North Seattle Community College's
PHYSICAL GEOLOGY 101
Instructor:  Tom Braziunas

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GEL101 Key to
Sedimentary Rock Identification

@2002 -- The information contained in this document is copyrighted.
No reproduction may be made without prior approval from the author.

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I. Introduction

This lab exercise is my attempt to help you to identify the sedimentary rocks in your lab kit.  The most important first step is to read our text section on "Classifying Sedimentary Rocks" (pages 124-130), especially paying attention to the charts and images.  You will need to be familiar with basic terminology for composition and texture of sedimentary rocks. 

Table 6.1 and the chart on page 130 distinguish three main categories of sedimentary rocks:  (1) Clastic (also called Detrital) Inorganic Sedimentary Rocks; (2) Chemical Inorganic Sedimentary Rocks; and (3) Chemical Organic (also called Biogenic) Sedimentary Rocks.  Be sure that your understand these terms.

As he created for igneous rock classification, Richard Harwood has created an interactive website which presents excellent photographs of various sedimentary rocks and lets you practice identifying them and grading your own results.  Please thoroughly examine his website before attempting our own lab:

http://www.bhc.edu/academics/science/harwoodr/Geol101/Labs/Sediment

Then use the key below to separate your sedimentary rock samples according to their textural and compositional characteristics.  Use the submission form to report your findings at every step (each question is worth 2 points).  You don't need to be correct in your findings but you will be asked to justify the selections that you have made.

Hopefully, this activity will make the subsequent multiple-choice lab manual exercise (Mike's Sedimentary Rock Lab) much easier to step through.

TOTAL POINTS = 30

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II. Identification Key

Step 1: Carefully examine each of your 8 sedimentary rock specimens (#22 through #29, with green labels)  Divide them into three groups according to their textures. 

  • If the grains (fragments) that comprise the rock are visible to the eye or you can see fossils (pieces of shell, for example), put the rocks into Group 1
  • If the grains are very hard to see with your eye but are visible using your hand lens, put them into Group 2.  If you are not sure, put the sample into this group as well.
  • If the grains are invisible (you cannot see individual fragments), or the rock looks crystalline rather than detrital, put the rocks into Group 3.

Question 1 (on the submission form for this homework):  Which rock samples fit into each category? 

  • REMEMBER TO USE THE ROCK NUMBERS -- NOT THE NAMES -- TO ANSWER THIS QUESTION. (I found 4 specimens that fit in Group 1, 1 specimen in Group 2, and 3 specimens in Group 3.   You may find a different distribution based on your different rock specimens and observations -- don't worry, specimens can fit into several categories below.)

Follow the appropriate steps below and try to answer the associated questions (use the order of the questions as a guide for each step to take).

IMPORTANT:  Once you have made an identification of a sample, place that sample aside.  You no longer need to work on that specimen.  In this way, each sample is identified and placed aside until you have completed all identifications by the final question (and none of the rocks are left to identify).

 

KEY TABLE FOR GROUP 1
Coarse-grained Detrital Sedimentary Rocks

For rocks that fit in Group 1 :  Are fossils visible?

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Yes, I see fossils.  

Question 2A (on the homework submission form):  Which rock samples (use their numbers) fit into this subgroup? Report them on the submission form.   (I found 1 specimen that fit in this subgroup.)

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No, I do not see fossils. 

Question 2B:  Which rock samples (use their numbers) fit into this subgroup? Report them on the submission form.  (I found 3 specimens that fit in this subgroup.)

NEXT STEP:  Do any of these rocks effervesce when acid (concentrated lemon juice) is applied?

NEXT STEP:  Are the grain sizes in any of these rocks greater than 2 mm?  (Use a ruler with centimeters.  2 mm is two-tenths of a centimeter.)

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Yes, I see "fizz".  

Question 3A:  Which rock samples (use their numbers) fit into this subgroup?  (I found 1 specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Fossiliferous limestone

  • Coquina

Read the descriptions of these specific rocks in your textbook in order to answer the next part of this question.

Question 3A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, I do not see "fizz". 

