Sedimentary Processes: How Sediment Becomes Rock

Sediment Production and Transport

Sediment originates primarily from the weathering and erosion of pre-existing rocks. Mechanical weathering breaks rock into progressively smaller fragments, from boulders to gravel, sand, silt, and clay. Chemical weathering dissolves soluble minerals and transforms others into new minerals, particularly clay minerals. The products of weathering are then transported away from their source by agents of erosion: rivers, waves, wind, glaciers, and gravity.

The distance sediment travels and the degree to which it is modified during transport depend on the energy of the transporting medium and the durability of the sediment particles. Rivers sort sediment by size as flow velocity changes: fast-flowing mountain streams carry boulders and cobbles, while meandering lowland rivers transport fine silt and clay in suspension. During transport, sediment grains collide with each other and with the channel bed, becoming progressively rounder and smaller through abrasion. Quartz, the most durable common mineral, can survive thousands of kilometers of river transport and multiple cycles of erosion and deposition. Less resistant minerals like feldspar and micas tend to break down into clay during prolonged transport.

Wind sorts sediment even more effectively than water, carrying only the finest particles (silt and fine sand) while leaving coarser material behind as desert pavement. Glaciers are the least selective transport agent, carrying everything from house-sized boulders to microscopic clay particles mixed together in an unsorted mass called till. The characteristics of the sediment, its grain size, sorting, rounding, and mineral composition, record the transport history and can be used by geologists to interpret the depositional environment.

Depositional Environments

Deposition occurs when the transporting agent loses energy. A river slowing as it enters a lake or the sea drops its sediment load, coarsest particles first. The specific conditions of deposition determine the type of sedimentary rock that will eventually form and the features it will contain.

Continental depositional environments include river channels and floodplains (producing sandstone and mudstone with cross-bedding and ripple marks), alluvial fans (coarse, poorly sorted conglomerate deposited where mountain streams emerge onto plains), lakes (fine-grained mudstone and evaporite deposits in closed basins), deserts (well-sorted, wind-blown sandstone with large-scale cross-bedding), and glacial environments (unsorted till deposits and well-sorted outwash sand and gravel). Transitional environments, where land meets sea, include deltas (thick wedges of sediment built by rivers entering the ocean), beaches (well-sorted sand shaped by wave action), tidal flats (alternating sand and mud deposited by tidal currents), and lagoons (fine sediment and evaporites deposited in sheltered coastal waters).

Marine environments range from shallow continental shelves (where carbonate sediments, shell beds, and fine-grained clastics accumulate in warm, sunlit waters) to the deep ocean floor (where extremely fine clay and the microscopic shells of planktonic organisms settle slowly through the water column to form deep-sea oozes and pelagic clay). Turbidity currents, underwater avalanches of sediment-laden water triggered by earthquakes or slope failures, can transport coarse sediment far out onto the deep ocean floor, producing distinctive graded beds called turbidites. Each depositional environment leaves a characteristic signature in the sediment: specific grain sizes, sedimentary structures, fossils, and mineral compositions that allow geologists to reconstruct ancient environments from the rock record.

Lithification: Turning Sediment into Rock

Lithification is the process by which loose sediment is converted into solid sedimentary rock. It involves two primary mechanisms: compaction and cementation. As sediment accumulates, the weight of overlying layers compresses the deeper sediment, squeezing out pore water and reducing the space between grains. Compaction is most effective in fine-grained sediments like mud and silt, which can lose 50 percent or more of their original volume during burial. Clay particles, which are flat and platy, align parallel to each other under compressive stress, giving the resulting mudstone or shale its characteristic tendency to split into thin layers.

Cementation occurs when dissolved minerals precipitate from groundwater flowing through the pore spaces between grains, binding them together. The most common cements are calcite (calcium carbonate), silica (silicon dioxide), and iron oxides. The type of cement affects the hardness and durability of the resulting rock. Silica-cemented sandstone is extremely hard and resistant to weathering. Calcite-cemented sandstone is softer and dissolves in acidic water. Iron oxide cement gives rock a characteristic reddish or yellowish color. The cementation process can take thousands to millions of years, depending on the rate of groundwater flow and the concentration of dissolved minerals.

Sedimentary Structures

Sedimentary rocks contain a variety of structures that record the conditions under which the sediment was deposited. Bedding (stratification) is the most fundamental sedimentary structure, consisting of horizontal or near-horizontal layers that reflect changes in sediment supply, grain size, or composition over time. Cross-bedding consists of inclined layers within a bed, formed by the migration of sand dunes, ripples, or delta fronts. It indicates the direction of current or wind flow at the time of deposition. Ripple marks are small-scale wave-like features on bedding surfaces, produced by water or wind currents. Symmetric ripple marks indicate oscillating wave action, while asymmetric ripple marks indicate unidirectional current flow.

Mud cracks form when wet mud dries and shrinks, creating polygonal patterns that are preserved when the next layer of sediment buries them. They indicate subaerial exposure and are common in tidal flat and playa lake deposits. Graded bedding, in which grain size decreases from coarse at the bottom to fine at the top of a single bed, records a single event of decreasing energy, typically a turbidity current or flood. Trace fossils, including burrows, tracks, and feeding traces, record the activity of organisms on and within the sediment surface. These structures, combined with the grain size, composition, and fossil content of the rock, allow geologists to reconstruct in remarkable detail the environments that existed at the Earth surface millions or billions of years ago.