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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 52 (1968)

Issue: 3. (March)

First Page: 536

Last Page: 536

Title: Modes of Formation, Sedimentary Associations, and Diagnostic Features of Shallow-Water and Supratidal Evaporites: ABSTRACT

Author(s): D. J. J. Kinsman

Article Type: Meeting abstract

Abstract:

Production of a brine depends critically on favorable geometry and climate. The required geometry rarely is achieved on a large scale but commonly is developed on a small scale in the form of lagoons, pans, and supratidal areas, during processes of nearshore sedimentation; large brine basins normally will exhibit these environments peripherally. The small size and exposed nature of shallow brine bodies and supratidal surfaces give them an inherent instability, e.g., short-term, extreme temperature fluctuations, brine dilution, or brine concentration. These instabilities may be reflected by features of the sediments and evaporite minerals.

Stability and kinetic data indicate that gypsum is always the first calcium sulfate mineral precipitated and that anhydrite is either an early or late post-burial diagenetic mineral. Gypsum crystals precipitated from brine bodies normally lack inclusions and are elongated whereas those precipitated diagenetically within supratidal sediments or below the sediment-brine interface in brine pans commonly contain inclusions of host sediment and are lensoid and stubby. Rate of crystal growth is thought to account for these differences in crystal morphology. Anhydrite in supratidal sediments exhibits a nodular structure. Recent occurrences in the Persian Gulf and Baja California indicate that anhydrite is precipitated where mean annual air temperatures exceed 22°C., only gypsum being pr cipitated below this temperature. Gypsum and anhydrite emplaced diagenetically within supratidal sediments are in places crudely layered but not laminated; brine-pan environments may exhibit sediment-evaporite laminae, each couplet representing a period of dilution and evaporation. Elongate gypsum crystals occur in the evaporite laminae whereas early diagenetic stubby gypsum crystals may occur in both evaporite and sediment laminae.

Several other evaporite minerals may be developed in these environments. Celestite occurs as a trace mineral. Halite may be dominant under high net evaporation conditions and a triplet of sediment-gypsum-halite may be developed as a result of one dilution-evaporation cycle, the halite normally being redissolved, however, at the next influx of dilute water. Polyhalite may replace early gypsum under very high net evaporation conditions. Small amounts of bassanite may result from near-surface dehydration of gypsum in supratidal environments. If local sediments are carbonate, then dolomitization is common. Magnesite and huntite are other recorded minor carbonate minerals. Some of these minerals are stable for only limited periods of time or under a limited set of conditions and are not ca ried into the subsurface.

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