Gamma-ray log correlation and stratigraphic architecture of the Cambro-Ordovician Alum Shale Formation on Bornholm, Denmark: Evidence for differential syndepositional isostasy

The Cambro–Ordovician Alum Shale Formation on Bornholm, Denmark, is in total 26.7 to ≥ 34.9 m thick in nine boreholes, but may be up to ~39 m thick. The well sections are correlated using gamma-ray logs supplemented in some boreholes with resistivity and sonic logs. The gamma radiation of the ‘hot’ Alum Shale Formation primarily reflects the uranium content, which is moderately high in the Miaolingian (≈ middle Cambrian) and Tremadocian (Lower Ordovician), and very high in the Furongian (≈ upper Cambrian). The log pattern is calibrated with the detailed biozonation established in the Gislövshammar-1 and -2 wells in south-eastern Skåne, Sweden. Except for the Eccaparadoxides oelandicus Superzone, all superzones known from the Alum Shale in Scandinavia are also developed on Bornholm, but not all zones. On Bornholm, the Miaolingian interval is 7.2–11.9 m thick, the Furongian is 16.4–22.8 m thick and the Tremadocian is 2.5–4.0 m thick. The Miaolingian strata exhibit no systematic thickness variations across southern Bornholm, whereas the Furongian Parabolina, Peltura and Acerocarina Superzones and, less pronounced, the Tremadocian, show increased condensation towards the south-east. In comparison with Skåne, the Alum Shale Formation is overall strongly condensed on Bornholm, but different stratigraphic levels show variable developments. The Miaolingian Paradoxides paradoxissimus Superzone is thus extremely condensed and incomplete, whereas the Paradoxides forchhammeri Superzone has almost the same thickness as in Skåne, and locally is even thicker. The Furongian Olenus and Parabolina Superzones are slightly thinner than in Skåne while the Protopeltura, Peltura and Acerocarina Superzones are half as thick or less. The Tremadocian is also much thinner on Bornholm. The Furongian Olenus scanicus–O. rotundatus and Parabolina brevispina Zones seem to be developed on Bornholm, and a thin ‘Leptoplastus neglectus’ Zone is also possibly present. The ‘Parabolina megalops’ Zone in the upper part of the Peltura Superzone appears to be absent. It is impossible to distinguish the individual thin zones in the lower part of the Acerocarina Superzone using wireline logs. A thin veneer of the Lower Ordovician Tøyen Formation, hitherto considered absent on Bornholm, is described from the Billegrav-2 core. It may also be present in the uncored Sømarken-3 and -4 wells. The Middle Ordovician Komstad Limestone Formation thins from c. 4.0–4.7 m in the Læså area to 0.1– c. 2.5 m in the Øleå area. The general decrease in thickness of Cambro–Ordovician strata from Skåne to Bornholm and also within Bornholm from the Læså to the Øleå area is inferred to reflect isostatic uplift of the southern margin of Baltica commencing with the terminal 'early' Cambrian Hawke Bay Event and lasting until the Late Ordovician. In detail, several uplift and subsidence phases can be discerned. The isostatic adjustments are surmised to reflect stress changes related to ongoing plate tectonic processes in the adjacent closing Tornquist Sea.