Sedimentation control on growth of stromatoporoid reefs in the Silurian of Gotland, Sweden

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Abstract
Silurian stromatoporoid-dominated reefs of Gotland, Sweden, are bioherms and biostromes formed during phases of reduced clastic supply. Well-exposed examples are in the early Wenlock Högklint Formation and middle Ludlow Hemse Group. Reef development culminated in shallow water biostromes, laterally expanded over a stabilized substrate and associated with frequent erosion surfaces. Högklint reefs began as bioherms, to be replaced by biostromal phases as water shallowed, with vertically zoned communities. Biostromal phases are considered equivalent to the whole of individual stromato-poroid biostromes (without biohermal phases) of the Hemse Group, which developed in shallow, low to moderate energy water, on stabilized substrate. Low clastic supply associated with these reefs reduced not only the clay input, which is otherwise common in most non-reef environments on Gotland, but also is interpreted to have lowered available nutrient levels. Modern reefs develop best in conditions of low nutrients and also show (a) suppressed bioerosion, and (b) increased symbiosis between corals and algae which conserves nutrients in the reef biota. Högklint reefs show reduced bioerosion while Hemse reefs show almost complete lack of bioerosion. Symbiosis between stromatoporoids and corals, interpreted as a response to the need for nutrient conservation, occurs in the reefs, but is much more common in Hemse reefs. Both Högklint and Hemse reefs are regarded as having grown in nutrient-deficient conditions in line with modern reefs although the effect on Hemse reefs is believed to have been greater because they appear to have grown in quieter conditions on a broad shelf and may have had reduced access to nutrients in a regressive regime throughout the Ludlow. Reduced clastic supply while the Gotland reefs grew could have been due to regional tectonism controlling local sea-level change. However, the Högklint Formation and Hemse Group reef phases coincide with times of proposed eustatic sea-level falls. Reef growth on Gotland could therefore relate to eustatic tectonism by subduction-related or plate flexure processes, or climatic control. An oceanographic model for the Silurian proposed alternating wet and dry episodes, interpreted as CO 2 -driven, and could have controlled clastic input, permitting the development of fossiliferous reefs and the carbonate platforms on which they formed. These interpretations provide a revised framework for examining the reef growth.