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Geological Habitat of Malta's Aquifers

 

The Maltese archipelago is made up almost entirely of exposed marine Tertiary sedimentary rocks deposited from 40 to 5 million years ago. Deep borehole information indicates a geological history going back 200 million years plus.

Malta forms, with SE Sicily, a spur on the northern edge of the African tectonic plate. For much of the 200 million years, it was located in warm shallow seas on the southern margin of an ancient ocean known as Tethys. In the warm seas, mainly shallow water carbonate rocks were laid down. As plate tectonic movements brought Africa and Eurasia closer and closer to each other, Tethys closed to a narrow seaway and eventually was cut off to leave the Mediterranean Sea. Eventual collision of the plates led to mountain building in Europe and NW Africa. The spur of Malta and SE Sicily contributed to mountain building as it directly collided with a microplate plate in NE Sicily and thus the present plate boundary runs close to Malta.

During plate collision, beginning around 6 million years ago, in an event known as the Messinian Crisis, the Straits of Gibraltar closed and reopened several times, the Mediterranean Sea repeatedly evaporated away to salt flats in its deepest parts and faunal and floral exchange took place between Africa and Europe. The Straits of Gibraltar finally reopened 5.3 million years ago and the Mediterranean Sea was re-established isolating the Islands.

Around 5 million years ago, plate front irregularities caused tensional development in the Pantelleria rift system to the south-west of Malta. A NE-SW trending fault system was imprinted on the Maltese Islands which controls the geomorphology today.

One million years ago the sea rose to 77 metres over present sea level and partially inundated the Islands.  Finally, a series of glacial and interglacial events created a fluctuating sea level around the Islands, with the development of a land bridge to Sicily, in glacial periods, which allowed migrating faunas to reach Malta.  The repeated exposure of the Islands to erosion and meteoric infiltration causes the geology of this period to be expressed by incised river valleys, caves, sink holes and localised river, lake and cave deposits.

Malta's exposed sediments consist of five principal layers, from oldest to youngest; Lower Coralline Limestone, Globigerina Limestone, Blue Clay, Greensand, and Upper Coralline Limestone. The resistant Lower Coralline Limestone was deposited in shallow seas in which reefs formed with protected lagoons and open areas with sand bars. In front of the reefs were deeper water where shells of surface floating micro-organisms died and sank to form sediments.

The softer Globigerina Limestone represents the spreading of deeper waters at least 100 metres deep to bury the reefs, lagoons and sand bars. On the sea floor were some colonies of burrowing organisms.

The soft Blue Clay marks a continuation of deep water conditions, but the clean clear water conditions were interrupted by the introduction of suspended clay minerals shed from mountains being built well to the north of Malta. Subsequently, the seas shallowed, marked by the Greensand layer which formed in a higher energy sea where numerous fossils were deposited. The  hard Upper Coralline Limestone represents a return to similar shallow sea conditions of the Lower Coralline Limestone time.

Around 6 million years ago deposition ceased as the seas retreated following the tectonic closure of the Straits of Gibraltar and the Mediterranean dried out.

Owing to the heterogeneous depositional environments, the Corallines would have had highly variable primary porosity and permeability from to high to low values. Following deposition, dissolution and re-precipitation of calcium carbonate would have modified further the very heterogeneous nature of the carbonates. The Globigerina Limestone would have had more even aquifer properties, but limited permeability. The Blue Clay is impermeable.

Block faulting would have been variable in impact, opening sub vertical fractures and forming barriers in other places. The faulting and fracturing would play a significant role in the Glacial Periods' aiding meteoric invasion. The limestones  have been affected over millions of years by rain and infiltrating waters The exposed landscape has been eroded and dissolved into irregular karst topography, a form known locally as garigue. At sea level, sea caves are being cut today, but there are older sea caves which were formed both during periods of higher sea level and lower sea level (now drowned). Infiltrating rain water dissolved the limestone below ground, particularly along faults and fractures, to form microscopic to macroscopic voids of which the largest are manifested as caves; some of which had subterranean rivers which concentrated the bones of extinct animals as at Għar Dalam. As caves developed in size, some could no longer support their roofs and spectacular collapses took place, to form sink holes, one of which is recorded in history at Il -Maqluba. Much older ones dot the landscape as shallow depressions which have been in filled by soil, or form circular depressions along the coast now invaded by the sea.

Rapid rainwater infiltration takes place via faults, fractures, solution voids, caves and sink holes of various ages, or more slowly over decades through the matrix of the limestones themselves. Where  dissolution structures originate in the Upper Coralline Limestone and do not cut the Blue Clay, as in NW Malta and on Gozo, the latter acts as a barrier to the deeper infiltration of rainwater. Elsewhere, particularly where the Blue Clay is absent, infiltrating waters penetrate down to older layers.

Essentially two fresh water bodies distinguished: an upper minor aquifer, perched at the base of the Upper Coralline Limestone on top of the Blue Clay formation in the north-west Malta and on Gozo; and much larger aquifers on Malta and Gozo, referred to as the Main Mean Sea Level Aquifers situated mainly within the Lower Coralline Limestone. In the latter, fresh water lenses float on seawater, owing to their lesser density within the pores and voids of the aquifer. The Mean Sea Level Aquifers are the main aquifers in Malta and Gozo and volumetrically dwarf other aquifers in terms of fresh water volume.

The perched aquifer has a visible expression in numerous fresh water springs particularly in the Rabat area, where the boundary between the Upper Coralline Limestone and Blue Clay is exposed.

The Mean Sea Level Aquifer is not visible except in boreholes and galleries which penetrate it.

The Upper Coralline Limestone has been continuously exposed and opened by rain water invasion and recharge of the perched aquifer can vary from instantaneously to a few years, whereas the Mean Sea Level Aquifers are mostly protected by overlying formations and are recharged over a few days to several decades, depending on whether recharge is via open fissures or via matrix, the bulk of the recharge is slowly via matrix.

In conclusion, the Maltese carbonate aquifers are complex and heterogeneous in primary depositional character. Block faulting complicated this further, which was then overprinted by meteoric water invasion and dissolution. Today, further changes may be occurring related to human intervention and water extraction via WSC infrastructure and private boreholes, whereby the natural water balance and salinity is being changed which further affects aquifer heterogeneity.

Thus, the water behaviour model for the aquifer will also be complex. 

 

Review by Gordon Knox.