The geomorphological context
The western limit of Bahía Inglesa is formed by the cliffs of granodiorite of the Morro de Copiapó and some reliefs of the Bahía Inglesa Fm partially covered by Quaternary marine terraces. The block of Morro de Copiapó is characterized by flattened surfaces bordered by abrupt scarps forming straight cliffs over the Pacific Ocean; while towards Bahía Inglesa there are tectonic steps bordered by faults. These structures give place to the formation of two small elongated depressions NNE-SSW oriented (western basins) and to another depression NNW-SSE oriented following the El Morro inverse fault (Figs. 2a, 2b). The last one is one of the most active faults affecting very young Quaternary deposits (Castro Correa, 2014). Due to its softer lithological composition, the Bahía Inglesa Fm was severely eroded by the fluvial network with the formation of rills, deep ravines, wide waddis or badlands.
The most important geomorphological features are the Quaternary marine terraces stepped since the El Morro towards the coastline. These terraces reach between 0.9 and 3 m thickness of gravels, sands, and mollusk shells, sometimes forming compacted lumachelas. Normally, sand forms the matrix in the mollusk layers. These formations are known since one and a half century, but was Paskoff (1970) who started with the researchers in the area during the 60s and 70s. Higher and older terraces are located further inland from Bahía Inglesa as pointed by Marquard et al. (2004) and Quezada et al. (2007) at 139, 205-210, and 300-320 m a.s.l. In the study area, the higher level is located at 110-120 m a.s.l. forming a flatten area affected by tilt and fluvial erosion (Figs. 2a, b). This terrace was formed during the MIS9 interglacial, and was dated around 330±10 Ky (Marquardt et al., 2004). The level is extended along the graben side of the Chorrillos ravine, in the east side of El Morro. It forms a paleocliff, together with some hard layers of Neogene calcarenites of the Bahía Inglesa Fm. On its foot, there are younger marine terrace levels. These younger accumulations are at +70-90 m, +45-50 m, +30-40 m, +16-23 m, +7-10 m a.s.l., although the last one has smaller surface (Figs. 2a, b). There are several deformations due neotectonic activity, highlighting the faults uplifting the terrace of +70-75 m to +80-90 m (Fig. 2a). Marquardt et al. (2004), using the U/Th and ESR datings of Radtke (1987) and Leonard et al. (1994) in areas close to Bahía Inglesa and the correlation of warm water bivalves (Ortlieb et al., 1997) propose a chronological sequence of marine terraces. The +70-90 m terrace would belong to MIS 7 (around 210±10 ky), the +45-50 m terrace was fomed during the MIS5e (125±5 ky), the +30-40 m terrace during the MIS5c (105±5 ky), the +16-23 m terrace during the MIS5a (83±3 ky), and the lowest was formed during the Holocene (6±2 ky).
The surface of the marine terraces shows features characteristic from aeolian deflation, giving place to the development of a reg of rounded gravels due to the loss of the fine matrix. The pavement of gravels is widely extended; they have siliceous composition, well rounded and large flattening index. Some gravels have badly developed rock varnishes and aeolian polish, there are also many broken and superficially corroded specimens due to lichens and mosses biochemical action that use to locally form biological crusts (Castro Correa, 2014). In the slopes between the steps of the marine terraces, the Bahía Blanca Fm appears, although there are also coverings composed by eroded gravels coming from the immediately upper terrace. In the case of the +110 and +70 m terrace, giving its position next to the Morro, there are alluvial cones (Fig. 2a).
Boulder field, morphometry and distribution of the boulders
Over the marine terrace located +70-75 m a.s.l., the satellite images (Fig. 1d,e) as well as the photographs taken from El Morro (Fig. 3a,b) show large boulder field at the top of the cliff accompanied by three large lobes of sandy sediments. These lobes show elongated shape NE-SW oriented over the terrace and the alluvial fans of the depression formed in the contact with the El Morro fault.
