GLS 100 Lab: Sedimentary Rocks
Physical Geology Lab – Dr. Lindley S. Hanson
Although sedimentary rocks comprise only 5% of the Earth’s lithosphere they cover nearly 90% of the
Earth’s surface, where the lithosphere, atmosphere, biosphere, and hydrosphere overlap and interact.
These rocks are therefore instrumental to unraveling past climates, changes in sea level, earth-surface
processes and lastly the evolution of life on Earth.
Pre-lab Assignment: Read Chapter 5 in Essentials of Geology
Objectives

Discuss the significance of sedimentary rocks

Identify the components of sediment and how they are created

Explain how sediment is lithified

List and describe the characteristics of sediment that reflect maturity and depositional setting

Define the three types of sedimentary rocks

Recognize macro-crystalline, microcrystalline, clastic, bioclastic, oolitic, and amorphous
sedimentary textures

Identify common sedimentary minerals such as quartz, calcite, hematite, halite, kaolinite, and
gypsum

Identify and define the major types of detrital (clastic) sedimentary rocks

Discuss the formation and appearance of fissility

Identify and define the organic and inorganic sedimentary rocks

Interpret depositional environments
What is sediment?
Sediment is the disaggregated rock and organic debris that accumulates on the Earth's surface.
Mostly it’s they byproduct of the chemical decomposition and mechanical disintegration of rocks.
These processes, collectively known as weathering, create rock fragments (detritus), new
minerals (e.g. kaolinite and hematite), and ions that are dissolved in water. Nearly all sediment
goes through one or several cycles of erosion, transportation, and deposition before ever settling
long to be buried and hardened rock. This hardening, known as lithification, is accomplished
partially through compaction and more completely through cementation and/or recrystallization.
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Terms: clastic and detrital: Both of these terms refer to fragmental material. However,
detrital refers to rock-derived particles only, whereas clastic refers to fragmental material
without regard to composition or origin (e.g. rock or shell fragments).
Why are sedimentary rocks important?
1. Important nonmetallic resources, such as gas, coal, oil, clay, potash, nitrate, lime, gypsum, salt,
and borax are obtained from sedimentary rocks. Many metallic resources like aluminum, iron, and
gold, originally derived from igneous or metamorphic rocks, are concentrated by weathering and
sedimentary processes into economically minable deposits, such as bauxite, banded iron ore,
bog iron, and placer gold.
2. Past climates and geologic events are recorded in sedimentary rocks. The climate and geologic
processes prevailing when sediment is deposited is recorded in the composition, texture,
sedimentary structures and fossils contained in its rock form.
3. Fossils preserved in sedimentary rocks document the evolution of life throughout geologic time,
and reveal the age of a rock and the events it records.
Transportational history of sediment
Once produced through weathering, detrital sediments are eroded and transported by
gravity, glaciers, rivers, waves and wind until they are permanently deposited and buried.
Some sediment may be picked up and deposited again and again, experiencing several
cycles of erosion and deposition before ever being lithified. The longer detrital sediment
is exposed to weathering, erosion, and transportation the more mature it becomes and
the farther it is carried from its source.
The maturity of sediment is revealed by its sorting, grain roundness, and its relative
abundance of stable (i.e. quartz and clay) and unstable (i.e. ferromagnesium and
feldspars) mineral grains. A well-sorted sediment containing rounded grains of pure
quartz sand is considered very mature, and is typical of beaches along the Gulf of Mexico
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where its source rock lay far away in either the Rockies or Appalachians.
Agents of transportation and deposition
Clastic sediment: Agents that carry sediment also deposit it. Sediment may settle from an
ocean current, river, lake, melting glacier, debris flow, or desert wind. The texture,
composition, and relict features such as ripples, mudcracks, shell fragments, and varves
will reveal the agent responsible and the depositional environment.
Deposition by precipitation: Precipitation can take many forms. Mineral material
dissolved in groundwater will cement clastic sediment if allowed to crystallize in pores
between grains. In arid lakes and inland seas layers of inorganic chemical sedimentary
rock such as rock salt and gypsum crystallize from evaporating water. These are
understandably known as evaporite deposits. Chemical sedimentary rocks precipitated
