|
Discussion |
1 Department of Civil Engineering, University of Malta, Msida, Malta (e-mail: mepcons@hotmail.com)
2 Institute for Masonry and Construction Research, University of Malta, Msida, MSD06, Malta (e-mail: joann.cassar@um.edu.mt)
3 Department of Chemistry, University of Malta, Msida MSD06, Malta (e-mail: alfred.j.vella@um.edu.mt)
 |
Introduction |
|---|
 TOP Introduction StratigraphyEffect of non-carbonate content...AcknowledgementsReferences |
|---|
|
|
Stratigraphy |
|---|
|
TOPIntroductionStratigraphyEffect of non-carbonate content...AcknowledgementsReferences |
|---|
Sampling at 1 m intervals, the Authors rejected 19 core samples showing colour or textural differences and retained the remaining 90 samples of ‘soll’ and ‘franka’ rock, which they claimed to be ‘visually identical’ (pale yellow colour). Using their geochemical method they concluded that ‘in all cores studied, the lowest layers always consist of "soll"’. Figure 2 compares data from table 5 of the Authors with logs presented by Wardell Armstrong (1996). The overall depths of grey and mottled orange-coloured facies described by Wardell Armstrong (1996) actually coincide with substantial parts of the cores claimed by the Authors as being visually identical. The visually distinguishable grey facies at the base of the cores weathers differently from both ‘soll’ and ‘franka’, is very low in compressive strength and consequently is never quarried. This makes the proposed geochemical method redundant in this case.
|
Several studies (e.g. Boyd et al. 1994) have reported a cyclic increase of Si, Al, Fe, K and Ti in global pelagic carbonates as a result of greater inputs of detrital clay. Maltese outcrops may show a similar increase, which correlates with the cyclic sedimentation of ‘soll’ stone. However, non-cyclic siliciclastic inputs such as clinoptilolite of volcanic origin (John et al. 2003) and clays accumulating in areas of local tectonically driven deepening (producing grey rock at the base of the cores) are superimposed on the cyclic pattern.
Clearly, a range of geochemical signals within a restricted area in Malta are being reported by the Authors, although some of these are unrelated to cyclic sedimentation of ‘soll’ stone and its depositional environment.
|
Effect of non-carbonate content on stone weathering |
|---|
|
TOPIntroductionStratigraphyEffect of non-carbonate content...AcknowledgementsReferences |
|---|
The Authors then applied their claim based on quarry samples directly to 90 freshly cut core samples without running weathering tests on these samples to support their argument. Therefore, their central hypothesis that a geochemical method can be used to differentiate between less and more durable fresh stone samples that are visually identical actually remains untested. Ultimately, the Authors do not explain how a marginal increase in non-carbonates is responsible for severe weathering in ‘soll’. In fact, Fitzner et al. (1997) concluded that there is no relationship between the non-carbonate fraction and weathering in Maltese limestone masonry.
Gatt (2007) showed that the greater intensity and diversity of bioturbation in ‘soll’ stone (Goldring et al. 2002) creates a more heterogeneous stone with a wide distribution of pore sizes responsible for differential weathering and loss of volume around trace fossils by cyclic salt crystallization. On the other hand, the marginally higher non-carbonate content in ‘soll’ is only coincidental to its form of rapid weathering, and not constrained to only this type of stone in Malta.
The geochemical method proposed by the Authors to identify ‘soll’ stone remains highly debatable in view of their limited geographical area of study and the inability to establish a causal relationship linking the marginal increase in non-carbonate content with the form of weathering that characterizes ‘soll’ stone.
J. Cassar and A.J. Vella reply: Our paper was the culmination of almost 20 years of research on the deterioration of Globigerina Limestone, as well as studies on the chemical and physical properties of Malta's building stone (e.g. Vannucci et al. 1994; Fitzner et al. 1997; Vella et al. 1997; Cassar & Vannucci 2001; Cassar 2002, 2004a, b, 2007; Rüdrich et al. 2005; Rothert et al. 2007). These studies have dealt with the characterization of Globigerina Limestone including, among other aspects, geochemical characterization. This latter line of research has led to us trying to answer an intriguing question: whether the geochemical composition of the insoluble part of the Lower Globigerina Limestone can be used to distinguish the durable ‘franka’ from the less durable ‘soll’. This work started with a University of Malta dissertation by Testa (1989), and culminated in our paper. In the intervening period, several other dissertations on this subject were written (Zammit 1991; Camilleri & Tabone Adami 1992; Gauci & Sapiano 1993; Rizzo & Serracino Inglott 1994; Cassar 1999), four of which resulted in a publication by Vella et al. (1997). All of this work, based on samples obtained from a number of abandoned quarry faces and natural outcrops, and all showing different forms of deterioration, has shown that the geochemical composition of the insoluble fraction of the Lower Globigerina Limestone can be used to identify ‘good’ and ‘bad’ stone.
