Introduction Part 1: The stratigraphy of the Maltese Islands

The exposed sedimentary rocks of the Maltese Islands date back from the late Oligocene, in the Chattian period, to the late Miocene, in the Messinian period. Strata as old as the Cretaceous are present beneath the oldest exposed layers, evidence for these was obtained through examination of fossilized spores (Pedley et al., 1976).

The first Oligocene stratum exposed in Malta is the Lower Coralline Limestone (LCL). This rock is composed mainly of rhodophytes with occasional coral horizons. The predominant rhodophyte genera are Lithothamnion and Archaeolithothamnion, both of which still exist. Since these require light for autotrophic processes, the environment was probably largely shallow water with calm conditions. In some areas where this layer is exposed, gigantic foraminifers such as Heterostegina and Lepidocyclina are preserved. The top of the stratum consists of the so-called Scutella bed (Spratt, 1843), which is composed of tests of the burrowing echinoid Scutella subrotunda. A phosphorite layer (C0) is also present in some areas (Gatt, 2005).

The stratum immediately above the LCL may date from the late Oligocene (Janssen, 2004). This layer is the Lower Globigerina Limestone (LGL), which marks a deepening of the sea-level since it is composed of mainly of planktonic foraminifera, which require a deep water column for such large populations to arise. Other deep-sea species such as the endemic echinoid Coelopleurus melitensis also occur (Zammit-Maempel, 1969).The LGL is succeeded by the similar but paler Middle Globigerina Limestone (MGL), which does not contain many fossils where exposed. Finally, the Upper Globigerina Limestone (UGL) caps the deep-sea limestone unit. The C1 phosphorite layer divides the LGL and MGL while the C2 phosphorite layer divides the MGL and the UGL. Phosphorite horizons represent a slowing-down in the deposition rate of sediments and a welling of nutrients from deeper areas of the Mediterranean. Several fossils, mostly of holoplanktonic molluscs and echinoderms, as well as teeth of Chondrichthyes, are frequent.

The Blue Clay Formation (BCF) succeeds the UGL. This still shows a deep-sea deposition environment, but instead of carbonate particles, there is a shift towards clayey minerals probably derived from volcanic detritus in a nearby area, creating a muddy area. The BCF contains limonite (iron compound) nodules, and fossils found in this layer are frequently composed of the same mineral.

The Greensand Formation (GF) lies directly above the BCF. This thin layer is almost entirely absent in Malta, though quite frequent in Gozo, especially in the Gelmus area where it reaches a thickness of about 11m (Pedley et al., 1976). It is lithologically a soft sandstone containing high proportions of the mineral glauconite (a complex silicate). The foraminifer Heterostegina appears again, but as another species several times smaller than that found in the LCL. Echinoids, mostly Clypeaster spp., are extremely common, if not always intact. Some authors regard the GF as part of the Upper Coralline Limestone (UCL). This marks a return to shallow water conditions and is composed of organisms similar to those in the LCL, though this time the species Mesophyllum commune is the most important and abundant coralline alga (Bosence, 1983). Some brachiopods and molluscs with algal habitats can also be found together with relatively shallow-water species such as the bivalve Lima lima. Fossilized wave impressions in some areas of the UCL exposure shows that currents were extremely strong, confirming the shallow water hypothesis.

Further sedimentary rocks of Quaternary origin, formed after the Maltese Islands had emerged from the water, are also present at some areas. The Maghlaq coast is the location of an alluvial fan formed by the delta of a large river, while Fiddien Valley in Rabat has a considerable area of lacustrine (lake-formed) tufa with pulmonate gastropod fossils (Pedley, 1980) and imprints of tracheophytes such as Laurus nobilis (Zammit-Maempel, 1977). However, the most important Quaternary deposit is that at Ghar Dalam, which shows the successive remains from the early Pleistocene fauna to the arrival of Neolithic Man in Malta (Zammit-Maempel, 1989).

Figures:

Fig. 1. Heterostegina cf. depressa from the Lower Coralline Limestone, Xghajra, Zabbar, Malta

Fig. 2. Globigerinoides ruber from the Upper Globigerina Limestone at San Lawrenz, Gozo, Malta

Fig. 3. Heterostegina costata from the Greensand Formation at Gelmus Hill, Gozo, Malta

References:

Boehme, W. & Zammit Maempel, G. (1982), Lacerta siculimelitensis sp. n. (Sauria: Lacertidae), a giant lizard from the Late Pleistocene of Malta. Amphibia-Reptilia, 3 (2-3), pp. 257-268.
Bosence, D. W. J. (1983), Coralline algae from the Miocene of Malta. Palaeontology, 26, pp. 147-173.
Gatt, P.A. (2005), Syntectonic deposition of an Oligo-Miocene phosphorite conglomerate bed in Malta. The Central Mediterranean Naturalist, 4 (2), pp. 109-119.
Janssen, A. W. (2004), Fossils from the Lower Globigerina Limestone Formation at Wardija, Gozo (Miocene, Aquitanian), with a description of some new pteropod species (Mollusca, Gastropoda). The Central Mediterranean Naturalist, 4 (1), pp. 1-33, 4 pl.
Pedley, H. M. (1980), The occurrence and sedimentology of a Pleistocene travertine in the Fiddien valley, Malta. Proceedings of the Geologists’ Association, 91, pp. 195-202.
Pedley, H. M., House, M. R., & Waugh, B. (1976), The Geology of Malta and Gozo. Proceedings of the Geologists’ Association, 87, pp. 325-341.
Spratt, T. A. B. (1843), On the Geology of the Maltese Islands. Proceedings of the Geological Society, 4 (2:97) pp. 225-230.
Zammit Maempel, G. (1969), A New Species of Coelopleurus (Echinoidea) from the Miocene of Malta. Palaeontology, 12 (1), pp. 42-47, 6 pl.
Zammit Maempel, G. (1977), An Outline of Maltese Geology. pp. 1-44, Progress Press, Malta.
Zammit Maempel, G. (1989), Ghar Dalam – Cave and Deposits. pp. 1-74, PEG, Malta.