Page 68 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 4: CARBONATE FACIES MODELS 59

Fig. 4.5.— Wilson’s standard facies
belts applied to a ramp. A) Complete fa-
cies succession. Facies 2, deeper-water
A) 6 7 or 8 9 or land shelf, is the dominant element. Land-
ward of belt 2 follows a high-energy sand
2
shoal of facies belt 6 that protects a la-
goon; depending on the degree of re-
1slope striction caused by the sand shoal, the
lagoon falls in facies belt 7 or 8. The
1floor spectrum ends with supratidal flats of
belt 9. On the seaward side, belt 2
grades into facies belt 1. If the sea
floor becomes steep enough for signif-
B) 6 9 or land icant sediment-gravity transport, facies
1 may be differentiable into slope fa-
2
land cies, 1-slope, and basin floor facies, 1-
6 floor. B) Common variations of the ramp
theme. Facies 7 through 9 need not be
1
present. The succession may terminate
with a high-energy sand belt that ends in
a beach or a sea cliff. On the seaward
side, belt 2 may imperceptibly grade into
the basin floor.
strongly influenced by tidal currents. Sediments: Clean lime features. Sediments: Calcareous or dolomitic mud or sand,
sands, occasionally with quartz; partly with well-preserved with nodular or coarse-crystalline gypsum or anhydrite;
cross bedding, partly bioturbated. Biota: Worn and abraded intercalations of red beds and terrigenous eolianites in
biota from reefs and associated environments, low-diversity land-attached platforms. Biota: Little indigenous biota ex-
in-fauna adjusted to very mobile substrate. cept mats of cyanobacteria, brine shrimp, abnormal-salinity
ostracodes, reworked marine biota.
7) Platform interior – normal marine. Setting: Flat platform
top within euphotic zone and normally above fair-weather 9B) Platform interior – brackish. Setting: poor connection
wave base; called lagoon when protected by sand shoals, with the open sea just like 9A but with a humid climate
islands or reefs of platform margin; sufficiently connected such that fresh water runoff dilutes the small bodies of
with open sea to maintain salinities and temperatures close ponded seawater and marsh vegetation spreads in the
to those of adjacent ocean. Sediments: Lime mud, muddy supratidal flats. Sediments: Calcareous marine mud or
sand or sand, depending on grain size of local sediment sand with occasional freshwater lime mud and peat layers.
production and the efficiency of winnowing by waves Biota: shoal-water marine organisms washed in with storms
and tidal currents; patches of bioherms and biostromes. plus abnormal-salinity ostracodes, freshwater snails and
Terrigenous sand and mud may be common in platforms charophytic algae.
attached to land, absent in detached platforms such as
oceanic atolls. Biota: Shallow-water benthos with bivalves, The internal variability of facies belts may be consider-
gastropods, sponges, arthropods, foraminifers and algae able. Gischler and Lomando (1999) contributed an instruc-
particularly common. tive example on variability of the platform interior facies
(belt 7) of the isolated platforms off Belize and its connec-
8) Platform interior – restricted. Setting: As for facies 7, but tion to antecedent karst relief and variations in subsidence.
less well connected with open ocean so that large variations Wilson’s (1975) standard facies can also be applied to car-
of temperature and salinity are common. Sediments: Mostly bonate ramps (Fig. 4.5). On attached ramps, the succes-
lime mud and muddy sand, some clean sand; often tidal sion includes facies 7 (platform interior) passing into facies
flats (Fig. 4.8); early diagenetic cementation common; ter- 2 (deep shelf). On detached ramps, the succession starts on
rigenous admixtures common. Biota: Shallow-water biota the landward side with facies 8 or 7, depending on the de-
of reduced diversitym e,g, cerithid gastropods, miliolid gree of restriction, includes the barrier as facies 6, followed
foraminifers. on the seaward side by facies 2. The critical difference to
rimmed platforms remains even if the same facies categories
9A) Platform interior – evaporitic. Setting: largely arid are used to describe the situation. The diagnostic criteria are
supratidal flats with only episodic influx of normal marine the slope facies 3 and 4. On rimmed platforms, facies 2 is
waters such that evaporites commonly alternate with the separated from the rim facies 6 by a slope with facies 3 or 4.
carbonates. Sabkhas, salt marshes and salt ponds are typical Homoclinal ramps lack slope facies altogether, whereas dis-