A stromatoporoid reef in the Ludlovian Sundre Beds at the sea stack field of Holmhällar I on southernmost Gotland was subjected to subaerial exposure during its period of deposition. The sea stack field shows a subcircular belt 500 m in diameter and 50 m wide which exposes part of a reef complex. One distinct erosional surface can be traced through all the localities. The surface cuts the stromatoporoid framework of a lower unit and undulates with a general dip 0–15° away from the centre of the subcircle. On the low‐lying parts of the undulating surface, limestone boulders and sediments containing a number of subordinate erosional surfaces occur. The geometry of the platform results in a complicated facies mosaic of the upper unit composed of stromatoporoid boundstone and its lateral facies, crinoidal debris. The boundstone generally rests on the topographic rises of the inner part of the platform. The crinoidal debris largely occurs on the outer part. Stromatoporoid skeletons occupy 50–65 per cent of volume of the boundstone. This value far exceeds the proportion of framebuilders in Quaternary reefs. The stromatoporoid assemblages are recognized. The assemblage of the boundstone is strongly dominated by Plectostroma scaniense which is laminar‐domical and mostly larger than 50 cm in basal diameter. In the other facies, the commonest species is Parallelostroma typicum which often exhibits a certain growth form as a result of growth on soft bottom and periodical sedimentation.
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