Abstract: Review of Recent Carbonate Lagoonal Sedimentation

Clifton F. Jordan

For interpreting ancient depositional environments, a "lagoon" is considered to be a shallow body of marine water that may be brackish, marine, or hypersaline; that is protected from high marine energies by any of several types of barriers; that has somewhat restricted water circulation; and that collects muddier and/or finer grained sediment than adjacent environments.

Barriers on at least some sides of a lagoon are essential for dampening marine currents, swell, and wind-driven waves, and thereby provide some degree of shelter for the lagoonal environment. Barriers also restrict water circulation and can have considerable effects in altering lagoon salinities. In humid climates, lagoon barriers permit less mixing of fresh and marine waters by confining freshwater runoff from swamps. In arid climates, extensive barriers permit deposition of evaporite sediments. Consequently, these variations in salinity affect the fauna and flora developed in the lagoon.

Reviewing of 20 Holocene lagoons from around the world indicates five distinctive types: coastal, atoll, shelf, backreef/backbar, and interreef lagoons. The most significant indigenous lagoonal plants and animals include Halimeda, Penicillus, Thalassia, mangroves, mollusks, and foraminifers. Thalassia and mangroves serve as dense vegetative baffles to trap sediment, whereas the other forms contribute their calcareous skeletons to the sediment.

Lagoonal deposits are characterized by a high lime mud content and abundant molluscan and algal particles. Equivalent lagoonal sediments in the geologic record are described as molluscan or algal wackestones and mudstones, with minor amounts of packstones.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC