Clear Shallow Seas

The seas have no suspended sediment, so there are a wide range of organisms. The rocks that form in clear, non-clastic shallow marine environments are biologically and chemically formed limestone. All limestones consist largely of calcium carbonate so react with dilute HCl. They often contain visible macrofossils but microfossils and fossil fragments are also common.Although most limestones are formed from the remains of organisms living in the sea, some are composed of calcium carbonate that has been directly precipitated from sea water by chemical processes.Bioclastic or fossiliferous limestoneAs much as 75% of a bioclastic rock may be composed of the remains of invertebrate skeletons from organisms such as crinoids, bivalves , brachiopods, gastropods or a mixture of fossils. The rest of the rock is formed of calcite mud and ordinary detrital mud. The rock itself is commonly grey in colour, such as the Carboniferous Limestones of Derbyshire. They are hard, well jointed and in thick massive beds. They may contain the broken remains of stem segments (ossicles) of crinoids and so are called crinoidal limestones.Bioclastic limestones are also common from the Middle Jurassic. They are often crumbly and fossils such as Ostrea, a bivalve, can be picked out whole. These limestones are often cream in colour. Skeletal remains are broken into fragmets during transport by the action of marine currents or waves.Reef LimestoneCorals provide the main framework of a reef, but other organisms also live on and around reefs. Carbonate secreting algae encrust the reef, cemeting it together. Crinoid, brachiopod, echinoid, bivalve, gastropod and tribolite remains can all be found in reef limestones. Reef limestones are typically unbedded because reefs form by growing upwards.Reef building corals live in warm, shallow, high-energy, well oxygenated tropical waters. Water temperatures need to be between 25 and 29 degrees. Reef building corals do not grow well at depths greater than 25m. Although reefs amd atolls occur in the middle of oceans, the corals only live in the shallow water near landmasses. In the Pacific, volcanic islands are colonised by corals and as the islands sink the coral may grow up at the same rate so that the coral forms a barrier reef around the island. Eventually the island sinks and a ring of coral called an atoll forms.ChalkChalk is white, hard and often massive and well jointed. It is biologically formed limestone composed of coccoliths, the tiny calcareous disc or oval shaped platelets measured in microns that formed part of the skeletons of single celled algae. It may contain the microfossils of foraminifera such as Globigerina. There may also be macrofossils of echinoids, belemnites, brachiopods and bivalves. Chalk is formed in low energy deep water shelf environments (<200m), but only where very little sediment was being supplied from the land. Flint nodules are common in chalk.Oolitic LimestoneThese rocks are formed from ooliths. They form in shallow seas where tiny grains of sand, shell fragments or pellets are rolled in carbonate mud by tidal currents and wave action. Around this nucleus concentric layers of calcium carbonate, in the form or aragonite, are precipitated from the sea water. The ooliths are therefore formed by chemical processes in warm tropical climates. Most ooliths form in water less than 2m deep, a depth where wave agitation and tidal movements are active.Oolitic limestones are typically white, but may be yellow or orange if iron stained. Current and wave action means that they are often cross bedded.Fossils are common but the main organisms such as brachiopods and bivalves are those which were attached to the sea floor to withstand the high energy conditions. Some organisms, such as irregular echinoids and bivalves, survived by burrowing. Many fossils are broken by the high energy conditions.MicriteMicrite is a fine, hard crystalline limestone formed from calcite mud, which has undergone diagenesis. The calcite mud is produced by chemical precipitation from sea water or by carbonate secreting algae. There are normally no fossils visible.

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