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

Ripple marks on a beach. If these sediments become cemented together,

the ripple marks may be preserved

1

Figure 2: Sedimentary Structures

1

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.

Figure 3: Onandaga Formation

Rock
Formation
(Member) / Name (Facies) / Rock Types, Grain Sizes, Sedimentary Structures / Fossils / Environment of Formation
Seneca
member / New Scotland / Shale that contains calcium carbonate and fine-grained limestone that contains much clay
Less pure ( contains much clay) with several thin volcanin ash beds (Tioga beds) / Brachiopods, including some with pinkish shells called Chonetes lineatus / Deeper water, below motion of fair-weather waves.
Bottom agitated by storm waves
Moorhouse
member / Kalkberg / Fine- to medium-grained limestone
Thin to medium-thick layers
Varying amounts of chert / Many fossil of sea-floor animals / Shallow, quiet water at or near lowest point reached by motion of fair-weather waves
Bottom occasionally agitated
Nedrow
Member* / New Scotland / Medium-grained limestone (upper)
Shale that contains calcium carbonate (lower) / Platyceratid gastropods and sparse fossils of sea-floor animals / Similar to New Scotland facies
Edgecliff
member / Coeymans / Medium- to coarse-grained limestone
Medium to thick layers
Chert
Blanketlike layers built by corals; scattered coral reefs / Similar to those found in Coeymans formation
rugose and tabulate corals,
pelmatozoans,
brachiopods,
trilobites / Shallow-water shelf
Vigorous wave motion
Well-agitated bottom

* 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.

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.

Data Chart

Specimen
Number / Clastic, Nonclastic, or Organic / Grain Size or Appearance / Texture, Rock Characteristics, Minerals Observed / Name of Rock

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: