Primary Mineralogical and Chemical Characteristics of the Major K/T and Late Eocene Impact Deposits

Three well-characterized, distal impact deposits at the K/T boundary and in upper Eocene sediments serve as a baseline for understanding other proposed impact deposits. All contain abundant spherules, evidence of shock metamorphism, and the largest have significant extraterrestrial components (ETCs). The K/T and the Eocene cpx-spherule (cpxS) deposits are global - likely from the events that produced the 180 km Chicxulub and 100 km Popigai craters. The Eocene North American microtektite (NAM) deposit is regional and likely from the event that produced the 45 km Chesapeake Bay crater. These deposits all contain abundant spherules formed from both shock-melted target and mixtures of target and projectile in the ejecta plume. Spherules constitute most of the mass of the distal ejecta. K/T spherules in regional deposits around the Gulf of Mexico are from low-velocity, target-rich ejecta. These can be a few mm in size and form deposits 10s of cm thick. Globally deposited K/T spherules from the plume (typically a few hundred micron size) are both target- and projectile- rich. When well preserved, the global deposits are 3 mm thick. Eocene cpxS deposits are similar to distal K/T with both target- and projectile-rich varieties (i.e., glassy microtektite, and cpx spherules). They are smaller on average than K/T spherules, concentrated in the 125-250 micron and smaller fractions. They are invariably bioturbated, but the initial deposit was probably less than 1 mm thick. The NAM are composed entirely of target-rich glass. They are similar in size to the cpxS. Size is an important criterion for distal ejecta because droplet size in the impact plume is proportional to the energy of the impact. Both the K/T and cpxS deposits are characterized by well-defined ETCs, commonly measured by Ir. The total Ir deposited is about 55 ng per square cm in K/T sediments, and about 11 ng for the cpxS layer. This 5/1 proportion in Ir is generally consistent with the ~1.8/1 ratio in crater diameters. The NAM have no significant ETC. This may be a function of the smaller impact. It indicates there was no significant projectile-rich plume deposit. All three deposits also contain evidence of shock metamorphism, including quartz with planar deformation features, and coesite. K/T and NAM deposits are also known to contain shocked feldspar and zircon. Shocked minerals are not as ubiquitous as spherules, although in K/T deposits they are found in the Pacific, North America, and in trace amounts in Europe. Shocked minerals are only a small fraction of the total mass (typically less than 1 mg/g). These diagnostic criteria are clearly demonstrated by numerous labs on samples from a large number of K/T and Eocene sites. At present, such evidence of impact is not ubiquitous in P/T or T/J boundary sediments. Scattered reports of very small spherules (less than 100 microns) in P/T boundaries do not include abundance data. There are no convincing Ir anomalies that would represent a large ETC. Reported traces of meteorite fragments or anomalous noble gases, while intriguing, could be derived from non-impact sources (e.g., interplanetary dust particles). A few reports of shocked quartz in P/T boundaries are also intriguing, but this author won't be convinced of their accuracy until confirmed by TEM analysis. A problem with searching for evidence of impact at the P/T and T/J boundaries is the paucity of good localities with continuous sediment records and the fact that they are unavailable to most researchers. Those who wish to advance impact at the T/J and P/T need to work to get key samples distributed the broader impact community.