Draft version: Copyright © Atlantic Geoscience Society 2005

3.   Minas Basin: South shore

OVERVIEW of the Geology

Previous mapping

Because of its accessibility, spectacular coastal exposure, and the presence of significant mineral resources, the south shore of the Minas Basin has been the subject of numerous mapping efforts, though the patchwork of maps with varied objectives and scales makes regional synthesis difficult. Various parts of the area of study were mapped by the Geological Survey of Canada (Weeks 1948, Bell 1960, Boyle 1957, Stevenson 1958, Crosby 1962) and the Nova Scotia Department of Natural Resources (Ferguson 1983, Giles & Boehner 1982, Moore 1986, 1993a, 1993b, 1994, 1996, Moore & Ferguson 1986, Moore et al. 2000). Fig. 3-1 represents a summary of the principal features of the geology.

Fig. 3-1:    Simplified geological map and cross-section of the south shore of the Minas Basin. Box encloses area of Fig. 3-2.

Field relationships around Cheverie

Relatively undeformed strata characterize the type sections of the Horton Bluff and Cheverie Formations, respectively west and east of the Avon estuary (Fig 2-1). To the northeast the gently folded Cheverie Formation and overlying Macumber Formation plunge beneath a broad region of Pembroke Breccia, which underlies Cheverie Harbour. Scattered outcrops of highly deformed Windsor gypsum and anhydrite occur in adjacent coastal cliffs. The Cheverie Formation reappears farther northeast in antiform at the Johnson Cove (Fig. 3-1, 3-2). On the north flank of this structure, the Cheverie Formation again dips north beneath Macumber limestone and Pembroke Breccia. However, immediately north, where a Windsor succession would be expected, is a region of coast within which Windsor and Horton lithologies are intermixed and have highly variable bedding orientations, interpreted as a megabreccia. Moore ( 1996) interpreted a north-dipping thrust fault through this zone. The thrust fault is overlain by Horton Bluff Formation is exposed in a succession of ENE and WSW-trending folds with axial plane cleavage increasing in intensity north, as far as Split Rock (Fig. 3-2).

Fig. 3-2: Schematic geologic map and cross-section of Johnson Cove and Split Rock, after Johnston (1999). Toothed lines indicate low angle faults; wavy lines are high-angle faults. Unornamented lines show trace of bedding. Box encloses area of Fig. 3-5.

Relationships between Bramber and Walton

East of Cheverie, Horton Bluff Formation strata continue to be exposed in coastal cliffs, between regions of unconformable Triassic cover. Typically, the Formation is folded by ENE-trending folds, and cut by steep faults that strike northwest, typically marked by streams valleys.

Inland, the Horton Bluff Formation appears to pass stratigraphically upward into poorly exposed, steeply-dipping, generally south-younging Cheverie Formation. A belt of lower Windsor Group rocks, including Macumber Formation, marks the SE edge of the belt of Horton rocks. It is marked by sink holes (formed over gypsum) and outcrops of steeply-dipping carbonates, indicating that this belt represents the steep limb of a somewhat sinuous monocline. Extensive mineralization of the Macumber Formation at Walton, and elsewhere on this zone, has led to a history of drilling and mining operations.

Regional structures

To the southeast of the coastal belt of highly deformed Horton Group rocks, the Windsor Group occupies a broad basinal area extending as far south as the Rawdon fault, which marks the north margin of a horst cored by Meguma basement. Around this basin, to the east and west, Horton and low Windsor Group rocks show relatively gentle inward dips. In contrast, stratigraphically higher units towards the middle of the basin are extensively deformed, with highly variable, steep dips. Outcrops in gypsum quarries (Ferguson 1983, Moore & Ferguson 1986) show folding and overturning. Although this central area of the Kennetcook basin is interpreted by Moore and Ferguson as a mosaic of steep block-faults, we infer from the decrease in deformation down-section that many of the faults actually sole into Windsor evaporites. At one point, the stratigraphic position of the evaporites is marked by a mega-breccia recorded by Moore and Ferguson (1986) lending support for the idea of a detachment at this level.

