Aeolian Abrasion of the Coastal Deposits on the Western Crete

Crete is located in one of the Mediterranean Sea’s most seismically active regions – in the African plate’s subduction zone under the Aegean microplate. The island coast was, therefore, often shaped as a result of violent tectonic phenomena, mainly earthquakes. These changes included uplift/subsidence and the tsunami’s rapid displacement of coastal deposits.

In historical times, namely between the 4th and 6th centuries AD, there occurred a period of unusually high seismic activity, referred to as the early Byzantine tectonic paroxysm (Pirazzoli 1986). In 365 AD, near the south-western coast of Crete, the epicentre of a major earthquake measuring 8.3 on the Richter scale (Pararas-Carayannis, Mader 2010) caused a significant upward displacement of the earth’s crust (Thommeret et al. 1981, Pirazzoli et al. 1996a, b, Price et al. 2002). The coast of western Crete was elevated by an average of 6.66 m above sea level, with a maximum of 9.15 ± 0.20 m, according to the results obtained in the study of Mourtzas et al. (2015). Stiros (2010) states that all subsequent earthquakes were events of much smaller scale and modified the coastal landscape formed by the 365 AD earthquake relatively little. The sea level then was –1.25 ± 0.05 m, and the current level was stabilised during two episodes of subsidence: the first by 0.70 m due to the 1604 earthquake; and the second by 0.55 m during the last 400 years (Mourtzas et al. 2015).

The coast of Crete was also within reach of the tsunami. Papadopoulos (2011) documented 19 extreme tsunamis, caused mainly by earthquakes but also by the eruption of the Thera volcano (Santorini). The coastal geomorphology was strongly influenced by five tsunamis that occurred in the following years, listed here in the order of youngest to oldest: 1965 AD, 1650 AD, 1303 AD, 365 AD and 1628 BC (Thommeret et al. 1981, Pirazzoli 1986, Stiros 2001, Scheffers, Scheffers 2007, Werner et al. 2018). According to Pearson et al. (2018), the last of the mentioned dates (1628 BC), referring to the tsunami caused by the eruption of the Thera volcano, should be moved from the 17th to the 16th century BC.

One of the geomorphological consequences of these geological phenomena is that the modern beaches of western Crete are fragments of the former seabed with its littoral deposits. Some of these deposits have been subjected to wind activity for several centuries. Morphological manifestations of aeolian processes, from fine ripplemarks to dunes, are observed in various parts of the west coast of Crete. One may ask whether these deposits already have the characteristics of aeolian deposits. Did wind activity lasting more than 1500 years give the marine deposits the features of aeolian deposits? An attempt to answer this question is the purpose of this article.

Coastal processes and deposits are widely described in the world literature, for example Carter (1988), Bird (2000), Nordstrom (2000), Carvalhido et al. (2014), Łabuz et al. (2018), and Bertran and Fouéré (2020). Studies referring directly or indirectly to aeolian abrasion of coastal deposits are conducted in various parts of the world, and in different climatic, geological and geomorphological conditions (Setlow 1978, Mycielska-Dowgiałło 1988, Araújo 1994, Pyökäri 1999, Kurowski 2002, Cherian et al. 2004, Kasper-Zubillaga et al. 2005, Kasper-Zubillaga 2009, Costa et al. 2012, Dullek, Olszak 2013, Garzanti et al. 2015, Bellanova et al. 2016, Whitmore, Strom 2017, Kalińska-Nartiša et al. 2018). The degree of quartz abrasion is determined by different methods and for different fractions, which makes direct comparison of results difficult. Still, the quartz grains from the aeolian environment are generally more rounded and matted than source deposits from the high-energy beach environment.

The beach deposits of Crete have been the subject of a detailed study by Pyökäri (1999). The author presented a broad set of data on coastal deposits’ texture, composition and roundness for 22 beaches (with and without dunes) located in the western and eastern parts of the island. In the context of the question posed above, three statements by Pyökäri (1999) are relevant: (1) less resistant carbonate deposits predominate over more resistant non-carbonate deposits on many beaches; (2) less resistant carbonate grains have better roundness than more resistant quartz grains; and (3) textural features of beach deposits are the result of local environmental conditions and the influence of coastal drift determined by the prevailing winds and waves.

