m/ s and 13.5 m/ s. Ostracods were common in the upper layer along with a small number of broken benthic foraminifera and a single planktonic foraminifera. Diatoms were abundant throughout. Species assemblages represent a mixture of diatoms from differing environments, life forms and substrate preferences. We attribute the mixed assemblages to turbulence within the water column during tsunami inundation, with erosion of beaches and salt marshes followed by redeposition of sand and mud inland. Breakage of fragile diatom valves in the lower layer may also support transport by turbulent flow. A higher abundance of diatom species with mud substrate preferences in the upper layer implies a decrease in flow velocity from lower to upper layers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Mw 8.8 Chile earthquake on February 27th 2010 ruptured a 500 km-long, 100 km-wide section along the Nazca-South American plate boundary (Fig. 1). In this region of south central Chile, the Nazca plate converges toward the northeast, subducting beneath South America at a rate of almost 70 mm/year (Ruegg et al., 2009; Vigny et al., 2009). The 2010 earthquake is the most recent in a series of Mw N8 subduction earthquakes that damaged Spanish settlements such as Talcahuano and Concepción in 1570, 1657, 1751 and 1835 (Lomnitz, 2004), and produced tsunami sand layers that have been recovered from estuarine sediments (Cisternas et al., 2005). During the 1835 earthquake, described by Charles Darwin (1846) during the voyage of the Beagle, shorelines of offshore islands were raised as much as 3 m and the Maule River estuary at Constitución subsided. Larger than historical earthquakes on the same part of the plate boundary in 1835, 1906, and 1928, the 2010 earthquake produced 120 seconds of strong ground motion that peaked above 0.5 g
⁎ Corresponding author at: Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104, USA. Tel.: +1 215 573 5388; fax: +1 215 898 0964. E-mail addresses: [email protected] (B.P. Horton), [email protected] (Y. Sawai), [email protected] (A.D. Hawkes), [email protected] (R.C. Witter). 0377-8398/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2011.02.001
(Geo-Engineering Extreme Events Reconnaissance, GEER, 2010). The earthquake was widely felt throughout Chile. At Pichilemu intensities reached VII on the MMI scale (Modified Mercalli Intensity scale) and strong shaking (intensity VI) extended over a 900- km-long region. The 2010 Chile earthquake was shortly followed by a tsunami that spread across the Pacific. The tsunami flooded areas along 500 km of the south-central Chile coast between Tirúa and Pichilemu causing severe damage in areas where inundation was enhanced by bathymetry and geomorphology (Earth Engineering Research Institute, EERI, 2010). Vertical coseismic and postseismic tectonic deformations along the south central Chilean coast mitigated tsunami impacts in areas uplifted by the earthquake but exacerbated inundation in areas that subsided (GEER, 2010). Eye witnesses reported that the first tsunami wave reached the coast within about 30 minutes of the earthquake, followed by one or more waves. In many areas the highest waves arrived 5 to 7 hours after the earthquake when wave crests were superimposed on a rising tide. Maximum wave height varied considerably along the coast, reaching 11.2 m in Constitución and 9.4 m in Dichato (EERI, 2010). Economic loss caused by the earthquake and tsunami have been estimated at $30 billion (USD) and human loss confirmed at 521 casualties (EERI, 2010), although less than one quarter of the fatalities were caused by the tsunami. Most coastal residents connected the ground shaking with the likelihood of a tsunami prior to government warnings and
B.P. Horton et al. / Marine Micropaleontology 79 (2011) 132–138
evacuated low-lying coastal areas. Such awareness of tsunami hazards came from modern and historic experiences with great earthquakes and tsunamis in Chile and from effective tsunami education in coastal schools (e.g., Atwater et al., 1999). Catastrophic tsunamis are too infrequent for their hazard to be characterized by historical records alone (e.g., Atwater et al., 2005). Long-term geologic records provide opportunities to assess tsunami hazards more fully (e.g., Mamo et al., 2009). Telltale deposits left by tsunamis help assess water depth and velocity of past inundations, estimate source locations, and aid in understanding how tsunamis affect the ecology and geomorphology of coastlines (e.g., Shepard et al., 1950; Kitamura et al., 1961; Bernard and Robinson, 2009). Identification of tsunami deposits is often based on finding anomalous sand deposits in low-energy environments that have the best preservation potential such as coastal ponds, lakes, and marshes. The deposits are evaluated using several criteria such as area, landward thinning of deposit thickness and single or multiple layers with normal grading (e.g., Gelfenbaum and Jaffe, 2003; Jaffe et al., 2003). Evaluating the microfossil assemblages, such as diatoms and foraminifera, from tsunami deposits may provide additional techniques to either support existing lithostratigraphic characteristics or importantly, distinguish tsunami sediment from other sediments where lithostratigraphic characteristics are inadequate (Hawkes et al., 2007). Hemphill-Haley (1996) emphasized that analysis of diatoms works particularly well in freshwater environments, because brackish-marine diatoms present in the tsunami sand stand out in striking contrast to freshwater diatom assemblages in the host sediment. Diatoms have identified the source of tsunami lain sand. For example, sandy intertidal and subtidal tsunami sediments were deposited above a forest soil subsided by the AD 1700 earthquake at the Copalis River in southwestern Washington (Atwater, 1992). The superior preservation of diatom tests in the sand has indicated rapid burial of a modern diatom assemblage that was transported inland rather than reworking of older deposits (Atwater and Hemphill-Haley, 1997). Diatoms also proved useful for mapping the inland extent of the tsunami deposit by establishing a seaward source for fine-grained sediment where sandy laminae were not observed (Hemphill-Haley, 1995, 1996).
