Page 47 - Carbonate Facies in Geologic History

34 The Stratigraphy of Carbonate Deposits

Special environment for carbonate deposition (warm, clear, shallow water of photic zone)
Production rate Stabilization processes
controlled by: A. Amount and form of frame-
A. Volume of in situ organic growth building organisms (variable in
~ ----+ (enhanced by upwelling) + geologic time).
e B. Water agitation creating ooids, B. Amount of sediment baffling and
~ grapestone, hardened pelletoids trapping by rooted organisms and
e through accretion by infiltering or in-pumping into +
t framework cavities.
~ C. Organic cementation
~ _______________ -+1 D. Organic encrustation and binding.
§ E. Inorganic cementation - mainly
... due to meteoric water.
tl
.....
Q)
.2:;
~ Piling by mechanical movement Removal processes
Q)- of sediment A. Bioerosion - scraping and boring
~
(accessory process only) B. Solution by meteoric water
C. Wave abrasion and erosion
D. Wave and current winnowing

I Ultimate volume of accumulated sediment I

Fig.II-11. Processes forming and shaping carbonate accumulations at shelf margins from
Wilson (1974), with permission of American Association of Petroleum Geologists

Each of these areas has its own stratigraphic peculiarities and problems of
correlation. The shelf margins and major basinal buildups are areas of pro-
nounced depositional topography, steep slope, sharply defined facies, and thick
sections.

Natural Construction of Carbonate Ramps-Platforms

The standard sequence of facies is based on the sedimentary construction of a
wedge-shaped platform or ramp on a gently subsiding planar surface and the
consequent development of submarine topography with a seaward face of vari-
able steepness. Modern barrier and fringing reefs typically have this configuration
even though most are built on paleotopography inherited from the Pleistocene.
This commonly developed form results from the basic organic origin of carbonate
sediment whose production is speeded up over preexisting high areas as well as
over those created by local production or sediment piling.
Carbonates are produced most abundantly in marine water which is warm,
shallow, clear, sunlit, and free of clayey contamination. The special processes
operating to enhance the biological productivity on such sea bottoms are numer-
ous and result from an interplay of hydrologic and biologic factors. The ultimate
volume of accumulated sediment is dependent upon the increased production
rates plus the stabilization processes and mechanical piling less the normal pro-
cesses of sediment removal. Fig. II -11 diagrams these processes.