The Coral Reef of the Yonne Valley
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The coral reef of the Yonne valley When rocks tell us about Earth’s History 5 Table of contents The higher reef complex of Bourgogne 3 A different world Welcome to the Yonne valley 4 Corals The landscapes, the rocks, the fossils... are witnesses of planet Earth’s history. In the valley, the river erosion and various phenomena revealed several outcrops that are real geological time machines. Fossilized remains of a coral 6 Coral reefs reef that existed 4 to 5 million years ago can still be seen today. 8 Spatial organization This guide will help you learn about the reef through its story, its wildlife, how it grew and lived. It will also introduce you to the geology of various sites of 10 The sites the Yonne valley. 11 The site of Châtel-Censoir Each site recounts a specific time period through several layered “slices of life“. 12 The “Quatre Pieux” quarry The oldest evidence comes from the deepest horizons. 13 The “Bois du Parc“ quarry 14 The “Rochers du Saussois“ 16 The “Roche aux Poulets“ 18 “La Rippe“ 19 Lexicon 2 Coral formations can Today A different world still be seen in today’s + 2000 years landscape, on the reserve cliffs. 160 million years ago, the placement of the continents and sea levels were very different than they are today. Bourgogne was then located in the intertropical zone CENOZOIC : The climate alternates ERA QUATERNARY of the globe, its climate hot and humid. between very cold and temperate episodes. - 1,8 Ma During the Jurassic age, the current Since then, the plate tectonics have moved During the temperate Yonne county was beneath a warm and Europe towards more temperate latitudes. episodes, the erosion The period is shallow sea. The fossilized reef of the “Bois du Parc“ is a of the rivers, the Yonne tectonically active, included, causes the causing geological vestige of that tropical period. carving of today’s changes : valleys. emergence of Bourgogne, formation of the CENOZOIC : CENOZOIC : Alps and the TERTIARY ERA TERTIARY Bressan trough Latitude corresponding France’s current position Location of the European territories - 65 Ma Tropic of Cancer The sea retreats from the Bourgogne region. Equator CRETACEOUS - 145,5 Ma The sea partly Tropic of Capricorn retreats to return The coral reef of the again. current Yonne valley MESOZOIC : MESOZOIC : Based on lesdinos.free grows for 4 to 5 million years, during the late ERA SECONDARY JURASSIC The sea floods Jurassic period. the majority of the current French territory. - 200 Ma Placement of the continents during the Jurassic age TRIASSIC - 250 Ma The lands, until then forming one giant continent, Pangaea, started drifting apart during that time period and oceans like the Atlantic were born. The continental drift at the end of the Triassic period was what determined the positions that these lands have today. PALEOZOIC PALEOZOIC PRIMARY ERA PRIMARY 3 Corals Colours stemmed from a pact The coral is an animal usually living in large colonies of individuals called “polyps“, Many corals live symbiotically* with attached to a base. Belonging to the cnidarian group, corals share the same tiny algae, mostly zooxanthellae*, sac-like body structure of the jellyfish, made of two distinct layers of tissue, and responsible for their specific colour. with a single opening surrounded by a ring of stinging cells Taping light energy, these algae produce organic molecules essential to the growth of the polyp and its The polyps of some species can secrete limestone skeleton. In return, the coral Tentacles et stinging cells catching their own external skeletons called coral- provides the algae with a stable envi- zooplankton to feed lite. There are hard corals with limestone ronment and the elements required for their survival. Zooxanthellae Mouth skeletons, soft corals with horny-whip like skeletons and soft corals without any real skeleton. The accumulation of the limestone skele- tons are the bedrock of a coral reef forma- tion when the hard corals die. Pharynx The living corals then keep growing on Coral reef Pixabay Living tissue top of it. Inside a coral reef, many coral species - nearly 300 species on the Reunion island Gastrovascular for example - coexist, with many different cavity Skeleton shapes and colours. A specific species can Based on Encyclopædia 2010 Britannica Inc. Polyp anatomy adopt different shapes depending on the environmental conditions - depth, light, water currents… 4 The different types of coral J.P. Quod J.P. Quod B. Martin-Garin Branching coral colony Table or tabular coral colony Lamellar coral colony The colony displays an architecture of more or The colony forms broad flat surfaces com- The colony grows in multiple horizontal layers. less fused main branches, with short or slender monly referred to as table-shaped corals. It can The thinner layers can look like a stack of plates. sub-branches. be side-attached or rest on a central leg. F. Mazeas F. Mazeas F. Mazeas Massive coral colony Encrusting coral colony Pillar coral colony The colony is characterized by a thick spherical The layers of the colony cover the substrate The colony grows upward in cylindrical forms shape. It allows the coral to be more resistant and adhere to it firmly, forming a crust that that have been compared to candles. to ocean currents. sometimes has a few lumps. 