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Aims of project and progress

The aims of this project are to detect and characterize shale cycles of different order and magnitude in two selected successions in Scandinavia, the Alum and the Tøyen Shale Formations. Field data were taken in August, 2010, during a three-week stay describing outcrops and cores in Scania and Västergötland, southern Sweden, as well as in the Oslo region of southern Norway. For characterizing the Alum Formations, two cores were measured in greatest possible detail, and a continuous sample was taken for the Peltura scarabeoides trilobite Biozone from the Tomten core at the University of Lund, and from the Holsbrotten outcrop at Hunneberg, Västergötland (cf. Egenhoff and Maletz 2007). Furthermore, one continuous section was sampled and measured in the Kakeled quarry in Kinnekulle, Västergötland (cf. Terfelt 2003). The Tøyen Formation was documented by measuring part of the Lerhamn KBH core at the University of Lund (cf. Maletz and Ahlberg, 2010) and one outcrop section in Slemmestad, Norway. 

This study has just begun - the student has been recruited as part of this project in May 2010, and the PI as well as the student have returned from their field season on August 21, 2010. The results summarized here are the work of a little more than 5 weeks, are therefore preliminary and will be greatly expanded over the next few month.

During our one-week visit to Lund, Sweden, we started a cooperation with Prof.Mikael Calner and Prof. Per Ahlberg from Lund University, who provided access to the core material from both formations. The samples are now unpacked and cut, and in part on its way to the thin section preparation lab. However, a first batch of ten samples has already been converted into thin sections, studied by the student working on the project and the PI, and preliminary results are in review by AAPG to be presented on their annual meeting in April, 2011 in Houston.

Preliminary results

The results from this research project so far can be summarized as follows:

(a) Thin section work shows that the ten Alum Formation shale samples investigated microscopically are characterized by abundant horizontal and vertical bioturbation regardless of facies. However, coarser-grained facies such as siltstones are generally less affected by bioturbation than mudstones with finer overall grain sizes. The bioturbations are not equally distributed throughout the succession but are rather concentrated in distinct sub-millimeter-thick layers. All of the Alum Formation bioturbation features are only visible in ultra-thin thin sections (20 micrometers). The Tøyen Formation shales, however, show also bioturbations of millimeter-size visible in well-exposed outcrops (e.g. Slemmestad in Norway, south of Oslo).

The abundant bioturbations suggest that the black shale portion of the Alum Formation environment was not completely anoxic (see Graphic Table of content "TOC"). Frequent traces of organisms rather support a more dysoxic to occasionally oxic setting that nevertheless preserved high amounts of total organic carbon.

(b) A detailed facies analysis of cores, outcrops and thin sections provided enough data to develop a preliminary facies model for the Alum Formation (see Graphic Table of content "TOC") as a basis to detect shale cyclicity. The facies recognized in only ten thin sections already shows a remarkable complexity. The proximal to distal facies transect during Alum Formation deposition varied significantly based on the availability of siliciclastic detritus in proximal settings. During times of abundant supply of quartz detritus the nearshore area was dominated by a siliciclastic facies belt. Quartz silt was also transported offshore onto the deeper shelf, preferentially during storms. The amount of siliciclastics therefore gradually decreased away from the shoreline towards the offshore with the deepest facies being a black organic-rich mudstone without any visible silt-sized detrital grains.

During times without detrital quartz input from the hinterland no siliciclastic sediment characterized the proximal shelf. Nearshore facies then changed to carbonate deposition with shell-rich pack- to grainstones representing the most proximal facies. Towards the offshore the pack- and grainstones gradually graded into lower energy carbonate mudstones, and at the distal end into dark organic-rich black siliciclastic mudstones. A mechanism for this distinct change back and forth has not yet been proposed,. However, the availability of siliciclastic detritus is likely a function of uplift in the hinterland and therefore mirrors synsedimentary tectonics on the Baltic plate (see below).

(c) The Tøyen Formation shows well developed small-scale cycles, likely in the Milankovitch band. These cycles are in the range of some decimeters thick and show a distinct coarsening-upward internal architecure. The Alum Shale Formation, in contrast, does not exhibit macroscopically visible cycles but may still show microscopically detectable cyclicity. However, the fist ten thin sections reflect only abundant event sedimentation within the Alum mudstones and not regular or "cyclic" internal architecture.

(d) Our study detected micro-scale slump structures. These have been caused by synsedimentary tectonic movements on the Baltic plate in Late Cambrian times which has been regarded as tectonically quiet by most previous workers (Cocks and Torsvik 2005; Lindström 1971).

References

Cocks, L.R.M. and Torsvik, T.H., 2005. Baltica from the late Precambrian to mid-Palaeozoic times: The gain and loss of a terrane's identity. Earth-Science Reviews, 72: 39-66.

Egenhoff, S. and Maletz, J., 2007. Graptolites as indicators of maximum flooding surfaces in monotonous deep-water shelf successions. Palaios, 22: 373-383.

Greiling, R.O. and Garfunkel, Z., 2007. An Early Ordovician (Finnmarkian?) foreland basin and related lithospheric flexure in the Scandinavian Caledonides. American Journal of Science, 307: 527-553.

Lindström, M., 1971. Vom Anfang, Hochstand und Ende eines Epikontinentalmeeres. Geologische Rundschau, 60: 419-438.

Loucks, R.G. and Ruppel, S.C., 2007. Mississippian Barnett Shale: lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas. American Association of Petroleum Geologists Bulletin, 91: 579-601.

Maletz, J., and Ahlberg, P., 2010. The Lerham drill core and its nearing for the graptolite biostratigraphy of the Ordovician Tøyen Shale in Scania, southern Sweden. Lethaia, 19 pages,  in press.

Schieber, J., 1998. Simple gifts and buried treasures - implications of finding bioturbation and erosion surfaces in black shales: The Sedimentary Record 1: 4-8.