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Tektite-bearing, deep-water clastic unit at the Cretaceous-Tertiary boundary in northeastern MexicoThe hypothesis of Cretaceous-Tertiary (K-T) boundary impact on Yucatan, Mexico, predicts that nearby sites should show evidence of proximal impact ejecta and disturbance by giant waves. An outcrop along the Arroyo el Mimbral in northeastern Mexico contains a layered clastic unit up to 3 m thick that interrupts a biostratigraphically complete pelagic-marl sequence deposited at more than 400 m water depth. The marls were found to be unsuitable for determining magnetostratigraphy, but foraminiferal biostratigraphy places the clastic unit precisely at the K-T boundary. We interpret this clastic unit as the deposit of a megawave or tsunami produced by an extraterrestrial impact. The clastic unit comprises three main subunits. (1) The basal "spherule bed" contains glass in the form of tektites and microtektites, glass spherules replaced by chlorite-smectite and calcite, and quartz grains showing probable shock features. This bed is interpreted as a channelized deposit of proximal ejecta. (2) A set of lenticular, massive, graded "laminated beds" contains intraclasts and abundant plant debris, and may be the result of megawave backwash that carried coarse debris from shallow parts of the continental margin into deeper water. (3) At the top, several thin "ripple beds" composed of fine sand are separated by clay drapes; they are interpreted as deposits of oscillating currents, perhaps a seiche. An iridium anomaly (921 +/- 23 pg/g) is observed at the top of the ripple beds. Our observations at the Mimbral locality support the hypothesis of a K-T impact on nearby Yucatan.
Document ID
20040090310
Document Type
Reprint (Version printed in journal)
Authors
Smit, J. (Free University of Amsterdam Netherlands)
Montanari, A.
Swinburne, N. H.
Alvarez, W.
Hildebrand, A. R.
Margolis, S. V.
Claeys, P.
Lowrie, W.
Asaro, F.
Date Acquired
August 21, 2013
Publication Date
February 1, 1992
Publication Information
Publication: Geology
Volume: 20
ISSN: 0091-7613
Subject Category
Geosciences (General)
Funding Number(s)
CONTRACT_GRANT: EAR-9105297
Distribution Limits
Public
Copyright
Other
Keywords
NASA Discipline Number 52-40
Non-NASA Center
NASA Discipline Exobiology
NASA Program Exobiology