Carbonate sediments are commonly subjected to subaerial exposure and surficial alteration relatively early in their history through real or relative changes in sea level. The resultant diagenetic overprint may be superficial or pervasive, subtle or profound, depending upon the interplay between a number of controlling variables. It is particularly important to realize that, to a first approximation, the nature and extent of near-surface subaerial diagenesis is not exclusively a time-dependent function, but rather commonly reflects the prevailing subaerial (climatic) regime. Because of this, the course of such diagenetic modification is highly variable.

Studies on Pleistocene deposits from localities such as Barbados and Florida demonstrate a tremendous range in exposure-surface fabric. These fabrics encompass lithified but otherwise little-altered horizons, thin but texturally variable calcrete crusts, thick complex caliche profiles, in-situ psuedo-breccias, and solution karst with pits and channels of high relief. It is common to find sharply contrasting fabrics developed in close proximity on the same exposure surface.

To recognize such breaks in outcrop or core, it is necessary to maintain a critical appreciation that variability is the rule rather than the exception. There is no question about the importance of such breaks, for they serve as major time-stratigraphic datums, provide insight into basin history, and may either locally enhance porosity or act as substantial vertical permeability barriers, commonly within stratigraphic sequences that otherwise appear to be lithologically uniform.

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