Page 159 - Carbonate Facies in Geologic History

146 The Advent of Framebuilders in the Middle Paleozoic

in Devonian reefs (Renalcis and Chabakovia) but are not seen in the Siluro-Devonian. This
organic bindstone is much like that of the Late Carboniferous and Permian.
7. Crinoidal flank beds are not so abundant in Devonian as in Silurian strata but reap-
pear importantly in the Early Carboniferous.

Generalizations about Water Depths and Turbulence on Tops of Middle
Paleozoic Buildups

Several indications exist that these carbonate accumulations may have generally
grown to wave base but not necessarily close enough to the sea surface to cause a
surf zone. Presumably, except for certain Devonian buildups, the favored position
for the assemblage was in water about 5 or 10 m deep.
1. The extensive flank beds developed around shelf buildups are for the most part purely
bioclastic, composed of organisms which inhabited flank or top of buildups. Many of these
(crinoids, blastoids, bryozoans, and long, thin, straight nautiloids) were of construction so
delicate that they could not withstand extensive wave battering on the tops of the buildups.
2. Breccia beds with reworked, previously cemented, clasts eroded from the reef front are
rare, though present in Australia and Canada in buildups with steeper slopes of 2-30 degrees.
More typically, flank debris is composed of organisms growing at the wave base.
3. Middle Paleozoic organisms with frame-building capabilities are small compared to
Mesozoic and Holocene forms. Most large colonies are of dendroid-fasciculate forms living
rather well down slopes. The most impressive assemblages of encrusting boundstone is the
low-lying massive irregular stromatoporoids and algal? Renalcis from the crest of Western
Australian Devonian reefs.
4. The general absence of sabkha evaporite deposits or tidal flat deposits in the interior of
most of the faros (shallow atolls) and great stromatoporoid-rimmed banks in Australia and
Canada is significant, particularly when one considers that the Devonian was a time of
common evaporite deposition. The characteristic Amphipora pellet-calcisphere wackestone-
packstone assemblage represents a restricted lagoonal sediment and indicates that sea water
had some access over the barrier rim to the bank interior. Some backreef or shelf areas behind
reefs, e.g., Lennard Shelf, Australia, and external shelf of Eifel region, Germany, even contain
corals and echinoderm beds.

Diagenesis in Middle Paleozoic Buildups

1. Many buildups of coral and stromatoporoids contain large vugs and cavities inherited
from the original constructed megaporous reef fabric, extensively bored and rotted. Smaller
pore space results from construction by Renalcis boundstone. Most cementation of these
Middle Paleozoic fabrics seems to have been from organisms and within a totally marine
environment. The exact origin of the coarse drusy linings and sparry in-fillings of cavities
within boundstone fabric remains, however, a mystery. It is possible that it formed in the
marine splash zone like the aragonite druse seen today on arid coastlines of the Trucial Coast.
But, because it is also seen filling stromatactoid cavities in mud mounds, it must have
originated in quiet-water conditions-either submarine or within buried sediment. It has
even been considered a cave deposit from meteoric water. It is not, therefore, a particularly
safe indicator for high water energy in the marine environment. This problem is also dis-
cussed in later chapters.
2. Stromatactoid structures, patches of spar with flat bases and digitate tops, are com-
mon within buildups on shelf areas and in basins but only where cores and flank beds contain
much lime mudstone. The structure is not known in normally bedded lime mudstones. A