Volcanic ash
Volcanic ash layers occur in the Bass Formation. They consist of layers of white, very fine-grained tephra, that are interbedded with dolomite and argillite toward the base of the formation. These layers are characterized by sharp basal contacts, some graded-bedding, and a diverse silicate mineral assemblage that differs from the carbonate or calcsilicate dominated mineral assemblages of adjacent strata. Zircons from one of these volcanic ash beds has been dated using Uranium-lead dating techniques.