Deriving Bedform Phase Diagrams

Thomas Hickson
University of St. Thomas
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Students derive bedform phase diagrams from Middleton and Southard (1984) by plotting the original data (stolen using DataThief software from the publication) in Excel and defining bedform field boundaries. These diagrams are then linked to cross stratification.

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A junior level sedimentology and stratigraphy course.

Skills and concepts that students must have mastered

They should have some exposure to what happens as a unidirectional water flow interacts with a sand bed: the progression from lower plane, to ripples, to dunes, to upper plane, to antidunes. I take them to a lab that has a flume and we play with the flume to make different bed states, but a video that describes what happens would work fine too. A lecture might also work.

How the activity is situated in the course

This exercise is part of a much larger project where students must interpret cross stratification from an actual section they've measured.


Content/concepts goals for this activity

Bedforms and interpretation of cross stratification

Higher order thinking skills goals for this activity

This activity requires students to synthesize a large dataset and evaluate/interpret it. They must also synthesize a range of ideas and concepts because they must link what they do in this activity to an actual sequence of strata. I also think this activity emphasizes estimation and approximation techniques.

Other skills goals for this activity

Data plotting in Excel. Working in groups.

Description of the activity/assignment

Students prepare for this activity by working with a unidirectional flume with a sand bed. We adjust water depth, flow velocity, and channel slope to achieve a range of bed states, in an effort for them to understand the controls on bedforms. This portion of the activity could be done in lecture or via another exercise that makes use of digital video of actual experiments. The activity itself is a jigsaw: students form groups of three, each group responsible for plotting depth vs. velocity plots of bedform state for a single sand grain size range (0.10-0.14 mm, 0.5-0.64 mm, and 1.3-1.8 mm). These data are provided to them as Excel files and the data were directly 'stolen' from the original depth vs. velocity plots in Middleton and Southard (1984), Mechanics of Sediment Movement, SEPM Short Course Number 3. Datathief software (available free on the web) was used to steal the data. The data are arranged in columns: depth, velocity, and bedform type. Students must plot each of the different bedform types with a different symbol, then they have to define field boundaries. It is critical that they have never seen the original plots in their textbook. The goal is for them to derive them on their own, not to regurgitate what is in their textbook or elsewhere. After they complete their plots for each grain size range, the groups re-arrange themselves into groups of three with one representative from each of the grain size groups. They then must try to evaluate the effects of changing grain size on bedform state. Finally, after completing the exercise, the bedform analysis is linked to the cross stratification that is produced under conditions of high sediment fallout rates and the given bed state. The activity gives students practice working with realistic datasets, exposure to the role of physical modeling in sedimentary geology, and a chance to plot and interpret real data. Furthermore, it really solidifies the link between cross stratification and its dynamic interpretation from the rock record.

Determining whether students have met the goals

Students hand in worksheets that I look over and grade. But more importantly, the degree to which they integrate information from this activity into the overall project in which it is situated is more critical.

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