Scotland Chalk nomenclature (Scottish Chalk Province) - Inner Hebrides Group: Coire Riabhach Phosphatic Formation

Name

The name first appears in Mortimore et al. (2001) and is substantially unrecognised in previous descriptions.

Type section

Stream section on the west bank of Coire Riabhach [NM 697 564] on the northeast flank of Beinn Iadain.

Primary Reference Section

As above

Formal subdivision

None

Lithology

Up to five beds of glauconitic sand with phosphate nodules and fossil casts.

Definition of upper boundary

Unconformable (or a non-sequence at an erosion surface) with the overlying Gribun Chalk Formation.

Definition of lower boundary

Conformable (or a non-sequence at an erosion surface) with the underlying Lochaline White Sandstone Formation.

Thickness

Usually less than one metre.

Distribution

Limited to the Beinn Iadain area.

Previous names

None

Parent

Inner Hebrides Group.

Age and biostratigraphy

Upper Cretaceous, Coniacian to Santonian in Mortimore et al., (2001).

References

Mortimore, Wood and Gallois (2001). (In this volume a number of the correlations in the earlier works are adjusted and the dating of the sequence refined. The term Coire Riabhach Phosphatic Formation appears first in this volume as one of the adjustments to the terminology and correlation of the sequence proposed by Braley, 1990).

NOTE: - Because of its relative thinness it is questionable as to whether this bed is a ‘mappable’ unit and as such worthy of formational status, however it is prominent in sections logged in Morvern and may prove the correlative of the basal unit of the Gribun Chalk Formation as mapped on Mull where glauconite is seen in the sequences associated with erosional surfaces. It may also be a key link with the Northern Ireland succession where Mortimore et al. correlate it with the Cloghfin Sponge Formation at the base of the Ulster White Limestone Group.