Carbonate lake deposits interbedded with coal seams exist in perennial fluvial floodplain deposits during the Phanerozoic. Such lakes require that a protected water body must contemporaneously exist with a minimum of siliciclastic input. Meandering and braided river systems do not have areas shielded from siliciclastic bedload during floods, so that lacustrine carbonate or peat accumulation over time in these systems is interrupted. However, anastomosing river systems have flood basin areas surrounded by relatively high levees that protect those basins, allowing them to receive mostly suspended and dissolved load during most floods. This protection from siliciclastic input favors enhancement of carbonate precipitation. The water table must remain high to reserve peat for coal formation. This hydraulic control on carbonate and coal sedimentation in a fluvial system is dependent on flooding and groundwater characteristics. The other important control on carbonate and coal sedimentation is provenance. Carbonate accumulation in continental settings is dependent on the influx of ions from the weathered drainage area; sedimentary material, whether bedload, suspended load, or dissolved load, must come from the basinal source area. Bedrock with calcium-rich rocks can contribute sufficient quantities of dissolved to suspended load to allow for bio-mediated precipitation in protected carbonate ponds and lakes in association with plants.
In order to test this hypothesis that indeed carbonate sedimentation on perennial siliciclastic floodplains can primarily occur in anastomosing river environments, a database of over 200 examples mostly of perennial anastomosing and meandering river systems was compiled. Information regarding the fluvial parameters and facies characteristics of each Phanerozoic river deposit, the tectonics of its region, as well as the provenance was used to recognize carbonate sedimentation patterns through time and space. Difficulties in collecting data on single fluvial systems versus data averaged across successions containing multiple fluvial systems reduced the size of the dataset. Overall, 56 anastomosing river deposits were found to have carbonate floodplain lakes and a carbonate provenance. This means that 46% of all definitively anastomosing river deposits accumulated flood basin carbonates and 100% of these river deposits had a carbonate provenance. 66 anastomosing river systems (54%) did not have carbonates in their provenance nor in their flood basins, and no anastomosing systems had a carbonate provenance without carbonate deposits on their floodplains. No meandering river systems (8) had carbonate deposits, despite the fact that four of the meandering entries had carbonates in their source area. This research contributes sedimentologic criteria helpful to coal exploration and the refinement of fluvial depositional system models.