GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.2 647 Malta and Gozo in the Mediterranean Sea. Final interpretation and modelling will be completed by University of Malta. The general aim of this study is to provide a new contributions and improvements to the geological model of the area, so as to better plan and design the final proposed route. Specific objectives of the project are as follows: to characterize the nature, thickness and spatial variability of the geological formations below the seafloor; to characterize the geologic, geotechnical and hydraulic properties of these formations; to detect features of geologic and geomorphic relevance within them (e.g. faults, Karstic formations); to build a 3D geologic model of the study area. Geological settings of the study area and its implications for tunneling. The islands are composed almost entirely of marine sedimentary rocks, mainly limestone of Oligo-Miocene age. Some minor quaternary deposits of terrestrial origin are also present. The rock sequence comprises five main distinct formations which, although slightly disturbed by almost vertical faults displacements, lie almost horizontally across the islands with a minor generalized NE dip. The five main rock types are (in order of decreasing age): - Lower Coralline Limestone: this is the oldest exposed rock type in the Maltese Islands and started being laid down during Oligocene age. It is a 140m thick, hard limestone unit that forms sheer cliffs that may be from ten to over a hundred meters high; - Globigerina Limestone: this formation is subdivided into three units (Lower, Middle and Upper Globigerina Limestone) by two pebbles beds. It is a softer fine-grained limestone unit that forms irregular slopes, and that start being deposited during Miocene (Langhian to Aquitanian). Its thickness varies from 23m to 207m; - Blue Clay: this is a very soft unit that within the islands normally forms rolling low slopes that are mostly covered by carbonate raw soil or scattered rubble. The age is Langhian to Tortonian, in the Miocene. Blue Clay is generally impermeable and holds water, with a thickness up to 65m; - Greensand: even if not sufficient thick (12m), this formation proved distinctive enough to have deserve a separate name. It consists of massive, friable, intensely burrowed marly limestone, deposited during the Tortonian age in the Miocene; - Upper Coralline Limestone: this formation is a complex association of limestone. The deposition of these marine sediments stopped around 10 million years ago when the seabed rose above sealevel. This formation constitutes the coralline plateau that top most of the hills of Gozo and Malta. It overlies the Blue Clay in an irregular pattern and its thickness can reach 162m. A system of horst and graben structures of E-NE trend characterizes the area interested by the project. These structures are indicated by prominent ridges and valleys, with sub-horizontal strata dominating. Rock faulting and displacements are widely present in north Malta and south Gozo and in the channel between them, where the geology is not clearly understood yet. There are a series of geological implications for tunneling in this area. The most relevant are: 1) The presence of faults, that degrade the rock quality and its mechanic characteristics. This represents a potential risk for the tunnel excavation in areas where the faults positions are either unknown or poorly mapped; 2) The presence of Blue Clay formation, that would be preferable to avoid during the tunneling because of its softness and its generally poor mechanic characteristics. Where possible, in fact, the tunnel alignment should be developed below the impermeable Blue Clay layer, and extend as far as possible within the Globigerina and Lower Coralline Limestone formations, that are harder and where karsts formations are understood to be scarce. Plan of work and methodologies. The geophysical investigations have been completed in two different phases. The first phase took place in autumn 2016, and comprised all offshore operations. The second phase was planned after the borehole drillings, and was completed during winter 2017-2018.