Palaeoecology of Jurassic coral reefs in the swiss Jura Mountains by Christophe Dupraz

		2. REEF COMPOSITION Oxfordian (Early Jurassic) reefs have been studied in detail. The different section location is shown on the map (above) and the type of detailed presentation of patch-reef is presented below (this is a small part of the reef of Hautes-Roches). 2.1 Microbialite (all benthic microbial deposits; also called Microbolite) Microbial crusts develop in marine, fresh-water, and terrestrial depositional environments. RIDING (1991) presents a classification and introduces the word "microbolite" in place of "microbialite" (BURNE & MOORE, 1987) as a general term for benthic microbial deposits. Autors continue to use the microbialite term by widening the definition which was preferentially defined for laminated deposits (e.g. REITNER, 1993).
						The term automicrite can be used for autochthonous micrite (mineralization in situ) in contrast to allomicrite composed from allochthonous (transported) micrite (WOLF, 1965). Organomicrite is a subdivition of automicrite which are related to Ca-binding organic macromolecules (REITNER & AL, 1995). Chromatography or fluorescence microscopy not being discussed in this paper, organomicrite will not be used. In the definition of microbolite, macro- and microfabrics are included. Macroscopically, three different fabrics are distinguished: laminated (stromatolitic), clotted (thrombolitic), and structureless (leiolitic). Microscopic fabrics are described as containing trapped and agglutinated particles, as peloidal, or as densely micritic. Peloidal and micritic fabrics are formed through Ca-binding matrix and bacterially-induced mineralisation and/or biomineralisation . Intermediate stages between these different end-members are common. The micropeloid formation is one of the most sensible problem. Many microbial crusts display a peloidal microfabric with a diameter of approximately 30µm. Although autors give preference to abiogenic precipitation for the genesis of this peloids (e.g. MACINTIRE, 1985; MARSCHAL, 1983), evidences for biogenical precipitation have been found. CHAFETZ (1986) favors bacterial mediation, whereas REITNER (1993), in modern reef cave from the Great Barrier, have studied precipitation via an organic Ca-binding mucus without necessarily bacteria activity. This mucus being often composed by decayed bacteria, both processes are not opponent and are probably associated (REITNER, 1993). 2.2 Micro-encrusters A semi-quantitative analysis of the relative abundance of microbolite types and associated micro-encrusters permits to better constrain the processes leading to a reef crisis. Four micro-encruster associations can be distinguished, and each follows an evolutionary trend in the studied section: Terebella-Tubiphytes dominated, Serpula-Berenicea dominated, Lithocodium dominated, and Bacinella dominated. These trends are interpreted to reflect changes in environmental conditions. Bioerosion generally is at its maximum before and after abundant growth of microbolite. 2.3 Corals Coral are the main builder of reef construction. They can be classified by their corallite morphology and organisation, what have a great meaning for environmental interpretation. Modern reefs prefer nutrient-poor water (e.g., HALLOCK, 1997). The fact that the studied Oxfordian reefs occur in environments with a relatively high terrigenous content suggests that they were quite tolerant to nutrients. This may be explained by a weakly developed relation between symbiotic algae and the coral hosts, which is confirmed by the low growth rates and weak growth-band contrasts (NOSE & LEINFELDER, 1997). Microsolenid corals display a dense structure of perforated septa with penulae suspected to have supported a complex gastro-vascular system, interpreting to permit an adaptation to a heterotrophic (suspension-feeding) mode of life comparable to the modern deeper-water Leptoseris fragilis (GILL & SANTANTONIO, 1995; SCHLICHTER, 1992). Microsolenids thus is interpreted as support light and nutrient fluctuations and are found in extreme environments such as mudy and clay-rich lagoons (AILLUD & DUPRAZ, 1998; FÜRSICH et al, 1994; INSALACO, 1996a). Stylinid corals sensu Gill 1977 seems to have also a great importance for Jurassic reef ecological interpretation Dupraz & Strasser, 1999. They are present when the conditions are relatively good, and could be interpreted to have a opposite behavior in regard to the microsolenids.
		First part of Hautes-Roches section. For full section see Dupraz & Strasser (1999).
			3. Trophic and oxygen fluctuation Periodic and relatively high input of siliciclastics and associated nutrients and high alkaline water are the major controlling factors for reef development. Bathymetry played a subordinate role on the very shallow Jura platform, but opening and closing of lagoons linked to low-amplitude sea-level fluctuations as well as fluctuating terrigenous run-off created periodic changes in trophic conditions. An ideal reef sequence can be reconstructed (cf. animations at the begining of this page): (1) A well-oxygenated lagoon with low water turbidity allows for coral growth with relative high diversity. Microbial crusts and micro-encrusters are associated with the living corals. Autotrophic organisms dominate, and thrombolite is formed only inside the patch-reef framework. (2) The second stage is the confining of this lagoon, forcing the evolution toward dysaerobic and low-mesotrophic conditions. Water turbidity increases due to terrestrial run-off and algal-microbial blooms, and the photic zone consequently does not reach the sea floor any more. Bioerosion activity increases as well as the abundance of heterotrophic micro-encrusters. Thrombolite progressively reaches the surface of the bioconstructions. (3) The lagoon becomes more restricted, high nutrient concentration induces high primary productivity which in turn induces oxygen consumption through bacterial decomposition of organic matter. The resulting dysoxic to anoxic environment favors pure thrombolite-dominated microbolites associated with Terebella and Tubiphytes. (4) In extreme cases, high clay and nutrient input strongly reduces carbonate production.
				Eutrophication of a shallow lagoon
			4. Sequence stratigraphy and reef crises Several major and smaller periods of environmentally controlled crises in coral-reef growth could be identified. Lateral correlation by high-resolution sequence stratigraphy and cyclostratigraphy suggests that these crises are probably orbitally controlled: insolation cycles induced by the precession of the equinoxes (20-ky periodicity) caused low-amplitude sea-level fluctuations which opened and closed lagoons, and climatic changes which controlled siliciclastic run-off and nutrient input. The studied Oxfordian reefs generally grew well during high sea level and dry climate, but suffered when low sea level and high rainfall in the hinterland caused eutrophication, water turbidity, and dysoxic conditions. Once coral growth slowed down and bioerosion further weakened the bioherm, microbolites took over and finally killed the reef. Consolidated through bacterially mediated cementation, however, microbialites also served as substrate for renewed coral growth once the environmental conditions allowed it.