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Geology - Sedimentary Rocks

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    Study Guide Geology – Sedimentary Rocks 1. Clas Ɵ c Sedimentary Rocks Clastic sedimentary rocks form from pieces of pre - existing rocks that were broken apart, transported, deposited, and then compacted and cemented together. Geologists classify these rocks mainly by grain size and by the types of rock fragments they contain. 1.1 Grain Size, Distance, and Transport Grain size depends largely on how far sediment traveled before it was deposited. • Short travel distance → particles stay large and angular • Long travel distance → particles become smaller and more rounded As rock fragments move, they bump into each other and wear down. This smoothing process is called rounding .

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    Study Guide 1.2 Sor Ɵ ng of Sediment Sediment is also affected by sorting , which describes how well sediment grains are separated by size. • Large, coarse, angular fragments settle close to the source • Sand - sized particles travel farther before settling • Silt, mud, and clay travel the farthest and settle in calm water In general, longer transport = better sorting and smaller grains . 1.3 Coarse - and Medium - Grained Clas Ɵ c Rocks 1.3.1 Breccia • Made of large, angular gravel - sized fragments • Deposited very close to the source • Angular shape shows little transport 1.3.2 Conglomerate • Made of large, rounded gravel - sized fragments • Indicates longer transport than breccia 1.3.3 Sandstone • A medium - grained rock • Made mostly of sand - sized particles • Most grains are quartz 1.3.4 Special Types of Sandstone • Quartz sandstone : at least 90% quartz

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    Study Guide • Graywacke : a “dirty” sandstone with more than 15% silt or clay , giving it a darker, speckled look • Arkose : a sandstone with over 25% feldspar grains 1.4 Fine - Grained Clas Ɵ c Rocks Fine - grained clastic rocks form from very small particles that usually settle in quiet water . 1.4.1 Shale • Made of silt and clay • Thinly layered and very smooth • Fissile , meaning it splits easily along layers 1.4.2 Siltstone • Made mostly of silt - sized grains • Looks similar to shale • Not as fissile (does not split as easily) 1.4.3 Mudstone • Finest - grained clastic rock • Contains more clay • Poorly layered compared to shale and siltstone Most shale, siltstone, and mudstone are tan, brown, gray, or black . 2. Chemical Sedimentary Rocks Chemical sedimentary rocks form when minerals crystallize out of water . This can happen when water evaporates or when chemical and biological processes cause minerals to settle and harden.

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    Study Guide Unlike clastic rocks, these rocks are not made from broken rock fragments , but from dissolved materials . 2.1 Limestones Limestone is the most abundant chemical sedimentary rock . It is made mostly of the mineral calcite (CaCO ₃ ) and usually forms in shallow, warm seawater . Most limestone forms through biochemical processes , meaning living organisms play a big role: • Corals and algae are especially important limestone builders. • Many marine organisms use calcite to make shells or skeletons. • When these organisms die, their remains settle on the sea floor and slowly turn into limestone. Special Types of Limestone • Oolitic limestone Forms in shallow ocean water where waves roll tiny grains of calcite into small, sand - sized spheres called oolites . These build up in tidal zones. • Coquina Made from large, broken shell fragments that have been cemented together. • Bioclastic and skeletal limestone Contain a wide mix of shell fragments and fossils, ranging from fine to coarse grains. • Chalk A very fine - grained limestone made from the microscopic skeletal remains of tiny marine organisms that slowly settled to the ocean floor. Because these limestones form from shell fragments, they can be thought of as chemical, clastic, and organic sedimentary rocks at the same time.

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    Study Guide 2.2 Dolomites (Dolostones) Many limestones are later changed into dolomite , also called dolostone . This transformation happens during the early stages of: • Compaction • Loss of water (dewatering) • Lithification (turning sediment into rock) The process is called dolomitization : • Calcium in limestone is partially removed • Magnesium - rich fluids replace it • The new mineral formed is dolomite , with the formula CaMg(CO ₃ ) ₂ 2.3 Cherts Chert is a hard, glassy rock made of silica that crystallized from water. Common varieties of chert include: • Flint • Agate • Jasper Chert often forms as: • Nodules , also called geodes , inside limestone • These nodules fill cavities that once held gas or organic material • Chert can also form as thin layers within sedimentary rock beds Chert is also a major part of iron formations , layered rocks that formed in shallow seas long ago. These rocks are extremely important because they are a major source of iron worldwide.

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