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A number of studies have employed foraminifera to further define tsunami sediment characteristics (e.g., Hawkes et al., 2007; Dahanayake and Kulasena, 2008; Mamo et al., 2009). For example, Hawkes et al. (2007) analyzed the foraminiferal assemblages of tsunami sediment from the 2004 Indian Ocean tsunami along the Malaysia–Thailand Peninsula. Foraminiferal assemblages in the tsunami sand revealed at least two separate episodes of deposition that contained species from the inner-shelf (e.g., Elphidium hispudilum) recording uprush and species from an inland mangrove environment (e.g., Haplophragmoides spp.) recording backwash. In this short note, we used lithostratigraphic (grain size distribution and loss on ignition) and microfossil analyses to characterize the tsunami sediment from a trench at Pichilemu to contribute information on tsunami sediment source and deposition style, and provide a modern analog to aid in interpretation of prehistoric tsunami deposits in the geologic record. 2. The tsunami at Pichilemu, south-central Chile Less than two weeks after the earthquake a reconnaissance team visited Pichilemu (Fig. 1b) to collect perishable data on the impacts of the 2010 Chile earthquake and tsunami, including preliminary observations of vertical tectonic deformation and the effects of tsunami inundation (GEER, 2010). Pichilemu has approximately 12,400 residents and is located 170 km southeast of Santiago and 175 km north of the earthquake epicenter. Comparison of pre- and post-earthquake imageries and field observations suggests the vertical deformation produced by the earthquake in the Pichilemu area probably amounted to b0.2 m uplift (Farías et al., 2010). Eye witnesses reported multiple tsunami waves inundated Laguna Petrel, a small (0.6 km2) estuary directly north of Pichilemu (Fig. 1c and d), leaving a sandy deposit 3 to 5 cm thick. The leaves and stems of dead marsh reeds (Scirpus and Typha species) dangling from limbs of trees south of the lagoon indicate minimum flow depths of about 3 m (Fig. 2a and b). A wrack line of plant debris south of the lagoon shows that tsunamis extended several hundred meters inland from the beach (Fig. 1d). Landward of the shoreline, north of La Puntilla
Fig. 1. Maps of study area along the south-central coast of Chile. (a) Plate-tectonic setting where the Nazca plate is subducting below the South American plate. Paired arrows indicate the direction of plate convergence at the rate of 68 mm/ year west of Concepción (Vigny et al., 2009). (b) Area (in pink) of the 2010 Mw 8.8 Maule earthquake in south-central Chile; star marks the epicenter (USGS, 2010). (c) Google Earth vertical imagery (dated December 2004) of Laguna Petrel before the 2010 earthquake. Location of city water tower shown in c and d. (d) Oblique aerial photograph (view to the south) showing the extent of inundation by the 2010 tsunami in Laguna Petrel (photo credit: K. Kelson; date of photo: March 12, 2010). Circle indicates the field site in a marsh on the southern shore of the lagoon where we sampled the sediment deposited by the 2010 tsunami.
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(Fig. 1c), fine to coarse sand strewn over roads and minor building damage indicates that the tsunami inundation was primarily limited to beachfront properties. 3. Sedimentology of the tsunami deposit On the southern shore of Laguna Petrel, a shallow trench revealed a 5 cm-thick tsunami deposit over rooted salt marsh plants (Fig. 2c). Variations exist at the local to regional scale including deposit thickness and geometry, landscape conformity, deposit elevation, inland inundation distances, and sediment–transport distances (Morton et al., 2007). For example, Morton et al. (2010) also documented relatively thin tsunami deposits (b25 cm thick) along the central coast of Chile. Exceptions were the thick tsunami deposits near the mouths of Rio Huenchullami (La Trinchera) and Rio Maule (Constitucion), where the sediment supply was abundant (Morton et al., 2010). At Pichilemu the contact between tsunami and pre-tsunami deposits was a sharp erosional boundary marked by changes in
color and lithology. The tsunami deposit was coarser and had a lower organic content than the pre-tsunami salt marsh sediment. The stems and leaves of reeds flattened by incoming waves, point in the direction of tsunami flow. The tsunami deposit was composed of a lower layer (4.5 cm-thick) of black-brown, medium to fine sand containing rock clasts capped by a 0.5 cm-thick light gray-tan, silty very fine sand (upper tsunami layer). Tsunami deposits typically have 1 to 3 layers, although they can have 12 or more layers (Jaffe et al., 2003). Layers are commonly assumed to form by either different tsunami waves and/or from differing flow regimes during wave inundation and outwash (Nanayama et al., 2000; Choowong et al., 2008a, b; Morton et al., 2007). Both sediment layers at Pichilemu contain angular to sub-angular quartz, feldspar, and mica grains similar in mineralogy to beach sand to the west of Laguna Petrel. The provenance of the larger clasts within the lower layer was probably from the eroded bedrock within a channel less than 50 m away. The lower tsunami layer was poorly sorted whereas the overlying silty sand was very poorly sorted. Mean grain size in the lower tsunami
Fig. 2. Photographs along the southern shore of Laguna Petrel. (a) View to the east showing marsh plants pushed over in the direction of tsunami flow during inundation. Site of tsunami sand deposit sampled for analysis shown in c. (b) The minimum flow depth of the tsunami (~3 m) as indicated by marsh plant leaves and stems draped over tree branches. (c) Shallow trench in marsh surface exposing 3-to-5 cm-thick sandy layer deposited on a marsh by the 2010 tsunami.
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Fig. 3. Summary of lithostratigraphy, particle size, eukaryote, foraminiferal, and diatom analyses of the tsunami deposit at Pichilemu.
layer is 200 μm whereas the grain size of the upper layer decreases to 125 μm (Fig. 3). For both layers 90% of the tsunami deposit (D90) is finer than 401–408 μm. For a 5-cm-thick deposit, this implies tsunami velocities of up to 13.5 m/ s (Jaffe and Gelfenbuam, 2007). Following Jaffe and Gelfenbuam (2007), tentative flow velocity estimates for the lower tsunami layer are likely about 7–8 m/ s but may be up to 13.5 m/ s and in the upper layer are likely b2 m/ s but as high as 7 m/ s. Comparable flow velocities (10–18 m/ s) were calculated by Titov and Synolakis (1997) for the July 12th 1993 Hokkaido-Nansei-Oki tsunami. Further, Titov et al. (2001), and Jaffe and Gelfenbaum (2002, 2007) proposed flow velocities between 4 and 17 m/s for the July 17th 1998 Papua New Guinea tsunami. Actual measurements of tsunami flow velocity were carried out based on a survivor's videos in December 26th 2004 Indian Ocean tsunami (Fritz et al., 2006). In these measurements, the velocities were within the range of 2 to 5 m/s. Thus, further discussion is needed for more precise estimates of the tsunami flow velocity based upon numerical simulations using sedimentological data.
