CARBONATE LAKES ON DISTAL PERENNIAL RIVER FLOODPLAINS

Genesis of carbonate lakes on distal siliciclastic floodplains of perennial fluvial systems below the regional spring line has been misinterpreted in the literature. The presence or absence of carbonate deposits in many nonmarine settings has been used to document arid vs. humid depositional paleoconditions. In addition, spring deposition is cited as the source for carbonate precipitation in any part of the landscape. For continental deposits, provenance generally determines the type of sediment that accumulates, not climate. Long-term isolation from siliciclastic input is also an important criterion for promoting carbonate accumulation.

Floodplains of anastomosing river systems with carbonate provenance that contain perennially protected areas on their floodplain are ideal localities for carbonate lake deposits. Hydrodynamic features create and maintain isolated areas on the floodplain for the precipitation of carbonate. Concave-upward interchannel flood basin areas mostly receive dissolved and suspended load during floods, because of relatively high levee systems in an aggradational system. Bedload only enters these areas as crevasse splays. Anastomosing perennial river floodplains are the best candidates for carbonate accumulation because they are stable over long periods and preservation potential of interchannel lake areas is high.

To test the hypothesis that carbonate lakes generally form within anastomosing perennial river systems with carbonate provenance in distal areas below the regional spring line, a database of these perennial river systems in the geologic record was compiled. From 136 systems, no anastomosing fluvial system contained carbonate lake sediments without carbonates present in the watershed. Next, carbonate deposition was modeled based on Ca-rich discharge of surface waters vs. spring waters since spring deposition is an alternate origin. For meters thick accumulation of lacustrine carbonate within floodplains, overland input from surface waters, not spring deposition, will allow for carbonate accumulation over extensive areas during the short time period (thousands of years) these floodplain lakes are active. Existence of these lakes below the regional spring line also indicates input of Ca-rich surface waters contributing ions for carbonate accumulation.