Marine Geological Research on Santorini: Preliminary Results

Some detailed work has also been carried out on the sea-floor sediments (e.g. Petersen and Müller 1974, 1978; Butuzova 1965; Smith and Cronan 1983). Very little effort, however, has been devoted to the study of the stratigraphy and structure of the sub-bottom layers (Hoskins and Edgerton 1971; and a description of the profiles by Heiken and McCoy 1984). Later research was conducted by the French research vessel Noroit in 1980, but only a short report was published (Got et al. 1981).

Recently (1987) the Marine Geology Department of IGME has carried out a marine geological research project on Santorini, in the framework of a general geological study in the broader area. The project resulted in the collection of 46 surface sediment samples and 150 km of sparker and 3.5 kHz seismic profiling, carried out simultaneously (Fig. 1). The geochemical and sedimentological analyses of the samples are presented in other papers in this volume (Boström et al. 1990; Perissoratis et al. 1990). A preliminary evaluation of the seismic data is presented here.

It is of course difficult fully to evaluate the seismic data collected, because of the successive geological events, which have been occurring in the Santorini area up to a few tens of years ago. It is believed, however, that the results of this research, even in a preliminary form, will contribute to the better understanding of the relationship between the geology of the land and the adjacent sea floor, especially in the caldera area. In this effort, some of the results of previous research are also used, related mainly to the bathymetry of the caldera basin.

MORPHOLOGY - BATHYMETRY

On the sea floor around Santorini there are two sectors where there is a change in the general morphological-bathymetric picture (Fig. 2). One is to the north of the Thera-Therasia strait, where a steep-sided elongated depression is formed with a maximum depth over 300 m, which is an apparent extension of the caldera basin. The other is south of Cape Exomitis on Thera, where a ridge and an adjacent valley occur, having an approximately north-south orientation and correlated with similar features on land.

Within the Santorini caldera, the Kameni islands separate the area into two deep sectors, one to the north and another to the south. To the north, one deep basin is formed, with an oval shape and a depth slightly exceeding 390 m. The sides are very steep, but after the 350 m isobath the slope diminishes and at about 380 m the floor becomes almost flat, exhibiting a very slight gradient to the South-east. South of the Kameni islands there are three minor basins, the eastern, southern and western basins (Fig. 2). The eastern basin has an elongated shape orientated in a NE-SW direction, and a maximum depth of about 290 m; its floor grades slightly to the east-north-east. It is separated from the southern basin by a 230 m deep north-south trending ridge. The latter has the same greatest depth (290 m) and a slight slope to the east. Finally, the published information (Hoskins and Edgenon 1971; Heiken and McCoy 1984) indicates that the western basin, for which we have no seismic or bathymetric data, has a maximum depth of over 320 m, and its bottom slopes slightly tothe north-east.

EXAMINATION OF SEISMIC DATA

There follows an examination of the seismic profiles around Santorini and in the caldera area. In the calculation of the various depths of the reflectors and other features a sound velocity of 1500 m/sec was assumed.

To the west of Therasia (Fig. 3a), a series of descending hills is observed, having heights up to 20 m and being only locally covered by a thin layer of loose sediments (2-3 m thick; Fig. 3a, 1), mainly on the flat or nearly flat surfaces. This layer pinches off at about 95 m of water depth. The rest of the sea floor gives a strong reflection indicating a hard, sandy bottom. In the sub-bottom layers, a series of sub-horizontal reflectors is observed, pinching off at about 110 m (Fig. 3b, r). The rocks above the reflectors show an irregular structure, with a slightly wavy character, while below, the layering is more continuous. To the north-west and south of Therasia the same picture is obtained, with the sub-horizontal continuous reflectors cropping out at 120 and 170 m respectively.

To the north of the Thera-Therasia strait, the elongated basin has fault-generated eastern and western sides (Fig. 4). A westward thickening of loose sediments is observed at the basin bottom. Elsewhere the bottom consists of hard formations cut abruptly into the basin walls.

To the east of Thera, the sea floor is quite hummocky, with alternating ridges and valleys of up to 35 m of relief (Fig. 5a), usually barren of loose sediments. However, the sea floor becomes smooth further to the south, off Mesa Vouno, with slight undulations and a distinct 2-3 m thick transparent layer over a strongly reflective substratum (Fig. 5a, 1). In the lower horizons we also distinguish two different pictures, one at the northern and one at the southern part of the profile.

In the northern part, the sub-bottom layers lack consistent internal structure and layering, and only in the deeper sectors can semi-prolonged reflectors be traced.

In the southern part, however, the stratification is prominent and a series of prolonged reflectors can be observed. Some channel-like features are also noted, filled with sediments.

The same seismic picture as that observed in the southern part of the profile in Fig. 5 is present in the profile just south of Cape Exomitis (Fig. 6), while further to the south (Fig. 7) a series of prominent north-south trending faults was noted, which apparently formed the ridge and valley features described in the morphology. The stratification of the rocks is also prominent, with pinch-off of layers, fault offsets, etc.

Within the caldera the geological picture depicted in the seismic profiles is different. In the basin north of the Kameni islands (Fig. 2, NB, 8) the sides are very steep to the depth of 350 m. From 350 to 380 m the slope diminishes and an area of rough hummocky relief is formed, apparently due to the presence of blocky material deposited by slumping. After 380 m the sea floor becomes nearly flat, tilting slightly to the south. In the 3.5 kHz picture, a sub-bottom reflector with rather anomalous relief is distinguished (Fig. 8a), with thickness increasing to the south (up to 5 m). In the sparker profile (Fig. 8b) we can distinguish a pocket of reflectors filling the basin and having a maximum thickness of up to 90 m. In the lower part of this pocket, the stratification is not continuous; there are many broken reflectors present, filling minor depressions. In the middle part there is a more transparent section about 15 m thick, and above it a more opaque one about 10 m thick. The beds below the basin fill lack internal structure and are the same as the rocks that form the sides of the basin.

