Page 187 - Laboratory Manual in Physical Geology

Hand Sample Analysis and Ancient Environments and
Interpretation Ecosystems
The complete classification of a sedimentary rock requires Sediments are deposited in many different environments.
knowledge of its composition, texture(s), and other Some of these environments are illustrated in FIGURE 6.10 .
distinctive properties. The same information can be used Each environment has characteristic sediments,
sedimentary structures, and organisms that can become
to infer where and how it formed ( FIGURE 6.10 ). Follow
these steps to analyze and interpret a sedimentary rock: fossils (any evidence of prehistoric life). The information
gained from grain characteristics, sedimentary structures,

Step 1: Determine and record the rock’s general
and fossils in rocks can be used to infer the ancient
composition as biochemical (bioclastic), chemical, or detrital
environment ( paleoenvironment ) in which they formed.
(siliciclastic) with reference to FIGURES 6.2 and 6.9 , and
The process of understanding where and how a body
record a description of the specific kinds and abundances
of sediment was deposited depends on the Principle of
of grains that make up the rock. Refer to the categories for
Uniformitarianism —the assumption that processes that
composition in the left-hand column of FIGURE 6.9 .
shaped Earth and its environments in the past are the
Step 2: Record a description of the rock’s texture(s) same as processes operating today. This principle is often

with reference to FIGURE 6.3 . Also record any other stated as, “the present is the key to the past.” You can think

of the rock’s distinctive properties as categorized in of processes operating in modern ecosystems and then
the center columns of FIGURE 6.9 . imagine how those same processes may have operated in

past ecosystems with different organisms. You can also

Step 3: Determine the name of the sedimentary
look at sediment, sedimentary structures, and fossils in
rock by categorizing the rock from left to right
a sedimentary rock and infer how it formed on the basis

across FIGURE 6.9 . Use the compositional, textural,
of where such sediment, sedimentary structures, and
and special properties data from Steps 1 and 2 (left
organisms are found together today.

side of FIGURE 6.9 ) to deduce the rock name (right

side of FIGURE 6.9 ).
Indicators of Ancient Environments
Step 4: After you have named the rock, then you

can use FIGURE 6.10 and information from Steps 1 Think of a goldfish. Chances are that your brain put

and 2 to infer where and how the rock formed. the goldfish into context, and you imagined it in a
See the example for sample X ( FIGURE 6.11 and the bowl of water. Now if you saw a goldfish bowl on

Activity 6.7 worksheet). your neighbor’s kitchen table, you would probably
think that the neighbor is getting a goldfish. Whether
you think of the goldfish or the bowl, you cannot
help but imagine the goldfish in a bowl of water—a
goldfish ecosystem. The same process is used to analyze
ACTIVITY sedimentary rocks and infer how and where they may
have formed. If the rock has a fossil of a freshwater fish,
6.9 Using the Present to Imagine then the sediment must have accumulated under water,
the Past—Cape Cod to Kansas in a stream or lake. If the rock is made of rounded
gravel with pieces of tree bark, then the sediment in
| there were both trees and rounded gravel—like the
THINK What can sedimentary rocks tell the rock must have accumulated in an ecosystem where
About It us about Earth’s history and past edge of a river. Fossils and sedimentary structures are
environments and ecosystems? good indicators of the paleoenvironments. It is up to
you, the geologist, to place the structures and fossils

OBJECTIVE Infer characteristics of an ancient into context, and infer an environment or ecosystem in
environment by comparing present-day seafl oor which they could have formed together.
sediments with sedimentary rock formed on an
ancient sea fl oor. Fossils
PROCEDURES Fossils are any evidence of ancient life. Body fossils are
fossils or the body parts of organisms. Soft body parts of
1. Before you begin , read about Ancient
Environments and Ecosystems and Indicators organisms (skin, leaves of trees) decay easily, so they are
of Ancient Environments below. Also, this is rarely fossilized. Hard body parts like shells and bones
what you will need : are much easier to fossilize. Trace fossils are any evidence
of the activities of organisms, such as their footprints
___ Activity 6.9 Worksheet (p. 185 ) and pencil
and burrows or other structures that they made when
2. Then follow your instructor’s directions for living. Both kinds of fossils are useful as clues about the
completing the worksheets. ancient environment of deposition. Trace fossils cannot
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