Use the flow charts on pages 3 and 4 to identify your samples. List your answers in the table below.
GEOL4
Sedimentary Rocks
Name_____________________________
Overview
Sedimentary rocks are rocks composed of pieces of rock (sediment). They are deposited in a
number of environments by moving air, water, and in some cases by organisms themselves. We
primarily identify and classify sedimentary rocks by their composition. Texture is also used to
classify sedimentary rocks but it will be used in a different sense than it was used to identify
igneous rocks.
Sedimentary rocks record their depositional environment. Often time the shells of organisms
become part of the rock, recording what organisms were there. Sometimes we see traces of life
without seeing the bodies of the organisms. We may see traces made from the feeding pattern of
the organism but not the animal itself. Why, you ask? Because the animal moved away from the
area or they were made of soft tissue which does not preserve well in the rock record.
In arid environments we will see evidence of water when we find rocks with evaporate mineralsor minerals that formed when a body of water dried up such as halite and gypsum. In these
environments we can often find traces of other process such as fossilized raindrops, or ripples
from slow moving water. When wind or water move sediments, we often see a pattern called
cross stratification as ripples move downstream. (See cartoon below).
feeding
(left),
Trace
pattern
f r o m
impression left
Cross stratification from ripples migrating downstream.
Texture
1|Page
fossil
made
from
raindrops
(right).
GEOL4
Sedimentary Rocks
The texture of sedimentary rocks is used to indicate the origin or type of sediment found in the
rock. We will use three textures- clastic, chemical, and biological.
Clastic Rocks
Clastic sedimentary rocks contain clasts. These are fragments or pieces of rock or minerals. The
composition of clastic sedimentary rocks is divided into three types based on the size of the
clasts- clay/ silt, sand, and gravel. Clay and silt are less than 1/16mm. Clay will feel smooth to
your touch and you will not be able to see the grains without a hand lens. Silt size particles will
feel gritty in your hand but you will not be able to see the individual grains. Sand has clasts
between 1/16 and 2 mm in size, and gravel is greater than 2 mm.
Chemical Rocks
Chemical sedimentary rocks are c by identifying the mineral from which they are composed. In
this lab there are four minerals that may need to be identified: quartz, halite, gypsum, and
calcite. Quartz has a hardness of 7 and is very difficult to scratch, even with a good quality knife
blade. Gypsum is relatively soft (Hardness =2) and can be scratched easily with a fingernail.
Halite is common table salt and is most easily identified by taste. However, this is not a sensible
practice in a large lab with many different people handling the samples. Halite has a hardness of
2.5 and cannot be scratched by a fingernail (unpolished fingernail). Calcite readily reacts with a
small drop of HCl. Dolomite will also react with HCl but much less vigorously than calcite and it
usually requires that you powder the sample
Biologic Rocks
Biologic sedimentary rocks form as the result of the accumulation of organic material or biologic
activity. Coal is usually obvious to most students even though few people seem to have ever
actually examined it up close. The dark brown to black color is the most obvious characteristic.
Coquina and limestone are both composed of the mineral calcite. Coquina is composed almost
entirely of shells or fossil fragments- it is weakly cemented together. Limestone may or may not
contain fossil fragments. Both will react to HCl. Limestone containing fossils is referred to as
fossiliferous limestone.
Using the following flow charts and the samples provided by your instructor, identify the texture,
grain size (clastic rocks), mineral composition and rock name for the samples provided.
Depositional Environment
Using the tools we developed in lecture to examine the size, shape, and sorting of clastic rocks.
What information does size, shape and sorting give you about the distance from the source of the
sediments? If you have a chemical or biologic rock, what else can you infer about the
depositional environment? (Example: Lake, River, Ocean, Beach, Reef, Coastal Swamp)
2|Page
GEOL4
3|Page
Sedimentary Rocks
GEOL4
Sedimentary Rocks
Sedimentary Rock Identification
Follow the link and watch each of the videos in the folder for each sedimentary rock:
4|Page
GEOL4
Sedimentary Rocks
https://www.dropbox.com/sh/dcd06vs4dpw6pw2/AAB1AqUOqU0nPasfMlUKG5eGa?dl=0
(Note: You do not need to identify the sedimentary structures, only the rocks.)
Use the flow charts on pages 3 and 4 to identify your samples. List your answers in the table
below.
Sampl
e
#
1
2
3
4
5
6
7
5|Page
Texture
(see list
above)
Grain Size
(if clastic)
Depositional
Environment
Composition
Rock Name
GEOL4
8
9
10
11
12
13
14
6|Page
Sedimentary Rocks