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Abstract

Jurassic reef patterns reflect the fulminant global and regional changes initiated by the breakup of northern Pangea. The pattern of reef distribution across the Jurassic reflects a complex mix of (1) different and changing tectonic styles along the continental margins and adjacent shelf seas; (2) sea-level rise and its modulating influence on extrinsic sedimentation; (3) oceanographic and climatic reorganizations related to general sea-level rise and the new plate-tectonic configurations; and (4) evolutionary changes in the ecological demands and abilities of reef biota, which, in part, appear to have been triggered by the extrinsic changes during the breakup of northern Pangea. Rifting and onset of drift in the central Atlantic as well as in the western Tethys resulted in a distinct sea-level rise, which transformed Jurassic shelf seas along the northern Tethys margin from dominantly siliciclastic to dominantly carbonate settings. The opening of the ocean passageway from the Tethys to the Caribbean and Pacific completely reorganized global oceanic circulation patterns. During the Late Jurassic, shelf seas were considerably deep, increasing the areas of settings suitable for development of siliceous sponge mounds on the northern Tethys margin. In contrast, many parts of the southern Tethys margin underwent strong morphological changes due to rift tectonics within the Triassic carbonate platforms, which resulted in a completely different pattern in Jurassic reef distribution relative to the northern Tethys. After the end-Triassic extinction event, Jurassic reefs recuperated gradually during the Early Jurassic, with a first major reef domaindeveloping in Morocco Their temporal distribution through the Middle Jurassic was more balanced, but reefs occurred in scattered domains often distant from each other (e.g., Portugal, France, Madagascar, Iran). Late Jurassic reefs expanded rapidly in the course of the ongoing sea-level rise and the oceanographic reorganization, resulting in mostly interconnected domains. A pattern of waxing and waning of reef abundanceand spatial reef distribution through time is superimposed on this trend. It is again, at least to a large extent, correctable with sea-level fluctuations of greater magnitude. Jurassic reef growth had pea ks during the transgressivc episodes of the Sinemurian-Pliensbachian, Bajocian-Bathonian, and Oxfordian-Kimmeridgian, with superimposed higher-frequency peaks. The Jurassic represents the peak not only of development of Mesozoic coral reefs but equally of development of sponge mounds. Sponge mounds represent siliceous sponge-microbolite mud mounds, which expanded enormously during the Oxfordian along the European part of the northern Tethys. A peculiar type of bivalve reefs, the Lilhiothis reefs, were widespread particularly during the Sinemurian and Pliensbachian, and they might have partially filled a potential reef-growth habitat not occupied by corals, owing to the reduced availability of coral taxa at that time. Bivalve reefs, in particular oyster reefs, also occurred scattered in Middle and Latejurassic times, mostly representing marginal marine environments. Sea-level rise and tectonic opening of new seaways hod a pronounced influence on climate and marine circulation patterns, which were the principal factors in Jurassic reef development. Particularly in the Late Jurassic, coral and stromatoporoid reefs occurred in high paleolatitudinal settings (e.g., Argentina, Patagonia, japan) evidencing strong climatic equilibration of marine and coastal areas, despite the fact that strong seasonal contrasts should have prevailed in the Gondwana interiors. There are only a few records of low-latitude reef sites, despite the availability of carbonate platforms, which might reflect overheated waters in this area. Humidity was probably higher than previously thought. Siliciclastic influx was partially high during the Kimmeridgian, owing to fluvial runoff and renewed tectonic activity, and reduced the number of reef sites and domains considerably, despite ongoing global sea-level rise. Jurassic, chiefly Upper Jurassic, reefs not only grew within the expanding carbonate settings but also thrived in temgeneously influenced environments. This is particularly obvious in the North Atlantic rift basins, such as the Lusitanian Basin of west-central Portugal, but itisalsodiscernibleinmany other Jurassic reef domains. Occurrence of cora! associations in fine siliciclastics, ratios of skeletal low-density vs. high-density banding, morphological adaptations towards sedimentation, high proportions of bioerosion, and overlap of many coral domains with proposed upwelling areas suggests that there was a considerable stock of Jurassic zooxanthellate corals with a distinct heterotrophic proportion of feeding, thus living in mesotrophic settings. In contrast, reefs on the isolated, oceanic shallow-water Apulia-Adria platforms differ considerably, being dominated by stromatoporoids, chaetetids, and corals. We propose the theory that these oceanic faunas might have already had a more advanced photosymbiotic relationship than the other forms and thus could thrive in presumably strongly oligo trophic settings. Such associations, which might have occurred similarly on oceanic platforms in the Pacific, are thought to have been the stock for Cenozoic development of coral reefs into superoligotrophic settings, whereas the more nutrient-tolerant reefs a long the continental margins vanished in the course of latest Jurassic and Berriasian sea-level drop. Sediment-stressed, nutrient-rich shallow-water settings might then have been reconquered by rudist bivalves in the course of the Cretaceous.

Jurassic reefs not only constitute widespread and important hydrocarbon reservoir rocks; their manifold characteristicsand related dependence on basin tectonics, sea-level change, and ecological parameters makes them valuable basin-analysis tools for potential hydrocarbon plays.

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