Geology Reference
In-Depth Information
and water depths, and nutrients and siliciclastic input
are the major controls on the distribution of modern
zooxanthellate coral reefs. Cool-water reefs are dis-
cussed in Sect. 16.4.1.
city of rapidly growing calcareous frame-building or-
ganisms and the absence of aragonite (which would
favor early lithification) hinder the development of plat-
forms with wave-resistent margins and steep flanks.
Non-tropical shelf sediments are primarily controlled
by bottom currents which distribute the bioclastic sedi-
ment over extended areas. The results are homoclinal
ramps, developing in low-water energy, and distally
steepened ramps in high-energy environments.
Non-tropical carbonates are found almost continu-
ously from mid to high latitudes in shelf areas with low
or missing clastic input. Similar settings which occur
along a latitudinal transect from warm to cold water
conditions, may be developed along downslope pro-
files at any latitude (including tropical carbonate plat-
forms, Brazil shelf).
2.4.4.3 Non-Tropical Shelf and Reef Carbonates
Shallow-marine temperate carbonate sediments consti-
tute about one third of modern global shelf sediments
(Nelson 1988). Modern temperate cool-water carbon-
ates differ from tropical carbonates in the type of skel-
etal grains, mineralogy and diagenesis, oxygen and car-
bon isotopes and in the range of trace elements. Non-
tropical and tropical shallow-marine carbonates differ
significantly in their prevailing geometries: The scar-
Plate 3 Modern and Pleistocene Carbonates: ShallowMarine and Windblown Sands; Beachrocks
The samples shown on this plate document the common composition of calcareous sands deposited in shallow-
marine (-> 1-3) and subaerially exposed environments (-> 4, 5, 7, 8). Both the Bahamas and the Bermudas offer
excellent possibilities for studying the controls on the deposition of calcareous sediments and the processes
involved in their transformation into carbonate rocks. The Bahamas are a large isolated platform, 700 x 300 km
in size. The subtidal sediments exhibit
specific distributional patterns
(see map on next page). Lithofacies
characterized by skeletal grains (coralalgal), ooids and reefs (see Pl. 51/7) occur at the platform margins, whereas
sediments rich in aggregate grains (grapestone) or carbonate mud (pellet mud) are platform-interior facies.
Modern shallowmarine carbonate sands
formed in tropical and subtropical warm-water environments consist
predominantly of skeletal grains (foraminifera, -> 6; calcareous algae, -> 5; mollusk shells, -> 2, and other
biota), associated with non-skeletal grains including peloids (-> 2), ooids (-> 1, 3) and composite grains represented
by aggregate grains (-> 1; Pl. 15/1-3), and reworked and redeposited sediment (intraclasts, -> 1). Grains are
usually aragonite or High-Mg calcite. The average size ranges between < 0.5 and about 2.0 mm. Sand
accumulations are common on or near the seawards edge of banks, platforms or shelves (Halley et al. 1983).
Sand production and the development of modern bank-margin sand bodies are strongly controlled by antecedent
topography, physical processes (wind, waves, tides, and storms), and sea-level changes. Various encrusting
organisms contribute to the stabilization of the sediment binding of grains and to the formation of beachrocks
(-> 8). Submarine cementation within inter- and intragranular pores as well as early diagenetic cementation of
subaerially exposed sediments lead to an incipient lithification (-> 4). The microstructure of many grains becomes
highly altered due to the activity of microborers (-> 2; 'micritization
) and diagenetic alterations. These grains
are formed in specific high- and low-energy environments (see map).
Eolianites,
consisting of windblown marine carbonate grains, are known from modern and ancient tropical
and subtropical coastal environments (see Fig. 15.1). Eolianites are commonly associated with coastal marine
deposits and paleosols. Because the sediment consists of grains, originally formed in shallow-marine environments,
eolian grainstones may be confused with marine calcarenites. Important diagnostic criteria of eolianites are
lamination and stratification with high- to low-angle beds (see Fig. 15.1), fine- to medium-sized grains, good
overall sorting (-> 4), sorting differences between laminae or beds, inverse grading, rounded tabular grains, and
meteoric vadose meniscus and microstalactitic calcite cements (-> 4). Other cement types are fibrous cements
consisting of long calcite crystals, and micrite rim cement that superficially resemble micrite envelopes. Calcified
plant root systems in calcarenites associated with paleosols and carbonate crusts (see Pl. 128) suggest a pause in
sedimentation under subaerial conditions.