5 The coral reefs Threatened ecosystems Coral reefs are mineral structures essentially built by animals. They are primarily associated For the last few decades, the coral with warm seas, and are mainly made of hard corals. Their growth is controlled and populations have been decreasing at optimized by : an alarming rate, for multiple reasons : • the water temperature between 18 and 29 °C ; • an adequate salinity nearing 35 grams of salt per litre of water ; • coastal urbanization, clearing and • a depth not exceeding 165 ft and clear waters allowing light to penetrate the surface, intensive farming causing reefs to be which is essential to the algae living in symbiosis with corals ; covered by sediments, • ocean currents guaranteeing a good water oxygenation and food supply, though if they’re too violent, the risk of the colonies being destroyed is high ; • chemical pollution, • marine waters with low phosphate and nitrate levels. The reefs serve many roles, such as catering to a considerable biodiversity. Even if they • overfishing, poaching and the intro- only take up 0.2 % of the ocean floor, the reefs host more than 25 % of marine species, duction of exotic species upsetting plants and animals. the natural biological balance, • climate change, about to become the Shape and organisation of a reef main issue. The temperature variations of the oceans lead to coral bleaching The various shapes of coral colonies are that can kill the reefs. determined by the necessity to both build a structure able to resist water currents, and maximize the light-capturing surface. In wave-battered areas, the corals have massive “dome“ morphologies. In calm waters, they can settle for a thinner and more branch-like skeleton to better capture light. A coral reef Coral bleaching J.P. Quod Frank Mazeas 6 The reef formation of Yonne Fossil formations Today, this reef formation is spread out over 47 miles between the Loire and the Cure rivers. Its outcrop width is but a few miles, though its true distance can’t be determined Coral skeletons, shells of numerous to the North or East due to its erosion during the Cenozoic era. The thickness of the reef organisms but also the soft parts of formation ranges from 197 to 328 feet. certain organisms can be preserved The reef formation developed on a large platform, a vast and shallow marine expanse through time under specific conditions : that used to host a complex network of channels and passes where the reefs grew. This spatial distribution can still be observed today in diverse places on the planet. • the dead organism must be quickly protected from degradation by a layer of sediment (no oxygen or degrading organisms) Patch reefs Main reef system • the protective sediment layer must be thin and light enough so as not to crush the remains of the dead organism. Back-reef Mineral salts replace the organic matter depression of the animal or plant trapped in the Passes Open sea rocks. Through time, organic debris and sedi- ments settle and form successive strata. Sometimes, some fossils allow one of the strata to be dated. Based on http://planet-terre.ens-lyon.fr/ SOUTH-WEST NORTH-EAST Towards the current city of Clamecy Towards the current city of Vermenton Representation of the environmental conditions of the Yonne valley coral reef development. 7 Spatial organisation South-West “Roche aux Poulets“ 2 1 Châtel-Censoir level crossing 623 ft Yonne river Merry-sur- Yonne Based on J.C. Menot - 1991 This diagram is not to scale. Callovian Lower reef slope > Building arena limestone substratum Upper reef slope > Fore reef 8 “La Rippe“ 5 North-east 4 “Bois du Parc“ quarry 6 “Quatre Pieux“ quarry 3 “Rochers du Saussois“ 558 ft Yonne 560 ft Old open sea river Yonne river Mailly-le-Château Yonne river This diagram is not to scale. Reef bedrock : Colonization area structure and size unknown Emerging terminal facies Main reef Back reef or “ heart of the reef “ Lateral non-reef sediments 9 Sites of the Yonne valley 1 Châtel - Censoir For safety reasons and lack of access to private plots, a few sites are not open level crossing to the public. Please be respectful of (no public allowed) this restriction. 6 These sites have been developed to 5 2 “ Roche aux Poulets “ allow you to discover this geological 4 heritage. They’re accessible to all. 3 “ Rochers du Saussois “ Coral fossils and other organisms are common heritage.