Tektites are glass objects believed by many authors to have been formed by meteorite (or cometary) impact. The areas on the earth's surface where tektites are found are called strewn fields. Thus, tektites found in Texas and Georgia belong to the North American strewn field. Microtektites (< 1-mm diameter tektites) have been found in upper Eocene sediments from one piston core and in cores from nine Deep Sea Drilling Project sites in the Caribbean Sea, Gulf of Mexico, equatorial Pacific, and eastern Indian Ocean. Based on their fission-track age (34.6 ± 4.2 m.y.), geologic age, geographic location, and chemistry, these microtekties are thought to be part of the North American strewn field. The North American tektites have fission-track, K-Ar, and ⁴⁰Ar-39Ar ages of ~ 34 m.y. The North American microtektite layer

End_Page 470------------------------------

occurs in the Thyrsocyrtis bromia radiolarian Zone, the P15 foraminiferal Zone, and the Discoaster barbadiensis calcareous nannoplankton Zone. At each site, the layer is associated with the last appearance of several species of Radiolaria (e.g., Thyrsocyrtis bromia, t. tetracantha, t. rhizodon, Calocyclas turris). Previously published oxygen isotope data indicate a drop in temperature at about this time, which may be related to the tektite event and may have been responsible for the radiolarian extinctions. An iridium anomaly has recently been found associated with the microtektite layer that supports an impact origin for the tektites. The extent of the strewn field and the calculated mass of microtektite material (~ 10¹² kg) indicate that the North American tektite event w s a relatively large event.

End_of_Article - Last_Page 471------------