8 Sedimentary rocks

Formation

• Old igneous, sedimentary or metamorphic rocks are broken up by weathering, carried by transporting agents to depositional basins, and deposited in parallel layers one over another.

Characteristics

• They cover about three quarters of the crust in thin layers, but make only $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">5</span>}\mathrm{\%}$ of the crust by volume.
• Most have economic value: limestone, phosphate, coal, iron deposits and sandstone.
• Muddy rocks are where oil, natural gas and kerogen form; porous rocks such as sandstone and limestone store oil, gas and groundwater.
• Their types are few; mudstone, sandstone and limestone make about $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">90</span>}\mathrm{\%}$ of them.

Types by mode of formation

• Clastic (detrital) rocks: classified by the dominant size of the fragments, lithified by cementing material.

• Chemical sedimentary rocks: salts dissolved in water precipitate because of evaporation (higher concentration) or chemical reactions.
• Calcareous rocks: limestone and dolomite.
• Silicate rocks: flint.
• Evaporites: gypsum (hydrated calcium sulphate), anhydrite (calcium sulphate) and rock salt (halite, NaCl).
• Biogenic (biochemical) rocks: from living organisms.
• Fossiliferous limestone: marine organisms build shells and skeletons of calcium carbonate taken from seawater; the remains accumulate after death.
• Phosphate rocks: fossilised remains of marine vertebrates that contain phosphate, plus phosphate minerals.

Energy resources in organic sedimentary rocks

• Coal: plants are buried away from oxygen for a long time (for example in swamps behind river deltas); the tissues lose volatile parts and carbon becomes concentrated.
• Oil and gas: hydrocarbons formed from the remains of marine micro-organisms buried with fine muddy sediments away from air.
• The source rocks are matured at $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">2</span>}$ to $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">4</span>}$ km depth at $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">70</span>}$ to $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">10</span>}{\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">0</span>}}^{\circ }$C; the hydrocarbons then migrate and gather in porous reservoir rocks (sand, sandstone, sometimes limestone).
• Oil shale: a muddy rock rich in solid waxy kerogen of mainly plant origin, which turns into oil when heated to about $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">48</span>}{\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">0</span>}}^{\circ }$C; it is kept as a reserve.