Maltese Miocene Stratigraphy

By virtue of the mode of formation of Maltese rocks, the stratigraphy follows a definite sequence as indicated in Table 1. This table suggests that the Islands should be covered by an Upper Coralline Limestone cap, however because of the various erosive mechanisms the deeper strata has been exposed.

TABLE 1: MIOCENE STRATIGRAPH

*`SERIES`*	*`FORMATION`*	*`DEPTH`*	*`AGE`*
`Tortonian`	`Upper Coraline Limestone`	`250ft`	`28 million`
`Helvetian`	`Greensand`	`50ft`	`30 million`
`Schlier`	`Blue Clay`	`30ft`
`Burdigalian`	`Globigerina Limestone`	`200ft`	`34 million`
`Aquitanian`	`Lower Coralline Limestone`	`+500ft`	`37 million`

LOWER CORALLINE LIMESTONE [LCL]

This strata, the oldest formation belonging to the Aquitannian series of the Miocene, has a total thickness of over 500 feet and undoubtedly exceeds the thickness of all the upper strata together. The rocks of this horizon are more compact and more crystalline than any other Maltese rock. It is also more resistant regards denudation and weathering than any of the others. It is simply subdivided into an upper semi-crystalline and a lower non-crystalline variety. It colour may vary from pure white to red, crean or grey. The permeability and porosity varies considerably, permeability often depending on joints.

Analysis of typical rock specicems shows an average ofd more than 97% calcium carbonate. The reddish residue after acid treatment is found to consist of clayey matter, iron oxide and mineral fragments, the largest of which are smaller than 0.1 mm diameter. These minerals originated from igneous or metamorphic rocks in the not very distant continental land mass. Phosphoric acid and magnesia were found in small quamntities. The rocks of this horizon may take a good polish and have been sold as Gozo or Malta Marbles. This rock is not a marble in the full petrological sense but merely a polished limestone, since the crystallization of the calcite particles is not sufficiently coarse while the rocks's hardeness is below that of a true marble.

Because of the rock's hardness fossils are difficult to extract in a perfect condition. Large portions of rock are composed of small particles of shells, corals and coralline. The organic remains of this strata suggests that it was laid down at sea depths 30-180 feet. Towards the end of this series the water became deeper, as is indicated by the smaller fragments and the globigerina shells of the upper beds of the strata. The SCUTELLA and NODULE BEDS are often considered as forming the boundery line or transition between the Lower Coralline Limestone and the overlying Globigerina Limestone. associated with the Scutella Bed is BLACK LIMESTONE.

The Lower Coralline Limestone formations have been subdivided into four main beds: [1] Maghlaq Member comprising biomicrites; [2] Attard Member comprising Rhodolite biosparites; [3] Xlendi Member comprising Scutella beds and cross-bedded biosparites; and [4] Il Mara Members comorising Bryozoan and Lepidocyclina beds.

GLOBIGERINA LIMESTONE [GLS]

The end of the Aquitanian series saw a sinking of the land mass as the overlying strata of the Burdigalian series is entirely composed of Globigerina and related deep-sea micro-organisms, all of which suggest sea-depths of about 600 feet. No mineral fragments are found in these beds - another indication of deep sea origins. The stratum has been subdivided by various authors into a variable number of substrata. Some authors have divided this strata into four layers, while others into as much as twelve. A simplified subdivision is indicated in the Table 2 below. The transition towards the overlying clays is gradual and the general opinion is that the two horizons lithologically and palaentologically belong together.

TABLE 2: GLOBINGERINA SUBSTRATA
Following the SCUTELLA or PHOSPHATIC TRANSITION BED
1. Freestone: A pale yellow soft limestone often used for building.
2. A thick seam of Phospahatic nodules (where present).
3. Whitish-yellow fine-grained rock with chert nodules.
4. Soft very fine-grained rock, blue when fresh becoming grey or white.
5. An irregular band of green coloured nodules.
6. Second seam of brown phosphatic nodules.
7. A thin layer of soft stone with nodules of selenite, manganese, etc.
8. A seam of phosphatic nodules.
9. Pale grey soft limestone, dark blue when fresh.

Light reddish grey (1) and bluish-green globigerina (2) specimens from New Dock were analysed by C.H. Colson. The insoluble remains contained iron in the ferrous form, abundant alumins and small quantities of lime. Due to the soft nature, and at times even clayey nature, of this matrix, this stratum is very advantageous for fossil collecting.

