Page 180 - Carbonate Facies in Geologic History

Conclusions 167

perimeter of such piles was progressively colonized by crinoids and fenestrates
which thrived as relative sea level rose. Their colonies gradually formed a wall-
like thicket which surrounded and grew atop an increasing mound of mud. The
quiet water within the growing ring of organic thickets accumulated mud and the
coarser bioclastic debris drifted down the flanks. Exposed to gentle currents on
the slope and around the outside of the mounds, this flank material was win-
nowed clean of lime mud. A steadily rising sea level is envisaged, the top of the
mound never reaching very close to wave base and even remaining below the
photic zone (Fig. V -15).

Conclusions

In several ways the Waulsortian facies is unique among carbonate buildups.
Buildups made chiefly oflime mud are reasonably common in the geologic record
principally in pre-Cretaceous time. Most of these, however, possess at least one
type of fairly large organism whose bulk dominates the faunal assemblage and
could form a bafflestone texture in the buildup, e.g., sponges in the Cambro-
Ordovician, Permo-Triassic and Jurassic, tabular stromatoporoids in the Late
Devonian, phylloid algae in the Permo-Pennsylvanian and the organ-pipe coral,
"Thecosmilia" in the early Mesozoic.
Waulsortian mounds, which are just as large and more steep-sided than most,
possess no major large organisms, only tiny fragments of crinoids and bryozoans
constituting hardly more than 20% of the bulk. Additionally, most Waulsortian
mounds appear to lack the ecological zonations described in other mounds.
Waulsortian mounds commonly contain strikingly multiple, regular and par-
allel stromatactoid structures (sheet spar) compared with such features seen in
other calcite mud mounds. This appears especially true in Mississippian to Early
Pennsylvanian mounds, although such regular stromatactoids are known in Silu-
rian and Devonian buildups. One may argue that during the Middle Paleozoic a
particular soft-bodied organism, with stromatactoid shape, was able to encrust
and stabilize a sloping mud bottom. On the contrary, perhaps the multiple and
parallel stromatactoids result from regular slumping and transmission of sheer
stress in a more or less homogeneous lime mud accumulated without any large
amount of internal shelly debris to disrupt the fabric.
The Waulsortian facies are essentially characteristic of Lower Carboniferous
strata. However, very similar micrite mounds with bryozoans are known in strata
as young as Middle Pennsylvanian age in the Arctic of Canada (Davies and
Nassichuk, 1973). It remains to be demonstrated how important a role fenestrate
bryozoans play in construction of such mounds. Do they provide necessary baf-
fles for construction or are they merely accessory organisms growing on crinoids?
Could their mesh-work be instrumental in stabilizing the steep slopes?
A basic but unsolved problem is what localizes the individual mounds. This is
not unique to Waulsortian accumulations but is made more difficult by lack of
any obvious baffling or framebuilding organisms. Possibly the answer lies in the