Dr. Kalin McDannell will give a talk on

Thursday 2/23 at 4:30pm in Roux 307.

All welcome!

 

Title: The Great Unconformity: One of Earth's "greatest" (un)solved mysteries

"The Great Unconformity is a gap of hundreds of millions to billions of years in the geologic record and its origin has remained a mystery for over a century. The term was first coined by Clarence Dutton (1882) to describe the dramatic juxtaposition of younger flatlying strata overlying older tilted and deformed rocks in Grand Canyon.

 Unconformities are common features in Earth's sedimentary rock archive that denote relatively brief periods of either nondeposition or erosion. However, the "missing time" represented by the Great Unconformity is considerable, found globally, and has been long associated with the occurrence of the earliest complex animal fossils marking the Cambrian explosion of life. The simplest explanation for this hiatus is one of gradualism, but recent hypotheses challenge that assumption. 

Global ?Snowball Earth? glaciations between 717?635 million years ago (Ma) and plate tectonic activity from ca. 800?550 Ma related to the breakup of supercontinent Rodinia have both been put forth as causes of the Great Unconformity. These hypotheses, however, are not mutually exclusive. 

We reconstructed the history of the ancient bedrock from multiple sites across North America using thermochronology, which is a radiometric dating technique that allows us to estimate the temperatures that a mineral crystal has experienced over time and its position in the continental crust given an assumed thermal structure. Common minerals like zircon and apatite are sensitive to changes in temperature and thus, by proxy, record periods of erosion [cooling] or burial [heating]. 

I will discuss how this allows us to constrain the temporal and spatial patterns of erosion in the Neoproterozoic era (1000-539 Ma) leading up to preservation of the Great Unconformity surface. Ongoing work is focused on compilation of other physical evidence for deep Precambrian erosion and a global thermochronological assessment of the Neoproterozoic.