Abstract

Porosity-permeability values of sandstones of the Lower Triassic Manda Beds have been severely reduced by authigenic laumontite. The formation of laumontite took place preferentially in bioturbated arkosic sandstones, both as interstitial cement and as replacement nodules. R 0 values on coal implying maximum temperature of 75 degrees C and disruption of laumontite cement by roottubes suggest that laumontite formed during early diagenesis. The Manda Beds were deposited in climatically-controlled upward fining and desiccating cyclothems of a fluvio-deltaic depositional regime. Homoaxial quartz overgrowth which is the first diagenetic phase was effected during modification of initially low pH pore waters to more alkaline conditions. This event was followed by the formation of laumontite reflecting a low P CO2 and a change in pH values from acid to alkaline. In regions of higher P CO2 , calcite cement was formed in preference to laumontite. Frequently, laumontite was also displaced by a later calcite phase.

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