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Published:
January 01, 1973

Lennoxville Till of the Lac-Mégantic region is homogeneous with respect to fabric, texture, clay-mineral composition, and color. The homogeneity resulted from erosion and transportation by a glacier that moved southeastward over bedrock that had little petrologic variation. Lennox-ville Till is, however, also characterized by southeast-trending bands with high concentrations of certain rocks, minerals, and chemical components derived from scattered igneous bodies.

Chromium, nickel, and magnetite are dispersed in ribbonlike bands at least 50 km southeast of their principal source areas in the ultrabasic-basic rock complex at Thetford Mines. Concentrations of surface erratics derived from these same sources have dispersal patterns similar to those of Cr, Ni, and magnetite.

Plagioclase grains and granodioritic erratics are dispersed in bands southeast of the granodiorite stock of the Little Megantic Mountains. Granodioritic erratics that occur on the surface are thought to have been let down onto lodgment facies of Lennoxville Till during melting of the Lennoxville glacier, because they mantle the surface of its grano-diorite-poor clay-till facies. By extension, most of the boulder mantle that characteristically rests on Lennoxville lodgment facies may be an ablation deposit with dispersal characteristics locally reflecting glacier deflections and lobations that occurred during deglaciation.

Dispersal data strongly confirm ice-flow patterns inferred independently from striation, fabric, and geomorphic data. Most till components discussed show evidence of long-distance transport, except where topographic prominences have blocked or deflected ice-transported sediment.

No evidence was found to support the concept of glacier flow into Quebec from late-glacial highland centers of outflow located south or east of the area.

Texture of till has an important influence on observed concentrations of trace elements. Zr is concentrated preferentially in the silt fraction of silt + clay; Ti, Cu, V, Zn, Pb, Cr, and Ni are concentrated preferentially in the clay fraction. Concentrations of Cr, Ni, and Ti are thought to be texture-independent in samples located down-ice from igneous sources rich in these components.

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GSA Memoirs

The Wisconsinan Stage

Robert F. Black
Robert F. Black
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Richard P. Goldthwait
Richard P. Goldthwait
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H. B. Willman
H. B. Willman
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Geological Society of America
Volume
136
ISBN print:
9780813711362
Publication date:
January 01, 1973

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