TRANSLATION AND BREAKUP OF SUPRADETACHMENT BASINS: LESSONS FROM GRASSHOPPER, HORSE PRAIRIE, MEDICINE LODGE, MUDDY CREEK AND NICHOLIA CREEK BASINS, SW MONTANA

Two adjacent Eocene to Oligocene supradetachment basins in SW Montana grew in the hanging wall of the west-dipping Muddy-Grasshopper detachment. The northern Grant and southern Nicholia-Muddy (NM) supradetachment basins have distinctive stratal architectures due to their evolution above a detachment fault. Early translation on the detachment fault produced the accommodation space for the bulk of the basin fill (Eocene-Oligocene Medicine Lodge beds). Later the hanging wall broke up internally (break-up phase) to produce 5 subbasins in the modern landscape. I refer to the two reconstructed supradetachment basins as “protobasins”. Seismic lines and field relationships reveal nearly conformable translational strata. This geometry arises from the great width of the original protobasin. Both protobasins preserve a small volume of coarse basin margin facies near the master detachment fault and show evidence for later structural partitioning (break-up). The NM protobasin broke up into three subbasins, whereas the Grant protobasin formed two subbasins in the south and 4-5 in the north. In the south, the proximal, eastern Muddy Creek subbasin lacks hanging wall (west) derived coarse clastics whereas the two half graben in the distal Nicholia Creek subbasin lack footwall-derived clastics. These relationships show that these separated subbasins are the east and west halves of the NM protobasin. Farther north in the Grant protobasin, mapping, stratigraphic analyses, and provenance data show that translational synrift strata correlate across younger normal faults, and the protobasin was >40 km wide. When the hanging wall of the Grant protobasin began to break up internally ~27 to 26 Ma, smaller fault blocks tilted eastward above the master detachment. Portions of the older Grant basin were uplifted and recycled into smaller, localized half graben, and progressive unconformities formed. The presence of tuffaceous siltstone clasts derived from early translation-phase deposits within younger breakup-phase conglomerates provides compelling evidence for reorganization and dismembering of the Grant protobasin. Subtle fanning dips in early synrift deposits coupled with clear fanning dips in younger synrift deposits are common in supradetachment basins, and reflect their similar evolution.