Tsunami sediments and their grain size characteristics

Characteristics of tsunami deposits are very complex as the deposition by tsunami is very complex processes. The grain size characteristics of tsunami deposits are simply generalized no matter the local condition in which the deposition took place. The general characteristics are fining upward and landward, poor sorting, and the grain size distribution is not unimodal. Here I review the grain size characteristics of tsunami deposit in various environments: swale, coastal marsh and lagoon/lake. Review results show that although there are similar characters in some environments and cases, but in detail the characteristics in each environment can be distinguished; therefore, the tsunami deposit in each environment has its own characteristic. The local geological and geomorphological condition of the environment may greatly affect the grain size characteristics.


Introduction
Grain size analysis is one of the main parameters for tsunami deposits identification [1]. Most of paleotsunami studies use grain size in the analysis. Although grain size characteristics alone cannot be used to directly identify the paleotsunami, the general grain size characteristics of the studied tsunami deposits or paleotsunami is the important analogue for identification. Grain size characteristics of palaeotsunami deposits reflect both the origin of the displaced sediment and the hydrodynamic conditions of sedimentation; therefore, palaeotsunami (and tsunami) deposits usually display common characteristics with normally graded sand layers related to the decrease of the hydrodynamic energy during sedimentation [1]. Another common characteristic of paleotsunami are fining landward [2], poorly sorted [3], and the grain size distribution must not be unimodal [4]. As the tsunami deposits were the result of a complex transport and deposition processes, these common characteristics are overly simplifying the actual characteristics. Furthermore, the local condition, including the tsunami depositional environment, greatly affects the tsunami deposit characteristics. Here I review the sedimentological characteristics of tsunami deposit in various environments: swale, coastal marsh and lagoon/lake. In addition to summarizing the tsunami geology literature that describes the sedimentology analysis, the aim of this paper is to obtain a specific grain size characteristic in every environment. Another aim is to review the gaps in our understanding of grain size within paleotsunami studies.

Studies of tsunami deposits/paleotsunami that report grain size
Selected literatures are reviewed and summarized in the tables according to the environmental deposition from which the tsunami sediments deposited. Table 1 provides comprehensive selected literatures that describe sedimentology and grain size of tsunami deposits/paleotsunami in the coastal marsh environment. Literatures of the coastal lagoon/lake and in the ridge and swale environments are summarized in the Table 2  place, the study of grain size may be grouped into two. The first group is the grain size characteristics which are only visually observed without any laboratory analysis. The second group is the studies which performed grain size analysis.  [12] 4 tsunamis up to All four layers display internal stratification in the form of multiple sand-~2100 BP mud couplets. Progressively finer landward, fining upward. 12 [13] 1700, 1.2, 1.7 Ka Deposits fine upward and landward. No rip-up clast.
13 [14] 1960, 1575 The tsunami deposits seem thinning and fining landward, no analysis. tsunami also graded. Moderately to poorly sorted sand with erosional base. The deposit is also in the form of coarse organic detritus with rip-up clasts, termed 'organic conglomerate'. The clasts of the organic conglomerate normally have an irregular form and are commonly from 0.5-6 cm across, but may also be larger (at least 20 cm). The tsunami deposits show a generally fining-inland trend along the 3.4 km long transect. Normal grading is common, with pebble-sized clasts and mud cap. In some locations, parallel and cross-laminations occur in the upper part, with a massive in the lower part. Unimodal and bimodal grain size distributions. The thick tsunami deposits were observed at sites immediately landward of beach ridges. The deposits mostly poorly sorted.

[27] 2004 tsunami
The tsunami deposited a sand sheet, ranged from 0-30 cm in thickness, with an average thickness of approximately 10 cm. Sedimentary structures include ripples, graded bedding, parallel lamination, and double-layered deposits.

[28] 200tsunami
A discontinuous sand-dominated sheet was prevalent to about 2800 m from the shoreline where mud content then gradually increased further landward resulting in a mud-dominated deposit ranging from 3.5 cm to a few mm thickness. The overall thinning and fining of the deposit was often interrupted by localized features that led to complex sedimentological relationships over short distances. The deposits are poorly sorted inland.

[29] 2004 tsunami
The 2004 tsunami deposits thickness ranged from 8 to 12 cm, composed mostly of medium to and older very coarse sand, moderately to poorly sorted, fining upward. Sharp lower contact with the (late Holocene) soil. The older deposits were generally composed from medium to very coarse sand, in some parts the most of the sediments consisted of laminated. No sorting information.
6 [30] 1000 -1200 BP The sediment size ranges from fine-to-medium, moderately well-sorted-towell-sorted, and tsunami exhibit positive skewness with platykurtic-to-leptokurtic nature. Abrupt winnowing or back and forth motion including unidirectional transport of these sediments with positive skewness. Fining upward and landward. The deposits were also thinning landward with bimodal distribution. 7 [31] 2011 tsunami Immediately landward of the coastal dunes the tsunami deposit was more than 20 cm thick, but thinned markedly inland from this point. At sites more than half the inundation distance inland, the thinner tsunami deposit consisted mainly of fine sand with some upward fining. The deposit massive at most sites, and showed lamination and bedding only at those near the sea. The deposit thinned inland. No apparent laminae or grading were observed at the sites more than half the inundation distance. Tsunami sediments are generally fine inland, well to moderately sorted.
8 [32] 700-500 BP, The sediments are thick in the swale and thinned across the ridges. The tsunami deposit is coarser 1350-1180 BP better sorted sand compared with the soil. Two normally graded sublayers dominated by rather homogeneous sand. Fining upward, rip-up clasts no fining inland. Moderately to poorly sorted. 9 [33] 2004 and older The 2004 tsunami deposit varies in thickness, structure and preservation potential. The deposit tsunamis shows no internal structure and contains one fining upward sequence. More fining upward in the swale. The 2004 tsunami sediments show prominent bimodal distribution.

