Breccia Is not Just Breccia

It is beautiful here at the Sudbury Basin but all of the gorgeous vegetation is covering our data. We hunt for data that comes in the form of rocks. We are confined to the world of road cuts where large sections of rock are cut open to a steady stream of passing traffic. These road cuts yield a great side benefit for geologists, they let us see 3D relationships in rock structures that would otherwise be impossible to analyze without doing some dynamite blasting of our own. As you might guess, dynamite blasting for geology is not looked upon favorably or funded like new roadways. So here in Sudbury we have spent much of our time plastered against interesting rock structures on the sides of busy highways.

We are looking for signs of an ancient asteroid impact. Geologists look for signatures in the rock record such as mineral content, fractures, and structural relationships between different rock types. Here at the Sudbury Basin the geology before and after the impact is complex. It lies within the bounds of ancient mountain roots from the Greenvillian mountain building event as well as the more recent deformation that left the crater itself more elliptical than circular. The rock types involved include meta-sedimentary, intrusive-igneous, meta-igneous, shocked metamorphic, and extrusive igneous among others. When compared to the more simple structure of Meteor Crater that is nearly pristine in appearance and is located solely in sedimentary rock, interpretation of Sudbury Crater is challenging to say the least. Difficult scientific interpretations always seem to lead to scientific disagreements.

Our expedition leader, Paul Lowman, believes the size of the crater has been over estimated and thus the size of the asteroid creating it also over estimated. So we hunt for a particular rock that Dr. Lowman believes has been mis-catagorized. Breccia is a type of rock made up of a variety of other angular to subangular rocks. Sudbury is known for this type of rock that was created in abundance as the energy of the impact shattered parts of the lithosphere and melted other sections. The result is a rock of shattered parts held together by other melted rocks. However, this is not the only way breccia can be formed. Lowman believes that some of the Sudbury breccia was created by process after the impact and not directly caused by it.

So we hunt for breccia. We trace it back in formations and try to find its contact with other rocks. We hunt for cross cutting dykes made of basalt that may have pulled off chunks of rocks it flowed through before cooling. We have collected many great samples and documented many great sites. We have taking magnetic susceptibility reading and radiation readings. After all of the data has been analyzed, we may have a good answer or just a new set of questions. Such is the journey of a scientist.

-Andrea