Formed by the compaction and cementation of sediments (small pieces of rock and living things).
Depositional Environment Sedimentary rocks can tell you what the environment was like when they formed. This is called the “Depositional Environment.” For example, a sandstone’s depositional environment was likely a beach or a desert since that is where you find sand today.
Where did these rocks form? Sandstone with cross-beddingMudstone with ripple marks Fossil FishFossil Plant Impressions
Types 2 major types: – A. Clastic: Made of smaller pieces of rocks called clasts. (sandstone for example) – B. Nonclastic (Either Chemical or Organic): Made of once living things or from evaporated/precipitated minerals. (Coal and Limestone for example).
Examples of Clastic Sediments Breccia Conglomerate Sandstone Shale
Texture of Sedimentary Rocks Grain Size Grain Type Sorting Grain Shape
Clastic Clastics - made from fragments of other rocks called clasts. These clasts are either compacted when sediments build up on top of them or they are cemented together by some sort of solution that acts like glue. Form layers that are easily seen with the naked eye.
Size Fine grained: Small, can’t see individual grains Medium grained: Sand sized, can see grains Coarse grained: Pebble sized, can easily see individual grains
Sorting Well Sorted: Grains are similar shape and size Poorly Sorted: Grains are all shapes and sizes Well or poorly sorted? Well or Poorly Sorted?
Fossils Sedimentary rocks are the most likely rock to have fossils. The fossil can tell you where the rock formed.
Brachiopod fossils in limestone Imprint of a leaf Trilobite fossils – over 250 MILLION years old!!! Trilobites
Nonclastics Made from chemical or organic processes A. Chemical: Precipitates - Minerals combine and settle out in the ocean. Examples include: limestone (CaCO3), dolomite (MgCO3), and chert (SiO2) & some ores like hematite concretions. -Limestone and Dolomite form in shallow, warm, tropical oceans. Chert forms in colder, deeper ocean waters. Evaporites - left behind when water evaporates. Rock Salt (NaCl)
Nonclastics B. Organic: Limestone – Formed in shallow, warm, tropical oceans from shells of organisms and corals reefs that are cemented or compacted together. Bituminous coal – Form from build up of dead plants. Formed in swamps and marshes. Coquina – Formed from cemented shells. Likely formed in a beach environment.