Pebble lithology of the tills of southeast Nova Scotia

Abstract

This preliminary investigation of coastal non-stratified glacial deposits was planned to study the regional pattern, and provide basic information necessary for the interpretation of Scotian Shelf sediments. It is a detaiied-reconnaissance survey aimed at establishing the geographic distribution of till types as characterized by their pebble associations. The scope ranges from detailed pebble analysis to stratigraphy and chronology. One hundred and sixty ground moraine samples were col lected along road traverses over a 6000 square-mile area. The 5.6mm - 22.2mm pebble fraction was dry sieved in the field and wet screened in a rotary washing mill. Number frequency pebble counts were made on 500-grain samples and the data are examined variously as; unit percentages of all rock species, bedrock components versus foreign elements, relative proportions of the foreign species, and as relative source area influences. Fifty rock species are traced to seven source regions, for which special topographic and geographic features pertinent to interpretation are set out. A bar diagram map of source area lithology, and triangular diagrams reveal that three mono lithologic till types form the bulk of the ground moraine. Each contains at least 80 per cent bedrock material, and has a distribution matching the ,corresponding local source - granite quartzite or slate. Minor hybrid or mixed tills occur near source area contacts. Isopleth maps oft he foreign elements, recalculated to 100 per cent, reveal a lobate dispersal pattern. In the anomalous areas of foreign-pebble enrichment occur red clay drumlin fields which were sampled subsequently. Ternary compositional diagrams reveal a characteristic lith ology for each field. Lithology of individual drumlins can be related to distance of travel over the underlying bedrock, and away from the distant sandstone area. The anomalous fine grained red, 'drumlin' till matrix is interpreted as former alluvial, lacustrine, and tidal flat deposits winnowed during an interstadial from pre-existing red tills. The lobate foreign-pebble distribution is interpreted as the result of currents created in the thin ice sheet margin, either by topographic channeling or confluence of local ice caps. A tentative sequence of glacial events is based on stratigraphic correlation of sections and radiocarbon dates. Linear outcrops of cemented outwash are assigned to a pre Sangamon terminal moraine. 'Early' Wisconsin tills are pres ent, followed by pre-classical Wisconsin regoliths underlying the compact sub-drumlin Tazewell till. The drumlin advance dates from the 'Port Stanley' episode, and the drumlins were overridden during the 'Port Huron' glaciation. A parallel study of beach sand heavy minerals reveals an identical dispersal pattern of indica~or augiteo The mis leading occurrence of 'transported' _geoch~mical anomalies in drumlin fields is emphasized. Applications to oceanographic sampling and sedimentary petrographic interpretation are cited.