Relative mineral frequencies (number or weight) are not suitable to study variations in the absolute frequencies of individual heavy-mineral species in a turbidite. A particular grain size can be selected as the “nominal grain” and numbers of grains of a particular mineral in all size classes can be converted to nominal grains. The total number of nominal grains per gram of sediment sample has been defined as the “nominal grain abundance”, which is a function of the volume (or weight) of the total mineral in the sample.

Study of a simple, graded, Tertiary turbidite indicates that the least vertical change in heavy-mineral content occurs at, or slightly above, the middle part of the bed. Heavy minerals in the distal part of the turbidite have more uniform vertical distribution than in the proximal part.