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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 74 (1990)

Issue: 2. (February)

First Page: 119

Last Page: 134

Title: Occurrence and Significance of Magnesite in Upper Permian (Guadalupian) Tansill and Yates Formations, Delaware Basin, New Mexico (1)

Author(s): R. A. GARBER, P. M. HARRIS, and J. M. BORER (2)


Magnesite (MgCO[3]) occurs pervasively in a 270-ft (82-m) cored interval of Upper Permian (Guadalupian) shelf deposits from the northern rim of the Delaware basin portion of the Permian basin, New Mexico. In our core example, magnesite is found in tidal flat/lagoon and pisolite shoal dolomites and siltstones of the Tansill and uppermost Yates formations. The interval is overlain by magnesite-bearing anhydrite and a thick halite section of the (Ochoan) Salado Formation. The basinwide extent of magnesite is unknown.

The texture of magnesite in core slabs, thin sections, and SEM (scanning electron microscope) mounts is nearly identical to that of dolomite. The magnesite consists of anhedral to euhedral crystals ranging in size from 1 to 4 mm. Within the dolomite intervals of the core, magnesite occurs in three forms: (1) patches of crystals and as replacement of allochems, both in an otherwise dolomite matrix, (2) pervasive replacement of host dolomudstone and peloidal and pisolitic dolowackestone/packstone, and (3) discrete layers interbedded with anhydrite. Magnesite rarely fills pore spaces in dolomite, but within siliciclastic beds, magnesite occurs both as a first-generation cement and as a replacement of carbonate grains. Magnesite and dolomite are mutually exclusive in siliciclastic beds wi hin the core, and magnesite occurs only where anhydrite is abundant.

Magnesite may have formed either (1) during Ochoan deposition or thereafter, after burial of the Tansill and Yates formations, from dense brines originating from the overlying Salado evaporites; or less likely, (2) syndepositionally with the Tansill and Yates sediments. Preliminary measurements of stable carbon and oxygen isotopes for magnesite yield normal Permian values for (sigma){13}C averaging +6.84 parts per mil (PDB) and slightly evaporitic values for (sigma){18}O averaging +1.04 parts per mil (PDB); corrected {87}Sr/{86}Sr isotope composition averages 0.70687.

Because a high content of associated uranium in the magnesite-rich part of the core causes large gamma-ray deflections similar to those for shale, and because the density of magnesite is close to that of anhydrite, the presence of magnesite could lead to improper evaluation of lithology and porosity from logs and could ultimately result in failure to recognize potential reservoir zones.

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