Download Lab 7: Sedimentary petrography

NAME:___________________________________
Introduction to Sedimentary Petrology: Common Sedimentary Particles
The purpose of this lab is to get you acquainted with using a petrographic microscope and identifying some of the
most common sedimentary particles in rocks. Chapter 5 in Boggs, especially Figures 5.1, 5.2, and 5.13, will be
useful references for what these grains and cements look like in thin section. First, we will review how to use the
microscope and all the terminology used by sedimentary petro logists.
Part I. Clastics: Quartz
Quartz is one of the most abundant minerals in sedimentary rocks. Quartz is recognized in thin section by these
properties: low relief, clear color in plane-polarized light, 1st order birefringence (grey to white color in cross polarized light), concoidal fracture, and commonly, undulose extinction. Quartz comes in different forms, as grains
and cements.
a) Examine THREE of the following thin sections to identify which forms of quartz are in each of these thin
sections: CH-3; CH-19.1, 278.4; 296-ADM; 280-ABQ; 278.11. All of these thin sections contain at least one form
of quartz. Use a Y for yes, N for no.
CH-3
Monocrystalline quartz clasts
Polycrystalline quartz clasts
Chert clasts
Chalcedony cement
Quartz overgrowths
Chert cement
CH-19.1
278.4
296-ADM 280-ABQ
278.11
b) Sketch examples of each of the following types of quartz in the space provided below. Label the magnification of
the field of view you used to make your sketch. A high magnification may be useful to make you sketches. Use the
same thin sections as in part (a) to make your sketches.
Quartz Overgrowth Cement Around
Monocyrstalline Quartz Grain
Magnification _____________
Chert Clast
Magnification _____________
Polycrystaline Quartz Grain
Magnification _____________
Part II: Clastics: Feldspars
Feldspars are also common sedimentary particles found in sedimentary rocks. In thin section, quartz and feldspars
share many of the same optical properties. Both have low relief and 1 st order birefringence. However, unlike quartz,
feldspars have cleavage and twinning. Your handout has some sketches of the different types of twinning. Twinning
in feldspars is described as the appearance of symmetrical planes (twin lamellae) parallel to a crystal face and
usually identical with the compositional plane. Twinning is recognized only in cross -polarized light.
1. Sample 316-MPC is a feldspathic arenite. Identify and sketch an example of potassium feldspar and plagioclase
feldspar in XPL. Label the twinning style for both grains and the magnification you used to make your sketch.
Potassium Feldspar
Twinning Style ___________________
Magnification _____________
Plagioclase Feldspar
Twinning Style ___________________
Magnification _____________
Part III: Lithic Fragments
Not all lithic fragments are polycrystalline. Some lithic fragments are monocrystalline such as muscovite, biotite,
and chlorite. Polycrystalline lithic fragments tend to be fine-grained because coarser-grained rocks would break up
into individual mineral grains .
1. Sketch examples of each of the following types of lithic fragments using samples 304-AK or 335-AKG
Schist Fragment
Magnification _____________
Rhyolite Fragment
Magnification _____________
Mica (list which one _______________)
Magnification _____________
Quartzite Clast
Magnification _____________
Part III: Carbonate Rocks in Thin Section
There are many important differences between carbonate and siliciclastic rocks. First, most
carbonates are “born,” not made. Siliciclastic material can be transported great distances whereas
most carbonate particles remain in or near the same place it was created. As a result, the grain
size of the majority of carbonate rocks is a function of the type of skeletal particle or non-skeletal
grain size, not hydraulics. Secondly, carbonate rocks are only common in clear, shallow, warm,
tropical to subtropical marine environments (in the “carbonate factory”).
Limestones make up approximately 25% of sedimentary rocks in the stratigraphic record. The
common carbonate minerals fall into three main groups: aragonite, calcite, and dolomite.
