Cementation and Neomorphism: Incorporating the Basics of Diagenesis into Any Sedimentary Geology Course
Central Michigan University
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This page first made public: Jun 5, 2006
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Diagenesis is an important sedimentary process and undergraduate students greatly benefit from learning how to recognize and interpret common diagenetic features. This can be accomplished with a two to three week unit that includes lectures and lab activities that focus on cementation and neomorphism, but also includes dissolution, compaction, and paragenetic sequences.
400-level undergraduate course in Sedimentology and Stratigraphy (3-credit course and only course in sedimentary geology that our deptartment offers; a requirement for geology major)
Skills and concepts that students must have mastered
1) Basic sedimentary petrology (both in hand sample and using basic microscope to examine thin sections; know how to describe major types of sediment grains and how to classify basic siliciclastic rock types and carbonate rock types), 2) Basics of relative dating (especially cross-cutting relationships)
How the activity is situated in the course
Either a two or three week long course unit that includes lectures and two to three lab assignments. It is done approximately 2/3 of the way thorugh the semester. Diagenesis is then considered background knowledge for the final two to four weeks of the course and incorporated into lab activities in which students must consider diagenetic history in solving larger problems such as for global change studies.
Content/concepts goals for this activity
Understanding, recognizing, and interpreting diagenetic processes and products.
Higher order thinking skills goals for this activity
Hand sample and thin section interpretation, establishing a paragenetic sequence, formulation of a hypothesis, critical evaluation of possible history of sedimentary rock.
Other skills goals for this activity
Using a microscope, making hand sample descriptions, communicating data and interpretations in written form.
Description of the activity/assignment
Two or three weeks of the course are dedicated to studying diagenesis. Lectures start with a general definition of diagenesis, the range of conditions under which it occurs, and examples of diverse diagenetic environments and features. I use rice crispy cereal and rice crispy treats to introduce cement (the marshmellow is the cement that "glues" the rice krispies together). I also incorporate basic hydrogeology to show how pores filled with (or partially filled with) groundwater provide both the space and the material for cementation. As part of this lecture, I show the students various rock samples and photomicrographs in which they can see cement examples. I outline the different cement minerals and shapes and how they can be used to interpret past diagenetic conditions (eg., gravitational "pendant" calcite cements indicate that the host sediment was once in a vadose zone with groundwater rich in calcium and carbonate). I also discuss types of pores during these lectures and the ways that pores form. We also discuss criteria for recognizing cements. After two one-hour lectures about cements, we have a lab exercise in which the students are given ~10 samples (including hand samples and thin sections) and asked to sketch and describe the cement types. The next one-hour lecture focuses on neomorphic processes and their products, including replacement, recrystallization, and polymorphic transition. As part of the lecture, we look at photomicrographs and hand samples that illstrate various neomorphic features, such as replacement dolomite and replacement chert. We establish criteria for distinguishing cements from neomorphic fabrics. This lecture is followed by a lab exercise that presents the students with ~10 rocks and thin sections and asks them to sketch and identify neomorphic fabrics. This lab is follwed by another one-hour lecture on compaction features, dissolution evidence, and determining paragentic sequences. If I am short on time, that is all I do for diagenesis. However, ideally, I continue with a lecture focused on the "dolomite problem" and some case studies of other types of diagenesis, as well as a third lab assignment that combines cementation, neomorphism, compaction, dissolution, and paragenetic sequences. As part of this section, I also try to incorporate examples of methods other than petrology (eg., fluid inclusion studies, stable isotope studies, dating) that are used for diagenetic studies. Later in the course, we take several field trips in which the students examine diagenetic features.
Determining whether students have met the goals
The two to three lab assignments (out of ~10-12 during the course) are evaluated by me in detail and graded. These grades used to determine the final lab grade (~35% of final course grade). In addition, the end of the course builds upon and incorporates knowledge about diagenesis.
More information about assessment tools and techniques.
Download teaching materials and tips
Diagenesis, ed. by I.A. McIlreath and D.W. Morrow, Geoscience Canada Reprint Series 4, 1990
Jay Gregg's web page about carbonate petrology:
Photographic Gallery of Carbonate Petrology
A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities, by P.A. Scholle, AAPG Memoir 27, 1978
A Color Illustrated Guide to Constituents, Textures, Cements, and Porosities of Sandstones and Associated Rocks, by P.A. Scholle, AAPG Memoir 28, 1979