![]() ![]() ![]() As the water drains downwards, picking up oxidised iron and clay material, and the soil dries, the dissolved calcium carbonate precipitates as a thin iron-rich layer of calcite on the underlying limestone. This forms when aragonitic mud and sand dissolve in damp organicrich peaty soil. Coring has shown that the oolite is underlain by the older coralline limestone.īoth these limestones have undergone alteration and diagenesis due to subaerial exposure, which has produced widespread minor karstification and in places a reddish-brown laminated calcrete crust, known as caliche. The younger oolitic Miami Limestone is composed of millions of ooids, formed in clear current-swept waters supersaturated with calcium and carbonate that precipitated out around a nucleus grain to form these tiny spherical pellets. corals which deposit hard calcareous material to form the stony framework of the reef) with intra- and interbedded calcarenites and thin beds of quartz sand, and is thought to be up to 60m thick. The former is described as an elevated coral reef rock that was submerged as sea levels rose. Two Pleistocene formations form the Keys: The glaciers gradually melted, sea levels rose again, until now only the highest points of the ridges are visible, and both fossil and modern coral reefs lie submerged to the east and south of the arc of islands.Ĭoral & Oolite Formations of the Florida Keys Sea levels fell during the last ice age, so that about 20,000 years ago these reefs and ridges were exposed and vegetation grew on the once-submerged areas. At that time the sea was warm enough to allow coral to grow and reefs and sandy bars formed on the shallow shelf. It is these variations in sea level, starting 125,000 years ago when the sea level was about 7m higher than at present, which have been primarily responsible for the geology and topography of the Florida Keys. ![]() Modified from Shinn and Lidz, 2018.Ĭarbonate sedimentation continued through the Pleistocene and Holocene and on into the present, interspersed by glacial and interglacial periods and subsequent marine incursions and withdrawals. Sketch cross-section roughly north-west to south-east across the Florida Keys. Subsidence of the passive continental margin resulted in carbonate deposition about 160 million years ago in the late Jurassic, which continued through the Cretaceous and Tertiary to create the Florida Platform, an accumulation of nearly 5,000m of shallow-water carbonates and evaporites with only minor components of terrigenous sediments. Precambrian-Cambrian igneous rocks were overlain by Ordovician-Devonian sediments and Triassic-Jurassic volcanics, deposited as the Florida basement separated from the African plate to join North America. The relatively young Florida Keys have much older foundations. However, this unique environment, the result of repeated episodes of flooding and exposure during the Pleistocene, also allows us an insight into limestone formation throughout the world. Made up of over 800 islands, none more than 5.5m above sea level, and stretching in a gentle arc nearly 200 km from Miami south-westwards into the Gulf of Mexico, the Florida Keys are known for their distinctive ecology, clear waters and beautiful coral reefs a famous playground for fishermen and snorkellers alike. Source: Google Earth.Ī string of pearls in a turquoise sea: as well as being picturesque, the unique Florida Keys offer the petroleum geologist a masterclass on carbonate sedimentary processes. Satellite image of the Florida Keys identifying places mentioned in the text. The picturesque Florida Keys are not just a favourite holiday destination, they also offer geoscientists a masterclass on carbonate geology and the sedimentary processes associated with creating carbonate reservoirs. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |