STRATIGRAPHY AND STRUCTURE OF THE ST. ALBANS AREA, NORTHWESTERN VERMONT

Abstract

The Cambro-Ordovician sequence in northwestern Vermont includes two Lower Cambrian formations, Dunham Dolomite and Gilman Quartzite; Parker Slate is both Lower and Middle Cambrian, and Rugg Brook Dolomite is Middle Cambrian. Six formations of the Woods Corners Group (new name) are tentatively assigned to the upper Middle Cambrian because accepted evidence demonstrates that the Cedaria zone (lower Dresbachian) correlates with the standard Middle Cambrian of Europe rather than with the Upper Cambrian. The Upper Cambrian (post-Dresbachian), Lower Ordovician, and Middle (?) Ordovician are each represented by one formation: Gorge and Highgate formations and Morses Line Slate, respectively. The old names Georgia Slate, Mallett Formation, Winooski Dolomite, Russell Slate and Grandge Slate are abandoned; the Corliss Conglomerate is regarded as a member of the Morses Line Slate, and nearly all formations are substantially redefined. The faunas of each formation are listed and reviewed. Structurally, the St. Albans area, which is basically a large northward-plunging syncline, is part of the Rosenberg thrust slice of Clark. East of the Champlain thrust six additional thrusts (four newly described) are recognized and cut the northern part of the area into five imbricated thrust slices. Minor folding and the flow cleavage are related to the major syncline rather than to the thrusting, although it is believed that the thrusting was a late stage in the same period of deformation that began with the synclinal folding. One tear fault seems to be genetically related to the minor folding. Eight interformational unconformities and at least as many intraformational unconformities, particularly in the Ordovician, point out the structural instability of the St. Albans area in Cambro-Ordovician time. The predominance of slate contrasts to the carbonate-quartzite sequence to the south in the Hinesburg and Middlebury synclinoria. Sedimentary evidence suggests both northwestern and southwestern sources for the sands in the area.