Summer Field Trips 2001

TGI: Targeted Geoscience Initiative - South-central Cape Breton Island

Cape Breton TGI Horton Group Field Trip
July 5 and 6, 2001

Click on image to enlarge - Use browser "BACK" button to return.
(Go across the row to see the stops in order of the field trip.)

Field Trip to Examine Horton Group Stratigraphy, Cape Breton Island and Adjacent Mainland Nova Scotia,
July 5 and 6, 2001¹

P. S. Giles, NRCan-GSC
R. D. Naylor, NSDNR
P. J. Ténière, Acadia University

¹Funded by Natural Resources Canada and Nova Scotia Department of Natural Resources under the Targeted Geoscience Initiative (TGI) project: Geological Mapping for Mineral Development, South-central Cape Breton Island (2000-2003)

Introduction

Stratigraphic mapping of Carboniferous strata in south-central Cape Breton Island is a significant component of the Targeted Geoscience Initiative (TGI) currently entering its second year of operation. In early June, the TGI Project provided an opportunity to view several deep cores from the central and southern portions of Cape Breton Island where the Windsor Group hosts significant deposits of salt with associated potash, as well as commercial gypsum deposits. This field trip focuses on strata of the underlying Late Tournaisian Horton Group which, within the TGI Project area (NTS 11F/1 and 11F/14), remains largely undivided and has therefore been stratigraphically problematic. Activities which address this thick succession of largely continental strata include graduate study of the area stretching from Antigonish south to Guysborough and easterly to the Strait of Canso area (Paul Ténière - Acadia), and mapping of 11F/11 (2000-2001) by Peter Giles (GSC-Atlantic) and Rob Naylor (NSDNR). This trip will provide an overview of progress to July, 2001, and will also allow a brief look at equivalent strata in southwestern Cape Breton Island, mapped in the mid-nineties by Giles et al. (1997) as part of the Canada-Nova Scotia Cooperation Agreement on Mineral Development. The TGI Horton Group stratigraphy - Cape Breton Island and adjacent mainland Nova Scotia Field Trip was organized and run on July 5 and 6, 2001, by P. S. Giles, NRCan-GSC, R. D. Naylor, NSDNR, and P. J. Ténière, Acadia University. It is the third field trip event of the South-central Cape Breton Island TGI Project 2000-2003, and a complement to the earlier Windsor Group drill core field trip in May 2001.

Regional Stratigraphic Overview

The Horton Group has been subdivided into a three-part succession over much of Atlantic Canada, with a variety of names applied to the stratigraphic subdivisions. At its base, the Group is typified by coarse-grained pebbly sandstone and conglomerate, interbedded with both grey and red siltstone and fine - to medium-grained sandstone. Grey to black shale may interfinger but are volumetrically minor. The top of this coarse-grained interval is marked by the upwards dominance of grey to black shale with interbedded fine- to medium-grained sandstone, typically greenish or grey, and dominated by quartz. In some exposures, the interbedded sandstones are feldspathic as well, and locally are pebbly (rarely truly conglomeratic). The uppermost division of the Horton Group is marked by a return to sandstone-dominated rocks with lesser red and grey shale. These upper sandstones are variable in grain size and in some areas are coarse, pebbly, and even conglomeratic, mimicking the lower portion of the Horton Group.

A remarkable feature of the group is the approximate time-equivalence, regionally, of the dark shale, medial portion of the Horton Group. These rocks constitute the major source-rock interval which has traditionally driven much of the onshore hydrocarbon exploration in pre-Windsor Group strata of eastern Canada. Names applied to these source rocks include the upper Horton Bluff Formation of the Horton type area, the well-known Albert Formation of eastern New Brunswick, and the Strathlorne Formation of Cape Breton Island. In the area south of Antigonish, the term Tracadie Road Formation has been applied to rocks that we believe to correlate with the Strathlorne. In the TGI project map area, this interval is not only characterized by black and dark grey shale and siltstone, but also is locally calcareous with strata best described as black limestone. Interbedded sandstones are ubiquitous and show features typical of small deltaic cycles culminating in rooted horizons.

