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Shallow-Water versus Deep-Water:

Stratigraphic Geometries in the

Organic-Rich Shale/Mudstone Debate

David M. Petty, retired petroleum geologist, dpusa555@gmail.com

ABSTRACT INTRODUCTION depositional systems have recently been
In the central Williston basin, USA, the There has been a +100-year debate over proposed for Devonian black shales in New
Bakken Formation and overlying lower depth of deposition interpretations for fine- York and suggested as a worldwide model
Lodgepole Formation both have fine- grained, organic-rich, upper Devonian– for similar black shale depositional systems
grained, organic-rich stratigraphic units that lower Carboniferous stratigraphic units in (Smith et al., 2019). Key shallow-water indi-
have been interpreted sedimentologically to North America (Conant and Swanson, 1961; cations include onlap onto subaerial uncon-
represent deep-water deposition in a low- Ettensohn and Barron, 1981; McCollum, formities and the absence of a base-clinoform
energy, distal-marine environment; how- 1988), and this debate has accelerated in setting. However, the regional stratigraphic
ever, these formations display vastly differ- recent years. The upper Devonian–lower relationships for these black shales remain
ent stratigraphic geometries that challenge Carboniferous debate is part of a larger debated, and additional support data from
the conventional sedimentology interpreta- worldwide discussion on the origin of fine- comparable black shale stratigraphic sys-
tions. The Bakken Formation spans the grained, organic-rich stratigraphic units, tems is required to further define shallow-
Devonian-Carboniferous boundary and as represented by session #144 at GSA water relationships.
includes black, organic-rich (2%–26% total Connects 2021, titled “Broken paradigms: Research on upper Devonian–lower
organic carbon [TOC]) shale units. Strat- Shallow-water deposition of organic-rich Carboniferous black shale units in the
igraphic characteristics strongly support facies through Earth history” (Landing et Bakken Formation in the North Dakota
deposition of all Bakken sediments in shal- al., 2021). Regional stratigraphic relation- portion (Fig. 1) of the Williston basin illus-
low water, as indicated by (1) the Bakken ships that define shallow-water organic-rich trates the shallow-water versus deep-water
stratigraphic position overlying a major sub-
aerial 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 mud-
stone, 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 mud-
stone to packstone slope facies, which transi-
tions seaward into seaward-thinning (10 m to
1 m), black, organic-rich (1%–8% TOC) car-
bonate mudstone in a basin-floor facies,
inferred to have been deposited in water as Figure 1. Landward limits for Bakken Formation members, with cross section
deep as 140 m. locations.

GSA Today, v. 32, https://doi.org/10.1130/GSATG536A.1. CC-BY-NC.

4 GSA TODAY | July 2022