CONTROLS ON CARBONATE SEDIMENTATION PATTERNS IN SILICICLASTIC FLOODPLAINS

Carbonate lake deposits interbedded with coal seams have been found to exist associated with perennial fluvial floodplains since the Devonian. A protected water body and vegetation must contemporaneously exist with a minimum of siliciclastic input. Meandering and braided river systems do not have areas shielded from siliciclastic bedload during floods, not allowing for lacustrine carbonate or peat accumulation over time. However, anastomosing river systems have flood basin areas with high levees that receive mostly suspended and dissolved load during floods that do not breech levees. As these areas remain protected and collect water, carbonate precipitation can be enhanced, despite a siliciclastic milieu. In addition, the water table must remain high to preserve peat for coal formation. This hydraulic control on carbonate and coal sedimentation is based on river type and its flooding and groundwater characteristics. The other important control on 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 rich in carbonates can contribute a dissolved load rich in calcium carbonate, allowing for bio-mediated precipitation in protected ponds and lakes in association with plants.

Other explanations for carbonate/coal associations in the literature include intermittent dry climate, groundwater springs, metamorphic provenance, and faulting on floodplains. Flaws in these sedimentary models can be readily recognized. In order to statistically test the idea that indeed carbonate sedimentation on siliciclastic floodplains within non-arid climates can only occur in anastomosing river environments, a database of nearly 200 examples 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. This research contributes sedimentologic criteria helpful to coal exploration and the refinement of fluvial depositional system models.