A STRATIGRAPHIC PERSPECTIVE ON THE HEART MOUNTAIN PROBLEM

The Heart Mountain Detachment (HMD) in northwest Wyoming is large, enigmatic, complex, well-studied, controversial, and unfortunately still poorly understood. The various aspects of the HMD have resulted in two principal contexts for studying and understanding the problem. Some workers have used the principles of structural geology as their context; others use the principles of stratigraphy. The context of structural geology is more appropriate for the proximal areas of the HMD. Here, the basal contact almost everywhere follows the same Ordovician bedding plane. Allochthonous volcanic and Paleozoic rocks are considered to be part of an upper plate. The boundaries between components of the upper plate are faults. The lower contact of allochthonous rocks is viewed as a detachment. Relief on this detachment is attributed to footwall and side-wall ramps.

A stratigraphic context is more appropriate for the distal areas of the HMD. Here, the allochthonous rocks can be viewed as a teramictite (Sundell and Fisher, 1985), and a lithostratigraphic unit within the Eocene Absaroka Volcanic Supergroup. The allochthonous rocks are similar in texture, thickness, structure, composition, and areal extent, as debris-avalanche deposits which are reported in many Quaternary volcanic terranes. The geometric features within allochthonous rocks can alternatively be regarded as textures and primary structures within a stratigraphic unit. Blocks, individually as large as several km in maximum dimension, are separated by matrix. Some blocks are volcanic and Eocene in age; others are limestone and Paleozoic in age. The basal contact is irregular and has more than 1 km of relief, and underlying rocks range in age from Mississipian to Eocene. The allochthonous rocks are sometimes older, where they are Paleozoic carbonates, and sometimes younger, where they are Eocene volcanic, than the underlying strata. The footwall and side-wall ramps can be viewed as erosional paleotopography.