Mull Lava Group

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Mull Lava Group
Stratigraphic range: Palaeocene epoch, Palaeogene
Type Group
Sub-unitsMull Central Lava Formation, Mull Plateau Lava Formation, Staff Lava Formation
Overlies Moine Supergroup and Mesozoic sedimentary rocks
Area840sq km
Thickness1800 m (estimated)
Lithology
Primary basalt lavas
Location
CountryScotland
Extent Isle of Mull, Ardnamurchan, Morvern
Type section
Named for Isle of Mull

The Mull Lava Group is a Palaeogene lithostratigraphic group (a sequence of rock strata) in the west Highlands of Scotland. The name is derived from the Isle of Mull where they are most extensively seen, forming the bedrock across much of the island. They extend into the mainland peninsulas of Ardnamurchan and Morvern and also out to sea.

Lithology and stratigraphy

The Group consists of around 1800 m thickness of lavas erupted from the Mull Central Volcanic Complex and display a range of chemistries. The Group includes (in descending order, i.e. oldest last):

These three formations were formerly known as the Staffa, Main and Pale ‘suites’ of the Plateau Group [1]

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Geology of Mull

The geology of the Isle of Mull in Scotland is dominated by the development during the early Palaeogene period of a ‘volcanic central complex’ associated with the opening of the Atlantic Ocean. The bedrock of the larger part of the island is formed by basalt lava flows ascribed to the Mull Lava Group erupted onto a succession of Mesozoic sedimentary rocks during the Palaeocene epoch. Precambrian and Palaeozoic rocks occur at the island's margins. A number of distinct deposits and features such as raised beaches were formed during the Quaternary period.

Loch Spelve is a sea loch on the southeast coast of the Isle of Mull, off the west coast of Scotland. It is almost landlocked with a relatively narrow opening onto the Firth of Lorn. The A849 road runs past the northern arm of the loch and a minor road runs south from it and around the shore of the western arm via Kinlochspelve as far as the settlement of Croggan. Raised beaches are notable features of the loch, particularly on the shores of the narrows through which it enters the open waters of the firth. There are a few rocky islets within the loch, the largest of which is Eilean Amalaig on which are the ruins of a castle. There is a terminal moraine at Kinlochspelve dating from the last ice age. The southern and eastern shores of Loch Spelve are formed from basalt lava flows of Palaeogene age, intruded by numerous dolerite dykes. The northern and western shores are formed in granophyres and Triassic sandstones, the latter intruded by olivine-dolerite cone sheets associated with the Palaeogene volcanism. Though unseen at the surface, the Great Glen Fault is usually considered to run beneath this loch and nearby Loch Buie.

Loch Don is a sea loch on the east coast of the Isle of Mull, off the west coast of Scotland. It is an intricately shaped loch opening onto the Firth of Lorn near the easternmost point of Mull. Its waters are shallow and the loch dries almost completely at low tide. The community of Lochdon sits on the A849 at the head of the loch. A minor road runs south from Lochdon and around to Grass Point at the point where the loch meets the open waters of the firth. There are both a terminal moraine and deltaic sand deposits around the northern end of the loch, a product of meltwater flow at the end of the last ice age. The shores of the outer part of the loch are formed from basalt lava flows of Palaeogene age whilst the inner parts of the loch extend across Palaeozoic basalt and also Mesozoic sedimentary rocks including Triassic sandstones and Jurassic Lias strata.

References

  1. Emeleus, C.H.; Bell, B.R. (2005). British Regional Geology: the Palaeogene Volcanic Districts of Scotland (Fourth ed.). Nottingham: British Geological Survey. pp. 69–77. ISBN   0852725191.