Abstract

Volcanic sedimentary rocks of the Veija Group of Trans-Pecos Texas contain zeolite, montmorillonite, and silica minerals that formed during diagenesis in an open hydrologic system. Volcanic glass shards dissolved in ground water to provide constituents for authigenic minerals. Diagenic mineral zones, from top to bottom, are (1) montmorillonite-opal-glass, (2a) montmorillonite-opal-clinoptilolite, (2b) montmorillonite-quartz-clinoptilolite, (3a) montmorillonite-quartz-analcime, and (3b) analcime-quartz. During diagenesis the original vitroclastic texture of volcanic sediment was preserved. Montmorillonite formed coatings on glass shards that preserved their outline during replacement by clinoptilolite. Clay or clinoptilolite cement filled much of the remaining inter-granular space. Analcime replaced both pseudomorphs of shards and interstitial cement that first formed as clinoptilolite. All authigenic minerals formed at low temperature and low pressure at burial depths no greater than a few hundred meters. Rocks in each zone were buried less deeply, and boundaries between zones are stratigraphically higher in the northeastern than in the southwestern part of the area. Distribution of clinoptilolite and analcime were controlled locally by permeability of the host rocks. Diagenesis converted sediment with an original composition similar to that of rhyolite into rock composed of Na2O, CaO, Al2O3, and SiO2, but relatively depleted in K2O. Field evidence shows that diagenesis occurred in early Oligocene time, but zeolites began to form only after a critical thickness of a few hundred meters of sediment had accumulated.

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