March/April Geological Society of America Bulletin media highlights

Keywords: diagenesis, opal, cement, subduction, Ocean Drilling Program.

Spinelli et al. have determined that a very small amount of opal acts as a cement that strengthens and dramatically controls the deformation of sediment offshore southern Japan. Opal comprises less than 1% of the sediment, but it coats sediment grains and cements grain contacts. At temperatures above 55 ˚C, the opal begins to dissolve and the weight of the overlying sediment breaks the cement. The cemented sediment is approaching the Nankai Trough subduction zone off southern Japan. The anomalously strong cemented zone deforms differently as it enters the subduction zone. The cemented zone has very few fractures in it, while sediments above and below the cemented zone have numerous fractures. This has implications for the distribution of deformation and fluid flow in subduction zones. Similar cementation and anomalous sediment strength may be important at other locations, including offshore southern Alaska, northern California, and Guatemala.

Terrestrial records of a regional weathering profile at the Paleocene-Eocene boundary in the Williston Basin of North Dakota
Elizabeth R. Clechenko, University of Wisconsin–Madison, Geology and Geophysics, Madison, WI 53706, U.S.A.; et al. Pages 428-442.

Keywords: Paleocene-Eocene Thermal Maximum, Williston Basin, kaolinite, Golden Valley Formation.

This paper describes a regional weathering profile preserved in continental strata of the Golden Valley Formation in western North Dakota (Williston Basin). Combined palynostratigraphic and chemostratigraphic evidence suggest that this episode of continental weathering occurred during an ancient (ca. 55 Ma) global warming event referred to as the Paleocene-Eocene Thermal Maximum, or PETM. Clay mineral assemblages within these weathered strata are dominated by thermodynamically stable kaolinte, which lends credence to the hypothesis that continental weathering intensified during the greenhouse climatic conditions of the PETM.

Geology and complex collapse mechanisms of the 3.72 Ma Hannegan caldera, North Cascades, Washington, USA
David Tucker, Western Washington University, Geology, Bellingham, WA 98225, U.S.A.; et al. Pages 329-342.