The Maltese Archipelago consists of a group ofsmall, low-lying islands located in the centralMediterranean, approximately 96km South ofSicily and 320km north of North Africa. Thearchipelago extends for 45km in a NW-SEdirection. The largest islands are Malta andGozo. The other islands of the archipelago aremuch smaller and comprise Comino, St Paul'sIslands, Cominotto, Filfla and General's Rock.

The rocks of the Maltese Islands are allsedimentary and result from the accumulationof carbonate sediments in a relatively shallowmarine environment. Various types of rockscorrespond to different palaeo-environments ofdeposition. A second class of rocks, quaternarydeposits, represent sediments that weredeposited in a terrestrial environmentfollowing the emergence of the Maltese Islandsabove sea level. The origin of the MalteseIslands is related to the formation of theMediterranean basin and may be traced to theTriassic. During the Triassic era, rifting in Pangaea in aneast-west direction produced a transverse oceancalled the Tethys Sea. River sediments and reefdeposits laid down on this early ocean bedwere to become the precursors of the rocks thatcomprise the Maltese Islands. Further rifting inPangaea resulted in the northward movementof what is now the African continent, with theconsequent destruction of most of Tethys. Thisrotational movement is ongoing and hasformed a zone of collision between Eurasia andAfrica. The effects of this collision on thegeology and geomorphology of the MalteseIslands are evident as faulting, uparching andsubsidence of the sediments deposited in theproto-Mediterranean.

General Structural Geology Rifting in the vicinity of the Maltese Islands hasresulted in alternate uplifting of various regionsof the Maltese Islands. The most recent suchepisode has given the archipelago a tilt towardsthe north-east. The western regions of theislands are uplifted to form high cliffs while theeastern coastlines are drowned. The highest

point on the archipelago is 253m above meansea level and is situated at Dingli Cliffs on thesouth-western coast. The tilt of the archipelagois responsible for the predominant north-eastern trend of drainage channels on Malta.

Malta is crossed by two main fault systemsrepresenting the effects of two separate riftingepisodes in the vicinity of the archipelago. Theolder of the two, the Great Fault, trends SW toNE, while the Maghlaq Fault system trendsapproximately NW to SE along the southerncoast of the island and has been responsible forthe downthrow of Filfla to sea level. A systemof horst and graben structures of East-Northeast trend characterises Malta north of theGreat Fault. These structures are indicated byprominent ridges and valleys. No well-definedhorst and graben systems occur south of theGreat fault.

Several circular subsidence structures aredistributed throughout the islands. The originsof these structures are various, but are mainlyassociated with solution of limestone bypercolating acidified groundwater leading toroof collapse of subterranean or submarinecaverns.

Stratigraphy Although the sedimentary platform on whichthe Maltese Islands are situated was formedduring the Triassic, there are no surfaceoutcrops of this age. All exposed rocks weredeposited during the Oligocene and Mioceneperiods of geological time. No rocks of Plioceneage have been conclusively identified,indicating that definitive emergence of thearchipelago above sea level probably occurredat this time. The rocks of the Maltese Islands arearranged in a simple layer-cake succession fromoldest to youngest as follows:

Lower Coralline Limestone.Globigerina LimestoneBlue ClayGreensandUpper Coralline Limestone

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GEOLOGY OF MALTAVictoria Grif�ths

These rocks are sporadically overlain byterrestrial, aeolian and alluvial deposits laiddown following the emergence of the MalteseIslands above sea level. Much of the central andsouth-eastern portion of the Maltese comprisesoutcrops of Globigerina Limestone while thenorthern and north-western regions arecharacterised by highlands on which UpperCoralline Limestone is the dominant outcrop.

The geology of Gozo is more varied than that ofMalta, with more frequent outcrops of BlueClay being a characteristic feature. Theabundance of Blue Clay on Gozo makes the soilparticularly fertile. Gozo therefore producesmost of the agricultural produce for the islands.

Lower Coralline Limestone is the oldestexposed rock in the Maltese Islands,outcropping to a height of 140m in the verticalcliffs near Xlendi, Gozo. It is mainly composedof the tests of coralline algae indicatingdeposition in a shallow gulf environment.Younger beds show evidence of deposition inmore open marine conditions.

This lithology acts as the main aquifer for theisland. It provides water for domestic andindustrial use. Water supply is a big problemfor the island especially during the touristseason. The government also uses severaldesalination plants to provide fresh water butthis is very expensive.

Globigerina Limestone is the second oldestrock and outcrops over approximately 70% ofthe area of the islands, eroding to give a broad,gently rolling land-scape. Variations in thethickness of this formation are considerable,ranging from 23m near Fort Chambray, Gozo to207m around Marsaxlokk, Malta. This rockconsists of yellow to pale-grey limestonescomprising tests of planktonic globigerinidforaminifera. The formation is divided intoLower, Middle and Upper GlobigerinaLimestone by two beds of phosphorite pebbles.

This lithology provides most of the buildingstone for the island. It also contains some verywell preserved fossils.

Blue Clay overlies the Globigerina Limestoneformation. It erodes easily when wet and formstaluses which flow out over the underlyingrock. Variations in thickness are considerable

ranging from 75m at Xaghra, Gozo to nil ineastern Malta, where Upper CorallineLimestone rests directly on GlobigerinaLimestone. Deposition of the Blue Clay mayhave occurred in an open muddy waterenvironment with water depths up to 150m forthe lower part of the formation.

The Blue Clays act as an aquiclude to the water-bearing Greensand and Upper CorallineLimestone aquifers. These lithologies alsoprovide some of the water for the island.

Greensand consists of bioclastic limestones richin glauconite deposited in a warm sea.Unweathered sections are green but areoxidised to an orange colour when exposed.The deposit attains a maximum thickness of11m in localised depressions at Il-Gelmus inGozo, but elsewhere is less than 1m thick. There are many fossils to be found in theGreensand mainly bryozoa, gastropods andsharks� teeth. Larger fossils can be found butthey are generally not very well preserved.

Upper Coralline Limestone is the youngestTertiary formation in the islands, reaching athickness of approximately 160m in theBingemma area, Malta. Several significantdepositional breaks occur towards the top ofthis formation indicating emergence on at leasttwo occasions and in general the top of theformation is dominated by facies which implydeposition in vary shallow water ranging fromshallow subtidal to possible intertidal andsupratidal environments.

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