Download Lab 2: Intro to Chem/Biochem rocks

GEOS 302, LAB 2: INTRODUCTION TO CARBONATE AND CHEMICAL
SEDIMENTARY ROCKS
Purpose:
1. Be able to identify carbonates and describe them using either Folk or Dunham.
2. Understand the origin of different carbonate rocks
3. Be familiar with evaporates, coals, cherts
Reference material for carbonates and other chemical sedimentary rocks
(see also Boggs, chapters 6 and 7)
CARBONATE ROCKS
What are Carbonate rocks?
(bio)chemical rocks
a. The matrix and clasts that make up these rocks were formed through
chemical or biochemical reactions (precipitation)
b. Precipitation occurs within the basin in which the sediments are deposited
(almost always)
2. CaCO3 makes up limestones
a. aragonite or calcite (polymorphs of CaCO3)
b. soft, rhombohedral cleavage, clear to milky in color
c. fizzes with HCL
3. MgCa(CO3)2 makes up dolomites
a. Magnesium replaces some calcium in the crystal structure
b. Evaporates, groundwater mixing, interaction between carbonates and Mg-rich
waters
c. Must scratch to get the rock to fizz with HCl
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Many sandstones can have calcium carbonate cement – watch what is fizzing!
Carbonate Classification – general notes
carbonates are mono-mineralic, so texture is going to rule in classification
to name a carbonate rock, you look at the MATRIX and CLASTS
a. if there are clasts, what are they?
b. what kind of matrix do you see?
c. what is the proportion of matrix to clasts?
3. there are two classification schemes for carbonate rocks: Folk and Dunham - we
will learn both in this lab
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In general, Folk’s classification works best for thin sections under high magnification
(with a petrographic microscope), while Dunham’s scheme is more useful in the field.
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Naming Carbonates: Folk's Classification
Classification based on characteristics of matrix (orthochem) and clasts (allochems)
orthochem types (matrix)
a. micrite (microcrystalline calcite)
i. dark, opaque, tiny tiny crystals
ii. a mud composed of microcrystalline calcite
iii. most carbonate rocks have micrite as their matrix
b. sparry calcite (sparite)
i. clear, shiny (sparkly), larger crystals, translucent
ii. often replacement when directly precipitated,
iii. usually implies little carbonate mud in the area, and an agitated
depositional environment
2. allochem types (clasts)
a. bioclasts (bio-)
i. fossils, chunks, fragments, who/e etc
ii. sometimes encompasses fragments of eroded limestone rocks
b. coated grains (oo-)
i. ooids: sand-sized spheres of CaCO3 created when a tiny particle
nucleates calcite precipitation in , to and fro movement, sand sized,
more regular concentric lamina
ii. oncoids: larger, accretion aided by algae, irregular or rough looking
concentric lamina
c. intraclasts (intra-)
i. semi-consolidated hunks of carby material (often ripped up)
ii. often angular looking, can have lamiae within the clasts
d. pelloids (pell-)
e. everything else
f. opaque, can look like an elongated sphere
g. often fecal matter or bio-degredated shells
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Folk's Formula: alIochem prefix(es) + orthochem word = one word
 e.g. biomicrite, oosparite
 when there are multiple allochem types, the most common type is the last prefix
e.g. biointrasparite = more intraclasts than bioclasts
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Naming Carbonates: Dunham's Classification
Uses the relative abundance of the allochems (clasts), and how they are supported in
the matrix (orthochem)
Terms to describe allochem abundance:
 Mudstone: <10% allochems
 Wackestone: >10% allochems, but matrix supported (not many clasts)
 Packstone: >10% allochems, clast supported; still has some micrite/mud for matrix
 Grainstone: no micrite/mud orthochem, can have sparite, and is clast supported
Important modifiers describe the allochems:
 Fossiliferous
 Oolitic
 Intraclast
 Pelloidal
Dunham’s Formula: allochem type + allochem abundance
e.g. fossiliferous packstone; oolitic mudstone
Naming Carbonates: reef rocks
Those carbonate rocks where the allochems weren't really clasts but were part of a
cohesive unit prior to deposition, e.g., a piece of coral reef
Folk classification: biolithite
Dunham classification: boundstone
Note that there is some interpretation inherent in these names (not just descriptive)
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OTHER CHEMICAL ROCKS
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coals: diagenesis of organic material
a. anthracite: high grade coal, looks shiny
b. lower grades (bituminous, sub- bituminous): progressively “duller” and softer
c. bituminous sandstone: a sandstone with a large amount of organic material,
or a “sandy” coal.
