The Plio-Pleistocene nonmarine volcanic sandstones of the Cagayan basin, Philippines, have been significantly altered by early dissolution and cementation processes. The amount and type of alteration vary by formation, depth, and age of the deposit. Plio-Pleistocene fluvial sandstones (litharenites and feldspathic litharenites) buried to depths of 400-900 m, are only slightly compacted, but contain significant amounts of authigenic pore-lining clay and zeolites. Dissolution of plagioclase, heavy minerals, and volcanic rock fragments has occurred in nearly all samples, dissolving up to one-half the framework grains and increasing thin-section porosity to as much as 40%. The overlying Pleistocene sandstones are compositionally different (lithic arkoses and arkoses) and have not been as extensively affected by diagenetic processes. the more extensive alteration of the Plio-Pleistocene sandstones reflects increased diagenetic alteration with burial depth and time as a result of relatively high pore-fluid flow rates in shallow alluvial deposits.

The diagenesis of the Cagayan basin Plio-Pleistocene sandstones indicates that significant secondary porosity can develop in nonmarine volcaniclastics as a result of early silicate dissolution during shallow burial diagenesis. Early dissolution and secondary porosity development have important implications for studies of nonmarine volcaniclastics. Early dissolution processes distort provenance, tectonic setting, and depositional environment interpretations based on the detrital mineralogy of older volcaniclastic sediments. Secondary porosity increases the reservoir quality of volcaniclastics prior to more extensive compaction and cementation. Recognition of similar shallow volcaniclastic reservoirs in the past may have been limited because of low resistivity sand identification proble s caused by authigenic smectite.