4. Micropaleontology of the tsunami deposit Similar to the sedimentology, the micropaleontological characteristics can vary on a local to regional scale, including abundance, salinity preference, life form and habitat (Mamo et al., 2009). At Pichilemu, foraminifera and mollusca were rare in the tsunami deposit, whereas ostracoda were abundant (Fig. 3). One broken gastropod and one ostracod valve were found in the lower layer. The upper layer contained 16 ostracod halves of which 9 were broken, with a single planktonic foraminifera (Orbulina sp.) and a small number of broken tests of benthic foraminifera (Ammonia sp. and Elphidium sp.). This foraminiferal assemblage is typical of sandy substrates with insignificant fine grained material and/or high wave energy, especially if the source was local beach sand, as suggested by the lithology (Horton et al., 2009). In contrast, diatoms were abundant throughout the deposit (Table 1). In theory, marine diatoms should dominate tsunami deposits, because they are transported by rapid marine incursions. However, diatoms within tsunami deposits are generally composed of mixed assemblages, because tsunamis inundate coastal and inland areas, eroding, transporting and redepositing brackish and freshwater sediments including associated species (e.g., Dawson et al., 1996). The species assemblages at Pichilemu differed between lower and upper layers (Fig. 4). The lower layer has fresh, brackish and marine diatoms.
The assemblage consists of planktonic and benthic epiphytic (attaching on aquatic plants; e.g., Cocconeis placentula var. lineata; Vos and de Wolf, 1988) and epipsammic (attaching on sand; e.g., Fallacia tenera; Witkowski et al., 2000) species. In the upper layer the relative abundance of epiphytic and epipsammic species decrease and salt marsh diatoms (e.g., Navicula tenelloides and N. salinicola; HemphillHaley, 1995; Sawai and Nagumo, 2003) increase. The mixed nature of the diatom assemblages from differing environments with different life forms and substrate preferences is comparable to other tsunami deposits worldwide (e.g., Dawson et al., 1996; Bondevik et al., 2005). Although, in contrast to Pichilemu, freshwater species were nearly absent in the diatom assemblages in the lower part of 2004 SumatraAndaman tsunami deposit at the savanna beach ridge plain of Phra Thong Island (Sawai et al., 2009). Sawai et al. (2009) attributed this to a very strong current which allowed only epipsammic diatoms attaching to heavy sandy substrata to settle out during the first stage of deposition. About 70% of diatom valves in the tsunami deposit are complete (Plate I), although there is evidence of selective breakage of diatoms with relatively fragile valve constructions (e.g., Tabularia fasciculata) in the lower layer. Breakage of diatoms in the lower layer implies turbulent transportation. Overall the breakage is moderate compared to the poor preservation of diatoms observed in tsunami deposits such as 1998 Papua New Guinea (Dawson, 2007) where greater than 60% of the diatom valves were broken. In contrast, good preservation of diatom valves was found within tsunami deposits of Washington State, USA from the AD 1700 Cascadia earthquake (Hemphill-Haley, 1996) and Thailand from the 2004 Sumatra-Andaman earthquake (Sawai et al., 2009).
5. Conclusion Modern analogs provide geologic criteria for distinguishing ancient tsunami deposits and encompass a broad range of stratigraphic and geomorphic evidence. We collected the sedimentological and micropaleontological data from sandy sediment at Laguna Petrel, Pichilemu to serve as an example of the deposits from the tsunami of the great Mw 8.8 Chile earthquake. Post-earthquake reconnaissance of the southern shore of Laguna Petrel identified an anomalous sand deposit (2 to 5 cm thick) overlying a former salt marsh. The tsunami deposit at Pichilemu was composed of a lower layer of medium to fine sand containing rock clasts, overlain by a thin, silty very fine sand. Mollusca and foraminifera were either absent or in low
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Table 1 Diatoms indentified in a sandy layer generated from the 2010 Chile tsunami. †Bold; reported from salt marsh. With *; reported from tidal flat. With **; reported as epiphytes (attaching on plants). Underline; reported as planktonic species. With §; no detailed information on habitat. Species name
Reference
Freshwater taxa Eunotia formica Ehrenberg Krammer Eunotia praerupta Ehrenberg Krammer Eunotia spp. Krammer Planothidium lanceolatum (Brébisson ex Kützing) Round & Krammer Bukhtiyarova Sellaphora pupula agg. (Kützing) Mereschkowsky Krammer Fresh and Brackish water taxa† Cocconeis placentula var. lineata** (Ehrenb.) Van Heurck Cyclotella meneghiniana Kützing
and and and and
Lange-Bertalot Lange-Bertalot Lange-Bertalot Lange-Bertalot
(1991a), Patrick and Reimer (1966) (1991a), Patrick and Reimer (1966) (1991a), Patrick and Reimer (1966) (1991b), Patrick and Reimer (1966)
and Lange-Bertalot (1986), Patrick and Reimer (1966)
Krammer and Lange-Bertalot (1991b), Patrick and Reimer (1966), Vos and de Wolf (1988) Håkansson (1990), Snoeijs et al (1993), Vos and de Wolf (1988)
Brackish and marine water taxa† Amphora holsatica Hustedt Amphora pseudoholsatica§ Nagumo & Kobayasi Amphora salina* W.Smith Amphora ventricosa* Gregory Bacillaria paxillifer (Müller) Hendy Cyclotella striata (Kützing) Grunow Diploneis didyma* (Ehrenberg) Ehrenberg Diploneis interrupta* (Kützing) Cleve
