In the previous lecture, we discussed the shoreline, where the action of waves and tides predominated. This lecture, instead, will be concern environments that are below wave base. Down here, energy to move sediments comes from gravity (in the form of landslides - strong.)

General topography of ocean floor: This has been worked out by echo sounding and gravitational anomaly techniques, and is best expressed by schematic profiles.

We will consider the geology of these regions in order of increasing depth:

Continental shelf and surroundings: Shallow ocean waters roughly correspond to the photic zone - the region penetrated by sunlight. This extends from 50-200 m. depending on the clarity of the water.

Note: "Continental shelf" may be a useful term to describe topography, but from the standpoint of geologic processes, the photic zone is more important because the major depositional processes in this depth zone are mediated by living things.


Such organisms encrust and build up on any stable surface. This invariably results in the deposition of layers of limestone. The remains of the Central Cretaceous Seaway in the Interior Plains of North America, for example. Sometimes, organisms build impressive piles of biogenic sediment that stand out topographically. These take two forms:

  • Reef development: Many reef building critters, including corals, contain photosynthesizing symbionts. As a result, they thrive in sunlight and grow right up to the water's surface. In fact, if sea level rises too quickly for the reef-builders to keep up, a reef can "drown." (Actually, it dies for lack of sunlight.) These facts constrain the manner in which reefs grow.

  • Planktonic microorganisms: Reef-builders are rooted to the bottom, however many microscopic critters float freely in photic zone waters throughout the ocean. When they die, their remains rain downward in shallow and deep water alike. Important ones include: Continental slope and rise: Below the photic zone living things typically become scarce and aren't major factors in deposition. This is the realm of gravity-driven deposition.

    The abyssal plain: Even turbidity currents only reach the edges of the abyssal plain. Sediment here exists as a thin drape of material that has rained down from above. Three major sources: Carbonate compensation depth: Remember carbonic acid?


    Key concepts and vocabulary: