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Atoll lagoon patch-reefs in modern (Enewetak, Marshall Islands) and Middle Jurassic (High Atlas Mountains, Morocco) settings exhibit geologically instantaneous cementation.
Three kinds of cements are pervasive in a shallow-water (<= 7 m, 23 ft) patch-reef in the lagoon at Enewetak, precipitated in skeletal and other voids in samples that radiocarbon date from 700 to 1,800 years B.P. Aragonite, blocky Mg-calcite and nannocrystalline Mg-calcite all are present in the same thin sections. The latter appears as micrite in thin section, but is clearly crystalline cement as seen with the SEM. At least two of the cements commonly occur in the same void, but they never appear intergrown, suggesting that subtle (and not understood) changes in chemical environment control episodes of cementation. The same sequence of cements commonly occurs in adjacent voids (cavities, boring, corallite cavities, intergranular, etc); where a typical sequence is: blocky Mg-calcit rims followed by aragonite sprays (usually nucleated around loose carbonate debris) and/or nannocrystalline Mg-calcite cementing fine-grained loose debris. An important in-situ process involves development of borings which became filled with rapidly cemented debris, converting parts of the reef from skeletal carbonate to cemented debris in situ. Petrographically, the fabric of the reef is converted to packstones and wackestones, with little of the original boundstone preserved for the petrographer.
The exhumed Middle Jurassic atoll near the town of Rich in the Central High Atlas of Morocco exhibits a rim of pinnacle reefs surrounding a lagoon with hundreds of patch reefs, identical with modern atolls. Evidence for pre-burial marine cementation includes multiple episodes of cross-cutting borings that have been filled with debris and cemented in the time between borings; attached epibiontic organisms over cemented reef-and-mud masses; cascaded beds of crinoid, brachiopod, bivalve, etc, debris down the flanks of cemented reef-and-debris mounds: and over-steepened flanks of these mounds. A thorough recrystallization makes determination of cement types difficult in the Moroccan examples. However, by comparison with modern examples, we reinterpret the "mud" fillings of cavities, coral ites, etc, as representing original Mg-calcite cement rather than low-energy muds.
At both localities no evidence exists for subaerial cementation. Both are associated with abundant muddy substrates, although well ventilated by tidal currents and choppy waves in the lagoon at Enewetak, and affected to some extent by waves and currents in Morocco. These examples indicate that lagoon reefs can be tightly bound by marine cements, losing porosity before burial, as can reefs in higher energy surf zones as documented in Belize and elsewhere.
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