STRONTIUM-ISOTOPIC SIGNATURES REFLECT AN ORIGIN OF DOLOMITE BY FRESH-WATER EFFLUENT - THE PINE PLAINS FORMATION (WAPPINGER GROUP, CAMBRIAN)OF SOUTHEASTERN NEW-YORK

Dolostones of the Pine Plains Formation of Late Cambrian age from a co ntinental margin of the Appalachian Basin formed in a peritidal settin g. The strontium-isotopic signature of this hypersaline dolostone show s that the waters responsible for precipitating this dolomite were fre sh-water effluents derived from a granitic continental crust. In inter bedded black dolostones of reducing or anoxic setting the carbon-isoto pic ratio shows enrichment in the lighter isotope. In the absence of l and plants such C-13 depletions indicate that photosynthetic communiti es, such as fresh-water, terrestrial or soil cyanobacteria or bacteria were present. If delta(13)C reflects depositional conditions, so shou ld the strontium-isotopic composition. Cambrian sea water was not inti mately involved in precipitating this dolomite. Dolomite formed by fre sh-water effluent under conditions of low stand of sea level. An alter native interpretation is that the original isotopic composition of the marine dolomite was not retained, and that the elevated strontium-iso tope composition reflects fabric-retentive replacement of precursor do lomite in a meteoric fluid system. The studied dolostones exhibit the highest Sr-87/Sr-86 ratios ever reported in Paleozoic carbonate deposi ts.