Petrologic and electronprobe microanalysis of the Upper Cambrian Little Fails Formation, New York, has revealed the presence of an unusual fabric--syntaxial calcite borders on dolomite crystals. One type of border, herein designated "calcite rim," is even and uniform in thickness, ranging between 5 and 25 mu . A study of the fabric geometry and mineral paragenesis strongly suggests that the rims have formed by marginal dedolomitization. Another type of border consists of coarse, void-filling calcite also optically continuous with the dolomite; these are very irregular in thickness and bear some resemblance to the "calcite envelopes" of Goldman (1952). Apparently, these envelopes formed by passive precipitation of calcite syntaxial with dolomite "nuclei." Solutions with relatively high Ca (super ++) /Mg (super +) + ratios provided by meteoric water passing through overlying Ordovician limestones may have been the dedolomitizing agent for the calcite rims. On the other hand, although there is no direct evidence of evaporites, sporadic pockets containing length-slow chalcedony may represent the former presence of gypsum or anhydrite which could have contributed calcium ions to ground water. In view of the common occurrence of hematite after pyrite, it is very likely that oxidation of the pyrite could have produced sulfate ions which effected dedolomitization.

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