Chapter 3.3 Sedimentary Rocks

Sedimentary Rocks The word sedimentary comes from the latin word sedimentum, which means “settling”. Sedimentary rocks form when sediments settle out and are deposited, usually from water, but occasionally from wind or glaciers.

Weathering, Erosion, and Deposition The first three processes involved in the formation of Sedimentary Rocks are Weathering, Erosion, and Deposition. Weathering means to break rock down into smaller parts called sediment. Erosion is when the broken rock fragments are carried away by water, wind, or ice (glaciers). Deposition is when the water, wind, or ice slows down enough to deposit the sediment.

Weathering, Erosion and Deposition This River is Eroding away the Sediments of this Canyon Example of Weathering Sediment is being Deposited By this River Delta Environments of Deposition Summary of the Process

Deposition and Sorting Because gravity plays a factor when sediments are transported by water, wind, and ice, the heaviest and largest sediments settle out first and the smallest settle out last. This causes a process in geology known as sorting.

Compaction and Cementation After the sediments become deposited, they must first get compacted or squeezed by the pressure and weight of more sediment on top. Compaction squeezes the water out, leaving behind minerals in the spaces left behind. Those dissolved minerals act like a glue to cement the sediments together.

Compaction and Cementation

Classification of Sedimentary Rocks Sedimentary Rocks are grouped by Geologists into two different groups based on the way they were formed. Clastic Sedimentary Rocks are made up of broken rock fragments called clastics. Chemical Sedimentary Rocks are usually formed when dissolved minerals separate out of water. But they can also be formed by biological processes.

Clastic Sedimentary Rocks Clastic Sedimentary Rocks can further be classified based on grain size. Clastic Sedimentary Rocks, classified by grain size can also be subdivided by grain composition, maturity and sorting.

Classification of Clastics by Grain Size, Maturity, Composition and Sorting

Classification of Clastics

Clastic Sedimentary Rocks Breccia Quartz Sandstone Conglomerate Arkose Sandstone – sand grains mixed with feldspar Siltstone – Feels like fine sandpaper Shale – Very fine sandpaper with the consistency of flour

Chemical Sedimentary Rocks Chemical Sedimentary Rocks are a little more complicated than the clastics. Chemical Sedimentary Rocks with the mineral calcite can be further classified by texture. Crystalline Limestone and Travertine have a coarse crystalline texture. Coquina, Fossiliferous Limestone, and Chalk all have seashell of various sizes mixed in them.

Chemical Sedimentary Rocks made of the mineral Calcite Coquina – Loosely Compacted shells cemented together Crystalline Limestone Travertine The nice thing about all of these rocks is that all of them react with acid, because they are made of the mineral calcite. Fossiliferous Limestone Chalk

Other Chemical Sedimentary Rocks Flint and Chert are both made of Quartz so they are very hard and can scratch glass. Conchoidal fracture is also a property that can help identify both Flint and Chert. Gypsum and Rock Salt both have a crystalline texture, but only rock salt has a salty taste, and neither one of these reacts with acid. Bituminous Coal has a dark black color.

Other Chemical Sedimentary Rocks Chert Chert and Flint Arrowheads Gypsum Rock Salt Bituminous Coal Coal Seams in Alaska