A1: PRECAMBRIAN BASEMENT AND CAMBRIAN-ORDOVICIAN STRATA, AS DISPLAYED IN THREE PROVINCIAL PARKS OF CANADA
J. Allan Donaldson
Department of Earth Sciences
Carleton University
Ottawa, Ontario, Canada K 1 S 5B6
Jeffrey R. Chiarenzelli
Department of Geology
State University of New York
Potsdam, New York 13676
INTRODUCTION
General overviews of the region covered by this excursion are provided by Wilson (1946) and by Williams (1991). A regional sedimentological study of the Nepean formation was carried out by Wolf and Dalrymple (1984). The relationship of Paleozoic strata of eastern Ontario to the Frontenac Axis remains a problem not yet resolved: Was the arch a positive structure during sedimentation, or did it become so as a result of elevation through faulting/subsequent crustal upwarp/compactional draping or some combination of these processes? This question will be considered during the trip, which was designed to provide an overview of Paleozoic strata and the underlying Grenvillian basement along a traverse from the Ottawa River to the St. Lawrence River.
One highlight will be a visit to a shallow underground mica-apatite mine at Murphys Point Provincial Park to view workings abandoned in 1920. Another will be a visit in Charleston Lake Provincial Park to view a giant cylindrical structure in Nepean quartz arenite, which appears to represent the outline of an exhumed dewatering structure. The Paleozoic-over-Grenville unconformity will be viewed at two localities, both showing evidence of paleo weathering of the Precambrian basement. Stops will be made to view a variety of abundant trace fossils, including Skolithos and Diplocraterion, as well as several Cambrian and Ordovician stromatolite localities (complementary to those seen during Trip F-1). Local sedimentary units have been used extensively as building stone, as will be seen in several historic examples. Teachers and other educators are particularly encouraged to attend this trip, as an introduction to the geoheritage potential of Ontario's Provincial Parks.
This trip has been modified from a series of three guided geoheritage hikes in Ontario Provincial Parks (July 10th, 17th, 24th, 2004), organized by Friends of Canadian Geoheritage, a group founded in 2003 to encourage greater public appreciation and understanding of Canada's geological diversity. The first of these hikes attracted 35 participants, the second drew 40 and the final one drew 80 participants, ranging in age from 3 to 90. The educational aspects of geology and geography provided by visits to these three parks, which offer different slants on the varied geoheritage of this part of North America, should be of particular interest to teachers. The geological heritage of eastern Ontario spans over 1 billion years of Earth history. Ontario's provincial parks provide significant examples of this heritage, ranging from bedrock that shows evidence of billion year-old mountains and ancient sea-floor deposits, to more recent signs of glaciation, succeeded by development of postglacial lakes and river systems that were initially much larger.

