EVIDENCE FOR THE ORDOVICIAN METEORITE EVENT IN OKLAHOMA, USA

Approximately 470 million years ago, the breakup of the L-Chondrite Parent Body led to an increase in the amount of extraterrestrial material being delivered to Earth. Studies from Sweden have identified the long lasting (~20-50 my) nature of this Ordovician Meteorite Event (OME), while studies from China and Russia have confirmed it as a global occurrence. Thus far little work has been done to conclusively identify evidence of this event in the Western Hemisphere.

Two field areas in Oklahoma were considered for their connection to the OME. The Ames Astrobleme in northwest Oklahoma is a buried impact structure thought to have formed in the mid-Ordovician based on stratigraphic relationships. The Arbuckle Anticline in southeast Oklahoma exposes rock units ranging in age from the Cambrian to Pennsylvanian along 11.7 kilometers of I-35.

At Ames Astrobleme, impact breccia (suevite) samples were taken from drill core supplied by the Oklahoma Geologic Survey. Zircons and feldspars were extracted and analyzed via U-Pb and ⁴⁰Ar/³⁹Ar dating respectively, with the hope that the analysis would show thermal resetting at the time of the impact. The initial, pre-Cambrian implement age of the regional bedrock was confirmed, but unfortunately the most recent thermal reset age was ‘too young’ to identify the age of crater formation, assuming the stratigraphic relationships are correct. This mirrors previous failed attempts to date impact glass from the crater.

From the Arbuckle Anticline, limestone and dolostone samples were taken from a location most likely to fall within the window of the OME. Small grains of chromite were extracted from the samples and analyzed to determine their origin (terrestrial or extraterrestrial). This was compared with the results of a detrital zircon analysis to confirm a mid-Ordovician formation age. Based on preliminary results, the number of recovered chromite grains confirms an OME presence in the area.