TABLE 3: GLOBINGERINA & BLUE CLAY CHEMICAL ANALYSIS
....SPECIMENS....................1...........2............3
Calcium Carbonate..............80.24%......78.39%.......2-67%
Calcium Phosphate...............3.57%.......2.70%.......4-30%
Magnesium Carbonate.............1.63%.......0.44%.......traces
Calcium Sulphate................0.06%.......0.33% ......traces to 2%
Iron Oxide].....................1.13%........-..........4-10%
Alumina...]............................................25-58%
Dilute HCl (1:10) insoluble....12.88%......17.87%.......3-10%
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TOTAL..........................99.51.......99.73

BLUE CLAY [BC]

The Schlier series of the Miocene marked the start of a continuous land rise from a sea depth of about 600 feet to a shallow depth of about 30 feet. The land lift is first indicated by the comparatively thin (30 feet) layer of the Blue Clay strata. This argillaceous layer is composed of blue, yellow, green and grey clays. In spite of being clays, this strata is never completely free of calcium carbonate. The clay is compact when dry. It contains minute grains of quartz, augite, hornblend, felspar, zircon and tourmaline. Selenite and gypsum occur in considerable quantities either in isolated masses or in aggregations of small crystals. Iron nodules of a dark red colour are also found. The minute crystal grains of minerals originating from the mainland are another indication of the land lift and relatively shallow waters. This stratum is impermeable to water, and this fact alone makes this strata the most important from a practiocal and technical point of view, since it forms the basis of the Maltese water table. Analysis of Blue Clay specimens (3) were carried out by J.H. Cooke and outlined in Table 3. Organic remains are very easy to extract, but many of these are difficult to identify sine the majority are completely mineralized.

GREENSANDS [GNS]

This facies consists either of a rather compact red or yellowish-red sandstone, or at other localities it may be soft and loose. Green and black grains are intermixed, though one variety usually predominates. The appearance may also be patchy. The upper layers are yellow through oxidation of the glauconite grains. This stratum contains various nodules and mineral grains, the largest being glauconite with a mean diameter of about 0.5 mm. This strata is typical of sediments laid down in shallow waters. The transition from Blue Clay is quite definite, but this is not the case with the overlying strata - Upper Coralline Limestone. According to Murray, the upper limit of this horizon is marked by a bed consisting almost entirely of grains of calcite, each having in the center a minute particle of limonite among which a few formaminifera may be traced. Due to the rather slippery character of the Blue Clay Strata below, it is very common to find large blocks of greensands among the clay talus below. Organic remains are numerous and easy to extract.

UPPER CORALLINE LIMESTONE [UCL]

This strata is very similar to the lowest stratum in the Maltese Islands. It is so named because of the abundance of the fossil algae species Coralline. It resembles the Lower Coralline Limestone both on chemical and palaentological grounds, indicating deposition in shallow waters. The transition from the underlying greensands is gradual, sometimes merging into red and black granular sandstone; or red and white Coralline rich limestone which passes into a white calcareous sandstone - compact, soft or porous but always rich in organic remains. Though some layers are completely crystalline and have lost all traces of the organisms from which they originated, other portions are highly fossiliferous containing casts of shells and other organisms. The stratum may be subdivided into three main substrata.

TABLE 4: UPPER CORALLINE SUBSTRATA
Gradual transition from GREENSAND
1. Soft white limestone, very rich in organic remains including cetacean bones, sharks teeth, and various mollusca species.
2. Reddish coarse-grained limestone very noteworthy for the echinodermata genus Schizaster which may often be found in fragments strewn in long lines, resemblings washings of the sea-coast.
3. White rubbly limestone

While Maltese stratigraphy is overall limited to five horizons arranged in a layer-cake pattern, detailed studies have shown these to be composed of various varieties. A summarised list of Maltese stratigraphy is outlined below.

TABLE 5: MALTESE STRATIGRAPHY

LOWER CORALLINE LIMESTONE [LCL - violet]	*1. LOWER NON-CRYSTALLINE* *2. UPPER SEMI-CRYSTALLINE*	*Rock compact and crystalline. Colour varies from white to red, cream or grey.* Transition Bed: Phosphatic Nodules or Scutella Bed (+ Black Limestone)
GLOBIGERINA LIMESTONE [GLS - yellow]	*1. Pale yellow soft limestone.* *2. Thick seam of Phosphatic nodules.* *3. Whitish yellow fine-grained rock with chert.* *4. Soft fine-grained rock. Fresh - blue; later grey.* *5. Green coloured nodule bed.* *6. Brown phosphatic seam.* *7. Thin layer of soft stone with nodules of selenite, etc.* *8. Seam of Phosphatic nodules.* *9. Pale grey soft limestone; dark blue when fresh.*	Transition Bed: Gradual towards Blue Clay. No real demarcation
BLUE CLAY [BC - blue]	*1. Yellow clays* *2. Green clays* *3. Grey clays* *4. Blue clays*	*MINERALS: Gypsum (hardness 2.0); Anhydrite (3.0-3.5); Iron nodules* *NOTE: talus formation* Transition Bed: Quite definite
GREENSANDS [GNS - green]	*1. Compact red or yellow-red sandstone* *2. Soft and loose sandstone with green and black grains.* *3. UPPER LIMIT: Bed with grains of calcite*	Transition Bed: Red and white limestone passing into a white calcareous compact, soft and porous sandstone Red and Black granular sandstone
UPPER CORALLINE LIMESTONE [UCL - green]	*1. Soft white limestone.* *2. Reddish coarse-grained limestone.* *3. Soft white limestone*	*NOTE: Arrangement of fossils to resemble shore washings*

Geological Map after A.L.Adams, 1870