3 Deposition and Sedimentary Landforms

How transport energy controls deposition

Sediment is transported by water, wind, ice, or gravity. Deposition begins when a transporting agent loses enough energy that it can no longer carry some of its load.

As a stream slows, it may drop gravel and sand while finer silt and clay remain suspended longer and settle in quieter water farther downstream. The order and distance in which grains settle depend on grain size, density, shape, and flow conditions—not on grain size alone.

Grain size and layers

A graded deposit can form when transport energy decreases: coarser grains settle at the bottom, with progressively finer grains above them. A sudden loss of energy, such as when a flood or debris flow stops, can instead deposit a mixture of grain sizes at once. Repeated transport and reworking can winnow away fine particles, leaving a more uniform deposit.

Sorting as evidence

Sorting describes how similar or varied the grain sizes in a deposit are. A well-sorted deposit contains grains of roughly similar size, whereas a poorly sorted deposit contains a wide range of sizes.

Persistent waves or wind can sort sediment by repeatedly moving some grain sizes and leaving others behind. Rapid deposition from a landslide or debris flow commonly leaves poorly sorted material. Sorting provides clues about how sediment was transported and deposited, but it does not identify an environment by itself.

Accumulation in beds

Sedimentation is the settling and accumulation of sediment, often in layers called beds. When conditions change, successive layers can differ in grain size or composition. Examples of such changes include a river shifting course, a flood subsiding, or waves changing strength. The layers preserve evidence of changing transport and depositional conditions.

Landforms built by sediment

Deposited sediment can build distinctive landforms. Their formation depends on where transport slows or sediment is redistributed.

• Deltas form where a river enters a standing body of water, slows, and deposits sediment. Channels may divide and shift as a delta grows.

• Alluvial fans form where a stream or debris flow leaves a steep, confined valley and spreads across flatter ground, losing energy and dropping sediment.

• Floodplains and bars develop as rivers deposit sediment during floods or where currents weaken within channels.

• Beaches and dunes form as waves and wind redistribute and sort sediment along coasts and across dry surfaces.

These landforms may remain loose sediment, be eroded and redeposited, or eventually become rock.

From loose sediment to rock

As sediment is buried beneath newer deposits, pressure compacts it and reduces the pore spaces between grains. Groundwater can carry dissolved minerals into those spaces; the minerals precipitate and bind the grains together through cementation.

Compaction and cementation together commonly lithify loose sediment into sedimentary rock. Sand may become sandstone, while mud rich in clay and silt may become mudstone or shale. Lithification can preserve layers and other features of the original deposit, although later changes may modify them.