We interpret these relationships as indicating that the highly deformed coastal units of Horton Group were thrust over a more autochthonous succession of Horton Bluff, Cheverie, and Macumber Formations. The Pembroke Breccia, and the highly deformed evaporites on the coast of Cheverie Harbour, probably represent a thick deformation zone - effectively a décollement in Windsor evaporites - above which Horton Bluff Formation was transported.

To the southeast, in the central Kennetcook basin, there is no duplication of stratigraphy, though the structural contrast remains, so the thrust is interpreted to cut up-section southward in its hangingwall until it comes to lie between deformed Windsor Group (above), and relatively less deformed lower Windsor and Horton Groups (below). The hangingwall cut-off of the Cheverie Formation must lie between Johnson Cove and the central part of the Kennetcook basin; it is inferred to underlie the monoclinal structure inland just southeast of the Cheverie-01 well, that passes through the Walton mine (Fig. 3-3).

The inferred thrust does not outcrop within the area of Fig. 3-1. Folds in the hangingwall are likely detachment folds, which absorb some of the shortening, so it is likely that the thrust loses displacement to the south. Displacement may decline to zero within the area of Fig. 3-1, or farther south.

Fig. 3-3:    Schematic cross section along line A'B' (Fig. 3-1), after Roselli (2004).

Previous authors have invoked thrusting in this area. Boehner (1991) interpreted Chevron Canada seismic lines as showing a low angle, north-dipping thrust fault, the Kennetcook thrust, with a mapped trace southeast of the Walton monocline. The interpretation favoured here is similar, but we infer that the Kennetcook thrust does outcrop at the surface, and therefore is categorized as a 'blind' thrust.

Horton Bluff: Horton Bluff Formation type section

Access

From Halifax take highway 101 towards Wolfville. Take Exit 9 at Avonport.

0.1 km In Avonport turn immediately left.

0.5 km T junction with Oak Island Road. Turn Right.

0.7 km Bend to right on road marked Bluff Road.

2.0 km Cross railway tracks.

2.3 km Turn left on dirt road to beach.

2.4 km Beach. Turn right (southeast) and walk ~200 m to first area of outcrop.

Details

This stop is intended to show the undeformed characteristics of the Horton Bluff Formation, a unit that will be encountered in various deformed states at many of the later stops on the trip.

The Horton Bluff Formation is characterized by interbedded shales, sandstones, and impure dolomitic carbonates that probably represent paleosols. The section was described by Martel and Gibling (1991, 1996) and interpreted as a classic lacustrine succession, dominated by successive shallowing-upward cycles representing the filling of the lake following each subsidence episode. Although relatively undeformed (at least compared with later stops) the succession contains a number of soft-sediment features. Most notably, synsedimentary dykes are common, and frequently feed upward into conspicuously thickened, lenticular units of overlying sandstone. These were interpreted by Hesse and Reading (1978) as feeders overlain by sand volcanoes. However, Martel and Gibling (1993) reinterpreted the sand lenses as hummocky cross-strata formed by storm waves; the underlying sedimentary dykes were interpreted as products of transient pressures generated by waves impinging on a lake shore.

If the tide is low enough, we may be able to see a large, roughly elliptical region of disturbed bedding on the foreshore. This is also interpreted as some kind of dewatering structure, though its origin has not been entirely satisfactorily explained.

Retrace steps to highway 101 at Avonport and turn south towards Halifax

SPLIT ROCK: Johnson Cove to Mutton Cove

Access

Travelling south on highway 101 toward Halifax. At exit 4 turn off and take highway 215 east toward Newport (0 km).

3.8 km             Newport Corner; - turn left just after the military communications antennae on the left.

9.3 km             Brooklyn; turn right following highway 215 and 14. This is the first of a rather complex series of intersections over the next 2 km, through which we will steadfastly follow highway 215.

10.0 km           Bear left to follow highway 215 as highway 14 turns off right.

