Page 19 - Carbonate Facies in Geologic History

6 Principles of Carbonate Sedimentation

basins. Currents and waves winnow fine-grain sediment creating sand and gravel
lag deposits on the open shallow shelves. These may vary from coastal sand flats
in areas with great tidal range (2-3 m) to the broad shelves of the Campeche and
west Florida areas now under 50-100 m of water. Waves and currents also tend to
pile up carbonate sand and gravel deposits. Coquina beaches, littoral spits and sub-
marine bars due to longshore drift, tidal deltas and bars in tidal passes constitute
well-known bodies of mechanically deposited carbonate sediment. Similarly, tidal
bars of oolite and peloid grains are known at the edges of major Holocene
carbonate banks.
Finer carbonate sediment, winnowed from shelves, tends to accumulate in two
preferred locations relative to shelf margins: off the shelf in deeper water marginal
to the shore and in protected areas in quiet water behind barriers. In the axis of
the Persian Gulf, calcareous muds occur off the northern edge of the Great Pearl
Bank, and accumulations of fine calcareous mud sediments are known in the
Sigsbee Deep, just north of the Campeche Bank, Gulf of Mexico. Contrasting
with these occurrences are shallow lagoons enclosed by coastal spit accretion
along the northeast Yucatan coast, which contain 30 feet or more oflime mud, the
vast muddy tidal flats on the leeward (western) side of Andros Island, the tidal
lagoons along the western side of the Persian Gulf, and the mud-choked Florida
Bay. In the latter cases, much fine mud sediment has been brought from outer
shelves into lagoons and onto tidal flats by storm and tidal currents and trapped,
although in situ accumulation is also proceeding in these areas.
Despite the similarity of hydrologic processes in carbonate and terrigenous
sedimentation, some additional effects of water movement occur in the deposition
of carbonates, because oftheir predominantly organic origin. These effects can be
differentiated into those occurring in marginal basins, with exposure to open sea
and those of interior or epeiric seas. Water movements induced by strong currents
and crashing waves at coastal margins have an important positive effect on
growth rates of carbonate-producing organisms. Removal of CO 2 through
wave action and change of pressure, and the bringing of nutrients by fresh
marine water to stimulate growth of organisms, encourages CaC0 3 precipi-
tation. Modern reefs form best along such areas, particularly where a shelf break
occurs. Even in periods of quiet weather or those with onshore winds, upwelling
along steep slopes brings fresh nutrients to the shelf margin. Rapid growth to sea
level occurs and much debris is shed rapidly from the centers of organic growth.
Thus strong water movement itself indirectly creates great volumes of carbonate
sediment despite its tendency to erode. Persistently strong to moderate water
movement (such as over shallow, tropical shelves) also creates sand size-particles
such as ooids, grapestones, and hardened fecal pellets by submarine accretion and
cementation. These processes are partly organic and partly physico-chemical, and
they not only create hardened sand-size particles, but may also stabilize the
sediment through pervasive submarine cementation.
In quiet water areas, behind reef or sand barriers, or across shallow water over
wide flat shelves, restricted circulation and climatic factors combine to influence
strongly the type of carbonate sedimentation in a different way. These conditions
have been exhaustively described by Irwin (1965) and Shaw (1964) and applied to
the geologic record by Roehl (1967) and Lucia (1972). Stagnant circulation results