Composition, Structure and Environmental Setting of Silurian Bioherms and Biostromes in Northern Europe

ABSTRACT

Silurian reefs of northern Europe occur in cratonic sedimentary sequences which have been relatively well documented stratigraphically and paleontologically although the reefs have generally been less closely examined than the level bottom communities. Reef development adjacent to the Caledonian Belt was restricted by siliciclastic sedimentation and this is also reflected in the low proportions of carbonate rocks in the Welsh Borderland and Oslo successions. Marine sequences in the Baltic areas of Gotland and Estonia are relatively thin and contain much higher proportions of both carbonates and reefs.

Four main types of reef are recognizable on Gotland. Axelsro (previously termed Upper Visby) and Hoburgen Reefs are essentially tabulate coral and stromatoporoid dominated bioherms of moderate to high diversity. Their dense structure and argillaceous matrix made them locally unstable and prone to marginal collapse and internal displacement. Important accessory reef builders include rugose corals, calcareous algae, Problematica, and bryo-zoans. Similar bioherms, particularly of the smaller tabulate rich Axelsro type, are well developed in the Wenlock Limestone of the Welsh Borderland of England where good examples occur at Wenlock Edge. They are also present in the Oslo Region of Norway, together with tabulate dominated bioslromes and Rothpletzella-Wetheredella bioherms, and occur at several horizons, sometimes very extensively, in the Llandovery and Wenlock.

In Gotland, Hoburgen reefs are especially widespread at numerous horizons, but a unique feature is the occur-rence of Kuppen and Holmhällar type stromatoporoid biostromes which have rigid dense to frame structures and relatively low diversity. They are interpreted as shallow water, high energy linear reefs which developed prefer-entially'in the cratonic interior.The Estonian Silurian sequence shows close similarities to that in Gotland. Reefs are developed at a number of horizons but are generally little documented in detail.

Reef geometry and organic composition were controlled by environmental factors. The size and morphology of the organisms in turn determined the internal structure of the reefs. The bioherms show internal displacement and differential compaction of adjacent sediments in response to their own weight and to subsequent overburden. The biostromes behaved more rigidly and compaction was taken up mainly by stylolitization of adjacent large skeletons.