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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 3. (March)

First Page: 469

Last Page: 469

Title: Carbonate Rocks of Montoya Group (Middle and Upper Ordovician) of Trans-Pecos Texas: ABSTRACT

Author(s): Herbert J. Howe

Article Type: Meeting abstract

Abstract:

In the petroliferous Permian basin of west Texas the subsurface Montoya Formation consists of a monotonous sequence of dolomitic limestones. However, 100 to 200 mi (160 to 320 km) west in the Franklin and Hueco Mountains of Texas and the Cooks Range and Sacramento Mountains of New Mexico, the Montoya is divided into three distinctive carbonate formations (in ascending order): the Upham Limestone, the Aleman Limestone, and the Cutter Limestone.

Study of microscopic sections shows several carbonate lithologies including: (1) crinoidal calcarenite with calcareous mud matrix (biomicrite); (2) crinoidal calcarenite with clear calcite cement (biosparite); (3) micrites with abundant cherty nodules and layers of interbedded chert; (4) laminated micrites without chert; (5) shelly limestones (mainly brachiopodal biomicrites); (6) autochthonous reef rock (coralline biolithite); and (7) partly or completely dolomitized equivalents of any of the former.

Crinoidal calcarenites with a calcareous mud matrix characterize the Upham Limestone except for the uppermost beds. There, shallow-water, high-energy conditions apparently winnowed out the calcareous mud, which is replaced by clear calcite cement. Cherty and chert-free micrites and biomicrites form the dominant lithologies of the overlying Aleman and Cutter Limestones.

At some localities, especially near faults, dolomitization is massive, cutting across all facies and rock layers. Dolomitization progresses from sporadic small crystals embedded in the original matrix to total replacement where original features are obscured or destroyed. Layered dolomites are most common in the Cutter Formation.

Montoya deposition is cratonic, averaging only 320 ft (96 m) over about 10 m.y. Individual rock units representing specific shallow-water epineritic environments may be traced widely (in some places more than 100 mi; 160 km).

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