Geological Society of America

T178 Impact Stratigraphy

Tuesday, 29 October 2002
1:30 PM - 5:30 PM

A TALE OF TWO BOLIDES: COMPARING THE CRETACEOUS-PALEOGENE (K-T) AND NEWLY IDENTIFIED MIDDLE DEVONIAN EIFELIAN-GIVETIAN IMPACTS

Paper number: T178-3
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Brooks B. Ellwood Presenting Author
Louisiana State Univ, Baton Rouge, LA
Stephen L. Benoist
Louisiana State Univ, Baton Rouge, LA
Ahmed El Hassani
Institut Scientifique, Rabat Agdal, N/A
William D. MacDonald
SUNY at Binghamton, Binghamton, NY
Rex E. Crick
Univ of Texas at Arlington, Arlington, TX

Abstract

Earth has clear evidence of a long history of extraterrestrial impact events, but only a few of these have been shown to exhibit strong evidence to support a direct cause and effect relationship between impacts and mass extinctions. The best known of these is the K-T boundary impact at which time the dinosaurs went to extinction. Such impact events may leave very little evidence in the rock record and it is often very difficult to correlate this evidence between geological sections. Identifying these generally very thin stratigraphic beds and then correlating these beds in widely separated sections is extremely difficult when no clear evidence of these horizons is visually evident in outcrop. This can be accomplished using a magnetic susceptibility (MS) field method to optimize identification of such beds, and here we use the K-T boundary interval exposed in Oman as a test of the method. After identifying the K-T interval in the field, the geochemical analyses we performed, showing Platinum group element enrichment including iridium and a large negative carbon isotope anomaly, confirmed the interval selected was correct.

As a further test using the method we have discovered evidence for a hither-to-fore unidentified impact that struck Earth during the Middle Devonian, immediately preceding the Eifelian-Givetian stage boundary (~380 Ma). The impact evidence has been found in the well defined Eifelian-Givetian GSSP (Global Boundary Stratotype Section and Point) located in the Anti-Atlas of Morocco. Because of the abundant extinctions at the level where the impact evidence is found, we believe that this impact could have been responsible for the extinctions and bioevents leading to recognition of the Givetian geological stage. The geochemical, magnetic and physical evidence, including shocked quartz, spheres, high iridium and other elemental anomalies, and a large negative carbon isotope shift is essentially identical with that observed within K-T boundary intervals.