Echinoderm limestone boulders from the Åland Islands, Finland (mid Sandbian, Ordovician) – earliest evidence of Baltscandian shallow water patch reefs

On the Åland Islands, Finland, rare erratic boulders with a pelmatozoan pack-, grainstone lithology occur. These boulders contain predominantly crinoid columnal fragments with columnal diameters of up to 12 mm. Based on petrographic characteristics, the boulders can be interpreted as representing clinoform flank deposits, probably of small patch reefs. The stratigraphic age of one of the boulders can be precisely constrained as belonging to the mid-Sandbian Baltoniodus viirae conodont zone, Dalbyan Regional Stage, Ordovician. It corresponds in age to the pelmatozoan-rich Dalby Limestone of Sweden. The occurrence of erratic boulders of pelmatozoan limestone on the Åland Islands suggests a mid-Sandbian existence of pelmatozoan-rich patch reefs on the proximal, now eroded part of the carbonate platform. This is significantly older than the previously known oldest echinoderm-rich reefs of Baltoscandia.