Evidence from the northwestern Venezuelan Andes for extraterrestrial impact: The black mat enigma

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• 作者：W.C. Mahaney ; V. Kalm ; D.H. Krinsley ; P. Tricart ; S. Schwartz ; J. Dohm ; K.J. Kim ; B. Kapran ; M.W. Milner ; R. Beukens ; S. Boccia ; R.G.V. Hancock ; K.M. Hart ; B. Kelleher
• 关键词：Asteroid impact ; Black mat ; Younger Dryas ; Paleoclimate
• 刊名：Geomorphology
• 出版年：2010
• 出版时间：15 March 2010
• 年：2010
• 卷：116
• 期：1-2
• 页码：48-57
• 全文大小：1680 K

文摘

A carbon-rich black layer encrusted on a sandy pebbly bed of outwash in the northern Venezuelan Andes, previously considered the result of an alpine grass fire, is now recognized as a &#x2018;black mat&#x2019; candidate correlative with Clovis Age sites in North America, falling within the range of &#x2018;black mat&#x2019; dated sites (~ 12.9 ka cal BP). As such, the bed at site MUM7B, which dates to < 11.8 ka ¹⁴C years BP (raw dates) and appears to be contemporaneous with the Younger Dryas (YD) cooling event, marks a possibly much more extensive occurrence than previously identified. No fossils (megafauna) or tool assemblages were observed at this newly identified candidate site (3800 a.m.s.l.), as in the case of the North American sites. Here, evidence is presented for an extraterrestrial impact event at ~ 12.9 ka. The impact-related Andean bed, located ~ 20 cm above 13.7&#x2013;13.3 ka cal BP alluvial and glaciolacustrine deposits, falls within the sediment characteristics and age range of &#x2018;black mat&#x2019; dated sites (~ 12.9 ka cal BP) in North America. Site sediment characteristics include: carbon, glassy spherules, magnetic microspherules, carbon mat &#x2018;welded&#x2019; onto coarse granular material, occasional presence of platinum group metals (Rh and Ru), planar deformation features (pdfs) in fine silt-size fragmental grains of quartz, as well as orthoclase, and monazite (with an abundance of Rare Earth Elements&#x2014;REEs). If the candidate site is &#x2018;black mat&#x2019;, correlative with the &#x2018;black mat&#x2019; sites of North America, such an extensive occurrence may support the hypothesized airburst/impact over the Laurentide Glacier, which led to a reversal of Allerød warming and the onset of YD cooling and readvance of glaciers. While this finding does not confirm such, it merits further investigation, which includes the reconnaissance for additional sites in South America. Furthermore, if confirmed, such an extensive occurrence may corroborate an impact origin.