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

摘要

Streamlined features, rat-tails, glacial striae, and dispersal patterns for till clasts and matrix geochemistry define glacier dynamics and flow directions across the Petitcodiac map area, southeastern New Brunswick. These data indicate that a single till sheet was deposited as the dominant ice-flow direction fluctuated 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 flowing ice. Three phases of glacier flow are recognized: (1) an early topographically-controlled, thin-ice phase; (2) regional flow during a phase of maximum ice growth and; (3) a late phase of thinning ice. The late phase is characterized by: deviation of flow around major obstacles; ice-sheet drawdown toward outlets; flooding of low lying valleys due to eustatic rise in sea level; and glacier melting. No evidence was found indicating northward or radial flow 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 identification of additional local mineralized zones along some faults. Sand content in the till reflects increased erosion and incorporation at locations where the glacier climbed over major topographic obstacles, oriented transverse to the regional flow 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.