The morphometric values of the boulders are shown in Table S1 and Figure 1S. The average weight of the 54 boulders was 1.15 t; and their average distance from the coastline was 112.78 m, with an average orientation of 129º (Figure 1Sa). The shape of the boulders is mainly equant and elongate unfavorable to rolling transport (Figure 1Sb), in fact, the boulders did not show any signs of wave abrasion or friction marks. Conversely, they have been affected by post-depositional desquamation processes. Regarding shape, mean Corey Shape Factor (CSF) (Corey AT 1949) was found to be 0.57, with a value of 1 representing a perfect sphere. Mean altitude of the boulder field was 68.79 m a.s.l. on the cliff-top of the marine terrace at +70-75 m a.s.l., while at the base of the cliff there is a marine terrace t +45-50 m a.s.l. (Fig. 2). The orientation of the boulders shows a significant correlation with the distance to the sea (rs = -0.342; p-value < 0.05), the most distant boulders to sea (120 – 127.6 m) are clearly oriented to the NE (0 - 80º), but the weight and the shape of the blocks does not show significant correlation with the distance to the sea. Therefore, the tsunami that originated this deposit in Bahía Inglesa occurred in the interval that both terraces were formed between two interglacial periods MIS 7 (ca. 210±10 ky) and MIS5e (ca.125±5 ky) according to the chronological frame proposed by Marquart et al. (2004). The combination of the disposition of the deposit on the cliff-top in the hyper-arid environment of the Atacama coast, only affected by aeolian processes and the historically low human presence have preserved this paleotsunami deposit (Spiske et al., 2013; Ishimura and Miyauchi 2015).
The study of boulders transported either by tsunamis or by storms is of great value for reconstructing this type of events occurred in the past (Nott, 2003), as well as for estimating the hydraulic properties that have given rise to these accumulations (Imamura et al., 2008). The distinction between them is based on a set of sedimentological, morphological, chronological, stratigraphic, and organizational criteria (Roig-Munar, 2016). Goto et al. (2011) proposed that boulder measurements and their spatial distribution can be used to estimate the velocities of wave flows generated by storms or tsunamis. The interlocking boulder deposits aligned along the coasts, either forming a single strip or several overlapping strips, are clear indicators of tsunami-associated transport, according to Browne (2011), Weiss (2012), Scheffers and Kinis (2014), and Roig-Munar et al. (2017). In the last decade, the discussion on boulders transport with the aim of discerning between those originated by tsunamis and by storms has led to a more thorough consideration of the role of storms on rocky shores (Etienne and Paris, 2010; Switzer and Burston, 2010; Roig-Munar et al., 2017).
The formula applied by Scheffers and Kelletat (2003) simplifies the equations, thus allowing for an approximation to limits and conditions to obtain the storm column of a tsunami wave. According to Scheffers and Kelletat (2003) when applying the TF to their research, TF values up to 230 correspond to boulders transported by storm surges, while boulder with TF > 250 correspond to those transported by tsunami waves. In this case, the criteria of Roig-Munar et al. (2015) for tsunami boulders in the Balearic Islands have been used. These authors established a threshold of TF > 1000 to consider a tsunami-related origin, which offers a greater certainty by establishing a higher margin than that considered by Scheffers and Kelletat (2003).
Applying the Transport Figure equation, an average TF value 2607.53 was obtained for the 54 boulders analyzed. Forty-five blocks had TF values < 1000; these boulders showed average dimensions (L * I * S) of 112, 73, and 48 m and an average weight of 1.12 t (Fig. 3), and they were located at an average distance of 112.50 m from the coastline and at a mean altitude of 68.72 m above the current sea level, with an average orientation of 125.33°. The theoretical calculations using the Scheffers and Kelletat (2003) formulas determines that a tsunami wave can move boulder of 1 t at a height of between 20-30 m, so the boulder field is in this theorical range at an altitude of 23.79 m from the paleocoast line following the lower marine terrace at +45-50 m a.s.l. (Fig. 2). This theoretical possibility of transport of large boulders by tsunami events has been verified in the Paleotsunami deposit of Bahía Cisne in nearby coastal areas (Fig. 1), with a boulder field on the shore at +20 m from actual sea level (Abad et al., 2019). The rose diagram (Fig. 4) shows that the A-axis presents dominant SE-NW direction, parallel of the paleocoast, and NE-SW direction like the sand lobes orientated with the possible wave flood direction, with a predominant landward movement with a rapid loss of transport capacity of the wave flow inland from the cliff edge.
