Lithology and geochemical dispersal in till: Petitcodiac area, New Brunswick

Abstract

Streamlined features, rat-tails, glacial striae, and dispersal patterns for till clasts and matrix geochemistry defi ne glacier dynamics and fl ow directions across the Petitcodiac map area, southeastern New Brunswick. These data indicate that a single till sheet was deposited as the dominant ice-fl ow direction fl uctuated between east, south-southwest, and east-southeast across the study area. The Anagance Ridges and Central Plateau formed major obstructions, but were eventually glaciated by southward fl owing ice. Three phases of glacier fl ow are recognized: (1) an early topographicallycontrolled, thin-ice phase; (2) regional fl ow during a phase of maximum ice growth and; (3) a late phase of thinning ice. The late phase is characterized by: deviation of fl ow around major obstacles; ice-sheet drawdown toward outlets; fl ooding of low lying valleys due to eustatic rise in sea level; and glacier melting. No evidence was found indicating northward or radial fl ow from the Central Plateau.Clast trains are traceable from known outcrops, southward over distances of 10 km, whereas distinct till geochemical dispersal patterns are commonly lost within 5 km of transport from the known source unit. Geochemical dispersal suggests a potential for the identifi cation of additional local mineralized zones along some faults.Sand content in the till refl ects increased erosion and incorporation at locations where the glacier climbed over major topographic obstacles, oriented transverse to the regional fl ow direction. The clay content increases primarily because of the incorporation of sediments occupying valleys and low-lying coastal areas at the time of glacier advance. These observations indicate that the basal sediment load did not establish an erosive equilibrium between the glacier and the underlying bedrock for all locations across the study area. Preferential incorporation occurred at points of increased erosion and in areas of thick preglacial sediment accumulations, altering matrix composition and obscuring some geochemical dispersal patterns. These observations have implications for glacial prospecting in sub-alpine terrains elsewhere. RÉSUMÉLes caractéristiques laminaires, les queues-de-rat, les stries glaciaires et les traînées de dispersion des clastes de till de même que la composition géochimique de la matrice, défi nissent la dynamique du glacier et les directions de l'écoulement à l'intérieur du secteur cartographique de Petitcodiac, dans le Sud-Est du Nouveau-Brunswick. Ces données révèlent qu'une nappe de till unique s'est déposée pendant que la direction prédominante de l'écoulement des glaces fl uctuait entre l'est, le sud-sud-est et l'est-sud-est dans le secteur d'étude. Les chaînons Anagance et le plateau Central ont constitué les principales obstructions, mais ils ont fi nalement été recouverts par des glaces s'écoulant vers le sud. On distingue trois phases d'écoulement glaciaire : 1) une phase précoce de glace mince topographiquement canalisée; 2) un écoulement régional au cours d'une phas...

Comparison of humus and till as prospecting material in areas of thick overburden and multiple ice-flow events: An example from northeastern New Brunswick

“…Dreimanis, 1956;Shilts, 1976;DiLabio, 1990;Broster and Huntley, 1995). Broster et al (2004) note that the nature of the glacial dispersal pattern is less clear when the resulting till has been deposited shortly after changes in glacier dynamics, variation in topography or underlying material, or the glacier is not in erosive equilibrium with the bedrock (e.g. bedrock was covered by thick deposits of pre-glacial sediment).…”

“…The distance between the point of origin and the highest concentration in a till, represents the lag between incorporation of the source rock, its comminution, and mixing with the glacier, enabling deposition of source-materials in a near-surface part of the till sheet. While the end result is dependent on several variables involving ice dynamics and sub-glacial topography, discussed in greater depth by Broster et al (2004), the occurrence of a gradational dispersal pattern indicates that the glacier was locally, in erosive contact with the substrate. The distance between the underlying bedrock mineralization and its geochemical signature in the overlying surficial sediments, increases with increasing till thickness (Miller, 1984;Paulen, 2001).…”

“…These dispersal patterns, called fans, plumes or trains (e.g. DiLabio, 1990), are often wedge-, amoeboid-or ribbon-shaped concentration patterns elongated parallel to ice-flow and decreasing in content inversely with distance from the source (Shilts, 1976;McClenaghan and Kjarsgaard, 2001;Broster et al, 2004). The identification of first derivative glacial dispersal trains can often be confused by: (a) multiple ice-flow events producing palimpsest (Parent et al, 1996) or composite dispersal patterns (e.g.…”

Comparison of clast and matrix dispersal in till: Charlo-Atholville area, north-central New Brunswick

“…The high correlations amongst the lanthanide elements (Ce, Eu, La, Lu, Sm, Tb, and Yb) are commonly found in till investigations in New Brunswick (e.g., Munn et al 1996;Broster et al 2004). Lithophile and rare earth elements have been reported to be elevated slightly in some ablation tills (Stumpf et al 1997) but this is not considered to be relevant here.…”