Department of Geology and Geological EngineeringVan Tuyl Lecture Series- Fall 2014

4:00-5:00 p.m. in Berthoud Hall Room 241  Thursday, October 9, 2014Dr. Sven EgenhoffColorado 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 silt-rich 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 paleo-basin 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 fine-grained siltstone, shows nearly exclusively one fecal string type. This well-developed trace-fossil-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.