The boulder field at the top of the cliff in Bahía Inglesa is analogous to the boulder transport by tsunami events documented in other paleotsunami deposit on the near coast of Bahía Cisne (Abad et al., 2019), or on the shore in coastal areas of the Central Chile (Spiske and Bahlburg 2011; Bahlburg and Spiske 2012, 2015; Aedo et al., 2021) and the Peruvian coast (Spiske et al., 2013), but also the boulders field on the shore with a tsunamigenic origin are documented on other Pacific coastal areas, such as the Solomon Islands (McAdoo et al., 2009), Sumatra Island (Indonesia) (Paris et al., 2010), Sabusawa Island (Japan) (Goto et al., 2012) or Sanriku Coast (Japan) (Ishimura and Miyauchi 2015).
Composition and geochemical properties of the sand lobes sediment
The major mineralogical composition of sandy lobes is siliceous, with no major changes in depth. The mean mineralogical composition of the sand lobes sediment is as follows: SiO2 76.31 %, Al2O3 5.58 %, Fe 5.50 %, MgO 3.78, K 2.14, NaAlSi3O8 1.80% and Ca 1.57 %. The gravels have a percentage between 2.54 – 6.54 %, with an increasing proportion in depth with between 5.23 and 6.54 % in the samples of 10-15 and 15-20 cm respectively. The fraction minor than 2 mm sand was the dominant fraction (92.99-95.00 %). Clay and silt represented < 2% (Table 1).
The pH was alkaline, ranging between 9-9.5 and the electric conductivity was very low (< 100 mS cm-1) according with the low ionic concentration observed in the extract (Table 1). Sediment reaction (pH), electric conductivity (EC) and soluble ionic concentration of the sediments (Table 1) have relatively low values with respect to seawater except for pH, also the Na: Cl ratio of 1.37 - 2.87 is higher than seawater (0.86), however, the conductivity and the majority ions are not always good indicators due to its high solubility can wash out over time (Chagué-Goff 2010).
Table 1. Chemical parameters analyzed from the samples of the sandy lobes of the paleotsunami deposit of Bahía Inglesa.
Depth
|
Clay and slits
|
Fine sand
|
Medium sand
|
Coarse sand
|
Gravel
|
pH
|
EC
|
Cl-
|
SO4=
|
Na
|
Ca
|
Mg
|
K
|
TOC
|
TN
|
TP
|
(cm)
|
%
|
|
µS·cm-1
|
mg·l-1
|
%
|
0-5
|
1.63
|
74.08
|
16.1
|
5.65
|
2.54
|
9.57
|
77.7
|
6.01
|
1.90
|
11.2
|
< 1
|
0.68
|
2.20
|
0.20
|
<DL
|
939
|
5-10
|
1.32
|
47.61
|
28.43
|
18.89
|
3.75
|
9.46
|
34.3
|
3.98
|
< 1.0
|
6.40
|
< 1
|
0.64
|
1.59
|
0.11
|
<DL
|
680
|
10-15
|
0.67
|
43.64
|
24.09
|
25.76
|
5.84
|
9.12
|
34.3
|
5.60
|
< 1.0
|
6.39
|
< 1
|
0.39
|
1.79
|
0.16
|
0.07
|
821
|
15-20
|
0.47
|
42.78
|
13.43
|
36.78
|
6.54
|
9.07
|
56.2
|
10.7
|
< 1
.0
|
9.52
|
< 1
|
0.65
|
2.23
|
0.18
|
<DL
|
859
|
The organic C content was extremely low (< 0.20%) and, accordingly, so was the total N (TN) content, generally lower than the detection limit (DL) (DL <0.01%), however, the content in total P (TP) is relatively high (Table 1). The high percentage of sands and low Total Organic Carbon (TOC) supports the removal of organic matter during high energy event consistent with a tsunamigenic origin (Chagué-Goff 2010).
The isotopic signature of the organic matter of the sand lobes sediment deposit in Bahía Inglesa is different between the upper layer BI1 and the lower levels BI2-BI4. Isotopic signature of δ13C shows important differences between the upper layer BI1 with a value of 25.4 ‰ and the lower layers BI2-BI4 with a range of values of - 22.8 to - 21.4 ‰. The δ13C value of BI1 is close to the values of the coastal samples CO1-CO3 of - 26.1 to - 27.6 ‰, while the lowest values are soil samples AT1-AT2 of -30.3 to -31.6 ‰ (Fig. 6). Plot of the δ13C signature and C:N ratio (Fig. 6) shows the grouping of samples BI2-BI4 in the range of marine origin, BI1 and CO1-CO3 in the range of coastal origin, and AT1-AT2 in the range of terrestrial origin (Middelburg et al., 1997; Carreira et al., 2002; Barros et al., 2010; Mackensen et al., 2019), while the upper layer BI1 have a coastal origin possibly by subsequent eolian deposition in a hyper-arid environment. The geochemical signature of the tsunami deposits is poorly conserved due to post-depositional erosion processes or washing of more soluble chemical compounds (Chagué-Goff 2010), however, the disposition, genetic origin and the exceptional hyper-arid conditions of the Bahía Inglesa paleotsunami deposit on the Atacama coast have played a key role in the preservation of the original chemical signals of organic matter (Spiske et al., 2013).
