The earliest gotland reefs: Two bioherms from the Lower Visby Beds (Upper Llandovery)

Abstract

Reef development on Gotland began in the lowest exposed unit, the Lower Visby Beds — a sequence of thin, interbedded green mudstones and mostly nodular limestones. The two reefs described were built up mainly by sediment-tolerant forms — halysitids, heliolitids and Syringopora — within which the remainder of the fauna was embedded. There was a marginal zone rich in rugosans (mostly radially disposed, their calyces pointing downslope from the reef centre) and a central/apical region where the halysitid frame gave way by degrees to one composed of favositids, heliolitids and stromatoporoids. Bryozoan genera also changed centripetally from delicate, stick-like forms to laminar and massive ones. The reefs probably had a low profile, not rising much more than one metre above the sea floor, and may have formed at or about a depth of 100 m.