Stratigraphy of the Maltese Islands

➢ Lower Coralline Limestone Formation

The Lower Coralline Limestone is the oldest visible unit of the rock formations on the Maltese Islands. It is a hard, pale grey limestone and contains beds with fossils such as corals and marine calcareous algae. Outcrops of this limestone are mainly restricted to coastal sections along the western coasts of Malta and Gozo (Fig. 2.5). It can be over 140 m. thick, forms sheer cliffs particularly on the southwest coasts of the islands and its base cannot be seen above the sea level. When found inland, this formation forms barren grey limestone-platform plateaux on which karst-land develops (Schembri, 1997). A characteristic geomorphological feature developed from this formation is the bare karstic plateau similar to these found in the west of Gozo (Fig. 2.6). The bedrocks comprising this formation are all indicative of sediments laid down in a shallow agitated sea and can be subdivided into five different facies¹ of limestones (Pedley, 2002). These facies are: a) the Reef Limestone, b) the fine–

grained Shallow Lime Muds, c) the Cross–bedded lime sands, d) the Foraminiferal Limestones and e) the “Scutella Bed”. Felix (1973) suggested that the deposition of the Lower Coralline Limestone had initially been in a shallow gulf-type environment. In addition, succeeding beds provided evidence of increasingly open marine conditions during which algal rhodolites developed. Finally, a shallow marine shoal environment followed and was the dominant environment in all areas except southeastern Malta. In this area, calmer conditions prevailed in a protected deeper water environment (Pedley et al. 1976).

1 “Facies” provides a specific characterisation of a group of rocks with distinct similar features. In sedimentary rocks, it embraces major features such as the main composition (e.g. quartz sand, clay or limestone facies), the sedimentary layering (e.g. cross bedded facies, etc.) or the main fossils. These are then related to an interpreted environment in which the sediments were deposited. Consequently, the rock can be referred to as beach-facies, lagoon facies, reef facies, and so on.

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Figure 2-5: Typical coastal outcrops of Lower Coralline Limestone, forming sheer cliffs at the southwest of Malta.

Figure 2-6: Characteristic geomorphological feature developed on Lower Coralline Limestone in western Gozo (Dwejra Point). The picture shows different sub-circular collapsed karstic features (a and b), while the green arrow points the

location of the chert outcrops (Map Coryright@2017 Google).

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➢ Globigerina Limestone Formation

The Globigerina Limestone is a softer, yellowish fine-grained limestone that forms irregular slopes and is the most extensively exposed formation on these islands. It is named after a type of microscopic, planktonic foraminifera, fossil shell (Globigerina) which is abundant in this limestone. The formation varies in thickness from some 20 to over 200 m., a characteristic which possibly signifies the onset of the slow warping of the sea bed and possibly the formation of depressions due to the collapse of the sea bed above underlying caverns (Pedley et al. 2002). Moreover, the size of the fine grains and the content of fossils show that this formation was originally deposited in a deeper water below the level of wave action. Felix (1973) thought most of the Globigerina Limestones were deposited in water depths between 40 and 150 m. The unexpected occurrence of the planktonic foraminifera, such as Globigerina, in this shallow-water depositional environment may be explained by a drift that brought these organisms into this shallower basin from the surrounding deeper water seas.

The Globigerina Limestone is divided into three units (upper, middle and lower) by two layers of conglomerates (also referred as C1 and C2), which do not exceed one meter in thickness. The upper and lower units have a pale yellow colour, while the middle one is pale grey (Fig. 2.7). The latter unit is considered to have been deposited during the time that the sea basin reached the deepest level.

This could also explain the presence of chert outcrops, which have been found intercalating with the middle Globigerina Limestone (see below). Although the sources providing the material are still not known, it is certain that only the middle Globigerina Limestone had the adequate conditions for these deposits to form.

The two conglomerate layers show evidence of erosion phases through the incorporation of many pebbles and cobbles of brown-colour limestones (Pedley et al. 2002). In addition, their presence indicates that the sea basin was influenced by water agitation and that the sea levels had probably fallen during the deposition of the formation. The colour of these layers is attributed to the concentration of the francolite (phosphatic mineral) in the cements. It was reported that francolite has replaced some fossils, the matrix of the pebbles and also the top surface of the limestone unit beneath the conglomerate layers. The presence of so much phosphate material in the cements suggests that the water streaming over this shallowed surface was rising from greater depths as an “upwelling”

current (Pedley et al. 2002). Furthermore, the examination of the size of the pebbles in the conglomerates indicates that they become smaller moving to the east, implying that the currents had a direction from west to east. This suggests that high nutrient water was rising from the depths of the western Mediterranean basin (Pedley et al. 2002). It is possible that these inputs were also supplying material during the deposition of the middle Globigerina Limestone and contributed to the formation of the chert outcrops.

