Craddock J.P.,Macalester College |
Konstantinou A.,Macalester College |
Konstantinou A.,Stanford University |
Vervoort J.D.,Washington State University |
And 6 more authors.
Journal of Geology | Year: 2013
We report the ages of detrital zircons (207Pb/206Pb LA-ICPMS; n p 1188) from 12 clastic units that are representative of the Mesoproterozoic Midcontinent Rift System (MRS) of North America. Magmatism of the MRS spanned 1115-1085 Ma. The Puckwunge, Nopeming, and Bessemer basal sandstones and the Oronto and Bayfield Group sandstones received a predominance of Keweenaw Rift-Grenville (1.3-1.0 Ga) zircons but also contributions of Wolf River (1.45 Ga), Yavapai (1.75 Ga), Penokean (1.8 Ga), and Archean (2.6-3.6 Ga) ages. Interflow sedimentary rocks in the North Shore Volcanic Group received only rift-aged zircons. The maximum depositional ages derived from detrital zircons also better constrain the age-stratigraphic relationship of the unfossiliferous rift section, in comparison to the overlying fossiliferous Franconia (n p 99 zircons), Mt. Simon (n p 116), and Munising (n p 110) late Cambrian transgressive sands. We also report the date of a granite xenolith (~1 km3) in the Beaver Bay Complex, which has a rift age (1091 Ma; n p 19) and is therefore not a rafted inclusion of Archean basement as previously suspected. © 2013 by The University of Chicago. All rights reserved.
Baumgardner D.,National Autonomous University of Mexico |
Avallone L.,University of Colorado at Boulder |
Bansemer A.,U.S. National Center for Atmospheric Research |
Borrmann S.,University of Mainz |
And 25 more authors.
Bulletin of the American Meteorological Society | Year: 2012
The workshop on in situ airborne instrumentation: addressing and solving measurement problems in ice clouds, June 25-27, 2010, Oregon, aimed to identify unresolved questions concerning ice formation and evolution in ice clouds, assess the current state of instrumentation that can address these problems, introduce emerging technology that may overcome current measurement issues, and recommend future courses of action to improve our understanding of ice cloud microphysical. Eleven presentations were made covering measurement challenges associated measuring the composition and concentration of all the modes of ice nuclei (IN), measuring the morphology, mass, surface, and optical properties of individual ice crystals over all sizes, and measuring temperature, humidity, and winds in clouds accurately. It was found that cloud lifetimes are sensitive to the sedimentation velocity of the ice crystals, as are the rates of aggregation and riming that depend on the relative fall velocities of ice crystals and supercooled water droplets.