Stone spiral

Cypraea sp.
Chattian, Oligocene

Lower Globigerina Limestone, Birżebbuġa, Malta

Dissertation - A student’s guide to the geology of the Maltese Islands

The citation and abstract to my B. Ed. (Hons) dissertation have been added to the links on the right-hand side of this blog. The page can also be accessed through here.

The 'Scutella Bed'

Specimen of Scutella subrotunda (Leske, 1778) from the British Natural History Museum, E16593

Leske's original description of the sea-urchin Scutella subrotunda, 1778

The rocks of the Maltese Islands formed beneath the sea in the period spanning from the late Oligocene to the late Miocene, from 28 to 5 million years ago.

By looking at the rocks and the fossils they contain, it is possible to deduce which kind of environment existed all those years ago, by comparing them to present marine habitats and environments.

The Lower Coralline Limestone (known in Maltese as Żonqor) is the first exposed rock of the islands. It was the first to form, 28 to 23 millions of years ago, in the Oligocene period.

Towards the end of the Oligocene, the consistency of the Lower Coralline Limestone changed considerably, passing from frequently semi-crystalline, very compact rock (Attard member) to about ten metres' thickness of relatively less rigid, coarser rock (Il-Mara member), with cross-bedding characteristics. This probably represents very suddenly shallowing water, and may suggest tectonic activity.

This changed environment meant that the seabed was now subjected to stronger currents than before, therefore waves would drag large amounts of debris from deeper waters and deposit them onto these shallow reef-like platforms nowadays called the Maltese Islands.

This debris consisted mostly of organisms and their skeletons, the most common of which was the now extinct sea-urchin Scutella subrotunda (Leske, 1778). The large calcium carbonate testas (usually fragmented, but not exclusively) of huge populations of this echinoid compose one or more layers (up to five) marking the end of the Lower Coralline Limestone phase, to the extent that they have been named 'Scutella Beds' by the British geologist T. A. B. Spratt.

Scutella Bed in Xlendi member Lower Coralline Limestone, Spinola Bay

Scutella fossil in Xlendi member Lower Coralline Limestone, Dwejra (Gozo)

References:

Leske, N. G., 1778. Additamenta ad Jacobi Theodori Klein naturalem dispositionem Echinodermatum et lucubratiunculam de aculeis echinorum marinorum. Lipsiae, Leipzig, 278 pp.
Smith, A. B. (ed.), 2005. The Echinoid Directory. [WWW document, link: http://www.nhm.ac.uk/research-curation/projects/echinoid-directory/index; last accessed: 24.V.2010]
Spratt, T. A. B., 1843. On the geology of the Maltese Islands. Proceedings of the Geological Society of London, 4 (2): 225-231.
Spratt, T. A. B., 1854. On the geology of Malta and Gozo [2nd edition]. Malta, 16 pp.

On some fossil brachiopods from the Maltese Islands

Brachiopods are filter-feeders usually indicating seabeds densely populated with algae. The calcium carbonate shell of these animals is fragile and therefore sufficient shelter from waves is usually needed. In fact, they are mostly found in the Upper and Lower Coralline limestones.

The Il-Mâra Lower Coralline Limestone in the north-east areas of Malta occasionally contains populations of a dwarf form of Terebratula vitrea (von Born, 1778). These fragile brachiopods are usually found in association with cidaroid sea-urchin fragments and large species of foraminifers, including Heterostegina.

Terebratula vitrea (von Born, 1778)

In the westernmost parts of Malta, conspicuous 10 to 20cm thick layers of brachiopod shells are often present in cliff faces composed of the Mtarfa Upper Coralline Limestone, amongst rhodoliths and bryozoan fragments. These fossils are often crushed and identification is difficult unless the shell is broken open. Material from these brachiopod layers was used by Cooper (1983) to describe a new genus and species both named for the type locality, namely Maltaia maltensis Cooper, 1983, considered by other writers (Gaetani & Saccà, 1983) to be a synonym of Terebratula sinuosa (Brocchi, 1814). The latter is also found in considerable numbers in the same strata. However, the former species is markedly different in form and is also found in areas where ‘normal’ T. sinuosa is absent .

Terebratula sinuosa (Brocchi, 1814) (lower picture - detail)

Maltaia maltensis (Cooper, 1983) (detail)

The small (6mm) Megathiris detruncata (Gmelin, 1790) and other small species like Argyrotheca sp. are also very common throughout some parts of the Upper Coralline Limestone Formation.

Megathiris detruncata (Gmelin, 1790)

Deep sea such as that in which the Globigerina Limestone formed also offers protection from strong currents that may damage brachiopods. Despite this, brachiopods in this layer are few and far in between. Disarticulated phosphatised valves of the still-extant Megerlia truncata (Linné, 1767) are infrequently found in the C2 Phosphorite Conglomerate Bed.

Megerlia truncata (Linné, 1767)

References:

Cooper, G. A., 1983. The Terebratulacea (Brachiopoda), Triassic to Recent: a study of the brachidia (loops). Smithsonian Contributions to Paleobiology, 50: 1-445.
Emig, C. C., 2006. Brachiopoda world database. [www document, url: http://www.marinespecies.org/brachiopoda, last accessed: 10.II.2010]
Gaetani, M. & Saccà, D., 1983. Brachiopodi neogenici e pleistocenici della provincia di Messina e della Calabria meridionale. Geologica Romana, 22: 1-42.

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.