Sven O. Egenhoff, PhD , Colorado State University
Aims of project and progress
This project uses detailed thin section analysis to establish a depositional model for the Alum and Tøyen Shales in Scandinavia and to search for a possible internal cyclic architecture of these two black shale units believed to be largely massive in character.
The basis of the investigations are: (1) detailed sedimentological documentation and sampling of the Andrarum and Tomten cores (Västergötland and Scania, Sweden) covering large parts of the Alum Shale, (2) complete Alum Shale outcrop sampling and documentation in two localities, Hunneberg and Kakeled (Västergötland, Sweden), (3) detailed sedimentological log of the Lerhamn and Krapperup cores (Scania, Sweden) showing a nearly complete section through the Tøyen Shale, and (4) outcrop investigations of Tøyen Shale in Slemmestad (Oslo area, Norway).
Alum Shale investigations followed two approaches: (a) document the Peltura scarabeoides trilobite Biozone in 3 different localities/cores (Hunneberg, Kakeled, and Tomten) and compare internal centimeter- and meter-scale cyclic architecture, and (b) investigate as much as possible of both the Alum and Tøyen Shales in order to identify the range of facies and facies stacking patterns, and thereby the variety of cycles types that characterize these two black shale units.
Field work was undertaken during two three-week long trips to Scandinavia in August 2010 and 2011. The cores are stored at the Geology Department in Lund, Sweden, where the graduate student working on this project and the PI stayed during most of their visits to Scandinavia. The Hunneberg, Kakeled, and Slemmestad localities have been studied during two week-long field trips, one on 2010 and one in 2011. The graduate student presented preliminary results from this project on the AAPG ACE meeting in Houston in April 2011, and the PI will give a talk on Alum Shale sedimentology on the GSA national meeting in October, 2011. Two larger manuscripts and one discussion paper based on data from this project are currently in progress. The field work in Scandinavia, especially the several week-long stays in Lund also initiated a close cooperation with professors Mikael Calner and Per Ahlberg at the Geology Department, University of Lund. The PI will likely spend part of his upcoming sabbatical in Lund and continue the studies on the Alum and Tøyen Shales initiated by this project, as well as their relationships to over- and underlying carbonate units.
While the project is still ongoing the results can be summarized as follows:
(a) Continuous thin section coverage over the entire Peltura scarabeoides trilobite Biozone and detailed documentation of younger and older Alum Shale strata show that this unit consists of multiple decimeter-thick carbonate intervals that are intercalated with decimeter- to meter-thick shale stacks. The shales show high variability in grain types, grain sizes and quantity of different grains, sedimentary structures, and burrow types and numbers. Fossil occurrence is patchy within the shales, but they are abundant throughout the carbonates (cf. Ahlberg et al. 2009). The Tøyen Shale generally shows higher amounts of carbonate and less black TOC (=total organic carbon)-rich intervals compared to the Alum Shale, but follows the same general trends.
(b) The depositional model for the Alum Shale shows a total of six distinct facies belts, three characterized by carbonate sediments, and three dominated by shales. The carbonates generally decrease in grain size downramp reflecting a decrease of energy with increasing paleo-water depth; however, a carbonate mudstone facies containing gypsum crystals is interpreted as a lagoonal setting and therefore represents the most proximal facies belt. The trend of decreasing grain sizes continues within the three shale facies belts. The millimeter- to sub-millimeter-thick layers consisting of sand- and silt-sized grains within the shales as well as layers with clay rip-up clasts represent tempestites deposited by bed-load transport. Those storm layers decrease in maximum grain size and frequency downslope but reach even the most distal portion of the Alum Shale shelf. The more clay-rich sediment in-between the storm layers was deposited by both bed-load and suspension processes mostly during fair-weather intervals.
All shale and carbonate facies show intense bioturbation features. However, proximal parts of the Alum Shale shelf transect were characterized by a more diverse fauna than the most distal shale facies belt. Nevertheless, even though a distinct bioturbation gradient developed it clearly shows that none of the shale facies was deposited in a completely anoxic setting but rather under dysoxic to even oxic conditions (cf. Schieber 1998). These findings are in remarkable contrast to previous interpretations describing the Alum Shale depositional environment (e.g. Andersson et al. 1985) and show that even well oxygenated to dysoxic environments can accumulate black shales with up to 17% TOC content. Our studies suggest a generally similar model for Tøyen Shale deposition but with overall higher amounts of carbonate mud input and not as extensive black shale development.
(c) Both the Alum and the Tøyen Shales show a well-defined decimeter-scale cyclicity. However, exclusively the most distal facies belt of the Alum Shale also records millimeter-thick shale cycles, mostly coarsening- but also fining-upward that represent short-term low-amplitude fluctuations of sea level and/or climate signals. Considering the duration of Alum/Tøyen Shale deposition (Andersson et al. 1985; Ahlberg et al. 2009) the larger cycles likely represent Milankovitch fluctuations, while the millimeter-thick repetitive shale stacks correspond to sub-Milankovitch cyclicity. These findings confirm the hypothesis of this project that distal shale environments, in contrast to their more proximal counterparts, preserve very short-term cyclic fluctuations reflected in distinct facies stacking in the rock record. Such signals may therefore be used for fine-scale correlations as they define time-lines within fine-grained rock successions for source rocks and potential unconventional reservoirs.
Ahlberg, P. et al., 2009. Cambrian high-resolution biostratigraphy and carbon isotrope geochemistry in Scania, Sweden: first record of the SPICE and DICE excursions in Scandinavia. Lethaia, 42: 2-16.
Andersson, A., Dahlman, B., Gee, D.G. and Snäll, S., 1985. The Scandinavian Alum Shales. Sveriges Geologiska Undersökning, Avhandlingar och Uppsatser, 56: 1-50.
Schieber, J., 1998. Simple gifts and buried treasures - implications of finding bioturbation and erosion surfaces in black shales: The Sedimentary Record 1: 4-8.