Limestone Formation
Limestones form usually close to the source of shelly debris although some significant transport can occur. Great sources for limestone are reefs. Reefs have been in existence for most of the history of life on Earth, but they have changed in the species that build them. Stromatolites, which are complex living structures of more than one organism (cyanobacteria and algae), formed the first reef like structures in the Cambrian Period. Early reefs in the Ordovician were composed of small crinoidal, bryozoan and brachiopod reef communities. In the Devonian, reefs became extremely large with tabulate and solitary corals starting to dominate, but brachiopods and crinoids still significant contributors. Some Cretaceous reefs really took on some huge proportions and were dominated by large, now extinct mollusks called rudist. Since those times, modern corals and bivalves (clams) have been the prime reef-building organisms.
All these carbonate shelled organisms needed the same requirements out of their ocean environments: sunlight, a food source, and enough turbulence to remove sand and clay. Where you find these conditions is usually the same, on the margins of flat littoral surfaces. Reefs tend to be offshore from sandy beaches but not in too deep of water to not have sunlight. In fact reefs often build upon the skeletal debris of former reef inhabitants to continually grow upward to the sea surface where turbulence keeps the reef "clean" from sand and clay debris. Ancient reefs and limestone’s are closely interconnected, although not all limestones indicate an ancient reef.
Because of limestone's biogenic origins, it is often the best rock for finding fossils. The organisms themselves leave fossils in the rock and entire communities and even entire reef structures can be preserved in a limestone bed. At times a limestone is entirely composed of fossils. The rock coquina is a variety of limestone and is composed entirely of fragments of sea shells.
But most limestones have a significant amount of carbonate mud. This mud matrix can even constitute 100% of the limestone rock. Origins of this mud are debated and may just be a fine grained mud left from the erosion and abrasion of calcite shells. There maybe a non-biogenic origin too. At times modern carbonate muds can accumulate in the oceans in thick layers that are destined for limestone formation. A limestone variety is caused by swift currents that rolled carbonate mud into small beads that (once solidified) look like tiny eggs. This limestone variety is called an oolite and is sometimes very ornamental.
Limestone is almost always marine (ocean water) in origin and is usually associated with other near shore rocks types. In a typical marine scenario, to the shore side of a reef is the silica mud of a lagoon and closer to shore is the sand of a beach. The silica mud will form a shale while the beach sand will form a sandstone. Farther inland might be a swamp whose organic debris might form a coal layer. Throughout the Carboniferous time period, coals were often interbedded with sandstones, shales, and limestones in repeating cycles. These cycles represent changes in ocean levels over thousands of years as swamps are flooded by a beach and then a lagoon and perhaps a reef. But as sea levels fall the limestone of the reef is replaced by the shales of a lagoon and then the sandstone of a beach and on and on. Hundreds of feet of repeating cycles like this can occur.
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