1.4 - Sedimentary Rocks
Sedimentary rocks are formed via the breakdown and redeposition of other materials, often older rocks. Sedimentary rocks are classified by their dominant particle size (sand, silt, or clay) and their mineral composition. Classic sedimentary rocks (i.e., sandstone or shale) are composed of minerals, grains and rock fragments derived from older rocks; while biogenic sedimentary rocks are made up of shells, their recrystallized remnants (i.e., limestone ) or plant fragments (i.e., coal). In general, the environment in which particles are deposited determines their grain size and mineral composition. On a beach or near the shoreline, chemical weathering (Lesson 2.3) and physical weathering (Lesson 2.2) are continuously breaking down mineral grains. Near-shore sediment tends to be well sorted sand, consisting mainly of quartz, which is very resistant to all types of weathering. Farther offshore, where water currents slow down, smaller particles can drop out of the water and form siltstones or shales. Sedimentary rocks make up the source material for more soils than igneous rocks do, because sedimentary rocks are more common at the earth’s surface.
Sandstone Properties
Specimen of standstone. Image by C. Geiss
Type: Sedimentary
Distinguishing Features: Made up of relatively coarse particles, which can often be seen without a hand lens; feels gritty, like sandpaper; sedimentary structures can be seen, such as layering or fossils. Layering is often hard to see in a hand specimen, but very noticeable in the outcrop (see image of sedimentary rocks in Canyonlands National Park, UT below).
Sedimentary rocks in Canyonlands National Park, UT. Image by C. Geiss
Main Minerals: quartz, some 'dirty' sandstones contain feldspars, muscovite
Weathering Behavior: Can erode easily depending on the mineral composition of the cement which holds single grains together; affected by chemical and physical weathering; individual grains often very resistant to weathering (i.e., quartz); physical weathering can crack rock along bedding planes (see image below).
Rock cracked along bedding plane. Image by C. Geiss
Impact on Soils: Usually low fertility (dominated by quartz), high permeability (sand).
How it Forms: Deposition of sediment on beaches, sand dunes, stream valleys.
Shale Properties
Specimen of shale. Image by C. Geiss
Type: Sedimentary
Distinguishing Features: Very fine grained, composed of clay-sized minerals, often breaks into platy fragments, some contain fossils.
Main Minerals: quartz
Weathering Behavior: Small particle size and poor cementation leads to rapid physical and chemical weathering. The image below shows weathering of sandstone and shale. The steep cliffs are made up of weathering-resistant sandstone, while the slope at the base of the cliff is composed of rock units containing a larger abundance of shale.
Weathering of sandstone and shale. Image by C. Geiss
Impact on Soils: Rapid disintegration generally leads to deep soils, high in clay-size particles, so slow permeability for water.
How it Forms: Shale forms by deposition of sediment in low-current environments, such as lakes or along ocean shores in deep water not affected by waves.
Limestone Properties
Specimen of limestone. Image by C. Geiss
Type: Sedimentary
Distinguishing Features: Relatively soft, can easily be scratched with a nail or pocket knife, reacts to hydrochloric acid (fizzes), often contains fossils.
Main Minerals: calcite, other carbonates
Weathering Behavior: Strongly affected by chemical weathering, leading to rounded edges (see image below).
Chemical weathering leading to rounded corners. Image by C. Geiss
Impact on Soils: Abundance of calcite makes for alkaline soils, which do not acidify rapidly.
How it Forms: In shallow oceans or lakes, by deposition of animal shells or by precipitation from solution.