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

Abstract

Wood-grained chert is an unusual type of banded nodular chert that is characterized by alternating thin dark-coloured bands and thicker light-coloured bands, which give the chert a texture reminiscent of the growth rings of wood. The wood-grained texture found in some chert nodules in the Portland Limestone Formation of southern England is the result of variations in the concentration of calcite inclusions within the chert. Light-coloured bands contain more abundant calcite inclusions than adjoining darker coloured chert bands. The Portland Limestone wood-grained cherts formed by the late diagenetic replacement of a highly compacted carbonate sand, in which abundant siliceous sponge spicules were the silica source. Oxygen and hydrogen isotope data (δ¹⁸O=27.0 to 29.3‰; δ¹⁸O=−71 to −91‰ V-SMOW), indicate that either chert formation or, more likely, the opal-CT to quartz transformation, occurred in meteoric or mixed marine and meteoric pore waters at temperatures in the range of 20–60°C. The wood-grained texture likely formed by a self-organization process. A feedback mechanism is proposed for the origin of the texture in which calcium and carbonate ions released during the replacement of the host limestone resulted in calcite supersaturation along the boundaries of growing chert nodule. Calcite inclusion-rich chert bands formed within the calcite supersaturated zone and inclusion-poor bands formed outside of the calcite-supersaturated zone. Wood-grained chert provides another example of how the coupling of reaction and transport can produce repetitive patterns in otherwise unordered sediments or rock.