Nagumo and Kobayasi (1990), Sawai and Nagumo (2003a) Nagumo and Kobayasi (1990) Sawai and Nagumo (2003a) Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Sawai and Nagumo (2003a), Sherrod (1999) Sawai and Nagumo (2003a), Sherrod (1999) Vos and de Wolf (1988) Hemphill-Haley (1995), Vos and de Wolf (1988) Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Riznyk (1973), Sherrod (1999), Vos and de Wolf (1988), Witkowski et al. (2000) Diploneis smithii (Brébisson) Cleve Sherrod (1999), Witkowski et al. (2000) Fallacia forcipata* (Greville) Stickle & D.G.Mann Vos and de Wolf (1988), Witkowski et al. (2000) Fallacia pygmaea* (Kützing) Stickle & D.G.Mann Hemphill-Haley (1995), Witkowski et al. (2000) Fallacia tenera* (Hustedt) D.G.Mann Sawai (unpublished data from Oregon, USA), Witkowski et al. (2000) Melosira nummuloides*, ** Agardh Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Sawai and Nagumo (2003b), Sherrod (1999), Witkowski et al. (2000) Navicula cincta* (Ehrenberg) Ralfs Atwater and Hemphill-Haley (1997), Sherrod (1999), Vos and de Wolf (1988), Witkowski et al. (2000) Navicula cryptocephala* Kützing Sherrod (1999), Vos and de Wolf (1988) Navicula cryptotenella* Lange-Bertalot Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Sherrod (1999) Navicula gregaria Donkin Sawai (unpublished data from Oregon, USA), Sherrod (1999) Navicula libonensis Schoeman Sawai (unpublished data from Oregon, USA), Witkowski et al. (2000) Navicula peregrina* (Ehrenberg) Kützing Sawai (2001), Sherrod (1999), Vos and de Wolf (1988), Witkowski et al. (2000) Navicula salinarum* Grunow in Cleve & Grunow Sherrod (1999), Vos and de Wolf (1988) Navicula salinicola Hustedt Round (1984), Sawai (unpublished data from Oregon, USA) Navicula tenelloides Hustedt Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995) Nitzschia pura Hustedt Sawai (unpublished data from Oregon, USA) Nitzschia sigma* (Kützing) W. Smith Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Vos and de Wolf (1988) Petrodictyon gemma* (Ehrenberg) D.G.Mann Hendey (1964), Riznyk (1973), Vos and de Wolf (1988) Petroneis marina§ (Ralfs in Pritchard) D.G.Mann Atwater and Hemphill-Haley (1997), Hendey (1964) Planothidium delicatulum* (Kützing) Round & Bukhtiyarova Hemphill-Haley (1995), Nelson and Kashima (1993), Sherrod (1999), Vos and de Wolf (1988), Sawai (2001) Rhaphoneis amphiceros* (Ehrenberg) Ehrenberg Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Vos and de Wolf (1988) Stauroneis amphioxys (Gregory) D.G.Mann Sawai and Nagumo (2003a) Surirella marina Krammer Sawai (unpublished data from Oregon, USA) Tabularia fasciculata** (C.Agardh) Williams & Round Vos and de Wolf (1988) Tryblionella apiculata* Gregory Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Sherrod (1999) Tryblionella compressa* (J.W.Bailey) M.Poulin Hemphill-Haley (1995) Marine water (plankton) † Chaetoceros spp. (resting spore)
Hendey (1964)
Others Amphora sp.1 Amphora sp.2 Amphora sp.3 Amphora sp.4 Amphora sp.5 Amphora sp.6 Cyclotella cf. atomus Hustedt Cyclotella spp. Navicula cf. tenelloides Hustedt Navicula sp. girdle view Nitzschia sp.1 Podosira sp.1 Rhopalodia sp.1
– – – – – – – – – – – – –
numbers in the tsunami deposit whereas ostracoda were abundant. The diatom assemblages were abundant in both the basal and upper layers with a mixed assemblage. There were differences between the diatom assemblages of the Pichilemu tsunami deposit. Most notably, species
attached to sand and plant materials decrease in the upper layer and taxa associated with suspended mud fractions increase, suggesting a decrease in the velocity of tsunami flow that is supported by velocity estimates derived from the grain size.
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Fig. 4. Species diagram of fossil diatom assemblages in the tsunami deposit at Pichilemu. Cocconeis placentula lives in a variable habitat. It is well known as an epiphytic species (Vos and de Wolf, 1988) but is sometimes found on other objects (Patrick and Reimer, 1966). It has a broad salt tolerance in fresh and brackish waters (Vos and de Wolf, 1988). In this study, we recognized one of its varieties (C. placentula var. lineata) as fresh and brackish water epiphyte.
Plate I. Diatoms from a sandy layer deposited by the 2010 Chile tsunami. 1. Amphora salina, 2. Cocconeis placentula var. lineata, 3. Cyclotella meneghiniana, 4. Diploneis didyma, 5. Fallacia pygmaea, 6, 7. Fallacia tenera, 8. Navicula salinicola, 9. Navicula salinarum, 10. Navicula tenelloides, 11. Tryblionella apiculata, 12. Tabularia fasciculata (fragment). Scale bar = 10 μm.
Acknowledgments We acknowledge vital support from the Geo-Engineering Extreme Events Reconnaissance (GEER) Association (Grant No. CMMI-Proposal no. 103483) and the Oregon Department of Geology and Mineral Industries. Jonathan Bray and David Frost effectively coordinated the GEER Team. Ramon Verdugo, Christian Ledezma and Terry Eldridge provided critical guidance and logistical assistance in Chile. Laurie Johnson and Leonardo Dorador assisted with field observations. This
research was also supported by the National Science Foundation award (EAR-0842728) to BPH and RCW. We thank Shigehiro Fujino and Yuichi Namegaya for discussion. BPH organized and drafted this publication. YS analyzed diatoms and provided Fig. 3. RCW made field measurements, collected samples, and provided Figs. 1 and 2. ADH did grain size, velocity, mollusca, ostracoda and foraminiferal analyses. We thank R. Jordan and two anonymous reviewers for their constructive comments. This paper is a contribution to IGCP Project 588 and is Earth Observatory of Singapore (EOS) publication number # 22.
