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GEOL 101 - Physical Geology Laboratory

SEDIMENTARY ROCKS – Lab #5

PRELAB SECTION – To be completed before labs starts:

I. Introduction & Purpose:

The purpose of this laboratory exercise is to become familiar with identifying common sedimentary rocks and understanding their depositional origin. In this lab you will learn to identify sedimentary rocks in hand samples from their physical properties. You will become familiar with the most common sedimentary rock-forming minerals and processes. The nature and origin of sedimentary rocks, the major types of sedimentary rocks, and their structures, and the connection between plate tectonics and sedimentary rocks in the rock cycle will be explored.

II. General Overview and Classification of Sedimentary Rocks

A. The classification of sedimentary rocks is based upon two major criteria (see page 110 in text)

1. Texture = grain size and rock “fabric”

2. Composition = mineralogy

Sedimentary rocks are divided into three major groups:

1) Detrital clastic

2) Biochemical crystalline

3) Chemical crystalline

Detrital sedimentary rocks consist of sediment grains (called clasts) that are cemented together; these rocks have a "clastic" sedimentary texture. The sediment grains consist of one or more mineral crystals that come from the weathering and erosion of preexisting source rock, such as granite or volcanic rock; any source rock type is possible. Detrital sedimentary rocks are classified primarily upon grain size, e.g. sand(-sized)stone versus silt(-sized)stone.

In contrast, biochemical and chemical sedimentary rocks consist mainly of mineral crystals that have crystallized directly out of aqueous solutions (water), either secreted by living organisms (biochemical), or by inorganic precipitation (chemical), respectively. Most of these chemically derived sedimentary rocks have a "crystalline" sedimentary texture, much like that of igneous rocks. Biochemical and chemical sedimentary rocks are classified primarily upon mineral composition, e.g. limestone (CaCO3) versus chert (SiO2).

B. Grain Size and Texture: Examine Figures 6.1 and 6.2 (pages 111-112) in your lab manual.

These images of sediment grains and the major sedimentary rock types show the variation in

sediment textures and chemical compositions by which sedimentary rocks are classified.

The 6 types of sedimentary grain sizes and the textures (1^st column of Fig. 6.1)

Grain Size Name Grain Size Description Notes (Not mandatory)

1. _____Gravel-size___ ______________________________________________________

2. _____Sand-size_____ ______________________________________________________

3. _____ Silt-size _____ ______________________________________________________

4. _____ Clay-size ____ ______________________________________________________

5. ___Microcrystalline__ ______________________________________________________

6. ___Macrocrystalline_ ______________________________________________________

C. Grain Shape: Detrital grains are typically rock fragments that have been weathered and

eroded from pre-existing rock and transported, over time, a certain amount of distance from its

source. As a general rule of thumb, the farther and longer the grains have been transported

form their source, the more rounded they become. Thus, the detrital grain shape gives an

indication of its “maturity” in the sedimentary cycle.

1. List the three types of sediment grain shapes exhibited in detrital rocks (2nd column of Fig 6.1)

Detrital Grain Shapes

1. ____________ __ 2. ______________ 3. ________________

2. Question: Which grain shape type would you expect to be the least “mature”? Why?

3. Question: Which grain shape type would you expect to be the most “mature”? Why?

D. Grain Arrangements: Transported sediment grains become sorted (according to size), over

time, as the fluid mediums that transport them, such as running water and wind, selectively

deposit some grains while continuing to carry the smaller grains ever greater distances from the

source region. The causes for sorting include systematic variation in flow rate and turbulence of

the transporting medium over distance and time. As a general rule of thumb, greatest sorting

occurs within consistently medium to high energy transport mediums over long periods of time

(effective winnowing of sediment), whereas the conditions of poorest sorting occurs where

either, flow rate changes drastically or is very inconsistent (effective dumping of sediment).

Additionally, non-fluid transport mediums such as glaciers do not have the capability to sort

sediment by size, and therefore sediments directly deposited by glaciers are virtually unsorted.

1. List the three types of sediment grain arrangements found in detrital rocks (3rd column of Fig 6.1)

Detrital Grain Arrangements

1. ______________ 2. ______________ 3. __ ______________

2. Question: Why would a river be good at sorting sediment and a glacier not?

3. Question: Would you expect there to be a direct relationship to exist between grain shape,

i.e. roundness, and grain arrangement, i.e. sorting? Explain your answer.

