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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 49 (1965)

Issue: 3. (March)

First Page: 338

Last Page: 338

Title: Vadose Pisolite in the Capitan Reef: ABSTRACT

Author(s): Robert J. Dunham

Article Type: Meeting abstract


New evidence on the origin of the pisoliths of the Permian Capitan Reef Complex indicates that they are not of lagoonal algal-nodule origin, but are early vadose concretions and probably Permian pisolitic caliche. If the reinterpretation is valid, the pisolite implies that (1) the climate was dry, (2) the reef complex was subaerially exposed repeatedly during its growth, with attendant opportunities for diagenetic alteration of porosity and for "inorganic binding," and (3) the paleotopographic crest on the complex was not in the so-called organic reef-rock, that is, in the sponge-bearing lime wackestone of the Capitan facies, but instead was in the dolomitized grainstone of the Carlsbad facies.

Basic to the older interpretation is the requirement that the pisoliths rolled about during growth. [Algae cannot grow downward so as to encrust the bottom side of objects at rest, due to their need for light and to the resistance of the substrate.] Evidence of in situ downward growth (consisting of fitted polygonal structure, downward elongation of pisoliths, and inclusions of silt perched in the upper parts of concentric growth layers), together with the lack of admixed sediment and of sedimentary structures characteristic of gravel-size deposits, indicate that the pisoliths are not algal nodules, nor even transported sediment, but are concretions.

Evidence that the growth of pisoliths was commonly interrupted by leaching and nontectonic fracturing and was closely associated with cementation and internal sedimentation of silt requires that the concretions grew in a diagenetic environment characterized by complex variability and by water moving rapidly enough to transport silt. The vadose zone along seems to meet this requirement. The Permian pisoliths significantly resemble known vadose concretions, especially those of pisolitic caliche.

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Copyright 1997 American Association of Petroleum Geologists