Cambrian stratigraphy and depositional dynamics based on the Tomten-1 drill core, Falbygden, Västergötland, Sweden

Sammanfattning: The Tomten-1 drilling at Torbjörntorp in Västergötland, southern Sweden, penetrated 29.85 m of Cambrian Series 2 and 3, Furongian, and Lower and Middle Ordovician strata. The biostratigraphy, sedimentology and carbon isotope (δ[sup]13[/sup]C[sub]org[/sub]) stratigraphy of the core have been analysed. The succession is interrupted by numerous stratigraphic gaps (i.e. hiatuses) of variable magnitudes. There also appear to be several stratigraphic gaps within the Exsulans Limestone Bed. In the Cambrian Series 3 through Furongian Alum Shale Formation, agnostoids and trilobites have been identified to species level and the succession is subdivided into nine biozones (in ascending order): the [i]Ptychagnostus gibbus[/i], [i]P. atavus, Lejopyge laevigata, Agnostus pisiformis, Olenus gibbosus, Parabolina spinulosa, Ctenopyge tumida, C. bisculata[/i] and [i]C. linnarssoni[/i] zones. Lithological characteristics have been studied macroscopically as well as microscopically through thin section petrography, allowing for identification of unconformities and general depositional environments. The characteristics of the strata have rendered it possible to divide the drill core into important lithostratigraphic units that reflect sea level changes and allow for a correlation with other drill cores in Sweden. Two negative δ[sup]13[/sup]C[sub]org[/sub] excursions have been recorded from the lowermost part of the Alum Shale Formation. The most distinctive of these has a net shift of 2.14 ‰ and occurs below the Exsulans Limestone Bed, in strata that seem to be equivalent to the [i]Ptychagnostus gibbus[/i] Zone. It may correspond to an un-named excursion that has been recorded from the Cambrian Stage 5 in Scania, southern Sweden, and South China, but in the absence of useful biostratigraphic evidence, this identification is problematic. Another, but very poorly developed excursion in the lower [i]P. atavus[/i] Zone possibly represents the Drumian Carbon Isotope Excursion (DICE) as described from the GSSP section at Drum Mountains in Utah, western United States. However, the values are not low enough to be considered diagnostic for the DICE. Detailed and dense sampling is required in order to delimit the range and amplitude of this minor excursion in the Tomten-1 drill core. The Steptoean Positive Carbon Isotope Excursion (SPICE) has not been recorded, largely because of lack of δ[sup]13[/sup]C[sub]carb[/sub]-sampling in the lithologically highly variable Kakeled Limestone Bed and the incompleteness of the lower Furongian (Paibian Stage). Collectively, the data suggest that the Tomten area was shallow enough to be exposed at low sea-level and that Västergötland was subaerial during long periods of time.