Page 179 - Carbonate Facies in Geologic History

166 The Lower Carboniferous Waulsortian Facies

CROSS SECTION VIEWS

STAGE 3
Wa •• , d •• p and quie,.
Complete ring of ctinoid ~ f.n.,"at.
futnhhe, bioclastic d.bril

Ril ing leo 1.",.1 _~ ________ ......... ______ .....-..

STAGE 1
Growth of orsonic bofft
up sides and around bordtrs
of growing mound curU-"'

Ri .. ;ng ,eo 1.".1 ~
STAGE I
AccumuJolion in I •• i "'J~. .
of "inoid . lone.trole .--
cur'ent I
thlckel

Fig. V -15. Theoretical diagram illustrating hypotheses of development of equidimensional
Waulsortian mounds through progressive colonization of current-induced pile of fine sedi-
ment in lee of crinoid-fenestrate bryozoan thicket. Rising sea level accounts for mud accumu-
lation in center of thicket growing on all sides of mound. Flanking beds develop coarse debris
derived from outer edges of thicket where finest lime-mud sediment is winnowed

or textural types in them. Most geologists, familiar with the Lower Carboniferous
bioherms, now consider them to be accumulations formed below wave base,
perhaps in water deep enough to be below the photic zone.
The third hypothesis of origin, that of accumulation of mud under and around
thriving masses of stalked crinoids and bryozoans, is precluded as a unique
explanation by the simple fact of the relatively great amount of micrite and small
amount of bioclastic debris within the mound core. Any explanation of the mud
accumulation must also take into account the surrounding halo of coarse, win-
nowed, crinoidal, lime sand.
The fourth possibility is a combination of processes 1 and 3. Perhaps thickets
of stalked crinoids, which supported masses of fenestrate bryozoans growing
intertwined with them, offered effective baffles to gentle currents which com-
menced to heap up fine mud in lee areas in roughly conical piles. Once started, the