Sedimentary Rocks, Part 1: Clastic rocks

I. Sedimentary rocks form from the interaction of rocks at Earth's surface with water and atmosphere. The deposition of mechanically and chemically weathered particles following erosional processes leads to the formation of detrital or clastic sedimentary rocks. The second major type of sedimentary rock is chemical sedimentary rock.
A. Detrital sediments become sedimentary rocks through the process of lithification. Lithification occurs through two processes.
1. Compaction reduces the pore space between sediments by the overburden, or weight of overlying sediment.
2. Cementation occurs when H2O solutions precipitate minerals in the pore spaces. Some common cements:
a. calcite
b. quartz/silica
c. iron oxide
B. Detrital sedimentary rocks can be classified by grain size

Size Range Particle name Sediment name Detrital rock
gravel breccia or
1/16-2 sand sand sandstone
mud shale or

C. Grain texture gives clues to the energy present in the environment of deposition.
1. Sorting means % of grains with a given particle size. If more grains in a rock are of the same size, the rock is better sorted. For instance:
a. 40% pebbles, 30% sand, 30% silt is a poorly sorted rock
b. 100% sand is a well-sorted rock
2. Grain shape refers to the roundedness of the grains
a. Angularity -- how rounded are the angular fragments?
b. Sphericity -- how spherical are the grains?
D. What are common sedimentary features found in sedimentary rocks?
1. Layering
a. Bedding planes = depositional surface
b. Ripple marks from waves or currents
c. Crossbeds = sloping layers inclined to bedding planes
d. Graded beds = larger fragments on the bottom of a layer, fining upward.
d. Mudcracks from drying of the depostional surface
2. Fossils of organisms that live on or in the sediment layer

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