In the central Williston basin, USA, the Bakken Formation and overlying lower Lodgepole Formation
both have fine-grained, organic-rich stratigraphic units that have been interpreted
sedimentologically to represent deep-water deposition in a low-energy, distal-marine
environment; however, these formations display vastly different stratigraphic geometries that
challenge the conventional sedimentology interpretations. The Bakken Formation spans the
Devonian-Carboniferous boundary and includes black, organic-rich (2%–26% total organic carbon
[TOC]) shale units. Stratigraphic characteristics strongly support deposition of all Bakken
sediments in shallow water, as indicated by (1) the Bakken stratigraphic position overlying a
major subaerial unconformity; (2) the restriction of Bakken strata to basinal areas; (3) the
absence of shale-equivalent landward deposits; (4) a layer-cake, onlap, landward-thinning
stratigraphic geometry for all Bakken units; (5) gradual landward shale pinchouts that occur by
intra-shale onlap and stratal thinning, not erosional truncation; (6) unequivocal evidence for
very shallow-water middle Bakken deposition; and (7) the absence of evidence for large
intra-Bakken sea-level changes. Lower Lodgepole strata in the Williston basin are characterized
by prominent sigmoidal clinoforms. In the lower Virden clinoform, argillaceous mudstone,
laminated microcrystalline dolostone, microbial-peloidal-intraclastic packstone, and
skeletal-oolitic limestone form a shelf facies that transitions seaward into a thick (maximum 80
m), skeletal-peloidal mudstone to packstone slope facies, which transitions seaward into
seaward-thinning (10 m to 1 m), black, organic-rich (1%–8% TOC) carbonate mudstone in a
basin-floor facies, inferred to have been deposited in water as deep as 140 m.
Manuscript received 2 Feb. 2022. Revised manuscript received 11 Apr. 2022.
Manuscript accepted 12 Apr. 2022. Posted 11 May 2022.
© The Geological Society of America, 2022. CC-BY-NC.