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 banded patterns concentrically concretions from Jurassic Navajo Sandstone (United States) claystone ‘zebra rock’ Neoproterozoic Johnny Cake Shale Member (Western Australia). Micro‐X‐ray fluorescence imaging both samples revealed uniformly asymmetrical Fe content peaks. proposes their formations were caused diffusion, with a pH buffering reaction 2+ O 2 due to dissolution earlier materials acidic fluids. Based model, it is suggested acid fluids for diffused towards inner side concretion, Kimberley they penetrated oblique or perpendicular bedding. In addition, precipitation has resulted rises because diagenetic alteration calcite concretion in pyrite‐bearing Kimberley. The interpretation based δ 34 S values (< –20‰) zebra rock, suggesting alunite supply SO 4 2– produced as result pyrite oxidation rocks. rate band was estimated constant width peaks expected diffusion coefficient through rock matrix, found be order years tens