13 The fine-grained particles comprising the Globigerina Limestone formation (upper and lower) are only lightly cemented and therefore are easily worked as building stone. Indeed, the lower Globigerina Limestone unit, called 'Franka' locally, has proven to be the most suitable building stone. This is related to its uniform texture and can explain why most of the buildings of the Maltese Islands were built from this unit. Its texture, in addition to its extensive exposure on Malta and Gozo, has contributed to the smoothing of the topography of the islands. The thin soils produced from this formation are intensively cultivated and terraced.

Figure 2-7: The middle Globigerina Limestone at the Xwejni coastline. It is one of the biggest outcrop of this unit and the orange lines highlight the two conglomerate layers, which are clearly presented in this location and signify the transition

to the upper and lower Globigerina unit.

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➢ Chert outcrops

The existence of the chert outcrops has been long reported (Cooke, 1893a), but little is known about their characteristics and the conditions under which they formed. Archaeological research has revealed that these chert rocks were used by the prehistoric inhabitants (Malone et al. 2009; Vella, 2009), and that a better understanding of these resources is necessary. The middle Globigerina has extensive exposures in both islands of Malta and Gozo, but not all of them present chert outcrops. The fieldwork surveys (conducted by Chatzimpaloglou in 2016 and 2017) on the islands revealed that chert outcrops were present only on the western parts of both islands. The exposures of middle Globigerina unit in these areas were in bedded form, which could possibly be attributed to the influence of tide cycles in a former shallow marine environment in this part of the Maltese basin.

The chert outcrops on Malta were located in the broader area of the Fomm-IR-RIĦ Bay area (Fig.

2.8) and are considered more extensive than on Gozo. It is probably not a coincidence that these exposures are located at the end of the Victoria Lines (Fig. 2.9), which is a major tectonic feature of the Maltese islands. The chert outcrops on Gozo were found at Dwejra Point, in an area close to Fungus Rock (Fig. 2.10). The area is characterised by massive karstic features (Fig. 2.6), which could have been enhanced by past tectonic activity. The investigation of both exposures showed that nodular chert was present at the top and bottom of the unit, while bedded chert and/or silicified limestone were found in the middle part of the unit (Fig. 2.11). Generally, the outcrops present similar macroscopic characteristics, but distinct outcrops have also been recorded. In addition, the bedded chert outcrops have a higher concentration of carbonate material than the nodules and are therefore softer.

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Figure 2-8: An overview of the area investigated in western Malta. It presents the locations with the chert outcrops (yellow lines) and the areas investigated during fieldwork (green lines). The upper right figure shows Malta Island and

the exact location of this area (Maps Copyright @2017 Google).

Figure 2-9: The end of the major fault system of Malta (Victorian Lines) at Fomm-IR-RIĦ, beside the location with the chert outcrops.

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Figure 2-10: An overview of the area in west of Gozo, where the chert outcrops were located. The yellow line orientates the internal valley, close to Fungus Rock. The location with the chert outcrops is highlighted with the blue line and the areas investigated during the 2017 fieldwork are marked with red lines. The upper right figure shows Gozo Island and the

exact location of this area (Maps Copyright @2017 Google).

Figure 2-11: Chert outcrops. a) Bedded and b) Nodular chert from Malta, c) Bedded chert and d) Nodular chert from Gozo.

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➢ Blue Clay Formation

The Blue Clay is a very soft formation and, when exposed on the surface, forms low or rounded slopes (Pedley et al. 2002). The thickness of the formation ranges from less than 20m to around 70m (at Fomm IR Bay), while the colour reported on the outcrops is bluish grey (Fig. 2.12). Although Blue Clay has macroscopic differences from the Globigerina Limestone, they have very similar characteristics. It is also composed of very fine-grained sediments, with a large proportion of them of carbonate origin. Hence, it can be suggested that this formation was originally deposited in a very similar deep-sea depositional setting (Pedley et al. 2002). It could be regarded as a continuation of the Globigerina limestone sedimentation in which clay material became progressively incorporated. This is also supported by the smooth and fast transition from the one formation to the other during sedimentation. There is only a small step in topography that highlights the change, and this transition is restricted to a layer just over one metre thick at the base of the Blue Clay Formation (Pedley et al.

