ORIGIN OF ALLOPHANE AND RETARDATION OF PEBBLE WEATHERING IN QUATERNARY MARINE TERRACE DEPOSITS

Quaternary marine terrace deposits consisting of gravels interbedded with thin sandy gravel layers have been subjected to subaerial weathering. Restrictedly within the sandy gravel layers, allophane gel either replaced bytownite sands to form a pseudomorph or coated pebbles. The allophane has an average Al/Si atomic ratio of 1.5 with 45% H₂O. The sandy gravels were originally rich in bytownite (av. An₈₆) sands derived from underlying Tertiary basaltic lapilli-tuff. The highly soluble and aluminous bytownite favored the formation of allophane. In the sandy gravel layers, pebbles coated with allophane gel were almost fresh while those in the gravel layers were highly weathered to form halloysite-rich clays. Allophane gels acted as a somewhat impermeable geochemical barrier impeding a mineral-water reaction in the bytownite-rich sandy gravel layers and thus significantly retarding pebble weathering, while prolonged weathering in the gravel layers resulted in the severe decomposition of pebbles. Bytownite sacrificed itself for protection of the pebbles from weathering, implying that minor soluble minerals might be one of the factors in the natural variation of the weathering rates of rocks and sediments.