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GEOLOGY: K-T boundary - The K-T boundary claystone layer at the Sussex locality, Wyoming, USA
At bottom of the image is the mudstone of the Lance Formation (Cretaceous). On top is a coal bed of the Tullock Member of the Fort Union Formation (Tertiary). The K-T boundary layer is the gray to white kaolinitic claystone, less than 2 cm thick, at about the middle of the photo. On top of it is a darker, laminated layer a few millimeters thick, containing sand grains, many of them showing multiple sets of parallel fractures ("shocked quartz") under the microscope. Iridium can be detected in the claystone layer and in the carbonaceous layer immediately above it.
© Francois Gohier / ardea.com