Introduction to Sedimentary Rock Texture and Composition in Thin Section

Introduction to Sedimentary Rock Texture and Composition in Thin Section Goal The main goal of this activity is to observe and classify a variety of textural and compositional characteristics of sedimentary rocks.

Learning Objectives By the end of this activity you should be able to:

 Identify the basic components of sedimentary rocks in thin section: framework grains/allochems, matrix and cement

 Describe the texture of a siliciclastic detrital sedimentary rock in thin section: grain size, rounding, sphericity, sorting, degree of grain supporting, and overall textural maturity

 Identify the composition of framework grains, matrix and cement in siliciclastic detrital sedimentary rocks in thin section and determine the overall compositional maturity

 Describe the texture of a carbonate detrital sedimentary rock in thin section: grain size, sorting and degree of grain supporting

 Identify the type of allochem and the amount of micrite vs sparry calcite in a carbonate sedimentary rock in thin section

 Use the Folk and Dunham classification schemes to classify carbonate sedimentary rocks in hand sample and thin section

Instructions to Students This laboratory assignment is an introduction to observing and classifying sedimentary textures and components. All the questions are based on slides from the UK Virtual Microscope. You will submit your answers to the online multiple-choice quiz. When you have finished the quiz and submitted your answers you will be able to access your responses, the correct answers and detailed feedback explaining why your answer was correct or incorrect as appropriate.

The assignment is divided into two sections and is worth 100 points. The first section covers siliciclastic detrital sedimentary rocks and the second section covers carbonate sedimentary rocks. The multiple-choice answer options are not provided in this document. You may wish to review the submission quiz as you work on the assignment as the limited options will help you.

Minerals that you should be familiar with that will appear in this activity are:

 Quartz

Activity developed by Dr. Rachel Walters ([email protected]) Department of Geological Sciences, University of Florida Introduction to Sedimentary Rock Texture and Composition in Thin Section

 Plagioclase

New minerals that will be introduced during this activity are:

 Calcite

 Chlorite

Section A: Common Detrital Sedimentary Grain Composition and Textures

All of these questions are based on slides from the UK Virtual Microscope

Main collection: http://www.virtualmicroscope.org/content/uk-virtual-microscope

You will need the following slides open from the UK Virtual Microscope as you will be switching back and forth. Some of them are not in the main collection so you will need to follow the links provided.

 Virtual Microscope – Arenite Sandstone  [Link: http://www.virtualmicroscope.org/content/arenite-sandstone-falkland-islands]  Virtual Microscope – Lithic Greywacke*  [Link: http://www.virtualmicroscope.org/content/lithic-greywacke-p19158]  Virtual Microscope – Carbonate Cemented Conglomerate  [Link: http://www.virtualmicroscope.org/content/carbonate-cemented-conglomerate]  Virtual Microscope – Subarkose Sandstone  [Link: http://www.virtualmicroscope.org/content/subarkose-sandstone]

*Note: the term greywacke or greywacke is no longer in use and should be replaced by the terms: lithicwacke or feldspathicwacke depending on the composition.

Framework Grain Composition There are four main compositional components of detrital sedimentary rocks: quartz and feldspar (plagioclase or alkali-feldspar), lithic fragments and mud. There are others (as always), but these are the four key types of grains that you should be able to identify in thin section.

Question 1: Feldspar and Lithic Fragments (5 points) Virtual Microscope – Lithic Greywacke Navigate your way on the virtual microscope to the XPL slide overview of the lithic greywacke. There are tons of plagioclase grains and lithic fragments in this rock. Observe the images provided online - a number of grains have been annotated. For each grain classify it as either feldspar or a lithic fragment.

Grain Feldspar or Lithic Fragment

A B C E F G

Question 2 (4 points): What mineral is grain D? Hint: Check out the interference colors and note that on rotation of the stage in PPL it is strongly pleochroic. Use the microscope to look around at similar grains as they may show you the interference colors more strongly. Answer:

Question 3: Feldspar and Quartz (4 points) How does the PPL appearance of the feldspar grains that you identified compare to the PPL appearance of the quartz in the arenite sandstone? Answer:

Question 4: Lithic Fragments (4 points) Virtual Microscope - Carbonate Cemented Conglomerate

Navigate your way on the virtual microscope to the XPL slide overview of the carbonate cemented conglomerate. Note: they have mis-labeled the PPL toggle as XPL.

