DERIVING BEDFORM PHASE DIAGRAMS: DEVELOPING A DEEPER INSIGHT INTO THE FORMATION AND INTERPRETATION OF PRIMARY SEDIMENTARY STRUCTURES

The formation of lamination and cross stratification by subaqueous bedforms is a typical component of many sedimentary geology courses. Usually, bedform phase diagrams such as those presented in Middleton and Southard (1984) are used to assist students in the interpretation of these bedforms and their sedimentary products. Rather than lecturing on and explaining these phase diagrams (depth-velocity-size plots), I saw an opportunity to develop student's analytical skills by having them derive these diagrams from the original data. In pursuing this strategy I hoped that students would gain valuable insight into how process sedimentology is done in practice and would develop a deeper understanding of the competing roles of depth, velocity, and grain size in the formation of clastic sedimentary structures. I used a Java-based shareware program (Datathief.jar) to cull depth-velocity-size data from digital scans from the original papers. Using digital video or an actual unidirectional flume, I then lead students through the progression from initiation of motion through ripples, dunes, upper plane bed, and antidunes for one grain size only. I then ask them to visualize doing this same experiment many times for many velocities, depths, and grain sizes. Finally, they are given the culled, raw data in Excel format and they must plot the data and derive bedform fields for the entire dataset. I typically do this as a jigsaw, where initial groups become ‘experts' on bedforms that develop at one grain size (silts/vf sand, medium sand, coarse sand), then members of these groups recombine to understand the effects of variable grain size on bedform development. In the end, students can compare their diagrams to the originals, which typically spurs a very interesting discussion about how the original bedform fields were defined.