Abstract

Thirty-four K-bentonites from the Marias River Formation (Upper Cretaceous) in the Montana disturbed belt have been studied in detail. A 2.5-m-thick K-bentonite is zoned mineralogically and chemically, the upper and lower contacts of the bed being more illitic and K-rich than the middle of the bentonite. K/Ar dates of illite/smectite separated from the bed are also zoned, yielding older ages at the contacts than at the center. A model for the formation of K-bentonites enclosed by K-rich shale is presented in which K-bentonites form from smectite bentonites during a thermal event (T = 100–200 °C). Potassium is derived from mineralogic breakdown (probably mica and K-feldspar) in the shale, and migrates by diffusion into the bentonite.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.