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Siliciclastic depositional environments are not normally favorable for the growth of reef-building organisms because of high turbidity, reduced salinity, or unfavorable substrate. Yet there are numerous examples, both living and fossil, of close associations, even intermixing, of the two kinds of deposits.
In the Red Sea (Gulf of Aqaba), Holocene coral reefs develop on the seaward margins of inactive alluvial fans of gravel. In the nearshore zones of Brazil (Abrolhos Bank), Mexico (Vera Cruz), and the northern Great Barrier Reef, there are reefs surrounded by siliciclastic sands and silty clays; locally some of this noncarbonate fraction occurs as internal sediment within the reefal frame. In the lagoonal areas of both the Belize (Central America) and Great Barrier Reef tracts, the positions and the geometries of some reefs were probably determined by the local relief (channel banks, bars, deltaic lobes) of the underlying siliciclastic foundations.
Throughout the Phanerozoic, there is a wide spectrum of interaction between reefal carbonates and siliciclastics. Reddish or greenish argillaceous internal sediments are common in some Triassic and Devonian reefs of western Europe. In the Phanerozoic of North America, there are numerous examples of reefs encased in shales or siltstones. In the Triassic of Europe and the Yukon (Canada), reefal carbonates are surrounded by and locally interfinger with volcaniclastics. In the Pennsylvanian, Permian, and Jurassic of North American and in the Permian of Japan, reefal carbonates are juxtaposed with deltaic and associated siliciclastics.
At least two factors relating to exploration emerge from this review of the connections between reefal carbonates and siliciclastics. One is the effect of local relief on the underlying siliciclastics in determining the locations and forms of reefs. The other concerns the combined source and seal provided by fine-grained, peri-reefal siliciclastics.
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