Offshore & Deep Water Environments

Away from land, the open sea divides into two main regions, the continental shelves (pale blue on the globe), where water is less than 200m deep, and the deep abyssal plains (dark blue) of the ocean floor, anything from 2,000m to 6,000m deep.

Between the two is the continental slope.

Continental Shelf

The sediment deposited here is mainly material eroded from nearby land, together with organic remains such as broken sea shells. Sands may be swept along and deposited by tidal currents, whilst finer mud settles out in calmer (deeper or more sheltered) water.

Abyssal Plain

On the deep ocean floor, sediment deposition is usually very slow indeed, as the only sediments to reach this far from land are wind-borne dust and volcanic ash. However, plankton in the surface waters of the ocean provide a gentle “rain” of organic remains; the microscopic shells of these organisms form fine-grained oozes that gradually settle out on the deep sea floor.

Towards the poles, “dropstones” – material melted out of icebergs - also add to the deep sea sediments.

Turbidity currents

Sediment deposited near the top of the continental slope is not in a good resting place! Occasional earthquakes may stir up the sediment. This mixture of sediment and water can flow rapidly down the continental slope and sometimes far out onto the deep ocean floor. These currents can deposit more material in a few hours than would usually be laid down over centuries.