Page 178 - Carbonate Facies in Geologic History

Theories of Mound Origin 165

organic matter when alive that during decay and before Mg calcite inversion and
calcite infill, they should have been very light particles, even floating in sea water.
Thus the clean-washed, steeply dipping, crinoidal flank beds do not necessarily
indicate very strong current action.

Theories of Mound Origin

Since many large Waulsortian buildups are in the form of spectacular conical
mounds, consist mainly of fine lime mud, and lack any abundant framebuilding
organisms, their origin has been puzzling to geologists. Any explanations of origin
for these accumulations must consider (1) that the Waulsortian beds are princi-
pally micrite with scattered minor bioclastic debris in which bryozoans, a few
crinoids and stromatactoid coarse spar-filling occur, (2) that paleogeograph-
ically they tend to occur below the edges of shelves in belts separating shelf and
basin, (3) that the large basinal mounds are roughly equidimensional and without
much marked orientation, unless perhaps in crudely defined belts, and (4) that
they are layered accumulations in some places. Layering is parallel to their con-
vex upper surface.
The mounds are not erosional remnants but are accretional in origin because
of: (1) their regular smooth form, (2) steep depositional slopes (up to 50 degrees)
with sheets of stromatactoid and rare bioclastic layers paralleling the external
configuration, and (3) essential lack of detrital fans or breccias around most (not
all) mounds but common presence of halos of winnowed crinoidal debris.
Possible ways in which such large micrite masses could have come into being
include (1) hydrologic accumulations shaped by currents, (2) entrapment and/or
precipitation by algae to create local piles of carbonate mud, (3) entrapment of
lime mud by baffiing action of bryozoans and crinoid, or (4) a combination of
baffiing and trapping by thickets of these organisms with accumulation in the lee
of such thickets induced by gentle currents.
That the mounds are not purely current-piled sediment is indicated by the
thorough mixture of the lime mud and sand-size bioclasts which indicates a lack
of sorting during accumulation. The smooth, rounded, and almost perfectly coni-
cal form of many mounds also argues for accumulation in very quiet water.
The first workers to describe the Mississippian mounds of the Sacramento
Mountains (Laudon and Bowsher, 1941), proposed that they originated through
disintegration of non-calcareous algae which trapped lime mud or precipitated it
through photosynthesis. Because of petrographic studies made during the last ten
years, this hypothesis seems untc;mable. Very few calcareous algae appear in these
mounds despite occurrence of abundant forms of these in other North American
Mississippian beds. Alleged stromatolitic algae are present in one mound in
Belgium at Colebi, (Dupont, 1969). On the other hand many micritic mounds
with preserved calcareous algae occur in shallow water Paleozoic strata in North
America. Why not in these Lower Carboniferous buildups? It is perhaps signifi-
cant that, in addition to the absence of calcareous algae in the Lower Carbonifer-
ous buildups, no workers have described a consistent vertical sequence of faunal