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The late Eocene clinopyroxene-bearing spherule layer: New sites, nature of the strewn field, Ir data, and discovery of coesite and shocked quartz

By
Shaobin Liu
Shaobin Liu
Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, USA
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B.P Glass
B.P Glass
Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, USA
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Frank T Kyte
Frank T Kyte
Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA
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Steven M Bohaty
Steven M Bohaty
School of Ocean and Earth Science, University of Southampton, European Way, Southampton SO14 3ZH, UK
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Published:
April 01, 2009

In order to better define the late Eocene clinopyroxene-bearing (cpx) spherule layer and to determine how the ejecta vary with distance from the presumed source crater (Popigai), we searched for the layer at 23 additional sites. We identified the layer at six (maybe seven) of these sites: Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) Holes 592, 699A, 703A, 709C, 786A, 1090B, and probably 738B. The cpx spherule layer occurs in magnetochron 16n.1n, which indicates an age of ca. 35.4 ± 0.1 Ma for the layer. We found the highest abundance of cpx spherules and associated microtektites in Hole 709C in the northwest Indian Ocean, and we found coesite and shocked quartz in the cpx spherule layer at this site. We also found coesite in the cpx spherule layer at Site 216 in the northeast Indian Ocean. This is the first time that coesite has been found in the cpx spherule layer, and it provides additional support for the impact origin of this layer. In addition, the discovery of coesite and shocked quartz grains (with planar deformation features [PDFs]) supports the conclusion that the pancake-shaped clay spherules associated with quartz grains exhibiting PDFs are diagenetically altered cpx spherules. An Ir anomaly was found associated with the cpx spherule layer at all four of the new sites (699A, 709C, 738B, 1090B) for which we obtained Ir data. The geometric mean of the Ir fluence for the 12 sites with Ir data is 5.7 ng/cm2, which is ~10% of the fluence estimated for the Cretaceous-Tertiary boundary. Based on the geographic distribution of the 23 sites now known to contain the cpx spherule layer, and 12 sites where we have good chronostratigraphy but the cpx spherule layer is apparently absent, we propose that the cpx spherule strewn field may have a ray-like distribution pattern. Within one of the rays, the abundance of spherules decreases and the percent microtektites increases with distance from Popigai. Shocked quartz and coesite have been found only in this ray at the two sites that are closest to Popigai. At several sites in the Southern Ocean, an increase in δ18O in the bulk carbonate occurs immediately above the cpx spherule layer. This increase may indicate a drop in temperature coincident with the impact that produced the cpx spherule layer.

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GSA Special Papers

The Late Eocene Earth—Hothouse, Icehouse, and Impacts

Christian Koeberl
Christian Koeberl
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Alessandro Montanari
Alessandro Montanari
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Geological Society of America
Volume
452
ISBN print:
9780813724522
Publication date:
April 01, 2009

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