SEDIMENTARY ROCKSSedimentary rocks form through the accumulation of sediments - which can be both clastic (bits of other rocks) or chemical (mineral grains that precipitate out of surface waters). All sediments form through process-es we can generally classify as ‘weathering’ processes. We will spend a good amount of our class thinking about sedimentary rocks and associated topics. For now, focus on using the information below to get yourself familiar with some common sedimentary rocks and minerals.

Clastic (Detrital)

Sedimentary Rocks

Chemical (Biochemical)Sedimentary

Rocks

Grain Size Typical Minerals Rock Name

Mineralogy Rock Name

Coarse Grained (> 2 mm)

Medium Grained (1/16 - 2 mm)

Fine Grained (< 1/16 mm)

quartz, feldspars, pieces of other rocks (lithics)

pieces of other rocks (boulders, cobbles,

pebbles) + sand

clay minerals, quartz

Conglomerate or Breccia

Sandstone

Shale

calcite

dolomite

halite

gypsum

microcrystalline quartz

limestone

dolostone

rock salt

rock gypsum

dolostone

TABLE 1. CLASSIFICATION OF COMMON SEDIMENTARY ROCKS

KEY TERMS AND CONCEPTS:The term ‘clastic’ comes from siliciclastic - and refers primarily to the mineralogy. Siliciclastic rocks are named and differentiatied by grain size, as you see above. Grain size varies within each clastic category shown above - so when we talk about sandstones sometime we will say “coarse-grained” or “fine-grained” as a modifier to describe the sand grains.

Calcite and dolomite are both carbonate minerals, so we will talk about them broadly in the sense of carbon-ate sedimentary rocks, which includes both limestone and dolostone.

Cementation: To turn sediment into a sedimentary rock you need minerals to precipitation out of the waters within pore spaces between adjacent grains of sediment. The minerals that precipitate are called ‘cement’. Common cements include carbonate minerals, quartz, and sometimes iron oxides (hematite).