directly from water have crystalline textures, composed of interlocking crystals.
Sometimes, precipitation is induced by organisms; shellfish, mammals, marine- and
fresh-water microorganisms, and corals all precipitate minerals to create shells, living
quarters, or internal hard parts. Sedimentary rocks formed from this material are
biochemical. The texture of these rocks reflects the organic structure of the organism that
created it. Given time many biochemical rocks will recrystallize, developing a more
interlocking crystalline texture. For example, a diatomite, composed of siliceous shells of
single-celled organisms, will recrystallize into a hard microcrystalline chert. Chalk,
composed of single-celled foraminifera, will recrystallize into a hard microcrystalline
limestone call micrite.
Lithification of Sedimentary Rocks
Some chemical and biochemical materials, such as rock salt and coral, are deposited as rocks.
However most sediment must go through some process of lithification to become a solid rock.
Lithification is accomplished by eliminating pore spaces and binding sediment particles. Processes of
lithification are:
1. Desiccation and compaction: Desiccation, or the drying of sediment, helps reduce pore space by
eliminating the water that fills them. Once the water is removed compaction through overburden
pressure becomes more effective in closing the pores. One or more of the following processes
are required to truly bind sediment.
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2. Cementation: Cementation is the binding of sediment by filling in the pores. Typically dissolved
material in the water flowing through the voids will precipitate in them.
3. Recrystallization: In the presence of pore water, soluble minerals particularly carbonate minerals
(fig 1) like aragonite and calcite will undergo local solution and precipitation until the original
porous sediment has recrystallized. This is the same process responsible for hardening table
sugar and salt that's allowed to sit for a while undisturbed.
Figure 1. Transformation of a shelly beachface sediment to a fossiliferous
limestone through progressive recrystallization.
Classification of Sedimentary Rocks
Sedimentary rocks and be divided into three categories:
1. Detrital sedimentary rocks: These rocks are composed of mineral grains and/or rock fragments
transported and deposited by some geologic agent. Detrital rocks are typically composed of
mineral grains derived from silicate rocks. Because these rocks are composed of particles they
have a clastic texture, and are classified first by grain size and second by composition (table 2).
Table 2. Classification of detrital (siliciclastic) sedimentary rocks.
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Clast size
Sediment
Rock Name
>2 mm
rounded gravel
conglomerate
angular gravel
breccia
(head of a
hat pin or
larger)
1/16
- 2 mm
<1/16 mm
sand
silt and/or
(not visible)
clay(<1./256
Contains all
mm)
matrix
clast sizes
supported
Sandstone
or arenite
Variations (p. 116-120)
quartz pebble --, lithic--, chert--, etc.
quartz sandstone (quartz grains)
arkose (feldspar, qtz., mica, lithic1
fragments, etc.)
mudstone (massive2)
shale (fissile3)
silty shale, shaley mudstone
paraconglomerate
Terms
1 lithic:
fragment of rock such as basalt, chert, quartizite, etc.
2 massive:
3Fissile:
uniform homogeneous appearance with no preferred parting or breakage.
Breaks into thin parallel layers. Property of fine-grained sedimentary rocks with
abundant clay or micaceous minerals.
________________
Notes:
2. Chemical and biochemical sedimentary rocks: These rocks (table 3) are chemically
precipitated and reflect climate where they form. They have a wide range of textures depending on mode
of precipitation and age. Older rocks typically are recrystallized and have denser crystalline textures.
Table 3. Classification of chemical and biochemical rocks.
Composition
Texture & other identifying
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Rock name
characteristics
bioclastic1 Composed
coquina
predominately of shell
carbonate
fragments.(looks like a granola
Micro-bioclastic Composed of
bar)
calcareous shells of
microorganisms(soft,
CaCO3
chalky)
oolitic3 (looks like tapioca)
(aragonite or calcite)
chalk
friable2,
Limestone
oolitic limestone
Micro-crystalline4 Recrystallized
calcareous mudstone having a
micrite
massive5 appearance and sub-
Halite
Gypsum
conchoidal fracture.
crystalline with fossils6
fossiliferous
Macro-crystalline
limestone
rock salt
macro- or micro-crystalline
(massive or fibrous habit)
quartz
Micro-crystalline
iron oxides with minor
variably crystalline and
quartz
amorphous
(iron oxides: limonite,
(typically vuggy7 and rusty in
hematite, and goethite)
appearance)
Hematite and quartz
Micro-crystalline
Evaporates
rock gypsum
Chert
bog iron
Banded Iron Formation (bifs)
