The geochemical composition of carbonate sediments reflects some of the variables in the chemistry of the waters in which they were deposited. An important factor determining the composition of trace elements in these sediments appears to be the availability of these elements in the waters of the deposi-tional environment. The trace element concentration of fresh water differs from that of sea water and the skeletal parts of organisms reflect this difference in their trace element content. The distribution of three trace elements was investigated: barium, iron, and manganese. Plots of barium against manganese concentration for fresh-water molluskan shells and marine carbonate sediments and mol-luskan shells show distinct groupings. A line of separation can be drawn between samples from the two environments. Fresh-water samples studied contain a greater abundance of these trace elements than marine samples. In a plot of barium against manganese for fresh water, lagoonal (brackish), and marine samples, lines of separation can be drawn between samples from these three environments. A plot of iron against manganese content for marine and lagoonal (brackish) samples shows distinct groupings indicating differences in iron and manganese concentration for carbonate sediments from these two environments. The concentration in both of these elements for lagoonal samples exceeds that of marine samples. Hence, a correlation exists between trace element concentration and depositional environment for carbonate sediments.
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Depositional Environments in Carbonate Rocks
One of the principal tasks of the geologist is to determine the depositional environments in which rocks are deposited. Although regional environmental interpretations of transgressions and regressions, movements of shoreline, and gross aspects of continental and marine sedimentation have been understood since stratigraphy became an established branch of geology, only recently has the science of sedimentology come up with criteria for environmental recognition of specific outcrops, wells, or even hand samples. This observation is especially true of carbonate rocks. The papers in this volume will provide a key to the subject of recognition of depositional environments in carbonate rocks. Based on a symposium held in Los Angeles, California, on April 1967, at the joint meeting of AAPG and SEPM.