Activity created by Prof. Karen Grove (@ 2002 K.Grove). Please send comments to: kgrove@sfsu.edu. Commercial use prohibited.

Sediment on the move

Learning Objectives

  1. To recognize the various types of sedimentary (i.e., primary) bedding structures.

  2. To connect sedimentary structures with the processes that created them.

  3. To use computer and physical models to understand sedimentary processes (i.e., how sediment is transported and deposited), and how sedimentary structures are created.


Problem

In preparing for your new job at EcoEarth Inc., you realize that describing sedimentary particles is only part of the process you will need to complete in the field. You must also be able to describe and identify the types of primary structures that are found in layers of sediment (that is, how sediment particles are organized to form cross bedding, plane bedding, and other features). So you get your friends together again to find examples of sedimentary structures and to figure out what they mean.


Stage 1—physical processes

Before you can interpret various types of sedimentary structures, you need to identify the processes that can transport and deposit sediment. Looking at your textbook (Chapter 3), you see that the author divides the processes that move sediments into three types: (1) dry (non-fluid-assisted) gravity-driven mass wasting processes; (2) wet (fluid-assisted) gravity-driven processes; and (3) processes that involve direct fluid flows of air, water, and ice. Consider the following questions while you read about the three processes.

  1. What are some specific examples of settings where each process would operate?
  2. Speculate about the types of deposits you would expect each process to create.
  3. How might you go about creating a physical model for your process?
  4. Figure 3.4 in the text illustrates some important aspects of sediment erosion, transport, and deposition. Which part of the diagram shows a linear relationship and why? Which part does not show a linear relationship and why?

Stage 2—connecting physical processes and sedimentary structures

Now that you have explored types of processes that transport and deposit sediment, you are ready to examine different types of sedimentary structures and to figure out which processes are responsible for forming each of the structure types. You and your friends have a few samples of sedimentary structures but, based on the list in your sedimentology book, you know that you do not have a very complete set. You decide to search for sedimentary structures on the web and come up with some good sites that have photographic catalogs and movies of bedform simulation.

Questions to consider (Chap. 4)

  1. Figure 4.3 is a bedform stability diagram. Explain how bedforms change with changing grain size and with changing current flow. Keep in mind that these diagrams are based on unidirectional flow experiments in the laboratory.
  2. There are many different types of structures that produce cross bedding / lamination. Begin to think about what information we can gain from different types of cross bedding (see Catalog below). .

Resources

Hjulstrom diagram


Catalog of sedimentary structures to recognize and correlate with processes


A variety of sedimentary structures are listed below. These are structures you should be able to recognize by the end of the semester. Over the next several weeks,.describe each structure and the process that forms it. Also include the types of environments within which they are likely to be found.


A. Bedding plane structures on the base of beds (i.e., sole marks) formed by fluid flow
1. Flutes
2. Tool marks


B. Bedding plane structures on the tops of beds
1. Mud (dessication) cracks
2. Raindrop impressions


C. Internal structures that produce cross bedding/lamination and associated terminology
1. Ripples, dunes, megaripples, sand waves
2. Antidunes
3. Climbing ripples
4. Herringbone cross bedding
5. Hummocky cross stratification (HCS)
6. Interference effects / superimposed ripples or dunes
7. Stoss side and lee side, crest and trough, foresets
8. Tabular versus trough bounding surfaces
9. Planar versus tangential foresets
10. Linear, sinuous, or linguoid crests
11. Flaser bedding, wavy bedding, lenticular bedding


D. Other internal structures associated with fluid/sediment flows
1. Planar or flat bedding/lamination
2. Normally graded beds
3. Reversely graded beds
4. Massive beds (no structures!)
5. Imbricated clasts


E. Sedimentary structures formed soon after deposition by sediment loading or fluid escape
1. Load casts (also ball-and-pillow structure)
2. Flames
3. Dish and pillar structures
4. Convoluted bedding (soft-sediment deformation)
5. Sand dikes (may form long after deposition)


F. Biogenic sedimentary structures (called trace fossils or ichofossils)
1. Skolithos ichnofacies
2. Cruziana ichnofacies
3. Zoophycos ichnofacies
4. Nereites ichnofacies


G. Also consider bedding contacts
1. Sharp or gradational
2. Planar, wavy, etc.


Evaluation for this activity

  1. There will be no formal write up for this problem, but we will continue to learn about sedimentary processes and structures throughout the next several weeks, as we study various types of sedimentary environments. Use of knowledge about processes and structures will be evaluated as part of subsequent activities.
  2. Exam I questions.