The Cambro–Ordovician stratigraphic successions encountered in the Vasegård-1, Hjulmagergård-1 and Gravgærde-1 shallow boreholes on Bornholm, Denmark

The Cambro–Ordovician successions penetrated by the fully cored shallow boreholes Gravgærde-1, Hjulmagergård-1 and Vasegård-1 are summarised; none has been described previously. All three boreholes are located south-west of Åkirkeby on Bornholm, Denmark. Gravgærde-1 drilled 4.1 m of the Alum Shale Formation and the upper 2.2 m of the Rispebjerg Member (Læså Formation). Hjulmagergård-1 drilled 7.65 m of the Dicellograptus Shale, 1.55 m of the Sularp Formation and 4.6 m of the Komstad Limestone before being terminated 3.7 m into the Alum Shale. The basal part of the Komstad Limestone is faulted and shows slight repetition, suggestive of reverse reactivation of the fault. The underlying Alum Shale is completely brecciated and clearly represents a fault zone. Vasegård-1 drilled the basal 2.0 m of the Dicellograptus Shale, the Sularp (1.65 m) and the Komstad Limestone (4.8 m) formations in their entirety, before being terminated 9.65 m into the Alum Shale. The Alum Shale succession in Vasegård-1 was analysed using a handheld XRF analyser, allowing construction of a core gamma-ray curve that can be correlated with wireline-logged boreholes as well as with the Alum Shale exposed at Limensgade, where a detailed tie between the gamma-ray log pattern and the Tremadocian graptolite zonation has been established using a handheld gamma-ray logger. The correlation indicates that the upper part of the Alum Shale Formation in the Vasegård-1 core represents the Tremadocian (4.9 m) and the Furongian Acerocarina (1.5 m) plus the uppermost Peltura (3.2 m) superzones. Gravgærde-1 confirms that the Alum Shale constitutes the bedrock in a belt extending from Soldatergård to north of Gravgærde. This belt is separated from the Ordovician strata to the east by a NW–SE striking fault with a vertical offset of around 40–50 m, which was intersected by the Hjulmagergård-1 borehole. A combination of data from Vasegård-1, Limensgade quarry and the nearby St. Bukkegård-1 borehole indicates that the Alum Shale Formation is at least 35.6 m thick in the Læså area but may reach 38 m. The maximum documented thicknesses on Bornholm of the Komstad Limestone (4.8 m) and the Ordovician Alum Shale (4.9 m) are recorded in the Vasegård-1 borehole.