Volcanic components in pelitic sediments

Abstract

The origin of the clay minerals present in pelitic sediments has been widely debated. Weaver¹ suggests that illite and interstratified illite/smectite (I/S) dominate. The association of illite with marine sediments² and the proposal^3,4 that illite forms from smectite (an expandable clay mineral sometimes referred to as montmorillonite) in the marine environment has also been discussed. Weaver⁵ argues for a continental origin of illite, produced by weathering of K-feldspar. Others^6–9 have shown that smectite and I/S (20–50% illite) are converted to I/S of higher illite content (60–80%) in pelitic sediments by low-grade metamorphism. Per cent illite will always refer to the per cent illite in I/S. The designation should not be confused with the content of discrete illite or mica commonly present in these sediments. Here we present clay mineral and K–Ar data for Cretaceous marine shales from the Western Interior of North America demonstrating a volcanic origin for much of these sediments. This observation is correlated with eustatic sea-level rise, increased volcanic activity, sedimentary preservation of organic matter, and incorporated into a petrogenetic/tectonic model for pelitic sediments.