In south-central Cape Breton, we are attempting to come to grips with nomenclature for the succession which has been applied in western Cape Breton Island as Creignish, Strathlorne and Ainslie formations in ascending order. South of Antigonish, similar rock units have been termed the Clam Harbour River, Tracadie Road and Caledonia Mills formations in ascending order. We have not yet determined which nomenclature is best applied to the Strait of Canso area. We have, however, confirmed that the Horton Group can be mapped as a three-part succession, as it can in most of eastern Canada. Because this field trip includes a brief visit to western Cape Breton Island, we will use the nomenclature established there for the sake of convenience, and apply it to the Strait of Canso area. As Ténière completes his assessment of the larger area to the west, the problems of stratigraphic nomenclature will be hopefully resolved to the satisfaction of the geological community at large.

Hamblin and Rust (1989) summarized the depositional setting for the subdivisions of the Horton Group in western Cape Breton Island and showed the general evolution of the Horton Group basins through time. Our work to date confirms that the facies assemblages that they described are essentially correct, although we may differ in the details of our local interpretation.

Strait of Canso Area

We will visit four key sections in the vicinity of the Strait of Canso, shown on the accompanying map (partially coloured only). In this area, we have the advantage of a well-defined base of the Windsor Group, the Macumber Formation laminated limestone, and can map down-section from this known stratigraphic marker.

Stop 1

At Port Hastings, exposures on the railroad tracks just south of the Canso Causeway show a complete section of the Ainslie Formation, in contact with the Macumber at the top, and lying on dark grey siltstone and shale of the Strathlorne Formation. The Ainslie Formation comprises approximately 150 m of sandstone with small amounts of both red and grey siltstone. Pebbly sandstones are intercalated as relatively thin beds and as lenticular bodies within sandstone. True conglomeratic beds are present but are volumetrically subordinate to sandstone. The contact with the Macumber Formation is very abrupt, and strata above and below the contact are parallel. Biostratigraphic data from western Cape Breton Island suggest that the youngest Horton Group is late Tournaisian in age, whereas the basal Windsor is mid-Viséan in age, so that this concordant relationship may mask a significant regional disconformity.

Stop 2

On the mainland side of the Strait of Canso, we will visit next a section of Strathlorne Formation, well exposed in a road cut leading to an industrial complex north of Mulgrave, adjacent to Porcupine Mountain. Here we will see highly calcareous black siltstone interbedded with black shale and thin sandstone. Up dip, this section passes into red strata of the Ainslie Formation with thin, lenticular beds of pebbly sandstone in red siltstone. Down-section in the same area, we will briefly visit small exposures of lowermost Horton Group rocks, characterized by pebbly sandstone, typically grey, and interbedded with blocky grey siltstone. The lateral continuity of these rocks is currently uncertain and is a mapping problem. The contact of these lower Horton beds with the basement rocks of Cape Porcupine is probably a fault in the local area.

Stop 3

This stop will show participants intermittent exposures of the Ainslie Formation in roadside exposures on a woods road following the Stora pipeline south of Mulgrave. Here we can see a three-part subdivision to the Ainslie Formation, with a lower pebbly sandstone interval, a medial dark shale interval, and an upper sand-dominated interval comparable to the sand-dominated interval seen at Port Hastings at the railway cut. The Macumber has been traced in this area, and overlies these upper sandstones in outcrops a short distance to the east of the woods road. As we travel a short distance to the west, we will see rocks very near the base of the Ainslie Formation in contact with greenish siltstone and underlain by grey calcareous siltstone and shale of the Strathlorne Formation. Bedding-cleavage relationships can be observed in small outcrops beneath the Ainslie pebbly beds, which suggest that the underlying Strathlorne is quite tightly folded, at least locally. At the base of the Ainslie Formation, we will seek opinions from the group on a possible tuffaceous rock which occurs in a single outcrop. If this rock type is confirmed, it would represent the first volcanic rock to be recognized within the upper portion of the Horton Group in eastern Nova Scotia.

