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Stratigraphic Breaks and Their Hierarchy

David T. King, Jr.

Traditionally stratigraphic breaks have been classified into two main categories: unconformities and diastems. The degree of significance with respect to loss of chronostratigraphic record is the fundamental difference between these two classes of stratigraphic breaks. Although both unconformities and diastems may be recognized using the same or similar sets of criteria, paleontological criteria are most commonly used to assess the degree of significance of stratigraphic breaks. To fully assess the degree of significance of a stratigraphic break, however, physical (including trace fossil), paleontological, and structural criteria must be given equal weight. Considering current research on episodic sedimentation and stratigraphic incompleteness, the need has emerged for a ierarchical ranking of stratigraphic breaks with respect to their genesis. The ranks are primary interparticle surfaces and surfaces between lamina (first-order breaks); intrastratal and interstratal surfaces of erosion, scour, and omission (second-order breaks); interfacies discontinuities (third-order breaks); transgressive or regressive discontinuities and punctuated aggradational cycle discontinuities (fourth-order breaks); and unconformities determined by physical, paleontological, and structural criteria (fifth-order breaks). The first- and second-order breaks correspond to the original concept of the diastem, a chronostratigraphically insignificant break with a ubiquitous presence in the rock record. Third- and fourth-order breaks correspond to an emerging class of stratigraphic b eaks having situational significance. This class corresponds approximately to the paracontinuity and paraconformity of other workers. Fifth-order breaks correspond most closely to the traditional concept of an unconformity.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.