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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 3. (March)

First Page: 503

Last Page: 503

Title: Field Relations of Some Pre-Tertiary Dolomites in Great Basin: ABSTRACT

Author(s): Kathryn M. Nichols

Article Type: Meeting abstract

Abstract:

In marine, pre-Tertiary, carbonate-platform deposits, primary and secondary dolomites are commonly associated. "Primary" dolomite is used here in the sense of penecontemporaneous replacement of calcareous sediments essentially at the time of their deposition; primary dolomite preserves microcrystalline textures and fabrics indicative of inner-platform sedimentation. "Secondary" dolomite is used here in the sense of post-depositional replacement of limestone or calcareous sediment by the progressive, slow growth and coalescence of discrete dolomite crystals. Secondary dolomite is generally characterized by saccharoidal texture, which may be formed at different diagenetic stages, as well as by hydrothermal processes. The boundaries of secondary dolomites may cross-cut strat fication surfaces, thereby making depositional and paleogeographic interpretation difficult.

Primary and secondary dolomites in the Great Basin commonly show a distinctive and recurrent pattern of spatial relations. In ascending order, marine limestone, which may be of any facies, is supplanted by sucrosic secondary dolomite, which in turn is overlain by primary dolomite. The boundary between the unaltered limestone and secondary dolomite is commonly a zone of mottled limestone and dolomite in which nodules or beds of secondary chert are present. The boundary between primary and secondary dolomite is abrupt and, in places, is an unconformity of regional extent. The regionally coextensive occurrence of primary and secondary dolomite indicates that secondary dolomitization is related to the formation of the overlying primary dolomite or to the surface on which it was formed, an is thus eogenetic. With the wide range of possible scenarios affecting the water chemistry of carbonate platforms, either the reflux or mixed-water hypothesis could be adapted to explain this pattern of coextensive occurrence of primary and secondary dolomites. For example, primary-dolomite areas could be associated with the concentration of magnesium or, through exposure, could be the avenue through which meteoric water is introduced into the system.

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