Banded red and gray
Terms:
1 bioclastic:
compose of fragmented material from plants or animals.
2 friable:
Grains are easily removed when handled
3 oolitic:
composed of spherical, sand-size carbonate concretions called ooid.
4 microcrystalline:
5 massive:
uniform homogeneous appearance with no preferred parting or breakage.
6fossiliferous:
7vuggy:
composed of interlocking crystals too small to be seen without a microscope
containing molds, casts, remains, etc. of organisms.
Containing irregular cavities.
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Notes:
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3. Organic sedimentary rocks: Organic rocks (table 1.4) are composed of the remains or altered
remains of plants and organisms. (Organic remains = hydrocarbon compounds that are the sole
product of life processes. I distinguish these from the inorganic remains of organisms, such as
carbonate material derived from shells.)
Table 4. Classification of organic sedimentary rocks
Composition
Texture
partially decomposed organic
bioclastic
matter
(brown, loose or friable)
carbon content <65%
completely altered organic
matter: macerals1
Name
peat
amorphous
(black, vitreous to dull luster often in visible layers, coal
carbon content >75%
blocky to conchoidal fractured)
EXERCISE 1: Identifying the maturity of sediment. Read the descriptions in Table 1.
Using numbers 1 (most mature) through 3 (least mature) arrange these sediments in
order of maturity. In the third column write a brief statement justifying your decision.
Table 1. Identifying sediment maturity
Description
Well-sorted quartz sand on a barrier island
in the Gulf of Mexico.
Order
Explanation
.
..
.
..
Pebbly, feldspathic sand with angular
clasts on an alluvial fan in Death Valley,
CA.
Well-sorted and -rounded cobble beach
along the New England Coast.
.
EXERCISE 2: Textures of macroscopic sedimentary rocks. Check the appropriate texture for
each sample listed.
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Table 5. Identification of sedimentary rock textures.
#
Clastic (Detrital)
Bioclastic
Crystalline
3
.
.
.
5
.
.
.
6
.
.
.
13
.
.
.
16
.
.
.
clastic: composed of detrital fragments held together by a matrix or cement. Microscopic rocks
that are fissile are composed of clay fragment and are clastic.
bioclastic: composed of plant, shell or other animal fragments.
crystalline: composed largely of interlocking crystals.
EXERCISE 3: Lab sample identification. Using the lab samples assigned in class fill out the table
below.
Table 6. Identification of sedimentary rocks.
#
Texture
3
,
4
Other physical
Composition
Rock
,.
,.
,.
,
,.
,.
,.
5
,
,.
,.
,.
6
,
,.
,.
,.
7
microcrystalline
,.
,.
,.
11
amorphous
,.
,.
,.
13
,.
,.
,.
,.
15
oolitic
,.
,.
,.
16
,.
,.
,.
,.
properties
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Column Instructions
1. Texture: put clastic (fine-grained, sand-sized, gravel-sized), bioclastic (micro- or
macro-), or crystalline (micro- or macro-), or amorphous (for coal).
2. Other properties: Hardness, color, fissility, friability, vuggy, etc.
3. Mineral composition: calcite (H<5.5, fizzes in HCl), quartz (H>5.5), iron oxides
(streak reddish brown), halite (H<5.5, greasy), detrital grains (mixed mineral
grains), lithic fragments, clay (fissile), etc.
Terms:
amorphous: non-crystalline (e.g. organic rocks)
bioclastic: compose of fragmented material from plants or animals.
friable: Grains are easily removed when handled
oolitic: composed of spherical, sand-size carbonate concretions called ooid.
Macrocrystalline: contains visable crystals
microcrystalline: composed of interlocking crystals too small to be seen without a
microscope
5massive:
uniform homogeneous appearance with no preferred parting or breakage.
fossiliferous: containing molds, casts, remains, etc. of organisms.
vuggy: Containing irregular cavities.
EXERCISE 4. Identify the rocks created when the following sediments are lithified.
Table 7. Rocks from sediments
#
Description
Sedimentary rock
Quartz sand from
1
Perdido Key, Gulf of
Mexico
Shelly beachface
2
sediments from Sanibel
Island Florida
Pebbly beach sediment
3
from Devereau Beach,
Marblehead, MA
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4
Laminated glacial lake
muds
Dessicated carbonate
5
mud from flood tidal
delta, Blindfish Pass,
Sanibel, Florida
6
Organic matter (from a
bog in Maine)
Stream deposited
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quartz pebble gravel
from the S. Atlantic
Coastal Plain
Oolitic sand from an
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offshore bar in the
Bahamas
List of possible candidates: lithic conglomerate, quartz pebble conglomerate, micrite, oolitic limestone,
coquina, quartz sandstone, silty shale, coal, pebbly sandstone, arkose
________________
Notes:
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