INTEGRATING SEDIMENTOLOGY, STRATIGRAPHY, HYDROGEOLOGY, AND CRITICAL THINKING THROUGH THE SUCCESSIVE INTERPRETATION OF SUBSURFACE GEOLOGIC CROSS SECTIONS
In the first semester, students work in teams to construct structural cross sections using borehole and natural gamma logs from a local groundwater contamination site. Students learn how to lay out a section line, choose a datum elevation, deal with vertical exaggeration, and hang wells on the cross section. Students then individually interpret the sections using conventional lithostratigraphic correlation techniques - deciding whether to lump or split lithologic units and how to interpolate between control points. With the students’ permission, we post a selection of their completed cross sections on the classroom wall to compare and contrast differing interpretations of the same data set. Discussions involve alternative correlation styles and sedimentary controls on the deposition of the sediments they have interpreted.
The activity is extended in Hydrogeology during the subsequent semester. Hydro students annotate uninterpreted cross section templates generated by Sed/Strat students with well screens, static water levels, and contaminant concentration data. Each student is then asked to subdivide the subsurface sediments into aquifer and aquitard units and to correlate these units from well to well. Their objective is to create a hydrostratigraphic interpretation that is constrained by both lithologic and hydrologic information. Again, multiple interpretations are generated and discussed. Students invariably ask which interpretation is right (or which is best), providing an ideal opportunity to discuss the formulation of conceptual geologic models and to illustrate the potential value of using multiple working hypotheses.
The experience culminates with a class field trip to the groundwater cleanup site to help students grasp the scale of the problem and to a nearby gravel pit where students can examine sediment exposures and think about implications for correlating lithostratigraphic or hydrostratigraphic units in the subsurface.