105 Lab - Sedimentary Rocks and Processes

Although sedimentary rocks are an extremely small portion of all rocks on the Earth; they are very common at the Earth’s surface. Sedimentary rocks form by the deposition and lithification of sediments or by the precipitation of minerals from aqueous solutions. Sediments are produced by the weathering of preexisting rocks— which can be igneous, metamorphic, or other sedimentary rocks. Weathering occurs through two basic means (or a combination of the two): (1) a rock can be broken into smaller fragments also referred to as clasts . This physical weathering occurs as rocks move downhill and crash into each other, when plant roots grow in cracks slowly widening the fracture, due to frost wedging during freeze/thaw cycles and by many other processes. (2) Chemical weathering is the process of dissolving the rock or altering the material by leaching or mineral transformations (for example, feldspars to clay minerals); in most cases water becomes mildly acidic and will react with the rock, but in some cases living organisms can produce acids which will dissolve the rock. If both physical and chemical weathering occur at the same time the rates of weathering increase significantly; physically breaking the rock creates more surface area for chemical weathering to act. Clastic sedimentary rocks are composed mainly of clasts of preexisting rocks. Clasts occur in many sizes and can be more angular or more rounded in shape. These clasts can be lithified (formed into a rock) by compaction and/or cementation. Identification of these sedimentary rocks is based mainly on the size of the clasts. With larger clasts; the shape of the clast will also be used. With smaller clasts the composition becomes more important and it will be used rather than the clast shape. Biochemical sedimentary rocks are composed of plant remains, shells, or fragments of shells which have been either compacted in the case of plants or cemented in the case of shells. In swamps, plant matter will accumulate over time burying older material deeper and deeper. Swamp waters and muds are extremely low in oxygen content, inhibiting decomposition and preserving the organic matter which is compressed and slightly heated converting it to coal as it is buried and lithified. In moderately shallow ocean environments, there can be an accumulation of shells, corals and algae which can be cemented together. Chemical sedimentary rocks are rocks which have been precipitated from a solution. Water has the capacity to dissolve materials and hold them in solution. When the water becomes saturated it cannot dissolve or hold any more material. If any water evaporates the solution becomes supersaturated and minerals will precipitate (crystallize) from the solution. Sedimentary Rocks GENERAL GEOLOGY 1

Classification Rock Texture Rock Name Clastic Rock and mineral fragments cemented or compacted Greater than 2 mm Angular Fragments Breccia Rounded Fragments Conglomerate 2 mm to 1/16 mm Mostly quartz grains Quartz Sandstone Mostly feldspar grains Arkose Mostly rock grains Lithic Sandstone Sand cemented with muds and clays Graywacke 1/16 mm to 1/256 mm Breaks into layers or blocks (feels gritty) Siltstone Less than 1/256 mm Breaks into blocks (feels smooth) Claystone Breaks into layers (feels Smooth) Shale Biochemical Plant fragments Brown porous rocks with visible plant fragments Peat Dull dark brown to black may have visible plant fragments Lignite Black brittle may be layered dull to slight gloss Bituminous Coal Shell fragments Mostly shell fragments with calcite cement Coquina Few shell fragments, impressions of shells in a calcite matrix Fossiliferous Limestone (Calcarenite) White compacted powder (reacts with HCl) Chalk White compacted powder (does not react with HCl) Diatomite Chemical Calcite Crystalline, micro-crystalline or spheres Mostly spheres about 2mm in diameter Oolitic Limestone Crystalline or micro-crystalline; may be porous; normally light colored, may have striped appearance Travertine Micro-crystalline dolomite Reacts to HCl when powdered (may react slightly when not powdered) Dolostone Crystalline halite Large cubic crystal (may be hoppered) tastes salty Rock Salt Crystalline or micro-crystalline Gypsum Color pink to tan to clear; can be scratched easily with fingernail Rock Gypsum Micro-crystalline quartz Tan to brown to grey to black in color typically mottled variations of those colors, hardness of 7.0, may break with conchoidal fracture Chert

Sample # Clastic, Chemical, Biochemical Composition Grain Size Sorting Rock Name 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

How do sediments move? Sediments formed by weathering are often moved hundreds of kilometers away from where they were formed. The carrying away of sediments is called erosion . Most sediments are moved by running water in streams and rivers. Over millions of years rivers and streams are able to erode entire mountains! As the water in a stream or river flows downstream, it carries different-sized sediments. The heavier sediments—boulders, sand, and gravel—are slowly pushed along the bottom of the riverbed. It can take hundreds of years for large boulders to make their way down from the mountaintop to the river’s mouth several hundred kilometers away. Lighter sediments, including plant remains, silt, and clay, float in the moving water. The floating sediments are suspended in the water. The mixture of water and floating sediments is called a suspension. In places where the velocity of the water slows down, the suspended sediments slowly start to sink and eventually settle to the bottom. Sediments are also moved by the wind. In deserts, the wind pushes sand around continuously to reshape sand dunes. When volcanoes erupt, many thousands of kilograms of ash and dust are sent into the air. These sediments can be carried halfway around the world by winds in the air! Eventually, as the winds lessen, the ash and dust settle on the surface of the earth. Types of Sediments Boulder Formed from mechanical weathering of rock. Pieces are > 15 cm in diameter. Pieces may be rounded or angular. Gravel Formed from mechanical weathering of rock. Pieces are 0.3 – 15 cm in diameter. Pieces may be rounded or angular. Sand Formed from mechanical weathering of rock. Pieces can be barely visible or up to 0.3 cm in diameter. They loosely stick together when wet. Silt Formed from mechanical and chemical weathering of rock. Individual particles are not visible. They stick together when wet and form mud. Pieces float in moving water, making water cloudy. Clay Formed from mechanical and chemical weathering of rock. Individual particles are not visible. Particles are more than 10 times smaller than silt particles. When dry, clay is powder. The particles stick together when wet, forming mud. Particles float in moving water, making water cloudy. Plant Remains Made of pieces of plant roots, stems, leaves, fruits, and flowers. Pieces have many sizes and are usually dark brown or black. Pieces float in moving water. Thick layers form muck. Animal Remains Made of seashells, tiny skeletons, bones, or teeth. Salts Formed from the evaporation of ocean water.

Q9: What causes the sediments to settle the way they do? Q10: What would happen if you left the Sedimentator upright overnight? Part IV: Sediments forming rocks 1. Leave the Sedimentator standing upright on one end. 2. Draw what you see inside the Sedimentator in the diagram on the right. 3. Label the sediments on the diagram. 4. Use the information from the sedimentary rock chart to find the rocks that are formed from the sediments in the Sedimentator. 5. On your diagram, write the rock name next to the sediment that forms the rock.