Page 381 - Carbonate Facies in Geologic History

368 Summary

2. Micritic bafJlestone core: The thickest part of the mound generally consists
of a micritic matrix replete with organisms capable of trapping or baffiing fine
lime sediment. These are commonly delicate or dendroid forms with upright
growth habits. Each geologic age has its special biota which serves in this role.
Often one form dominates almost to the exclusion of others:
a) Sponges and algae in Cambro-Ordovician
b) Bryozoans in Middle and Late Ordovician, Silurian and Early Carboniferous
c) Platy algae in Late Carboniferous (Pennsylvanian)
d) Large fasciculate-dendroid corals in Late Triassic shelf areas
e) Lithistid sponges in Late Jurassic
f) Rudists in Cretaceous shelf areas
g) Marine grasses at present time
Commonly this main core facies is extremely brecciated because it was origi-
nally a mixture of gelatinous, non-resistent mud, and rigid, brittle bioclasts. It
probably slumped under its own weight.
Many mounds, particularly those in very shallow water or those swamped by
argillaceous influx, never reached beyond this early stage of a micritic mound
core, developed few flank beds, and no crestal boundstone.
3. Crestal boundstone: When a baffiestone mound reached into wave base, the
topographic eminence formed by the stabilized soft sediment may have served as
a seat for organic boundstone. This may take two forms: (a) An ecologic reef may
have developed through colonization by large, sessile, massive invertebrates such
as corals, hydrozoans (stromatoporoids and spongiomorphs), sponges, some rud-
ists, richtofenid brachiopods, and red algae. Protected cavities showing vertically
oriented fabrics (geopetals) were formed. (b) Quieter water boundstone developed
with more lamellar forms and with protected cavities trending parallel to the
outer slope of the mound. As pointed out by Alberstadt and Walker (1973), initial
colonization may proceed to a high degree of biological diversification. In this
intrinsic process the development of a mesh-work of complexly shaped organisms
gives rise to a number of habitats and encourages a multitude of species-addi-
tional major framebuilders and binders as well as niche dwellers. The ecological
succession is similar in many buildups, despite great differences in taxa through
long geological periods. One logical explanation for the similar successions is a
prevailing substrate control of growth forms.
4. Organic veneers and fissure filling: Another possibility exists for coloniza-
tion of an earlier formed baffiestone mound. If conditions permitted only slow
deposition on the mound top, and no extensive growth of frame-producing organ-
isms occurred, the upper surface of the mound may have become coated with a
thin veneer of a variety of encrusting or adnate organisms. Examples of these
include Chondrodonta and oysters in the Cretaceous and certain sponges and
stromatoporoids in Pennsylvanian beds. Specialized forms growing in tidal pools
can be seen in certain Silurian buildups in Illinois. A second type of veneer occurs.
Laminated or stromatolitic beds are common at the top of many Ordovician and
Silurian mounds, noted by Alberstadt and Walker as a "Domination phase",
where one organism dominates all others by successful adaptation to stress condi-
tions. The same phenomenon has been described by Ahr (1971) in Late Cambrian
mounds in Texas. This type of veneer may be a response to very shallow water at
the tops of mounds in which wave action is essentially ineffective but where tides