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