Thick Nonmarine Carbonate Deposits: What You Need to Know about Clastic Carbonates
E. H. Gierlowski-Kordesch
In the interpretation of thick (decimeter to meter scale) carbonate sequences in lake (and fluvial) paleoenvironments, most researchers use incorrect assumptions based on marine dogma about the behavior of carbonates "on land". Nonmarine carbonates do not form only in evaporitic "closed" conditions and can accumulate in association with siliciclastics. Besides in saline lakes, carbonate precipitation also occurs in freshwater lakes when pH is affected by plants, algae, microbes, and invertebrates. If the source area contains carbonates or calcic basement rocks, then the water chemistry is conducive to the accumulation of carbonates in any nonmarine depositional system. Indeed, if the source area predominately contains carbonates, then an entire lacustrine to fluvial system, closed or open, can be composed of carbonate deposits (including substantial bedload deposits). Obvious clastic carbonates, such as carbonate conglomerates and grainstones, have been observed in small ancient lake basins. In larger basins, however, with distal reaches in the hundreds of kilometers, carbonate bedload could not survive. With its high reactivity at surface conditions, only suspended and dissolved carbonate load could reach distal regimes and precipitate biochemically. When a source area contains both siliciclastics and carbonates, both types of sediment can accumulate as contemporaneous distal deposits. Evidence for these geochemical and physical processes can be found in the rock record and in the modern, showing that nonmarine carbonates should be classified as biochemical as well as clastic rocks.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California