Despite a comprehensive presentation of the characteristics of beach deposits in the cited work, there are no references to the degree of their roundness due to the aeolian process, and even more so as a function of the duration of this process. Roundness is presented as one of the characteristics of deposits without specifying their origin. In addition, the Powers method (1953) used by the author is based on an analysis of only the grain shape without providing information about the nature of its surface, although the latter of these is a feature used in several studies of the aeolian environment (e.g. Goździk 1980, Mycielska-Dowgiałło, Woronko 1998, Woronko 2012, Zieliński 2016, Dulias 2023). For the above reasons, in this study, the degree of quartz grain rounding and matting was determined using the Cailleux (1942) method, as modified by Mycielska-Dowgiałło and Woronko (1998). The aim was to examine deposits representing three sedimentary environments – high-energy beach, aeolian, and beach with permanent or periodic fluvial supply, and to determine the differences between them regarding aeolian abrasion of quartz grains. To our knowledge, this is the first analysis of this type for coastal deposits in Crete.

Crete is the largest island of Greece, located in the Eastern Mediterranean. The island was formed from rising underwater mountain ranges. It is located in the collision zone of tectonic plates; therefore, numerous earthquakes occur here.

Crete is built of various rocks – limestones, dolomites, phyllites, quartzites, sandstones and volcanic rocks. Crete is predominantly mountainous (Timios Stavros – 2456 m above sea level) and is fragmented by numerous deep, rocky gorges. The island’s shores, over a 1000 km long, are rocky and interspersed with quite wide beaches. Due to its latitudinally elongated shape, the island is divided into a western and an eastern part. The research was carried out in the western region (Fig. 1).

Fig. 1.

Most of Crete has a Mediterranean climate – summers are hot and dry, and winters are cool. Some mountain peaks are covered with snow for most of the year. The average annual rainfall is about 640 mm. Generally, north-westerly and northerly winds predominate. The highest tides are around 0.3 m, and during storms, the sea level rises more than 1 m (Pyökäri 1999). The southern part of Crete belongs to the North African climate zone.

In the present study, the term coastal deposits has been treated as a reference to deposits occurring on the coast. Attention was focussed on deposits that are at least periodically exposed to wind transport, that is to say from the backshore and coastal dunes. Site selection criteria also varied depending upon the parameters of uplift scale, lithology and coastal morphology, including the presence or absence of a river mouth (permanent or periodic) (Table 1).

According to the above criteria, seven research sites located in the western part of Crete were established. They are: Seitan Limania, Stavros, Balos, Elafonisi, Orthi Ammos, Preveli and Matala (Fig. 1). One of the conditions of aeolian transport is the drying of deposits, which is why, on the coasts, this process can start in the backshore, which is submerged only during the highest tides and severest storms, and then it is also supplied with fresh marine material. In the study area, the sites located in this zone are Balos 1, Elafonisi 1, Orthi Ammos 1 and Stavros. The last of the mentioned sites is located on the coast with the mouth of a periodic watercourse, but it was established at a distance excluding the supply of fluvial material. Thus, the characteristics of the deposits from the mentioned sites represent the local high-energy beach environment. Three more sites – Seitan Limania, Preveli and Matala – are also located on the backshore, but the first is at the mouth of a gorge with periodic water flow, the second at the mouth of a permanent river from a large and deep rocky gorge, and the third close to the mouth of a periodic river. At these sites, beach deposits are constantly or periodically fed with fluvial deposits. Six sites were established within the deposits of aeolian origin – Elafonisi 2 in a dune on a small offshore island, Orthi Ammos 2 in a dune encroaching on a cliff and Balos 2–5 in an aeolian sand field on a cliff.

