Towards understanding the origin of massive dolostones

The origin of ancient massive dolostones, i.e. continuous dolostone sequence with a thickness >100 m and a platform-wide distribution, is the key issue of the 'Dolomite Problem' that cannot be clearly demonstrated by any existing dolomitization model individually or sequentially. It has been proposed that the massive dolostone could be generated by the stacking of multistage dolomitization events linked to the sea-level fluctuation, which results in repeatedly occurring of limestone precipitation-dolomitization cycles. However, the sequence of dolomitization events cannot be differentiated by any sedimentological or traditional geochemical techniques. Here we report Mg isotopic compositions of the massive dolostone (delta Mg-26(dol)) from the middle Cambrian Qinjiamiao Formation (QJM) in the Yangtze Platform, South China, which consists of cyclic depositions of shoaling upward sequences. The stratigraphic variation of delta Mg-26(dol) is coincident with the depositional cycles, suggesting the dolomitization might be periodic and be coupled with the sea-level oscillation. As dolomitization fluids experience changes in delta Mg-26 values during dolomitization processes, the intra-cycle stratigraphic delta Mg-26(dol) profile reflects the processes of dolomitization. Our study indicates that the massive dolostone could be generated by the temporal and spatial stacking of multiple dolomitization events that are associated with sea-level fluctuation. If this model can be verified by other massive dolostone successions, the origin of massive dolostone may be resolved. (C) 2020 Elsevier B.V. All rights reserved.