Sedimentary Rock Chapter 10.3

Sedimentary Rocks Sedimentary Rock Sedimentary rocks are formed when sediment deposits harden after being compressed and cemented together. Two process that form sedimentary rock are compaction and cementation.

Sedimentary Rock Compaction is when the weight of overlying sediments causes pressure, pushing the fragments together and squeezing out air and water from the fragments. Cementation is when water carries dissolved minerals through the sediments, the minerals are left between the fragments of sediment and provide a cement to hold the fragments together.

Compaction & Cementation

Sedimentary Rock Classification Sedimentary rocks are classified according to the kind and size of sediments that form them. Scientists have formed three classifications of sedimentary rock: Clastic Sedimentary, Chemical Sedimentary, & Organic Sedimentary.

Clastic Sedimentary Rock Clastic sedimentary rock forms when rock fragments carried away from their source by water, wind, or ice are left as deposits and compacted or cemented over time. A conglomerate is a clastic sedimentary rock composed of rounded gravel-sized fragments, or pebbles that are compacted or cemented together.

Conglomerates

Clastic Sedimentary Rock Breccia is a clastic sedimentary rock composed of angular, or rocks with sharp corners, that are compacted or cemented together. Sandstone is made mainly of sand-sized quartz grains that have been cemented together, but have large enough pores between the grains that water and crude oil can flow. Shale consists of flaky, clay-sized clay particles cemented and compacted under pressure.

Breccia, Shale, & Sandstone

Chemical Sedimentary Rock Chemical sedimentary rock form from minerals that were once dissolved in water. These minerals can precipitate (form a solid) out from water or settle out of the water to form chemical sedimentary rocks.

Chemical Sedimentary Rock Chemical limestone forms when cool currents lower the temperature of warm ocean water, calcite precipitates, settles, and solidifies on the ocean floor.

Chemical Sedimentary Rock Evaporates are dissolved minerals that are left behind when water evaporates and form rocks. Gypsum and halite, or rock salt, are two examples of rock formed from rapid evaporation. A large evaporate structure is the Bonneville Salt Flats in Utah.

Bonneville Salt Flats

Organic Sedimentary Rock Organic sedimentary rocks are formed from the remains of once living creatures. Coal and some limestone are examples of organic sedimentary rock. An example of organic limestone is chalk, which originally forms as mud on the floor of an ocean. The white cliffs of Dover, England are made up of chalk.

white cliffs of Dover, England

Sedimentary Rock Features Sedimentary rocks have many identifiable features which include stratification, ripple marks, mud cracks, fossils, and concretions. Stratification is the layering of sedimentary rocks that occurs when there is a change in the kind of sediment being deposited.

Stratification

Sedimentary Rock Features The strata, or layers, change for many reasons such as changes in river currents, sea level, and wind deposits. Graded bedding occurs when various sizes and kinds of materials are deposited within one layer.

Graded Bedding

Sedimentary Rock Features Ripple marks are formed by the action of wind or water on sand, and when the sand becomes sand stone the ripples may be preserved. Mud cracks result when muddy deposits dry, shrink, and crack. Fossils are the remains or traces of ancient plants and animals. Fossils are usually preserved in sedimentary rock, because of the way the sediments slowly cover, compact, and cement the fossil.

Sedimentary Rock Features Concretions form when minerals precipitated from solutions build up around an existing rock particle. Concretions can be formed from groundwater depositing dissolved quartz or calcite inside cavities in sedimentary rock leaving crystals behind. The crystal cavities are called geodes.