Page 129 - Carbonate Facies in Geologic History

116 The Advent of Framebuilders in the Middle Paleozoic

~ Eastern Baltic
Facies of

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Fig. IV -13. Silurian outcrops over Baltic shield showing marl and carbonate in Gotland belt
and across the shield and thicker dark shale section south of the neritic area

The first large buildups occur in Wenlockian strata on Gotland (the Holburgen
type of Manten). Some of these cover 100 square m and are of flattened lens-shape.
They comprise complexes of massive, highly fossiliferous, micritic "reefy"
limestone and are associated with horizontal to steeply dipping flank beds.
Depositional relief on the order of 5- 15 m can be ascertained from field relations
but is generally less. In some places the unbedded masses grade laterally into
horizontal thin-bedded strata lying between them. Elsewhere the masses are
onlapped by flanking beds. Such strata, which may dip up to 25 degrees, are halos
consisting mainly of crinoidal debris. Marine erosion of the tops of some buildups
can be observed.
These buildups resemble some of those in the middle stage of development in
North America but are smaller. The masses are linear, bread-loaf shaped, trend
northeast with long axes parallel to the belt of outcrops. The buildups apparently
trended with the Silurian coastline, which at times shifted to a more east-west
direction. The general belt containing these masses may be some kilometers wide.
The masses themselves contain a highly varied fauna: some 15 genera of