Abstract: Volcanoclastic Origin of Clay Minerals in Cretaceous Shales of the Western Canada Sedimentary Basin

J. Bloch, P. de Caritat, I. E. Hutcheon

Compositional and petrographic data from bentonites and shales indicate that clay minerals in Cretaceous Colorado Group marine shales from the Western Canada Sedimentary Basin are largely derived from volcanic ash delivered to the basin as fine-grained eolian sediment. The dominant clay minerals are mixed-layer illite/smectite (I/S) and kaolinite with minor detrital illite and chlorite. Bentonites and shale matrix I/S from correlative units show a compositional affinity indicating that bentonite-like material is the precursor to the matrix clay minerals in distal shales.

Petrographic observations indicate that volcanoclastic grains may be altered to K-feldspar, kaolinite and I/S. In addition, textures inferred to be remnant glass shards are completely altered to I/S.

The mechanism of alteration is dominantly hydrolysis of volcanic glass and phenocrysts. Mass balance considerations suggest that much of the shale matrix in distal portions of the middle Cretaceous Western Interior Seaway is derived from volcanic ash.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994