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 Unpublished MSc.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

Return to MS Theses completed in the Geological Sciences Program, University at Albany