Page 49 - Carbonate Sedimentology and Sequence Stratigraphy

40 WOLFGANG SCHLAGER

soft fill
soft flank sand, mud
mud, rubble
stiff rim Fig. 3.3.— Bucket structure of an attached platform. Produc-
organic frame,
cement tion at first comfortably exceeds accommodation creation and later
slowly approaches the limit of growth. This change is demonstrated
by decreasing rate of progradation and appearance of a deep la-
Fig. 3.2.— The bucket principle. The growth anatomy of rimmed goon.
carbonate platforms (the products of the T factory) resembles a
bucket. The platforms are held together by stiff rims of reefs or
rapidly cemented sand shoals, and filled with less consolidated that are only a few meters in diameter and height, as well
sediments of lagoons and tidal flats. The growth potential of a as for features measuring tens of kilometers across and over
platform is largely determined by the growth potential of the rim. a kilometer in height. Furthermore, it may be used for flat-
This atoll shifts from slow progradation to retrogradation as it ap-
topped accumulations, i.e. carbonate platforms, as well as
proaches its maximum growth rate. Raised rim and deep lagoon convex reefs or mounds.
(“empty bucket”) indicate incipient drowning. After Schlager (1981).
Reef . In carbonate sedimentology, the term reef denotes
platform rim is higher than that of the platform interior a wave-resistant buildup formed by the interplay of or-
such that the rim rises above the lagoon and sheds its ganic framebuilding, erosion, sedimentation and cementa-
excess sediment both downslope and into the lagoon. tion (Wright and Burchette, 1996, p. 368). Ecologic reefs are
Cool-water carbonates have no rims to speak of and defined by texture and composition. The geometry of reefs
therefore tend to form seaward sloping profiles in equi- is only a secondary attribute albeit an important one. Geom-
librium with wave action. Accumulations that form be- etry as observed in map view or cross sections of outcrops
low intensive wave action are convex rather than flat- or seismic data can aid in locating reefs and provides crucial
topped (Fig. 3.4). information on the effect of the reef on its surroundings.
➤ “Steepen the slopes”. Carbonate slopes steepen with The basic controls on reef geometry are upward growth of
height and are generally steeper than slopes of sili- the organic framework, current re-inforcement by this struc-
ciclastic accumulations. Several effects contribute to ture and sediment export by the reef factory. The interac-
this trend: Shoal-water carbonate production includes tion of the wave-resistant structure with the adjoining sea
much sand and rubble; these materials cannot be produces scours around the reef because the structure gen-
carried far and have high angle of repose. Slope lithi- erates extra turbulence, much like sand or snow are blown
fication retards slumping and stabilizes steep angles away from rock walls. Sediment export from the reef partly
once they are formed (see further on p. 43f). compensates for the scouring effect of waves and currents.
Reefs generally shed sediment of a wide range of sizes and
The terms used for describing the geometry of reefs and this material forms debris aprons that become thinner and
shoal-water carbonate accumulations are quite numerous finer with increasing distance from the reef core. If major
and not always self-explanatory (see Wilson, 1975; Wright scours are geometrically manifest, the reef probably suffered
and Burchette, 1996). However, in this plethora of words an extended period of greatly reduced production or it was
there are some that nicely capture the principal geometric drowned and scoured before it became buried.
trends just discussed. They will be given preference in this The degree of symmetry of reef core and aprons indicates
book and are explained below. to what extent a reef functioned as a barrier. Reef belts at
the platform margin are the prototype of a protective barrier.
Localised accumulations They are pounded by the ocean on one side and face calm la-
goon waters on the other. The result is pronounced lagoon-
Carbonate build-up. Build-up simply indicates that a carbon- ward transport of reef debris, commonly in the form of pro-
ate body rose above the adjacent sea floor according to the grading sediment tongues. Passages in the barrier may de-
principle of “ the rich get richer”. The term is unspecific velop curved spits of reef rubble that point lagoonward. The
about the origin of the carbonate material. It applies to a seaward reef front, on the other hand, remains bare. Reefs
wide range of scales. The term has been used for features with radially symmetric aprons indicate growth in a location