Rhythmic iron‐oxide bands of Navajo Sandstone concretions and Kimberley banded claystone: Formation process and buffering reaction rate by diagenetic alteration

Abstract Both igneous and sedimentary rocks affected by water–rock interaction commonly form rhythmic Fe‐oxide bands that provide information on palaeo‐groundwater. This study investigated the formation of Fe‐oxide banded patterns of concentrically banded concretions from the Jurassic Navajo Sandstone (United States) and banded claystone ‘zebra rock’ from the Neoproterozoic Johnny Cake Shale Member (Western Australia). Micro‐X‐ray fluorescence imaging of both samples revealed uniformly banded patterns of asymmetrical Fe content peaks. This study proposes that their formations were caused by diffusion, with a pH buffering reaction of Fe 2+ and O 2 due to dissolution of earlier materials by acidic fluids. Based on the model, it is suggested that the acid fluids for Navajo diffused towards the inner side of the concretion, and that for Kimberley they penetrated oblique or perpendicular to bedding. In addition, the Fe‐oxide precipitation has resulted from pH rises of acidic fluids because of diagenetic alteration of earlier calcite concretion in Navajo and that of earlier pyrite‐bearing claystone in Kimberley. The interpretation for Kimberley is based on the δ 34 S values (<−20‰) of zebra rock, suggesting that the alunite and Fe‐oxide resulted from the supply of and Fe 2+ produced as a result of pyrite oxidation in earlier rocks. The formation rate of the Fe‐oxide band was estimated from the constant width of the Fe peaks and the expected diffusion coefficient for Fe 2+ through the rock matrix, and it was found to be on the order of years for Navajo and on the order of tens of years for Kimberley.