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Physical Geology Lab -Sedimentary Rocks
Sedimentary rocks are formed on the earth's surface by the deposition and lithification of sediment. They
compose the weathered rind of the lithosphere.
Prelab Assignment: Read Chapter 5 in Essentials of Geology
What is sediment?
Sediment is any material deposited on the earth's surface that is derived from weathering. Weathering
creates rock fragments (detritus), new minerals, such as kaolinite and hematite, and dissolved ions, which
may later precipitate from water. All of these components, along with organic debris, comprise the material
that make up sedimentary rocks.
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, 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. Climatic and geologic events are recorded in sedimentary rocks.
3. Sedimentary rocks cover over 90% of the earth's surface and are therefore the most exposed.
However, this cover is so thin that the total volume of sedimentary rocks is small compared to
other rocks in the lithosphere.
Transportational history of sediment
Detrital sediments are carried by gravity, glaciers, rivers, waves and wind and are
eventually deposited. Some sediment may be picked up and deposited again and again,
experiencing several cycles of erosion and deposition before ever being incorporated into
a rock. The longer a detrital sediment has been exposed to weathering and transportation
the more mature it becomes.
Maturity is recognized by the degree of sorting, roundness of the grains, and the absence
of unstable rock fragments such as ferromagnesium minerals and plagioclase. A highly
mature sediment containing well-rounded sand grains of quartz, has experienced may
cycles of erosion and deposition and is far traveled from its original source. Quartz sand
deposited along the Gulf of Mexico may have originated from rocks exposed on the
eastern flanks of the Appalachian Mountains or Western margin of the Rocky Mountains.
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EXERCISE 1: 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 Order Explanation
Quartz sand on a barrier island in the Gulf of
Pebbly feldspathic sand on an alluvial fan in
Death Valley, CA.
Well-sorted and -rounded cobble beach along
the New England Coast.
Deposition of detrital sediment: Agents that carry sediment, such as water, wind and
ice, also deposit it. Sediment may be deposited in the ocean, on a river bed, in a lake,
beneath a glacier or in a number of other depositional settings. The texture and
composition of sedimentary rocks reflects their sediment source, transportational history,
and depositional environment.
Deposition by precipitation: Dissolved mineral material carried by water will eventually
crystallize from solution forming a chemical sedimentary rock, such as limestone, rock
salt, and gypsum. Chemical sedimentary rocks precipitated directly from water typically
have a crystalline texture, composed of interlocking crystals. However, some
precipitation is aided by organisms. For example, shellfish, mammals, marine micro-
organisms, and coral, induce mineral precipitation to form hard parts (either
endoskeletons, exoskeletons) or living quarters. Sedimentary rocks formed in this manner
are biochemical. The texture of these rocks typically reflect the organic structure of the
organism that created it. Over time these rocks will recrystallize to develop a more
interlocking crystalline texture.
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, turning
a sediment into a rock, is accomplished by reducing pore space and binding sediment particles. The
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.
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: Some minerals, particularly carbonate minerals (fig 1) such as aragonite and
calcite, will undergo local solution and precipitation until the original porous sediment is altered to
a massive rock composed of interlocking crystals. This is the same process that hardens table
sugar and salt that's allowed to sit for a while.
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Figure 1. Transformation of a shelly
beachface sediment to a fossiliferous
limestone through progressive
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.
Clast size Sediment Rock Name Variations (p. 116-120)
>2 mm rounded gravel conglomerate
quartz pebble --, lithic--, chert--, etc.
(head of a hat angular gravel breccia
pin or larger)
quartz sandstone (quartz grains)
1/16 - 2 mm sand arkose (feldspar, qtz., mica, lithic1
silt and/or mudstone (massive2)
silty shale, shaley mudstone
clay(<1./256 shale (fissile3)
matrix supported paraconglomerate
lithic: fragment of rock such as basalt, chert, quartizite, etc.
massive: uniform homogeneous appearance with no preferred parting or breakage.
Fissile: Breaks into thin parallel layers. Property of fine-grained sedimentary rocks with abundant
clay or micaceous minerals.
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
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Table 3. Classification of chemical and biochemical rocks.
Texture & other identifying
Composition Rock name
bioclastic1 Composed predominately
of shell fragments.(looks like a coquina
microbioclastic Composed of
calcareous shells of
oolitic3 (looks like tapioca) oolitic limestone (p. 120-123)
(aragonite or calcite) microcrystalline4 Recrystallized
calcareous mudstone having a
massive5 appearance and
crystalline with fossils6
Halite Macro-crystalline rock salt
macro- or microcrystalline Evaporates
Gypsum (massive or fibrous habit) rock gypsum (p. 119-126)
quartz microcrystalline Chert
iron oxides with minor quartz variably crystalline and amorphous
(iron oxides: limonite, (typically vuggy7 and rusty in bog iron
hematite, and goethite) appearance)
Micro-crystalline Banded Iron Formation (bifs)
Hematite and quartz
Banded red and gray ( p. 257)
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.
microcrystalline: composed of interlocking crystals too small to be seen without a microscope
massive: uniform homogeneous appearance with no preferred parting or breakage.
fossiliferous: containing molds, casts, remains, etc. of organisms.
vuggy: Containing irregular cavities.
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 Name
partially decomposed organic
(brown, loose or friable)
carbon content <65%
completely altered organic amorphous
matter: macerals1 (black, vitreous to dull luster often in visible layers, coal
carbon content >75% blocky to conchoidal fractured)
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EXERCISE 2: Textures of macroscopic sedimentary rocks. Check the appropriate texture
for each sample listed.
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: Identification of lab samples. Using the lab samples assigned in class fill out the
Table 6. Identification of sedimentary rocks.
# Texture Composition Rock
3 , ,. ,. ,.
4 , ,. ,. ,.
5 , ,. ,. ,.
6 , ,. ,. ,.
7 microcrystalline ,. ,. ,.
11 amorphous ,. ,. ,.
13 ,. ,. ,. ,.
15 oolitic ,. ,. ,.
16 ,. ,. ,. ,.
Texture: put clastic (fine-grained, sand-sized, gravel-sized), bioclastic (micro- or macro-), or
crystalline (micro- or macro-), or amorphous (for coal).
Other properties: Hardness, color, fissility, friability, vuggy, etc.
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
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EXERCISE 4. Identify the rocks created when the following sediments are lithified.
Table 7. Rocks from sediments
# Description Sedimentary rock
Quartz sand from Perdido
Key, Gulf of Mexico
2 sediments from Sanibel
Pebbly beach sediment
3 from Devereau Beach,
Laminated glacial lake
Dessicated carbonate mud
from flood tidal delta,
Blindfish Pass, Sanibel,
Organic matter (from a
bog in Maine)
Stream deposited quartz
7 pebble gravel from the S.
Atlantic Coastal Plain
Oolitic sand from an
8 offshore bar in the
List of candidates: lithic conglomerate, quartz pebble conglomerate, micrite, oolitic limestone, coquina,
quartz sandstone, silty shale, coal