Paleozoic Impact Spherule/Ejecta Layers
Although the Paleozoic Era lasted nearly five times longer than the Cenozoic, the number of Paleozoic distal impact ejecta layers that have been identified is much smaller (compare Table 6.1 with Table 4.1). Two probable Late Devonian distal impact ejecta
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Paleozoic Impact Spherule/Ejecta Layers
6.1 Introduction Although the Paleozoic Era lasted nearly five times longer than the Cenozoic, the number of Paleozoic distal impact ejecta layers that have been identified is much smaller (compare Table 6.1 with Table 4.1). Two probable Late Devonian distal impact ejecta layers have been reported. The youngest is a silicate glass spherule (microtektite?) layer and Ir anomaly found in Qidong (southern China) in Late Devonian deposits, about 1.5–2.0 Ma younger than the Frasnian–Famennian boundary. The second is an older layer of silicate glass spherules (microtektites) found just above the Frasnian–Famennian boundary at two sites (Senzeille and Hony) in Belgium. Four additional spherule occurrences of Frasnian to Famennian age have been reported in southern China: one above the Frasnian–Famennian boundary, one at the boundary, and two below the boundary. The most intensely studied Paleozoic ‘ejecta’ layer is at the Permian–Triassic boundary, but as of 2011, the impact origin of this boundary layer had not been established. Some researchers claim to have found evidence for an impact at the Devonian–Carboniferous boundary and at the Eifelian–Givetian boundary; however, the evidence is not yet convincing. Silicate glass spherules, interpreted as microtektites, have been found associated with the Devonian-Carboniferous boundary (also called the Devonian-Mississippian boundary) in southern China and shocked quartz has been reported from this boundary in northwestern Pennsylvania, USA. High concentrations of some siderophile elements, spherules, and shocked quartz in Middle Devonian (close to the Eifelian–Givetian boundary) deposits in Jebel Mech Irdane, Morocco, have been reported; but the presented evidence has been disputed by some researchers. At the end of the chapter, we have included a short section on the late Devonian (Frasnian Age) impact breccia (the Alamo Breccia) found in Nevada, USA. Some of the ejecta associated with this layer may be distal, but most of the Alamo Breccia appears to be proximal.
B. P. Glass and B. M. Simonson, Distal Impact Ejecta Layers, Impact Studies, DOI: 10.1007/978-3-540-88262-6_6, Springer-Verlag Berlin Heidelberg 2013
321
Taihu Lake (?) Wang (1992) ?
?
?
*382
*374 *376
*376
*376
Lower Frasnian Southwest USA SQ Alamo Breccia Probable/possible distal ejecta layers Famennian Qidong, South Sph China Near base of Nandong and Sph Famennian Xikuangshan, South China Frasnian– Nandong and Sph Famennian Xikuangshan, boundary South China Below Frasnian– Nandong and Sph Famennian Xikuangshan, boundary South China Ma and Bai (2002)
Ma and Bai (2002)
Ma and Bai (2002)
Siljan, Claeys and Casier (1994) Charlevoix, or Woodleigh Alamo Warme et al. (1991), Morrow et al. (2005)
*376
Selected referencesd
Sph
Well-established ejecta layers Base of Senzeille and Famennian Hony, Belgium
Table 6.1 Possible/probable Paleozoic impact spherule/ejecta layers Location Impact Age Possible Layera evidenceb (Ma) source craterc
(continued)
322 6 Paleozo
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