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Nield, 1982

The earliest Gotland reefs: two bioherms from the Lower Visby Beds (Upper Llandovery)

Nield, E. W.
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Typearticle in journal


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.

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