Limestone genesis is an important consideration in reconstruction of sedimentary basin history. We have designed a genetic classification of limestones based upon the energy that existed in the depositional environment, and one which permits us to construct geologic models of sedimentary history.

The energy spectrum at any depositional site, which is related to wave and current action, may range from quiet water through strongly agitated water. The Energy Index (EI) is an operational device for dividing the continuous energy spectrum into discrete energy levels. These steps are designated as limestone types in the EI classification as follows: Type I--quiet water sediments; Type II--intermittently agitated water sediments; Type III--slightly agitated water sediments; Type IV--moderately agitated water sediments; Type V--strongly agitated water sediments. The grading spectrum of energy and related water agitation is not directly related to depth, as quiet-water, low-energy sediments may be deposited in very shallow water.

We believe limestones are largely of biogenic origin and are comprised of carbonate materials which originate within the depositional basin. Such material may or may not be indigenous to its depositional site. Thus, recognition of energy levels and classification of a limestone in the EI spectrum depend upon evidence for mechanical transport of sedimentary particles by wave or current action. This evidence is found in the textural properties and biotic make-up of the granular components and in the textural properties of the finer grained matrix.

Carbonate sediments are easily altered by postdepositional processes. Thus, alteration textures must be recognized and understood, as they tend to obscure the depositional features we use for interpretation. In most limestones studied, however, primary features are sufficiently recognizable for successful interpretation of the energy spectrum.

A rock classification has a maximum utility if, in addition to its use as a medium of description and communication, it provides a means of constructing geologic models for interpretation of earth history. Our classification has this utility because it allows us to relate limestones genetically on a three-dimensional basis in space and time. Changes in depositional energy at one place as a function of time may be studied by means of an EI log. The change of EI with time at one locality is related to important developments in basin history such as transgression, regression, basin subsidence, or stillstand. Combination of several EI logs permits one to relate these energy-time changes on an areal basis for correlation purposes.

Finally, the limestone types, which essentially are lithofacies, provide a genetic framework for delineation of facies related to geomorphic environments of deposition.