North-Central Section - 49th Annual Meeting (19-20 May 2015)

Paper No. 21
Presentation Time: 8:00 AM-12:00 PM


JOHNSON, Cody R., Geology Department, Augustana College, 639 38th St., Rock Island, IL 61201,

The Great Unconformity is a section of rock that stretches across the globe where a billion years-worth of material is missing between layers. This phenomenon can be found across Laurentia, Gondwana, Baltica, Avalonia, and Siberia. The term “Great Unconformity” was first used in 1869 to explain the stratigraphy of the Grand Canyon which shows the ~525 Ma Cambrian Tapeats Sandstone, associated with a shallow marine environment, overlying the 1,740 Ma metamorphosed Vishnu Schist. There are a few different ideas about what has caused this extensive section to weather away billions of years of the rock record. One hyposthesis is water slowly seeped through the overlaying sandstone layer and remained on top of the granite layer, causing chemical weathering. The second hypothesis is the erosion was caused by a massive marine transgression, exposing the crystalline basement rock to erosion, followed by sedimentation. The main samples for analysis were collected from the Clarks Fork Canyon, Meadowlark Lake, and Owl Creek Mountains in central and northwestern Wyoming. Samples the size of two fists were collected above and below the contact as well as at the contact of the Great Unconformity in the summer of 2014. All 19 samples were crushed and prepped for X-Ray Fluorescence analysis to determine major and trace element compositions. Through microscope analysis of thin sections several occurrences of hydrothermal activity have been found as well as possible shifting of the crystal lattice due to high amounts of stress. If high amounts of sodium are found through XRF analysis it will be indicative of a marine transgression causing the erosion of the Great Unconformity, where as a lack of salts would indicate that water seeping through the sandstone layer causing the erosion of the crystalline basement. This hypothesis could be inaccurate however if rains have leached out the remaining sodium.