An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe any break in the sedimentary geologic record.
The phenomenon of angular unconformities was discovered by James Hutton, who found examples at Jedburgh in 1787 and at Siccar Point in 1788.
The rocks above an unconformity are, generally, younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents (a lot of) time during which no sediments were deposited and the local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus. There are four types of unconformities: disconformity, nonconformity, angular unconformity and paraconformity.
Siccar Point is historically the most important unconformity and is a rocky promontory in the county of Berwickshire on the east coast of Scotland. It is famous in the history of geology as a result of a boat trip in 1788 in which James Hutton, with James Hall and John Playfair, observed the angular unconformity which Hutton regarded as conclusive proof of the theory of geological evolution. During this year (2009) we are celebrating Charles Darwin bicentenary, it seems very important to remember that some Geological relevant information leads and helps the Darwin’s theory of evolution !!!!!!
One of the most important and well exposed unconformities along this SW Variscan Chain is the so called Telheiro beach unconformity. The problem involving the placing of an Earth cache in the local (Ponta Ruiva) is that the cliffs and the tides are very, very dangerous!!!
The unconformity of Praia de São Torpes (Sines - SW Portugal)
The unconformity of the São Torpes beach can be observed along the coastal boarder, between Sines and Vila Nova de Mil Fontes. It represents an enormous time interval (more then 100 M.y. – million years) during which a variety of complex geological processes took place. Along this angular unconformity two very different groups of rocks suddenly contact. These two rock types were formed at distinct times and by very diverse mechanisms.
The lower, older, group is composed by shales and graywackes while the upper, younger, group is represented by sandstones and basal conglomerates. According to the Tectonic Plate theory, between the sedimentation of the two groups of rocks took place the closure of a primeval ocean with the subsequent folding and metamorphism of the sediments he gathered during millions of years, giving rise to enormous mountain ranges which led to the constitution of the supercontinent Pangaea, at the end of the Paleozoic Era.
Afterwards erosion took place cutting through the mountains until a completely flattened surface was left, materialized by the soft contour of the unconformity surface.
The observation and interpretation of this type of geological evidences led XIX century naturalists, including Charles Darwin, to acquire a much more enlarged notion of our planet’s antiquity. An unconformity is defined whenever two very distinct types of rocks, formed a two separate moments (distant in time around 100 Million years, in this case) and by very diverse geological processes, come in contact with/to each other.
It is an angular unconformity because the underlying rocks became deformed and their layers form a certain angle relative to the overlaying rock layers. The shales and graywakes (Baixo Alentejo flysch Group) were formed under low grade regional metamorphism conditions from clays and clayish sands deposited more than 330 million years ago (Late Carboniferous), at the seabed of a deep ancient ocean.
The sandstones and conglomerates (Grés de Silves formation) formed by the sedimentation of very coarse conglomerate sands, 230 million years ago (Late Triassic), by small rivers and creeks in continental arid conditions with long dry spells alternating with sudden floods. Conglomerate is a sedimentary rock consisting of individual clasts within a finer-grained matrix that have become cemented together.
Conglomerates derived always from an erosive period of time and are formed of rounded fragments (some times very big – boulders) and are thus differentiated from breccias, which consist of angular clasts. Both conglomerates and breccias are characterized by clasts larger than sand (< 2 mm). (See also GC1D9RH - Conglomerate - DP/EC19 by danieloliveira, Lisboa)
This cache takes you to the southernmost part of São Torpes beach – South Sines cape. The outcrop in GZ could be easily identified. In order to log your find, your task is answered the following questions:
1- Measure the dimension of the biggest boulder of the conglomerate above the unconformity.
2- Tell us the colour of the ancient formation (more deformed)
3- And the colour of the matrix of the youngest litologies (the main cement of the conglomerate)
4- Finally – not a question! - take a picture of you and/or your GPS’r on the top of the biggest boulder (it seems a small seat), with two big electric power station chimneys behind.
Answers by the e-mail not in the log.(Using my profile to send).
The photo should be placed on the log after confirmation that the answers are correct. Log without photo or log without authorization will be erased. Please quote the reference/name of the cache in your answers.
Em Português - A cache
Esta cache vai levá-lo até ao extremo sul da praia de São Torpes – a Sul do Cabo de Sines. O afloramento rochoso no ponto 0 é facilmente identificável. Para reclamar o log desta cache, o objectivo é responder às seguintes perguntas:
1- Medir as dimensões do maior calhau de conglomerado acima da discordância angular.
2- Dizer-nos qual a cor da formação geológica mais antiga (mais deformada)
3- E a cor da matriz e do cimento das litologias mais recentes (o cimento do conglomerado)
4- Finalmente – não é uma pergunta! - tire uma fotografia consigo e/ou com o seu GPS'r por cima do maior dos calhaus do conglomerado (que até parece um "banquinho"), de modo a que também se vejam as duas chaminés da Central de Sines ao fundo.
As respostas devem ser enviadas por correio electrónico e não podem ser colocadas no log.(Usem o meu perfil para o envio).
A fotografia deverá ser colocada juntamente com o log depois da confirmação de que as respostas estão correctas. Log's sem autorização e sem fotografia serão apagados. Quando responder às perguntas por favor refira-se na resposta a qual Earthcache nome/número está a responder.