Abstract

Interpretation of energy level is an important part of the paleoenvironmental reconstruction of an ancient wave-dominated shelf, and it has a bearing on the potential for and distribution of reservoir rock and the paleoclimatology, paleo-oceanography and size of the depositional basin. Traditional approaches to interpreting energy levels include analysis of texture, sedimentary structures, the degree of bioturbation and the thickness of specific depositional facies in a progradational sequence. Numerous problems exist, however. The oceano-graphic definition of wave energy is inconsistent with most geological usage, and a quantitative distinction between high and low energy levels in the geological sense does not exist at present. Nonetheless, the concept of high and low levels on wave-dominated shelves is a useful geological distinction, although it is not always clear whether the concept applies to ambient energy conditions, those induced by infrequent highly energetic events, or some combination of the two.

The reconstruction of ancient energy conditions is further complicated by the ambiguity of most of the available evidence. The texture of a shelf sediment reflects not only the energy level but also the nature of the available material. Sedimentary structures (cross-bedding, planar lamination, hummocky cross-stratification) depend partly on substrate texture and water depth. The degree of bioturbation is partly a function of substrate character, depth, faunal community and rate of sedimentation. The thickness of facies in a progradational sequence depends on the combined rates of sea level change and sedimentation, as well as on the distribution of erosion forces. Ancient energy levels can be assessed, but only after consideration of water depth, nature of available sediment, rate of sedimentation, faunal community and sea level change.