Chapter 7: Sediment and Sedimentary Rocks

Transcription

1 Chapter 7: Sediment and Sedimentary Rocks Chapter Outline 7.1 Introduction 7.2 Sediment Sources, Transport, and Deposition 7.3 How Does Sediment Become Sedimentary Rock? 7.4 Types of Sedimentary Rocks 7.5 Sedimentary Facies 7.6 Sedimentary Rocks The Archives of Earth History GEO-INSIGHT 7.1: Fossilization GEO-FOCUS 7.1: State Fossils 7.7 Sedimentary Rocks and Economic Geology Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. Sediments are deposited as aggregates of loose solids that may become sedimentary rocks if they are compacted and/or cemented. Geologists use texture and composition to classify sedimentary rocks. A variety of features preserved in sedimentary rocks are good indicators of how the original sediment was deposited. Sedimentary facies can be traced across geographic areas to help geologists determine the geologic history of a region. Most evidence of prehistoric life in the form of fossils is found in sedimentary rocks. Sedimentary rocks host important resources like petroleum and natural gas, uranium, and iron. Chapter Summary Detrital sediment consists of weathered solid particles, whereas chemical sediment consists of minerals extracted from solution by inorganic chemical processes and the activities of organisms. Sedimentary particles are designated in order of decreasing size as gravel, sand, silt, and clay. During transport, sedimentary particles are rounded and sorted. Major depositional settings are continental, transitional, and marine, each of which includes several specific depositional environments. Lithification takes place when sediments are compacted and cemented and converted into sedimentary rock. Silica and calcium carbonate are the most common chemical cements. Sedimentary rocks are classified as detrital or chemical:

2 a) Detrital sedimentary rocks consist of particles (gravel, sand, silt, and clay) derived from preexisting rocks. b) Chemical sedimentary rocks are derived from substances in solution by inorganic chemical processes or the activities of organisms. A subcategory called biochemical sedimentary rocks is recognized. Carbonate rocks contain minerals with the carbonate radical (CO 3 ) -2 as in limestone and dolostone. Evaporites include rock salt and rock gypsum, both of which form by inorganic precipitation of minerals from evaporating water. Coal is a type of biochemical sedimentary rock composed of the altered remains of land plants. Sedimentary facies are bodies of sediment or sedimentary rock that are recognizably different from adjacent sediments or rocks. Vertical sequences of rocks with offshore facies overlying nearshore facies form when sea level rises with respect to the land, causing a marine transgression. A rise in the land relative to sea level causes a marine regression, which results in nearshore facies overlying offshore facies. Sedimentary facies are bodies of sediment or sedimentary rock that are recognizably different from adjacent sediments or rocks. Vertical sequences of rocks with offshore facies overlying nearshore facies form when sea level rises with respect to the land, causing a marine transgression. A rise in the land relative to sea level causes a marine regression, which results in nearshore facies overlying offshore facies. Sedimentary structure, such as bedding, cross-bedding, and ripple marks help geologists determine ancient current directions and depositional environments. Fossils provide the only record of prehistoric life and are useful for correlation and environmental interpretations. Depositional environments of ancient sedimentary rocks are determined by studying all aspects of the rocks and making comparisons with present-day sediments deposited by known processes. Many sediments and sedimentary rocks, including sand, gravel, evaporates, coal, and banded iron formations, are important natural resources. Most oil and natural gas are found in sedimentary rocks. Enrichment Topics Topic 1. I Shall Return. In this hilarious article, the author describes how difficult it is to become a fossil, by trying to arrange to become one himself. Originally printed in Earth, April Topic 2. The Grand Canyon. Of course there s no better place to learn about sedimentary rocks, facies and ancient environments than at the Grand Canyon. Learning about each rock layer, the metamorphism of the basement, the structures that break up the layering, can all help students to understand how sedimentary geology is a window into earth history.

