Detailed sedimentological study of till sheets in a stratigraphic section, Samson River, Quebec
Authors: Shilts W W 
Published: July 2015

More than 150 samples of till were collected at intervals averaging 12.5 cm over the face of a 4 m-wide by 10 m-high section on Samson River, Quebec. The face exposes two till members of Lennoxville Till. The upper member is separated from the lower by deformed
lacustrine sediments, and the lower till rests on a thick sequence of laminated lacustrine silty clay. The section is located near the axis of a southeastward trending dispersal train of ultrabasic components, approximately 60 km down-ice from massive outcrops of nickel-chromium-magnetite-serpentine
rich ultrabasic rocks at Thetford Mines. The upper till member has fabric parallel to the dispersal train, while the lower till member has fabric striking north-northwest, approximately 30 degrees from the trend of the dispersal train. Several textural, chemical, and mineralogical criteria were
measured on each till sample and on selected samples of lacustrine sediment. A comparison of these parameters for the two tills indicates that they differ from one another in most respects. The upper till is sandy, loose, massive, oxidized, weakly calcareous, and rich in ultrabasic components. The
lower till is compact, calcareous, jointed, unoxidized, and is overlain by a 0.5 to 0.75 m-thick sandy bouldery deposit thought to be ablation till. This sandy bouldery deposit, which is directly overlain by lacustrine sediment, has many compositional characteristics in common with the overlying
upper till. Because of the unique location of the section with respect to the dispersal train of ultrabasic components, many of the till properties measured are site specific, i.e. surface till deposited a few km away during the glacial event has none of the ultrabasic components (Ni, Cr, Co,
magnetic minerals, serpentine, etc.) that are so characteristically enriched in the surf ace till at this site and at other sites within the ultrabasic dispersal train. Other properties of the tills at this site are similar to those of tills located in the Appalachian section of Quebec and are,
therefore, area specific. Area specific properties include texture, gross mineralogy and chemistry, thin ablation facies, and textural, mineralogical, and chemical layering thought to be related to the stacking of shear plates of till, one on top of another; mos.t of these properties can be related
to topography, structure, metamorphism, and bedrock lithologies typical of the Appalachian region. Some properties of the tills in this section are independent, that is characteristic of the sediment or till in general; they are properties related to the various glacial environments of erosion and
deposition that form till. Independent parameters include such features as evidence of shear, skewing of textures towards those of underlying unconsolidated sediment, the presence of the ablation-lodgment couple, the presence of a soft, far travelled mineral (serpentine) in virtually all size
fractions of the till, etc. Finally, it is recognized that in any classification of till, structural, textural, and compositional parameters must be properly classified as site specific, area specific, or independent, the latter group being the only one generally useful for recognition of various
till facies within classification schemes.