CONODONTS – USEFUL BUT UNDERUSED TOOLS FOR DECIPHERING GEOLOGY IN VERMONT

Conodonts have been applied to stratigraphic, structural, and tectonic problems related to Vermont geology for the past half century. However, they have been underused there, partly because of the generally elevated levels of thermal alteration and partly because of the difficulties of extracting and identifying them from metasedimentary rocks. Conodonts normally are rare in the uppermost Cambrian and Lower Ordovician strata of the Laurentian shelf exposed in northwestern Vermont. These rocks commonly are dolomites, which not only can be difficult to process, but also frequently yield conodont elements with altered, ‘sugary,’ surface textures. Conodonts from coeval strata in continental slope facies in this same region tend to be preserved rather well. Conodonts in this region typically have color alteration index (CAI) of 5, indicating post-depositional heating of near 300 degrees C. In central and south-central Vermont, west and east of the Green Mountains, Middle to Upper Ordovician conodonts having CAI up to 7-1/2 to 8 have been recovered, and some of these still are identifiable to species. Conodont elements from [originally] shaly carbonates in this region either have been destroyed or have been deformed beyond recognition by shearing in these lithologies. However, some elements do survive in minimally-sheared purer limestone marbles and, more commonly, in dolomitic marbles. In some cases, almost-clear conodont elements completely overgrown by clear apatite have been identified, because of the thin concentration of carbon present between the conodont and its overgrowth. These case studies show that by concentrated collection of favorable lithologies (dolomitic marbles; non-sheared limestones and limestone marbles), and by collection and careful processing of large samples, conodonts can be applied to age-dating and to facies and thermal analysis of additional Paleozoic units and localities in Vermont and similar regions.