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