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Significance of a Halysitid-Heliolitid Mud-facies Autobiostrome from the Middle Silurian of Gotland, Sweden

¹ Department of Geology, Historical Geology and Paleontology, Lund University, Sölvegatan 13, SE-223 62 Lund, Sweden
² Institute of Geology, University of Tartu, Vanemuise 46, EE-2400 Tartu, Estonia

A low-diversity halysitid-heliolitid autobiostrome from the Late Wenlock Halla Formation at Blåhäll 1, Gotland, indicates a brief period of particular environmental conditions. The reef is 0.3–0.5 m thick, internally without bedding structures, and had a sheet-like distribution on a low-relief soft-bottom, in marginal platform, graptolitiferous strata (height-width ratio is 1:900 in outcrop). The dominating coral species building this mud-facies biostrome were the tabulates Stelliporella cf. parvistella and Halysites laticatenatus. The rugose coral fauna is dominated by fixosessile and rhizosessile forms with well developed holdfasts, for example Dokophyllum elegantulum. Coral growth forms generally indicate high background sedimentation rates during the growth of the biostrome.

Four successive stages of sea-floor development are discussed: (1) a pre-biostrome stage, (2) a pioneering community stage, (3) a climax-community stage, and (4) a post-biostrome stage. Stratigraphically, these stages show an increased carbonate-siliciclastic ratio, a decreased phosphorus content, somewhat lighter ¹⁸O stable isotopes, and an increased rate of bioturbation. The climax-community stage coincides with a bloom(?) in paleocopid and metacopid ostracodes.

The biostrome differs from all other Wenlock-Ludlow reefs on Gotland in its geographic and stratigraphic isolation on the seaward slope of the carbonate platform. It was the first re-appearance of a reefal stucture following the oceanic Mulde Event and associated eustasy. On a regional scale, this appearance coincided with an inferred highstand systems tract and the onset of widespread reef growth in the basin indicating that, at least, basin-regional factors controlled reef growth.

This article has been cited by other articles:

				M. Calner, M. CALNER, and L. JEPPSSON Carbonate platform evolution and conodont stratigraphy during the middle Silurian Mulde Event, Gotland, Sweden Geological Magazine, March 1, 2003; 140(2): 173 - 203. [Abstract] [Full Text] [PDF]