Notes from 6.2 Types of Sedimentary Rocks pages 141-144
Sedimentary rocks are classified by their mode of formation.
Clastic Sedimentary rocks – these are the most common and are formed from the abundant deposits of
loose sediments that accumulate on Earth’s surface.
Clastic means broken
Coarse-grained sedimentary rocks contain gravel sized rock and mineral
fragments
a. Conglomerates – have rounded, gravel-sized particles this is because
mountain streams, flooding water, ocean waves, and glacial melt-water have
a high energy flow and can carry larger pieces of sediment. The gravel
becomes rounded and abraded as the pieces scrape off each other. This is
why beach and river gravel-rocks have well rounded edges.
b. Breccias – are composed of angular, gravel-sized particles. This is
because the sediments did not have time to become rounded. Therefore the
particles were transported a shorter distance.
Medium-grained sedimentary rocks have sand-sized rock and
mineral fragments. These are often found in stream and river
channels, beaches and deserts. Sandstone can be used to map
ancient river and stream channels. These rocks are called
sandstone.
Porosity is the percentage of open space between grains in a rock. Sandstone
have porosities of up to 30%.
Fine-grained sedimentary rocks consist of silt and
clay-sized particles. These can be found in swamps and ponds which have
slow moving water. The sediments settle at the bottom and form thin layers
and have a very low porosity. Shale is an example
Biochemical sedimentary rock are rocks that have the remains of once
living organisms in them. These are most common in limestone.