Characterization of Thickness Anomalies within the Three Forks Formation, Lower Bakken Shale, and Middle Bakken Siltstone Members, North Central North Dakota, US and Southwestern Manitoba, CA

Dylan Cobb and Stephen Sonnenberg

Isopach maps of the Three Forks Formation, Lower Bakken Shale, and Middle Bakken Siltstone Members show peculiar thickness anomalies at or near their depositional limits in north central North Dakota and southwestern Manitoba. These thickness anomalies overlap spatially, suggesting that the accommodation mechanism was persistent from Three Forks through Middle Bakken deposition. These anomalies trend NE-SW in a sub-linear pattern until reaching average NNW trending thickness contours. This trend is roughly coincident with the dissolutional edge of the Prairie Salt Formation, suggesting that the thickness anomalies are salt collapse features. Groundwater flow through faults, fractures, and Winnipegosis reef mounds is considered to have influenced Prairie Salt dissolution. Isopach maps of the Winnipegosis Formation show a thickness anomaly directly underlying the study area. A residual map of the pre-Bakken surface shows that the Churchill-Superior Boundary was active as a paleohigh during deposition. This feature would have influenced faulting and fracturing of the underlying strata, as well as the depositional limits of the Lower Bakken Shale and Middle Bakken Silstone Members. Erosion of the Upper Three Forks and Lower Bakken Shale may have also contributed to the formation of these thickness anomalies. A combination of Prairie Salt dissolution, non-deposition over the Churchill-Superior Boundary paleohigh, and erosion of the Upper Three Forks would have influenced pre-Bakken paleotopography. If negative pre-Bakken paleotopography influenced paleodrainage patterns, this may have interesting implications for the sediment source of the Lower Pronghorn and proximal Middle Bakken sandstone facies.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012