THE ORDOVICIAN METEOR EVENT: EVIDENCE FROM IMPACT-DAMAGED ACRITARCHS AND THE 40AR/39AR DATE OF IMPACT SPHERULES FROM THE CROOKED CREEK STRUCTURE

The correlation of impact-damaged Ordovician acritarchs with the ⁴⁰Ar/³⁹Ar date of impact spherules from the Crooked Creek impact structure (Missouri, USA) provide support for the timing of impact and may suggest a link with the Ordovician Meteor Event (OME). The OME is believed to be the result of the break-up of a large L-chondrite asteroid with fragments that intersected Earth’s orbit and produced a cluster of impacts during the Ordovician (c. 470 +/- 6 Ma). This clustering phenomenon is linked to several impact craters around the world including some documented in North America, such as the Decorah (Iowa), Aames (Oklahoma) and Rock Elm (Wisconsin) structures in the US, and the Slate Island (Ontario) structure in Canada. Meteorite impact produces extreme conditions that result in disordered stratigraphy and the alteration and obliteration of flora and fauna. While damage to palynomorphs, such as breakage, abrasion and loss of ornamentation can occur due to oxidation and other processes, the fusing of processes and surface alteration (e.g., bubbling, coarsening, or smoothing) attributed to partial melting are unique to impact. These features can be used to infer areal and temporal proximity of the palynomorphs to impact, and the youngest of these damaged fossils are used to infer the timing of the event. Although Cambrian-Holocene aged fossils are present in the structure, Ordovician acritarchs are the only fossils that exhibit this unique thermal damage. Additionally, the inferred age provided by the damaged palynomorphs is coincident with the age provided by the ⁴⁰Ar/³⁹Ar stepwise heating of impact spherules (c. 485.4 +/- 9.1 Ma) obtained from core and cuttings samples within the Crooked Creek structure. Thus, the integration of age information provided by palynomorphs and radioisotopic dating better constrain the age of impact for the Crooked Creek structure and widen the scope of the largest known clustering event in Earth’s history.