A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments . This process is called sedimentation.

A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or bedding.

As the water from the stream combines with the water in the lake, its speed slows down sharply. Once this occurs, the larger grains of sediment become too heavy for the current to move. Sediment particles that have traveled farther from their source tend to be smoother, more rounded, and finer.

Cosmogenous sediment is derived from extraterrestrial sources, and comes in two primary forms; microscopic spherules and larger meteor debris. Like spherules, meteor debris is mostly silica or iron and nickel. One interesting form of debris from these collisions are tektites , which are small droplets of glass.

Hydrogenous sediments are sediments directly precipitated from water. Examples include rocks called evaporites formed by the evaporation of salt bearing water (seawater or briny freshwater).

There are three kinds of sea floor sediment: terrigenous, pelagic, and hydrogenous. Terrigenous sediment is derived from land and usually deposited on the continental shelf, continental rise, and abyssal plain.

Red clay, also known as either brown clay or pelagic clay, accumulates in the deepest and most remote areas of the ocean. It covers 38% of the ocean floor and accumulates more slowly than any other sediment type, at only 0.1–0.5 cm/1000 yr.

On the seafloor, sediments are thinnest near spreading centers (young seafloor) and thicker away from the ridge, where the seafloor is older and has more time to accumulate. Sediments are also much thickest near continents.

Pelagic sediments are the deposits of the open ocean that accumulate on the ocean floor protected from terrestrial influence (see Hüneke and Henrich, 2011, this volume). They are not necessarily deep but are usually located at great distance from the continents.

The majority of the ocean floors in the Atlantic and Indian Oceans have pelagic deposits that are made up of calcareous ooze. Ooze can also occur as siliceous ooze, which again can be classified into radiolarian ooze and diatom ooze.

Areas of the ocean that lie beneath the carbonate compensation depth (CCD), below which calcium carbonate dissolves, typically beneath 4-5 km, will be dominated by siliceous ooze because calcium-carbonate-based material would dissolve in these regions.

ocean gyres

In all modern settings the CCD is deepest beneath the high- productivity zone that is associated with equatorial upwell- ing and where the calcite production rate depresses the CCD. The observation of no clearly focused biological Figure 4. Details of the CCD drop at 33.7 Ma, the Eocene/Oligocene boundary.