Abstract

Structural and Stratigraphic evidence indicates that the Deer Creek Member (DCM) of the Wapiti Formation is the basal unit of the Absaroka Volcanic Supergroup throughout the proximal areas of the Heart Mountain Detachment (HMD). Giant mega-blocks of the older Lamar River and Cathedral Cliffs Formation are included in the Deer Creek Member as clasts. The Deer Creek Member is a laterally persistent and mappable lithostratigraphic unit that consists of mountain-size blocks of volcanogenic strata within a complexly deformed and heterogeneous matrix of volcaniclastic material. The DCM was first recognized in the distal areas of the HMD; it was later recognized in the upper South Fork Shoshone River valley. In light of these new correlations, the total areal extent and volume of the unit are conservatively estimated at 1250 mi² (3200 km²) and 165 mi³ (680 km³), respectively. In the proximal areas of the HMD, the DCM is thicker (~600 m) than in areas to the south and has a greater block-matrix ratio.

The volcanic rocks assigned herein to the Deer Creek Member have previously been viewed as in depositional (Pierce, 1987) or tectonic (Hauge, 1990) contact with the underlying detachment and allochthonous Paleozoic rocks. Assignment of these rocks to the DCM permits these volcanic rocks to be viewed simultaneously as in depositional or tectonic contact. The distinction thereof depends on the scale of observation. Outcrop-scale structures (striations, high-angle truncation of bedding, etc.) indicate tectonic contact while larger-scale stratigraphic relations suggest depositional contact in terms of a large debris avalanche. These correlations also, in part. solve the stratigraphic problem of distinguishing the Wapiti from the Lamar River Formation. All rocks previously in dispute in terms of stratigraphic unit are now assigned to the Deer Creek Member.