1976).

Basically, the main factor that distinguishes the Blue Clay formation from the Globigerina limestone is the presence of clay minerals. This clay content can only have come from a land source, although the possibility that part of the clay fraction originates from volcanic ash of an at the time, active volcano should not be excluded (Pedley et al. 2002). The quality of clay material mixed with the planktonic calcium carbonate detritus prevented the formation from reaching the same level of hardness as the other limestones and this is the main reason that Blue Clay is considered the softest rock formation of the Maltese islands. Blue Clay formation is important for agriculture since it produces most of the important fertile and water retentive soils found across Gozo and Malta, provided there is the level of plough technology to work these heavier soils. This would have been more likely only in Roman and later historical times.

The upper parts of the formation show an increase in brown phosphatic sand grains and the green grains of the complex mineral glauconite. Finally, this passes up into sand made up almost entirely of these green grains together with lime-rich fossils fragments. This is known as the Greensand Formation. This change indicates that the sea was becoming shallower and was probably related to the uplift of the Maltese basin on the north flank of the Pantelleria Rift (Pedley et al. 2002).

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Figure 2-12: Characteristic exposures of the Blue Clay Formation at the Fomm-IR-RIĦ Bay .

➢ Greensand Formation

The Greensand is lying between the Upper Coralline and the Blue Clay formations and was deposited under shallow-water, marine conditions. Moreover, much of the sediment was transported into the Maltese basin from foreign sources, which must have been areas of erosion (Pedley et al.

1976). The outcrops of this formation, when reported, are very thin and only in Gozo do they exceed the 10m (11m at Il-Gelmus).

The freshly exposed outcrops, mainly in man-made cuts, have a very characteristic green colour influenced by the presence of glauconite (a complex, silicate-based mineral). In contrast, the natural exposures have a chocolate brown colour, or the same mineral just altered by weathering (Pedley et al. 2002). These greensand outcrops have been upgraded to a Formation as they represent the residue of a long period of submarine erosion and winnowing of sediments (Pedley et al. 2002). The top part of the formation passes transitionally into the overlying Upper Coralline Formation and it acts simply as a base. However, lying above the Blue Clay, it acts as an important point of water seepage and springs in the stratigraphy of the Maltese Islands. The Greensand formation represents a final shallowing after the earlier deep-water situation and encompasses a period of active current activity.

During that period, all the clay and fine carbonate particles were swept away, leaving behind the larger

19 particles, many of which were fossils or their fragments. The characteristic green Glauconite is typical of such winnowed marine environments bathed by upwelling water (Pedley et al. 2002).

➢ Upper Coralline Limestone Formation

The Upper Coralline limestone is situated at the top of the stratigraphic sequence of the Maltese Islands. It is a hard, pale grey limestone and very similar to the Lower Coralline limestone formation.

This Coralline Limestone again forms sheer cliffs of varying height and includes a similar content of fossils such as corals and coralline alga. It can be over 160m thick, although it also forms thin hill cappings and limestone-platforms. Karstic geomorphological features have been reported on this formation, but not at the same scale as for the Lower Coralline limestone. The Upper Coralline Limestone is mostly comprised of shallow marine sediments which have characteristics of several different marine or intertidal environments (facies). Although there are five facies reported, these are slightly different in detail from those in the lowest formation. These facies are: a) the Reef Limestone, b) the Tidal Flat Limestone, c) the Oolitic Cross–bedded sands, d) the Muds with large Foraminiferal Limestones and e) the Planktonic muds. The Upper Coralline limestone is the only formation reported on Comino and Cominoto, while it is fully developed in western Malta and eastern Gozo (Pedley et al.

2002).

➢ Quaternary Deposits

Although the main sedimentation ended between the Miocene and Pliocene, geological research has recorded the presence of some Quaternary deposits (Trechmann, 1938). They are mainly reported as cavern, fissure infillings and thin hillside veneers of calcreted material (Pedley et al. 1976). The different layers of these sediments contain an abundant mammalian fauna, which provide insights into the climatic conditions of that period. These findings suggest a more temperate climate than today, with perennial stream-systems and abundant vegetation (Pedley et al. 1976). A land bridge with Sicily would have existed during part of this period, and indeed the separation of the Maltese islands only took place at about 14ka (Furlani et al. 2013). As sea level rose, the Maltese islands progressively formed broadly into their current configuration over the following 7000 years.

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