LITHOFACIES AND STRUCTURE OF THE TACONIC FLYSCH, MELANGE, AND ALLOCHTHON, IN THE NEW YORK CAPITAL DISTRICT WILLIAM S.F. KIDD, ANDREAS PLESCHl Dept. of Geological Sciences, University at Albany, Albany, NY, 12222 FREDERICK W. VOLLMER Dept of Geological Sciences, SUNY College at New Paltz, New Paltz, NY, 12561 INTRODUCTION The Taconic Allochthon of eastern New York is bordered to the west by a zone of mid-Ordovician shale and greywacke turbidites, which are appropriately characterised by the term "flysch". These synorogenic deep-water clastics are now interpreted to represent the fill of the migrating flexural basin created by the advance of the Taconic thrust sheets onto the Cambro-Ordovician passive margin of easten North America (Rowley and Kidd, 1981; Bradley and Kusky, 1986). The flysch is markedly diachronous, with a thick basal carbonaceous shale unit, the Utica Shale, and extends many hundreds of kilometres west of the present margin of the Taconic Allochthon. The Allochthon consists almost exclusively of sedimentary rocks that represent a sample of the continental rise part of the Cambro-Ordovician passive margin, and of the latest Precambrian-earliest Cambrian clastics of the late-stage rift fill, and rift to passive margin transition; all are strongly folded and have been transported westward on a complex system of thrusts at least 150 to 200 km relative to the North American craton (Bradley, 1989). In the New York Capital District, a zone about 16-20 kilometers wide of the Ordovician flysch adjoining the western margin of the Allochthon has also undergone strong deformation, including widespread conversion of once stratified rocks to melange, associated with the later stages of the Taconic Orogeny. It is the purpose of this guide and field trip to examine these rocks, to attempt to illuminate the structures which they contain, to try to roll back at least some of the nomenclatural and stratigraphic confusion they have suffered, and to place them in the larger regional context of the Champlain Thrust system. GENERAL GEOLOGICAL SETTING OF THE FIELD TRIP Before confusing readers and trip attendees with the "stratigraphic" terminology, we set out the distribution of basic rock types and their structural condition (refer to Figures 1 and 2, the geological maps). West from a NNE-trending line through a point on the Mohawk River in Niskayuna, just east of Schenectady, regionally flat-lying [very gently-dipping] Paleozoic strata are exposed, which consist, in the immediate area of the field trip, of the medial Ordovician flysch, that is greywackes and shales, in varying proportions. East of this boundary, the western limit of Taconic deformation, deformed medial Ordovician rocks occur in a zone 16-20 kilometers wide, bounded to the east by the [also] NNE-trending western border fault of the Taconic Allochthon, the Taconic Frontal Thrust. The deformed rocks of this zone dominantly consist of highly disrupted shales and greywackes, and these are appropriately termed shale-matrix melange. The western side of the deformed zone consists of a belt about 5-6 kilometers wide of folded and internally faulted rocks that are still largely bedded (we term this the Vischer Ferry Zone), and which can be interpreted as expressing a zone of increasing strain transitional from the undeformed flat-lying strata in the west to the highly-strained, disrupted melange in the east. Within the melange, there are lens-shaped belts of less­ deformed material, both of shale-dominated, and of greywacke-dominated proto lith, and ranging in structural condition from merely folded, to "broken formation", transitional to melange. These less-deformed lenses range up in size to regionally mappable; the most prominent in the area of the field trip is the bedded greywacke and shale of the Halfmoon Greywacke Zone (see Figures 1 and 2). South of the Capital District, 1 now at Te1egrafenberg C2, 14472 Potsdam, Germany fu Garver, 1.1.. and Smith. J.A. (editors). Field Trips for the 67th annual meeting of the New York State Geological Association, Union College. Schenectady NY. 1995. p. 57-80. 57 Ballston Spao / Devonian / / Taconic Allochthon / / / / / / .I melange 1FZ - Troy Frontal Zone WFl - Waterford Flysch Zone MRZ - Mohawk River central Zone (w, e parts) --~ / / / / / / flysch / / .; slices / / RWZ - Rip van Winkle Greywacke Zone RGZ - Ravena Greywacke Zone / / SSZ - Stillwater Shale Zone HGZ - Halfmoon Greywacke Zone / / RTZ - Rocky Tucks Greywacke Zone VFZ - Vlscher Ferry Zone / / / / / undeformed / / flysch and shale / / tectonic contact / / / / TFT - Taconic Frontal Thrust / / / / .; / / 50km ® Field Trip Stops / / / / / / / / / / / / / / / / / / / / / / Fig 1. Tectonic units in the Saratoga Lake - Capital District - Kingston segment of the central Hudson Valley 58 along the Hudson valley as far as Kingston, most of the exposed width of the flysch and melange belt is occupied by two of these bedded belts, with a narrow melange belt separating them, and another bordering the eastern side (Figure 1). The change of structural