HR: 0800h
AN: GP31B-03    [Abstracts]
TI: Magnetic Fabrics in Chicxulub Crater Impact Breccias: Results From UNAM-5 Breccia Sequence
AU: * Velasco-Villarreal, M
EM: fatima_miriam@hotmail.com
AF: Universidad Nacional Autonoma de Mexico, Laboratorio de Paleomagnetismo y Paleoambientes, Instituto de Geofisica, CU, Coyoacan, Mexico, 04510, Mexico
AB: Chicxulub crater is our best example of a large complex impact structure that preserve the ejecta deposits. Their study is important in understanding cratering, fragmentation and collapse mechanisms, firewall deposits, post- cratering events, etc. Our objective is to investigate fabric type and characteristics fabric in the impact breccias by means of anisotropy of magnetic susceptibility (AMS). This study is made in core samples from the UNAM-5 drill hole located 110 km from the structure centre. In UNAM-5, impact breccias have 172m thickness of polymictic ‘suevitic' breccias with clay matrix rich in carbonate, impact glass, melt, basament fragments and melt-matrix breccias. From macroscopic and textural analyses of fragment size, color, matrix we defined four breccia subunits: subunit-1 (332-347m), subunit-2 (347-412m), subunit-3 (412-464m), subunit-4 (464-503.9m). AMS study gives two fabric types characterized by: (1) Prolate fabrics (332.05-450.7m), 55 percent lineation, and variable low-field susceptibility (250-1000x10-6 SI). (2) Oblate fabrics (450.72-503.9m), and high low-field susceptibilities (~700-1500x10-6 SI). In general the anisotropy degree is less than 1.1, in the range for sedimentary and volcanic rocks. Principal susceptibility axial distributions show relatively complex patterns, which can be linked to the textural characteristics of breccias and relative amounts of melt, basement clasts and matrix type. We observe that in the first 118m amount of melt is small, and for the next 53m amount of melt and fragments increases. Secondary alteration processes affecting the fabric in each subunit, including hydrothermal processes and weathering may have been important and their effects on the magnetic fabric are examined.
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6022 Impact phenomena (5420, 8136)
DE: 8136 Impact phenomena (5420, 6022)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly