ABSTRACT
The belt of medial Ordovician deformed flysch west of the traditionally defined Taconic boundary thrust was investigated by detailed mapping in the Capital District, from Albany to Saratoga Lake, and large scale compilation of the central Hudson Valley, between Glens Falls and Middletown. Maps and detailed sections are presented. Mapping recognized a two-fold division into a western belt of folded and faulted flysch and an eastern belt which is dominated by tectonic melange. The Folded and Faulted Flysch unit shows northward fining from greywacke-dominated to shale-dominated. Isoclinal folding and incipient melanges are characteristic in the south, whereas in the north folding is mild and deformation more localized. The boundary of this belt with the melange-dominated belt appears abrupt, but is not well exposed as it coincides in many areas with a Pleistocene/Quaternary filled bedrock valley. A flatlying and unfolded body of black shale within the Folded and Faulted Flysch near Saratoga Lake is anomalous with respect to the lithology and structure of its surroundings.
The melange-dominated part could be subdivided from west to east into Western Exotic Melange, Halfmoon Greywacke Zone (HGZ) and its northern equivalent, Eastern Exotic Melange, Bedded Shale, Flysch Melange and Frontal Exotic Melange. The modifier "exotic" indicates assemblages of non-flysch lithologies within the melange, specifically pale green shale, sideritic mudstone and black chert. Preserved bedding and unusual lithology (abundant and thick greywackes) in the HGZ and its northern equivalent contrast distinctively with the surroundings. The structure of the southern HGZ is a large syncline, of which the eastern limb and hinge is cut by a thrust juxtaposing a complexly deformed terrane. Complex melange exposed along the Mohawk River at Cohoes Gorge is described in detail and recorded in a detailed cross-section. The Flysch Melange, comprised only of shale, siltstone and thin greywackes, has small slices (< 10m) of bedded material in contrast to the Western and Eastern Exotic Melange which have virtually none. The Frontal Exotic Melange has large slices of non-flysch material (<20m) and small bedded flysch slices. All units are inferred to be in thrust contact with each other.
Sedimentologically, paleo-current directions and frequent sets of climbing ripples indicate linear trench topography and reworking by strong contour currents for the deposition of flysch sediments. The assemblage of non-flysch lithologies (mainly black chert, sideritic mudstone and pale green shale) was deposited on the slope/rise of a former passive margin. Coarser, more immature, "exotic" greywacke is probably related to the assemblage of non-flysch lithologies. A difference between "exotic" greywackes and "normal" greywackes is suggested by point counting. Phacoidal cleavage is the dominating structural element. Its average plane and other foliations dip moderately to steeply to the east. It is crosscut by late slickensided veins which are probably still associated with melange formation since they do not occur in the large bedded slices.
The assemblage of non-flysch lithologies must be highly allochthonous and probably some flysch is also. An emplacement model consistent with field relations is proposed. Early in the history of emplacement of the Taconic allochthon a coherent slice of non-flysch lithologies and flysch was added in front of the detached but not completely transported Taconic allochthon. Afterwards little or no more flysch was accreted. The basal detachment essentially overrode the seafloor keeping the non-flysch lithologies and flysch slice at the thrust front. Severe disruption and melange formation resulted. The boundary between Western Exotic Melange and Folded and Faulted Flysch is the trace of the basal detachment. Only towards the final stage of shortening was flysch west of this basal detachment incipiently to mildly transported and deformed, the broad melange detachment imbricated, and younger flysch from below the main basal detachment brought up.
Compilation of geological data between Saratoga Lake and Middletown finds that the belt of deformed flysch is continuous and that the basal detachment can be traced to the south into the Ellenville quadrangle. A piggy-back basin (Quassaic group) and a piece of Taconic allochthon (in the Goshen quadrangle) are tentatively identified. The Frontal Exotic Melange and the Taconic Frontal Thrust are the most continuous features. This continuity confirms suggestions of their late definition by previous workers. Between Saratoga Lake and the New York-Vermont state border, the belt of deformed flysch continues and can be traced into and correlated with the Champlain Thrust system of Vermont.

Plesch, A., 1994. Structure and tectonic significance of deformed medial Ordovician flysch and melange between Albany and Saratoga Lake and in the central Hudson Valley, New York.  Unpublished MSc. thesis, State University of New York at Albany. 265 pp., +xix; Appendix 34pp.; 12 folded plates (maps)
University at Albany Science Library call number:  SCIENCE Oversize (*) QE 40 Z899 1994 P57

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