THE SOFT AND THE BRITTLE – RECONSTRUCTING TECTONICALLY ACTIVE AREAS DURING SILICICLASTIC MUDSTONE DEPOSITION, MISSISSIPPIAN UPPER BAKKEN SHALE, NORTH DAKOTA, USA

Reconstructing synsedimentary tectonic activity in shales has been difficult to impossible up to now. Here, we present a new method that allows to precisely pinpoint tectonic movements that affected two shale lithologies differently: siliciclastic mudstones seem to have reacted in a ductile way, and the interbedded radiolarites brittle leading to cracks. Where cracks in the radiolarites reach a maximum in abundance, the synsedimentary tectonic activity must have been the most intense.

This study therefore documented the distribution pattern of healed cracks in radiolarites described to have hardened soon after deposition throughout the North Dakota portion of the Williston Basin. The results show that most of Divide and Williams Counties in the west of show little to no crack occurrence, and so do larger parts of Burke, and Dunn Counties. However, along the border of Divide and Burke County as well as along the same border between Williams/McKenzie and Mountrail/Dunn Counties, crack abundance was high. Similarly, crack abundance was high in wells in Golden Valley, Billings, and Stark Counties in the south of the study area.

This is interpreted to reflect synsedimentary tectonic activity during upper Bakken deposition. Crack distribution shows that synsedimentary tectonics were frequent along a N-S running fault line along the boundaries of Divide/Burke/Williams/Mountrail, and McKenzie Counties. Similarly, synsedimentary faults were active in a NW-SE trend through Golden Valley, Billings, and Stark Counties as indicated by the cracks. Areas of the Williston Basin that were tectonically quiet such as large parts of Divide and Williams Counties are interpreted as not having any active faults during upper Bakken deposition.