Abstract: Abstract: Diagenesis of Tertiary Volcaniclastics, Guatemala

David K. Davies, William R. Almon

Tertiary volcaniclastic rocks from the fore-arc trough in Guatemala are moderately well cemented and faulted, but are not folded. Exposures in and near the Pacific coastal plain reveal exclusively terrestrial sequences, the sediments having been deposited principally from lahars and stream floods, with minor contributions from air-fall deposition and glowing avalanches. The rocks consist dominantly of fragments of pyroxene-rich andesite (55%), and subordinate amounts of free feldspar and pyroxene crystals (20%), set in extensive diagenetic pore fill (25%). Floating grains are common, and little evidence of compaction is present.

The diagenetic sequence is I hematite, II goethite, III montmorillonite plus hematite, IV heulandite plus montmorillonite. Thermodynamic considerations indicate that the pore fill originated as an aqueous precipitate from an acid (pH 4 to 6), silica-rich (10^-2 to 10^-3) water of probable meteoric origin. The diagenetic sequence is interpreted as reflecting the progressive weathering of rock components, commencing with the more labile constituents (ferromagnesian minerals) and progressing to relatively more stable constituents (feldspars). Cementation of these volcaniclastic sediments probably occurred under near-surface conditions (less than 300 m burial) at temperatures between 25 and 60°C.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado