Alteration of sandstone as revealed by spheroidal weathering

ABSTRACT Many phyllosilicate-bearing sandstones in central and southern West Virginia have undergone considerable spheroidal weathering. The formation of the concentric shells was apparently promoted by expansion of phyllosilicates as the shells developed in sandstones with micaceous and argillaceous material and are completely absent in the clean quartzose sandstones. Comparison of the outer shells of the spheroidally weathered sandstones with the fresh cores yields important information on alteration of sandstones. Slight expansion of phyllosilicates on weathering at shallow depths leads to weakening of bonds in sandstones so that relatively rapid breakdown occurs on exposure. Even though the sandstones may be stained brown from oxidation of iron-bearing minerals, they will be durable if the phyllosilicates have not altered significantly. Some sandstones are weakened by near surface alteration even though they remain gray in color. The weakened sandstones lack resilience when struck with a hammer and will break down relatively rapidly on exposure. Near surface weathering caused marked dissolution of feldspar without the formation of clay minerals so that appreciable secondary porosity resulted. Weathered zones in feldspathic sandstones preserved below unconformities could form significant porosity for potential hydrocarbon reservoirs. Dissolution of feldspar in crystalline rocks below unconformities could also lead to favorable reservoirs to trap hydrocarbons.