Sediment: broken rock material. Rocks are continually being weathered, eroded, transported and deposited by a variety of processes. The end result is sediment, or the small rock particles remaining after weathering, erosion, transportation and deposition.
Sediment particles come in different sizes:
Boulders: > 256 mm in diameter (larger than a basketball)
Cobbles: 64 mm - 256 mm in diameter (between a tennis ball and a basketball)
Pebbles: 2 mm - 64 mm in diameter (between a pea and a tennis ball)
Sand: 1/16 mm - 2 mm in diameter
Silt: 1/256 mm - 1/16 mm in diameter
Clay: <1/256 mm in diameter
Sediments are continually laid down in piles or layers by geologic processes and agents including streams, wind, ocean currents, glaciers. Sediment is transformed into rock through the process of Lithification.
Sediments are lithified in a number of ways:
1) Cementation: Water flowing through a sediment pile contains dissolved chemical compounds. Over time these dissolved compounds are precipitated (deposited) in the empty spaces (pore spaces) between the individual sediment grains. The chemical compounds act like a glue or cement to hold the sediment grains together. Two common cementing compounds:
Silica: very hard
Calcium Carbonate: easily dissolved after formation
Cementation commonly lithifies sediments that are sand sized or larger.
2) Compaction and Desiccation: Fine grained sediments (silts and clays) become compressed by overburden and the water between the individual grains is squeezed out and dried. The overburden pressure acts to compact the sediment transforming it into rock. Compaction and desiccation commonly lithifies silts and clays.
3) Precipitation: Groundwater flowing through cracks and crevices underground contains dissolved chemicals. Under the right conditions these chemicals precipitate out of the groundwater and are deposited into the cracks and crevices. Over time these chemical deposits build up and a sedimentary rock is formed. Silica and calcium carbonate are the two most common types of dissolved chemicals that will form sedimentary rocks through precipitation.
Classification of Sedimentary Rocks:
You will learn to identify common sedimentary rocks in lab.
1) Clastic Sedimentary Rocks: rocks composed of sediment particles that have been lithified through cementation or compaction/desiccation.
Conglomerate: rock composed of large rounded particles (sediments larger than sand sized)
Breccia: rock composed of large angular particles (sediments larger than sand sized)
Sandstone: rock composed of sand sized particles
Siltstone (Mudstone): rock composed of silt sized particles
Shale: rock composed of clay sized particles
2) Bioclastic (Organic) Sedimentary Rocks: rocks composed of the remains of once living organisms.
Chert: composed of silicate shells of microscopic organisms
Chalk: composed of carbonate shells of microscopic organisms
Coquina: composed of large shell fragments cemented together
Limestone: Bioclastic Limestone and Fossiliferous Limestone contain fragments of
carbonate organisms
3) Chemical or Nonclastic Sedimentary Rocks: rocks formed from precipitation of chemicals
Limestone: calcium carbonate material precipitated out of solution: cave deposits,
oolitic limestone
Geodes: silicate mineral crystals deposited in a void
Agate: silicate material
Halite: salt deposits
Sedimentary rocks are common on the Earth's surface. They are important because they provide valuable information about the Earth's past. Fossils are common in sedimentary rocks and geologists can use sedimentary rocks to interpret the Earth's past.
Sedimentary Structures: features preserved in sedimentary rocks that provide clues to the environment in which the sediment forming the rock was deposited. Was the sediment deposited by wind on a dune? by a stream? was there a current in a body of water?
Ripple Marks: wavy features that indicate current direction in a body of water when
the sediment was deposited.
Crossbedding: Criss-cross lines in the rock that indicated layering in a sand dune, delta
or stream deposit.
Mud Cracks: Cracks preserved in fine grained sedimentary rock, cracks formed when
fine sediment dried out.
Rain drop impressions: Preserved rain drop marks in fine grained sediment.
LINK TO THE ROCK CHART FOR LAB
