Sedimentology and sequence stratigraphy of a pronounced Early Ordovician sea-level fall on Baltica — The Bjørkåsholmen Formation in Norway and Sweden

The Bjørkåsholmen Formation consists of interbedded carbonates, shales, and glauconitic beds and is characterized by heavy bioturbation and few preserved sedimentary structures. The unit shows five facies shale, glauconitic packstone, and three predominantly mud-dominated carbonate facies. Carbonates and shales are arranged in small-scale deepening-upward cycles. A minimum of fourteen of these small-scale cycles are recognized in the Bjørkåsholmen Formation. They are arranged in stacks of 3 to 5, forming a total of four medium-scale cycles separated by decimeter-thick shale units.

Based on the predominance of mud-rich facies the succession is interpreted to have been deposited in an overall tranquil setting during one mayor sea-level fall and subsequent initial rise of third order. Time-estimates suggest that the 14 small-scale cycles fall into the Milankovitch band of precessional forcing, and the overriding medium-scale cycles likely represent short eccentricity. The sequence stratigraphic interpretation shows that the Bjørkåsholmen Formation is characterized by falling stage, lowstand and initial transgressive systems tracts. Consequently, the contact between the Bjørkåsholmen and the underlying Alum Shale Formation represents the basal surface of forced regression. The maximum regressive surface is defined by a hiatus in the Öland sections and by shallow-marine packstones within mud-rich distal ramp carbonates in Norway. The top of the Bjørkåsholmen Formation represents a flooding surface at the base of the transgressive systems tract.

A comparison of time-equivalent successions worldwide suggests that the Bjørkåsholmen Formation represents a tectonically-enhanced lowstand with two overriding short-term Milankovitch eustatic signals. Although deposition of the Bjørkåsholmen Formation coincides with the initiation of a foreland basin in the Caledonides of Norway it remains unclear how these tectonic movements may have lead to the widespread Bjørkåsholmen lowstand during the Early Ordovician. It is suggested in this study that a combination of compressional forces from Avalonia and the Caledonian margin may have acted in concert to produce an uplift of larger parts of the Baltica plate for a time-span of approximately 0.5 Myr.