Collaborative Research: High Resolution Dating of Sedimentary Successions Using Cyclostratigraphic Methods and Zircon Dating: The Controversial Triassic Latemar Limestone ....

George Washington University, Washington DC

Investigators

Abstract

John M. Hanchar (EAR 9909383) The Middle Triassic Latemar Limestone (Italy), a thick succession of hundreds of meter-scale shallow marine depositional cycles, is unique in that to date it is the only cyclic sedimentary deposit that has been intensively analyzed for Milankovitch forcing (Hinnov & Golldhammer, 1991) as well as radiometrically dated (indirectly) using the zircon single crystal technique on volcanoclastic interbeds in the coeval basinal deposits (mundil et al. 1996). Unfortunately, the published results of the two methods as applied to the Latemar Limestone are in serious conflict (Brack et al. 1996; Hardie & Hinnov 1997). In essence, the zircon dates have been interpreted to indicate that the Latemar cyclic succession was deposited in 2-4 m.y. whereas the cyclostratigraphic analysis indicates a Milankovitch origin which implies deposition over a ca. 10-12 m.y. period. These conflicting results present a serious dilemma that goes well beyond the Latemar problem in view of the fact that both zircon single crystal and cyclostratigraphic methods have come to be widely applied in chronostratigraphic studies. In an effort to resolve the problem, PIs propose to (1) re-examine the Latemar cyclostratigraphy for Milankovitch forcing using the latest time series modelling and analysis techniques, and (2) determine U/Pb ages of zircon crystals from volcanoclastic interbeds, the source ignimbrites, and the Predazzo granite that cuts trhough the Latemar massif. The important new approach PIs will bring to the zircon dating will be pre-screening of the zircons using CL and BSE imaging techniques (Hanchar & Miller, 1995) and the dating of microslices of selected parts of single zircon crystals. It is their hope that this proposed new study of the "Latemar controversy" will function as an important test case for high resolution dating of cyuclic sedimentary successions.