Pliensbachian (Lower Lias) sediments from Yorkshire contain semicontinuous bands of mudstone cemented by impure siderite. The assemblage of burrows, and faunas associated with them suggest that the siderite was selectivity precipitated close to successive sediment/water interfaces which mark minor nonsequences. These substrates were oxidizing in their upper portions, but reducing at depth. The reduced regions provided ferrous iron which was then precipitated as the oxide and hydroxide near the sediment/water interfaces. During later burial and renewed sedimentation, reduction of this concentrated iron allowed its reaction with CO 2 which resulted from the decay of organic matter. SO4 2 - ions from sea water were excluded by the overlying sediment which thus prevented the formation of much pyrite. Reduced and increased rates of sedimentation alternated to produce the rhythmic alternations of siderite bands and shale.