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Marine Micropaleontology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a r m i c r o
Research paper
Sedimentology and paleontology of a tsunami deposit accompanying the great Chilean earthquake of February 2010 B.P. Horton a,b,⁎, Y. Sawai c, A.D. Hawkes d, R.C. Witter e a
Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104, USA Earth Observatory of Singapore (EOS), Nanyang Technological University, Singapore 639798, Singapore Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Site C7 1-1-1 Higashi, Tsukuba 305-8567, Japan d Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA e Department of Geology & Mineral Industries, Coastal Field Office, Newport, OR 97365, USA b c
a r t i c l e
i n f o
Article history: Received 13 October 2010 Received in revised form 13 February 2011 Accepted 16 February 2011 Keywords: Micrfossils Tsunami 2010 Chile earthquake
a b s t r a c t At Pichilemu, in the northern third of the rupture area of the moment magnitude scale (Mw) 8.8 2010 Chile earthquake, deposits of the tsunami accompanying the earthquake consist of a lower layer of medium to fine sand (mean grain size of 200 μm) containing rock clasts, overlain by a thin, silty, very fine sand (mean grain size of 125 μm) layer. Based on a sedimentological model, most (90%) of the deposit is finer than 401–408 μm suggesting tsunami flow velocities were between 7 m/ s and 13.5 m/ s. Ostracods were common in the upper layer along with a small number of broken benthic foraminifera and a single planktonic foraminifera. Diatoms were abundant throughout. Species assemblages represent a mixture of diatoms from differing environments, life forms and substrate preferences. We attribute the mixed assemblages to turbulence within the water column during tsunami inundation, with erosion of beaches and salt marshes followed by redeposition of sand and mud inland. Breakage of fragile diatom valves in the lower layer may also support transport by turbulent flow. A higher abundance of diatom species with mud substrate preferences in the upper layer implies a decrease in flow velocity from lower to upper layers. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Mw 8.8 Chile earthquake on February 27th 2010 ruptured a 500 km-long, 100 km-wide section along the Nazca-South American plate boundary (Fig. 1). In this region of south central Chile, the Nazca plate converges toward the northeast, subducting beneath South America at a rate of almost 70 mm/year (Ruegg et al., 2009; Vigny et al., 2009). The 2010 earthquake is the most recent in a series of Mw N8 subduction earthquakes that damaged Spanish settlements such as Talcahuano and Concepción in 1570, 1657, 1751 and 1835 (Lomnitz, 2004), and produced tsunami sand layers that have been recovered from estuarine sediments (Cisternas et al., 2005). During the 1835 earthquake, described by Charles Darwin (1846) during the voyage of the Beagle, shorelines of offshore islands were raised as much as 3 m and the Maule River estuary at Constitución subsided. Larger than historical earthquakes on the same part of the plate boundary in 1835, 1906, and 1928, the 2010 earthquake produced 120 seconds of strong ground motion that peaked above 0.5 g
⁎ Corresponding author at: Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA 19104, USA. Tel.: +1 215 573 5388; fax: +1 215 898 0964. E-mail addresses: [email protected] (B.P. Horton), [email protected] (Y. Sawai), [email protected] (A.D. Hawkes), [email protected] (R.C. Witter). 0377-8398/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2011.02.001
(Geo-Engineering Extreme Events Reconnaissance, GEER, 2010). The earthquake was widely felt throughout Chile. At Pichilemu intensities reached VII on the MMI scale (Modified Mercalli Intensity scale) and strong shaking (intensity VI) extended over a 900- km-long region. The 2010 Chile earthquake was shortly followed by a tsunami that spread across the Pacific. The tsunami flooded areas along 500 km of the south-central Chile coast between Tirúa and Pichilemu causing severe damage in areas where inundation was enhanced by bathymetry and geomorphology (Earth Engineering Research Institute, EERI, 2010). Vertical coseismic and postseismic tectonic deformations along the south central Chilean coast mitigated tsunami impacts in areas uplifted by the earthquake but exacerbated inundation in areas that subsided (GEER, 2010). Eye witnesses reported that the first tsunami wave reached the coast within about 30 minutes of the earthquake, followed by one or more waves. In many areas the highest waves arrived 5 to 7 hours after the earthquake when wave crests were superimposed on a rising tide. Maximum wave height varied considerably along the coast, reaching 11.2 m in Constitución and 9.4 m in Dichato (EERI, 2010). Economic loss caused by the earthquake and tsunami have been estimated at $30 billion (USD) and human loss confirmed at 521 casualties (EERI, 2010), although less than one quarter of the fatalities were caused by the tsunami. Most coastal residents connected the ground shaking with the likelihood of a tsunami prior to government warnings and
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evacuated low-lying coastal areas. Such awareness of tsunami hazards came from modern and historic experiences with great earthquakes and tsunamis in Chile and from effective tsunami education in coastal schools (e.g., Atwater et al., 1999). Catastrophic tsunamis are too infrequent for their hazard to be characterized by historical records alone (e.g., Atwater et al., 2005). Long-term geologic records provide opportunities to assess tsunami hazards more fully (e.g., Mamo et al., 2009). Telltale deposits left by tsunamis help assess water depth and velocity of past inundations, estimate source locations, and aid in understanding how tsunamis affect the ecology and geomorphology of coastlines (e.g., Shepard et al., 1950; Kitamura et al., 1961; Bernard and Robinson, 2009). Identification of tsunami deposits is often based on finding anomalous sand deposits in low-energy environments that have the best preservation potential such as coastal ponds, lakes, and marshes. The deposits are evaluated using several criteria such as area, landward thinning of deposit thickness and single or multiple layers with normal grading (e.g., Gelfenbaum and Jaffe, 2003; Jaffe et al., 2003). Evaluating the microfossil assemblages, such as diatoms and foraminifera, from tsunami deposits may provide additional techniques to either support existing lithostratigraphic characteristics or importantly, distinguish tsunami sediment from other sediments where lithostratigraphic characteristics are inadequate (Hawkes et al., 2007). Hemphill-Haley (1996) emphasized that analysis of diatoms works particularly well in freshwater environments, because brackish-marine diatoms present in the tsunami sand stand out in striking contrast to freshwater diatom assemblages in the host sediment. Diatoms have identified the source of tsunami lain sand. For example, sandy intertidal and subtidal tsunami sediments were deposited above a forest soil subsided by the AD 1700 earthquake at the Copalis River in southwestern Washington (Atwater, 1992). The superior preservation of diatom tests in the sand has indicated rapid burial of a modern diatom assemblage that was transported inland rather than reworking of older deposits (Atwater and Hemphill-Haley, 1997). Diatoms also proved useful for mapping the inland extent of the tsunami deposit by establishing a seaward source for fine-grained sediment where sandy laminae were not observed (Hemphill-Haley, 1995, 1996).
