Origin of Cretaceous and Oligocene Kaolinites from the Iwaizumi Clay Deposit, Iwate, Northeastern Japan

C. Mizota1 and F. J. Longstaffe2
1 Faculty of Agriculture, Iwate University, Ueda 3-18-8, Morioka 020, Japan
2 Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7 Canada

Abstract: Hydrogen- (δD = −106 to −97‰) and oxygen-(δ18O = +14.0 to +16.6‰) isotope compositions of kaolinite from late Cretaceous and Oligocene deposits at Iwaizumi, northeastern Japan, indicate that these clays formed by weathering of volcanic parent rocks, rather than during hydrothermal (>100 °C) alteration. The Iwaizumi kaolinites also are depleted of D and 18O relative to kaolinite formed during modern, tropical weathering, suggesting that the kaolinite developed under cool or cool-temperate conditions. The oxygen-isotope compositions of the kaolinite increase slightly upward through the deposits, perhaps implying a modest increase in temperature from late Cretaceous to Oligocene time. The δD and δ18O results for kaolinite from the Oligocene deposits closely follow the kaolinite weathering line. However, a small but systematic deviation from this line for the Cretaceous kaolinites is most simply explained by post-formational, hydrogen-isotope exchange between these clays and downward percolating meteoric water.

Key Words: Climate • Cretaceous • Hydrogen-isotopes • Iwate • Japan • Kaolinite • Oligocene • Oxygen-isotopes

Clays and Clay Minerals; June 1996 v. 44; no. 3; p. 408-416; DOI: 10.1346/CCMN.1996.0440310
© 1996, The Clay Minerals Society
Clay Minerals Society (www.clays.org)