Question 3B:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimens that fit in this subgroup.  Did you?)

Your rock sample(s) might be:

  • Diatomite

  • Bituminous coal

Read the descriptions of these specific rocks in the lab manual tables in order to answer the next part of this question.

Question 3B (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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Yes, I see very large grains.  

Question 4A:  Which rock samples (use their numbers) fit into this subgroup?  (I found 1 specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Breccia

  • Conglomerate

Read the descriptions of these specific rocks in your textbook in order to answer the next part of this question.

Question 4A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, I do not see very large grains. 

Question 4B:  Which rock samples (use their numbers) fit into this subgroup?  (I found 2 specimens that fit in this subgroup.)

Proceed to the next key table.

 

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KEY TABLE FOR GROUP 1 (continued)

For rocks that fit in Group 1 that have no fossils and have grains smaller than 2 mm: 
Are the grains made up mostly of quartz? (Check with your hand lens)

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Yes, I see only quartz.  

Question 5A:  Which rock samples (use their numbers) fit into this subgroup? (I found 1 specimen that fit in this subgroup.)

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No, I see other minerals plus quartz -- or I do not see quartz at all. 

Question 5B:  Which rock samples (use their numbers) fit into this subgroup? (I found 1 specimen that fit in this subgroup.)

NEXT STEP: Do any of these rocks effervesce when acid (concentrated lemon juice) is applied?

NEXT STEP:  Does the rock contain abundant feldspar?

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Yes, I see "fizz".  

Question 6A:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Quartz Sandstone cemented with calcium carbonate (if white or gray)

 

 

Question 6A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, I do not see "fizz". 

Question 6B:  Which rock samples (use their numbers) fit into this subgroup?  (I found 1 specimen that fit in this subgroup.  Did you?)

Your rock sample(s) might be:

  • Quartz Sandstone cemented with silica (if white or gray)

  • Quartz Sandstone cemented with iron oxide (if deep red)

Question 6B (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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Yes, I see feldspars (pink and/or gray).  

Question 7A:  Which rock samples (use their numbers) fit into this subgroup?  (I found 1 specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Arkose

 

Question 7A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, I do not see feldspars. 

Question 7B:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimens that fit in this subgroup.)

None of the rocks in your textbook fit in this category.

Proceed to the next key table.

 

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KEY TABLE FOR GROUP 2
Fine-grained Detrital Sedimentary Rocks

We are finished with Group 1.  For rocks that fit in Group 2:
Does the surface of the rock look sugary and have a rough feel?

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Yes, it is rough.  

Question 8A:  Which rock samples (use their numbers) fit into this subgroup? (I found 1 specimen that fit in this subgroup.)

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No, it is not rough. 

Question 8B:  Which rock samples (use their numbers) fit into this subgroup? (I found no specimens that fit in this subgroup.)

NEXT STEP: Do any of these rocks effervesce when acid (concentrated lemon juice) is applied?

NEXT STEP:  THE TASTE TEST!
Does the rock powder taste gritty? 
-- P.S. I did not find any specimens in this category, so I did not conduct the taste test myself.  Don't feel you need to either! :)

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Yes, I see "fizz".  

Question 9A:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Siltstone cemented with calcium carbonate

 

 

Question 9A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, I do not see "fizz". 

Question 9B:  Which rock samples (use their numbers) fit into this subgroup?  (I found 1 specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Siltstone cemented with silica (if gray)

  • Bituminous coal (brown to black)

Question 9B (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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Yes, but I can't believe I tasted a rock.  

Question 10A:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimens that fit in this subgroup.)

Your rock sample(s) might be:

  • Siltstone (grains hardly visible, gritty taste)

Question 10A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, but I still can't believe I tasted a rock. 

Question 10B:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimens that fit in this subgroup.)

Your rock sample(s) might be:

  • Claystone (grains invisible, smooth taste)

If you have a rock like this, it belongs in Group 3 instead.  The grains are actually not visible with a hand lens.

Proceed to the next key table.

