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Geology of Jersey
The geology of Jersey is characterised by the Late Proterozoic Brioverian volcanics, the Cadomian Orogeny, and only small signs of later deposits from the Cambrian and Quaternary periods. The kind of rocks go from conglomerate to shale, volcanic, intrusive and plutonic igneous rocks of many compositions, and metamorphic rocks as well, thus including most major types.
The Brioverian rocks were formed between 900 and 700 mya. They were named after Briovère, the native name for St. Lô, in Normandy, which is the first area these rocks were described from. They start with turbidites in the west, centre and south of Jersey, including at St Ouens Bay, and St Aubin's Bay. The Brioverian sedimentary rocks are all well bedded and were originally the mid and outer parts of a submarine fan. This constitutes the Jersey Shale Formation. The shale is found at Gorey Harbour in the east coast, and La Belle Hougue Point, and Le Mont Mado granite has an occurrence. The shale is more easily eroded, and this has affected the shape of the island by the concave St Ouens's Bay and St Aubin's Bay. The deepest valleys (Valley of St. Peter and St. Lawrence) are cut into this soft rock. Other sediments associated with the shale are mudstone, and fine grained sandstone. Various sedimentary structures include flute and bounce castes, ripple lamination, graded bedding, cross bedding, and boudinage. The shale layers have been identified as Association IV in the submarine fan.
The sandstone is Association III of the submarine fan. Greywacke is termed Association IV along with the shale. It is found in the same areas as the shale. In an analysis of the greywacke it has fragments with 70% quartz, 10% to 15% plagioclase and microcline feldspar, 2% dark iron containing minerals, with carbon flakes, in a matrix of 10 to 20% clay. The minerals contained are very diverse indicating a broad source area.
Conglomerate, termed Association I from the upper parts of the fan, is found in lenticular bodies near St Peter's Valley, at Gargate Mill. The pebbles in the conglomerate are of multiple kinds of rock. The granite intrusions at the boundary of the formation have caused metamorphism and intrusion by dykes.
In the L'Êtacq area, the metamorphism has produced spotted hornfels with a grey colour. The spots are darker, containing cordierite and biotite. At St. Ouën's Bay the metamorphosis of greywacke has made hornblende-hornfels. Regional metamorphism has converted the shale to a low greenschist facies, where clay is converted to chlorite.
The interpretation of this area is that is a part of the north facing continental slope of Armorica facing a subduction zone, where the Celtic oceanic plate was converging and descending in the trench. The sediments were carried by rivers from the Le Vast Arc, a strip of land oriented east north east, that lay to the south of Jersey. The sea at this time was termed l' Océan de la Manche.
These sediments were uplifted and are overlaid by volcanic rocks of the Jersey Volcanic Group around 530 Mya. The magma came from the subduction zones below and to the north of the island. Andesite from St. Saviour's Andesite Formation is in the centre and in the north at Les Rouaux. It is coloured grey and includes some basalt and pyroclastic fallout such as tuff and agglomerate. Porphyritic andesite is dark grey with white crystals of plagioclase feldspar embedded. Outcrops occur on the coast at Giffards Bay on the north coast and Vicard Point on the east coast. The volcanoes were to the north east and south. Vicard Tuff contains bombs. The Long Echet Tuff contains large crystals of quartz and feldspar and strands of quartz. Les Rouaux Agglomerate and L' Homme Mort Agglomerate contain fragments of the surrounding rocks including shale and andesite embedded in a feldspar matrix. The Les Rouaux Agglomerate also contains pumice. This was eroded before the next phase of volcanism.
Porphoritic rhyolite of St. John's Rhyolite Formation is found to the north east of the centre. It contains banded flows and ignimbrite. Totally it is 950 meters thick. It contains shards of glass and pumice. Different flows exits with sections called Bonne Nuit Ignimbrite followed by Frémont Ignimbrite on the north coast. They contain xenoliths of the andesite and shale found in Jersey. Also there are local mudstone and conglomerate deposited on the flows. The conglomerate is called L'Homme Mort Conglomerate.
