LAB # 1--CLASTIC ROCKS
In this lab we will learn to identify several different types of sedimentary rocks, including mudstones, siltstones,
sandstones, conglomerates, and breccias. I have set aside the rocks that we will study. We will be working in teams.
Clastic sedimentary rocks - Rocks composed of fragments of pre-existing rocks. Sandstones, siltstones, mudstones,
conglomerates, and breccias are clastic rocks.
Classification of Sedimentary Rocks
Texture
Clastic sedimentary rocks are classified according to texture (grain size). For example, sandstone is a rock comprised
of grains ranging from .062 to 2 mm. A rock composed mostly of grains greater than 2mm is either a breccia or a
conglomerate, depending on the rounding of the grains. Make sure that you understand that the classification of
clastic rocks as conglomerate, sandstone, siltstone, mudstone, and claystone is based on texture.
Rounding
Sorting
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Classification of Sedimentary Rocks Based on Composition
Sandstones are also classified according to mineralogy or composition of the rock. The sandstones that you are working with
will fall into one of the following classifications based on the composition of the rock:
1. Arkose--usually red, contains greater than 25% feldspar, usually contains rock fragments, micas and clays. The high
percentage of unstable feldspars and lithic fragments usually indicates that the grains forming the rock have not been
transported great distances. The sediment was deposited close to the source area
2. Quartz arenite--contains greater than 95% quartz. May contain heavy minerals, which are more resistant to abrasion. To
form a quartz-rich rock such as a quartz arenite requires special conditions. A quartz arenite implies several cycles of erosion,
transport, and deposition to form. This way the unstable mineral grains such as the feldspars are removed.
3. Greywacke--usually gray, contains an abundance of clay, rock fragments, feldspar and quartz. The presence of feldspars and
rock fragments suggest that the sediment forming the greywacke was not transported great distances.
4. Lithic sandstone-- contains an abundance of rock fragments. Lithic sandstones are usually deposited close to the source
area
I) LABORATORY EXERCISE ---- EXAMINE THE FOLLOWING SEDIMENTS AND ROCKS. ANSWER THE
QUESTIONS POSED FOR THE SAMPLES.
1. 2JDM-Aeolian Sand --This sand is derived sand dunes in the Mojave Desert. The sand will eventually lithify to form a
sandstone. Aeolian refers to a sand dune environment. Examine the sand and make sure that you feel comfortable identifying
quartz. The quartz grains are glassy and show conchoidal fractures.
A. What is the texture (size, rounding, sorting) of the sand?
B. What is the textural and compositional maturity?
C. Use your textural description to characterize the sand e.g. coarse grained, poorly sorted, well-rounded sand.
2. 3 JDM --This sand is from the beach at Malaga Cove near Palos Verdes. The dark grains are heavy minerals such as
magnetite, ilmenite, etc. Again make sure that you can identify the quartz grains
A. What is the texture (size, rounding, sorting) of the sand?
B. What is the textural and compositional maturity?
C. Use your textural description to characterize the sand e.g. coarse grained, poorly sorted, well-rounded sand.
3. 1610 & 1610A-- Excellent example of a quartz arenite. Remember that quartz arenites contain greater than 95% quartz.
Confirm that these samples are quartz arenites. The red color of this sample results from iron oxide staining. Describe using
description sheets
4. 1610 G-- This is a typical arkose. Arkoses contain greater than 25% feldspar. The feldspar is commonly potassium
feldspar. Arkoses are derived from the breakdown of granites and gneisses. Many arkoses are red because of the pink
potassium feldspar and the iron staining. Identify the feldspar. Identify the granitic lithic (rock) fragments. The silvery
mineral is muscovite; it should flake off if you scratch it with your fingernail or a knife. Describe using description sheets
5. X111-- Lithic Sandstone -- This is a lithic sandstone. Note that the rock fragments are composed of plutonic igneous rocks.
A. Does the rock contain matrix?
B. If so, is it matrix or framework supported? A rock is matrix supported if most of the grains are floating in the
matrix without making contact with other grains. A rock is framework supported when a majority of the clasts/grains
is in contact with each other.
6. 7144 -- Graywacke -- Graywacke is a term that evokes much controversy. It is used for a hard, dense sedimentary rock that
contains greater than 15% matrix of typically recrystallized clays. Graywackes are typically gray. Make sure that you can
identify the grains and the matrix. Graywackes are texturally and compositionally immature.
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7. 410A-- Conglomerate
A. What is the composition of the clasts?
B. Is the rock matrix or framework supported?
8. 1770 A --Shale is a mudrock, which splits parallel to bedding planes. Splitting or parting of a mudrock parallel to the
bedding planes is termed fissility and the rock is said to be fissile.
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