3 Topic 3. The Good and Bad of Methane Hydrates. Ice-like solids composed of gas molecules largely methane that have become trapped in the crystal lattice of water are known as gas hydrates. Gas hydrates that are abundant in the pore spaces of deep sea sediments and arctic permafrost could provide vast amounts of natural gas that could become important natural resources. Thousands of gigatons of methane are located in the oceans, equal to the world s total amount of coal. The U.S. and other countries have a rich supply just offshore. The downside is that methane is a potent greenhouse gas, about 23 times more effective at trapping heat than CO 2. Releasing methane into the atmosphere would exacerbate global warming. Perhaps more frightening is that methane hydrates need the correct temperature to keep them stable. If water temperatures rise beyond a certain threshold, the icy methane hydrates melt and methane gas is released. This is another positive feedback mechanism for global warming. This mechanism may be responsible for rapid increases in global temperature that occurred in the past. Oceanus, Fall-Winter, Common Misconceptions Misconception 1. Oil and gas deposits accumulate in large underground caverns or pools. Fact: Oil and natural gas exist in the pores of sediments and sedimentary rocks. Large reserves of fossil fuels accumulate by upward migration of these materials, because of their lower specific gravity, to geologic structures which serve as traps. Misconception2. Petroleum is formed from the remains of dinosaurs. Fact: Petroleum and natural gas form from the remains of microscopic organisms that exist in the seas and some large lakes. When these organisms die, their remains settle to the sea or lake floor where little oxygen is available to decompose them. They are then buried by sediment, heated by depth of burial, and transformed into petroleum and natural gas. Lecture Suggestions 1. Stress that detrital sedimentary rocks are classified primarily according to particle size, not composition. Note that sand is not a compositional term indicating quartz particles, but a size category. 2. Conglomerate may be effectively compared to a natural concrete. Perhaps the latter could be used in a demonstration of the larger particle size, and the finer grained, poorly sorted matrix. 3. Note that clay may be used as a size or compositional term depending on the context in which it is discussed, and that mud may be used as a size term when clay is meant to refer to clay minerals. 4. Point out that, unlike detrital sedimentary rocks, the shell and skeletal particles that occur in biochemical sedimentary rocks are not transported by streams to depositional sites in marine environments, but that these form from the organisms that lived on, in, and above the sea floor.

4 5. Be sure students understand how sedimentary facies reflect the different locations of a shoreline as sea level rises and falls. 6. Perform a simple demonstration of how graded beds form using a sturdy glass or clear plastic jar. Into the jar, place various sizes of sediment some small gravel, coarse sand, fine sand, clay. Then, partially fill the jar with water. Thoroughly shake the jar then set it on the table, allowing the sediment to settle in a graded fashion. This shows how graded bedding can be used to determine the top direction, and tells something about the environment of deposition. 7. To illustrate how certain depositional environments are recognized by geologists, offer a few sets of sedimentary rock types, textures, structures, and types of fossils (e.g., mud cracks, raindrop prints, and fossil plants or vertebrates) and have the students determine which depositional environments are most likely represented by the collective evidence. Consider This 1. If a sedimentary facies is deposited during a transgression or regression, is that facies of the same age everywhere? If so, why? If not, how might age equivalence within the facies be demonstrated? 2. Has the abundance of some sedimentary rock types changed over the duration of Earth s existence? 3. What types of sedimentary rocks are evidence of arid conditions and of tropical climates? 4. If soil is the terrestrial material that combines some components of the hydrosphere, atmosphere, biosphere, and lithosphere and thus, is an embodiment of the ecosphere what is the aquatic equivalent that combines all components of the marine ecosphere? 5. If fossils did not exist, would geologists have discovered that Earth has a history longer than recorded human history? Important Terms biochemical sedimentary rocks carbonate rock cementation chemical sedimentary rock compaction cross-bedding depositional environment detrital sedimentary rock evaporite fossil graded bedding sorting strata (beds) lithification marine regression marine transgression mud crack ripple mark rounding sediment sedimentary facies sedimentary rock sedimentary structure

