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

Name

The Gribun Chalk Formation was formally proposed by Braley (1990).

Type section

The Gribun Boulder [NM 457 357] and Gribun Stream Boulder [NM 456 355], Isle of Mull.

Primary Reference Section

Clachandhu Boulders [NM 4550 3533, NM 4550 3531, NM 4550 3528] and at Allt na Teangaidh [NM 453 328] near Gribun, Isle of Mull.

Formal subdivision

None

Lithology

Silicified Chalk. Reworked Chalk and conglomerate as (?) debris flow. Flinty in part.

Definition of upper boundary

Unconformable with the Clach Alasdair Conglomerate Member of the Strathaird Limestone Formation or Beinn Iadain Mudstone Formation (basal Paleocene age).

Definition of lower boundary

Unconformable on the Coire Riabhach Phosphatic Formation or the Lochaline White Sandstone Formation.

Thickness

c. 5 m at Gribun, about 1 m elsewhere.

Distribution

Principally on the island of Mull and within the Morvern district.

Previous/equivalent names

Silicified limestone with flint (Torosay) (h5’’’)(Senonian = Santonian to Maastrichtian) on Eastern Mull (44W and pt 44E) and Lismore (44E).

Silicified Chalk (≡ Gribun Chalk) on Staffa (43N).

Chalk

Senonian Chalk

Gribun Chalk

Chalk of Morvern, Judd (1878).

Silicified Chalk (Beinn Iadain), Scott (1928).

NOTE: - Hancock (2000), confusingly, utilised the term Gribun Formation or Gribun Conglomerate Formation to describe rocks above the Chalk (silicified Chalk) of? Santonian age which is the named Gribun Chalk Formation. This unit is probably equivalent to the Clach Alasdair Conglomerate Member of Mortimore, Woods and Gallois (2001).

Parent

Inner Hebrides Group.

Age and biostratigraphy

Upper Cretaceous, Cenomanian in Lowden et al, (1992); Santonian in Hancock, (2000); Santonian to Campanian in Mortimore et al., (2001).

References

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