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-Stage 1.5-

Sedimentary Rocks

 

Sedimentary Rock Cycle

Definition - Sedimentary Rocks form from the cementation of fragments of other rocks including igneous, metamorphic and other sedimentary rocks. They also form from the cementation  and precipitation of animal byproducts.

 

 

Sedimentary Rocks Page Layout

 


Categories of Sedimentary Rocks


Identifying Clastic Sedimentary Rocks

 

Grain Size Chart

 


Basic Types of Sedimentary Rocks

 

Types of Rocks

Grain Size

Roundness Sorting Rock Name

Clastic

Clay Well Rounded - Angular Very Well Sorted Shale (Layered)
Claystone (Massive)
Silt Well Rounded Very Well Sorted Siltstone
Sand Well Rounded Very Well Sorted Quartz Arenite
Angular Well Sorted Arkose Sandstone
Sub-Rounded Well Sorted Lithic Sandstone
Gravel Angular Poorly Sorted Breccia
Well Rounded Poorly Sorted Conglomerate

 

Types of Rocks Principle Mineral Diagnostic Properties Rock Name
Chemical Biogenic Quartz Scratches glass Chert
Carbon Leaves black film on hands (looks like coal) Coal
Calcite/ Dolomite/ Aragonite Contains shells and other fossils (Reacts with acid) Fossiliferous Limestone
Calcite/ Dolomite/ Aragonite Contains loosely cemented shells with little matrix (Reacts with acid) Coquina
Calcite/ Dolomite/ Aragonite Able to write on a chalkboard (Reacts with acid) Chalk
Calcite/ Dolomite/ Aragonite Reacts with acid but has no visible crystals Micrite
Precipitate Calcite/ Dolomite/ Aragonite Contains large crystals of calcite (Reacts with acid) Crystalline Limestone
Calcite/ Dolomite/ Aragonite Is made up of tiny spheres (Reacts with acid) Oolitic Limestone
Calcite/ Dolomite/ Aragonite Often layered with no crystals (Reacts with acid) Travertine
Gypsum Easily scratched by a fingernail Rock Gypsum
Halite Tastes salty Rock Salt

 

Petrogenesis (Creating Rocks)

 

Sedimentary Rocks Formation Locations

 

Sedimentary Rock Formation Localities

 


Clastic Sedimentary Rocks

Most Clastic Sedimentary Rocks are formed in or near standing water due to the processes of running water. Here are some descriptions of Clastic Sedimentary rocks starting with depositional environments at the start of the running water cycle:

 

Breccia - Breccias often form near mountains, where landslides occur. The landslides pile up large piles of debris that contain large and small angular rock fragments. The large angular fragments are sometimes cemented into a rock. Other times the debris is carried downstream.

 

Breccia

 

Conglomerate - Conglomerates form along braided rivers where the debris from the mountain peaks are rounded in the streams and sorting starts to take place. Usually the large pebbles are left behind while smaller debris is carried further downstream.

 

Conglomerate

 

Sandstone - There are multiple varieties of sandstone that are identified by the type of sediment in the area and how long the sediment has been transported.

Arkose Sandstone (Arkose) - Arkose is defined as having at least 25% Potassium Feldspar (K-Spar) causing the rock to be generally pink or red. They form in close relation to granites which have high amounts of K-Spar and are often associated with breccias. Since K-Spar breaks down fairly easily, arkose is typically found closer to the source.

 

Arkose

 

Lithic Sandstone - Formed a little downstream from Arkose. This results in the sediment content to have a higher ratio of rock (lithic) fragments.

 

Lithic Sandstone

 

Quartz Arenite (Quartz Sandstone) - Out of all the sandstones this is the one that has been transported the furthest. Usually consisting of >90% quartz. This is because quartz is very stable and very hard, preventing it from breaking down as fast as most other common minerals. Typically this rock forms in beach or desert environments, where quartz sand is abundant. The farther offshore you go the smaller the sand in the sandstone.

 

Quartz Sandstone

Mudstone - Generic term for a rock with both clay and silt. It is often massive when used as an identification term but it can be used as an all encompassing term for siltstones, shales, and claystones.

Siltstone - Siltstone forms in a couple of places but is usually rare. One place is along river banks. When a river floods, it drops the largest debris first directly next to the river creating a natural levee. This levee is often made up of silt. Another place is farther out on the continental shelf, beyond the formation of the sandstone but before the shale in the open ocean. The grains are not visible to the naked eye, but when rubbed against one's teeth it feels gritty, where as shale (clay) will feel smooth.

 

Siltstone

 

Shale - Shale typically forms in one of two different places. 1. The open ocean - due to settling of clay in the calm ocean water far from shore. The clay is carried there by rivers and streams draining the continents. 2. Floodplains surrounding meandering rivers - During floods the water in rivers overflow their banks and settle along floodplains creating temporary lakes where the material being carried settles out. The finer the material the farther out is goes. Shale forms thin, flat, layers that often break apart very easily.

 

Shale

 

Claystone - Claystone forms typically in floodplains. Made up of clay, like shale, but it doesn't form into thin layers. Found mostly as thick deposits.

 

Claystone

 


Chemical Sedimentary Rocks

Most chemical sedimentary rocks are formed on or near standing water.

 

Chert - Chert forms in the open ocean from the shells of animals that make their shells from silica (quartz). Usually only occurs in deeper waters, which are too cold for carbonates to form.

 

Chert

 

Coal - Coal forms in areas where there are very high densities of plant matter, like swamps. Usually when plants die they degrade due to the action of bugs and microorganisms but in swamps there is so much plant life that there is not enough oxygen to support all the life needed to break the plant material down. This results in an anoxic environment (no oxygen) where plant material just builds up and doesn't break down, over time forming coal.

 

Coal

 

Limestone - Limestone is often formed from the carbonate shells of aquatic animal life, since most shelly animals makes their shells out of a variety of carbonates. Carbonates also react with acid, making it an easy thing to identify. Identifying the specific type of limestone is a little more complex though.

Fossiliferous Limestone - Formed usually on the continental shelf from the large shells of animals like clams and snails cemented together with microscopic calcium carbonate shells, called lime mud.

 

Fossiliferous Limestone

 

Coquina - Typically formed in beach like environments where the number of animal shells far exceeds the surrounding sediment. Although some definitions vary, the typical definition is that coquina is made up of loosely cemented shells with little to no matrix (cement).

 

Coquina

 

Chalk - Formed from the compaction of microscopic plates called coccoliths, which come from animals called coccolithophores. Similar to fossiliferous limestone except the rock is typically all white and contains only one type of fossil.

 

Chalk

 

Micrite - This is just fossiliferous limestone without the fossils. A rock composed of mostly lime mud. It forms a little further out from the shoreline than fossiliferous limestone where the mud can float out in the water but is generally too deep for shelled animal life.

 

Micrite

 

Crystalline Limestone - An inorganic type of limestone that usually forms in shallow lagoons or lakes. The water in the lagoon becomes saturated in calcite and then the water starts to evaporate causing calcite to precipitate out of the rock. This is common where water is periodically added to the lagoon or the lake after a lot of calcite is precipitated out. It is also found when other types of limestone are altered in some way.

 

Crystalline Limestone

 

Oolitic Limestone - This is another inorganic type of limestone where limestone mud is deposited along the continental shelf edge and the naturally movement of the water rolls the mud around creating little balls of calcite. These little calcite balls are then cemented together.

 

Oolitic Limestone

 

Travertine - This is the type of limestone formed in caves from the deposit of calcite along stalactites, stalagmites, and other flowstones. Since the structures are built up layer by layer, this causes the rocks to often have a banded appearance with often a lot of holes within it.

 

Travertine

 

Rock Gypsum - Similar to crystalline limestone except the water is supersaturated with gypsum instead of calcite. Usually formed in lagoons or shallow lakes. Often the environment is going to be hot and dry as well.

 

Rock Gypsum

 

Rock Salt - Similar again to Rock Gypsum or Crystalline Limestone except the water is supersaturated with halite (aka salt). Usually formed in lagoons or shallow lakes also (think Great Salt Lake). Often the environment is going to be hot and dry as well.

 

Rock Salt

 


Maturity

 

When classifying clastic sedimentary rocks, it is often easiest to think of them in levels of maturity. Rock maturity is how far the sediment in the rock has traveled before becoming cemented together. Think of the mountain top of the picture as the beginning and the ocean basin as the end. The further sediment is towards the end, the more mature it will be.

 

Sedimentary Rock Maturity

 

Immature - The sediment at the beginning. The grains usually contain large pebbles, sometimes even cobbles and boulders mixed together with fine sediment like silt, sand, and clay. The roundness of the grains can also vary from angular to rounded. Minerals include quartz, feldspars, micas, clays, and rock fragments. Rock types include Breccia usually.

 

Submature - The sediment just passed the beginning but usually still along the mountain side. The grains are smaller and/or rounder but still contain a variety of sizes. Grains are usually angular through well rounded. Minerals include all the immature minerals like quartz, micas, clays, and rock fragments. Rock types include Conglomerate, Arkose, and Lithic Sandstone.

 

Mature - The sediment along the end of the system. The grains are often sorted completely into different sizes where sand is found only with sand, silt with silt, and clay with clay. The minerals are often just quartz or clay. Rock types include Siltstone, Shale, and Quartz Arenite.