Studies of modern carbonate sediments during the past 15 years have resulted in 4 revelations that should modify traditional stratigraphic concepts and definitions relating to unconformities: (1) brief subaerial exposure and attendant cementation of scattered small tracts, such as sand bars and tidal flats, are normal and continuing aspects of shallow-water carbonate deposition; (2) submarine cementation of carbonate sediments is a widespread, ongoing process; (3) submarine erosion and transportation of contemporaneously cemented fragments also are taking place constantly within the carbonate depositional realm; and (4) some features formerly thought to represent "soil" or vadose origin are essentially indistinguishable from marine-organic structure, or products or submar ne cementation.

By analogy, these same concepts probably apply to ancient carbonate rocks. Consequently many of the traditionally accepted criteria for outcrop and core recognition of unconformities are invalid or tenuous at best.

A new definition of "unconformity" is needed. "Missing section" is not satisfactory as a definitive criterion because an apparent hiatus in fact may be only a function of unrecognized lithofacies or biofacies patterns, depending on the magnitude of the interval involved. Clearly, the really important connotation of "unconformity" is that an area of regional extent was eroded, and the new definition should reflect both qualities: erosion and extent. Along the same line, a useful classification of nongradational stratigraphic boundaries can be constructed, based on the same two criteria.

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