Page 31 - Carbonate Facies in Geologic History

18 Principles of Carbonate Sedimentation

3. Basin configuration and water energy are the dominant controls on facies
genesis and differentiation (Laporte, 1968). Depth control of facies exists because
depth controls water energy. Light control is important, confining important bio-
logical production to the photic zone. Certain hydrographic controls operate on
carbonate sediments in a way different from the obvious mechanical ones of
transport, piling, and winnowing. Marine upwelling and water agitation induce
considerable in situ organic production on shelf margins, and lack of circulation
in restricted marine areas results in hydrographic conditions leaving strong im-
prints on the sediments. Such very shallow marine water and intertidal sediments
make up a great volume of carbonate rock.

Biological Influences

4. Carbonate formation is basically biochemical and organisms are all-impor-
tant in creating and modifying all types of carbonate particles from tiny grains to
large precipitated rock masses.
5. A corollary pointed out by Laporte and Ginsburg is that organic abun-
dance and diversity in carbonate sediments reflect original conditions despite a
bias caused by differences in preservation.
6. A consequence of dominant organic composition is that biofacies and
lithofacies often correlate. Organisms cause certain typical lithofacies (e.g., hard,
massive coral reefs) and the same environmental factors which cause inorganic
grain types affect the ever present organisms (e.g., oolite commonly possesses
thick-shelled gastropod fragments). Substrates in turn control types of organisms
inhabiting them.
7. The relative rates of physical and, particularly, biological reworking are
ascertainable from sedimentological study and have environmental significance.

Textural Influences

8. Since carbonate particles are produced approximately in situ and in a
variety of shapes and sizes, special sedimentary principles and textural classifica-
tions are necessary to further our understanding of them. Modern classifications
are based principally on presence or absence of interpreted original lime mud and
on types of grains; the amount of mud matrix is considered a better guide to water
energy and circulation than grain size or shapes of particles.
9. As corollary to the above concepts Laporte pointed out that since most
carbonate grains accumulate where produced and little net transport occurs, the
textures of many carbonate sands are more dependent on the nature of contribut-
ing skeletal producers than on external agents.

Rates of Sedimentation

10. Carbonate sedimentation can be extremely rapid when the special marine
environment is right. The measured rates of neritic sediment for the last 5000
years are on an order of magnitude too high for the maximum amounts of thick,