STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

folds in the Taconic sequence. Had the rocks of the Taconic sequence
lain above the rocks of the Valley sequence when the latter were first
deformed, it is probable that the Taconic sequence would now reflect
the large-scale folding. The superficially crumpled sheet composed of
the Taconic sequence thus lies with evident structural discordance
upon the folds of the Champlain Valley.
Unconformity hypothesis: The preceding evidence of discordance
between the two sequences could be interpreted as due to unconformity.
Such a hypothesis would require post-Hortonville folding followed by
erosion and then deposition of the sediments that are now the phyllites
of the Taconic Range. Under this hypothesis, in other words, the phyllites
of the Taconic Range must lie unconformably above the Cambrian
and Ordovician slates of the slate belt on the west and the Cambrian and
Ordovician carbonate rocks of the marble belt on the east. Although the
unconformity hypothesis is consonant with the evidence so far presented,
the following stratigraphic and facies evidence will show that the discordance
must be explained by overthrusting.
Stratigraphic evidence: The rocks of the Taconic sequence in the
Taconic Range south of the Castleton River are here called the Nassau
formation. North of the Castleton River Kaiser (1945) indicated the
presence of Mettawee slate at the base of the Taconic sequence. Inasmuch
as these formations are assigned to the Pre-Cambrian and Lower
Cambrian, their presence above the Trenton Hortonville slate on the
eastern flank of the Taconic Range can be explained only by overthrusting.
If one accepts the phyllites of the Taconic Range as Nassau and
Mettawee, the Taconic overthrust is demonstrated.
Isolated patches of phyllite belonging to the Taconic sequence lie
on the Hortonville slate in the area about Butler and Sargent Ponds.
Similar patches were reported west of the main Taconic mass by Kaiser
(1945, p. 1085) and Cady (1945, P1. 10). These features are interpreted
as klippen of probable Cambrian age lying on the Ordovician Hortonville
slate. Dana (1877, p. 340) reported finding Trenton fossils in a patch of
limestone exposed in the mass of Cambrian slate in Hubbardton township.
Cady (1945, P1. 10) interpreted this limestone and a similar patch
at the southern end of Beebe Pond as fensters of the Valley sequence
lying beneath the overthrust Taconic sequence. Kaiser (1945, p. 1087)
did not accept the limestones as fensters.
66