Abstract

The Bearpaw uplift of north central Montana is an east-west elongate igneous cored geanticline surrounded by shallow-rooted concentric thrust faults, radial normal faults, and by plutonic-cored satellite domes. Previous workers have ascribed the unusual concentric thrust faulting to gravitational sliding off the uplift.

Clay model studies suggest an alternate hypothesis. The new theory better explains subsurface data as well as recently discovered flank gas accumulations in the Eagle Sandstone (Cretaceous). Gas is trapped in both upthrown and downthrown thrust blocks, depending on geometry, geography and gas migration paths. Exploration and development are currently in progress and require an understanding of the fault patterns.

Structural genesis is envisioned as a two stage phenomenon as follows:

1. A hot-spot plume rose from the mantle forcing fluid magma to the surface and doming the uplift, creating a volcanic field and related radial fault pattern around the core in early Eocene, about 58 to 51 MYBP, followed by cooling and collapse of the uplift.

2. Renewed rise of the plume and uplift of the cool. now solidified core, creating a concentric fault pattern, again followed by partial collapse, in mid-Eocene about 52 to 51 MYBP.

Movement on the faults was normal during each uplift and reversed by each collapse. The radial faults are offset by the concentric faults indicating the latter are older and formed during Stage 1. The Concentric faults formed during Stage 2 and were critical to trapping gas.