Page 128 - Carbonate Facies in Geologic History

Silurian Buildups in Gotland 115

2. When sea level is stabilized, core or framework tends to grow over the flanking beds on
all sides of a patch reef, but more on the windward side. Satellite reefs grow preferentially on
windward sides.
3. Faunal differentiation occurs, forming parallel, semi-encircling belts which are nar-
rower and more numerous on the windward side. Special organic communities tend to grow
downflank on this side. These include coral thickets, crinoidal and brachiopod beds.
4. Crinoidal bioclastic debris from thickets growing on flanks surrounds the reef core.
These calcarenites are thicker on the windward sides. Coarser, less disarticulated fragments
occur here, despite the more widely spread sediment of this type on leeward flanks.
5. Breccia beds and coarse detritus are rarely present on these Silurian reef flanks but
may occur on the windward side.
6. More argillaceous, quieter water beds interfinger with reef calcarenite on leeward
flanks, wave action preventing clay deposition in the windward quadrant.

Silurian Buildups in Gotland

Silurian carbonate buildups and masses of several types have been extensively
studied in the Baltic islands of Gotland and off the coast of Estonia. An extensive
literature including studies by Hede (through the years 1917-1960), Hadding
(through the years 1927-1958), Jux (1957), and Manten (1971) is available. The
latter book discusses the Gotlandian strata in great detail and has a good bibliog-
raphy. The Baltic island outcrops are part of a belt of limestone and marl trending
northeast-southwest along the axis of the present Baltic Sea. These and older
Paleozoic beds were deposited in shallow basins or shelf environments south of
the Baltic-Scandinavian shield and east of the Caledonian geosyncline whose
thick argillaceous graptolite-bearing strata crop out in southern Sweden (Scania)
(Fig. IV-13) and are known in the subsurface south of the Baltic. The composite of
Silurian strata exposed on Gotland is about 600 m; biostratigraphically it em-
braces practically the whole system. The strata dip gently southeast and are
exposed in low cliffs in outcrops striking oblique to the long axis of the island,
which extends for about 100 km north-south. The elaborately subdivided section
consists of alternating limestones with shales and marls. The section varies litholog-
ically because of successive marine shallowing and deepening and clay influx. It is
a typical neritic section and is highly fossiliferous. Several horizons bear carbon-
ate buildUps. These occur in both regressive and transgressive sequences. Accord-
ing to Manten the buildups vary systematically from base to top of section.
The oldest and most northern Visby beds contain small subspherical to in-
verted conical steepsided knolls only a few meters in diameter. These are embed-
ded in marls and lack flank beds. They bear a limited number of species, main reef
builders being coral colonies of bell and tower shapes, with rare flattened stroma-
toporoids and, according to Manten, almost no algae. Crinoids are similarly
absent. The matrix is lime mud. Such buildups are believed to have formed in
muddy water some tens of meters deep. The Visby beds are of Late Llandoverian
age. Wenlockian Silurian strata in Estonia contain superficially similar small
buildups but of a different organic composition, consisting of bryozoans and
calcareous algae. Hadding (1959) described a similar but algal bioherm at Smojen
in northeastern Gotland.