style, from melange-dominated in the north, to "fold/thrust"-dominated to the south, was probably controlled by the change from shale-rich to greywacke­ rich strata, which in turn must have been a product of sediment supply and local basin geometry. PREVIOUS WORK Detailed and systematic studies of the medial Ordovician rocks of the Hudson River Valley are contained in Ruedemann's reports of mapping and stratigraphic and structural studies, Geology of Saratoga Springs and vicinity (Cushing and Ruedemann, 1914) and Geology of the Capital District (Ruedemann, 1930), which even now include the most detailed published maps of this area. Ruedemann identified, and marked on his maps, the western limit of deformation in the medial Ordovician flysch, meaning folding and pervasive faulting and melange formation, and devoted considerable space in his text to the structural condition of the deformed zone, which occupies most of the width of the exposure of these rocks in the Hudson River lowlands in this area. It is unfortunate that this pioneer structural work has been submerged by the choice of the compilers of the last several geological maps of New York State (Fisher et al., 1970, Rogers et al., 1990) and of the Albany County area (Fickes, 1982) not to indicate that the medial Ordovician flysch in most of this area is significantly deformed, in strong contrast to these strata farther west, and to have continued use of a stratigraphic nomenclature that, among other defects, actively works to obscure this fact. It is ironic that Ruedemann himself remains responsible for creating much of the stratigraphic nomenclature and confusion in the first place! Bird (1963, 1969) first clearly documented the regional extent of the melange, and its general structural significance in its relation to the emplacement of the Taconic Allochthon, although we reject the then-prevalent notion of gravity sliding for the emplacement mechanism of the Allochthon and the formation of the melange. More recent work in the Hudson Valley flysch by Vollmer (1981a), Bosworth and Vollmer (1981), Vollmer and Bosworth (1984), and Plesch (1994) includes detailed mapping of the area between Ravena and Saratoga Lake, and compilation of outcrop mapping farther south to the area of Middletown (in Plesch, 1994). This work reveals, more clearly than that of previous workers, the abundance of melange in the Capital District (see Figure 1), and that this is a product largely of subsurface shear strain, not of superficial slumping, and that the shearing was accommodated by significant brittle fracturing, largely on a small scale, besides more ductile behaviour. These melange zones of the Hudson Valley are a part of the southern extension of the Champlain Thrust and its subsidiary faults. Vollmer (1981a) and Vollmer and Bosworth (1984) pointed out that melange produced by this thrusting is unconformably overlain in the southern Capital District of New York by the earliest Devonian Helderberg Group carbonates. The unconformable contact constrains the formation of the melange, and the thrusting which produced it, to be a product of the Taconic Orogeny, and hence a purely Ordovician event. This unconformable relationship applies to the deformation in the westernmost (and hence youngest) part of the Taconic fold and thrust belt, implying that similar melange now east of the outcrop of the unconformity is also a product of Ordovician tectonism. The Champlain thrust system links the deformed fl ysch of the central Hudson Valley to similar sections in southern Quebec; it is a purpose of this guide and field trip to point out that there is much greater similarity between the marginal zone of the Taconic fold and thrust belt in these two areas than might be inferred from the existing published maps which, despite similar poor outcrop, clearly indicate the importance of melange and the extent of the marginal deformed zone in Quebec (St. Julien and Hubert, 1975; St. Julien et al., 1983; Avramtchev, 1989), but do not in New York. STRATIGRAPHIC TERMS Names and assigned ages ofrocks in the deformed belt of the Ordovician flysch of New York are badly confused, both because of the structural complexity and because of the application, by Ruedemann and subsequent workers, of biostratigraphic names to inadequately defined lithic units. To the west of the deformed belt, where strata are close to flat-lying, the black shales of the basal part of the foreland basin sequence are termed Utica shale [Canajoharie shale has been biostratigraphically distinguished as slightly older than the type Utica, but is lithologically not distinguishable from it]. This shale is overlain by rapidly coarsening-upward flysch, which is termed Schenectady Formation in the eastern Mohawk Valley. West of the central Mohawk Valley this is replaced by the Frankfort Formation which is not that different lithologically, although on average containing somewhat thinner-bedded greywackes, and in part somewhat 59 Fig 2. Geology of the Ordovician flysch Ubs and melange, or Albany-Saratoga Lake Ss area.