11.0 km           Turn left following 215

11.2 km           Bend right following 215

13.2 km           Kennetcook River Bridge. Note the outcrops of deformed Windsor Group on the south bank of the river

20.5 km           Centre Burlington Sandford's store

28.0 km           Avon Emporium

36.4 km           Road runs beside shore in Cheverie

38.1 km           Turn left on Sherman Lake Road

39.0 km.          Park at beach. Proceed on foot walking north along shore.

Although a number of 'stops' are identified in the 3 km coastal section (Fig. 3-4) between Johnson Cove and Mutton Cove, exposure is nearly continuous after the first covered interval. Be prepared to see an enormous variety of structures, including many that cannot be described in this account.

Vehicles will return to highway 21 and proceed to the end of the section

39.9                 Turn left and proceed north 900 m to Ocean Beach Road

40.8.                Turn left on Ocean Beach Road and proceed to the beach.

42.7                Park in beach parking area at Mutton Cove.

SR-1: Cheverie Formation

At the beach there are typical exposures of the Cheverie Formation (the upper portion of the Horton Group in this area, conformably overlying the Horton Bluff Formation). The Cheverie formation consists of red to yellow sandstones and mudstones. The laminated sandstones locally include sedimentary structures such as cross-bedding and climbing-ripple cross-lamination. Mudstones locally display mudcracks, plant fragments, and yellowish carbonate concretions interpreted as caliche. Both sandstone and mudstone locally contain upright fossil tree stumps, though typically not particularly well preserved. The Cheverie Formation is interpreted as a fluvial succession probably deposited in channels and arid floodplains. There are significant differences between this succession and that on the SW side of Cheverie Harbour, suggesting significant lateral variability.

SR-2: Uppermost Cheverie - Basal Windsor section

The uppermost beds of the Cheverie Formation are calcareous sandstones. The transition to overlying pinkish grey fine limestones of the basal Windsor Group (Macumber Formation) is inconspicuous but sharp. The Macumber Formation consists of laterally continuous laminated limestone with a yellow to reddish orange oxidized weathering surface. The resistant limestone forms a distinctive linear outcrop on the shore (Fig. 3-4). The overall stratigraphic thickness of the Macumber Formation is roughly 2-3 metres, with individual laminations of 1-2 mm in thickness.

Several different interpretations have been proposed for the Macumber Formation and associated facies at the base of the Windsor Group, summarized by Lavoie et al. (1995); interpretations include intertidal algal mats (Schenk 1967), deep water methane-vent bioaccumulations (von Bitter et al., 1992), saline lake deposits (Schenk et al. 1992), to deep water microbial mats (Lavoie et al. 1995).

250 m approx.

Fig. 3-4: Air view of shore in Split Rock area, showing stops.

The Macumber Formation passes transitionally upward into a poorly understood unit known as the Pembroke Breccia. The Pembroke Breccia consists of variably sized, randomly oriented, elongate angular blocks of thinly laminated yellow weathered limestone, of Macumber Formation origin (Lavoie et al. 1995), typically in a limestone matrix. The origin of the Pembroke Breccia is controversial. Four different processes may have contributed: (1) synsedimentary slumping produced a breccia consisting of cyclic patterns of decimetre to metre thick sequences of well bedded microbial mats, and slump folds in contorted mats (Lavoie et al. 1995). (2) Because of the absence of certain stratigraphic units above the Macumber in the area, Lavoie et al. (1995) also proposed that some of the breccia is tectonic, formed by bedding-parallel extensional shear. Stratigraphic omissions were linked to the proposed Ainsley detachment positioned at the Macumber / evaporite boundary (Giles & Lynch 1994, Lavoie & Sangster 1995). (3) A late karstic breccia can incorporate the tectonic breccia and synsedimentary breccia or the non-brecciated Macumber microbial mats (Lavoie et al. 1995). (4) Parts of the Pembroke Breccia can also be interpreted as solution collapse breccia.

