Geology 101 - Introduction to Physical Geology
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Lab - Sedimentary Rocks

Introduction

Sedimentary rocks are consolidated sediments, sediments that have been turned into rock by being buried, compacted, and cemented. Sediment consists of minerals or other solids deposited at the earth's surface, including sand on a beach, mud in a lake, or minerals precipitated at the bottom of an evaporating lake. Because the ocean is the largest, lowest, and most chemically rich part of the earth's surface, it is where much of the earth's sand, silt, mud and minerals accumulate as sediment and are buried to become sedimentary rocks. On land, the various environments where sediments accumulate and are buried to become rock are mostly basins or low areas.

Nearly all fossils are found in sedimentary rocks. This is because life on earth occupies surface environments, and sedimentary rocks have their beginnings on earth's surface. Sedimentary rocks contain a record of life through geologic time, preserved in layers like on pages in a book. Those same layers also contain evidence of earth's past climates and environments. Sedimentary structures such as beds, cross-beds, and graded beds provide clues about the environment in which the sediments were deposited and the dynamic processes that occurred there.

There are two main categories of sedimentary rock, clastic and chemical. Clastic sedimentary rocks, which are also called detrital sedimentary rocks, consist of minerals or pieces of rock that were eroded from pre-existing rocks. The mineral grains or pieces of eroded rock that compose a clastic sedimentary rock are known as clasts.

Chemical sedimentary rocks consist of minerals that precipitated from solution, such as salt that crystallized from a lake evaporating in a desert, or the minerals in the shells of marine organisms which were biochemically precipitated from sea water and preserved in a chemical sedimentary rock such as limestone.

A sedimentary rock name is shorthand for describing the process by which the rock orginated. Therefore, the more precise the name, the more information it provides about what kind of environment the rock came from and the more it aids in reconstructing the geologic history of its place of origin.

Advance Preparation and Materials Needed

Geology Learning Outcomes

By performing and completing this lab, you will progress toward the following learning objectives for this course:

Methods

Lab Procedures

To understand and identify sedimentary rocks, you cannot be shy about addressing the rocks. Remember what you learned in the minerals and igneous rock labs - you have to do more than look at the samples. You need to:

Refer to the Lab Assignments Grading Rubric for a reminder of what constitutes a well-performed lab.

Distinguishing clastic from chemical sedimentary rocks

What do you look for to decide whether a sedimentary rock is clastic or chemical?

Look to see if the rock seems to contain bits of earth materials that you might also see as loose sediments in a riverbed or at the beach or on the bottom of a lake - gravel, sand, or silt. If so, the rock is likely a clastic sedimentary rock. Conglomerate or breccia, depending on how round the clasts are, look like gravel turned into rock. Sandstone looks like sand turned into rock. Siltstone looks like silt (extra fine sand) turned into rock. Shale looks like layers of mud turned into rock.

The surfaces of sandstone and siltstone usually feel gritty to the touch. Under magnification, you can see that individual sand grains or silt are separate from each other. There may be either pore space or cement between the separate grains. Shale is made mostly of clay particles. The particles are too fine see without a very strong microscope. Shale is typically well-bedded (layered).

If a sedimentary rock does not look like gravel, sand, silt or mud and seems to be made of a single mineral it is probably a chemical sedimentary rock. A chemical sedimentary rock usually looks like it is made of a single substance.

What differences do you see in these rocks? Click on each thumbnail for a larger version of the image that opens in a new window.

photo of a rock showing visible calcite cleavages photo of rock that appears to be made of sand grains.

Which sedimentary rock is clastic? Which sedimentary rock is chemical?

What differences do you see in these rocks? Click on each thumbnail for a larger version of the image that opens in a new window.

photo of a dark, well-bedded rock that looks like layers of mud photo of rock that is shiny, carbon-black, and well-bedded

Which sedimentary rock is clastic? Which sedimentary rock is chemical?

Identifying minerals in sedimentary rocks

In getting to know all rocks, the place to start is determining what minerals they contain.

