STRATIGRAPHY OF THE EAST LA GRANDE AREA: A LAB EXERCISE

"Stratigraphy of the East La Grande Area" is a laboratory exercise designed for Historical Geology students at Eastern Oregon University. After an introduction to basic physical, biological, and chronostratigraphic concepts, the students are given a series of stratigraphic sections and a map showing the relative locations of the sections. They are told that the area is located somewhere to the east of La Grande, Oregon. One of the sections is divided into formations that are named after towns in the Grande Ronde Valley. Once the students familiarize themselves with the rock types and fossils in each formation in the type section, they divide the remaining sections into formations, choose a datum, and construct a stratigraphic section across the area. Then they carefully examine the types of foraminifera, pollen, and other fossils shown on the stratigraphic sections and draw in taxon range zones, concurrent range zones, and abundance zones on their cross-section. One of these fossils is the Mesozoic clam, Inoceramus, which allows the students to determine the approximate geologic age of the sequence. Rather than having students try to sort through the complicated distributions of foraminifera and pollen in the actual sequences that the sections are based on, the exercise uses a simplified sequence of foraminifera and pollen fossils that are named after the students in the class (e.g. Carlimorrisia and Storymillerensis) to make it clear that these are not the actual fossils in the area. Once the stratigraphic cross-section is completed, the students describe the overall geologic history of the area, including the types of environment present, the overall pattern of transgressions and regressions recorded in the sequence, and the history of uplift, sediment input, sea level changes, and subsidence of the basin in the area. When they are done, the location near Mesa Verde, Colorado, where the sections were measured, is revealed and the students put what they have learned into the context of what was happening along the western margin of the Cretaceous Interior seaway at the time of deposition. By changing the names, this exercise can easily be adapted for use at other universities and colleges.