Mesozoic Spherule/Impact Ejecta Layers

Abstract

The Cretaceous-Tertiary (K-T), or Cretaceous-Paleogene (K-Pg), boundary impact ejecta layer; the Late Cretaceous Manson crater ejecta layer; the Late Triassic spherule/ejecta layer; and a possible impact ejecta layer at the Triassic-Jurassic boundary are discussed in this chapter (Table 5.1). As previously mentioned, the K-T boundary ejecta layer is the most studied and best known distal impact ejecta layer. The Manson structure is a buried impact crater located in Iowa, USA, which at one time was thought to be the, or a, source crater for the K-T boundary layer. Distal impact ejecta from this structure have been found in South Dakota and Nebraska. An impact spherule layer containing shocked quartz has been found in Late Triassic deposits in southwestern England. Some researchers have reported finding evidence for an impact ejecta layer at the Triassic-Jurassic boundary, but others do not find the evidence convincing.