At Johnson Cove only a short interval of Pembroke Breccia is exposed. Locally, cavities within the breccia include soft red sandstone possibly derived by infiltration from younger Carboniferous or Triassic units above. To the north, there is only sporadic exposure in the beach, including both Windsor and Horton lithologies. Bedding orientations appear chaotic, suggesting that there is a significant thickness of megabreccia, involving material from both groups. This zone is interpreted as the base of "allochthonous" Horton Group.

SR-3: Horton Bluff Formation, and Triassic Fundy Group graben-fill

At the start of continuous exposure (just south of SR-3, Fig. 3-5) there are Horton Group shales and thin sandstones that are tightly folded, with incipient axial planar cleavage in some beds. Immediately north, the sedimentary rocks are cut by a diabase dyke, and by faults that bring down a small graben-like outcrop of the Triassic Fundy Group to beach level. The Fundy Group is much more friable than the adjacent Horton Group and is easily distinguished. A first interpretation might suggest that the intrusion is related to the graben formation. However, this seems not to be the case because the intrusion is closely similar in petrology to others in the area that have been dated as mid-Carboniferous, and quite unlike the Mesozoic basalts of the Fundy Group. Hence we have to regard the juxtaposition of the dyke and the graben either as a coincidence, or as a result of the localization of Mesozoic faults at pre-existing intrusions.

SR-4: Mid-Carboniferous intrusions

An approximately homoclinal, gently dipping succession of Horton Bluff Formation shales and subordinate sandstones is exposed on the south-facing cliff to the north of Stop 3 (Fig. 3-4). Spectacular wave-ripple marks are visible on the rocky shore. Despite the apparent low degree of deformation, there is a steep cleavage in places, and intersection lineations are visible on bedding surfaces. The preservation of sedimentary structures suggests that the strain is low.

Close to the top of the cliff, a diabase sill is more resistant to weathering than most of the Horton Bluff lithologies. At stop 4, the sill is abruptly connected with a dyke, which cuts across stratigraphy before connecting with another sill-like segment of the intrusion, that disappears under the beach. Lenticular intrusions of diabase are present low in the cliffs, and suggest that the magma filled a series of generally en-echelon tension gashes, interconnected fractures, and bedding-plane cracks.

This group of intrusions is one of several that occur within Horton Bluff Formation rocks in the area. Although the age relationships are unclear at this locality, about 2 km to the south, in Cheverie, Johnston (1999) was able to show in thin section that the intrusions clearly cross-cut the fabric of highly deformed shale (Fig. 3-5a), though it was itself cut by faults. Kontak et al. (2000) was able to date that another intrusion in this suite, located about 2 km inland, also cross-cutting structure in the Horton Group, at 315+/-4 Ma.

These relationships indicate that the Horton Group was deformed prior to ~315 Ma, when the intrusions were emplaced. The sediments of the Horton Group would have been less than 40 million years old at this time.

Fig. 3-5: (a) Left: Thin section showing diabase intrusion (upper part of slide) cross-cutting fabric in shale (right). Both are cut by faults (lower part of slide). (b) Below: Syncline at Split Rock.

SR-5: Folded Horton Bluff Formation

The coastal section to the north is unrivalled as a place to view folded sedimentary rocks. The folds are typically subhorizontal, upright to moderately inclined, with gently curved hinges that are locally spectacularly exposed in the beach (Fig. 3-4, 3-5b, 3-6). At some locations, folds are disharmonic, with shale intervals acting as detachment surfaces. Some folds are related to these detachments as detachment folds and fault-propagation folds. A spectacular example of the latter is exposed in the wave-cut platform.

Cleavage is weakly developed along this section of shore, though many bedding surfaces, especially in fissile shales, show a weakly developed crenulation lineation, parallel or sub-parallel with the main fold hinges.

 

Fig. 3-6:    Detailed map of folds in Split Rock area, after Johnston (1999)

Extensional structures are also much less conspicuous than those associated with shortening, but at numerous locations on this stretch of beach there are boudinage structures with axes either perpendicular to fold hinges or rotated slightly clockwise from this orientation.

Thin quartz and calcite veins mark some fractures, both parallel to layering and cross-cutting the bedding. Slickenfibres indicate the direction of slip. These have not been systematically investigated, but there appear to be veins that cross-cut the folds and veins that are folded, suggesting that several generations of fractures are present.