To the south of the Kameni islands (Fig. 9), in the eastern basin (EB), there are also steep slopes down to a depth of 285 m. The floor of this basin is irregular, with strong surface reflectors and no sub-bottom reflectors distinguished in the 3.5 kHz profile. Deeper in the sub-bottom structure, the loose sediments increase in thickness, up to 20 m, towards the east, with an upper opaque and lower transparent section. Below the transparent section the rocks exhibit a broken and disturbed character, with subtle stratification, probably as a result of intense folding and faulting. To a depth of about 60 m below the sedimentary basin, there is also a series of well-distinguished reflectors. The upper ones can be traced to the west (Fig. 9b), while the lower ones are offset by a fault.

This stratification can be followed in the ridge separating the eastern and southern basins, and also below the southern basin. In the latter, the sea floor is smooth, while in the 3.5 kHz picture a 2-3 m thick transparent layer is distinguished below (Fig. 9a). In the strata filling the basin, a lower, more transparent and an upper opaque set of beds are observed (Fig. 9b). The maximum thickness is about 30 m.

PRELIMINARY EVALUATION OF THE DATA

From the data presented above it seems that around the Santorini islands the thin (2-3 m) transparent layer which locally overlies the harder bottom structure corresponds to sediments deposited recently, most of them probably after the Minoan eruption. Their patchy and local character is due to the minor amount of sediments transported in the sea and redistributed by wave and current action.

Most of the sub-bottom structures outside the caldera consist of broken, discontinuous reflectors having a chaotic structure which can be tentatively correlated with the Thera Pyroclastic formation on land. The deeper, sub-horizontal stratified layers noted in the area west of Therasia could either correspond to Therasia volcanics or represent a different, more stratified phase within the Thera Pyroclastic Formation.

One area where the seismic picture is distinctly different is to the east and south of Mesa Vouno and south of Cape Exomitis. Here, the well-stratified section with the erosionally-filled channel features and the north-south trending faults are correlated with the Thera Basement Formation, which crops out in southern and south-eastern Thera (Fig. 1).

Within the caldera, the northern basin is filled by sediments deposited since its formation, which have a thickness of up to 90 m. The data show that the basin has undergone three phases of filling. During the initial phase, blocks were transported and laid on its floor by slumping and sliding (lower seismic section). In a second stage, the sediments filling the basin were finer (middle, more transparent seismic section), while in the third stage, the sediments became coarser (upper opaque seismic section). In both last stages the deposition was probably effected by tourbidites and settling. If, as is generally accepted, the northern basin is a recent phenomenon, formed during the Minoan eruption about 3500 years ago, then these seismic units can be tentatively correlated with the various Minoan formations deposited on the surrounding land, as described by Heiken and McCoy (1984) and by Druitt et al. (1989). Thus, the lower seismic unit might correspond to the massive Plinian Pumice Fall, or Minoan A unit, the middle seismic unit to the well-bedded Second (phreatomagmatic) Phase or Minoan B, and the upper seismic unit to the (also stratified) Third (phreatomagmatic) Phase or Minoan C. It is apparent that these formations were deposited in a short time span, as they were products of the eruption. As a result, the sediments deposited after the eruption and the settling of its products are represented by the thin transparent sub-bottom layer depicted in the 3.5 kHz profiles of the northern basin (Fig. 8a). This layer is from 3 m to 5 m thick, which gives a sedimentation rate between 8 cm and 14 cm per 100 year. A similar sedimentation rate was also given by Petersen and Müller (1974). On the other hand, the slight slope of the bottom surface to the south indicates a southward tilting of the basin, probably along a NE-SW trending fault (Fig. 2).

In the other two basins for which data exists, the eastern and southern basins, a different situation is noted. The basin-filling sediments lack the deeper chaotic structure. The sediments are well stratified and are finer in the deeper (more transparent) and coarser in the upper (more opaque) sections. Below the sediment fill a clear stratification is noted which can be followed in both basins as well as in the ridge separating them. The same stratification, noted in the seismic profiles east and south of Thera (Fig. 5, 6), is attributed to the presence of Thera Basement Formations (see below). This, in combination with the fact that basement rocks crop out at the eastern side of the eastern basin (Fig. 1), suggests that the floors of these basins consist of basement rocks. We must also note that the considerable amount of loose sediments deposited in the southern basin (up to 30 m) is not explained by the restricted source area present today around this basin. It might be that some of these sediments were deposited earlier, when the configuration of Santorini was different. Therefore, it seems that these basins may have been formed at an earlier time than the northern basin. Since we do not know the time of formation of these basins, it is not possible to estimate their sedimentation rate. The eastward slope of the basin bottom and the sub-bottom layers in both of them indicate a tilting of the basins, apparently along approximately north-south trending faults. This coincides with the fault direction affecting the Thera basement rocks (Fig. 2).

As stated in the morphology section, the only area where the deeper caldera is connected to the open sea is the Thera-Therasia strait. The elongated basin to the north is a fault bounded by sediments accumulating in the western part. These sediments are apparently not transported into the caldera basin because of the ridge separating the two.

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