A Silurian reefal succession in the Gotland area, Baltic Sea

Seismic reflection profiling east of Gotland has given information on a suc cession of four reefal units in the lower Ludlow stratal sequence ranging f rom the Klinteberg Formation to the Hemse Group in the Gotland sequence. In this paper they are named the Klinte Reef (Klinteberg Formation unit f), t he Hammarudden Reef (Hemse Group 'younger' units a-c), the Ostergarn Reef ( Hemse Group unit d) and the Millklint Reef (Hemse Group unit e). The reef u nits form a mainly regressive succession with younger reefs resting on the seaward slopes of older reefs. The reef succession is exposed on Gotland an d in the Baltic Sea east of Gotland, whereas towards the East Baltic coast it is overlain by Pridoli sedimentary bedrock. Each reef unit corresponds t o a well-defined seismic unit with distinct facies zonality ranging from la goon via reef barrier and biohermal slope to basin facies. The 'fore-reef' facies is in this paper termed biohermal slope to express its characteristi c reefal features. Reconstruction of the post-depositional tilting of the s equence indicates lagoonal depths of up to 10 m. The reef barriers develope d at depths of up to 20 m and the biohermal slopes at between 10 and 55 m. The reef barrier and the biohermal slope are generally separated by a debri s fan of waste products from the reef, 1-2 km wide. c reef barriers are gen erally 1-3 km wide, which is about the same width as their lagoons. The bio hermal slopes become successively wide towards the upper part of reef succe ssion, ranging from 5-9 km at the Klinte Reef to 15-18 km at the Millklint Reef. Comparisons with the Gotland sequence show that the reef barriers are biostromal stromatoporoid reefs in a matrix of crinoid debris. The biostro mes are of the Kuppen type. The bioherms are of the Axelro and Hoburgen typ es. The bioherms occur randomly on the seaward slope of the barriers.