Biological remains, diatoms and sponge spicules
Biological remains, diatom and sponge spicules, incorporated in sedimentary records of the sandy lobes (Fig. 7) constitute reliable stratigraphic evidence to determine the provenance of the deposit and estimate tsunami run-up beyond the landward limit of tsunami deposits (Dura et al., 2015; Pilarczyk et al., 2014; Hocking et al., 2017; Castillo-Aja et al., 2019). Also, the presence and abundance of diatoms and sponge spicules coincide with previous sedimentary and geochemical evidence in sand lobes (i.e., grain size, carbon content, trace elements, isotopes signature) (Fig. 5, 6) all of them indicative of a tsunamigenic origin. A total of 27 diatom taxa were observed in sedimentary deposits of Bahía Inglesa (Table S2). The upper levels BI1-BI2 (0 - 5 cm, 5 - 10 cm) showed no presence of diatoms, but the lower levels BI3-BI4 (10 - 15 cm, 15 - 20 cm) showed small diatom concentrations, estimated between 19.5 and 82.6 diatom valves per 30 g. Compositions of diatom communities were similar between both levels BI3-BI4: a mixture of freshwater, brackish, and marine taxa with a high percentage of brackish/marine diatoms (53.8 - 62.4%). Of particular importance were planktonic marine taxa such as Paralia sulcata, Aptinoptychus senarius, and Amphitetras antediluviana (Fig. 7). Alkaline conditions lead to a poor conservation status of diatom valves, and a large proportion of valves were fragmented (70.5 - 78.9%) (Fig. 7). Allochthonous diatom marine communities or a chaotic mixture of marine, brackish, and freshwater taxa in coastal or continental deposits may constitute evidence of marine tsunami events, as documented in other paleotsunami deposits in the Chilean coast and South America (e.g., Chagué-Goff et al., 2011; Goff et al., 2011; Chagué-Goff et al., 2015) or in other regions of the world (e.g., Hemphill-Haley, 1995; Sawai et al., 2002; Dura et al., 2015).
Mainly four morphotypes of marine sponge spicules were observed in sedimentary deposits of Bahía Inglesa (Fig. 7). The high fragmentation of spicules (90- 100 %) prevented their identification at the species level. Type-1 sponge spicules, the most abundant morphotype, were monactinal, monaxon, and slightly curved, showed a smooth surface, and they measured 715.6 ± 97.15 (592.7 – 810.4) µm long and 10.3 ± 2.61 (8.4 – 12.5) µm wide. Type-2 sponge spicules were long, acanthostyle, and slightly curved, with a microspine surface. Larger spicule fragments were 134.7 ± 48.35 (90.3 – 188.7) µm long and 5.2 ± 0.78 (4.1 – 6.0) µm wide. Type-3 sponge spicules were long, narrow, tylostyle, and slightly curved and showed a smooth surface. Larger fragments were 215.6 ± 37.15 (187.5 – 244.6) µm long and 4.5 ± 1.08 (3.2 – 5.8) µm wide. Type-4 sponge spicules were triradiate, and their average axis length was 18.77 ± 3.8 (15.2 – 26.1) µm. The upper level BI1 (0 - 5 cm) showed no presence of sponge spicules. The middle level BI2 (5 – 10 cm) showed a very low concentration (10.7 fragments of spicules per 30 g), while concentrations were slightly higher in the lower levels BI2-BI4 (10 - 15 cm, 15 - 20 cm), with 83.3 and 92.5 fragments of spicules per 30 g, respectively (Fig. 5). Also, the high fragmentation of diatom valves and sponge spicules (Fig. 5), could constitute evidence of a high-energy turbulent event, compatible with a tsunami (Dawson et al., 1996; Sawai et al., 2002; Smith et al., 2004; Witter et al., 2009).