Page 380 - Carbonate Facies in Geologic History

Mound Facies Sequence 367

~ Organ ic yeneer
. . . . .. .
o· .... • •
..

Fig.XlI-S. Ideal carbonate mound with seven commonly developed facies. See also Fig.IV-9
and VI-2S. The sequence offacies develops when the accumulation grows into wave base and
is controlled by varying rates of sediment production, rates of subsidence, and hydrographic
factors

2. Trapping and baffling of carbonate sediment produced locally at higher
than normal rates. Probably the most important process contributing to the
growth of the mound.
3. Stabilization of sediment by surface encrustation so that normal processes
of marine erosion do not remove it.
4. Protection by a veneer or wall of frame-building organisms at a late stage in
its development.
S. Protection by cementation. In lime mud deposited and remaining in the
marine environment, cementation is very slow. In shallow-water banks, where
chances of subaerial exposure are better, lithification of lime mud is more effec-
tive.
Although extrinsic, mainly hydrological, controls operated to cause a succes-
sion of changes in mound sediment as the structure builds, additionally intrinsic
changes operated within the organic communities inhabiting the mounds. These
were occasioned by creation of and competition for substrates by the mound
dwellers (Alber stadt and Walker, 1973).
Examination of Paleozoic, Cretaceous, and Holocene sedimentary bodies re-
ferred to in previous Chapters permits generalization of the sequence which is
summarized in diagrams on Figs. XII-4, XII-S, VI-2S, and IV-9. Not all mounds
progress through the whole sequence--only those in areas of moderate subsi-
dence. But mounds commonly begin growth below wave base and build up into it,
remaining there for sufficient time to develop crestal boundstone, flanking beds,
talus, and perhaps a capping bed. Figs. IV -27 a and b compare an arrested early
stage of mound formation with a more complete development. Seven distinctive
facies are commonly repeated in the examples despite vast differences in geologic
age. They are defined by both textural and biological differences and are
determined by organic and sediment reaction mainly to external factors.
1. Basal bioclastic wackestone pile: Most mounds begin with a very micritic
sediment with much bioclastic debris. In Early and Medial Paleozoic strata this
commonly contains considerable echinoderm fragments with less bryozoans, and
brachiopod shells. In several instances piles of shell debris are seen at the base.
The origin of these piles cannot be generally ascertained; presumably they are
heaped up by gentle currents. No pronounced baffling or binding organisms are
common in these beds.