Devonian Black Shales

Devonian Black Shales Eastern US Chattanooga, New Albany, Ohio Shales

A map of the eastern United States shows the geographic extent of the Devonian black shale complex in the Appalachian, Illinois, and Michigan Basins.

The Mid-Late Devonian black shales of the Appalachian, Illinois, and Michigan Basins have long been studied to provide clues to past climates, ocean ventilation, mass extinctions, and general earth history. Common misconceptions about these shales are that they represent distal deposits that accumulated in deep waters of stratified anoxic basins, and that they form a largely continuous, though condensed, succession. We have shown that these shales contain numerous discontinuities, and represent at least in part shallow water deposition. Truly anoxic conditions must have been exceedingly rare occurences.

Large scale erosion surfaces, traceable over large distances, partition the Devonian black shale succession into successively deposited packages.

These erosion surfaces reflect intermittent sea level drops followed by transgression and renewed black shale deposition. They are the basis for a coherent sequence stratigraphic framework for all of these shales.

A slide from the Eastern Black Shale Study shows two photographs of a rock outcrop with labeled erosion surfaces, illustrating how sections of the shale packages were truncated and removed over geological time.

Erosion can be documented (and is common) at any scale, from sub-mm scours, over removal of cm-scale shale beds (see picture), local truncations at the dm-scale, to deep m-scale erosion that is traceable for 100's of kilometers.

A slide from the Eastern Black Shale Study shows a microscopic view of a digitally enhanced shale sample, with labels highlighting five layers, two of which have been removed by erosion, and showing how bioturbation increases the amount of information available to sedimentologists.

References for above:

Schieber, J., 1994, Evidence for episodic high energy events and shallow water deposition in the Chattanooga Shale, Devonian, central Tennessee, U.S.A. Sedimentary Geology, v. 93, p. 193-208. download PDF file (left click on link and click on "Save as"...)
Schieber, J., 1998, Sedimentary features indicating erosion, condensation, and hiatuses in the Chattanooga Shale of Central Tennessee: Relevance for sedimentary and stratigraphic evolution. In: J. Schieber, W. Zimmerle, and P. Sethi (editors), Shales and Mudstones (vol. 1): Basin Studies, Sedimentology and Paleontology, Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, p. 187-215. download PDF file (left click on link and click on "Save as"...)
Schieber, J., 2003, Simple gifts and hidden treasures – Implications of finding bioturbation and erosion surfaces in black shales. The Sedimentary Record, v. 1, p. 4-8. download PDF file(left click on link and click on "Save as"...)

Aside of erosive features (above) shallow water conditions (10's of meters to 50 meters) are also indicated by storm deposits, such as HCS sandstones/siltstones, and coarse grained lags.

A presentation slide from the Eastern Black Shale Study shows a photograph of a rock outcrop with a geologic hammer for scale, highlighting a pinch and swell bed, which is an example of a hummocky cross-stratified sandstone that formed from storm deposits.

Fine grained storm indicators are graded silt/mud couplets, graded rhythmites (at right), and mud tempestites. The latter are rapidly deposited mud-blankets that "smother" bioturbated horizons (at right).

A slide from a study shows a diagram of a flume experiment, with a large white arrow indicating the direction of flow and smaller arrows pointing to three vertical structures within the tank.

References for above:

Schieber, J., 1994, Paleoflow patterns and macroscopic sedimentary features in the Late Devonian Chattanooga Shale of Tennessee: Differences between the Appalachian Basin and the American Craton. Canadian Society of Petroleum Geologists, Memoir 17, p. 763-772. download PDF file (left click on link and click on "Save as"...)
Schieber, J., 1994, Reflection of deep vs shallow water deposition by small scale sedimentary features and microfabrics of the Chattanooga Shale in Tennessee. Canadian Society of Petroleum Geologists, Memoir 17, p. 773-784. Link

Bioturbation is widespread and pervasive, but often missed because it is typically subtle and requires careful study of ground and polished surfaces. Digital image processing and X-radiography has revealed bioturbation in many shale intervals that were once thought of as unbioturbated.

A slide from a scientific study shows two magnified images of a geological sample, with annotations describing graded rhythmites and a burrowing trace fossil, which are identified as mud tempesites from storm deposits.

References for above:

Lobza, V., and Schieber, J., 1999, Biogenic sedimentary structures produced by worms in soupy, soft muds: Observations from the Chattanooga Shale (Upper Devonian) and experiments. Journal of Sedimentary Research, v. 69, p. 1041-1049. download PDF file (left click on link and click on "Save as"...)
Schieber, J., 2003, Simple gifts and hidden treasures – Implications of finding bioturbation and erosion surfaces in black shales. The Sedimentary Record, v. 1, p. 4-8. download PDF file(left click on link and click on "Save as"...)

Pyrite ooids were discovered in lag deposits overlying major erosion surfaces. Subsequent study showed them to have originated as chamosite ooids during low-stands of sea-level. During sea-level rise the lag deposits were draped by black shales and the chamosite was replaced by pyrite.

A scientific slide shows three magnified images of pyrite ooids, with text explaining that they are found in reworked horizons and indicate extended erosion, high-energy conditions, and an aerated water column.

Pertinent References:

Schieber, J., and Riciputi, L., 2004, Pyrite ooids in Devonian Black Shales record intermittent Sea level drop and shallow water conditions. Geology, v. 32, p. 305-308. download PDF file(left click on link and click on "Save as"...)

Early diagenetic pyrite infill of Tasmanites cysts produced spherical pyrite grains within slowly deposited black shales. Upon reworking and erosion these pyrite spheres were concentrated into pyritic lags that indicate high energy conditions via cross-lamination and scouring.

A presentation slide shows two magnified images of geological samples, highlighting fine bioturbation features in the rock, and notes that subtle burrows can be revealed through digital image processing and X-radiography.

Reference for above:

Schieber, J., and Baird, G., 2001, On the origin and significance of pyrite spheres in Devonian black shales of North America. Journal of Sedimentary Research, v. 71, p. 155-166. download PDF file (left click on link and click on "Save as"...)

Detailed petrographic investigations of quartz grains in these shales showed an appreciable proportion of authigenic quartz grains. The silica was derived from dissolving radiolarians and reprecipitated in pores, especially the cysts of Tasmanites. In places 80% or more of the visible quartz is of authigenic origin. This quartz can be detected easily with scanned cathodo-luminescence, and its authigenic origin was further confirmed through ion probe determination of oxygen isotope ratios.

A slide from the Eastern Black Shale Study shows three magnified images of pyritic microspheres with a caption that describes them as lag deposits that indicate high-energy conditions, substantial erosion, and an aerated water column.

References for above:

Schieber, J., 1996. Early diagenetic silica deposition in algal cysts and spores: A source of sand in black shales? Journal of Sedimentary Research, v. 66, p. 175-183. download PDF file (left click on link and click on "Save as"...)
Schieber, J., Krinsley, D., and Riciputi, L., 2000, Diagenetic origin of quartz silt in mudstones and implications for silica cycling. Nature, v. 406, p. 981-985. download PDF file (left click on link and click on "Save as"...)

Continued study of petrographic and geochemical features of these black shales provides us with an ever more detailed understanding of their depositional setting. Work in progress is an examination of geochemical anoxia proxies to assess their reliability (or rather the lack thereof), an in depth examination of framboid diameter distributions, and experimental work on the formation of black shale rhythmites. Stay tuned.