Identifying minerals in clastic sedimentary rocks

As discussed in the minerals section of the sedimentary rocks Basics page, winnowing occurs during the weathering-to-deposition portion of the process that forms clastic sedimentary rocks. Quartz becomes increasingly abundant during this winnowing.

Therefore, in clastic sedimentary rocks, it is a good idea to check for the presence of the mineral quartz. Remember that you can recognize quartz by its glassy luster and its lack of cleavages. Quartz tends to be colorless in clastic sediments (with some exceptions). Quartz has either irregular or conchoidal fracture and lacks flat, consistently bright and shiny cleavage surfaces. Quartz as clastic sediment tends to look like clear pieces or particles of glass. You need to use magnification to see it well.

The physical properties of the feldspars make them distinguishable from quartz. Feldspars are also common in clastic sediments, because they are the most common type of mineral in source rocks and because they are hard enough to last awhile in the weathering-to-deposition process (even though they don't last as long as quartz). Feldspar physical properties include color (pink, white, or clear) and pearly luster, which is seen best on shiny cleavage faces. The cleavage of feldspar is especially telling, with two perfect cleavages that meet at about 90 degree corners. Imagine looking through a hand lens at a light-colored mineral with shiny flat surfaces and square corners. What is the mineral likely to be?

Feldspar gradually breaks down on earth's surface and produces, among other things, clay minerals. During the winnowing process of sedimentation, there is a gradual decrease in feldspar and increase in clay in the mix of minerals that compose clastic sediments. Clay is very soft, with dull luster and white, tan, gray, or red colors. In shale, which is made of clay-size grains, the most abundant mineral is almost always clay, though tiny grains of quartz and other minerals may be present as well.

Other minerals in source rocks that undergo weathering and erosion to become clastic sediments, such as amphibole, biotite, muscovite, olivine, and pyroxene, can be present in immature sediments that become clastic sedimentary rocks. In such rocks, these minerals or the metal oxides and clay minerals they often become during during weathering-to-deposition can appear as dark specks known as "lithics." Clastic sediments containing lithic grains are mineralogically immature sediments.

Identifying minerals in chemical sedimentary rocks

Most chemical sedimentary rocks are predominantly a single mineral (or mineraloid, as in coal and amber). The name of chemical sedimentary rocks is based mainly on the mineral each type of rock is made of. Limestone, for example, is made mainly of the mineral calcite. Amber is made of an organic substance that originated as tree sap. Coal is made mostly of carbon in mineraloid form. Gypsum rock is made mainly of (no surprise) gypsum. Therefore, analyzing chemical sedimentary rocks usually involves identifying the one mineral that makes up all or nearly all of the rock, as shown in the Sedimentary Rock Classification Table.

What analytical technique might you use to distinguish whether a chemical sedimentary rock is composed of silica or calcite? They can look a lot alike. What chemical test would provide additional confirmation?

Describing sedimentary rock textures and distinguishing characteristics

Because the same minerals can occur in different types of sedimentary rocks to completely characterize a sedimentary rock, it is necessary to analyze other features of sedimentary rocks that set them apart from each other.

Describing clastic sedimentary rock textures

Clastic sedimentary texture is described in terms of the size of the clasts, the sorting of the clasts, and the rounding of the clasts. See clastic textures on the sedimentary Basics for definitions and illustrations.

When examining a clastic sedimentary rock, you can analyze its texture in terms of answering three questions:

You will need to examine the rock using a hand lens or, better yet, a lower power microscope to answer these texture questions properly. If the rock is made entirely of clay size clasts, too small to make out individually even using a low-power microscope, you cannot say anything about the rounding of the grains other than "not visible," but you can state the size of the clasts and how well sorted they are.

Describing chemical sedimentary rock distinguishing characteristics

Because chemical sedimentary rocks do not usually contain grains of sediment that were rounded or sorted, they are treated differently from clastic sedimentary rocks. Each type of chemical sedimentary rock has its own set of distinguishing characteristics.

