Chapter 6 Sedimentary Rocks

Chapter 6 Sedimentary Rocks - 2009 Allan Ludman and Stephen...

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CHAPTER 6 © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORY PURPOSE • To become familiar with sedimentary rock textures and mineral assemblages • To learn how to use sedimentary rocks to interpret ancient geologic, geographic, and environmental settings. MATERIALS NEEDED • A set of sedimentary rocks. • A magnifying glass or hand lens and a microscope and thin sections of sedimentary rocks. • Standard supplies for identifying minerals (streak plate, glass plate, etc.) 6.1 INTRODUCTION The story of igneous rocks begins deep in the Earth and gives geologists important information about conditions and processes of a part of our planet that we are unable to visit or even drill to. In contrast, the story of sedimentary rock is largely written at the surface and tells of ancient processes, conditions, and the animals and plants that lived there. For example, sedimentary rocks tell us that North America was very different in the past: • 50,000 years ago, a continental ice sheet extended as far south as the Ohio River before melting only 10-15,000 years ago. At that time, the northern United States looked like Antarctica or Greenland. • at about 60 million years ago (Ma), the ancestral Rocky Mountains were nearly as high as the Himalayas are today; the Great Plains were forming as a blanket of sediment eroded from the Rockies; log onto Google Earth or World Wind to see the Himalayas today, and the blanket of sediment shed southward into Bangladesh from the mountains. • at 340 Ma the mid-continent was covered by a shallow-water inland sea. • 420 Ma, the east coast was oriented E-W and was located close to the equator. For a comparable modern environment, log onto Google Earth or World Wind to look at the Bahamas or the Great Barrier Reef of Australia. 6.2: FORMATION OF SEDIMENTARY ROCKS Sedimentary rocks form when igneous, sedimentary, or metamorphic rocks exposed to physical and chemical weathering at Earth’s surface break down, the resulting sediment is transported away and deposited somewhere else, and the sediment is converted to a new sedimentary rock (Figure 6.1) When geologists study sedimentary rocks we want to know What kind(s) of rock(s) were weathered? What 1
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