Page 17 - Carbonate Facies in Geologic History

4 Principles of Carbonate Sedimentation

It is noteworthy that most major deposits of limestone or dolomite are re-
markably pure carbonate and contain only a few percent of argillaceous or silty
insoluble material. Such "contaminants" are obviously detrimental to deposition
of calcareous sediment.
Two other processes are important, though probably not necessary for car-
bonate deposition: agitation of water and strong evaporation. The former process
is discussed later. The role of evaporation in concentrating sea water in basins or
on tidal flats is well established. Biochemical or wholely inorganic removal of
CaC0 3 from sea water must occur, before concentration of dissolved salts be-
comes sufficient for CaSO 4 to be precipitated. Interbedded, unfossiliferous, fine
grained, homogeneous carbonates, with anhydrite or gypsum in basinal evaporite
deposits, attest to this process.
The above factors, which cause formation of marine CaC0 3 may be viewed
collectively as a system of combined special natural processes. All other sedimen-
tologic processes operate to modify in one way or another the products of this
system. Thus the type of available organisms, sea level fluctuation, rate of subsid-
ence, hydrographic factors, and climate, operate on this "carbonate factory" to
produce the kinds of limestone and dolomite recognized in the geologic record.
See Fig. XII-1 in the Summary Chapter.

Carbonate Production Is Basically Organic

Organisms contribute to detrital deposits of widely varying grain size and, as well,
form huge masses of precipitated limestone.

Most Lime Mud Is Organically Derived
Sedimentologists have extensively discussed its origin and modes of accumula-
tion. It is a common product of shallow, tropical water or the upper layers of the
open ocean. Cool, marine waters of shallow temperate zones produce only shelly
and silty carbonate biogenic debris, and planktonic accumulations in ocean basins
are composed mostly of silt-size material formed in the photic zone in lower
latitudes.
Fine lime sediment settles to the bottom without appreciable compaction. A
few centimeters below the surface the soft sediment consists of about equal por-
tions of CaC0 3 and water. The carbonate in modern shallow water lime muds is
generally aragonite, but may contain substantial proportions (up to 50%) of high
Mg calcite (generally with greater than 10 mol percent Mg in the crystal lattice)
and as much as 10-15% low magnesium calcite (generally with less than 5 mol
percent Mg).
Lime mud has been demonstrated to derive in several different ways: from
death and decay of benthonic organisms (mainly calcareous algae), detritus
abraded from larger carbonate particles, accumulation of planktonic biota and
possibly from direct precipitation from sea water (perhaps biochemically induced
by phytoplankton blooms). Both low and high Mg calcite deposited in sea water