Category:Scotland Chalk nomenclature (Scottish Chalk Province) - Inner Hebrides Group

Name

The Inner Hebrides Group was formally proposed by Braley (1990). See also Lowden, Braley, Hurst, and Lewis, (1992) and Mortimore, Wood, and Gallois, (2001) in which volume a number of the correlations in the earlier work are adjusted and the dating of the sequence refined.

Type section

At various localities for the constituent formations in the Inner Hebrides and Morvern. Principal exposures are at Beinn Iadain and Loch Aline Mine in Morvern, at Gribun, Carsaig and Auchnacraig on Mull, at Clach Alasdair and Laig Gorge on Eigg and Strathaird on Skye.

Primary Reference Section

At the type-sites of the constituent formations and members.

Formal subdivisions

Divided into seven formations and a number of constituent members. These are from the oldest the Morvern Greensand Formation, Lochaline White Sandstone Formation, Coire Riabhach Phosphatic Formation, Gribun Chalk Formation, Feorlin Sandstone Formation, Strathaird Limestone Formation (including the Clach Alasdair Conglomerate Member and Laig Gorge Sandstone Member) and the Beinn Iadain Mudstone Formation. This latter probably principally of basal Palaeogene in age a description is included herein for completeness.

Lithology

Glauconitic sandstones, pure silica sands, silicified chalks, flints and conglomerates.

Definition of upper boundary

Unconformable beneath the Paleocene lavas or conformable (?) beneath the Beinn Iadain Mudstone Formation of presumed Paleocene age.

Definition of lower boundary

Unconformable at the erosion surface below the Morvern Greensand Formation.

Thickness

The group comprises a highly condensed sequence and is variable in thickness depending on the relative development of its constituent parts. The group may be as little as a few metres thick but is between 10 to 15 m on Mull and between 11 to 22 m in Morvern.

Distribution

Known from the Inner Hebrides islands of Mull, Eigg, Skye, Scalpay, Soay and Raasay and also from the Morvern district of the Scottish mainland.

Previous names

Generally the Cretaceous rocks of the Inner Hebrides have been described using that title or, because of their limited outcrop, as individual constituent units attached to a locality i.e. Morvern Greensand. Some of these gained greater acceptance and were applied more widely.

Parent

None

Age and biostratigraphy

Upper Cretaceous, Cenomanian to early Campanian.

References

Braley (1990); Lowden, Braley, Hurst and Lewis (1992); Mortimore, Wood and Gallois (2001).