Although these minerals have similar chemical formulas, their structures are very different and
can be distinguished in thin section. Aragonite can be distinguished from calcite and dolomite
because of its lack of rhombohedral cleavage. Dolomite has a higher index than calcite. Table 1
(Boggs, p.160) lists a number of features that can be used to differentiate these three carbonate
minerals. Most of the samples you will see in today’s lab are made up of calcite. There are a
number of thin section photomicrographs in Boggs (pp. 175, 178, and 179) that will be good
references.
I. The Orthochems
There are two major types of othochems: micrite and sparite. Micrite is microcrystalline
carbonate (super fine-grained), which means that the edges of the individual crystals are not
visible. Spar refers to the larger crystals (coarser-grained than micrite). Spar is usually more
translucent (clear). Typical spar is 0.02-0.1 mm in diameter. Spar can form in a variety of
cementation fabrics such as fibers, blades, and equant crystals. Compare the two types of
orthochems. Look at samples 668-TE-4 and UNK and sketch an example of both micrite and
sparite in the appropriate circles below.
Sample Number ____________________
Sample Number ____________________
Magnification
Magnification
____________________
Orthochem type ____________________
____________________
Orthochem type ____________________
II. The Allochems – Non-skeletal Carbonate Grains
Most non-skeletal grains are coated except for peloids, intraclasts, and extraclasts. Coated grains
include ooids, pisoids, and oncoids. Figure 1 depicts some diagrams of each carbonate grain
type. The best way to identify the grain type is by the size of the grain, shape of the grain, and
the characteristics of the laminae inside the grain. Because coated grains have a polygenetic
origin, processes of formation are poorly understood. Choose one sample from each group to
sketch and describe in the space below. For each sample, indicate the scale of your sketch of
the grain. For all your chosen samples, name the grain type. Name the rock according to
Folk. If the grain is a coated grain, indicate what the composition of the nucleus is. Indicate
the one probable depositional environment for each sample. How do you think these grains
formed? Be specific. Look up the answers in Boggs. Explain in the space provided below
for each group.
Group 1:
Group 2:
601-JAR, FOI
758-AHO, 665-APK
Sample ____________________________
Sample ___________________________
Magnification ______________________
Magnification _____________________
Grain type _________________________
Grain type ________________________
Folk ______________________________
Folk ______________________________
Nucleus composition _________________
Nucleus composition ________________
Depositional Environment ____________
Depositional Environment ___________
____________________________________
__________________________________
Mechanism of Formation:
Mechanism of Formation:
Group 3:
Group 4:
690-APP
682-AOL
Sample ____________________________
Sample ___________________________
Magnification ______________________
Magnification _____________________
Grain type _________________________
Grain type ________________________
Folk _______________________________
Folk ______________________________
Nucleus composition _________________
Nucleus composition ________________
Depositional Environment ____________
Depositional Environment ___________
___________________________________
__________________________________
Mechanism of Formation:
Mechanism of Formation:
III. The Allochems – Skeletal Carbonate Grains
Skeletal grains are the most abundant and important kinds of grains that occur in limestones.
These grains will be either whole fossil organisms or fragments that have been rounded to
various degrees by abrasion. Fossils yield vital information about environmental conditions such
as water depth, salinity, turbidity, and energy levels because the organisms were adapted to
specific ecological tolerances. Tables and figures have been provided for you to help guide you
through fossil identification. We’re only looking at a few marine fossils in thin section.
1. Sample 669-T-5 Bivalves (clams) are abundant in limestones, especially in the Mesozoic and
Cenozoic. The wall structure provides the most effective means to differentiate bivalve shells
form the other shelly fauna. Make a sketch of the shell in the space provided below.
Magnification _______________________
Orthochem Name ____________________
Based on the orthochem type and the taphonomic state of the shell (Is the fossil abraded? Is it a
whole fossil?), what is the energy of deposition of this rock? High, medium or low? Why? Cite
specific reasons for your interpretation.
2. Sample 617-PRO. This fossil is an important time marker because this group of foraminfera
is characteristic of the Late Carboniferous and Permian. The have coiled "shell" forms called
tests that are similar in shape to gastropods (snails) and ammonites. But, there are a few major
differences between those coiled forms and foraminfera. Name one.
