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The identification and quantification of surface change on limestone shore platforms of the Maltese Islands

Student thesis: Doctoral Thesis

  • Ritienne Gauci
Despite the extensive international research on shore platforms over the past 190 years, very little is known about the platforms skirting the micro-tidal coasts of the Mediterranean region. This study investigates the surface change dynamics operating at supratidal levels on five shore platforms of the Maltese Islands, situated in central Mediterranean Sea. The lithology of the five selected shore platforms is Globigerina Limestone, with two platforms formed in the Upper Globigerina Limestone Member (UGLM) and another three having exposures in Lower Globigerina Limestone Member (LGLM) intermixed with conglomerate beds of variable thickness. The selected sites were considered as representative of the Maltese shore platforms in terms lithology and are situated in five different locations on mainland Malta.
The study was undertaken over a period of four years and used a multi-method approach aimed primarily to understand the behaviour of rock surface change on limestone supratidal surfaces. Morphological characteristics of each platform, in terms of weathering and erosion forms, were described from geomorphological field observations. The spatial and temporal variability of surface hardness was calculated with a N-type Schmidt Hammer. The mineralogical composition of the platforms was assessed with near-infrared spectroscopy (NIR) to determine whether the mineral component has a role in surface change dynamics. In addition, a rock exposure experiment with sixteen slabs was undertaken over a period of a year and a half in order to monitor the level of limestone susceptibility to inland subaerial weathering processes. In situ rates and modes of surface change were measured and quantified using the Traversing Micro-Erosion Meter (TMEM) on a spatio-temporal sampling network of 31 measurement stations.
Though the five selected platforms were generally known to be lithologically similar, differences in morphological forms were observed on each platform, with site-specific presence of weathering and erosion forms. Surface hardness results ftom the Schmidt Hammer indicated a lithological variability of the surface hardness, with the UGLM platforms (Blata l-Bajda and Ras il-Fenek) being the most heterogenous of the group and controlled by the presence of hardground beds within this member. The surface resistance of the LGLM platforms (Ponta tal-Qammieħ, Ponta tal-Munxar and Ponta tal-Miġnuna) were less divergent than those in UGLM, but still exhibited variability in surface hardness at spatial scale on each platform.
The weathering rates recorded in the subaerial weathering experiment confirmed the overal surface hardness properties measured on the platforms. The main differences were observed on the UGLM slabs, with Blata l-Bajda slabs experiencing the highest rates of rock weathering, whilst those at Ras il-Fenek were the least responsive in their alterations. The weathering results of the LGLM slabs from Ponta tal-Qammieħ, Ponta tal-Munxar and Ponta tal-Miġnuna mirrored the same mix of results recorded for surface hardness for their respective exposures.
A good part of the surface morphologies observed on the platforms point to visible evidence of a salt-weathered environment in a supratidal conditions. Mineralogy results recorded a strong presence of calcite as the main mineral type but they also confirmed the evidence of variable levels of sodium chloride, which suggest that platforms are characterised by variable levels of salt dominance. The samples from the UGLM platform of Blata l-Bajda was found to be the most salt-dominated. The mineralogy results of the platform samples were further confirmed by those done on the sixteen weathered samples, which recorded loss of sodium chloride after a year and half of inland subaerial exposure.
The mean rate of surface change was measured to be -0.237 mma-1 and ranged from 0.097 mma-1 to -3.247 mma-1. The rates were close to other results for limestone surfaces at supratidal levels in the Mediterranean, such as those published by Swantesson et al. (2004) and Mayaud et al. (2014). They were however found to be on the low side for rates published by Micallef and Williams (2009) for LGLM platforms. Analyses showed no spatial-temporal patterns in the rates of surface change across platforms or between platforms, although the behaviour of surface change of front and middle sections of the platforms were observed to be more comparable. The rates of surface change calcuated over a semi-annual and annual time-scale are more comparable than those calculated over a tri-monthly time-scale. The conclusions are that the rates of surface change on these platforms are largely lithologically controlled and that that lithological surface heterogeneity between and within platforms has influenced measured rates of surface change.
Original languageEnglish
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Award dateMar 2018

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