Stop 14: Ballantynes Cove Section: Type Section for Northward Ice Flow from the Scotian Ice Divide

Wave-cut sections in the lowland regions that flank the Antigonish Highlands generally display two to three till units. The type section is at Ballantynes Cove south of Cape George. From this vantage point the Sangamon wave-cut platform cut into differing rock types, is clearly visible under three Wisconsinan tills. The underlying rocks of the section belong to the McAras Brook Formation (basalt, conglomerate) and Wilkie Brook Formation (limestone). The section was thoroughly sampled from top to bottom and the pebble types analyzed for provenance, a key to interpreting ice flow directions in Nova Scotia.

Summary for non-geologists: The Ballantynes Cove section shows three layers of glacial deposits, each corresponding to a different glacier advance. The lowest reddish till with shells was deposited by a large glacier advancing to the east-southeast and ending offshore near Sable Island. The middle greyish till was deposited by a Nova Scotian glacier flowing northeastward into St. Georges Bay and ending in the Cabot Strait, where the glacier floated off as a stream of icebergs. The final glacier advance was from a small ice cap in the Northumberland Strait that briefly expanded at the end of the last ice age.

Description

The lower till (T1) is reddish-brown, massive and fine-grained containing Mercenaria shell fragments. It can be correlated with other shell-bearing basal till units along the Northumberland Strait. One of the distinguishing characteristics of the three till units at Ballantynes Cove is the stone content. The lowest till unit contains less than 10% pebble-sized particles and exhibits little sample to sample variation. A ship's boiler (3 m high) is washed up on the beach in front of the 31 m thick section.

Till 1 shows a relative abundance of reddish sandstone and conglomerate clasts, and the presence of well-rounded plutonic clasts. Till fabric at the base of the section shows a strong east-west orientation with a minor southeast mode.

The middle greyish red-brown silty till (T2) has sand lenses and partings and contains >10% pebble-sized particles. It is characterized by a marked increase in percentages of black metasiltstone and volcanic clasts, and the inclusion of diorite. Till fabric is oriented predominately northeast-southwest.

The upper till (T3) is a greyish, slightly fissile, silty-sandy till. It is separated from the middle till by a boulder layer containing black metasiltstone and diorite. Till fabric is oriented northeast-southwest. Clasts of green, hard conglomerate and amygdaloidal basalt increase markedly in the unit.

Interpretation

Sources of tills and glacier movement directions

The relative abundance of arkosic sandstone and well-rounded plutonic clasts in Till 1 is the result of east-southeastward ice flow across Devonian to Carboniferous conglomerate and sandstone to the north and northwest. Till fabric at the base of the section shows a strong east-west orientation with a minor southeast mode. Shell fragments in the till and an overall low percentage of clasts confirm a correlation with till on the Northumberland Shore formed by east-southeastward ice during the Caledonia Phase.

The marked increase in the proportion of black metasiltstone clasts and volcanic clasts in Till 2 and the presence of dioritic clasts indicate erosion of NeoProterozoic Georgeville Group rocks to the south and southwest, and a significant change in ice flow from southeastward to northeastward. This flow emanated from a local ice divide over Nova Scotia called the Scotian Ice Divide. The northeastward-trending till fabric in Till 2 is aligned with this change in ice flow direction.

The source of the green hard conglomerate in Till 3 is the Livingstone Cove Formation and amygdaloidal basalt is found in the McAras Brook Formation, which underlies the section and outcrops in an east to west zone between the Georgeville Group rocks and Devonian conglomerate. A flow toward the east is implied, but till fabric measured in Till 3 is north-south. The till fabric may have been reset by later flow, or may represent a change in till depositional dynamics. The youngest eastward flow from the Northumberland Strait area may correspond to Younger Dryas Readvance from the Gulf of St. Lawrence area and possibly to Grant's (1994) onshore Phase F in southwest Cape Breton Island.

Processes of reworking and hybrid tills

Contacts between the till units in the section seem sharp to the eye, but the lithological boundaries are diffuse. Gradational variations between till sheets in the pebble lithology and source areas up-section can be explained by gradually shifting ice flow patterns, but the erosional record does not record sequential flow changes. Flow direction changes are abrupt, as local striae are grouped regionally into well-defined flow patterns. A more likely explanation for these variations are processes of reworking. "Hybrid" tills (cf. Grant 1963) are created in the areas where there are interactions between the glacier bed and previously deposited tills through reworking. Reworking can modify the basic characteristics of the original or "end member" tills to the extent that the resulting hybrid tills no longer reflect (to greater or lesser degrees) the properties of the original tills. Two distinct reworking processes can be defined: inheritance and overprinting (Stea and Finck, 2001).

Inheritance is the incorporation of till components and/or fabric into a younger till by erosion and entrainment of material from an older till.

Overprinting is the injection or mixing of matrix and clast components and imprint of fabric on older tills by overriding ice.

Each till unit starts with an "end member" clast composition which is altered by processes of either inheritance, overprinting or both. The original composition reflects the dynamics of the depositing glacier and bedrock sources. The resultant clast composition of any particular sample will be the sum of end members and reworking processes, in effect the compositional evolution of the tills through several glaciations. The reworked components will form a background population of clast types, generally subordinate to the influx of fresh bedrock types, provided there is access to outcrop by ensuing glaciers. At the Ballantynes Cove Section overprinting is apparent at the contact regions of Tills 1 and 2, and 2 and 3, where metamorphic siltstone and green, meta-conglomerate (lithologies 3 and 7) were injected down-section into earlier tills derived from glaciers that did not traverse the parent rock outcrops. The persistence of well-rounded igneous clasts (lithology 6) derived from the arkosic conglomerate to the north of the section throughout all three till units strongly suggests inheritance. See a model of the evolution of tills and reworking over several glaciations in Nova Scotia.

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