Print this pageCarbonate Lakes from Siliciclastic Floodplains: Meandering Versus Anastomosing
The genesis of carbonate lakes on continental siliciclastic floodplains of fluvial systems is explained four ways in the literature, using assumptions more appropriate for marine systems. A dry climate is supposed to allow carbonate precipitation in lakes, even though lacustrine and palustrine carbonates can form in all climates in the continental realm. Groundwater springs are supposed to deposit carbonates all across a basin, but this is hydrologically impossible, especially in rift and foreland basins. Metamorphic provenance is supposed to suppress siliciclastic input for a basin to allow carbonate precipitation, but there is not enough calcium and carbonate ions released from most metamorphic rocks, except marble, to allow for thick accumulations of carbonates. Faults on a floodplain are supposed to isolate areas for carbonate deposition, but these low-lying areas are targets for siliciclastic deposition as well. Isolation is certainly an important criterion for promoting carbonate precipitation on a siliciclastic floodplain, but in continental systems, a source for the needed ions is also a key consideration. Freshwaters derive their highly variable chemistry directly from the watershed in contrast to the well-defined chemistry of marine waters. Provenance is a key indicator for the genesis of carbonate lakes as well as type of fluvial depositional paleoenvironment. River systems which contain perennially protected areas on their floodplain would be ideal localities to accumulate and preserve carbonate lacustrine and palustrine deposits.
Two types of rivers contain significant lakes on their floodplains: anastomosing and meandering. The hydrodynamics of flooding in anastomosing river systems appear to be the most conducive to extensive carbonate precipitation in protected flood basins. To test this hypothesis, a database of over two hundred examples of meandering and anastomosing river deposits was constructed, collecting information on facies, including crevasse splay deposits, fluvial parameters, such as channel width and depth, and provenance. This study is useful in refining fluvial system recognition in the fossil record since differentiating between anastomosing and meandering forms is sometimes difficult. In addition, this research contributes to coal exploration because many carbonate lakes on fluvial floodplains are associated with coal deposits.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009