Heath-Amsden Strata in Central Montana

Abstract

In central Montana the so-called formation consists of three lithologic divisions--an upper dolomite unit, a middle brownish limestone unit, and a lower sequence of shale and sandstone beds. The dolomite unit is lithologically, stratigraphically, and paleontologically equivalent to the carbonate part of the Amsden formation at its type locality in northern Wyoming. The Amsden dolomite overlaps toward the south the underlying brown limestone and shale beds and also the formations of the Big Snowy group. Unconformably(?) underlying the Amsden dolomite of Atokan age is a brown ostracodal limestone unit which is probably of Chester age but may be all or in part very early Pennsylvanian. This brown limestone was named the Alaska Bench formation by Freeman and his terminology is recommended for future use. The Alaska Bench formation is restricted areally to the Big Snowy trough and can be traced eastward in the subsurface at least as far as the Montana-North Dakota state line. Conformably underlying the Alaska Bench formation and unconformably overlying the Heath formation is a and dark gray shale sequence which is commonly included as an upper portion of the Heath formation. This unit contains the prolific oil sands at the Northwest Sumatra and other central Montana oil fields. Tyler formation is the name herein applied to the unit; the term is used because it is available and applicable, and because a type locality has been previously designated from surface exposures. The shales at the top of the Tyler formation are included in the formation as a red shale member but a more formal designation may be desirable in the future.