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Geology of Jersey
The geology of Jersey is characterised by the Late Proterozoic Brioverian volcanics, the Cadomian Orogeny, and only small signs of later deposits from the Cambrian and Quaternary periods. The kind of rocks go from conglomerate to shale, volcanic, intrusive and plutonic igneous rocks of many compositions, and metamorphic rocks as well, thus including most major types.
The Brioverian rocks were formed between 900 and 700 mya. They were named after Briovère, the native name for St. Lô, in Normandy, which is the first area these rocks were described from. They start with turbidites in the west, centre and south of Jersey, including at St Ouens Bay, and St Aubin's Bay. The Brioverian sedimentary rocks are all well bedded and were originally the mid and outer parts of a submarine fan. This constitutes the Jersey Shale Formation. The shale is found at Gorey Harbour in the east coast, and La Belle Hougue Point, and Le Mont Mado granite has an occurrence. The shale is more easily eroded, and this has affected the shape of the island by the concave St Ouens's Bay and St Aubin's Bay. The deepest valleys (Valley of St. Peter and St. Lawrence) are cut into this soft rock. Other sediments associated with the shale are mudstone, and fine grained sandstone. Various sedimentary structures include flute and bounce castes, ripple lamination, graded bedding, cross bedding, and boudinage. The shale layers have been identified as Association IV in the submarine fan.
The sandstone is Association III of the submarine fan. Greywacke is termed Association IV along with the shale. It is found in the same areas as the shale. In an analysis of the greywacke it has fragments with 70% quartz, 10% to 15% plagioclase and microcline feldspar, 2% dark iron containing minerals, with carbon flakes, in a matrix of 10 to 20% clay. The minerals contained are very diverse indicating a broad source area.
Conglomerate, termed Association I from the upper parts of the fan, is found in lenticular bodies near St Peter's Valley, at Gargate Mill. The pebbles in the conglomerate are of multiple kinds of rock. The granite intrusions at the boundary of the formation have caused metamorphism and intrusion by dykes.
In the L'Êtacq area, the metamorphism has produced spotted hornfels with a grey colour. The spots are darker, containing cordierite and biotite. At St. Ouën's Bay the metamorphosis of greywacke has made hornblende-hornfels. Regional metamorphism has converted the shale to a low greenschist facies, where clay is converted to chlorite.
The interpretation of this area is that is a part of the north facing continental slope of Armorica facing a subduction zone, where the Celtic oceanic plate was converging and descending in the trench. The sediments were carried by rivers from the Le Vast Arc, a strip of land oriented east north east, that lay to the south of Jersey. The sea at this time was termed l' Océan de la Manche.
These sediments were uplifted and are overlaid by volcanic rocks of the Jersey Volcanic Group around 530 Mya. The magma came from the subduction zones below and to the north of the island. Andesite from St. Saviour's Andesite Formation is in the centre and in the north at Les Rouaux. It is coloured grey and includes some basalt and pyroclastic fallout such as tuff and agglomerate. Porphyritic andesite is dark grey with white crystals of plagioclase feldspar embedded. Outcrops occur on the coast at Giffards Bay on the north coast and Vicard Point on the east coast. The volcanoes were to the north east and south. Vicard Tuff contains bombs. The Long Echet Tuff contains large crystals of quartz and feldspar and strands of quartz. Les Rouaux Agglomerate and L' Homme Mort Agglomerate contain fragments of the surrounding rocks including shale and andesite embedded in a feldspar matrix. The Les Rouaux Agglomerate also contains pumice. This was eroded before the next phase of volcanism.
Porphoritic rhyolite of St. John's Rhyolite Formation is found to the north east of the centre. It contains banded flows and ignimbrite. Totally it is 950 meters thick. It contains shards of glass and pumice. Different flows exits with sections called Bonne Nuit Ignimbrite followed by Frémont Ignimbrite on the north coast. They contain xenoliths of the andesite and shale found in Jersey. Also there are local mudstone and conglomerate deposited on the flows. The conglomerate is called L'Homme Mort Conglomerate.