Page 130 - Carbonate Facies in Geologic History

Silurian Buildups in Gotland 117

N-NE s-sw

Fig. IV-14. Silurian reef on Gotland, north of Lundsklint in Hogklint beds. Hoburgen reef
type of Manten. From Manten (1971, Fig.49), with permission of author and Elsevier Pub-
lishing Company, Amsterdam

Tabulates and Heliolitids and 25 genera of Tetracorals are recognized, but
volumetrically stromatoporoids replace the corals as major framework builders.
Algae are also common. Altogether 260 species of organisms have been recognized
from these Wenlockian buildups and their flank beds. The environment is
interpreted as shallow water, photic zone, but not far into active wave base
(Fig. IV-14).
The highest carbonate buildups in the Gotlandian Silurian (Holmhiillar type
of Manten) are of Ludlovian age and are large, flattened, irregular masses built
chiefly of large stromatoporoids and algae with almost no corals. The fauna is
very limited; much crinoid reef flank debris occurs. These masses, not more
than a few tens of meters thick, occur in pure limestone strata and may be
greater than a square kilometer. They are wide and flattish, and in some places
with a cuspate shape. The convex arch of the crescent faces in the windward
direction away from the coastline. Their environment is considered to have been
shallow, very clear and perhaps slightly restricted marine water. Shallowness is
indicated by many interruptions in growth, giant fissure fillings, and coarse blocks
of talus on the flanks. In addition, major circular reefs occur on the Karls6
Islands with large corals, overlain by stromatoporoids. "Flank reefs" or satellite
buildups are also known here.
In some respects, the Gotlandian shelf reefs differ from those of the American
Middle West. Over all, the shelf must have had better circulation and more
normal marine water. Argillaceous influx was greater in Gotland. Depth may
have been shallower or subsidence slower for the buildups are smaller. Flanking
beds in Gotlandian reefs show less consistent wave and current direction. Yet,
coarser crinoid debris occurs closer to cores and there is some evidence of a
southeast wave direction based on the arcuate shape of some reefs.