A1: LOWER PALEOZOIC SEDIMENTARY SUCCESSION OF THE ST. LAWRENCE RIVER VALLEY, NEW YORK AND ONTARIO
Antun Husinec
Department of Geology
St. Lawrence University
Canton, NY 13617
J. Allan Donaldson
Department of Earth Sciences
Carleton University
Ottawa, Ontario
Canada K1S 5B6
INTRODUCTION
Our field trip to the St. Lawrence River valley in New York and Ontario will showcase the two Lower Paleozoic formations outcropping along the river (Fig. 1.), the middle to upper Cambrian Potsdam Sandstone, and lower-middle Ordovician Theresa (March in Ontario) Sandstone. We will be able to examine the non-conformable contact between Potsdam Sandstone and the underlying Proterozoic basement of the Grenville orogeny, the disconformable contact between Theresa and Potsdam Sandstones, different primary sedimentary structures, trace fossils, and microbial structures preserved within the formations.

A1: LOWER PALEOZOIC SEDIMENTARY SUCCESSION OF THE ST. LAWRENCE RIVER VALLEY, NEW YORK AND ONTARIO

STOP 1. Nonconformity Between Proterozoic Basement and Potsdam Sandstone at Alexandria Bay, New York
One of the best exposures of the non-conformable contact between the Potsdam Formation and the underlying Proterozoic basement in northwestern New York is in a road cut located approximately 1.5 miles northeast of Alexandria Bay, where New York State route 12 (NY-12), a two-lane undivided roadway cuts into a hill some 360 m (1,180 ft) west of the Cranberry Creek bridge (Fig. 5). The basal Potsdam Formation is in sharp contact with the Proterozoic gneiss that shows signs of alteration (illite, Fe-chlorite, and siderite; Selleck, 1993). The basal ∼2 meters of Potsdam Sandstone weather into thin and friable slabs that are composed of low-angle cross-laminated (Fig. 6), non-arkosic and non-conglomeratic quartz arenite that is overall moderately sorted and contains some coarse grains within the predominantly fine- to medium-grained framework. Sorting increases up-section, and the sandstone becomes more massive. No body fossils or trace fossils are present in the Potsdam Formation at this stop.

STOP 2. Primary Sedimentary Structures in Basal Potsdam Sandstone, Goose Bay, NY
The northwest face of this road cut nicely exposes lowermost approximately 2.5 meters of Potsdam Sandstone (Fig. 7). The basal ∼1.5 meters is a cross-laminated medium- and fine-grained quartz arenite showing tabular cross-bedding with curved bases and sharp erosive tops. The overlying ∼40-cm-thick tabular quartz-arenite bed is characterized by planar to very low-angle cross lamination. The topmost set exhibits discontinuous, faint wavy (possible erosive bases) and parallel stratification, and becomes more massive up-dip. The exposed section is barren of body and trace fossils and contains no obvious microbially formed structures.

STOP 3. Nonconformity Between Proterozoic Basement and Potsdam Sandstone at Goose Bay, NY
The road cut on the southeast side of the road exposes the nonconformity between Potsdam Sandstone and the underlying Grenville basement rock (Fig. 8). The contact is sharp but irregular, and displays heavily weathered, friable Proterozoic basement rock below the unconformity. Basal Potsdam Sandstone is a quartz arenite composed of poorly sorted and angular grains without any fossils. The lower 50-60 cm is white to light gray in color; the color changes in the upper part of the outcrop to pink and red. The reddish color around detrital grains and within secondary silica and illite cements is due to presence of finely crystalline hematite, goethite and anatase that formed by breakdown of detrital magnetite and ilmenite grains (Selleck, 1993). This basal, low-angle cross-laminated sandstone weathers more easily than the overlying, more massive quartz arenite that we observed at Stop 1, but is missing at this outcrop.

STOP 4. Potsdam Sandstone at Schermerhorn Harbor, Hammond, NY
Note that between this stop and Alexandria Bay, all the outcrops are either Proterozoic Grenville basement or Potsdam Sandstone, indicating that Potsdam Formation blanketed topographic lows of the Proterozoic surface. Topographically higher areas likely were not sites of Potsdam deposition, or alternatively, sandstone was subsequently eroded from these areas. At this an unconformity within Potsdam Sandstone is exposed, (Fig. 9). The basal 3 meters above the unconformity is characterized by cross bedding (Fig. 10), with alternating poorly sorted, coarse- (up to very coarse in places) to medium-grained quartz arenite. This basal sandstone characteristically weathers into thin slabs. Upward in the section, the sandstone becomes more massive.

