Название: FLANK MARGIN CAVE DEVELOPMENT AS SYNDEPOSITIONAL CAVES: EXAMPLES FROM THE BAHAMAS
Источник: 15th International Congress of Speleology
Место публикации: USA
Том:
Выпуск:
Страницы: 533-539
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№ полки:
Добавлено: Elmaz
Ключевые слова: FLANK MARGIN CAVE, SYNDEPOSITIONAL CAVES, THE BAHAMAS, Багамские острова, генезис пещер
Источник:
Реферат: In 1964 Joe Jennings introduced the term syngenetic karst to describe karst formation occurring in carbonate sands as those sands consolidated. The term “consolidation” was somewhat open-ended and Jennings treated it as any carbonate rock undergoing initial lithification and early diagenesis, a rock category carbonate geologists now call eogenetic. Joe Jenning’s definition was based on work in eolian calcarenites of southern and western Australia. His syngenetic karst term was subsequently reviewed in 2007 by Australian karst scientists Ken Grimes and Susan White, in conjunction with the coauthors, to create a hierarchy of caves based on age and place in the diagenetic cycle. They stated that the earliest cave genesis is constructional cave formation, as in tufa caves and reef macro-porosity, which occurs simultaneously with carbonate rock deposition but without in situ dissolution. Syngenetic caves, according to their view, form in two ways: by dissolution in unlithified and still depositing sediments, as syndepositional caves; and by dissolution in lithified but diagenetically-immature carbonate rocks, as eogenetic caves. With time, carbonate burial results in diagenetic maturity, and creates mesogenetic caves produced, by definition, through hypogenic processes. Re-exposure of diagenetically-mature carbonates on the earth’s surface results in the most common cave type, telogenetic caves. Marble caves form in metamorphosed carbonates derived from mesogenetic or telogenetic conditions. Work in the Late Pleistocene carbonates of the Bahamas has demonstrated that syndepositional caves exist. Altar Cave and Chinese Fire Drill Cave, San Salvador Island, and A-Survey Cave, New Providence Island, are all flank margin caves developed in carbonate rocks of the Cockburn Town Member of the Grotto Beach Formation. These rocks were deposited during the last interglacial sea-level highstand (MIS 5e) from 131 to 119 ka. The Bahamas are tectonically stable, and for dry phreatic caves to exist in Cockburn Town Member carbonate rocks requires that the fresh-water lens subsequently invaded these carbonates on the same sea-level highstand that deposited the carbonate sediments. The carbonate units involved are progradational, they resulted from excess carbonate sediment supply infilling lagoons, allowing beach, back beach, and dune carbonates to prograde into the lagoonal depositional environment. The fresh-water lens then advanced into this new subaerial environment, and dissolutional hydrology was established in the lens margin. The carbonate sediments were precipitated, transported and deposited, then flank margin caves were dissolved within them, in a time span of less than 12,000 years. Cementation must have been sufficient to allow voids to withstand collapse, yet dissolution occurred in the very same environment where carbonate deposition was on-going. We consider this situation to be an example of syndepositional cave development.