Following the work on naturally weathered stone samples, which clearly showed that ‘franka’ and ‘soll’ types did have differing geochemical composition in the insoluble fraction, we proceeded to extend this line of research to include a number of cores obtained from the main quarry area of the Maltese Islands, as explained in our paper (Cassar & Vella 2003, figure 2). The experimental work was carried out on three cores originally extracted by Wardell Armstrong (WA) and subsequently donated to the University of Malta, where they have always been freely available to anyone who wanted to examine them. The report then written by Wardell Armstrong, on the other hand, was not published, but was available to interested readers with an academic rather than commercial interest.
From the three cores, samples were taken from the parts comprising the Lower Globigerina Limestone Formation. The distinctly blue–grey or mottled orange-coloured rock was discarded; no samples were analysed that were grey coloured, whereas some mottled stones were included for comparative purposes. The 90 samples studied were taken, as stated in our paper (Cassar & Vella 2003, p. 87) ‘every 1 m’. In all cases, sampling started at the top of the available cores, and not, as the Discusser suggests in Figure 2, from the bottom of the core, such that visually identical stone samples were obtained. It is unfortunate that, because of a typing error on our part, the three cores as indicated in table 5 of our paper are numbered differently from the numbering given in the text, which clearly states that B2 was the longest core at 47.5 m, and B3 the shortest core at 28.3 m (p. 87). This is also obvious from the WA logs shown in Figure 2a. As a result, the Discusser's correlation between our figure 5 and the WA logs in Figure 2 is erroneous. However, if we take as correct, in isolation, the ‘simplified logs based on Wardell Armstrong’ presented by the Discusser (Fig. 2a), it can be evidently seen that, starting from the top of the cores, 90 samples, taken at 1 m intervals, can easily be obtained from the three cores. These are areas indicated by the Discusser as ‘pale yellow facies’. This is illustrated in Figure 3. This confirms that no stones from the grey facies were analysed; the distinctly blue–grey rock was never taken into consideration. It is also apparent from Figure 3 that the transition from ‘franka’ to ‘soll’, by our approach, occurred at a point that was located well within the homogeneous part of each core.
|
Although the aim of our paper was to determine the how, and not the why of the phenomenon, the geochemical information obtained was discussed and interpreted in a separate publication (Cassar 2002). Quoting from the abstract of that paper:
‘The Globigerina Limestone occurs as two types of building stone: the resistant "franka" and the easily weathering "soll". Research on both fresh and weathered samples has led to an understanding of the main differences in these two types of stones. The causes and mechanisms of deterioration have also been established. "Franka" and "soll" differ in geochemical and mineralogical composition and in physical properties. The "soll" is richer in the non-carbonate fraction, which occludes some of the pore space, resulting in a lower overall porosity and a higher proportion of small pores. The ambient local environment, heavily loaded with sea salt, particularly sodium chloride and sulphates, readily induces deterioration in "soll", whereas "franka" tends to resist better in this aggressive environment.’
The Authors reiterate that the technique was developed through the analysis of a large number of homogeneous stones, which were in fact ‘visually identical in appearance’, thus developing a geochemical method of recognizing Globigerina Limestone types that is innovative and functional. The method has also been validated by the testing of many additional samples, both weathered and unweathered. Besides, although not within the scope of the research being carried out at the time, a link between the non-carbonate content and the weathering of ‘soll’ stone was in fact subsequently hypothesized, a hypothesis that is currently being researched.
P.A. Gatt1, J. Cassar2& A.J. Vella3
1Department of Civil Engineering, University of Malta, Msida, Malta (e-mail: mepcons@hotmail.com)
2Institute of Masonry and Construction Research, University of Malta, Msida MSD 06, Malta (e-mail: joann.cassar@um.edu.mt)
3Department of Chemistry, University of Malta, Msida MSD06, Malta (e-mail: alfred.j.vella@um.edu.mt)
|
Acknowledgements |
|---|
|
TOPIntroductionStratigraphyEffect of non-carbonate content...AcknowledgementsReferences |
|---|
Received for publication 22 November 2006. Accepted for publication 7 February 2007.
|
References |
|---|
|
TOPIntroductionStratigraphyEffect of non-carbonate content...AcknowledgementsReferences |
|---|
Boyd, R., Huang, Z., O'Connell, S., Milankovitch cyclicity in the Late Cretaceous sediments from Exemouth Plateau off NW Australia. In: DeBoer, P.L. & Smith, D.G. (eds) Orbital Forcing and Cyclic Sequences. International Association of Sedimentologists, Special Publications, 19 1994. 145–166.
Camilleri, A. J. & Tabone Adami, J. P. 1992. Geochemical study of ‘soll’ facies in Lower Globigerina Limestone, Malta. BSc dissertation, University of Malta.
Cassar, J. 1999. Geochemical and mineralogical characterisation of the Lower Globigerina Limestone of the Maltese Islands with special reference to the ‘soll’ facies. PhD thesis, University of Malta.