E. Composition of Sedimentary Rocks: The mineral composition of a sedimentary rock is a

reflection of 1) source material and 2) sedimentary processes. Sources include virtually all types of geologic, biologic, hydrologic, and cosmologic materials such as: 1) land-derived materials such as weathered and eroded igneous, metamorphic and sedimentary rocks; 2) hard-part remains (shells) of marine organisms; and 3) seawater chemical precipitates. Sedimentary lithification processes, termed “diagenesis” can both, alter and add chemicals and minerals to the rock, such as rock cement. A review of the composition of all the major sedimentary rock types shows a surprising conclusion: that there are only a small number of major sedimentary rock-forming minerals and rock fragment detrital types. The vast bulk of sedimentary rocks have one or more of the following mineral constituents: quartz/ silica, feldspar, mica, clay, iron oxide, amphibole, calcium carbonate, and various minor amounts of sulfate, phosphate, and halide minerals. The primary reason for this compositional simplicity, compared to those of igneous and metamorphic rocks, is the fact that most of the sedimentary rock-forming minerals are stable or meta-stable at Earth surface conditions; many of the igneous and metamorphic rock-forming minerals are unstable at the surface and with sufficient time will alter to minerals such as the clays, silica, and carbonates.

The major source materials for each of the three sedimentary rock types (see Figure 6.2 page 114)

Detrital (Clastic-origin)

1. Rock fragments 2. Quartz 3. Feldspar 4. Clay 5) Dark silicates and oxides

Biochemical (Organic-origin)

1. Shells and Shell and coral fragments (carbonates and silica) 2. Carbon and Charcoal

Chemical (Inorganic-origin)

1. Calcite 2. Dolomite 3. Quartz_ 4. Gypsum_ 5. Halite 6. Iron-bearing minerals

1. Question: Clay is the most common sedimentary mineral. How is it derived? Why so much?

2. Question: Calcium carbonate is another major sedimentary constituent. How is it derived?

F. Naming the Detrital Rocks: Detrital rocks are named based primarily upon their grain size,

while their composition, which is a reflection of the source rocks and subsequent weathering

erosion history, is secondary to naming a detrital rock, e.g. arkose and wacke.

1. The five major detrital (clastic) rock types that are based on grain size and shape.

List the grain size and typical mineral(s)/material(s) associated with that rock type. Note:

siltstone and shale are lumped together as “mudstones”. (See top of Figure 6.8, page 118).

Rock Name Grain Size Texture and Distinctive properties

1. _____Breccia______ _________ ______________________________________

2. ___ Conglomerate__ _________ ______________________________________

3. ____Sandstone____ _________ ______________________________________

4. ____ Siltstone_____ _________ ______________________________________

5. ______Shale______ _________ ______________________________________

2. List four types of cement that bond sediment grains together in detrital rocks (Fig. 6.4, page 114)

a. ____________ b. _____________ c. _____________ and d. ______________

3. Question: How might you test whether calcite is the cementing agent in sandstone?

4. Question: Based on your experience with mineral hardness, which of the above bonding

agents would you expect to be the strongest? The weakest?

G. Naming of Biochemical Rocks: Biochemical rocks are named based primarily upon their

composition, e.g. calcium carbonate fossil shell or plant material, and secondarily upon their

texture, e.g. sandy, shelly, crystalline, microcrystalline, etc. Limestone is a sedimentary rock

named primarily for being rich in calcium carbonate. The types of limestone are named by the

type and texture of the calcium carbonate. As an example, coquina is a poorly cemented mass

of large-sized shell fragments, whereas, chalk is a super fine-grained mass of microfossils.

1. Five major biochemical rock types are based primarily on mineral composition. List the

distinctive mineralogy and texture of each rock type. (Middle section of Figure 6.8, page 116)

Rock Name Rock Mineralogy Textural and Distinctive properties

1. ______Coal______ _________________ _________________________________

2. _Coquina Limestone_ _________________ ________________________________

3. Fossiliferous Limestone _________________ ________________________________

4. __Chalk Limestone__ _________________ ________________________________

H. Nomenclature of Chemical Rocks: Chemical sedimentary rocks are also named based

primarily on composition. However, all chemical sedimentary rocks have crystalline textures

that reflect their direct precipitation of ions from an aqueous fluid such as seawater.

1. Seven major chemical (inorganic) rock types that are based mainly on mineral composition,

and describe the distinctive properties of each rock type. (Bottom far-right column of Fig. 6.8 page 116)

Rock Name Rock Mineralogy Texture and Distinctive properties

1. _Oolitic Limestone_ _________________ ________________________________

2. Travertine Limestone _________________ ________________________________

3. ____Dolostone____ _________________ ________________________________

4. ____Rock salt_____ _________________ ________________________________

5. __Rock Gypsum___ _________________ _______________________________

6. _____Chert_______ _________________ ________________________________

2. Question: What is the primary difference between a chemical and biochemical limestone?

III. Depositional Settings of Sedimentary Rocks

A. Sedimentary rocks retain a memory of the conditions in which they formed in, and that information is recorded by the rock’s texture, composition, fossils, and structure. By observing and studying today’s depositional environments and the type and structure of the sediments that collect there, we can infer the depositional setting and history of sedimentary rock assemblages by comparing their sedimentary characteristics to that of modern day depositional systems.

