Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Understand the processes by
which sedimentary rocks form
■■
Understand how geologists
classify sedimentary rocks
based on their origin
■■
Become familiar with
sedimentary rock textures and
mineral assemblages
■■
Use texture to identify the
agent of erosion involved in
formation of sedimentary
rocks
■■
Use sedimentary rock textures
and minerals to interpret
ancient geography and
climate
MATERIALS
NEEDED
■■
Set of sedimentary rocks
■■
Magnifying glass or hand lens,
microscope, and thin sections
of sedimentary rocks
■■
Mineral-testing supplies
(streak plate, glass plate. etc.)
■ ■ Glass beaker, concentrated
Ca(OH) 2
(calcium hydroxide)
solution, straws
6.1 Introduction
Like igneous rocks, sedimentary rocks form from previously existing rocks—but by
very different processes. If you’ve ever seen shells on a beach, gravel along a riverbank,
mud in a swamp, or sand in a desert, you’ve seen sediment. Sediment consists
of loose grains (clasts) that may be made of individual minerals or fragments of
previously existing rock containing several minerals. Sedimentary rock forms at or
near the Earth’s surface by the cementation and compaction of accumulated layers
of these different kinds of sediment.
Sedimentary rocks preserve a record of past environments and ancient life and
thus record the story of Earth history—once you know how to read it. For example,
the fact that a sedimentary rock called limestone occurs throughout North America
means that in the past, a warm, shallow sea covered much of the continent. Geologists
have learned to read the record in the rocks by comparing features in sedimentary
rocks with those found today in environments where distinct types of sediment
form (remember, the present is the key to the past). In this chapter, you will develop
the skills needed to read the history recorded in sedimentary rocks.
6.2 Sediment Formation and Evolution
6.2.1 The Origin of Sediment
The materials from which sedimentary rocks form ultimately come from weathering—
the chemical and physical breakdown of pre-existing rock. Weathering produces
both loose pieces, called clasts, and dissolved ions, which are charged atoms in a
water solution. Once formed, clasts may be transported by water, wind, or ice to
another location, where they are deposited and accumulate. Dissolved ions, meanwhile,
enter streams and groundwater. Some of these ions precipitate from groundwater
in the spaces between clasts, forming a cement that holds the clasts together.
Other ions get carried to lakes or seas, where organisms extract them to form shells.
Finally, some dissolved ions precipitate directly from water to form layers of new
sedimentary minerals. In some environments, sediment may also include organic
material, the carbon-containing compounds that remain when plants, animals, and
microorganisms die.
6.2.2 Weathering and Its Influence
on Sediment Composition
The minerals in sediment depend both on the composition of the source rocks and
on the nature of the weathering the source rocks were exposed to. Let’s look first at
weathering processes and how they influence sediment composition.
Physical weathering breaks rock into fragments called clasts, as does hitting a
rock with a hammer, but does not change the minerals that make up the rock. Initially,
clasts are simply small rock pieces and may contain many mineral grains or crystals
that retain their original sizes and shapes. But as physical weathering progresses,
the clasts break into smaller pieces, each consisting of a single mineral grain. While
moving along in wind and water, clasts grind and crash against one another and
become progressively smaller. As a result, the sizes and shapes of transported clasts
do not tell us whether the source rock itself was coarse- or fine-grained, or whether
grains in the source rock interlocked or were cemented together (FIG. 6.1). But
because physical weathering does not change the composition of clasts, we can
get a sense of the composition of the source rock by looking at what minerals the
144 CHAPTER 6 USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORY