GLS 100 Lab 9: 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.

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?

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.
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.
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 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 inorganicchemicalsedimentary rock such as rock salt and gypsumcrystallize 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:

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.
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.
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.
Clast size / Sediment / Rock Name / Variations (p. 116-120)
>2 mm
(head of a hat pin or larger) / rounded gravel / conglomerate / quartz pebble --, lithic--, chert--, etc.
angular gravel / breccia
1/16 - 2 mm / sand / Sandstone
or arenite / quartz sandstone (quartz grains)
arkose (feldspar, qtz., mica, lithic1 fragments, etc.)
<1/16 mm
(not visible) / silt and/or
clay(<1./256 mm) / mudstone (massive2) / silty shale, shaley mudstone
shale (fissile3)
Contains all clast sizes / matrix supported / paraconglomerate
Terms
1 lithic: fragment of rock such as basalt, chert, quartizite, etc.
2 massive: uniform homogeneous appearance with no preferred parting or breakage.
3Fissile: Breaks into thin parallel layers. Property of fine-grained sedimentary rocks with abundant clay or micaceous minerals.

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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 characteristics / Rock name
carbonate
CaCO3
(aragonite or calcite) / bioclastic1 Composed predominately of shell fragments.(looks like a granola bar) / coquina / Limestone
Micro-bioclastic Composed of calcareous shells of microorganisms(soft, friable2, chalky) / chalk
oolitic3(looks like tapioca) / oolitic limestone
Micro-crystalline4Recrystallized calcareous mudstone having a massive5 appearance and sub-conchoidal fracture. / micrite
crystalline with fossils6 / fossiliferous limestone
Halite / Macro-crystalline / rock salt / Evaporates
Gypsum / macro- or micro-crystalline
(massive or fibrous habit) / rock gypsum
quartz / Micro-crystalline / Chert
iron oxides with minor quartz
(iron oxides: limonite, hematite, and goethite) / variably crystalline and amorphous
(typically vuggy7 and rusty in appearance) / bog iron
Hematite and quartz / Micro-crystalline
Banded red and gray / Banded Iron Formation (bifs)
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: composed of interlocking crystals too small to be seen without a microscope
5 massive: uniform homogeneous appearance with no preferred parting or breakage.
6fossiliferous: containing molds, casts, remains, etc. of organisms.
7vuggy: Containing irregular cavities.

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Notes:

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 matter
carbon content <65% / bioclastic
(brown, loose or friable) / peat
completely altered organic matter: macerals1
carbon content >75% / amorphous
(black, vitreous to dull luster often in visible layers, blocky to conchoidal fractured) / coal

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 / Order / Explanation
Well-sorted quartz sand on a barrier island in the Gulf of Mexico. / . / ..
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.

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 / Other physical properties / Composition / Rock
3 / , / ,. / ,. / ,.
4 / , / ,. / ,. / ,.
5 / , / ,. / ,. / ,.
6 / , / ,. / ,. / ,.
7 / microcrystalline / ,. / ,. / ,.
11 / amorphous / ,. / ,. / ,.
13 / ,. / ,. / ,. / ,.
15 / oolitic / ,. / ,. / ,.
16 / ,. / ,. / ,. / ,.
Column Instructions

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 (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
1 / Quartz sand from Perdido Key, Gulf of Mexico
2 / Shelly beachface sediments from Sanibel Island Florida
3 / Pebbly beach sediment from Devereau Beach, Marblehead, MA
4 / Laminated glacial lake muds
5 / Dessicated carbonate mud from flood tidal delta, Blindfish Pass, Sanibel, Florida
6 / Organic matter (from a bog in Maine)
7 / Stream deposited quartz pebble gravel from the S. Atlantic Coastal Plain
8 / Oolitic sand from an 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

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