1
Class_____________ Name: ________________________________________________________________
Sedimentary Rocks
Background Information
The silicate minerals found in igneous and metamorphic (and even some sedimentary,) rocks weather to form
the materials that lithify to become sedimentary rocks. Solid particles derived from weathering accumulate to
form broken pieces or clastic sediments. Other particles come from the precipitation of dissolved material in
lakes, streams or oceans. These are known as chemical, or nonclastic sediments. Clastic or broken fragments
exhibit a history of transportation and deposition that often includes evidence of running water. Clastic rocks
are further classified on the basis of particle size. The natural cement or glue that holds rock particles together is
commonly calcium carbonate, iron oxide, or silica. A third sedimentary classification includes materials derived
from the life processes of water-dwelling plants and animals. Many marine organisms secrete hard calcium
carbonate shells, which, when the organisms die, can dissolve in the water and precipitate as solid rock when the
water dries up, or stay as shells that become cemented together to form fossil limestone.
Calcareous (lime-rich) and silica materials cover large areas of the ocean floor. If long-term evaporation of
shallow waters takes place, deposits of the evaporates gypsum, limestone and salt may be formed.
With time and compaction, often on the ocean floor, large, continuous areas of sedimentary rock form while
incorporating characteristics of the prevailing environment. Ripple marks, mud cracks, fossils, and bedding
structures tell the story of the formative stages of the rock. Coal, considered to be an organic sedimentary rock,
is the plant world’s contribution to the sedimentary inventory. Figure 1 shows the relationship of particle size
to sedimentary rock classification. Figure 2 on the next page, illustrates sedimentary structures. Ripple marks
suggest various types of water motion. Cross bedding indicates changes in the agents or mode of deposition.
Formed by Compaction and Cementation
Particle Size
Land-derived Sediments
Marine-derived Sediments
(Evaporites that precipitate from
evaporating water include
limestone, dolostone, gypsum
and salt)
Figure 1: Relationship of particle size to sedimentary
rock classification
2
Ripple marks on a beach. If these sediments become cemented together,
the ripple marks may be preserved
Figure 2: Sedimentary Structures
It is interesting to note that sedimentary rock makes up only 5 percent of the volume of the Earth's
crust—the outer 15 km—but accounts for 75 percent of' the rock exposed at the Earth's surface. This
percentage includes metamorphic rocks as either igneous or sedimentary, depending on their origin.
Figure 3 shows information on the members of the Onondaga formation, which is a series of
sedimentary rocks, described here as seen in New York State. The Onondaga limestone is a rock that is
relatively resistant to weathering. Onandaga limestone runs east-west across the state and is seen in
many localities as an escarpment or steep, cliff-like rock face. The various rock layers, or sediment
materials that formed under similar conditions, reflect past geologic environments. Note that in the
Seneca member there is volcanic ash, similar to ash found in midwestern North America, indicating
that large-scale volcanic eruptions occurred that affected large areas of the continent. Also shown in
Figure 3 are fossils similar to those found in the rocks of the Onondaga formation. Fossils are
especially helpful for determining paleoenvironments, or the ancient conditions under which the rocks
formed.
3
Figure 3: Onandaga Formation
Rock
Name (Facies)
Formation
(Member)
Seneca
New Scotland
member
Moorhouse
member
Kalkberg
Rock Types, Grain
Sizes, Sedimentary
Structures
Fossils
Shale that contains
calcium carbonate and
fine-grained limestone
that contains much clay
Brachiopods, including
some with pinkish shells
called Chonetes lineatus
Less pure ( contains
much clay) with several
thin volcanin ash beds
(Tioga beds)
Fine- to medium-grained
limestone
Nedrow
Member*
Edgecliff
member
New Scotland
Coeymans
Shale that contains
calcium carbonate
(lower)
Medium- to coarsegrained limestone
Deeper water, below
motion of fair-weather
waves.
Bottom agitated by
storm waves
Many fossil of sea-floor
animals
Thin to medium-thick
layers
Varying amounts of
chert
Medium-grained
limestone (upper)
Environment of
Formation
Shallow, quiet water at
or near lowest point
reached by motion of
fair-weather waves
Bottom occasionally
agitated
Platyceratid gastropods
and sparse fossils of seafloor animals
Similar to New Scotland
facies
Similar to those found in
Coeymans formation
Shallow-water shelf
Vigorous wave motion
Medium to thick layers
Chert
rugose and tabulate
corals,
pelmatozoans,
brachiopods,
trilobites
Well-agitated bottom
Blanketlike layers built
by corals; scattered coral
reefs
* The Nedrow member occurs in Central New York. To the east and west, it gradually becomes a “cleaner” limestone with chert in it.
There, it is more like the rest of the Onandaga limestone and less like a separate member.
4
Problem: What are the characteristics of sedimentary rocks?
How do these characteristics reflect the environment in which the rocks formed?
Materials (per group): set of sedimentary rocks, hand lens
Procedure and observations:
1. Review the Background Information section of this investigation and page 7 of the ESRT.
2. Each sedimentary rock has been labeled with a number. Please enter this number on your Data Table. Look
for the features that will help you to identify the rock as a sedimentary rock and the features that will help you to
figure out exactly which type of sedimentary rock it is. Record the information in the Data Table. The
sedimentary rock identification key is reproduced below for your convenience. Look carefully for structures,
mineral content, and fossils.
3. Complete the Data Table. Use the hand lens for observing the texture and the minerals present in the samples.
4. Observe the additional samples, including large specimens, if available. Draw features of the rocks or make
notes about things you observe that will help you to remember what each kind of rock looks like in the spaces
provided in the Data Table.
5
Data Chart
Specimen
Number
Clastic,
Nonclastic, or
Organic
Grain Size or
Appearance
Texture, Rock
Characteristics, Minerals
Observed
Name of
Rock
6
Analysis and Conclusions
1. Why are rocks that contain fossils more likely to be sedimentary rocks, rather than igneous or
metamorphic rocks?
2. What is the difference between "bioclastic" limestone and "evaporitic" limestone?
3. Why is there such a variation in the appearance of sedimentary rocks of the same type, (like
sandstone) from one location to another?
4. Note several features of each of the following sedimentary rocks that would help you identify that
type of rock in the future:
Sandstone:
Shale:
Conglomerate:
Limestone:
Gypsum:
Halite:
Coal: