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
thesis scanned
text with colour pictures) - 18MB pdf file
Plate
1 - Lithostructural units between Albany and Saratoga Lake, NY
(scale 1: 69,000)
Plate
2 - Bedrock outcrop between Albany and Saratoga Lake, NY
(scale 1: 69,000)
Plate
3 - Cities, water bodies and roads between Albany and Saratoga
Lake, NY
Plate
4
- A part of Ruedemann's map of the Capital District
Plate
5
- The Troy South quadrangle in Elam's map
Plate
6
- Geologic map of the southern HGZ, Colonie
Plate
7
- Sketch geological map, north of Cohoes Falls to dam, N Cohoes
Plate
8
- Frontal thrust, Poesten Kill (geological sketch map)
Plate
9
- Section along the Mohawk from Cohoes Falls to the Rt. 32
bridge
Plate
10
- Section from Cohoes Falls to the Rt. 32 bridge
Plate
11A
- Simplified geological map of the central Hudson Valley - N
sheet 8 MB pdf file
Plate
11B
- Simplified geological map of the central Hudson Valley - S
sheet 12MB pdf file
Return to MS Theses completed in the
Geological Sciences Program, University at Albany