Page 131 - Carbonate Facies in Geologic History

118 The Advent of Framebuilders in the Middle Paleozoic

No vertical biological sequence is observed within most of the individual cores
of the buildups. Perhaps they existed in water so shallow and quiet that differ-
ences of relief were insufficient to cause biological zonation. In larger reefs, on the
Karlso Islands, large corals at the base give way to more stromatoporoids and
Hadding (1959) suggested that here a sequence of mud-trapping globular stroma-
toporoids and tabulates formed a base for micrite accumulation. Such cores built
up into fragmental limestone with abundant algae but hardly into the surf zone.
All in all, biological zonation is not prominently displayed. In this and in the
small size of most buildups, Gotlandian masses more resemble the early to middle
growth stages of North American Silurian mounds and reefs. Occasionally,
smooth stromatoporoids, compact corals, and patches of derived conglomerates
in flanks and interruptions of growth indicate development into active wave base,
but this type of reef is not very common. Most flank sediment is fine-grained.
Many times Hadding (1959) stressed the role of algae in binding other sessile
organisms and debris in some buildups. Solenopora nodules and encrusting
spongy stromatolites occur. Marly intermound beds are noted for their abundant
algal balls (Sphaerocodium and Spongiostroma) so that algae are of general impor-
tance in these strata.
Strangely, no stromatactoid structure is reported from the Gotlandian build-
ups.
Environmental conclusion: these buildups must have formed in very quiet but
normal marine water with not much consistent wave and current direction. The
area was far removed from the basin and close to a fine clastic source area.
Buildups began on soft substrates. Water was of moderate depth, a few tens of
meters. In places buildups grew to wave base in later Silurian times. Some accu-
mulated up into water 5 m or so deep and were much like Illinois-Indiana reef
masses.
Silurian buildups, both in North America and in the Baltic regions, are typi-
cally stable shelf deposits much like those described from the Ordovician. They
formed as mud mounds, replete with bryozoans and often with a stromatolitic
algal cap; apparently they were capable of beginning on soft substrate in quiet
water of moderate depth. Growth into wave base resulted in cores of corals and
stromatoporoid boundstone and extensive flank beds consisting of debris or or-
ganisms living on the tops of the buildups. Probably framework organisms grew
only into wave base and remained covered by water some meters deep. Larger
banks and shelf margin buildups, if they existed at all, were composites of individ-
ual cores or patch reefs. The Silurian features typify a sequence of growth stages
seen in many other mounds, e.g., in the Permo-Pennsylvanian and Jurassic. The
typical sequence is reviewed in this Chapter and Chapter XII.

Devonian Buildups

Older Devonian buildups consist of coral and stromatoporoid patch reefs much
like those in the Silurian. No continuous shelf-margin developments are known in
strata this old. In the Middle and Late Devonian, however, a bloom of coral and