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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 763

Last Page: 763

Title: Cyclic Hydrologic and Diagenetic Events in San Andres Formation: Geologic Implications: ABSTRACT

Author(s): Alonzo D. Jacka, William E. Barone, Nancy J. Muir

Article Type: Meeting abstract


In the San Andreas Formation in Roosevelt, Curry, Quay, and DeBaca Counties, New Mexico, cyclic hydrologic and diagenetic events reflect rhythmic fluctuations in sea level, climate, and sedimentation.

Depositional cycles in the San Andres record: (1) rapid transgressions which accompanied eustatic rises in sea level; (2) seaward progradations of subtidal, intertidal, and supratidal (sabkha) facies during stillstands of sea; (3) subaerial exposure during lowstands of sea level; and (4) continuous subsidence of the area. Effects of fluctuation in sea level, climate, and sedimentation were transmitted to San Andres carbonates through changes in hydrology which produced distinctive diagenetic overprints.

The following cyclic hydrological events are recorded in many San Andres intervals: (1) dolomitizing fluids dolomitized initially unstable carbonates; (2) hypersaline brines emplaced anhydrite as partial to nearly complete replacement of carbonate (anhydritization) and as cement; and (3) low-salinity ground waters dissolved much anhydrite, brought about replacement of anhydrite by gypsum, hemihydrate, calcite, and silica and also caused dedolomitization. Many San Andres carbonate intervals bear evidence of having been subjected to overprints of two cyclic fluid invasion sequences.

Nearly all primary and secondary porosity in dolostone has been occluded by dolomite, anhydrite, and calcite cements. In limestones (exclusive of dedolostones) nearly all primary and secondary porosity has been occluded by calcite and anhydrite cements. Many San Andres limestones were formed by paramorphic replacement of dolomite by calcite (dedolomitization). Secondary intercrystalline and moldic porosity in the original dolostone have been preserved after dedolomitization.

Diagenesis and porosity relations within the San Andres reflect the relative flux of dolomitizing, anhydritizing, and low-salinity fluids within a given interval. Tertiary porosity is extensively preserved in intervals in which the low-salinity stage of a "hydrologic cycle" has not had its overprint superseded by dolomitizing or anhydritizing fluids of a subsequent "cycle." The most highly porous intervals contain both secondary intercrystalline and anhdrite moldic porosity.

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