Page 150 - Carbonate Facies in Geologic History

Canning Basin. Western Australia 137

stone texture. The sediments represent gravity debris-flows down steep slopes.
The forereef strata are extremely fossiliferous and contain many silicified forms.
Fragments of fossils and of eroded and reworked reef rock and debris from
upslope are common, e.g., corals and stromatoporoids. Even more common are
fossils of indigenous slope dwellers: brachiopods, living a few tens of meters
downslope in some atoll areas, sponges, crinoids and bryozoans, and abundant
nautiloids. Forereef encrinites are known. Mixed with this benthonic fauna are
accumulations of nektonic open-sea forms, such as goniatites and conodonts. The
dipping forereef limestone grades basinward to the off-reef equivalent siltstones
which also include fine-grained lime wackestone-packstone strata. The fauna here
very largely consists of goniatites and conodonts (Gogo beds). Higher up the
reddish Virgin Hills siltstone, a shale and lime wackestone-mudstone sequence,
displays prominent stromatactoid structures which are interpreted by Playford as
calcite filling and matrix replacements of openings along thin organic layers,
perhaps under bulbous layers of Renalcis. The structure is not associated here
with bryozoans as in the Silurian strata. Coarse calcite drusy cement is common
in some fragmented and brecciated layers. Bulbous stacked hemispheroidal stro-
matolites, are known in these slope deposits. If sea level remained stable during
forereef deposition, such stromatolites must have been deposited in water about
45 m deep. These algal bioherms are capped by the reef-encrusting form Renalcis
(Plate XX B) which must have lived in much shallower, more agitated water. An
unsolved problem in these Devonian strata is how far down the foreslope sea level
may have dropped from time to time. Coarse calcite filling of stromatactoids and
inter breccia spaces may also indicate deposition from much shallower water than
indicated by assuming stable sea level, and projecting down the preserved deposi-
tional dip.
Interestingly, as in forereef breccias known in the Permo-Triassic beds of West
Texas-New Mexico and the Dolomites in Italy, very little dolomitization has
occurred. In all these examples, however, dolomite is prominent in the top and
backreef of the buildup. Clearly, such dolomitization was not syndepositional'
Exposures of the relatively narrow reef front proper are marked by an abrupt
change from strongly dipping foreslope to massive unbedded strata. The contact
is almost vertical in places. This reef front belt is hardly more than 100-200 m
wide and has gaps within it just as modern barrier reefs do. These may have been
a 100 m or so wide. In many places it can be seen that the barrier was a freestand-
ing, wave-resistant vertical wall. The narrow belt of growth furnished an immense
pile of debris both lithoclastic and bioclastic. No detailed biological zonation is
seen in the reef but certain important generalizations can be made. The reef crest
consists on an average of only 60% organic framework. Detritus can actually
range up to 90% of the volume. It is mostly fine bioclastic debris. The most
striking organism is massive irregular forms of stromatoporoids, capped by the
supposed alga or foraminifera Renalcis which was microporous, looks white in
hand specimen and forms irregular encrusting chambers. A related form Chaba-
kovia is also present. These algae (?) are considered to be the major binders of the
reef. They much resemble the tubular encrusting foraminifera of the Late Paleo-
zoic. Many Solenopora and other types of algae are seen in the reef. The large
dasycladacean-like form Receptaculites is common. Sponges are present. The