Page 156 - Carbonate Facies in Geologic History

Paleoecology of Middle Paleozoic Reef Associated Organisms 143

a) Massive-irregular and encrusting: Highest water energy at or just into wave base.
Matrix calcarenitic, often of broken dendroid stromatoporid fragments.
b) Tabular-lamellar: Developed below wave base encrusting or binding micrite matrix.
Found consistently downslope of buildups and commonly with corals.
c) Globular (cabbage-head) forms: Found in biostromes commonly with micrite matrix.
A platform or shelf-adapted form. Known in this environment in Devonian beds from Aus-
tralia, USA, Canada, and Eifel region of Germany where such strata are termed "Knollen-
riffe." The subspherical shape probably represents an adaptation to prevent fine sediment
accumulation on the colony. Similar shapes are seen in F avosites and some colonial Rugosa
as well as in modern brain corals which live in areas of moderate water agitation.
d) delicate irregular stick-like growth. This is represented exclusively by the genus Am-
phipora, which formed mats of "spaghetti rock" in quiet restricted marine backreef conditions
("Rasenriffe" of German authors).
e) The stubby dendroid form of Stachyoides, the same form taken by several Devonian
Tabulata (e.g., Thamnopora), had a wider bathymetric distribution. Abundant fragments of
such stromatoporoids accumulated down the slopes of banks, occur in ancient bank margins,
and grade into the bank-interior facies which consists of Amphipora, calcispheres, and peloids.
These forms probably dwelt in protected areas in turbulent water, existed over a wide range
of depths, and perhaps formed thickets just below wave base on the forereef slope. The
elongate stick-like form is also considered an adaptation to prevent smothering by mud
suspended in water.
4. Algae.' The role of algae as binders and encrusters on Middle Paleozoic reet
tops was not appreciated until petrographic study was thoroughly accomplished.
The algae are present in different forms (Machielse, 1972).
a) Renalcis and Chabakovia: These are tiny-chambered irregular encrusting organisms
with relatively thick and porous wall structure which are generally considered algal, but are
possibly foraminifera. They are structured much like the encrusting tubular foraminifera of
Late Paleozoic. The irregularly plumose fabrics characterize boundstones from Cambrian to
Devonian age, much as do the cornuspirids of the Permo-Pennsylvanian (Plate XX). True
reefy boundstone is only well developed by these forms on Devonian shelf margins with steep
slopes and detrital breccias in Canada and Australia.
b) Red algae, such Parachaetetes and Solenopora are common as nodules in buildups
from all Devonian areas particularly on shelf edges. Neither in the Silurian nor Devonian do
these form impressive encrusting boundstones but they are Ubiquitous. They indicate normal
marine water. In Chazyan mounds probable Solenopora combines with nodules of blue-green
filamentous algae and bryozoans to form small buildups.
c) Blue-green filamentous algae coat particles and form nodules and balls (onkoids), and
occur just as commonly as in all other parts of the geological record. They occur both in
buildups and interreef sediments.
d) Mud-trapping stromatolitic algae make shallow-water buildups of varying size from
Precambrian through Silurian.
e) Giant dasycladaceans, Nidulites and Receptaculites which have been traditionally con-
sidered sponge-like in affinity, possess structure much more like that of large dasycladacean
algae. They are common in light-colored shelf limestones and in carbonate buildups from
Middle Ordovician through Devonian. In the Ordovician they are associated with stromato-
poroids and heads of colonial corals, the large gastropod Maclurites and nautiloids, an
assemblage probably indicating very shallow, warm and somewhat agitated water on shoals
and banks.
5. The flanking communities.' A wide variety exists.
a) Pelmatozoans. Normally early and Middle Paleozoic buildups of all sizes must have
been bordered by thickets and meadows of sessil echinoderms. Even in early Ordovician
mounds, flank beds are rich in such debris. Crinoids, blastoids, and cystoids contributed most
of the surrounding debris in the Silurian shelf mounds and continue sporadically in the
Devonian and Mississippian to playa major role. These lime sands are generally winnowed
and foreset-bedded and may dip quaquaversally off mounds or downslopes oflinear buildups.