Department of Geology and Geological Engineering
Van Tuyl Lecture Series- Fall 2014
4:00-5:00 p.m. in Berthoud Hall Room 241
Thursday, October 9, 2014
Dr. Sven Egenhoff
Colorado State University
“Depositional Environments of Black Shales and the Anoxic-dysoxic Dispute – the
Williston Basin of North Dakota, USA, During Upper Bakken Times as a Key Example”
Abstract: Black shales are excellent source rocks for hydrocarbons but remain a mystery in
terms of the exact environment in which they were deposited. Some geochemical studies
argue for an anoxic or even euxinic setting for black shale deposition and typically envision
sedimentation in a tranquil environment purely by suspension settling. In contrast, the
sedimentological community is becoming increasingly convinced that at least dysoxic
conditions prevailed at times during the deposition of black shales, and sedimentation was
partly through bed load transport with a significantly diminished importance being placed on
suspension settling. That dysoxic conditions existed during deposition of black shales is
supported by evidence of bottom water currents moving and depositing sediment as well as
an abundance of bioturbation/cryptobioturbation; the significance of trace fossils in black
shales is considered crucial as an indicator for some oxygenation of bottom waters.
This study focuses on recognizing depositional events as well as the stratigraphic and spatial
distribution of bioturbation within the upper shale member of the Devonian-Mississippian
Bakken Formation, an important source rock and potential unconventional petroleum
reservoir, Williston Basin, US and Canada. Facies analysis of the upper shale member reveals
that this depositional system is characterized by at least three distinct facies belts with
amorphous organic material occurring in all of them in variable abundance. On a transect from
proximal to distal, these facies belts are: (1) a heavily bioturbated mudstone, with scours and
local fossil lag deposits, (2) a laminated silt-rich mudstone with horizontal burrows and fecal
strings, and (3) a radiolarian-rich mudstone with varying content of silt and clay. The highest
amounts of organic matter occur in facies belt #1.
Evidence of event deposition exists in all facies belts, in the form of sub-millimeter-thick fine
siltstone laminae interpreted as distal tempestites, and lag deposits from weak currents. The
presence of bedding-parallel burrows as well as multidirectional fecal strings in laminated siltrich mudstones, which forms the bulk of the sediment in the unit, clearly points to the
presence of burrowing organisms present during and after deposition, which thereby argues
against persistently anoxic conditions even some millimeters below the sediment-water
interface. Only some of the most distal radiolarian-rich facies, which contain very limited
bioturbation and are largely devoid of tempestite-formed structures, may have been
deposited under temporarily anoxic conditions. That even some distal sediment contains
ripples indicates bottom current reworking occurred at least during portions of their
depositional history.
Burrow and fecal string diversity does show a correlation to grain size and interpreted paleobasin depth. Proximal sediments containing some sand-size grains show up to four different
burrow and fecal string types whereas the most distal facies, composed of clay and finegrained siltstone, shows nearly exclusively one fecal string type. This well-developed tracefossil-diversity trend suggests that an oxygen gradient existed during deposition with generally
higher levels of oxygen present in proximal settings and relatively lower oxygen levels in distal
settings. The very high overall abundance of trace fossils in these rocks suggests that the
producers of burrows and fecal strings lived in this environment, at least at times, and could
not have been swept in through occasional storm events. Deposition of these organic-rich
mudstones must therefore have occurred under largely dysoxic conditions and not under
persistent anoxia.