Samples weighing 200–400 g were collected from the surface, usually 2–3 at individual sites at a distance of 10–80 m from each other, and the obtained results were averaged. After drying, the samples were sieved through a set of sieves with the following mesh diameters: 0.1 mm, 0.25 mm, 0.5 mm, 0.8 mm and 1.0 mm. For all samples, the primary grain size indices were determined according to the formulas of Folk and Ward (1957), using in this analysis mainly the average grain diameter Mz and sorting δ.

From the 0.8–1.0 mm fraction, 1–2 teaspoons of sand were taken for mineral composition analysis and processing. The mineral composition was determined in a simplified way, but this was sufficient to assess the impact of the aeolian process on the deposit. The following were separated: less resistant carbonate minerals, more resistant quartz, and other minerals (without marking, with different resistance). It was assumed that the greater the share of quartz grains, the longer the aeolian process lasted (Mycielska-Dowgiałło, Woronko 1998, Woronko 2012). The number of quartz grains counted for testing ranged from 200 to 1000.

The abrasion of quartz grains was examined using the Cailleux (1942) morphoscopic method, as modified by Mycielska-Dowgiałło and Woronko (1998) with the use of an optical microscope. In this method, the following grains are separated: broken C, fresh and angular NU with a degree of rounding of 0.1–0.2 on the Krumbein scale (1941), very well-rounded and shiny EL with a degree of rounding of 0.7–0.9, very well-rounded and mat RM with a degree of rounding of 0.7–0.9, moderately rounded and shiny EM/EL with a degree of rounding of 0.3–0.6 and moderately rounded and mat EM/RM with a degree of rounding of 0.3–0.6.

It was assumed that the measure of the duration of the aeolian process is the ratio of RM and EM/RM grains – if RM grains are more than EM/ RM, then the aeolian processes operated for a long time, and vice versa (Mycielska-Dowgiałło, Woronko 1998, Woronko 2012).

The investigated deposits are mostly medium-grained, with average Mz values of 0.277– 0.496 mm. In terms of this feature, they do not differ from deposits on other coasts, as mentioned in results of research into modern beach sands in various regions, for example the Pacific, the Gulf of California, the Caribbean and the Gulf of Mexico (Edwards 2001), as well as the Baltic Sea (Tylkowski, Samołyk 2011, Žilinskas et al. 2018). Only dune deposits in Orhi Ammos 2 south of Crete are fine-grained (Mz, 0.227 mm). The most coarse-grained deposits are at the Seitan Limania site on the Akrotiri Peninsula and at the Balos 2 site in the north-west of Crete – in both places, the average value of Mz is 0.907 mm. In the first case, it is associated with the beach’s location in a narrow rocky bay, into which coarse material is periodically supplied from a steep gorge. At the Balos 2 site, the coarseness of the deposits is probably due to the proximity of multi-ton boulders, which, according to Scheffers and Scheffers (2007), were brought to the beach by the tsunami after the 365 AD (or later) earthquake. In addition, the site is located near the former shoreline. There are also coarse-grained beach deposits at the mouth of the Preveli Gorge (Mz, 0.722 mm) and on the beaches of Matala (Mz, 0.599 mm) and Stavros (Mz, 0.536 mm).

Most investigated deposits are characterised by moderate sorting with average δ values in the range of 0.53–0.80. The beach deposits at the Elafonisi 1 and Orthi Ammos 1 sites are the least sorted, for which the δ values are 1.24 and 1.42, respectively. In the case of Elafonisi beach, this can be explained by its protrusion into the sea, exposure to stronger winds, and the supply of fresh material. Orthi Ammos beach is very narrow, stretches along the cliff and is also within the range of significant waves. Only beach deposits in the Preveli site (δ, 0.40) and aeolian deposits in the Balos 5 site at an altitude of about 45 m above sea level are very well-sorted.

