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Abstract:

Field and petrographic studies of middle? to upper Miocene carbonate rocks near the town of Níjar (Almería Province, southeastern Spain) document the effects of relative sea-level change on sedimentation and biotic composition. Erosion, prograding and down-stepping beds (producing down-lap surfaces), or alteration due to subaerial exposure signify relative sea-level fall. Onlapping geometries, extensive bioturbation, channeling, or crossbedding characterize periods of relative rise.

Geometries of and within seven depositional units, variation in grain size and fossil constituents, sedimentary structures, and slight variation in color help differentiate the carbonate rocks in the field. The seven recognizable depositional units can be grouped into three broad associations: pre-reef, reef, and post-reef. Reefs did not develop during deposition of the older half of the section at Níjar; reef complexes (fringing types) are evident only in the younger half of the section. Reef-slope sediments are well preserved and exposed, but erosion truncated part of the reef core. The coral Porites and associated micritic coatings (probably microbial) constructed the reef framework. A variety of mollusks, coralline algae, serpulid worms, and bryozoans lived in association with Porites. The green calcareous alga Halimeda and the coralline alga Mesophyllum seem to have colonized the reef slope.

Syndepositional alteration (micro- and macro-boring) and cementation, fabric-selective dissolution of predominantly aragonitic components, dolomitization, and calcite cementation modified original mineralogy, fabric, and porosity.

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