ABSTRACT

The Wolsey Shale is a transitional unit between the coarsely clastic Flathead Sandstone below and the Meagher (Limestone) Formation above. The Wolsey Shale consists of predominantly green and brown fissile shale, interbedded with thin lenses of sandstone and siltstone, and contains a thick carbonate unit in the upper portion. Dioctahedral illite and chlorite are present in the shale. The sandstones and siltstones are cross bedded, calcareous, and marked by worm trails and borings, and they contain rounded glauconite pellets and clay galls. The glauconite pellets are composed of ordered 1 M dioctahedral illite and are allochthonous in origin. Three types of limestone can be found in the carbonate unit: silty sparites, arkosic sparites, and intrasparrudites. These limestones are locally oolitic, pelletoidal, dolomitic, and glauconitic, and they contain interbedded fossil hash and limestone pebble conglomerate.

The Wolsey Shale was deposited on a stable shelf during a period of eastward transgression of the seas. Sedimentation was slow, and varying amounts of terrigenous detritus were brought into the environment. Shoals and low islands were frequently exposed and influenced local sedimentation.

The Wolsey Shale as a unit has not undergone extensive diagenetic alteration. Alteration of the glauconite and minor recrystallization of the carbonate are the most notable changes.