Self-organization origin of wood-grained chert, Portland Limestone Formation (Upper Jurassic), southern England

Wood-grained chert is an unusual type of banded nodular chert that is chara cterized by alternating thin dark-coloured bands and thicker light-coloured bands, which give the chert a texture reminiscent of the growth rings of w ood. The wood-grained texture found in some chert nodules in the Portland L imestone Formation of southern England is the result of variations in the c oncentration of calcite inclusions within the chert. Light-coloured bands c ontain more abundant calcite inclusions than adjoining darker coloured cher t bands. The Portland Limestone wood-grained cherts formed by the late diag enetic replacement of a highly compacted carbonate sand, in which abundant siliceous sponge spicules were the silica source. Oxygen and hydrogen isoto pe data (delta(18)O = 27.0 to 29.3 parts per thousand; delta(18)O = -71 to -91 parts per thousand V-SMOW), indicate that either chert formation or, mo re likely, the opal-CT to quartz transformation, occurred in meteoric or mi xed marine and meteoric pore waters at temperatures in the range of 20-60 d egrees C. The wood-grained texture likely formed by a self-organization pro cess. A feedback mechanism is proposed for the origin of the texture in whi ch calcium and carbonate ions released during the replacement of the host l imestone resulted in calcite supersaturation along the boundaries of growin g chert nodule. Calcite inclusion-rich chert bands formed within the calcit e supersaturated zone and inclusion-poor bands formed outside of the calcit e-supersaturated zone. Wood-grained chert provides another example of how t he coupling of reaction and transport can produce repetitive patterns in ot herwise unordered sediments or rock.