TIMING AND ORIGIN OF DEDOLOMITE IN UPPER WAPPINGER GROUP (LOWER ORDOVICIAN) STRATA, SOUTHEASTERN NEW-YORK

Detailed petrographic-, geochemical-, and geologic (including surface- and subsurface) study of the massive regional-scale dolostones of the upper Wappinger Group (Lower Ordovician part of Sauk Sequence, southe astern New York) enables us to recognize the products of three stages of dolomitization and of one episode of dedolomitization. The three do lomite stages are: (1) earliest syndepositional dolomite, very fine cr ystalline dolomite (10 to 30 mu m) and some tiny cores of coarser rhom bs and displaying delta(18)O mean value of -7.2 parts per thousand and delta(13)C mean value of -1.2 parts per thousand; (2) second-stage, s hallow-burial dolomites including both replacement-type phases (fine- to coarsely crystalline dolomite), and some dolomite cement that was p recipitated between crystals of replacement origin and displays delta( 18)O mean value of -7.6 parts per thousand and delta(13)C mean value o f -1.7 parts per thousand; and (3) late-stage dolomite that fills frac tures and crosscuts earlier-stage dolomites and that probably formed a t intermediate depth of burial. The dedolomite displays progressive de grees of replacement fabric from intercrystalline through pseudomorphi c to massive. With its bright-yellow CL color, dedolomite is easy to d istinguish from the dull-CL precursor calcite, from dull-CL veinlet ca lcite, and from the reddish-CL-colored dolomites. Dedolomite and its r elated fracture-filling bright-CL-colored calcite display delta(18)O m ean value -7.4 parts per thousand and delta(13)C mean value of -2 part s per thousand. The evidence that dedolomite, with its high concentrat ion in Mn2+ and low content of Fe2+, depleted oxygen isotope-ratio, an d distribution restricted to the top three formations that were eroded at the post-Sauk, pre-Tippecanoe surface of unconformity formed sugge sts that dedolomitization took place early in the Middle Ordovician as a result of post-Early Ordovician uplift, -subaerial exposure, and ka rstification when meteoric water circulated within the porous doloston es near the Earth's surface. Petrographic textural data prove that ded olomite replaced second-stage dolomite; oxygen-isotope data indicate t hat dedolomite and second-stage dolomite are not co-precipitated miner als. Therefore, the second-stage dolomite must have been stabilized du ring the Early Ordovician initial episode of subsidence that was assoc iated with a passive-margin tectonic regime. The maximum possible dept h of burial was several hundred meters. Subsequently, the top three fo rmations of the Wappinger Group were deeply buried and late-stage dolo mite in places filled fractures. This subsequent deep burial did not s ignificantly alter either the second-stage, shallow-burial dolomites o r the dedolomites.