Page 154 - Carbonate Facies in Geologic History

Paleoecology of Middle Paleozoic Reef Associated Organisms 141

Even in shelf interiors, patches and individual mounds occur. These include sim-
ple micritic masses of algae, sponges, and bryozoans, mounds capped with corals
and stromatoporoids, and circular faros and atolls of boundstone with crinoidal
flanking beds. Fig. IV -27 illustrates these common forms of shelf buildups.

Latitudinal Range of Middle Paleozoic Buildups

It is worth noting how great the latitudinal range of Middle Paleozoic reefoid
assemblage really is, based on the present equator. Banks Island, Northwest
Territories of Canada, with the reefs described by Embry and Klovan (1971), is
within the Arctic Circle (74°), and in North America the southern range is to
Arizona, at almost 30° latitude. In Europe the same wide range is demonstrable
from Novaya Zemlya (75°) to 25° in Spanish Sahara. About the same range is
demonstrable in Silurian buildups which in North America are known from
northwestern Greenland (about 80°) to about 32° in West Texas. Comparable
ranges for Silurian reefs exist in the eastern hemisphere. Gotland is about 57°
North and Nowshera in northern Pakistan is 34°. Evidence suggests a drastic
relocation of the Middle Paleozoic equator, particularly in North America. This is
because the spread of 45° is much greater than present tropical conditions permit
reefoid biota to flourish, and because the north-south extension of such faunas
may occur within the same cratonic plates.

Paleoecology of Middle Paleozoic Reef Associated Organisms

1. Tabulate corals: Growth forms in Ordovician and Silurian times were small,
nodular to platy colonies in chains or in clusters, particularly when developed in
marly sediments. Devonian colonies were hardly ever more than one meter across
and usually smaller. The Devonian genus Alveolites had many growth forms; on
lower slopes it was small and lamellar, thin and encrusting on other organisms,
and it occurs generally with micrite matrix. In higher parts of buildups it may be
large, irregularly massive to spherical. The Devonian small, stick-like genus,
Thamnopora, forms biostromes ("Rasenriffe") or is intergrown with larger sup-
porting organisms. It may also form globose colonies. The ubiquitous genus
F avosites varies from small, flattish, dish-like forms in quiet water or on marly
substrates to large, irregular, or globular masses in buildups.
Since many marly beds contain biostromes of Tabulata they seem to have
adapted to life in somewhat turbid water. Particularly the domical-globular to
occasionally elongate forms are considered to result from convex and/or upward
growth to avoid being smothered by a persistent rain of fine sediment. Tabulates
occur in buildups commonly downslope from stromatoporoids but also occur
with them higher in the buildups in "cleaner" water.
2. Rugose colonial corals: There seems little doubt that the elongate curved, or
twisted cones of the large solitary tetracorals were an adaptation to life on a soft
substrate. Tsien (1971) has shown how some of these forms tended to become
adnate on local hard spots on these bottoms. Solitary tetracorals are most com-