Abstract

The lithologic units recognizable in the fossiliferous succession along southern Lake Champlain are structurally continuous with and traceable eastward into the “marble belt” of west-central Vermont immediately west of the Green Mountain Front. They are also traceable northward through west-central Vermont into a succession in northwestern Vermont bounded on the east and west by major thrusts, where they pass laterally northeastward into fossiliferous shales, the faunal zones of which are correlated with those along southern Lake Champlain and in the Hudson and Mohawk valleys. The Cambrian strata are traced into west-central from northwestern Vermont whereas the Ordovician correlation is with rocks in the Mohawk-Hudson-Champlain region. Certain of the Upper Cambrian strata can be correlated with established formations of this age in both of the outlying areas.

The structural pattern reflects movements dependent upon the original distribution of sedimentary facies. Interbedded Carnbro-Ordovician limestones and dolomites grade westward into foreland sandstones and eastward into geosynclinal shales. Cambrian and early Ordovician sandstone tongues extend far to the east. Later Ordovician strata of the shale facies overlie the calcareous and sandy succession and are locally unconformable on it. In the Taconic Range allochthonous Cambrian strata of shale facies are superposed upon the autochthonous Ordovician beds. The rocks of the klippe were derived from a zone at least 50 miles east of the present westernmost exposures, arriving there by movements confined largely to the shale facies. Flexural folding and thrusting, effects of alternating competency and incompetency of the foreland sequence, affected the latter succession and possibly the Taconic Allochthone. Breaking of competent strata in the flexures initiated the thrusts; thrusting continued by the stripping of competent from incompetent beds. This was accompanied by counterclockwise rotation of the thrust slices around pivotal zones bordering on a foreland massif to the southwest. Thus rock cropping out in the north-south thrust slices originally extended northeast-southwest. The thrusts are concentric to the Adirondack crystallines and are cut by the Adirondack normal faults; probably they were warped during normal faulting and uplift of the Adirondacks.

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