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A number of studies have employed foraminifera to further define tsunami sediment characteristics (e.g., Hawkes et al., 2007; Dahanayake and Kulasena, 2008; Mamo et al., 2009). For example, Hawkes et al. (2007) analyzed the foraminiferal assemblages of tsunami sediment from the 2004 Indian Ocean tsunami along the Malaysia–Thailand Peninsula. Foraminiferal assemblages in the tsunami sand revealed at least two separate episodes of deposition that contained species from the inner-shelf (e.g., Elphidium hispudilum) recording uprush and species from an inland mangrove environment (e.g., Haplophragmoides spp.) recording backwash. In this short note, we used lithostratigraphic (grain size distribution and loss on ignition) and microfossil analyses to characterize the tsunami sediment from a trench at Pichilemu to contribute information on tsunami sediment source and deposition style, and provide a modern analog to aid in interpretation of prehistoric tsunami deposits in the geologic record. 2. The tsunami at Pichilemu, south-central Chile Less than two weeks after the earthquake a reconnaissance team visited Pichilemu (Fig. 1b) to collect perishable data on the impacts of the 2010 Chile earthquake and tsunami, including preliminary observations of vertical tectonic deformation and the effects of tsunami inundation (GEER, 2010). Pichilemu has approximately 12,400 residents and is located 170 km southeast of Santiago and 175 km north of the earthquake epicenter. Comparison of pre- and post-earthquake imageries and field observations suggests the vertical deformation produced by the earthquake in the Pichilemu area probably amounted to b0.2 m uplift (Farías et al., 2010). Eye witnesses reported multiple tsunami waves inundated Laguna Petrel, a small (0.6 km2) estuary directly north of Pichilemu (Fig. 1c and d), leaving a sandy deposit 3 to 5 cm thick. The leaves and stems of dead marsh reeds (Scirpus and Typha species) dangling from limbs of trees south of the lagoon indicate minimum flow depths of about 3 m (Fig. 2a and b). A wrack line of plant debris south of the lagoon shows that tsunamis extended several hundred meters inland from the beach (Fig. 1d). Landward of the shoreline, north of La Puntilla
Fig. 1. Maps of study area along the south-central coast of Chile. (a) Plate-tectonic setting where the Nazca plate is subducting below the South American plate. Paired arrows indicate the direction of plate convergence at the rate of 68 mm/ year west of Concepción (Vigny et al., 2009). (b) Area (in pink) of the 2010 Mw 8.8 Maule earthquake in south-central Chile; star marks the epicenter (USGS, 2010). (c) Google Earth vertical imagery (dated December 2004) of Laguna Petrel before the 2010 earthquake. Location of city water tower shown in c and d. (d) Oblique aerial photograph (view to the south) showing the extent of inundation by the 2010 tsunami in Laguna Petrel (photo credit: K. Kelson; date of photo: March 12, 2010). Circle indicates the field site in a marsh on the southern shore of the lagoon where we sampled the sediment deposited by the 2010 tsunami.
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(Fig. 1c), fine to coarse sand strewn over roads and minor building damage indicates that the tsunami inundation was primarily limited to beachfront properties. 3. Sedimentology of the tsunami deposit On the southern shore of Laguna Petrel, a shallow trench revealed a 5 cm-thick tsunami deposit over rooted salt marsh plants (Fig. 2c). Variations exist at the local to regional scale including deposit thickness and geometry, landscape conformity, deposit elevation, inland inundation distances, and sediment–transport distances (Morton et al., 2007). For example, Morton et al. (2010) also documented relatively thin tsunami deposits (b25 cm thick) along the central coast of Chile. Exceptions were the thick tsunami deposits near the mouths of Rio Huenchullami (La Trinchera) and Rio Maule (Constitucion), where the sediment supply was abundant (Morton et al., 2010). At Pichilemu the contact between tsunami and pre-tsunami deposits was a sharp erosional boundary marked by changes in
color and lithology. The tsunami deposit was coarser and had a lower organic content than the pre-tsunami salt marsh sediment. The stems and leaves of reeds flattened by incoming waves, point in the direction of tsunami flow. The tsunami deposit was composed of a lower layer (4.5 cm-thick) of black-brown, medium to fine sand containing rock clasts capped by a 0.5 cm-thick light gray-tan, silty very fine sand (upper tsunami layer). Tsunami deposits typically have 1 to 3 layers, although they can have 12 or more layers (Jaffe et al., 2003). Layers are commonly assumed to form by either different tsunami waves and/or from differing flow regimes during wave inundation and outwash (Nanayama et al., 2000; Choowong et al., 2008a, b; Morton et al., 2007). Both sediment layers at Pichilemu contain angular to sub-angular quartz, feldspar, and mica grains similar in mineralogy to beach sand to the west of Laguna Petrel. The provenance of the larger clasts within the lower layer was probably from the eroded bedrock within a channel less than 50 m away. The lower tsunami layer was poorly sorted whereas the overlying silty sand was very poorly sorted. Mean grain size in the lower tsunami
Fig. 2. Photographs along the southern shore of Laguna Petrel. (a) View to the east showing marsh plants pushed over in the direction of tsunami flow during inundation. Site of tsunami sand deposit sampled for analysis shown in c. (b) The minimum flow depth of the tsunami (~3 m) as indicated by marsh plant leaves and stems draped over tree branches. (c) Shallow trench in marsh surface exposing 3-to-5 cm-thick sandy layer deposited on a marsh by the 2010 tsunami.
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Fig. 3. Summary of lithostratigraphy, particle size, eukaryote, foraminiferal, and diatom analyses of the tsunami deposit at Pichilemu.
layer is 200 μm whereas the grain size of the upper layer decreases to 125 μm (Fig. 3). For both layers 90% of the tsunami deposit (D90) is finer than 401–408 μm. For a 5-cm-thick deposit, this implies tsunami velocities of up to 13.5 m/ s (Jaffe and Gelfenbuam, 2007). Following Jaffe and Gelfenbuam (2007), tentative flow velocity estimates for the lower tsunami layer are likely about 7–8 m/ s but may be up to 13.5 m/ s and in the upper layer are likely b2 m/ s but as high as 7 m/ s. Comparable flow velocities (10–18 m/ s) were calculated by Titov and Synolakis (1997) for the July 12th 1993 Hokkaido-Nansei-Oki tsunami. Further, Titov et al. (2001), and Jaffe and Gelfenbaum (2002, 2007) proposed flow velocities between 4 and 17 m/s for the July 17th 1998 Papua New Guinea tsunami. Actual measurements of tsunami flow velocity were carried out based on a survivor's videos in December 26th 2004 Indian Ocean tsunami (Fritz et al., 2006). In these measurements, the velocities were within the range of 2 to 5 m/s. Thus, further discussion is needed for more precise estimates of the tsunami flow velocity based upon numerical simulations using sedimentological data.