Cassar, J.Deterioration of the Globigerina Limestone of the Maltese Islands. In: Siegesmund, S., Weiss, T. & Vollbrecht, A. (eds) Natural Stone, Weathering Phenomena, Conservation Strategies and Case Studies. Geological Society, London, Special Publications, 205 2002. 33–49.
Cassar, J.Composition and property data of Malta's building stone for the construction of a database. In: Prikryl, R. & Siegl, P. (eds) Architectural and Sculptural Stone in Cultural Landscape., 2004a. 11–28 Karolinum Press, Prague.
Cassar, J.Comparing visual and geochemical classification of limestone types: the Maltese Globigerina Limestone. In: Kwiatkowski, D., Löfvendahl, R. (eds) Stone 2004, 10th International Congress on Deterioration and Conservation of Stone, 27 June–2 July, 2004, Stockholm., 2004b. 569–577 ICOMOS, Sweden.
Cassar, J.Classifying Maltese prehistoric limestone megaliths by means of geochemical data. In: 7th International Conference of the Association for the Study of Marble and Other Stones Used in Antiquity, ASMOSIA VII, Thassos, Greece, 15–20 September 2003., 2007. in press.
Cassar, J., Vannucci, S., Petrographic and chemical research on the stone of the megalithic temples. Malta Archaeological Review 2001. 40–45.
Fitzner, B., Heinrichs, K., Volker, M., Model for salt weathering at Maltese Globigerina Limestones. In: Zezza, F. (ed.) Origin, Mechanisms and Effects of Salt on Degradation of Monuments in Marine and Continental Environments. Proceedings, European Commission Research Workshop on Protection and Conservation of the European Cultural Heritage. Research Report, 4 1997. 333–344.
Gatt, P.A.Model of limestone weathering and damage in masonry: sedimentological and geotechnical controls in the Globigerina Limestone Formation (Miocene) of Malta. Xjenza, 11 2007. (in print) www.xjenza.com.
Gauci, D. A. & Sapiano, M. 1993. Geochemical anomalies in Globigerina Limestone and the ‘soll’ facies. BSc dissertation, University of Malta.
Goldring, R., Gruszczynski, M., Gatt, P.A., A bow-form burrow and its sedimentological and paleoecological significance. Palaios, 17 2002. 622–630.
Gonzalez, I.J., Scherer, G., Effect of swelling inhibitors on the swelling and stress relaxation of clay bearing stones. Environmental Geology, 46 2004. 364–377.[GeoRef]
John, C., Mutti, M., Adatte, T., Mixed carbonate–siliclastic record on the North African margin (Malta)—coupling of weathering processes and Miocene climate. Geological Society of America Bulletin, 115 2003. 217–229.
Pedley, H.M.A new lithostratigraphical and palaeoenvironmental interpretation for the Coralline Limestone formations (Miocene) of the Maltese Islands. Overseas Geology and Mineral Resources, 54 1978. 1–17.
Rizzo, C. & Serracino Inglott, K. 1994. Geochemical study of the Globigerina Limestone formation (Malta). BSc dissertation, University of Malta.
Rothert, E., Eggers, T., Cassar, J., Ruedrich, J., Fitzner, B., Siegesmund, S., Stone properties and weathering induced by salt crystallisation of Maltese Globigerina Limestone. In: P
ikryl, R. & Smith, B.J. (eds) Building Stone Decay: From Diagnosis to Conservation. Geological Society, London, Special Publications, 271 2007. 189–198.
Rüdrich, J., Rothert, E., Eggers, T., Cassar, J., Fitzner, B., Siegesmund, S., Rock characteristics and salt-conditioned decomposition behaviour of Maltese Globerigina Limestone. In: Siegesmund, S., Auras, M. & Snethlage, R. (eds) STEIN Zerfall und Konservierung., 2005. 194–200 [in German] Edition, Leipzig.
Testa, S. J. 1989. A down-column geochemical study of Lower Globigerina Limestone with special reference to the ‘soll’ layers. BEd(Hons) dissertation, University of Malta.
Vannucci, S., Alessandrini, G., Cassar, J., Tampone, G., Vannucci, M.L., Prehistoric megalithic temples of the Maltese Islands: causes and processes of deterioration of Globigerina Limestone. In: Fassina, V., Ott, H. & Zezza, F. (eds) Conservation of Monuments in the Mediterranean Basin. Proceedings of the 3rd International Symposium, Venice., 1994. 555–565 [in Italian] Sopritendenza ai Beni Artistici e Storici di Venezia, Venice.
Vella, A.J., Testa, S., Zammit, C., Geochemistry of the soll facies of the Lower Globigerina Limestone Formation. Xjenza, 2 1997. 27–33.
Wardell Armstrong Mineral Resource Assessment Report., 1996. Malta Environment and Planning Authority, Malta.
Zammit, C. V. 1991. The analysis of Lower Globigerina Limestone for silicon, iron and aluminium. BSc dissertation, University of Malta.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
