Geology Reference
In-Depth Information
Box 14.1.
Definitions of Standard Facies Zones (continued).
Biota:
Almost exclusively benthos. Colonies of frame-
builders, encrusters, and bafflers along with large volumes
of loose skeletal rubble and sand containing benthic mi-
crofossils (e.g. foraminifera, algae).
Common lithofacies:
Framestone, bafflestone, bind-
stone, wackestone and floatstone, grainstone and rudstone.
Platform-edge and platform sand shoals
FZ 6 Platformmargin sand shoals
Setting
: Elongate shoals, tidal bars and beaches, some-
times with eolianite islands. Above fair-weather wave base
and within the euphotic zone, strongly influenced by tidal
currents. Very narrow facies belt.
Sediments:
Clean calcareous, often rounded, coated and
well-sorted sands, occasionally with quartz. Sand grains
are skeletal grains, or ooids and peloids. Partly with well-
preserved cross-bedding, sometimes bioturbated. Suscep-
tible to subaerial exposure. Rock color: light.
Biota:
Worn and abraded biota from reefs and associ-
ated environments. Low-diversity infauna adjusted to mo-
bile substrate. Common biota are large bivalves and gas-
tropods and special types of foraminifera and dasyclads.
Common lithofacies
: Grainstone, packstone.
Open-marine platform
FZ 7 Platform interior normal marine (open marine)
Setting:
Flat platform top within euphotic zone, nor-
mally above fair-weather wave base. Called lagoon when
protected by sand shoals, islands or reefs of the platform
margin. Sufficiently connected with the open sea to main-
tain salinity and temperature close to that of the adjacent
ocean. Moderate circulation. Water depths a few meters to
tens of meters. Wide facies belt.
Sediment:
Lime mud, muddy sand and clean sands, de-
pending on the grain size of local sediment production and
the efficiency of winnowing by waves and tidal currents.
Medium to coarse bedded. Locally patch reefs or organic
banks. Terrigenous sand and mud may be common in at-
tached platforms, but are generally absent in detached plat-
forms such as oceanic atolls. Rock color: light and dark.
Biota:
Shallow-water benthos with algae, foraminifera,
and bivalves; gastropods, particularly common. Areas with
marine grasses and with patch reefs.
Common lithofacies:
Lime mudstone, wackestone and
floatstone, packstone, grainstone.
Restricted-marine platform
FZ 8 Platform interior restricted
Setting:
As for Facies Zone 7, but less well connected
with the open ocean, causing large variations in salinities
and temperatures. Within the euphotic zone. Typically
strongly differentiated tidal zones with freshwater, salt-
water and hypersaline conditions as well as subaerially ex-
posed areas. Shallow, cut-off ponds and lagoons with re-
stricted circulation and hypersaline water. Lagoons behind
barrier reefs, within atolls or behind coastal splits. Water
depths below one meter and a few meters to a few tens of
meters. Wide facies belts.
Sediments:
Mostly lime mud and muddy sand; some
clean sand. Terrigenous influx common. Early diagenetic
cementation common. Limestones and dolomites. Rock
color: light.
Biota:
Shallow-water biota of reduced diversity, but
commonly with high number of individuals. Typical are
miliolid foraminifera, ostracods, gastropods, algae and cy-
anobacteria. Marine and freshwater vegetation.
Common lithofacies:
Lime mudstone and dolomite
mudstone, wackestone, grainstone, bindstone. Sedimen-
tary breccia.
Arid near-coast evaporitic platforms
FZ 9A Arid platform interior evaporitic
Setting:
As in Facies Zone 7 and 8, yet with only epi-
sodic influx of normal marine waters and arid climate so
that gypsum, anhydrite or halite may be deposited beside
carbonates. Supratidal. Sabkhas, salt marshes, salt ponds.
Wide facies belt.
Sediments:
Calcareous or dolomitic mud or sands, with
nodular, wavy or coarse-crystalline gypsum or anhydrite.
Intercalations of red beds and terrigenous eolianites in land-
attached platforms. Rock color: highly variable; light, yel-
low, brown, red.
Biota:
Little indigenous biota except cyanobacteria; os-
tracods, mollusks, brine shrimps adapted to high salinities.
Common lithofacies:
Laminated lime and dolomitic
mudstones and bindstones alternating layers with layers
of gypsum or anhydrite.
Humid near-coast brackish regions
FZ 9B Humid platform interior brackish (humid)
Setting:
Poor connection with the open sea as in FZ 9A
but with a humid climate so that water runoff dilutes small
bodies of ponded seawater and marsh vegetation spreads
in the supratidal flats. Narrow facies belt.
Sediments:
Calcareous marine muds or sand with oc-
casional freshwater lime mud and peat layers. Rock color:
gray, light, brown, dark.
Biota:
Shoalwater marine organisms washed in with
storms plus organisms adapted to brackish-water and fresh-
water (ostracods; freshwater snails; charophycean algae).
Paleokarst, caliche and other terrestrial and terrestrial-
marine settings
FZ 10 Humid and arid often subaerially exposed, me
teorically influenced limestones
Setting:
Subaerial or subaquatic, formed under mete-
oric-vadose and marine-vadose conditions. Abundant in
karst settings and pedogenic carbonates (continental and
near-coast areas), and supratidal and intertidal environ-
ments.
Sediments:
Limestones affected by early diagenetic
meteoric dissolution predominantly during phases of sub-
aerial exposure (e.g. paleokarst). Common in caliche crusts.
Typically occurring in limestones rich in carbonate cement
crusts, but also occurring in micritic caliche or as reworked
grains in restricted environments (e.g. coastal ponds or la-
goons).
Biota:
Indigenous biota lacking except cyanobacteria
and microbes.