5 Internet Sites, Videos, Software, and Demonstration Aids Internet Sites 1. Sedimentary Rocks: Picture Gallery of the Most Common Rock Types Photos and articles covering all types of sedimentary rocks from geology.com. 2. American Association of Petroleum Geologists About careers in petroleum geology, with videos and other items for sale. 3. Grand Canyon Geology The National Park Service has a Geology Training Manuel for people interested in the sedimentary rocks and the formation of the Grand Canyon. 4. Sediment Thickness of the World s Oceans The thickness of sediments in the oceans yields very interesting information on the age of the ocean crust and on seafloor spreading. This site includes information and maps. Videos 1. America s National Monuments: The Geologic West. Insight Media DVD (2008, 4-40 min. DVDs) Touring the national monuments of the Pacific Northwest in search of geologic features, such as fossil beds and lava flows. 2. Elements of Earth Science: Rocks, Minerals and Soils. Insight Media DVD (2005, 30 mins.) The rock cycle, the main types of rocks and how fossil fuels are used. 3. Rock Cycle. Insight Media DVD (2003, 30 mins.) How minerals form rocks and how rocks alter from one type to another. 4. Sedimentary Environments. Insight Media DVD (1995, 19 mins.) Sedimentary rock formation, names and classifications. 5. Sedimentary Rocks and Their Formation. Insight Media DVD (2004, 18 mins.) Weathering, erosion, deposition and the formation of clastic and non-clastic sedimentary rocks. 6. From Rock to Sand to Muck: All the Dirt on Soils. Insight Media DVD (1996, 63 mins.) The breakdown of rock into sediments and their decomposition into soils; soil types. 7. The Once Good Earth: Understanding Soil. Insight Media DVD (2005, 46 mins.) The chemical and ecological features of soil. 8. Earth Revealed. Annenberg Media (1992, 30 mins., free video): #8: Earth s Structures. Exploring rock layers, sedimentation, structures and petroleum using the Grand Canyon as a study side.

6 #17: Sedimentary Rocks. Using the rocks of the Grand Canyon to understand the geologic past. Processes of sedimentation and sedimentary rock formation are discussed. #19: Running Water I. Rivers, Erosion and Deposition. Landscapes formed by rivers, parts of a river and other information about streams. Slides and Demonstration Aids 1. Rocks, Minerals and Resources. Insight Media., (2001, Mac/Windows CD-ROM) The main rock and mineral types and how they are identified. 2. Society for Sedimentary Geology slide sets, a. Rivers and their Deposits b. Coastal Erosion 3. Educational Images, Ltd. slide sets, a. Fossils and Fossilization b. Sediments, Faults and Unconformities c. Geomorphology and Computer Programs d. Erosion, Slides and Surface Features e. Rocks and Topography 4. National Geographic images of erosion and weathering 5. Science Stuff, a. Sedimentary Rock Collection Answers to Figure-Related Critical Thinking Questions Critical Thinking Question Figure 7.7 Coquina is made up of fragmented seashells, so would you expect it to accumulate on a Florida beach, on the deep seafloor, in a lake, or on a river s floodplain? Coquina is a Clastic Sedimentary Rock, made up of pieces or clasts. The St. Augustine, Florida, deposit is a high-energy, littoral deposit sourced by wave action in the fore shore. The Anastasia Formation coquina originated from beach action. Critical Thinking Question Figure 7.8 Do you know of any commercial uses of rock salt and rock gypsum? Both rocks (one or more minerals = rock) are mined in Michigan. Halite is mined both physically underground (Detroit Salt Company) in one mine while other developers in the state use solution mining, bringing brine up in deep wells. Detroit s product is rock salt dedicated to road use in the winter to melt snow and ice. The solution miners provide the table salt we eat. Gypsum, still mined by surface methods in Michigan, is used for Plaster of Paris, wallboard, and is added to cement. There are other uses such as in the chemical industry. Critical Thinking Question Figure 7.15

7 How can you determine whether the flow that formed the ripples shown in part (b) was from right to left or from left to right? In the photo (b), it is hard to tell which side has the steep face of these asymmetrical ripple marks. I think I can see the steep face on the left side of each ripple mark, therefore: the current would be from right to left. Critical Thinking Question Figure 7.16 What inferences can you make about the environment in which the mud in the image below was deposited. Mud was deposited in Montana in a wet, lucustrine environment; the lake dried up and in the desiccation of the sediment, the cracks formed, only to be preserved by a fine grained overlying deposit. Suggested Answer to Selected Short Answer Question (Answers to question 7 and question 9 provided in the appendix to the text) 8. De s c r i be how de pos i t s of mud a nd s a nd a r e l i t hi f i e d. Sugges t ed Ans wer: Lithification is the transformation of loose sediment into solid rock through compaction and cementation. The detrital deposits of mud and sand are lithified through the same general process. In each case, the sediment consists of solid particles and pore spaces. These deposits are compacted by their own weight and the weight of any additional deposits. This compaction reduces the size of the pores. For mud, compaction alone is sufficient for lithification, but for sand, the added processes of cementation occurs from minerals precipitated into the pore space, acting as a cementing agent.