Stop 4

We will finish our examination of the Mulgrave area with a stop where Naylor has recognized multiple coarsening-upwards cycles within the Ainslie Formation, with facies ranging from black shale (open lacustrine), through shallower facies, to delta top and braided channels.

Paul Ténière will take the group on an excursion to the area south of Monastery during the afternoon of Day 1 where we will examine strata typical of his Tracadie Road Formation and associated rocks.

We will return to the Strait of Canso area for an overnight stay on the evening of July 5, and meet on the morning of the 6 for a trip to the Port Hood - Mabou area to see equivalent Horton Group beds in that area.

Port Hood - Mabou Area

Introduction

As in the Mulgrave area, we can see the base of the Windsor Group in outcrop at many localities in western Cape Breton Island, and can map that marker with confidence. The relationship between the Macumber Formation and the underlying Horton Group, however, is variable compared with our observations so far in the Strait of Canso area. Our trip will illustrate some of this variability.

Stop 1

Our first stop will be at Mabou Mines, where the Macumber rests with angular unconformity on strata typical of the basal Horton Group. The Macumber is not particularly thick at this locality. The Creignish Formation comprises pebbly sandstone and conglomerate thought to represent deposition in a distal alluvial fan environment, presumed to have been deposited by intermittent braided stream flow on the surface of a fan originating in the Mabou Highlands. As we proceed southerly from the first exposures on the beach south of the fishing harbour at Mabou Mines, we will see small-scale fault displacements of the Horton-Windsor contact in cliff exposures, and if slumping has not covered key sections, we will see some spectacular folding of basal Windsor strata.

Stop 2

Our second stop will change position stratigraphically dramatically to the youngest known Horton Group rock unit (regionally), the Wilkie Brook Formation, which presumably overlies the Ainslie Formation, but has not been observed in field exposure in this position. At our stop at Northeast Mabou, we will see a 30 m thick carbonate bioherm, largely algal in nature, which rests on a thin oolitic limestone and is flanked by grey shales. A short distance below these strata are rocks of the Mabou Highlands, without intervening Horton Group beds. Downstream, in a small brook that drains the hill at Northeast Mabou, one observes red sandstone, minor red and grey shale, and finally a capping succession of white-weathering calcareous-cemented conglomerate, capped in turn by the Macumber Formation. This local succession is very comparable to the type section of the Wilkie Brook Formation north of Antigonish. At that locality, the Wilkie Brook is unconformable with rocks biostratigraphically comparable to the Creignish Formation. The Wilkie Brook Formation itself contains rich palynomorph assemblages which lack any Viséan indicators, assigning the Wilkie Brook to the highest (youngest) Tournaisian like the Ainslie Formation which it is presumed to overlie. In the Northeast Mabou area (NTS 11K/03), Giles et al. (1997) previously mapped the Wilkie Brook Formation as part of the Ainslie Formation.

After leaving Northeast Mabou, we will visit several exposures on the Southeast Mabou River. The first will illustrate a virtually complete section of the Ainslie Formation, overlain by the Macumber Formation. Sand-dominated, the Ainslie also contains some interbeds of red siltstone. Some of the sand units are thick, show reasonable porosity, and have been of interest to the hydrocarbon exploration community as possible reservoir sandstones. Compared with the Ainslie Formation in the Strait of Canso area, this stop is notable for its lack of pebbly sandstone, save for rare lag deposits in one or two of the thicker sandstone beds.

This stop will take some time, since the Ainslie is several hundred metres in thickness, and we will travel downstream to the Macumber contact. The base of the Ainslie at this locality is unfortunately faulted against strata of the middle portion of the Windsor Group. Participants should be prepared to get their feet wet at this stop, since the Southwest Mabou is a big river flowing on bedrock for almost 1.5 km at this stop.

We will next visit exposures farther upstream on the same river, where we can see beds of the upper Creignish Formation, with relatively thick intercalated red siltstone and more typical pebbly sandstone, and some limited exposures of Strathlorne Formation grey shale and interbedded sandstone. Each of these formations is thick in the area, and access to good sections is rather limited. Hopefully we will see enough in these exposures to demonstrate similarities between the local Horton Group and the current map area in south-central Cape Breton Island.

