Abstract

The interpretation of paleoenergy levels can be an important, but difficult, part of paleoenvironmental reconstruction of ancient wave-dominated shelves. The wave energy at the sea surface is only partly reflected by the kinetic energy at the sea floor, which also depends on the factors of water depth, wave period and any superimposed unidirectional flow. The energy at the sea surface and the sea floor can be related through wave theory if no other flow is present. Energy levels defined here to facilitate the interpretation of paleoenergy are based on threshold velocities for movement and sheet flow of quartz sand. Low-energy conditions are those normally below the threshold of movement of sand (orbital velocities < 10 cm/s). Moderately low-energy conditions are those under which sand (but not coarser material) can be moved, but not put into sheet flow (10-50 cm/s). Under moderately high-energy conditions (50-100 cm/s), pebbles up to 1 cm in diameter can be moved and fine sand put into sheet flow. Under high energy conditions (> 100 cm/s), clasts larger than 1 cm can be moved and medium or coarse sand can be put into sheet flow.

A number of assumptions are involved in the assessment of paleoenergy levels using this method. The existing grain sizes must be assumed to be in equilibrium with the oscillatory currents generated by shoaling waves. Superimposed unidirectional currents are assumed to be insignificant. A reasonable water depth must be inferred before the energy level at the sea floor can be effectively related to that at the sea surface. The effects of variation in energy levels, particularly due to large storms, must be taken into account. If these and other issues are met, paleoenergy levels can be reconstructed with acceptable accuracy for many ancient shelf deposits. Because of the many assumptions and problems involved, however, precise estimates should not be expected at our present level of knowledge.