Page 157 - Carbonate Facies in Geologic History

144 The Advent of Framebuilders in the Middle Paleozoic

BACK- FORE- OFF-
ORGANISMS REEF REEF REEF REEF
MASSIVE ~
STROMATOPOROIOS - ,...-
TABULAR ,----
STROMATOPOROIDS ,- ~
STACHYODES - TYPE -
STROMATOPOROIDS ."".--- ...... ~
AMPHIPORA - TYPE v-----...
STROMATOPOROIDS
TABULATE CORALS ---- ,
.......
RUGOSE CORALS
~ ----
BRACHIOPODS '-...
- ------
ECHINODERMS ./'"" ~

GASTROPOOS
~ .----..
GQNIATITES
/'
BRYOZOANS
~
OSTRACODS --..... - ~
f--
CONODONTS ~
V-
TENTACULITES
CALCISPHERES
~
"
ALGAE COATINGS
AND CRUSTS
Fig. IV -28. Distribution of organisms in Devonian reef and ofT-reef areas in central Europe.
From Krebs (1974, Fig. 14), with permission of Society of Economic Paleontologists and
Mineralogists

Studies have been made on size and degree of disarticulation of crinoidal particles in the two
areas: on Silurian buildups in Gotland by Manten (1971), and by Ingels (1963) in Illinois. In
both cases size diminishes and breaking apart increases away from the core. On this basis the
windward front of the buildup may be discerned. A leeward crinoid meadow filling a lagoon
and a leeward expansion of such bioclastic debris is outlined by Ingels for the Thornton
buildup.
b) Brachiopods were also capable of forming substantial meadows on flanking slopes of
buildUps. These are commonly cited as accessory flank communities. One of the most spec-
tacular of these is the large terebratuloid Stringocephalus of the Middle Devonian. Pentamer-
ids are known in similar flank positions in Silurian strata. Philcox (1970) notes flanking
communities of rhynchonellid brachiopods in the Silurian of Iowa. Brachiopods in some
places are known to inhabit similar environments in later buildups (Composita in Upper
Carboniferous, rhynchonellids in certain Jurassic and Triassic reefs).
c) Convex, smooth-shelled trilobites such as the Silurian genus Bumastus, probably in-
habited upper slopes of the buildUps. These may be found in shell accumulations on tops and
upper parts of the flanks.
d) Nautiloids. These forms are generally oriented downslope or accumulated in fissures
in the sides of cores or flanking beds. They appear even within the micritic cores of Lower
Ordovician buildups. Orthoconic forms have been reported in Chazyan strata by Pitcher