evaporites: chemical precipitation in an evaporitic setting
a. often salts (gypsum. anhydrite. NaCl)
chalk: billions of microfossils called coccoliths
a. made of CaCO3 and can fizz
soil carbonate
a. CaCO3 (usually) precipitation in the soil column.
b. nodular looking
c. preserved in paleosols
chert: microcrystalline silica
a. tiny organisms with silica shells like radiolarians or diatoms
b. or SiO2 replacement of other material
c. often nodular, can be many colors
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NAME: _______________________________________
GEOS 302: SEDIMENTOLOGY/STRATIGRAPHY
LAB 2: INTRODUCTION TO CARBONATE AND CHEMICAL
SEDIMENTARY ROCKS
PART I. CARBONATE IDENTIFICATION
A. Sample 302-II-lA: What is the grain size of this rock? What is the likely energy of
deposition (calm, turbulent, etc.)? In what water depths was it likely deposited?
Most carbonate rocks have a light grey color. Why do you think this rock is dark grey?
Name this rock using the classification schemes of Folk and Dunham.
Folk:
Dunham:
B. Sample 669-T-2: Name this rock using the classification schemes of Folk and Dunham.
Folk:
Dunham:
Is this rock matrix- or grain-supported?
C. Samples 12, 758-AHO, and Vial 16: There are two allochem types in these rocks (each rock
contains only one type). Although they are similar in appearance, they form by different means.
What are they called?
How do they form? (If you don't know, consult Boggs). Hint: The use of drawings is always
helpful.
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Name each of these rocks, using the classification schemes of Folk and Dunham.
SAMPLE
FOLK
DUNHAM
12
357-AM3
D. Sample APL-IR: If this were a siliciclastic rock, what would you name it?
How far were the clasts (called "intraclasts") in this rock transported before they were
deposited?
Name this rock using the classification schemes of Folk and Dunham.
Folk:
Dunham:
E. Sample 28: Name this rock using the classification schemes of Folk and Dunham.
Folk:
Dunham:
F. Sample TC169: There are two names for this rock in both Folk's and Dunham's
classification schemes-an objective textural one, and a genetic/interpretive one. Name both.
FOLK
DUNHAM
Textural
Interpretive
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PART II. CARBONATE CLASSIFICATIONS
Name each of the following rocks using both Folk and Dunhams classification scheme.
Sample
Folk
Dunham
302-ll-2F
DB 00
302-ll-2C
DB 01
754-DMG
302-ll-2G
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25
26
736 - KPL
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PART III: OTHER CARBONATES
A. Soil Carbonates
Carbonate rocks are not only found in marine environments. Soil carbonates are common in both
modern and ancient soils. Paleoclimatic studies often rely on these rocks as paleo-indicators of
past temperature, precipitation and seasonality. In what sort of climate does soil carbonate
typically form? Explain why.
B. Fresh-Water Carbonates
Lacustrine carbonates are also significant in the geologic record. Tufa is a porous carbonate
material often formed in springs and lakes. Marls are carbonate-rich mudstones which can be
deposited on a lake floor. Match the samples with their appropriate rock type (soil
carbonate, tufa or marl)
Sample
Lithology
PF-20
61804-2
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PART IV: CHEMICAL AND CARBONACEOUS SEDIMENTS
A. Peats and Coals
Match the following rock names to their corresponding samples; write the NAME in the
blank
302-"-4a
______________________________
302-"-4b
______________________________
302-"-4c
______________________________
1. Coalified tree; 2.Bituminous sandstone; 3. Anthracite
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B. Evaporites
Describe SAMPLE 302-II-4f and the characteristics of the different layers. Speculate as to
the composition of the layers and their origin (think about seasonal cycles; Boggs ch.7 may be
useful).
If you begin with a sample volume of standard seawater and then evaporate the water,
what minerals should form? List them in the order of formation and include the percentage
of the original volume of water present upon the start of precipitation of each. (see Boggs,
7.2)
C. Chalk and Tephra
Chalk is composed of parts of microscopic algae called coccoliths. They accumulate in deep-sea
settings and were abundant in the Cretaceous period. Tephra is a term used to identify
volcaniclastic sediments. Which sample (302-ll-4h or 302-ll-4i) is which? How did you
distinguish the samples from one another?
D. Chert
What is the grain size of the sample X-75? Does the specimen react when tested with HCI?
Why or Why not? What is chert and how does it form? Where does it form?
Chert takes on many different colors. Using your knowledge of coloration differences within a
given mineral type (e.g., rose quartz, smoky quartz, amethyst, etc.), propose an explanation for
the coloration of chert.
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