Upper Eocene Impact Ejecta/Spherule Layers in Marine Sediments

Abstract

Abstract Microtektites with compositions and ages similar to those of the North American tektites have been found in deep-sea deposits in the Gulf of Mexico, Caribbean Sea, northwestern Atlantic Ocean and on Barbados. Unmelted impact ejecta (including shocked quartz and feldspar with multiple sets of planar deformation features, coesite, stishovite, and reidite, a high-pressure polymorph of zircon) have been found associated with this microtektite layer. These microtektites appear to belong to the North American tektite strewn field based on their geographic location, age (∼35 Ma), and composition. Clinopyroxene-bearing spherules (cpx spherules) are closely associated with the North American microtektitestherefore, occurrences of cpx spherules in the Indian Ocean and equatorial Pacific were originally interpreted to indicate a major extension of the North American strewn field. However, at a few sites it is clear that the cpx spherules are older than the North American microtektites and it is now accepted that they belong to a different impact event. The major crystalline phase in the cpx spherules is clinopyroxene, as their name implies, but they also contain Cr- and Ni-rich spinels. The cpx spherules generally have lower SiO 2 and Al 2O 3 and higher FeO, MgO, and CaO contents compared with the North American microtektites. The cpx spherules also have high Ni, Cr, Co, and Ir contents compared to the North American microtektites and to the average upper continental crust. The cpx spherule layer is associated with a positive Ir anomaly and with the extinction of several taxa of Radiolaria. Flattened pancake spherules composed of clay have been found in upper Eocene deposits at Massignano, Italy, associated with a positive Ir anomaly. The pancake spherules are believed to be diagenetically altered cpx spherules. Shocked quartz with multiple sets of planar deformation features have been found associated with the spherule layer at Massignano. The number of upper Eocene microtektite/spherule layers is debated, but most authors agree that there are probably only two: the North American microtektite layer and the cpx spherule layer. Geographic variations in abundance, age, composition, and Sr and Nd isotopic data are consistent with the North American microtektites (and tektites) being derived from the 90 km-diameter Chesapeake Bay structure. Based on the nature of the shocked quartz found associated with the spherule layer at Massignano and the age and Sr and Nd isotopic composition of the cpx spherules, it has been suggested that the 100 km-diameter Popigai impact structure in northern Siberia may the source crater for the cpx strewn field.