Geochemical and mineralogical characterisation of the Lower Globigerina Limestone of the Maltese Islands with special reference to the 'soll' facies

Abstract

A detailed geochemical and mineralogical characterisation, supported by petrographical and micro-textural studies, of the Lower Globigerina Limestone was carried out on 109 samples from three cores taken from the largest quarry area of the Maltese islands. In addition, a limited number of samples were obtained from the Middle Globigerina Limestone, the C1 phosphatic nodule bed, the Scutella Bed and the Lower Coralline Limestone. For the Lower Globigerina Limestone, results showed that SiO2, Al2O3, K2O, TiO2 and Fe2O3 increased in concentration within older rocks. These elements also occur in high concentrations in the Middle Globigerina Limestone, within the phosphatic nodule bed and in the "grey" lithofacies located at the base of the Lower Globigerina Limestone. Mineralogical results and petrographical studies in thin section confirmed a progressive increase in insoluble residue down section. There was also a sudden decrease in concentration of the non-carbonate fraction at the Lower Coralline Limestone. Geochemical data for Lower Globigerina Limestone were subjected to statistical analysis, including Principal Component Analysis and Cluster Analysis. This led to a characterisation of the "soll" and "franka" types within this member. The variables SiO2, A1iO3, K2O, TiO2, Fe2O3 were identified as indicators to distinguish between "soll" and "franka" cluster groups. This was confirmed through the subsequent analysis and clustering of additional samples derived from both weathered and non-weathered areas on abandoned quarry faces and from the Hagar Qim prehistoric temples. It was thus found that, for the "soll'' samples under investigation, SiO2 concentrations were> 3.4 ±0.3%, AliO3 > 0.78 ± 0.05%, K2O > 0.13 ±0.01%, TiO2 > 0.06 ±0.002% and Fe2O3 > 0.47 ±0.02%. However, some overlap was found to occur between the "soll" and "franka" stone types for all 5 elements, demarcating another stone type designated as "intermediate". These geochemical differences are also reflected in the mineralogy of the Lower Globigerina Limestone. It was seen that generally the "soil" type contains more quartz (up to 8%) and more phyllosilicates (up to 12%) than the "franka" type. This was reflected as a higher insoluble residue for "soll", of up to 14%. These results were subsequently placed within a geological context. The high concentrations of insoluble residue found at the bottom of the three cores analysed confirmed that the base of the Lower Globigerina Limestone was formed in quite shallow waters where a large amount of fine siliciclastic detrital material was accumulating. In addition, the formation of the "soll" facies probably corresponded to periods of deposition of greater quantities of elastic material or to a slowing down of carbonate sedimentation, where the number and distribution of bioturbations probably also played a determining role. Finally, the appearance of "grey" limestone at the bottom of the Lower Globigerina Limestone member in all 3 cores suggests that shortly after the Lower Globigerina Limestone started to form, a reducing, anaerobic environment occurred, where there was also an increase in the arrival of elastic terrigenous material. This was probably associated with areas of greater depth where the rate of sedimentation, or its duration, also varied, as shown by the varying thickness of this lithofacies in the three different cores. Here, the low energy of the environment led to the accumulation of organic matter and of clay minerals.