A1: PRECAMBRIAN BASEMENT AND CAMBRIAN-ORDOVICIAN STRATA, AS DISPLAYED IN THREE PROVINCIAL PARKS OF CANADA

STOP 1 - TERRACES TRAIL, FITZROY PROVINCIAL PARK
OXFORD AND ROCKCLIFFE FORMATION (ORDOVICIAN)
Park entrance kiosk
From the Terraces Trailhead, a one-hour hike provides a snapshot view of the principal Paleozoic strata comprising the bedrock beneath this park (Figures 2 and 3). A succession of carbonates, sandstone, and shale record a succession of changing environments marginal to and within a sea that episodically covered the area from about 500 million years ago. Low-angle crossbeds in sandstones of the Rockliffe Formation indicate unimodal southward transport, consistent with fluvial accumulation in a system feeding into a basin to the south. Low dispersion about the mean suggests deposition as sandbars in a braided river system. Overlying green shale in the upper part of the Rockliffe Formation gives way upwards to dull red shale. Both varieties contain abundant bedding-parallel vermiform trace fossils, as well as rare desiccation cracks, suggesting transition to relatively quiet-water deposition on tidal flats or in a delta.
The present landscape is largely the product of modification by ice and meltwaters related to the Wisconsinan ice sheet that retreated from the Ottawa Valley less than 12,000 years ago. As the ice sheet melted, it left deposits of glacial drift containing boulders of Precambrian rock derived from the Canadian Shield (mainly from the adjacent Grenville Province, but a few from the more remote Superior Province). In addition, many large angular slabs were ripped up from nearby stromatolitic limestone and dolostone. As the once much larger post-glacial ancestor of the Ottawa River cut its way through the Ottawa Valley, it carved increasingly narrower channels in the Paleozoic rock, exposing a succession of bedrock cliffs along Terraces Trail.
On completion of the Trail loop, we will visit the shore of the Ottawa River, just north of the most northerly bathing beach, where a few small outcrops of Oxford dolostone that underlie the Rockliffe Formation display domal stromatolites. These stromatolites, near the top of the Oxford, are laterally linked (Figure 4), generally with smooth, gently domed calcilutitic laminae (some of the smaller stromatolites show incipient branching). The best bedrock exposures occur in shoals and along the shores of several nearby offshore islands in the Ottawa River, where individual stromatolite domes commonly exceed one metre in diameter, have a synoptic relief of up to 40 cm, and display a distinct north-south elongation trend. Discontinuous patches of silicification commonly accentuate the laminae. Locally derived stromatolitic blocks have been placed throughout the park along interior roads, and on both sides of the road at the park entrance. These large blocks provide opportunities to observe three-dimensional morphologies of the stromatolites.

Look to the north to see the Ottawa-Bonnechere Graben.
As we leave the park, look to the north to see the Gatineau region of Quebec, where Precambrian Grenville gneisses have been uplifted along a series of faults that form the Ottawa-Bonnechere Graben. This ancient tectonic feature controls the course of the Ottawa River for more than 600 kilometers to the northwest.

STOP 3. Oxford Formation Stromatolites (Ordovician)
STOP 3 - OXFORD FORMATION STROMATOLITES (ORDOVICIAN), ALMONTE
Victoria Woolen Mill
This stop will allow an opportunity to see some of the various stones used to construct the many historic buildings in this village of approximately 4500 inhabitants. Depending on the water level we will observe several units of laterally linked stromatolites in Ordovician carbonate strata (Oxford Formation), right beneath the centre-town dam on the Mississippi River, in downtown Almonte. These stromatolite beds are best viewed from the observation deck behind the Victoria Woolen Mill on Mill St., immediately east of Slide Bridge (Main Street/Hwy 49).
Lunch may be obtained in any of several restaurants along Mill St. Information pamphlets are available from the Visitor Centre, immediately east of the Victoria Woolen Mill (note the stromatolite-bearing block near the entrance).

Murphys Point Provincial Park - Check In

STOP 4. Murphys Point Provincial Park
SILVER QUEEN APATITE-MICA-FELDSPAR MINE, LALLY HOMESTEAD
Murphys Point Provincial Park lies within the Grenville Province, on the eastern side of the Frontenac axis. A wide variety of both igneous and metamorphic rocks are exposed within the park. Some of the metamorphic rocks show evidence of multiple stages of folding, best seen in exposures of the several belts of crystalline limestone that traverse the park. We will park at the Lally Homestead, where an outcrop of marble displays folding, foliation, possible relic bedding and a variety of positive-weathering inclusions (Figure 7).
Closed in 1920, the Silver Queen Mine was an important source of phosphorous for munitions during World War I. A part of this mine has been restored to allow safe underground entry, where large crystals of mica and apatite stand out on the partially mined rock walls.

STOP 5. Trace Fossils in Nepean Sandstone
Old Stone Mill, Delta
Built in 1810 of Nepean (Potsdam) sandstone, this is the only stone gristmill in Canada designated as a National Historic Site. The quartz arenite blocks used to construct this building are remarkable for their almost universal abundance of the trace fossils, Skolithos and Diplocraterion. (Figure 8). We are currently unaware of the provenance of this building stone, but are attempting to track down where it was quarried. Some Nepean/Potsdam and March/Theresa beds have such traces in abundance (cf. Bjerstedt and Erikson, 1989).

