Characterization of Thickness Anomalies within the Three Forks and Bakken Formations, North Central North Dakota, USA

Cobb, Dylan G.; Sonnenberg, Stephen

Isopach maps of the Three Forks and Bakken formations show peculiar thickness anomalies at or near their depositional or erosional limits in north central North Dakota. Several of these thickness anomalies overlap spatially, suggesting that the accommodation mechanism was persistent from Three Forks through Bakken deposition. These thickness anomalies are roughly coincident with the dissolutional edge of Prairie salt, suggesting that they are dissolution-collapse features. Faults, fractures, and Winnipegosis reef mounds are considered to have influenced the ingress of subsurface waters, Prairie salt dissolution, and egress of the resultant brine. Cemented vertical fractures observed in core of the Nisku Formation, directly underlying Bakken and Three Forks thickness anomalies, may be evidence for dissolution-collapse fracturing, and vertical escape of brine. The Churchill-Superior Boundary and the Lonetree-Glenburn trend are roughly coincident with the dissolutional edge of Prairie salt, suggesting that fracturing and faulting associated with these tectonic features may have influenced Prairie salt dissolution. In addition, erosion of the upper Three Forks during a period of uplift and exposure may have influenced pre-Bakken paleotopography and subsequent Lower Bakken Shale sedimentation. Whether a result of dissolution-collapse or erosion, negative pre-Bakken paleotopography may have influenced paleodrainage patterns, and this could suggest a fluvial sediment source for the basinward Pronghorn sandstone facies. Spatial distributions of clastic grain size and mineralogy suggest a northeasterly sediment source for the Bakken Formation. This supports a sediment transport path trending NE-SW through the study area, towards the basinward Pronghorn sandstone facies.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013