The framework grains in this detrital rock are almost entirely one type of framework grain: quartz, feldspar or lithic fragments? Answer:

Question 5: Conglomerate Classification (4 points) Is this an oligomictic or polymictic conglomerate? You may find the hand sample particularly useful (click on “VIEW OBJECT”). Answer:

Question 6: Framework Grain Identification (4 points) Virtual Microscope – Subarkose Sandstone

There are two main types of framework grains in the subarkose sandstone. What are the two main types? Note: the sandstone classification here may not be completely accurate. Answer:

Question 7: Cement (4 points) This carbonate cemented conglomerate has been cemented by interlocking crystals of calcite. Familiarize yourself with the variation of expression of 4th order white in XPL. It can appear to look like first order interference colors, but it often looks like a dirty or creamy white. There is a

Activity developed by Dr. Rachel Walters ([email protected]) Department of Geological Sciences, University of Florida Introduction to Sedimentary Rock Texture and Composition in Thin Section good example of a crystal showing a rainbow-like effect in rotation 1. Calcite often does not achieve full extinction and instead has a dark grey mottled appearance.

A screen shot taken from the slide overview is provided online. The arrow is pointing to the two sets of bright parallel strips within a calcite crystal.

What kind of structures do you think the strips are? These are very common in calcite and you can usually see at least one set (one orientation), and sometimes two. Answer:

Question 8: Cement and Matrix (4 points) Compare the lithic greywacke and the subarkose sandstone to the conglomerate. Which one would you describe as well cemented, which one poorly cemented and which one has a mud matrix?

Rock Type Cement or Matrix Description Carbonate Cemented Conglomerate Lithic Greywacke Subarkose Sandstone

Question 9: Compositional Maturity (4 points) Which two components of detrital sedimentary rocks indicate compositional immaturity? Answer:

Question 10: Compositional Maturity (4 points) Put the following rock types in order of compositional maturity from A to C where A is the most mature and C is the least mature.

Rock Type Compositional Maturity Subarkose Sandstone Arenite Sandstone Lithic Greywacke

Question 11: Grain Size (4 points) Using the Udden-Wentworth Scale in the “Sedimentary Rock Basics”, classify the grain size of framework grains that make up the majority of the subarkose sandstone. Analyze the slide as a whole to identify the predominant grain size. Measure multiple grains and average them to account for variation. Answer:

Question 12: Sorting (4 points)

Is the carbonate cemented conglomerate well sorted, moderately sorted or poorly sorted? Remember that sorting applies to the framework grains only when the rock has cement rather than matrix. Answer:

Question 13: Supporting (4 points) Is this a clast-supported (framework-supported) or matrix-supported conglomerate? Hint: using the hand sample in conjunction with the thin section will be helpful. Answer:

Question 14: Sorting (4 points) Even though sandstones are by definition composed predominantly of sand-sized grains they can still be described as poorly sorted if there is a significant grain size differential (including high mud content) and it is placed in the reference frame of other sandstones. Place the following rocks in order of sorting using A, B, C with A being well sorted, B moderately sorted and C poorly sorted). Use each letter only once.

Rock Type Sorting Subarkose Sandstone Lithic Greywacke Arenite Sandstone

Question 15: Rounding and Sphericity (5 points) Using the figure in the Sedimentary Basics section, match the following rocks with the descriptions of rounding and sphericity provided. Each description should only be used once.  Rounded grains with low sphericity.  Angular to very angular with low sphericity.  Sub-angular to poorly rounded with low sphericity.  Rounded with moderate sphericity.

Rock Type Rounding and Sphericity Subarkose Sandstone Lithic Greywacke Carbonate Cemented Conglomerate Arenite Sandstone

Question 16: Textural Maturity (4 points) With the help of your previous answers place the following rocks in order of textural maturity from A to D with A being the most mature and C being the least mature.

