Abstract

Field data accumulated during the past decade at both ends of the Taconic allochthon indicate a need for revisions of the interpretation of the tectonic history and local structural relations at its north end in western Vermont. A three-deformation chronology is here proposed for the allochthonous rocks: Event I, of Trenton age, involved large-scale soft rock submarine gravity sliding. Event II, tentatively dated as Late Ordovician to Early Silurian, involved isoclinal recumbent folding, formation of slaty (early) cleavage, emplacement of the Sudbury slice and the Florence nappe, and low-grade regional metamorphism. The folds, in general, have amplitudes and wavelengths of hundreds of meters to a few kilometers and define the principal map pattern. Event III, tentatively dated as Acadian, produced small-scale (wavelengths of a few tens to a hundred meters) upright or overturned to recumbent folds having generally shallow-plunging, north-trending axes. These late folds refolded the early slaty cleavage, developed late slip cleavage, and possibly imposed a second metamorphic event in the higher-grade areas along the Taconic Range.

Other revisions are: (1) The arcuate map pattern of the recumbent folds of the Giddings Brook fold complex is interpreted as a real change in the trend of major fold axes, rather than the effect of topography on a west-facing system of recumbent folds having a south component to the dip of the axial surface. The Giddings Brook fold complex is now considered a lateral continuation of the Great Ledge–Porcupine Ridge fold complex, both products of Event II now separated by an axial culmination that expresses an anticlinal folding of Event III. (2) The rocks east of the village of Sudbury, consisting mainly of black slate but including other rock types, are no longer considered the erosional remnant of a once-extensive nappe. Instead, the area is now mapped as a field of wildflysch-type conglomerate; the black slate is interpreted as the autochthonous (possibly parautochthonous) Middle Ordovician Hortonville Slate.

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