SR-6: Split Rock

The syncline at split rock is one of the best exposed on the shore (Fig. 3-4, 3-5b). This syncline has a near-horizontal hinge, but because of the slope of the beach, it is possible to view the fold in 'axial projection' from a suitable (if muddy) vantage point on the foreshore. A bed that is characterized by large 'cannonball' concretions of dolomitic carbonate can be located easily on both sides of the main fold. North of the fold hinge, bedding is vertical to overturned, and cleavage becomes more intense. A penetrative slaty cleavage is developed in black mudrocks; weak spaced cleavage occurs in some sandstones.

The dependency of buckle fold wavelength on layer thickness is well illustrated in muddy sections just north of the main syncline. Thick sandstone layers show long-wavelength buckles while thin layers are folded at a much smaller scale.

SR-7: Split Rock to Mutton Cove west side

The hike along shore to Mutton cove (Fig. 3-4) is less spectacular than that to the south because the cliffs are roughly parallel to fold hinges, so folds are not seen in profile. However, the wave-cut platform displays multiple examples of folds that plunge both to the SW and to the NE. Mutton Cove itself is marked by a zone of poor exposure that probably corresponds to a zone of NW-striking fractures. Examples of minor fractures in this orientation may be seen in the cliffs on either side of the cove. Mutton Cove makes an appropriate and accessible lunch stop.

SR-8: East side of Mutton Cove

At the resumption of outcrop on the east side of Mutton Cove (Fig. 3-7), folds are again prominent in the rocks of the wave-cut platform. Notice that the mudrocks are strongly cleaved, and there is a conspicuous, non-penetrative bedding-cleavage intersection lineation visible on the surfaces of sandstone beds. At several locations the cleavage is not parallel to the axial traces or hinges of the folds: - the folds are 'transected' by the cleavage in a counter-clockwise sense.

There are two 'classical' interpretations for this phenomenon. In the first, the folds and cleavage are regarded as separate generations of structures. Shortening occurred first in a more north-south direction, producing folds. Subsequently the shortening direction changed to a more east-west sense, overprinting the folds with cleavage. A second explanation involves dextral strike-slip or transpressional motion. Folds nucleated early, and underwent clockwise rotation with progressive deformation. Cleavage, on the other hand, reflects the bulk strain, acquired during the whole deformation history including components of shortening acquired up to the last moment of deformation. Cleavage therefore undergoes less rapid clockwise rotation, and ends up transecting the folds in a counterclockwise sense. It is not possible to choose absolutely between these alternatives based on the evidence at this spot, but the presence of major strike-slip features just to the NE (stop SR-9) favours the second explanation.

Fig. 3-7:    Aerial photo and geological map of the east side of Mutton Cove.

SR-9: Strike-slip faults and positive flower structure

Several faults cut the beach to the east of Mutton Cove (Fig. 3-7, 3-8), and can be traced into the cliffs where they are seen to have steep dips (Fig. 3-9). Mineralized sheets with slickenfibres of quartz and calcite are found on the fault surfaces, and in some cases allow the sense of slip to be determined as predominantly dextral strike-slip. There to not appear to be any unambiguous piercing points that allow a clear evaluation of the amount of slip.

Folds are present in bedding adjacent to the largest, most continuous fault. The folds are oriented in an en echelon relationship to the fault, and die out into surrounding bedded sedimentary rocks. Cleavage is also present, oblique to the fault, and becomes more nearly parallel to the fault in the immediately adjacent wall rocks.

Several fault-bounded slivers are located in the fault zone. One of these forms a prominent section of cliff, being 'popped up' relative to the surrounding rocks, and forming a structure closely resembling 'positive flower structures' often interpreted at much larger scale in seismic profiles from transpressional basins (Fig. 3-9c).

Fig. 3-8:    Detailed map of faults at stop SR-9, after Roselli (2004)

Fig. 3-9: Photographs of structures on east side of Mutton Cove (Roselli 2004).