Glauconitic peloids and chamositic ooids - favorable factors,constraints, and problems

Glauconitic minerals constitute a family ranging from green smectite to a 10Ådioctahedral mica (glauconite). Chamositic minerals include a 7Åtrioctahedral serpentine (berthierine) and a 14Åtrioctahedral chlorite (chamosite). These green iron-rich, neoformed or transformed clay minerals are most commonly concentrated in sand-size granules.Recent berthierine and Recent and ancient glauconitic minerals occur mainly in structureless peloids, most of which are believed to have been fecal pellets. In contrast, most of the ancient chamositic minerals are in multi-coated ooids generally assumed to have been made by gentle rolling on the sea floor.Glauconitic and chamositic granules accumulated most commonly in marine shelf environments during episodes of reduced influx of sediment. In modern deposits chamositic peloids predominate on the inner shelf, whereas glauconitic peloids are most abundant on the middle and outer shelf. In general, ancient glauconitic and chamositic deposits had a rather similar environmental distribution; in detail, however, they reflect more varied and overlapping marine habitats.Glauconitic greensands and chamositic ironstones commonly occur above a coarsening- or shoaling-upward facies sequence. Many of them are cross-bedded and burrowed, and some are interbedded with a ferruginized or phosphatized hardground. Although differing in detail, their temporal distributions throughout Phanerozoic time were rather similar. Both attained a maximum when cratonic blocks were widely dispersed and sea level was high in Early Paleozoic and Late Mesozoic time. In addition, recurring development of chamositic ooids commonly coincided with repeated regional transgressions.This review of current information and differing interpretations leads to significant questions that are essential subjects for future research. Moreover, some of these relate to unsolved problems of phosphorite genesis.