caa a22 4500 469045396 CHVBK 20180323132823.0 cr unu---uuuuu 170328e19921201xx s 000 0 eng 10.1007/BF02539791 doi (NATIONALLICENCE)springer-10.1007/BF02539791 Leinfelder Reinhold Institut für Geologie und Paläontologie, Universität Stuttgart, Herdweg 51, D-7000, Stuttgart aut A modern-type Kimmeridgian reef (Ota Limestone, Portugal): Implications for Jurassic reef models [Elektronische Daten] [Reinhold Leinfelder] Summary: Upper Jurassic reefs rich in microbial crusts generally appear in deeper (sponge—‘algal' crust reefs) or in very shallow but protected settings (coral or coral-coralline sponge meadows with ‘algal' crusts). Upper Jurassic high-energy reefs (coral reefs and coral-stromatoporoid reefs) normally lack major participation of microbial crusts but rather represent huge bioclastic piles with only minor framestone patches preserved. An exception to this rule is represented by the high-energy, coral-‘algal' Ota Reef from the Kimmeridgian of the Lusitanian Basin (Portugal). The narrow Ota Reef tract rims a small intra-basinal carbonate platform exhibiting perfect facies zonation (from W to E: Reef tract, back reef sands, peritidal belt, low-energy shallow lagoon). The reef is dominated by massive corals (Thamnasteria, Microsolena, Stylina). Complete preservation of coral framework is rare: like other Upper Jurassic high-energy reefs, the Ota Reef is very rich in debris; however, this debris is largely stabilized by algal and microbial crusts, what contrasts the other examples and gives the Ota Reef the appearance of a typical modern high-energy coral-melobesioid algal reef. Further similarities to modern reefs are the likely existence of a spur-and-groove system, the perfect sheltering of inner platform areas and the occurrence of small islands, as indicated by local blackenings and early vadose and karstic features. The exceptional character of the Ota Reef is thought to be due to the establishment of an equilibrated sediment balance. In a modern high-energy reef, physical and biological genereration of debris is compensated by (1) stabilization of loose debris by binding organisms, chiefly melobesioid algae, (2) accelerated coral-growth in order to keep pace with sedimentation, and (3) export of surplus material into back reef and fore reef areas through wave action. Since during the Late Jurassic, fast growing corals were rare and only less effective microbial binders were available, sediment balance could normally not be achieved in a comparable high-energy setting. This resulted in the increasing loss of stable bottom due to sedimentation and eventual suffocation of reef patches. However, in the Ota example the existence of a steep by-pass margin accounted for equilibrated sediment balance: (1) along the steep margin additional gravitational export was possible; (2) the steepness of the margin kept the reef tract narrow, so that it could be well winnowed by a longshore current system along the east side of the basin; (3) the remaining loose debris could, to a large extent, be stabilized by the available algal-like and microbial binders (thrombolitic-peloidal crusts,Bacinella, Lithocodium, Thaumatoporella, Tubiphytes). Institut für Palaentologie, Universitat Erlangen, 1992 Coral Reefs nationallicence Microbial Crusts nationallicence Sediment Balance nationallicence Palaeo-Oceanography nationallicence Reef Models nationallicence Portugal nationallicence Upper Jurassic (Kimmeridgian) nationallicence Facies Springer-Verlag 26/1(1992-12-01), 11-34 0172-9179 26:1<11 1992 26 10347 https://doi.org/10.1007/BF02539791 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/BF02539791 text/html Onlinezugriff via DOI NATIONALLICENCE 100 1- Leinfelder Reinhold Institut für Geologie und Paläontologie, Universität Stuttgart, Herdweg 51, D-7000, Stuttgart aut NATIONALLICENCE 773 0- Facies Springer-Verlag 26/1(1992-12-01), 11-34 0172-9179 26:1<11 1992 26 10347 Metadata rights reserved Springer special CC-BY-NC licence nationallicence SWISSBIB 23698487X BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer