6.3 SEDIMENTARY MODELS
241
Fig. 6.48. (A) Cross-section to indicate the critical parameters of continental shelves. (B) Cross-sections
showing how fluctuating sea level and the mudline control shelf deposition. Note that the depth to the mudline is controlled by tidal, storm, and wave processes. It also varies seasonally and geographically.
diminish with depth until a critical surface is reached below which mud may settle from
suspension. This surface has been termed the effective wave base, but this is not wholly
appropriate because other processes are also involved. The term mudline is to be preferred because it is purely descriptive (Stanley and Weir, 1978). As global sea level
varies with time continental shelves may be emergent or submergent. During submergent phases the mudline may move up from the continental margin to encroach across
the shelf itself (Fig. 6.48).
Other important continental shelf parameters include vertical variations in light and
oxygen. Several studies show that in modern oceans oxygen content is high in the shallow zone where planktonic algae photosynthesize, diminishes rapidly to almost zero at
some 200 m, and then increases slightly right down to the ocean deeps (e.g., Dietrich,
241
Fig. 6.48. (A) Cross-section to indicate the critical parameters of continental shelves. (B) Cross-sections
showing how fluctuating sea level and the mudline control shelf deposition. Note that the depth to the mudline is controlled by tidal, storm, and wave processes. It also varies seasonally and geographically.
diminish with depth until a critical surface is reached below which mud may settle from
suspension. This surface has been termed the effective wave base, but this is not wholly
appropriate because other processes are also involved. The term mudline is to be preferred because it is purely descriptive (Stanley and Weir, 1978). As global sea level
varies with time continental shelves may be emergent or submergent. During submergent phases the mudline may move up from the continental margin to encroach across
the shelf itself (Fig. 6.48).
Other important continental shelf parameters include vertical variations in light and
oxygen. Several studies show that in modern oceans oxygen content is high in the shallow zone where planktonic algae photosynthesize, diminishes rapidly to almost zero at
some 200 m, and then increases slightly right down to the ocean deeps (e.g., Dietrich,