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Explore the link between cross stratification and physical processes through deriving bedform phase diagrams from field measurements. Learn to interpret bedforms and make estimates of actual velocities and water depths. Utilize raw data to understand bedform progression and influence of grain size on bed state in unidirectional flows. Enhance Excel skills for data plotting and analysis in geology.
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Stealing data: Deriving bedform phase diagrams Tom Hickson University of St. Thomas St. Paul, MN Images of graphs from From Middleton & Southard have been removed from this presentation
Where they’re going • Interpreting cross stratification in a real section they’ve measured in the field • Section contains low angle planar lamina, trough x-strat at many scales, and HCS
The goals • I didn’t want to just lecture on bedforms and bedform phase diagrams • I wanted students to derive them on their own and really explore them • To drive home the link between cross stratification and physical processes • To get them to make estimates of actual velocities and water depths from observations of x-strat
Let’s work through what I do… … I sure hope that you have laptops…
The process • Show them the progression from initiation of motion to antidunes (in a flume, using video, whatever) • Now, they must imagine doing many experiments, with different grain sizes, depths, and velocities • Give them the raw data and have them derive the results
Show them the bedform progression • This part you’ll have to imagine • We go use a flume • Initiation of motion Small ripples Large Ripples/Dunes Upper plane bed Antidunes
Visualize doing this experiment many times • Different flow depths • Different flow velocities • Different grain sizes
Give them the raw data and have them derive the results Now is where stealing data comes in… I ‘stole’ the data from original data plots in Middleton and Southard (1983)
GETTING the raw data • DataThief III • Shareware • Java runtime so it works on just about every platform • http://www.datathief.org/
An excellent jigsaw • Three Excel data files correspond to three grain size ranges • Each group of 3 students is responsible for one of them 0.1-0.14_mm_sand.xls 1.3-1.8_mm_sand.xls 0.5-0.64_mm_sand.xls
In your groups: • First, understand what your data are… • Second, figure out a way to plot these data in Excel that will show the different bedform types in depth/velocity space • Make the plot (try it with linear and log-log axes) • Using Excel’s line-drawing capability, group the bedforms into logical ‘fields’ or ‘groups’
Give them the raw data and derive the results Medium sand data file
Depth/velocityphase diagrams From Middleton & Southard
Re-arrange the groups • One representative from each grain size group • Derive the full three dimensional relationship for bedforms by answering this question: • How does grain size influence the bed state in unidirectional flows?
What else do they learn? • A lot about Excel: sorting, plotting, grouping data in plots, how to you ‘help’, line drawing and labeling. • What makes a good graph? • How we use experiments to constrain our field observations • Can you think of others?
Other classic data sets? • QFL diagrams of the Great Valley Sequence and unroofing of the Sierra Nevada
LINKS • Data files at: http://serc.carleton.edu • Datathief Java Runtime app: http://www.datathief.org/ • Movies of different bedforms: http://faculty.gg.uwyo.edu/heller/sed_video_downloads.htm