Make a sketch of one of the foraminfera: .
Magnification _________________________
3. Sample S1017 There are three common marine allcohems in this sample: bryozoans,
brachiopods, and echinoderms. Make a sketch of each one in the space provided below. It is best
to look at two of the thin sections to see the variety of shapes.
Bryozoan
Magnification
Brachiopod
____________________
Orthochem type ____________________
Magnification
____________________
Orthochem type ____________________
Echinoderm
Magnification
____________________
Fossil name ________________________
Name the rock according to Dunham _______________________________
4. Sample ACC Look closely at the absence of any internal structure of this fossil. They are
often found fragmented in the fossil record. These fossils are now extinct. This fossil group
provides an excellent time marker for rocks. They only existed during the Paleozoic. Make a
sketch of one below.
Magnification
____________________
Fossil Name ________________________
Part 3: Sandstone Provenance Analysis
We will be learning a method of semi-quantitative analysis of sandstone composition and then
we will use these analyses to interpret different sandstones' provenances. Recall that sandstones
are detrital sedimentary rocks-they result from the physical and chemical breakdown of preexisting rocks, followed by the transport, deposition, and lithification of the resulting sediments.
Therefore, if we are able to account for the different susceptibilities of different rock types to
weather in the sedimentary environment, we can estimate the composition and/or tectonic
environment of the parent rock(s) (i.e., the provenance) from which the younger detrital
sedimentary rock was derived.
Our methods have two parts:
First, you will learn to conduct a point-count of the quartzose, feldspathic and lithic components
of a number of detrital sedimentary rocks.
Second, you will plot the resulting datasets on an accompanying QFL ternary diagram and then
compare the plot with a) the nomenclature QFL ternary diagram and b) Dickinson and Suczek's
(1979) sandstone provenance QFL ternary diagram.
Choose 1 sample from Groups I, II, III and, using the hand sample, give each a name. Then
conduct a point count on each sample and plot the samples on a QFL ternary diagram.
Now give the name according to the point count. It will likely be different. When all
samples are plotted, sketch the provenance fields over the sample points.
Group 1
Group 2
Group 3
218-PCA
407-TNY
328-AKW
269-AKL
311/310-TEW
ACA
261-KSU
299-AJN
309-KBC
112-APB
313-TDC
325-AKN
Sample Number:
Rock name from hand sample:
Point count tallies
Quartz:
Feldspar:
Lithics:
Textural and Mineralogical comments
Quartz:
Feldspar:
Lithics:
Rock name from point count:
Provenance field:
Comments:
Sample Number:
Rock name from hand sample:
Point count tallies
Quartz:
Feldspar:
Lithics:
Textural and Mineralogical comments
Quartz:
Feldspar:
Lithics:
Rock name from point count:
Provenance field:
Comments:
Sample Number:
Rock name from hand sample:
Point count tallies
Quartz:
Feldspar:
Lithics:
Textural and Mineralogical comments
Quartz:
Feldspar:
Lithics:
Rock name from point count:
Provenance field:
Comments:
How to conduct a point count:
1. Choose magnification such that ~30 grains are in field of view. Make sure that you are
looking at grains and not cement.
2. Pick 5 nonintersecting areas on the sample slide.
3. Divide field of view into quadrants, then count quartz, feldspar, and lithics in each.
1
2
3
4
4. Continue for each area until 150 grains have been counted. Then divide the count for each
grain type by the total number of grains counted to get percentages for quartz, feldspar, and
lithics.
5. Plot in QFL space on ternary graph sheet provided.
Some notes on provenance fields:
Stable Continental Craton: not much elevation – rivers, beaches, etc. Sediments are intensely
reworked.
Block Faulted Continental Basement: Mainly samples upper crust, so high percentage of
quartz and feldspar.
Recycled Orogen: Complicated setting, samples older/deeper rocks. Dominated by quartz and
lithics (metamorphic fragments).
Magmatic Arc: High percentage of lithics (volcanic fragments) and K-spar relative to quartz.