Sedimentary Rocks

Sedimentary rocks form at or near the Earth's surface from accumulated sediment, and because they preserve fossils and depositional features they are the principal archive of Earth's surface history.

Definition

A sedimentary rock is a rock formed at the Earth's surface by the deposition and lithification of weathering products, mineral precipitates, or organic material, divided mainly into clastic, chemical, and biochemical types.

Scope

This topic introduces sedimentary rocks from a general-geology standpoint: the main classes of clastic, chemical, and biochemical rocks, the processes of deposition, compaction, and cementation that turn sediment into rock, and the features that make these rocks the record of past environments. Detailed stratigraphic interpretation is treated under sedimentology-and-stratigraphy.

Core questions

• What are the main classes of sedimentary rock?
• How is loose sediment turned into solid rock?
• Why do sedimentary rocks preserve fossils and records of past environments?

Key theories

Clastic, chemical, and biochemical classification

Sedimentary rocks are grouped by origin: clastic rocks from physical fragments, chemical rocks from minerals precipitated from solution, and biochemical rocks from the remains of organisms, each with characteristic textures and components.

Lithification

Loose sediment becomes rock through compaction under burial and cementation by minerals precipitated in the pore spaces, a set of diagenetic processes that also reduce porosity and permeability.

Mechanisms

Weathering and erosion produce particles and dissolved ions that are transported and deposited. Burial compacts the sediment and expels water, while minerals precipitating from pore fluids cement the grains together. Chemical and biochemical rocks form where minerals precipitate directly from water or accumulate from organisms. Sedimentary structures and fossils generated during deposition are preserved through lithification.

Clinical relevance

Sedimentary rocks hold nearly all oil, gas, coal, and groundwater, host many evaporite, phosphate, and iron resources, and provide the fossil and environmental record used to reconstruct past life, climate, and sea level.

History

The systematic study of sedimentary rocks developed from nineteenth-century stratigraphy and twentieth-century petrography. Classification schemes such as Folk's, together with process studies of modern sediments, established the modern understanding of how these rocks form and what they record.

Key figures

• Robert L. Folk
• Robert Siever
• Johannes Walther

Seminal works

• folk1974
• boggs2014

Frequently asked questions

Why are most fossils found in sedimentary rocks?

Sedimentary rocks form at the surface from sediment that buries organisms gently, and their relatively low temperatures and pressures preserve remains and traces that would be destroyed by the heat and deformation involved in forming igneous and metamorphic rocks.