Page 255 - Sedimentology and Stratigraphy

242 Shallow Marine Carbonate and Evaporite Environments

continents were covered by shallow seas (Tucker & 15.5.1 Platform evaporites
Wright 1990). The water depth across an epicon-
tinental platform would be expected to be variable In arid regions the restriction of the circulation on
up to a few tens to hundreds of metres. Both tidal the inner ramp/shelf can lead to the formation
and storm processes may be expected, with the latter of extensive platform evaporites. On a gently sloping
more significant on platforms with small tidal ranges. ramp a sand shoal can partially isolate a zone of very
Currents in broad shallow seas would build shoals of shallow water that may be an area of evaporite pre-
oolitic and bioclastic debris that may become stabi- cipitation; the subtidal zone here often merges into a
lised into low-relief islands. Deposition in intertidal low-energy mudflat coastline. Shelf lagoons behind
zones around these islands and the margins of the rims formed by reefs or sand shoals can create similar
sea would result in the progradation of tidal flats. areas of evaporite deposition, although the barrier
The facies successions developed in these settings formed by a reef usually allows too much water cir-
would therefore be cycles displaying a shallowing-up culation. Evaporite units deposited on these platforms
trend, which may be traceable over large areas of the can be tens of kilometres across (Warren 1999).
platform.
15.5.2 Evaporitic basins (saline giants)
15.4.5 Carbonate banks and atolls
Evaporite sedimentation occurs only in situations
where a body of water becomes partly isolated from
Isolated platforms in areas of shallow sea surrounded
the ocean realm and salinity increases to supersatura-
on all sides by deeper water are commonly sites of
tion point and there is chemical precipitation of
carbonate sedimentation because there is no source of
minerals. This can occur in epicontinental seas or
terrigenous detritus. They are found in a number of
small ocean basins that are connected to the open
different settings ranging from small atolls above
ocean by a strait that may become blocked by a fall
extinct volcanoes to horst blocks in extensional basins
in sea level or by tectonic uplift of a barrier such as a
and within larger areas of shallow seas (Wright &
fault block. These are called barred basins and they
Burchette 1996; Bosence 2005). All sides are exposed
are distinguished from lagoons in that they are basins
to open seas and the distribution of facies on an iso-
capable of accumulating hundreds of metres of eva-
lated platform is controlled by the direction of the
porite sediment. To produce just a metre bed of halite
prevailing wind. The characteristics of the deposits
a column of seawater over 75 m deep must be evapo-
resemble those of a rimmed shelf and result in similar
rated, and to generate thick succession of evaporite
facies associations. The best developed marginal reef
facies occurs on the windward side of the platform, minerals the seawater must be repeatedly replenished
which experiences the highest energy waves. Carbo- (Warren 1999).
nate sand bodies may also form part of the rim of the Deposition of the thick succession can be pro-
platform. The platform interior is a region of low duced in three ways (Warren 1999) each of which
energy where islands of carbonate sand may develop will produce characteristic patterns of deposits
and deposition occurs on tidal flats. (Fig. 15.19).
1 A shallow-water to deep-basin setting exists where
a basin is well below sea level but is only partly filled
with evaporating seawater, which is periodically
15.5 MARINE EVAPORITES replenished. The deep-water setting will be evident if
the basin subsequently fills with seawater and the
Evaporite deposits in modern marine environments deposits overlying the evaporites show deep marine
are largely restricted to coastal regions, such as eva- characteristics such as turbidites.
porite lagoons and sabkha mudflats (15.2.2 & 2 A shallow-water to shallow-basin setting is one in
15.2.3). However, evaporite successions in the strati- which evaporites are deposited in salterns but contin-
graphic record indicate that precipitation of evaporite ued subsidence of the basin allows a thick succession
minerals has at times occurred in more extensive to be built up. The deposits will show the character-
marine settings. istics of shallow-water deposition throughout.