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Book Chapter

Karst Development on Carbonate Islands

By
John E. Mylroie
John E. Mylroie
Mississippi State University Mississippi State, Mississippi, U.S.A.
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James L. Carew
James L. Carew
University of Charleston Charleston, South Carolina, U.S.A.
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Published:
January 01, 1995

Abstract

Karst development on carbonate platforms occurs continuously on emergent portions of the platform. Surficial karst processes produce an irregular pitted and etched surface, or epikarst. The karst surface becomes mantled with soil, which may eventually result in the production of a resistant micritic paleosol. The epikarst transmits surface water into vadose pit caves, which in turn deliver water to a diffuse-flow aquifer. These pit caves form within a 100,000 yr time frame. On islands with a relatively thin carbonate cover over insoluble rock, vadose flow perched at the contact of carbonate rock with insoluble rock results in the lateral growth of vadose voids along the contact, creating large collapse chambers that may later stope to the surface.

Carbonate islands record successive sequences of paleosols (platform emergence) and carbonate sedimentation (platform submergence). The appropriate interpretation of paleosols as past exposure surfaces is difficult, because carbonate deposition is not distributed uniformly, paleosol material is commonly transported into vadose and phreatic voids at depth, and micritized zones similar in appearance to paleosols can develop within existing carbonates.

On carbonate islands, large dissolution voids called flank margin caves form preferentially in the discharging margin of the freshwater lens from the effects that result from freshwater/saltwater mixing. Similarly, smaller dissolution voids also develop at the top of the lens where vadose and phreatic freshwaters mix. Independent of fluid mixing, oxidation of organic carbon and oxidation/reduction reactions involving sulfur can produce acids that play an important role in phreatic dissolution. This enhanced dissolution can produce caves in freshwater lenses of very small size in less than 15,000 yr. Because dissolution voids develop at discrete horizons, they provide evidence of past sea level positions. The glacio-eustatic sea level changes of the Quaternary have overprinted the dissolutional record of many carbonate islands with multiple episodes of vadose, freshwater phreatic, mixing zone, and marine phreatic conditions. This record is further complicated by collapse of caves, which produces upwardly prograding voids whose current position does not correlate with past sea level positions.

The location and type of porosity developed on emergent carbonate platforms depend on the degree of platform exposure, climate, carbonate lithology, and rate of sea level change. Slow, steady, partial transgression or regression will result in migration of the site of phreatic void production as the freshwater lens changes elevation and moves laterally in response to sea level change. The result can be a continuum of voids that may later lead to development of solution-collapse breccias over an extended area.

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Contents

AAPG Memoir

Unconformities and Porosity in Carbonate Strata

David A. Budd
David A. Budd
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Arthur H. Saller
Arthur H. Saller
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Paul M. Harris
Paul M. Harris
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American Association of Petroleum Geologists
Volume
63
ISBN electronic:
9781629810843
Publication date:
January 01, 1995

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