4. Micropaleontology of the tsunami deposit Similar to the sedimentology, the micropaleontological characteristics can vary on a local to regional scale, including abundance, salinity preference, life form and habitat (Mamo et al., 2009). At Pichilemu, foraminifera and mollusca were rare in the tsunami deposit, whereas ostracoda were abundant (Fig. 3). One broken gastropod and one ostracod valve were found in the lower layer. The upper layer contained 16 ostracod halves of which 9 were broken, with a single planktonic foraminifera (Orbulina sp.) and a small number of broken tests of benthic foraminifera (Ammonia sp. and Elphidium sp.). This foraminiferal assemblage is typical of sandy substrates with insignificant fine grained material and/or high wave energy, especially if the source was local beach sand, as suggested by the lithology (Horton et al., 2009). In contrast, diatoms were abundant throughout the deposit (Table 1). In theory, marine diatoms should dominate tsunami deposits, because they are transported by rapid marine incursions. However, diatoms within tsunami deposits are generally composed of mixed assemblages, because tsunamis inundate coastal and inland areas, eroding, transporting and redepositing brackish and freshwater sediments including associated species (e.g., Dawson et al., 1996). The species assemblages at Pichilemu differed between lower and upper layers (Fig. 4). The lower layer has fresh, brackish and marine diatoms.
The assemblage consists of planktonic and benthic epiphytic (attaching on aquatic plants; e.g., Cocconeis placentula var. lineata; Vos and de Wolf, 1988) and epipsammic (attaching on sand; e.g., Fallacia tenera; Witkowski et al., 2000) species. In the upper layer the relative abundance of epiphytic and epipsammic species decrease and salt marsh diatoms (e.g., Navicula tenelloides and N. salinicola; HemphillHaley, 1995; Sawai and Nagumo, 2003) increase. The mixed nature of the diatom assemblages from differing environments with different life forms and substrate preferences is comparable to other tsunami deposits worldwide (e.g., Dawson et al., 1996; Bondevik et al., 2005). Although, in contrast to Pichilemu, freshwater species were nearly absent in the diatom assemblages in the lower part of 2004 SumatraAndaman tsunami deposit at the savanna beach ridge plain of Phra Thong Island (Sawai et al., 2009). Sawai et al. (2009) attributed this to a very strong current which allowed only epipsammic diatoms attaching to heavy sandy substrata to settle out during the first stage of deposition. About 70% of diatom valves in the tsunami deposit are complete (Plate I), although there is evidence of selective breakage of diatoms with relatively fragile valve constructions (e.g., Tabularia fasciculata) in the lower layer. Breakage of diatoms in the lower layer implies turbulent transportation. Overall the breakage is moderate compared to the poor preservation of diatoms observed in tsunami deposits such as 1998 Papua New Guinea (Dawson, 2007) where greater than 60% of the diatom valves were broken. In contrast, good preservation of diatom valves was found within tsunami deposits of Washington State, USA from the AD 1700 Cascadia earthquake (Hemphill-Haley, 1996) and Thailand from the 2004 Sumatra-Andaman earthquake (Sawai et al., 2009).
5. Conclusion Modern analogs provide geologic criteria for distinguishing ancient tsunami deposits and encompass a broad range of stratigraphic and geomorphic evidence. We collected the sedimentological and micropaleontological data from sandy sediment at Laguna Petrel, Pichilemu to serve as an example of the deposits from the tsunami of the great Mw 8.8 Chile earthquake. Post-earthquake reconnaissance of the southern shore of Laguna Petrel identified an anomalous sand deposit (2 to 5 cm thick) overlying a former salt marsh. The tsunami deposit at Pichilemu was composed of a lower layer of medium to fine sand containing rock clasts, overlain by a thin, silty very fine sand. Mollusca and foraminifera were either absent or in low
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Table 1 Diatoms indentified in a sandy layer generated from the 2010 Chile tsunami. †Bold; reported from salt marsh. With *; reported from tidal flat. With **; reported as epiphytes (attaching on plants). Underline; reported as planktonic species. With §; no detailed information on habitat. Species name
Reference
Freshwater taxa Eunotia formica Ehrenberg Krammer Eunotia praerupta Ehrenberg Krammer Eunotia spp. Krammer Planothidium lanceolatum (Brébisson ex Kützing) Round & Krammer Bukhtiyarova Sellaphora pupula agg. (Kützing) Mereschkowsky Krammer Fresh and Brackish water taxa† Cocconeis placentula var. lineata** (Ehrenb.) Van Heurck Cyclotella meneghiniana Kützing
and and and and
Lange-Bertalot Lange-Bertalot Lange-Bertalot Lange-Bertalot
(1991a), Patrick and Reimer (1966) (1991a), Patrick and Reimer (1966) (1991a), Patrick and Reimer (1966) (1991b), Patrick and Reimer (1966)
and Lange-Bertalot (1986), Patrick and Reimer (1966)
Krammer and Lange-Bertalot (1991b), Patrick and Reimer (1966), Vos and de Wolf (1988) Håkansson (1990), Snoeijs et al (1993), Vos and de Wolf (1988)
Brackish and marine water taxa† Amphora holsatica Hustedt Amphora pseudoholsatica§ Nagumo & Kobayasi Amphora salina* W.Smith Amphora ventricosa* Gregory Bacillaria paxillifer (Müller) Hendy Cyclotella striata (Kützing) Grunow Diploneis didyma* (Ehrenberg) Ehrenberg Diploneis interrupta* (Kützing) Cleve
Nagumo and Kobayasi (1990), Sawai and Nagumo (2003a) Nagumo and Kobayasi (1990) Sawai and Nagumo (2003a) Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Sawai and Nagumo (2003a), Sherrod (1999) Sawai and Nagumo (2003a), Sherrod (1999) Vos and de Wolf (1988) Hemphill-Haley (1995), Vos and de Wolf (1988) Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Riznyk (1973), Sherrod (1999), Vos and de Wolf (1988), Witkowski et al. (2000) Diploneis smithii (Brébisson) Cleve Sherrod (1999), Witkowski et al. (2000) Fallacia forcipata* (Greville) Stickle & D.G.Mann Vos and de Wolf (1988), Witkowski et al. (2000) Fallacia pygmaea* (Kützing) Stickle & D.G.Mann Hemphill-Haley (1995), Witkowski et al. (2000) Fallacia tenera* (Hustedt) D.G.Mann Sawai (unpublished data from Oregon, USA), Witkowski et al. (2000) Melosira nummuloides*, ** Agardh Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Sawai and Nagumo (2003b), Sherrod (1999), Witkowski et al. (2000) Navicula cincta* (Ehrenberg) Ralfs Atwater and Hemphill-Haley (1997), Sherrod (1999), Vos and de Wolf (1988), Witkowski et al. (2000) Navicula cryptocephala* Kützing Sherrod (1999), Vos and de Wolf (1988) Navicula cryptotenella* Lange-Bertalot Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Sherrod (1999) Navicula gregaria Donkin Sawai (unpublished data from Oregon, USA), Sherrod (1999) Navicula libonensis Schoeman Sawai (unpublished data from Oregon, USA), Witkowski et al. (2000) Navicula peregrina* (Ehrenberg) Kützing Sawai (2001), Sherrod (1999), Vos and de Wolf (1988), Witkowski et al. (2000) Navicula salinarum* Grunow in Cleve & Grunow Sherrod (1999), Vos and de Wolf (1988) Navicula salinicola Hustedt Round (1984), Sawai (unpublished data from Oregon, USA) Navicula tenelloides Hustedt Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995) Nitzschia pura Hustedt Sawai (unpublished data from Oregon, USA) Nitzschia sigma* (Kützing) W. Smith Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Vos and de Wolf (1988) Petrodictyon gemma* (Ehrenberg) D.G.Mann Hendey (1964), Riznyk (1973), Vos and de Wolf (1988) Petroneis marina§ (Ralfs in Pritchard) D.G.Mann Atwater and Hemphill-Haley (1997), Hendey (1964) Planothidium delicatulum* (Kützing) Round & Bukhtiyarova Hemphill-Haley (1995), Nelson and Kashima (1993), Sherrod (1999), Vos and de Wolf (1988), Sawai (2001) Rhaphoneis amphiceros* (Ehrenberg) Ehrenberg Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Vos and de Wolf (1988) Stauroneis amphioxys (Gregory) D.G.Mann Sawai and Nagumo (2003a) Surirella marina Krammer Sawai (unpublished data from Oregon, USA) Tabularia fasciculata** (C.Agardh) Williams & Round Vos and de Wolf (1988) Tryblionella apiculata* Gregory Atwater and Hemphill-Haley (1997), Hemphill-Haley (1995), Sherrod (1999) Tryblionella compressa* (J.W.Bailey) M.Poulin Hemphill-Haley (1995) Marine water (plankton) † Chaetoceros spp. (resting spore)
Hendey (1964)
Others Amphora sp.1 Amphora sp.2 Amphora sp.3 Amphora sp.4 Amphora sp.5 Amphora sp.6 Cyclotella cf. atomus Hustedt Cyclotella spp. Navicula cf. tenelloides Hustedt Navicula sp. girdle view Nitzschia sp.1 Podosira sp.1 Rhopalodia sp.1
– – – – – – – – – – – – –
numbers in the tsunami deposit whereas ostracoda were abundant. The diatom assemblages were abundant in both the basal and upper layers with a mixed assemblage. There were differences between the diatom assemblages of the Pichilemu tsunami deposit. Most notably, species
attached to sand and plant materials decrease in the upper layer and taxa associated with suspended mud fractions increase, suggesting a decrease in the velocity of tsunami flow that is supported by velocity estimates derived from the grain size.
B.P. Horton et al. / Marine Micropaleontology 79 (2011) 132–138
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Fig. 4. Species diagram of fossil diatom assemblages in the tsunami deposit at Pichilemu. Cocconeis placentula lives in a variable habitat. It is well known as an epiphytic species (Vos and de Wolf, 1988) but is sometimes found on other objects (Patrick and Reimer, 1966). It has a broad salt tolerance in fresh and brackish waters (Vos and de Wolf, 1988). In this study, we recognized one of its varieties (C. placentula var. lineata) as fresh and brackish water epiphyte.
Plate I. Diatoms from a sandy layer deposited by the 2010 Chile tsunami. 1. Amphora salina, 2. Cocconeis placentula var. lineata, 3. Cyclotella meneghiniana, 4. Diploneis didyma, 5. Fallacia pygmaea, 6, 7. Fallacia tenera, 8. Navicula salinicola, 9. Navicula salinarum, 10. Navicula tenelloides, 11. Tryblionella apiculata, 12. Tabularia fasciculata (fragment). Scale bar = 10 μm.
Acknowledgments We acknowledge vital support from the Geo-Engineering Extreme Events Reconnaissance (GEER) Association (Grant No. CMMI-Proposal no. 103483) and the Oregon Department of Geology and Mineral Industries. Jonathan Bray and David Frost effectively coordinated the GEER Team. Ramon Verdugo, Christian Ledezma and Terry Eldridge provided critical guidance and logistical assistance in Chile. Laurie Johnson and Leonardo Dorador assisted with field observations. This
research was also supported by the National Science Foundation award (EAR-0842728) to BPH and RCW. We thank Shigehiro Fujino and Yuichi Namegaya for discussion. BPH organized and drafted this publication. YS analyzed diatoms and provided Fig. 3. RCW made field measurements, collected samples, and provided Figs. 1 and 2. ADH did grain size, velocity, mollusca, ostracoda and foraminiferal analyses. We thank R. Jordan and two anonymous reviewers for their constructive comments. This paper is a contribution to IGCP Project 588 and is Earth Observatory of Singapore (EOS) publication number # 22.
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