Page 177 - Carbonate Facies in Geologic History

164 The Lower Carboniferous Waulsortian Facies

gentle collapse of mud and decay in organisms by causing a rigid cemented
framework. No modern analog to such cementation in calcite muds has been
found. Indeed, marine cementation seen to date either requires an inordinately
long time (deep sea crusts) or is observed in areas where rapidly moving water
permeates host rock (e.g., framebuilt reefs or shallow-shelflime sands). How could
moving water in sufficient quantity have cemented cavities in mud without re-
moving the host sediment? The coarse drusy calcite filling of Permo-Triassic mud
accumulations in the Dolomites and Permian Reef Complex present the same
problem (Chapter VIII). Is the cementation early marine or late vadose?
The most logical explanation for stromatactoids appears to be the filling of
cavities formed in mud through settling of plates and fronds, and through slump-
ing or brecciation of the mud mass. But the time of origin of the calcite in fill and
its diagenetic environment (in marine or connate water) presents a difficult prob-
lem.

Bryozoan Fronds and Crinoid Ossicles in Mounds

Not only regional stratigraphy and petrography, but also paleontology bears on
the origin of the peculiar Waulsortian facies. After thin section studies were
undertaken by several researchers in the 1950's, it was realized that fenestrate
bryozoans were an important faunal element in the mud mounds as well as
scattered echinoderm plates. Typically these calcified bryozoan fronds, along with
the ubiquitous stalked crinoids, show adaptation for food gathering in moder-
ately moving currents below active wave base. The common association of the
fenestrates and crinoids leads to the assumption that perhaps the long stems of
the echinoderms were rooted in mud and that the bryozoan fronds were growing
attached to them as prevention against smothering in the soft muddy substrate.
Estimates of bryozoan content range from 4 to 20% of rock volume. Pray (1958)
notes that in the micrite of the mound cores bryozoans are more abundant than
crinoids. Probably the light and lacy fronds easily floated in to collect with the
lime mud. The bryozoan fronds possibly trapped lime mud and stabilized it on
the steep slopes.

Flanking Encrinites (Plate XXI)

Large mounds in New Mexico and Montana are commonly flanked by steeply
dipping beds of coarse encrinite made of disarticulated stems, arm, and calyx
plates. Similar beds occur in Ireland and Britain but are not so common as in the
North American mounds. Battered fenestrate bryozoan remains also are abun-
dant with the crinoidal fragments. These flank beds are practically free of lime
mud, almost purely bioclastic and petrographically monotonous. Only in a few
places are lithoclasts important components of flanking beds despite the size of
the mounds and the steepness of their slopes. Apparently the mounds were diffi-
cult to erode or they were located below active wave base. Probably Paleozoic
crinoid segments like all echinoderm plates were so thoroughly impregnated with