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

Instructor:  Gwyneth Jones

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GEL101 Key to
Sedimentary Rock Exercise

@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, paying attention to the charts and images.  You will need to be familiar with basic terminology for composition and texture of sedimentary rocks. 

Tables 8.3 and 8.4 distinguish two main categories of sedimentary rocks:  (1) Clastic (also called Detrital) Sedimentary Rocks; and (2) Chemical Sedimentary Rocks.

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:

Sedimentary Rock Identification

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.

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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 or are invisible put them into Group 2.  

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.  

Look for shell fragments from bivalves (clam) or gastropods (snails).  The shells may have been replaced or might be impressions or molds of these shells. 

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No, I do not see fossils. 

Look for pebbles and sand grains.

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".  

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No, I do not see "fizz". 

Your rock sample(s) might be:

  • Diatomite-chalk-like, soft, friable, earthy, very fine-grained, siliceous sedimentary rock, usually light in color

  • Bituminous coal-a shiny black coal

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Yes, I see very large grains.  

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No, I do not see very large grains. 

 

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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.  

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

This sample is called a quartz sandstone.

NEXT STEP:  Does the rock contain abundant feldspar?  To determine this, look for white, gray, and/or pink minerals.

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KEY TABLE FOR GROUP 2
Chemical Sedimentary Rocks and
Detrital Sedimentary Rocks with Microscopic Grains

For rocks that fit in Group 2:
Do any of these rocks effervesce when acid (concentrated lemon juice) is applied?

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Yes, I see "fizz".  

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No, I do not see "fizz". 

 

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KEY TABLE FOR GROUP 2 (continued)

For rocks that fit in Group 2 and do not effervesce: 
Is the rock obviously crystalline with a shiny reflection?

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Yes, it looks crystalline, that is there are no layers when viewed from the side of the sample.

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No, it does not look crystalline, there are layers when viewed from the side of the sample

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.

 

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Yes, I do see "fizz" afterall.

Your rock sample(s) might be:

  • Micrite

  • Dolostone

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No, I do not see "fizz". 

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Coal-black or brown, thick layers of plant organic material.  You may see plant fragments.  

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Shale-green, gray, black, or sometimes red mudstone. 

 

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KEY TABLE FOR GROUP 2 (concluded)

For rocks that fit in Group 2 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.

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)

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)

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Answer the multiple-choice questions below: With your observations, determinations, and identifications of your igneous rock samples, proceed to answer your lab questions with relative ease (hopefully) and submit this last part of your lab homework. 

1.  Sample 22 contains mostly
     a.  quartz
     b.  calcite
     c.  organic material
     d.  halite
     e.  clay and silt grains

2.  Where does sediment like that in sample 22 typically accumulate?
     a. in evaporating desert lakes
     b.  on flanks of volcanoes
     c.  in rapidly flowing rivers
     d.  on the deep sea floor
     e.  all of the above

3.  Sample 22 is a
     a.  conglomerate
     b.  breccia
     c.  sandstone
     d.  shale
     e.  limestone

4.  The fossils in sample 23 are
     a.  plant debris
     b.  shell fragments
     c.  vertebrate bones
     d.  fish scales
     e. too small to see with the unaided eye

5.  Sample 23 contains  
     a.  quartz
     b.  calcite
     c.  organic material
     d.  halite
     e.  clay and silt grains

6.  Sample 23 is a
     a.  fossiliferous limestone
     b.  fossiliferous sandstone
     c.  fossiliferous shale
     d.  coquina
     e. chert

7.  How well-sorted is sample 24?
     a.  very well sorted
     b.  well sorted
     c.  moderately sorted
     d.  poorly sorted
     e.  very poorly sorted

8.  Sample 24 is a
     a.  conglomerate
     b.  breccia
     c.  sandstone
     d.  shale
     e.  limestone

9.  Sample 25 contains
     a.  coarse clastic grains
     b.  fine clastic grains
     c.  both fine and coarse clastic grains
     d.  no clastic grains:  it is organic
     e. no clastic grains:  it is inorganic

10.  Qualitatively, how hard is the material in sample 25?
     a.  A fingernail scratches it easily.
     b.  A cent scratches is easily.
     c.  A steel nail scratches it easily.
     d.  None of these tools scratches it easily.
     e.  I don't know, it is magnetic.

11.  Sample 25 is a
     a.  limestone
     b.  sandstone
     c.  shale
     d.  coquina
     e.  chert

12.  Compare that maturity of samples  26 and 29.
     a.  Sample 26 is much more mature than sample 29.
     b.  Sample 26 may be a bit more mature than sample 29.
     c.  Samples 26 and 29 are almost indistinguishable.
     d.  Sample 26 may be a bit less mature than sample 29.
     e.  Sample 26 is much less mature than sample 29.

13.  Sample 26 is a
     a.  limestone
     b.  quartz sandstone
     c.  arkose
     d.  micrite
     e.  chert

14.  Sample 27 contains
     a.  coarse clastic grains
     b.  fine clastic grains
     c.  both fine and coarse clastic grains
     d.  no clastic grains:  it is organic
     e. no clastic grains:  it is inorganic 

15.  Sample 27 is a
     a.  coal
     b.  arkose
     c. micrite
     d.  chert
     e. coquina

16.  One striking diagnostic feature of sample 28 is that it
     a.  is harder than glass
     b.  reacts with acid
     c.  is magnetic
     d.  contains prominent fossils
     e.  rapidly dissolves in water

17.  Sample 28 contains mostly
     a.  quartz
     b.  calcite
     c.  organic material
     d.  halite
     e.  clay and silt grains

18.    Sample 28 is a
     a.  coal
     b.  arkose
     c. micrite
     d.  chert
     e. coquina

19.  Sample 29 contains mostly
     a.  quartz
     b.  calcite
     c.  organic material
     d.  halite
     e.  clay and silt grains

 20.  Sample 29 is a
     a.  limestone
     b.  quartz sandstone
     c.  arkose
     d.  micrite
     e.  chert