ABSTRACT

Thin beds of altered volcanic ash, termed K-bentonite, are distributed widely in Ordovician strata of the eastern United States. Reported herein are the mineralogy and stratigraphy of eight K-bentonites of the Decorah and Galena formations (Middle Ordovician) of northeast Iowa and adjacent parts of Minnesota, Wisconsin, and Illinois.

Iowa K-bentonites consist almost entirely of clay-size, randomly interstratified mica-montmorillonite, the average mica: montmorillonite ratio being 3:1. Thus, they are similar to Ordovician K-bentonites reported from eastern states. The mica polymorph is dioctahedral 1M to 1M_d of low temperature origin. In contrast, the predominant clay mineral in the normal marine limestones and shales associated with K-bentonites is well crystallized, 2M₁, dioctahedral mica. Contacts between K-bentonites and limestones or shales are distinct; in a few exposures a thin zone of finely laminated, reworked volcanic debris is observed directly above the K-bentonite. K-bentonites themselves are unlaminated.

Accessory sand-size grains in K-bentonites are euhedral apatite, euhedral zircon, sanidine, pyrite, biotite, and hornblende; whereas those from shales are organic apatite, quartz, and calcite. The K-bentonite assemblage suggests volcanic origin, similar to those east of Iowa.

Differential thermal analysis suggests that the clay mineral of K-bentonites altered at low temperatures through a montmorillonite stage and became more mica-like by K-fixation. In contrast, DTA indicates that the clay mineral of shales originated as mica and was weathered slightly toward a montmorillonite-like phase.

Usefulness of K-bentonites as key-horizons for correlation within the Ordovician is limited by several difficulties. Removal of ash by contemporaneous reworking, the probable multiple centers of volcanic activity in the Appalachian region, and the great number of essentially identical K-bentonites within a stratigraphic interval are major problems.