In terms of mineral composition, the analysed deposits are significantly diversified. In the sites located on the northern and western coasts, the 0.8–1.0 mm fraction is dominated by carbonate minerals, which constitute an average of 77% (range, 46–100%), while on the southern coast, their share is many times lower and ranges from 5% to 16% (average, 9%). However, it does not seem to be a regional, but rather a local, feature. The Pyökäri (1999) study confirms a very high carbonate mineral content at the Stavros and Elafonisi sites. Still, their share is more diverse in other locations on the northern and western coasts, even decreasing to about 10%.

The content of quartz in the investigated deposits is generally low, on average 18%, and in two sites, on the beaches of Stavros and Balos 1, it was not found at all. The most quartz occurs in the sites located on the southern coast of Crete, with the maximum occurring in Matala (44%). Among the sites on the northern and western coasts, where quartz is present, the Elafonisi 2(dune) stands out with its 34% content. The share of minerals other than carbonates and quartz is the highest in sites on the southern coast, reaching a maximum of 77% (Preveli).

The high share of less resistant carbonate minerals, with a small amount of more resistant quartz grains, means that most investigated deposits are relatively young and were briefly in the range of aeolian processes.

Both beach and dune deposits are dominated by moderately rounded and mat grains EM/ RM with a share of 47–100%, on average 79%. The content of very well-rounded and mat grains RM is the highest in the Matala site (on average 28%); in four places, these grains do not occur at all, while in the remaining sites, their share is, on average, 8%. The higher content of EM/ RM grains in relation to RM grains is considered an indicator of short-term aeolian transport (Mycielska-Dowgiałlo, Woronko 1998, Woronko 2012). In the studied sites, this predominance is multiple, and thus it can be concluded that the deposits representing them have been in the aeolian environment for a very short time. This is indirectly confirmed by their young age, as most of the studied deposits emerged from the sea due to a several-metre uplift of western Crete resulting from an earthquake less than 1660 years ago (Thommeret et al. 1981, Pirazzoli et al. 1996a, b, Stiros 2001). The research conducted by Mourtzas and Kolaiti (2020) shows that in Matala Bay, sea level changes did not cover the zone where the Matala site was established. It means that beach deposits found here have been in the terrestrial environment longer than on other beaches studied. This probably explains the better abrasion of quartz grains, expressed by the highest share of RM grains among all sites. The research results of Pyökäri (1999) indicate that in western Crete, very well-rounded and rounded grains are relatively few – in the group of non-carbonate grains, they constitute an average of 4%.

A feature of the studied deposits is the negligible content of EL and EM/EL grains, characteristic of, among others, the high-energy beach environment (Krinsley, Doornkamp 1973, Lindé, Mycielska-Dowgiałło 1980, Mycielska-Dowgiałlo, Woronko 1998). EL grains were found only in the Seitan Limania site (average, 12%), while a few percent shares of EM/EL grains were found in the Preveli and Balos 3 and 4 sites (Table 3). In the beach environment, grains are constantly in motion, and thus they travel a long way, often in high-energy conditions (Krinsley, Doornkamp 1973, Mycielska-Dowgiałło 1988). The collision between the grains during waves and tides results in mechanical abrasion, but in warm climate zones, the grains are also subjected to chemical etching (Setlow 1978). As a result of these processes, the effect of matting of the whole grains can be created, which is already visible under the optical microscope (Goździk, Mycielska-Dowgiałlo 1982, Mycielska-Dowgiałło 1988). It cannot be ruled out that a certain part of the grains considered under the optical microscope as EM/RM grains, would, in an examination under the scanning electron microscope (SEM), be regarded as grains that are dull due to chemical etching. Nevertheless, the study of the surface microtexture of quartz grains from Japanese beach deposits using SEM conducted by Itamiya et al. (2019) showed that in high-energy beach conditions, mechanical features are more often recorded on the quartz surface than chemical features.