B. Examine Figure 6.12 (page 128) in your lab manual. This illustration shows most of the major types of modern sedimentary environments where sediments are depositing and sedimentary rocks are forming. Directions: List the depositional environments where each type of sedimentary rock forms as shown in Figure 6.12 in your lab manual.

Sedimentary Rock List of Associated Depositional Environments

1. Breccias and Conglomerates ___________________________________________________

2. Sandstones _________________________________________________________________

3. Mudstones ___________________________________________________________________

4. Limestones ___________________________________________________________________

5. Cherts _____________________________________________________________________

6. Rock Salt and Gypsum ________________________________________________________

SEDIMENTARY ROCKS IDENTIFICATION - IN-LAB SECTION

I. Preliminary Examination of the Sedimentary Rock Types:

A. Introduction: The instructor has assembled 3 reference collections of sedimentary rocks for

you to become familiarized with the three major types of sedimentary rocks.

Directions: Carefully study each rock’s: 1) physical characteristics, including texture and

composition, and 2) sedimentary origin – the rock’s original depositional environment.

Make observational notes of the samples. Discuss your observations with your group.

B. The Detrital Rocks: Sample Collection “A”

Observational Notes

Sample # A1 – Conglomerate _________________________________________________

Samples # A2 – Breccia _____________________________________________________

Samples # A3 and A4 – Sandstones ___________________________________________

Samples # A5 – Siltstone ____________________________________________________

Samples # A6 and A7 – Shales ________________________________________________

1 .Question: What is the primary difference between a breccia and a conglomerate?

C. The Biochemical Rocks: Sample Collection “B”

1. Directions: Carefully study the various samples of the biochemical rocks in your group. Make

observational notes of the samples. Discuss with your group your observations and ideas, and

the questions that are asked below. Note: Most bio-sedimentary rocks are fossil-rich limestone.

Observational Notes

Sample # B1 – Coal __________________________________________________________

Samples # B2 – Coquina (shelly) Limestone ______________________________________

Samples # B3, 4, 5 – Fossiliferous Limestone _____________________________________

Samples # B6 – Chalk _______________________________________________________

2. Questions: Which of the samples contain fossils you can see? _______________________

What types of fossils? _____________________________________________

D. Chemical Rocks: Sample Collection “C”

1. Directions: Carefully study the various samples of the chemical rocks in your group. Make

observational notes of the samples. Discuss with your group your observations and ideas, and

the questions that are asked below. Note that most chemical sedimentary rocks are either

limestone or chert; these rock types make “look” similar but have very different hardnesses.

Observational Notes

Sample # C1 – Travertine Limestone ____________________________________________

Sample # C2 – Oolitic (Sandy) Limestone _________________________________________

Samples # C3 - Chert _______________________________________________________

Samples # C4 – Rock Salt ____________________________________________________

Samples # C5 – Rock Gypsum _________________________________________________

2. Questions: Which of the “C” rocks fizzed in acid? ___________ Which is very hard? ______

3. Question: Which of the above samples can be scratched by a fingernail? __________.

II. Classification of Sedimentary Rock Samples:

Introduction: Sedimentary rock classification is done in a systematic manner, utilizing a step-by-step procedure. Sedimentary rocks are identified based upon 1) compositional make-up and 2) textural and structural qualities. A sedimentary rock analysis and classification chart is shown in Figure 6.8. The 3-step procedure for identifying sedimentary rock samples is as follows:

Step 1: Identify and record the rock’s composition (rock fragments? minerals?, fossils?)

Step 2: Identify and record the rock’s texture and other distinctive properties.

Step 3: Name the rock, including its most likely depositional setting

III. Analysis and Identification of 9 Unknown Sedimentary Rock Samples:

Directions: Identify the nine unknown sedimentary rock samples found in sample Collection “D”.

Be sure to check the following information about the rock: a) Composition (circle one or more);

b) Texture (grain type and size – circle one or more) c) Other distinguishing characteristics (fossils,

layering, fizz in acid, etc); d) Rock name; e) Rock origin: Speculate as to what type of depositional

environment the rock originated in? Depositional environments are shown in Figure 6.12, page 128