 

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KEY TABLE FOR GROUP 3
Chemical Sedimentary Rocks and
Detrital Sedimentary Rocks with Microscopic Grains

We are finished with Groups 1 and 2.  For rocks that fit in Group 3:
Do any of these rocks effervesce when acid (concentrated lemon juice) is applied?

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Yes, I see "fizz".  

Question 11A:  Which rock samples (use their numbers) fit into this subgroup?  (I found 1 specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Crystalline limestone (if you can see crystal faces)

  • Chalk

  • Micrite

Question 11A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, I do not see "fizz". 

Question 11B:  Which rock samples (use their numbers) fit into this subgroup?  (I found 2 specimens that fit in this subgroup.)

Proceed to the next key table.

 

 

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KEY TABLE FOR GROUP 3 (continued)

For rocks that fit in Group 3 and do not effervesce: 
Is the rock obviously crystalline with a shiny reflection?

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Yes, it looks crystalline.  

Question 12A:  Which rock samples (use their numbers) fit into this subgroup? (I found no specimen that fit in this subgroup.)

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No, it does not look crystalline. 

Question 12B:  Which rock samples (use their numbers) fit into this subgroup? (I found 2 specimens that fit in this subgroup.)

NEXT STEP: Check for other diagnostic features in order to identify each rock.  Is it hard or soft (scratched by your fingernail)?  Does it have conchoidal fracture?  Double check on whether it fizzes if you use your wire nail to make some powder on its surface.

NEXT STEP:  Check for other diagnostic features in order to identify each rock.  Is the rock fissile (read your text)?  Does the rock have black carbonized plant remains?  Is the sample hard with a waxy luster and with conchoidal (broken glass) fracturing?

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Yes, I do see "fizz" after all.

Question 13A:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Micrite

  • Dolostone

 

 

 

Question 13A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, I do not see "fizz". 

Question 13B:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Rock Salt (soft)

  • Gyprock (hard)

  • Chert (conchoidal)

Question 13B (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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Yes, I see fissile layering, carbonized matter, or conchoidal fracturing.  

Question 14A:  Which rock samples (use their numbers) fit into this subgroup?  (I found 2 specimens that fit in this subgroup.)

Your rock sample(s) might be:

  • Shale (fissile and usually dark gray)

  • Bituminous coal (brown to black)

  • Chert (hard, waxy, and conchoidal)

Question 14A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

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No, I don't see any of those features. 

Question 14B:  Which rock samples (use their numbers) fit into this subgroup?  (I found no specimen that fit in this subgroup.)

Proceed to the next and last key table.

 

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KEY TABLE FOR GROUP 3 (concluded)

For rocks that fit in Group 3 and do not fit with any of the categories above: 
Do you have a microscope with which to look at each rock sample?

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Yes, I do.  

NEXT STEP:  Look for some of the following features:  Are grains visible?  Are crystals visible?  Are microscopic fossils visible?  Although many possibilities exist, your rock kit only has a few choices to be one or more of these samples.

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No, I do not. 

NEXT STEP:  Look for some of the following features with your hand lens:  Are grains visible?  Are crystals visible?  Are fossils visible?  Although many possibilities exist, your rock kit only has a few choices to be one or more of these samples.

Question 15A:  Which rock samples (use their numbers) fit into this subgroup? (I found no specimen that fit into this group.)

Your rock sample(s) might be:

  • Diatomite (microscopic fossils)

  • Gyprock (crystalline, possibly with some banding of lighter and darker gray layers)

  • Claystone (detrital)

  • Siltstone (detrital and gritty)

Question 15A (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

Question 15B:  Which rock samples (use their numbers) fit into this subgroup? (I found no specimen that fit in this subgroup.)

Your rock sample(s) might be:

  • Diatomite (microscopic fossils)

  • Gyprock (crystalline, possibly with some banding of lighter and darker gray layers)

  • Claystone (detrital)

  • Siltstone (detrital and gritty)


Question 15B (continued): What are the identities (now use their names) for each of the rocks (if any) in this subgroup?  If you have no rocks in this subgroup, skip this part of the question.

 

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