The investigated deposits are diversified in terms of the content of fresh and angular grains NU. They were not found in any of the five sites on Balos, and in the remaining sites, their share in the 0.8–1.0 mm fraction is, on average, 17%. The largest share of NU grains is found in beach deposits at the mouth of the Preveli Gorge (34%) and in the rocky bay of Seitan Limania (26%). The analysis of diagrams from the work of Pyökäri (1999) shows that in western Crete, among non-carbonate grains (quartz and others), very angular and angular grains account for about 50% on average. Individual sites, however, are very diverse in this respect. The extreme values refer to two areas analysed in this study: for Stavros, they amount to several percent (in this study, no quartz grains were found in this site), and for Elafonisi, they oscillate around 80% (in this study, several times less, 17%). The above discrepancies can be partly explained by the high dynamics of processes in the coastal zone, multiple redeposition of deposits, and the variability of their textural features. It is worth quoting the results of research by Garzanti et al. (2015), who found that sand transported by coastal currents along the south-west coast of Africa remains angular for hundreds of kilometres and becomes rounded relatively quickly when it reaches the Namib Desert.

Scheffers and Scheffers (2007) found compelling evidence of a tsunami covering the Balos area, which rapidly remodelled the geomorphology and lithology of the coastal zone. However, without performing a study under a scanning microscope, it is difficult to determine whether this has been recorded in the texture of the quartz grains. The results of the research conducted by Costa et al. (2012) on the microtexture of quartz grains transported by the tsunami indicate the lack of any specific features in grain processing, but the share of fresh surfaces and traces of impact increases. According to Bellanova et al. (2016), who studied two tsunami deposits in Tirúa, Chile, there are no statistically significant differences between tsunami, beach, dune and river deposits, and microtextural analysis of quartz grains may not be a suitable method for identifying tsunami deposits.

When generalising the results, more or less clear differences can be seen between the deposits of the studied sedimentary environments. Deposits from beaches fed by alluvial rivers (permanent or periodic) stand out. Compared to aeolian deposits and beach deposits supplied only from the sea, they contain, on average, twice as much quartz (25%) and twice as much carbonate minerals (32%). In addition, they are characterised by the absence or a small share of shells, coarser grain (Mz is 0.743 mm on average) and a large share of fresh and angular NU grains (on average 21%). In turn, deposits from beaches without tributaries are characterised by a high content of carbonate minerals and well-rounded and polished shells. Most quartz grains are moderately rounded and mat EM/RM (average, 79%). Deposits from these beaches were the source of material for dunes and aeolian cover sands. This is evidenced not only by the mutual geomorphological location but also by the similar mineral composition and the presence of shells. On the other hand, the record of aeolian abrasion is a higher share of EM/RM grains (on average 89%).

The investigated deposits are young and generally characterised by poor and moderate quartz grain abrasion. It is not an isolated situation that the exposure of the deposits to relatively short wind activity (in this case for about 1500 years) only results in some aeolian retouching of the grains. One can cite the example of much older aeolian deposits in the Barguzin Basin in Transbaikal, 3000–4000 years old (Wyrkin 1998), with very poor abrasion. As research by Ovchinnikov et al. (2002) indicates, the share of moderately rounded and mat grains is, on average, 91.5%, and similar to their share in the bedrock deposits (90.9%).

Coastal deposits in western Crete occur in areas with a varied scale of uplift, lithology and coastal morphology. To determine the degree of aeolian abrasion of deposits that emerged from the sea after the 365 AD earthquake, attention was focussed on deposits from the backshore and from coastal dunes (deposits that were at least periodically exposed to wind transport). Deposits representing three sedimentary environments were examined – high-energy beach, aeolian, and beach with permanent or periodic fluvial supply. As a result, the following have been established:

Beach deposits are characterised by a high content of carbonate minerals (average, 64%; maximum, 100%) and well-rounded and polished shells. These are medium-grained sands (Mz is 0.437 mm on average) with moderate or poor sorting (δ ranges from 0.59 to 1.42). The share of quartz is small, ranging from 0% to 34%, with an average of 13%. No EL and EM/ EL grains were found, which are characteristic, among others, of the high-energy beach environment. Moderately rounded and mat EM/RM grains predominate (average, 79%). It means the beach deposits (from the backshore) have been subjected to aeolian abrasion.