Traces In the Dark--Sedimentary Processes and Facies Gradients In the Upper Devonian-Lower Mississippian Upper Shale Member of the Bakken Formation, Williston Basin, North Dakota, U.S.A.--Discussion

Abstract

I would like to comment on a recent paper by Egenhoff and Fishman (2013) that discusses observations made on cores of the Bakken Shale and presents a methodology to examine and describe these cores. In this contribution I would like to discuss some of their observations and conclusions related to perceived bioturbation features. The authors very eloquently came up with an attractive title for their paper “Traces in the Dark,” and it is this darkness that I would like to shine some light on. Some of the burrows that are shown, like the burrow tubes with contrasting fill in their figure 4, are rather clear, and there is no disagreement. Others, like those in their figure 5, may just as well be cross sections of pellets, and probably a thin section parallel to bedding would be required to sort out their actual origin (e.g., Macquaker et al. 2010). The aspects that I found the most troubling are shown in their figure 6 (A and B), where a laminated black shale is shown on which the authors drew white lines to highlight what they interpret to be burrows that penetrate this laminated shale. And there is the rub: without these white lines I would never have been able to “see” these “burrows,” and I have seen plenty of subtle bioturbation in shales before (e.g., Schieber 2003). Having worked on Devonian black shales for two decades, I actually have hundreds of thin sections like this one in my collection. The silt laminae in sections from my collection, especially if only a few grains thick, show gaps and discontinuities in the ten to hundred micron range quite commonly, but I personally would not interpret these gaps in terms …