[34] 2011 tsunami
The fining-inland tsunami deposits consisted of poorly to moderately sorted medium to coarse sand within 2 km of the coastline and very poorly to poorly sorted mud farther inland. There was a slight fining upward and coupled coarsening-fining upward trends.

General characteristics of the sedimentology
There are similar grain size characters in some environments and cases, but in detail the grain size characteristics in each environment can be distinguished. Therefore, the tsunami deposit in each environment has its own characteristic. As described above, the common characteristics are that the deposits tend to be fining upward with single or multiple bedding, thinning and fining landward, see e.g. [2,4,5,8,18,21,26,28] and relatively poorly sorted [3,18,20,36]. From the literature reviewed here (Table 1 to 3) it is undeniable that those characteristics are observed in each environment. The occurrence of rip-up clasts and mud cap may also be considered to be the common characteristics, e.g. [7,9,11,20,21,24]. Specifically, the most visible characteristic that characterizes the tsunami deposits in the ridge/swale environment is the occurrence of parallel lamination that cannot be observed in another environment (e.g. [27,31], Table 3). It is also observed that the sedimentary structures in the ridge/swale environment are the most complex as many sedimentary structures can be observed, such as current ripple and cross bedding [26,27]. Parallel lamination is also observed in the lagoon [22], but it is only presence as a minor structure and only in one location. Similar to parallel lamination, coarsening upward or inverse grading is intensively observed in the tsunami deposits in ridge/swale environment (Table 3). In this environment, the thickest tsunami deposits are observed and their occurrence is in relation to the parallel lamination that can be observed in immediate landward of the coastal dune. This condition is probably as a result of the bedload transport of tsunami [27]. The sediment type (clay, silt, sand or coarser materials) highly depends on the available source materials [1]. Thus, it is not applicable to distinguish the type of sediment in every environment. However, the occurrence of mud deposits (as rip-up mud and/or mud cap) is mostly visible when it is observed in the lagoon or lake environment. The tsunami deposits in the swale also commonly show this rip-up and mud cap, although the bigger rip-up is found in the tsunami deposits in the lagoon [24]. This characteristic is probably due to the soft and very fine lagoon sediments that could be easily lifted up and eroded by the tsunami waves. The sorting of the deposits is also connected to the available sediments source. If the available sources are in highly grain size variations, the resulting grain size might be relatively poorly sorted. In such case, we cannot differentiate the sorting of the deposits from each different depositional environment. From the reviewed literatures, however, the sorting of the deposits ranges from relatively well sorted to poorly sorted. It means that not all tsunami deposits are poorly sorted. For the grain size distribution, the controlling factors, whether the deposits distributed unimodal or not, is related to the types of source sediments and the tsunami transport and deposition processes. In general, the thickness of the deposits is thinning inland with some local variations, due to the local micro scale topography as an example. This characteristic is observed in every depositional environment.

Summary
In detail, the characteristics of tsunami deposits/paleotsunami in each environment can be distinguished, even though only based on the minor parameter, such as the occurrence of rip-up clasts that is the most common in the lagoon/lake environment, and based on the occurrence of a specific sedimentary structure. Although the sedimentological proxy is the main method for paleotsunami identification, there is no consistency in the sampling and methodology used. As described above (refer to Section 2), many literatures only visually describe the grain size characteristics. This approach is not recommended, because if it is from the visual observation only, the obtained information is very limited. On the other hand, most of the literatures that used grain size analysis only stated and described the mean grain size of the deposits. Some of them calculated the sorting of the deposit; however, there are many literatures that only visually observed the sorting. Then, the resulting sorting condition is not really valid. The other grain size parameters: skewness and kurtosis, are very rarely calculated. The complete calculation of grain size parameters, on the other hand, would give detail and complete grain size characteristics that not only will give more specific sediment characteristics but will also make it easier to identify paleotsunami. In addition, there is no consistency on the sampling resolution both horizontally along transect or vertically, that will be used for laboratory analysis. It is highly recommended that tsunami geologists make a scientific agreement on the deposits sampling and analysis methods. This scientific agreement will lead us to characterize the deposits more accurately and be able to compare the deposits from site to site and or event to event.