Identifying and Classifying Sedimentary Rocks in Thin Section

Content/concepts goals

Goal

The goal of this activity is to observe and describe the texture and composition of a range of sedimentary rock types.

Learning Objectives

By the end of this activity students will 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.

Higher order thinking skills goals

The higher order thinking goals for this activity are:

• Making inferences from observations
• Formulation of hypotheses
• Evaluation of multiple criteria for classification.

Other skills goals

• Increase confidence for mineral identification in thin section
• Reinforce basic understanding of the most common siliciclastic detrital and carbonate sedimentary rocks.

Type and level of course

This is an introductory activity designed for an online upper-level undergraduate majors course in Earth materials or petrology.

Skills and concepts students should have mastered

Prior to this activity students should be able to:

• Identify two of the most common major-rock forming minerals and lithic fragments in thin section. Specific minerals: quartz, and plagioclase feldspar
• Navigate the UK Virtual Microscope slide overview, PPL/XPL toggle, grains size measurement tool, rotational microscope views, and hand sample images
• Observe various optical properties to identify previously unmet common rock-forming minerals: calcite and chlorite.

How the activity is situated in the course

Prior to this activity students have completed extensive online laboratory assignments on the optical properties of minerals and the classification of igneous rocks in thin section and hand sample. Students complete two theory modules on sedimentary processes and rock classification that involve lecture videos, readings, a MasteringGeology^TM assignment on sedimentary rock basics, and multiple quizzes on lecture material. This activity is the first part in a series of five activities comprising a comprehensive online laboratory assignment on sedimentary rock classification in hand sample and thin section. The module that includes this five-part assignment (and the theory aspects of sedimentary rock classification) lasts ~8 days including weekends.

Activity Description

This is a laboratory-style activity designed for an online setting. The activity is based on a small sub-set of slides from the UK Virtual Microscope produced by The Open University, UK and free to use. Students are directed to various views of the slide and sometimes specific grains.

Siliciclastic Detrital Sedimentary Rocks

Students make observations of grain size, grain shape, and grain contacts and learn how to classify these observations in order to accurately describe the texture of sandstones and a conglomerate. Questions either provided limited options or require a ranking comparison to help students develop a visual library of the variation in textural characteristics. In addition to texture, students are directed to identify the composition of specific framework grains as quartz, plagioclase feldspar and lithic fragments. Finally, students determine whether the spaces between the grains are filled with matrix or cement.

Carbonate Sedimentary Rocks

Students make observations of the type of allochem observed: bioclasts, pelloids, and ooids. Again students are provided with limited options to help them develop their recognition of carbonate components. In addition to framework grain identification, students are asked to determine whether or not the rock is clast- or matrix-supported and whether it contains micritic matrix or a sparry calcite cement. Finally, students use those observations to complete a basic classification of several limestones.

Instructional Materials

Materials available for download:

• Sedimentary Rock Basics Reference (Word Document)
• Introduction to Sedimentary Rock Texture and Composition in Thin Section Student Worksheet (Word Document)
• Image(s) Specific to the Activity – Labeled Images of UK Virtual Microscope (zip)

Materials available on request from the author:

• Answer key including corrective feedback to insert into an online quiz
• Canvas LMS quiz export

Other resources:

• UK Virtual Microscope
• SEPM Strata

• Download Instructor Guide

The complete five-part laboratory assignment is set up as three online quizzes and one document submission (two parts). This activity is part one and is designed as a low-stakes un-timed "practice" quiz in a multiple-choice auto-graded format. The whole quiz is set up to be worth 100 points. Students are asked to score 35 or more points to receive 35 points toward their total laboratory assignment grade. This is an "all or nothing" set up to encourage students to properly engage with the activity and reward the work required to do so. However, the stakes are set very low to create a low stress environment where more effective learning can take place.

When students have finished the quiz and submitted their answers, they are provided with the correct answers and detailed feedback explaining why their answer was correct or incorrect. Students are encouraged to review this feedback before attempting the next step of the laboratory assignment. The advantage of setting up these multi-stage assignments with multiple-choice auto-graded quizzes is that students are provided with intermediate timely feedback to highlight misconceptions and aspects of their optical identification that are inaccurate. This intermediate feedback is particularly important in an online setting where students do not have an in-person instructor available on-demand for 1:1 corrective feedback, as they would during a residential laboratory class.

• The Open University UK Virtual Microscope User Guide
• UK Virtual Microscope Main Collection