ABSTRACT
The Taconic allochthon is a sequence of Cambrian or Precambrian to
Ordovician rocks. It is composed of predominantly deep water
argillaceous and subordinate arenaceous and calcareous rocks that
were deposited on the continental rise and slope. During the
Ordovician, sediments which were earlier deposited in the
slope-rise environment were incorporated into the accretionary
prism of an island arc that approached from the east, and
subsequently overthrust the carbonate platform.
The Hatch Hill Formation is part of the Taconic sequence. It
consists of dominantly black-gray slates, minor amounts of
sandstones and carbonates. Previous workers have recognized the
presence of dolomite and a siderite ore (the Burden Iron Ore) in
these sandstones. The stratigraphic position of the siderite ore
was not clear prior to this study. This study showed that the
Burden Iron Ore is the basal part of the Hatch Hill Formation in
the area studied, based on comparison with the northern Taconic
lithologic stratigraphy. It conformably overlies the Bomoseen
Formation. The contact between the Bomoseen Formation and the
Hatch Hill Formation is marked by a disconformity that has not
been noted elsewhere in the Taconics.
The origin of the iron ore is closely related to the origin of the
dolomite of the Hatch Hill Formation. It can be demonstrated that
both phases occur as cements that formed after the deposition of
the Hatch Hill arenites. The cements formed as a by-product of the
decay and fermentation of organic matter that was probably
deposited in the black-gray shales of the Hatch Hill Formation.
Isotopic evidence and geochemical considerations show that the
siderite cements formed after sulfate reduction was completed and
that the development of dolomite cements most likely took place in
the lower part of the zone of methanogenesis. Paleotemperatures
determined from oxygen isotope analyses indicate that the dolomite
cements probably formed at a temperature of approximately 750C, if
the pore fluid was not affected by meteoric or brine waters. This
would imply a depth of formation of 2-3 km, if present day
geothermal gradients for a passive continental margin sequence are
assumed. The formation of dolomite therefore took place during or
after the deposition of the Pawlet Formation (flysch sequence).
Hofmann, P.M., 1986. Dolomitization of the Hatch Hill Arenites
and the Burden Iron Ore. Unpublished MSc. thesis, State University
of New York at Albany. 180 pp., +x, +2p abstract, 1 plate (folded
map).
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 40 Z899 1986 H64
Hofmann MS thesis (scanned
text pdf - 10 MB)
Plate 1 - Geological
map of the Mt. Tom area (Columbia Co., NY)
(coloured outcrop map, scale
1: 10,000; cross-sections and stratigraphic column) - 6 MB pdf
file
Return to MS Theses completed in the Geological Sciences Program, University at Albany