Stones of Portugal Episode-7

Travel through the Limestone and slates of Portugal 

Limestones

In Portugal they occur mainly near the shore, to the north of Lisbon and in the Algarve region, in the south. The main mining district of ornamental limestones is the Maciço Calcário Estremenho (MCE), located north of Lisbon. It is a limestone mass with a well-known litho stratigraphy that is made up of a thick sequence of Mesozoic carbonated rocks tectonically elevated. The productive litho stratigraphic units of the main ornamental varieties are dated from the Middle-Jurassic ages. Mining is carried out by more than 50 quarries in 6 main mining districts: Pé da Pedreira municipal of Santarém), Moleanos (municipally of Alcobaça), Codaçal, Cabeça Veada and Salgueiras (Porto de Mós) and Fátima (Ourém). 

Exploitation of limestone in MCE is relatively recent and it started in the early eighties of the last century. From the 10 years, these rocks are the most demanded Portuguese ornamental stone, especially in the Chinese market.

The litho stratigraphic units have thicknesses of about 40 m to more than 150 m and the strata thickness ranges from 2 to more than 20 m. As the geological structure is very simple, the strata being sub horizontal, exploitability conditions are very favourable. Most of the MCE limestones are fine to coarse-grained calciclastic sparitic rocks (redstones and grain stones), that is, formed by grains cemented by small amounts of translucent calcite.

They are cream coloured with a texture marked by thin laminations, which are sometimes visible and not. It depends on the way the blocks are cut. The most traditional ornamental stones from MCE are referred to by their most common commercial names. The region of Pêro Pinheiro, just North of Lisbon, is one of the most traditional production centres of ornamental stones of Portugal. Quarrying in this region could have started in Roman times, as testified by the discovery of a roman quarry located about 10 km to the south. However, their intensive exploitation only began in the 18th century, for the reconstruction of Lisbon, after the big earthquake of 1755. Due to rapid urbanisation in the region the availability of the resource is threatened and most of the quarries are currently inactive. 

From one hundred registered queries, only 27 remain occasionally active.Ornamental limestones from Pêro Pinheiro are dated from the Cretaceous. In general terms they correspond to bioclastic and bio-edified rocks characterised by more or less abundant fossil remains of rudiments which give peculiar aesthetics to the ornamental varieties. 

In the Algarve region ornamental stones are exploited near the localities of Escarpão (municipality of Albufeira), Mesquita (municipality of S. Brás de Alportel) and Santo Estêvão (municipality of Tavira). Escarpão is a very small mining site of Upper Jurassic bluish grey limestone that is mainly exploited for aggregates. Bio-edified limestone of the Upper Jurassic age with coarse elements and variations of reddish and greyish colours give it a breccia appearance. 

Limestones do not exist in great varieties like ornamental stones. The cream coloured rocks form MCE dominate the market. 

Slates

The outcropping areas of slates in Portugal are quite extensive. However, the production of ornamental slates is small. The main mining sites are located in Valongo, Arouca and Vila Nova de Foz Coa, in the North of Portugal, and in Barrancos, in the Alentejo region, near the border with Spain.

In Valongo, slates are quarried in a very narrow strip of land constrained by urban households. They show a dark grey colour and a very fine granularity. Their superior textural homogeneity and the very well defined slaty cleavage gives them the quality for production of billiards tables. The productive unit of Valongo is of Paleozoic age (Ordovician) and is the same that is exploited at Arouca. In Vila Nova de Foz Côa the so called Xistos do Poio are mined since 200 years ago for building construction as dry set masonry and tiles, as well as support masts for vineyards.

Slate production of Portugal is very low but their ornamental stones can be used in a wide range of applications. Portugal produces a great diversity of ornamental stones. Besides the internationally known white and pink marbles, light cream limestones are also produced, as well as grey, yellow and pink granites, and dark grey slates. Among these limestones are the most wanted variety today, especially in the Chinese market.

Reference: 

Portuguese Ornamental Stones, Jorge M. F. Carvalho1, a, Cristina I. Carvalho1, b, José V. Lisboa1, c, António Casal Moura1,d, and Mário M. Leite1, e1Portuguese Energy and Geology Laboratory, Apartado 7586 – Alfragide, 2610-999 Amadora, Portugal

ajorge.carvalho@lneg.pt

bcristina.carvalho@lneg.pt

cvitor.lisboa@lneg.pt

dcasal.moura@lneg.pt

emachado.leite@lneg.pt