Rock Type Order of Textural Maturity Subarkose Sandstone Lithic Greywacke Arenite Sandstone Section B: Introduction to Common Carbonate Grain Composition and Textures

All of these questions are based on slides from the UK Virtual Microscope

Main collection: http://www.virtualmicroscope.org/content/uk-virtual-microscope

You will need the following slides open from the UK Virtual Microscope as you will be switching back and forth. These are all in the main collection.

Virtual Microscope – Carboniferous Limestone with Corals Virtual Microscope – Oolitic Limestone – Weldon Virtual Microscope – Carboniferous Limestone with Crinoids Virtual Microscope – Crinoidal Limestone Derbyshire

Question 17: Allochems - Fossil Fragments or Bioclasts (4 points) The image provided online is an XPL view taken from the Carboniferous Limestone with Corals. You can identify the corals as fragments containing multiple small chambers outlined in dark grey. There are also brachiopod shell fragments preserved in the rock and are long slender crystalline curves.

Several grains are annotated. Decide whether that grain is a fragment of coral or a brachiopod shell.

Grain Coral or Brachiopod A B C D

Question 18 (4 points): Which mineral has crystallized in the chambers left by the coral? Answer:

Question 19: Ooids (4 points) In the Oolitic Limestone – Weldon there are multiple types of allochem: ooids, pellets, intraclasts and bioclasts. Most of the pellets and fossil fragments have been used as nuclei for ooidal-type concentric precipitation of calcite. On the image provided online you should be able to easily identify ooids with pellets for a nucleus and ooids with a sparry calcite fossil fragment

Activity developed by Dr. Rachel Walters ([email protected]) Department of Geological Sciences, University of Florida Introduction to Sedimentary Rock Texture and Composition in Thin Section for a nucleus. Pellets look very uniform, they are composed of micrite (carbonate mud), are usually very well rounded and a consistent size. Micrite in thin section looks dark brown in PPL and looks a little fuzzy. It looks very similar in XPL as it does in PPL and is not opaque.

Several ooids have been annotated. Determine whether each ooid has a pellet or fossil fragment for a nucleus.

Grain Pellet or Fossil Nucleus A B C D

Question 20: Cement and Matrix (5 points) In addition to sparry calcite and micrite (carbonate mud) carbonate rocks can have small grains of calcite (grains deposited with the framework grains, not like the sparry calcite with precipitates later). And of course you can get mixtures of all of these or none at all.

Match the following descriptions of matrix or cement to the rock types from the virtual microscope. Each description is only used once. A. Little to no cement or matrix B. Sparry calcite C. Micrite D. Mixture of calcite grains and micrite

Rock Type Cement/Matrix Description Carboniferous Limestone with Corals Carboniferous Limetsone with Crinoids Crinoidal Limestone – Derbyshire Oolitic Limestone - Weldon

Question 21: Textures - Grain Support (4 points) Are the virtual microscope limestones grain-supported or matrix-supported? Complete the table classifying each rock as either grain-supported or matrix-supported. It can often be helpful to zoom out to consider the texture as a whole. Looking at just one small section can be mis- leading.

Rock Type Grain or Matrix/Cement Supported Carboniferous Limestone with Corals Carboniferous Limetsone with Crinoids Crinoidal Limestone – Derbyshire Oolitic Limestone - Weldon

Question 22: Sorting (4 points) None of these limestones are particularly well sorted. The oolitic limestone appears to have a bi- modal distribution of grain sizes with the ooids having a relatively consistent grain size combined with a population of large elongate shell fragments.

Which limestone is the most poorly sorted? Answer:.

Question 23 and 24: Folk and Dunham Classification Based on your observations of composition and texture classify the four limestones using the Folk and Dunham classification tables provided in the supplementary “Sedimentary Rock Basics” document (Appendix B). The Dunham classification scheme is usually only applied to hand samples, but we are able to apply it in this lab because the UK Virtual Microscope has very large slides and photographs of the hand samples.

Notes:  Cement is treated as pore space in the Dunham classification.  The oolitic limestone is exempt from the Folk classification.

Rock Type Q23: Folk (4 points) Q24: Dunham (5 points) Carboniferous Limestone with Corals Carboniferous Limetsone with Crinoids Crinoidal Limestone – Derbyshire Oolitic Limestone - Weldon

Activity developed by Dr. Rachel Walters ([email protected]) Department of Geological Sciences, University of Florida