STOP 5. Disconformity Between Potsdam Sandstone and Theresa Formation at Chippewa Bay, NY
The disconformity between the Theresa Formation and Potsdam Sandstone is spectacularly exposed near Chippewa Bay, New York (Fig. 11). An unconformity without angular discordance (dashed line in Figure 11) is marked by a thin, vegetated interval on top of the Potsdam Formation. The contact between two formations is sharp and indicated by a change in color from white to pale tan silica-cemented Potsdam below, to carbonate-cemented dark gray Theresa sandstone above. Unlike the lower part of the Potsdam Formation that on previous stops is devoid of any biogenous structures, its uppermost part exposed at this stop contains abundant Diplocraterion and Skolithos trace fossils (Fig. 12), clearly indicating a shallow-marine setting. The basal part of Theresa is extensively burrowed, and most primary sedimentary structures are completely obliterated. In addition to biogenous structures, Selleck (1993) recorded numerous fragments of the brachiopod Lingulepis in both the upper Potsdam and lower Theresa at this location.

STOP 6. Lower Theresa Formation North of Chippewa Bay, NY
The road cut on the northeast side of the NY-12 exposes extensively burrowed lower Theresa Formation (Fig. 13). Individual burrows, and possible pseudomorphs after evaporite minerals, are clearly visible as molds on weathered face of the outcrop (Fig. 14), and contribute to overall high porosity of this facies. Medium-grained quartz arenite is cemented by carbonate and is moderately to well sorted. The dark gray color is typical for the lower Theresa, with rare thin yellowish intervals that follow the bedding.

STOP 7. Microbial Structures in Theresa Formation North of Chippewa Bay, New York
The road cuts at this stop expose microbial structures preserved within the white, medium-grained quartz arenites of the middle Theresa Formation. These structures are exposed on both sides of NY-12, and are distinguished by wavy-laminated stromatolite growth structures that are common in the white quartz sandstones of the middle Theresa Formation. Vertical sections of stromatolites exhibit predominantly space-linked hemispheroids with close-linked hemispheroids as a microstructure in the constituent laminae (Fig. 15). Hemispheroids vary both in amplitude and in shape, i.e. from low-amplitude (5-10 cm) and gently convex, to higher-amplitude (up to 20 cm), steeply convex to slightly rectangular, vertically stacked hemispheroids. Sub-circular, concentrically stacked spheroids up to 30 cm in diameter, with laminae composed of close-linked hemispheroids are observed in plan view (Fig. 16). Their shape closely resemble intertidal forms from Shark Bay (cf. Tucker & Wright 1990, p. 150, fig. 4.50B).

STOP 8. Upper Theresa Formation at Morristown, NY
The upper Theresa is nicely exposed in road cuts on both sides of NY-37 immediately south of its intersection with High Street at Morristown, New York. The formation is characterized by thin beds of dark gray and white-yellowish calcareous quartz arenite that irregularly alternate, inter-finger and pinch out. Dark gray sandstone is thoroughly burrowed; extensive bioturbation resulted in complete obliteration of primary structures in this sub-tidal facies. On the contrary, although locally burrowed, the white to yellowish sandstone is characterized by well-preserved sedimentary structures indicating intertidal sand-flat setting with channel fills. Sedimentary structures include ripple cross-lamination, erosional (reactivation) surfaces, herringbone cross bedding, and local conglomerate-filled scours.

