A Silurian reefal succession in the Gotland area, Baltic Sea

Seismic reflection profiling east of Gotland has given information on a succession of four reefal units in the lower Ludlow stratal sequence ranging from the Klinteberg Formation to the Hemse Group in the Gotland sequence. In this paper they are named the Klinte Reef (Klinteberg Formation unit f), the Hammarudden Reef (Hemse Group ‘younger’ units a-c), the Östergarn Reef (Hemse Group unit d) and the Millklint Reef (Hemse Group unit e). The reef units form a mainly regressive succession with younger reefs resting on the seaward slopes of older reefs. The reef succession is exposed on Gotland and in the Baltic Sea east of Gotland, whereas towards the East Baltic coast it is overlain by Pridoli sedimentary bedrock. Each reef unit corresponds to a well-defined seismic unit with distinct facies zonality ranging from lagoon via reef barrier and biohermal slope to basin facies. The ‘fore-reef’ facies is in this paper termed biohermal slope to express its characteristic reefal features. Reconstruction of the post-depositional tilting of the sequence indicates lagoonal depths of up to 10 m. The reef barriers developed 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 debris fan of waste products from the reef, 1-2 km wide. The reef barriers are generally 1-3 km wide, which is about the same width as their lagoons. The biohermal slopes become successively wider towards the upper part of reef succession, 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 biostromes are of the Kuppen type. The bioherms are of the Axelro and Hoburgen types. The bioherms occur randomly on the seaward slope of the barriers.