Page 258 - Sedimentology and Stratigraphy

Further Reading 245

is sometimes referred to as the Messinian salinity Carbonate deposits can co-exist with terrigenous
crisis (Hsu ¨ 1972). clastic and volcaniclastic sediments under certain
conditions. Deltas built by ephemeral rivers in arid
environments may experience long periods without
15.6 MIXED CARBONATE–CLASTIC supply of debris and during these intervals carbonates
ENVIRONMENTS may develop on the delta front (Wilson 2005), for
example, in the form of small reefs that build up in
The depositional environments described in this the shallow marine parts of ephemeral fan-deltas
chapter are made up of ‘pure’ carbonate and evaporite (Chapter 17). Time intervals between eruption epi-
deposits that do not contain terrigenous clastic or sodes in island arc volcanoes (12.4.2) may be long
volcaniclastic material. There are, however, modern enough for small carbonate platforms to develop in
environments where the sediments are mixtures of the shallow water around an island volcano, giving
carbonate and other clastic materials, and in the rise to an association between volcanic and carbonate
stratigraphic record many successions consist of mix- deposition (Wilson & Lokier 2002).
tures of limestones, sandstones and mudstones. These Characteristics of shallow marine carbonates
typically occur in shallow-marine settings. The . lithology – limestone
changes from carbonate to non-carbonate deposition
. mineralogy – calcite and aragonite
and vice versa are the result of variations in the
. texture – variable, biogenic structures in reefs, well
supply of terrigenous clastic material and this is in
sorted in shallow water
turn determined by tectonic or climatic factors, or
. bed geometry – massive reef build-ups on rimmed
fluctuations in sea level.
shelves and extensive sheet units on ramps
Climate plays an important role in determining
. sedimentary structures– cross-bedding in ooliteshoals
the supply of sand and mud to shallow marine
. palaeocurrents – not usually diagnostic, with tide,
environments. Under more humid conditions, the
wave and storm driven currents
increased run-off on the land surface results in more
. fossils – usually abundant, shallow marine fauna
sediment being carried by rivers, which are them-
most common
selves more vigorous and hence deliver more sedi-
. colour – usually pale white, cream or grey
ment to the adjacent seas. A change to a wetter
. facies associations – may occur with evaporites, asso-
climate on an adjacent landmass will therefore result
ciations with terrigenous clastic material may occur
in increased deposition of sand and mud, which
will suppress carbonate production on a shelf. Alter- Characteristics of marine evaporites
nation of beds of limestone with beds of mudstone . lithology – gypsum, anhydrite and halite
or sandstone may therefore be due to periodic clim- . mineralogy – evaporite minerals
atic fluctuations of alternating drier and wetter con- . texture – crystalline or amorphous
ditions. However, other mechanisms can also . bed geometry – sheets in lagoons and barred basins,
cause fluctuations in the supply of detritus from the nodular in sabkhas
continent to parts of the shelf. Tectonic uplift of . sedimentary structures – intrastratal solution brec-
the landmass can also increase the sediment supply cias and deformation
by increasing relief and hence the rate of erosion. . palaeocurrents – rare
Tectonic activity can also result in subsidence of . fossils – rare
the shelf, which will make the water deeper across . colour – typically white, but may be coloured by
the shelf area: a relative sea-level rise will have the impurities
same effect. With increased water depth, more of the . facies associations – often with shallow marine
shelf area will be ‘starved’ of mud and sand, allowing carbonates
carbonate sedimentation to occur in place of clastic
deposition. Fluctuations in sea level (which are FURTHER READING
described in more detail in Chapter 23) may therefore
result in alternations between limestone and mud- Braithwaite, C. (2005) Carbonate Sediments and Rocks. Whit-
stone/sandstone deposition. tles Publishing, Dunbeath.