STRUCTURAL REFINMENT OF THE NORTHEASTERN CORNER OF THE LARAMIDE BEARTOOTH UPLIFT, MONTANA

The corner of the Beartooth Uplift near Red Lodge, MT, has perplexed structural geologists for at least 3/4 of a century. The prevailing interpretation was that apparent tear faults offset near-vertical frontal palisades of Paleozoic carbonates to define a map-view, keystone-shaped corner block in which erosion shifted the frontal thrust back toward the mountains by 0.5-4 km. Recently, the "tear faults" were shown to be normal faults bounding the Towne Point keystone and merging behind it as a fracture zone, thus allowing its reinterpretation as a raised block during later Laramide thrusting. Continuing problems remain. (1) Keystone blocks of near-vertical to strongly overturned Paleozoic beds are generally bounded above and below by thrusts. (2) Keystone Paleozoic rocks bear an uncertain relationship to adjacent Paleocene Fort Union Formation (FUF). The FUF includes carbonate-clast mudflows with massive carbonate slide-blocks overlain by a basement-clast facies. Adjacent to the keystone, unconformities pin the FUF carbonate-clast facies and basement to Paleozoic units but within the block these contacts are complicated by thrusts and poor exposure. Reinterpretation of corner evolution involves several stages. (1) Early thrusting rolled the leading edge of basement and its Paleozoic sedimentary cover into an exposed gently-dipping (20 degrees) frontal monocline. Erosion of uplifted carbonates, possibly assisted by earthquakes-induced block-slides, produced the FUF mudflow-dominated carbonate facies. Unconformities pin these units across the tilted Paleozoic beds. (2) Continued thrusting steepened the frontal zone and displaced it from the strongly overturned, attenuated footwall syncline. Deeper erosion of the uplift shifted deposition to the FUF basement-cobble facies. (3) Final-stage thrusting possibly involved changed transport direction, underthrusting, or duplexing, and created the Towne Point block by locally arching the frontal-zone master thrust. Keystone thrust-imbricates climbed structurally upward, abandoning blocks of early-tilted Paleozoic frontal beds and their FUF carbonate-clast-facies cover, to newer, higher splays and then to the trailing master thrust. Several hundred meters of structural uplift of the corner enhanced present-day erosional retreat and exposure of abandoned blocks from deeper levels of its thrust system.