STOP 9. Nonconformity Between Precambrian Basement and Potsdam Sandstone
Lyn Valley Conservation Area, Ontario
From the parking area follow the gravel road northward to the start of a footpath that leads eastward through bushes and trees to the base of a 20-meter vertical section of Potsdam sandstone (Fig. 20). This cliff displays almost flat-lying beds with pebble interbeds (clasts of quartz sandstone), ripple marks and low-angle crossbedding that reflect deposition in water rather than by wind. Elsewhere within the Potsdam, large-scale steeply inclined crossbedding in pebble-free Potsdam sandstone in areas to the west reflects eolian transport. Trace fossils plus a wide range of directions of cross-bed inclination indicate a shallow-water marine environment. Curled biofilm structures (Fig. 21) plus desiccation cracks reflect occasional subaerial exposure. Surface patches of gravel and sand cemented to the outcrop surface mark places where cross-bedded Pleistocene outwash was once in contact with this Paleozoic cliff, reflecting the rapid rate at which unconsolidated deposits can be locally lithified.
Distinctive roll-up structures, especially prominent near the western end of this outcrop (Fig. 21), record the curling of sheets of sand grains bound by tenacious films of cyanobacteria in response to desiccation. Although the sands were deposited in water, the presence of such structures indicates intermittent episodes of subaerial exposure sufficiently long to allow drying, shrinking and curling of such coherent thin sheets of biofilm-bound sand. Elsewhere in eastern Ontario and northern New York State, similar curled biomats are overlain by sequences of continuous biomat layers that trapped successive interlayers of quartz sand to form distinctive domal stromatolites, indicating more persistent subaqueous conditions. Impressive examples occur within the city limits of Ottawa, Ontario (Hilowle et al., 2000, Donaldson et al., 2002)
On the return trip to the bus, check a few of the numerous boulders and blocks (Fig. 22) arrayed along the west side of the gravel roadway. Extracted from nearby outwash deposits, they provide examples of Precambrian igneous and metamorphic rocks from the Canadian Shield in addition to samples from the Paleozoic cover. The Precambrian boulders display pegmatite dykes, crosscutting relationships, inclusions, foliation, lineation and fractures (some with minor fault offsets).
Between the margin of the gravel road and the telephone pole in Figure 22, immediately north of the largest boulder, a cluster of small glacially polished and striated outcrops of Grenville basement rock displays near-vertical foliation (Fig. 23). Distinct lithologic interlayers suggest a sedimentary provenance for these Precambrian rocks, with the compositional layering probably marking original bedding.

STOP 10. Heritage Stone Building on West Main St., Lyn Village, Ontario
Its walls of Potsdam sandstone are replete with an impressive array of trace fossils, with Diplocraterion and Skolithos predominating. A few blocks, especially those in the north wall, show syndepositional deformation of these trace fossils in vertical section, comparable to the syndepositional overturn of cross-beds.

STOP 11. Paleozoic-Precambrian Unconformity, Quabbin Hill
The rock cut on the north side of the highway on Quabbin Hill directly exposes the regional nonconformity between Potsdam sandstone and the underlying Grenville basement rock. The latter displays excellent spheroidal weathering of the basement (Fig. 26, 27), which here is a massive intrusive mafic rock. The Potsdam contains trace fossils perpendicular to bedding, just like those seen at STOPS 9 and 10, but the underside of beds on the south side of the highway display a different variety of trace fossils: sinuous trails parallel to bedding (Fig. 28).
Of particular interest in the south-side road-cut are two independent indications of repeated subaerial exposure: bedding-parallel nodular zones within bioturbated silty layers that probably represent tropical soil horizons (Fig. 29), and desiccation patterns on several bedding surfaces. The unconformity surface marks a gap in the geological record of at least half a billion years. During this time, the land may have been elevated so that no rock record accumulated, although it is possible that post-Grenville Proterozoic strata were indeed deposited, but subsequently removed by erosion before the onset of Paleozoic sedimentation.

STOP 12. Precambrian Basement Exposure Within Community at Rockport Boat Line
This outcrop displays lit-par-lit sills of pink to white granite pegmatite and aplite that were injected parallel to the foliation of fine-grained gray to black gneiss and schist of presumed sedimentary provenance (Figure 30). Similar magmatic fluids were injected along crosscutting dykes during multiple episodes that can be put in sequence by observing crosscutting relationships. The metamorphic foliation of the host rock for these intrusions may have developed along the bedding of an original clay-rich sedimentary rock, such as mudstone or siltstone. Although foliation is typically vertical or dips steeply in most exposures or Grenville metamorphic rocks, it here is almost horizontal, perhaps due to location along the hinge of a fold, or to low-angle thrust faulting. The pinch-and-swell morphology of some lit-par-lit injections indicates lateral extension that was locally sufficient to form several boudinage (Figure 31). Note also the prominent sub-ice hydraulic scouring that created sculpted and polished benches along the outcrop during deglaciation.