An alternative to the ‘clastic trap’ interpretation of oolitic ironstone facies

Abstract

SummaryThe rarity or absence of clastic grains in oolitic ironstones is often explained by the hypothesis of a ‘clastic trap’ in which the clastic grains are deposited before reaching the zone of iron oolith formation. If the hydrodynamic behaviour of the iron ooliths and clastic grains is considered, this is unnecessary. The two components are spatially separated due to their different densities and different behaviour, depending on whether they are in suspension or saltating, or rolling. Iron ooliths can be rolled more easily than the equivalent clastic grain size, whereas clastic grains are more easily saltated or moved in suspension than the equivalent size of iron oolith. These differences cause spatial separation of the two facies.