Question

How are the three types of sedimentary rocks different in their formation?

Answer 1

Sedimentary rocks form when loose particles, dissolved minerals, or organic matter are deposited and then buried and hardened. The three main types—clastic (detrital), chemical, and organic (biochemical)—differ in what is deposited and how it becomes rock:

1. Clastic (detrital) sedimentary rocks
- What forms them: fragments (clasts) of preexisting rocks produced by physical weathering (gravel, sand, silt, clay).
- How they form: clasts are transported by water, wind, or ice, deposited, then buried and turned into rock by compaction and cementation (minerals like silica or calcite precipitate and glue grains together).
- Typical textures and traits: grain-size controlled (conglomerate/breccia for coarse, sandstone for sand, siltstone, shale for fine), often well-sorted or poorly-sorted, may show layering and sedimentary structures (cross-bedding, ripple marks).
- Common environments: rivers, beaches, deserts, glacial deposits, deltas.
- Examples: conglomerate, sandstone, shale.

2. Chemical sedimentary rocks
- What forms them: minerals precipitated directly from solution (water) when chemical conditions change (evaporation, cooling, or chemical saturation).
- How they form: dissolved ions concentrate until minerals crystallize out of the water; mineral layers accumulate and lithify.
- Typical textures and traits: crystalline or massive texture, often lack clastic grains, may form evaporite beds or crystalline limestones.
- Common environments: evaporating inland seas and lakes (salt, gypsum), marine settings where chemical precipitation occurs, hot springs.
- Examples: rock salt (halite), gypsum, some limestones (when calcite precipitates chemically), some cherts.

3. Organic (biochemical) sedimentary rocks
- What forms them: the remains or biochemical products of living organisms (shells, skeletal debris, plant material).
- How they form: biological particles accumulate (e.g., shell fragments, plankton tests, plant debris), are buried, and are compacted and cemented; organic matter may transform into coal by heat/pressure.
- Typical textures and traits: can contain fossils or be composed largely of organic or carbonate skeletal material; may be fine-grained (chalk) or massive (coal seams).
- Common environments: shallow marine shelves and reefs (carbonate sediments), deep ocean where plankton accumulate, coastal swamps and peat bogs (coal precursors).
- Examples: fossiliferous limestone, chalk, coquina, coal.

Note: there is overlap—limestone, for example, can form both chemically (direct calcite precipitation) and biochemically (accumulation of shell debris). The key difference is whether the solid material originated as clastic rock fragments, chemical precipitation, or biological remains.