Chapter 6. Unconformity-bounded Units

A. Nature of unconformity-bounded units

Unconformity-bounded units are bodies of rocks bounded above and below by significant unconformities. They are composed of diverse types of any kind or kinds of rocks, but the lithologic properties of these rocks, their fossil content, or the chronostratigraphic span of the rocks on either side of the bounding unconformities are significant only to the extent that they serve to recognize the bounding unconformities.
Unconformity-bounded units are objective stratigraphic units established and identified without regard for the genetic or causal interpretation of their bounding unconformities.
The relation of unconformity-bounded units to other kinds of stratigraphic units is discussed in Chapter 10.

B. Definitions

1. Unconformity-bounded unit. A body of rocks bounded above and below by specifically designated, significant discontinuities in the stratigraphic succession preferably of regional or interregional extent.
The diagnostic criteria used to establish and to identify an unconformity-bounded unit are its two designated bounding unconformities.Unconformity-bounded units may include any number of other kinds of stratigraphic units.

2. Unconformity. A surface of erosion between rock bodies that represents a significant hiatus or gap in the stratigraphic succession. Some kinds of unconformities are:

a. Angular unconformity. An unconformity in which the bedding planes above and below the unconformity are at an angle to each other.

b. Disconformity. An unconformity in which the bedding planes above and below the stratigraphic break are essentially parallel.

c. Diastem. A short interruption in deposition with little or no erosion before resumption of sedimentation. Diastems are not an appropriate basis for establishing unconformity-bounded units.

C. Kinds of unconformity-bounded units

The basic unconformity-bounded unit is the synthem.

D. Hierarchy of unconformity-bounded units

E. Procedures for establishing unconformity-bounded units. See section 3.B.

Because the presence or absence of the bounding discontinuities is the single diagnostic criterion for establishing, defining, recognizing, and extending unconformity-bounded units, the definition of these units should emphasize the discussion of the nature, position, and characteristics of the discontinuities.
Unconformity-bounded units should be established only where and when they can fulfill a need that other kinds of stratigraphic units cannot meet.

F. Procedures for extending unconformity-bounded units

An unconformity-bounded unit should be extended laterally only as far as both of its bounding unconformities are identifiable.

G. Naming of unconformity-bounded units. See section 3.B.3.

H. Revision of unconformity-bounded units. See section 3.B.5.