Page 64 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 4

Carbonate facies models

Carbonate rocks often overwhelm the untrained eye The T factory produces nearly all its sediment in a narrow
by a bewildering variety of textures, structures and grain depth range that normally extends only tens of meters down
types. Patchy diagenesis adds to the impression of almost from sea level. The seaward perimeter of this highly produc-
chaotic diversity and irregularity. Upon closer inspection, tive zone is often protected by an elevated, wave-resistant
the situation is not nearly as bad. If carbonate sediments are rim. This production system generates a platform geometry
characterized by sedimentary structure, texture and grain with a particularly flat unda environment swept by waves
kind, a recurring succession of facies belts can be recognized and tidal currents, and a rapid transition, across the rim,
in shore-to-basin transects. These facies appear throughout into the clino environment. The clinoforms can be much
the Phanerozoic and with only slight modification also in steeper than in siliciclastics (Fig. 3.11). The clino environ-
the late Precambrian. This surprising persistence indicates ment passes basinward into the fondo environment of the
that the evolution of organisms in this time interval had flat basin floors. Both clinoforms and fondoforms may con-
only a modifying effect on the basic carbonate facies. The tain abundant slumps and debrites, often with meter-size
standard carbonate facies seem to capture trends dictated clasts. Where the M factory replaces the T factory, for in-
by other parameters such as the carbonate growth function, stance after major extinctions, it builds platforms with the
i.e. the distribution of growth rates as a function of depth same unda, clino and fondo differentiation as the T factory.
and distance from shore, the degree of protection from In its typical development, the M factory lacks the unda fa-
waves and tidal currents, and the degree of restriction in cies. The mud-mounds are upward-convex constructions
the water exchange with the open sea. On the slopes, the that are not planed by waves and form in the clino envi-
declivity and the the balance between sedimentation and ronment, even though the slope declivity may be very low
erosion are crucial controls. These principles are discussed (ramp setting).
in the next section, followed by a presentation of facies on In the C factory, the ability to build rims is weak and the
ramps and rimmed platforms. facies pattern resembles that of siliciclastic sediments. The
unda environment is a seaward sloping shelf that gradu-
ally bends down into the clino domain. Sediments reflect
UNDA, CLINO AND FONDO ENVIRONMENTS these gradual changes. Slumps and debrites are scarce, as
are large clasts. Deep-sea accumulations of the C factory lie
in the path of contour currents and are streamlined by these
One of the most fundamental classifications of depo- currents.
sitional environments and facies is the subdivision by In the unda environment of carbonate platforms, two pa-
Rich (1951) into unda (shallow, wave-swept), clino (slopes rameters control the further subdivision of depositional en-
shaped by gravity transport), and fondo (basin-floor) en- vironment and facies – the degree of protection from waves
vironments. Rich (1951) proposed the suffix “-form” for and currents, and the degree of restriction in the exchange
the morphologies associated with these environments and of water with the open sea. Both parameters are related
the suffix “-them” for the respective deposits. In practice, to a third characteristic of the system – the elevation and
the distinction between morphology and sediment body continuity of the platform rim. The rim index (p.42) pro-
has rarely been made. I will use the terms undaform, cli- vides a measure of the fraction of wave energy that passes
noform and fondoform for the deposits and the morphol- through the rim. Sediments deposited behind the rim obvi-
ogy inferred from these deposits. Rich’s (1951) classifica- ously “feel” this energy flux.
tion is broader (but analogous) to the subdivision of deltas In the clinoforms, the most important control on both ge-
in topset, foreset and bottomset beds (Barrell, 1912). The ometry and facies is the balance between sediment input
definitions of Rich (1951) apply to all depositional systems from above and sediment output onto the basin floor. Vari-
where transport of particulate sediment is important. In car- ations in the material balance lead to the subdivision into
bonate rocks, the expression of the unda, clino and fondo accretionary, bypass and erosional slopes decribed in chap-
domains varies somewhat among the three factories. ter 3.

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