Consider the several different types of chemical sedimentary rocks made of the mineral calcite. Although limestone is one of the most well-known rocks made from calcite, it is by no means the only one.

For example, chalk is a chemical sedimentary rock made of calcite. The calcite in chalk is in the form of tiny pieces of plankton which accumulated on the ocean floor and was preserved in the rock layers as a porous, powdery rock. The porous and powdery nature of the chalk are its major distinguishing characteristics.

Tufa is another chemical sedimentary rock made of calcite. It forms as a mound or branching set of growths where a spring flows upward into a lake, under chemical conditions in the water that cause the precipitation of calcite. The distinguishing characteristics of tufa include its rough, irregular surfaces, brown color, non-crystalline appearance (no visible calcite crystals or cleavage surfaces in the rock), and low density compared to limestone.

An obvious example of a distinguishing characteristic is the presence of visible fossils in fossiliferous limestone compared with limestone. Both are made of calcite. One type has visible fossils, the other does not.

A final example is coquina, which is a chemical sedimentary rock also made of calcite, in the form of slightly broken pieces of clam shells and snail shells. Coquina originates in tropical beaches or lagoons, where the waves and currents pile up shells of dead clams and snails. During lithification to become coquina, this chemical sediment is not buried very deeply or compacted very much, mainly just weakly cemented. In addition to being fossiliferous, it has the distinguishing characteristic of high porosity - lots of empty spaces between the broken shell pieces.

Naming sedimentary rocks

To name a sedimentary rock, first you must determine whether it is clastic or chemical. Next, you must determine which minerals it is made of. If it is a clastic rock, you must determine the size, sorting, and rounding of the grains, which describes its sedimentary texture. If it is a chemical sedimentary rock, you must note possible distinguishing characteristics, such as color, whether it is powdery, whether it is porous, whether it is crystalline (shows visible crystal faces or cleavage faces of minerals). For all sedimentary rocks, it is worth noting if it contains any fossils.

Once you have examined and analyzed the rock mad your observations about its features, refer to the Sedimentary Rock Classification Table to name the rock.

For example, you are analyzing a clastic sedimentary rock with an extra fine (clay) grain size in which the degree of roundness is not visible and the grains are well sorted. Matching those observations, from left to right in the table, tells you the rock is named shale or claystone. You observe that the rock has beds and is somewhat hard, and you correctly conclude that it is shale.

Similarly, you are analyzing a chemical sedimentary rock that is made of calcite and you have observed that it is white, porous, and powdery. Matching your observations to the table, you can name the rock chalk.

Notice that if you have determined that a rock is sandstone, you need to delve further and determine if it is arenite, arkose, or greywacke, based on its mineralogy and texture.

Determining depositional environments

To read the full story a sedimentary rock can tell us, we need to consider the environment in which its sediments were deposited. Once you have analyzed a sedimentary rock in terms of its minerals, texture, and distinguishing features, you can use your observations as clues to develop a plausible hypothesis about its depositional environment. In other words, you can describe the type of environment on earth in which the sediments were deposited. See the Basics page on Depositional Environments.

It is important to be as specific as possible in describing a depositional environment. For example, if the rock you were working with was fossiliferous limestone, including coral fossils, you can be sure the sediment originally formed in the ocean, but you can be much more specific than just stating "ocean"or "marine." Coral only grows abundantly in warm, shallow water where sunlight can penetrate, away from mud and clastic sediment in the water. Therefore, you can state a strong hypothesis that the depositional environment was the warm, shallow ocean water where tropical reefs form, some distance away from river mouths, continental shorelines and other sources of clastic sediment.

See the Table of Depositional Environments for likely settings in which the sediment of your rock may have originally accumulated.

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Geology 101 - Introduction to Physical Geology
Lab--Sedimentary Rocks
Created by Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, including figures unless otherwise noted
updated: 9/11/13

Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.

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