The Ordovician Orthoceratite Limestone and the Blommiga Bladet hardground complex at Horns Udde, Öland

Abstract: Blommiga Bladet, Flowery Sheet in English, is an Early Ordovician (basal Dapingian) hardground complex in the lowermost part of the Orthoceratite Limestone in southern Sweden. It formed in an epicontinental sea, across Baltoscandia and has been described also from Russia as “Steklo”, from Estonia as “Pustakkith” and from deep borings in Poland, thus covering an area exceeding 500,000 km². The Orthoceratite Limestone spans the Floian, Dapingian and Darriwilian stages and records an extremely low net sedimentation rate (2mm/ka). Recurrent periods of non-deposition allowed hardgrounds to form. These hardgrounds are numerous and probably represents a significant portion of time in the Orthoceratite Limestone. The Blommiga Bladet hardground complex displays distinct, clear-cut and abraded hardground surfaces that slowly have been polished by calcareous particles swept across the sea-floor by currents or waves, as well as a few more irregular omission surfaces that may represent firmgrounds. The uppermost hardground surface was colonized by a borer belonging to the ichnospecies Gastrochaenolites oelandicus. G. oelandicus borings cut trough indurated sediments and earlier omission surfaces, creating a sac-like pit with a small aperture. Blommiga Bladet was mineralized by several ferruginous compounds in the early diagenesis, making it skimmer in red, green and yellow. The cavities were subsequently filled with sediment. In the Early and Middle Ordovician, Baltoscandia was mainly affected by eustasy and Blommiga Bladet probably formed during a sea-level low-stand, when the sea-floor regionally was reworked by waves.