Self-guided Horton Group Localities (optional)

Because this trip was designed as a two-day excursion, we have not included some of the best-exposed sections in western Cape Breton Island, which occur in Cape Breton Highlands National Park just north of Chéticamp. For those interested, we recommend exposures at Grand Falaise picnic site where a high angle reverse fault juxtaposes a thin remnant of Fisset Brook volcanic rocks beneath Chéticamp Pluton granitoid rocks. Just to the north, where coastal exposures begin west of the highway again at I'le Boutreau, excellent exposures of both the Creignish and Strathlorne formations can be seen if one walks along the shore westerly from a small parking area near the shore. All beds are overturned here towards the west. The contact between the Creignish and Strathlorne formations is thought to be a fault (interpretation debatable). Kinematic indicators suggest that the fault functioned as a high-angle reverse fault. With all strata overturned in both the hanging wall and footwall, restoration to subhorizontal positions with normal facing restores the fault to an attitude where it could be interpreted as an extensional slide, still satisfying the kinematic indicators.

Proceeding along the highway from I'le Boutreau a short distance to the northeast, both Fisset Brook volcanics and basal Horton Group strata can be seen. The former are limited to a small brook draining into the western end of a small lake adjacent to the paved highway. Several basalt flows with intercalated red strata lie on beds of the Jumping Brook Formation in the headwaters of this brook, only a few hundred metres from the paved road. These beds disappear as they are traced to the east along the southern shore of the small lake, and basal Carboniferous strata comprising boulder conglomerate are exposed. Basalt boulders are common in this conglomerate. Where the paved road is closest to the shore again to the east and northeast, Creignish Formation beds are well exposed in coastal cliffs. Calcretes are well developed in distal alluvial fan beds, which pass upwards into typical fluvial sandstones. At the base of the coastal exposures, black shales with abundant fish scales underlie redbeds of the Creignish Formation. These are biostratigraphically older than any typical Strathlorne Formation shales, and are thought to document rapid facies change early in Horton deposition between alluvial fans and adjacent lacustrine environments.

One last stop is at a picnic park at Red Head, the next significant headland to the north. Here, the older black shales are well exposed, sandwiched between alluvial fan beds of typical Creignish Formation aspect. The shales are 20 m thick, and are notable for the presence near their base of two thin coal seams (<3 cm each).

At Pleasant Bay, on the shore northeast of the village, boulder conglomerates occur in the Creignish Formation in typical alluvial fan beds standing vertically and presumably fed from the adjacent Cape Breton Highlands. These outcrops are spectacular. If the tide is low, it is possible to observe from Pleasant Bay village the basal Windsor limestone just offshore.

If participants choose to visit these localities in Cape Breton Highlands National Park, they should remember that the rocks of the park are sacred! No hammers allowed, with serious penalties for breaking the rules. Be cautioned!!

References

Hamblin, A. P. and Rust, B. R. 1989:

Tectono-sedimentary analysis of alternate polarity half-graben basin-fill successions: Late Devonian-Early Carboniferous Horton Group, Cape Breton Island, Nova Scotia; Basin Research, v. 2, p. 239-255.

Giles, P. S., Hein, F. J. and Allen, T. L. 1997:

Bedrock Geology of Port Hood - Lake Ainslie (11K/04, 11K/03, 11F/13), Cape Breton Island, Nova Scotia; Geological Survey of Canada, Open File Map 3253, scale 1:50 000.

Picture Captions

Friday, July 6, 2001

Stop photos were not available from Thursday, July 5.
Stop photos for Friday, July 6, are courtesy of Lynn Baechler with thanks.

1. Group photo at the Northeast Mabou carbonate buildup locality stop.
2. Group photo at the Northeast Mabou carbonate buildup locality stop.
3. Northeast Mabou carbonate buildup locality stop.
4. Southwest Mabou River section stop.
5. Southwest Mabou River section stop.

Horton Group Stratigraphy Field Trip 2001 - Participants