Charleston Lake Provincial Park Check in

STOP 6. Giant Cylinder in Nepean Formation
We will drive to Sandstone Island Trailhead, and follow Sandstone Island Trail (Figure 9) to view basal conglomerate and sandstone of the nearly 500-million-year-old Nepean Formation in unconformable contact with billion-year-old Precambrian metamorphic and igneous rocks of the Grenville Province (Figure 10). This contact, representing more than half a billion years of elapsed time, is best displayed at Post 2, where weathered granite at the top of the Grenville basement may well represent a paleosol. Isoclinally folded units of crystalline limestone and intercalated siliciclastic metasediments are abundant within the park (see map of Hewitt, 1961).
Many of the well-rounded conglomerate clasts (readily seen on the undersides of undercut bedding surfaces, as well as in vertical sections at Posts 3 and 4) consist almost exclusively of vein quartz and Precambrian quartzite, but a few clasts are identical to the Nepean, indicating early cementation and cannibalism of basal Paleozoic units (some Covey Hill reworking?). Crossbedding shows a preferential southerly trend, but herringbone patterns (well-exposed at Posts 6 and 7) are locally abundant, suggesting alternation between fluvial and marine depositional environments. A few cross-sections of probable tidal channels are visible in close proximity to packages of strata displaying herringbone crossbedding. Similar successions, as well as facies containing spectacular aeolian dunes, occur on the west side of the Frontenac Axis (cf. Dalrymple and Wolf, 1988).
A particularly fascinating structure occurs at the site described as 'The Waterfall' (Post 8). Initially thought to have been formed by rapidly flowing meltwater cascading over a cliff during Wisconsinan deglaciation, it now is apparent that this hollowed segment of almost half a cylindrical structure (Figure 11), extending vertically through 5 m of well-indurated Nepean strata, is an exhumed dewatering structure (cf. Gruhman and Pederson, 1992). This interpretation, first presented here to account for 'The Waterfall', is provided by two lines of evidence: 1. A patch of sandstone on one side of the structure shows lamination parallel to the cylindrical surface; and 2. Below the level at which a conglomerate bed within the stratigraphic section intersects the cylindrical surface, pebbles and cobbles are cemented to the inner surface of this columnar structure by sand identical to sand in the horizontal source bed (Figure 11).
While still unconsolidated, forceful upward hydraulic overpressure could create such a structure, with streaming parallel to the walls accounting for creation of an annulus of nested concentric laminations in sand along the margin of the vertical dewatering column. During waning stages of dewatering, individual pebbles and cobbles from the intersected gravel bed could have readily sunk along the periphery of the dewatering cylinder, to then be concentrated along the cylinder's margin, and held in place by a matrix of sand until the walls and interior of the structure were completely cemented. Similar structures elsewhere within the Nepean/Potsdam have long been recognized and debated (Baker, 1916; Hawley and Hart, 1934; Hawley, 1935; Miser, 1935; and Deitrich, 1953).
We probably will return back along the path from this point, but if time permits, we will follow the trail forward to Post 11, where glacially polished surfaces on uppermost exposures of Nepean quartz arenite display high polish, parallel grooves and striae, and abundant chattermark clusters, all showing consistent facings of concave chattermark curvatures in a down-ice (southwestward) direction.

STOP 7. Unconformity
ORDOVICIAN LIMESTONE AND SANDSTONE UNCONFORMABLY OVERLYING GRENVILLE BASEMENT
Corestones in a variety of mafic and felsic basement rocks are preserved below the unconformity on the north side of the road. The overlying sandy carbonates, probably in the lower part of the March Formation, contain abundant trace fossils, as well as a few nodular zones that may represent soil profiles (Figure 12). The lowermost beds contain quartz sand and silt, and shaly beds become locally abundant upwards in the section. The lack of Covey Hill or Nepean beds suggests that this may have been a paleo high during early Ordovician time.