lecture_10 - LECTURE 10 - DIAMICTONS Nomenclature erratics...

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LECTURE 10 - DIAMICTONS Nomenclature erratics Okotoks erratic near Calgary 118,000 tons some traveled 1000 km indicators boulder train or indicator fan diamictons and diamictites unsorted, unstrat. seds. of uncertain origin glacial sediments included till unsorted, unstrat. seds. deposited directly by ice Lodgement Processes Associated with warm moving glacier Heat derived from friction during basal slip, internal friction, and from geothermal gradient Frictional heat > geothermal heat Heat distribution tends to be variable f(bed topog., bed perm., debris cont.)
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Pressure melting Tends to be variable Debris accumulation rates of 0.5 to 3 cm/yr Accumulates under high pressure, high stress, minimum water Thickest near thin ice margin Little under ice source area Matrix often sheared Clasts roll or slide on bed Some lodge on obstructions. -> rock clusters Develop preferred a-axis orientations parallel to direction of shear stress Drainage Distributed and channalized Low pore water pressure -> stiff deposition High pore water pressure -> fluidization possible Subsequent alterations Depositional breaks Erosional intervals Erosional surfaces -> Clast pavements Unloading -> Jointing
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Deposits ( lodgement till ) Distribution Continuous and sheetlike Matrix Rich in fines Firm, compact, overconsolidated Shear foliation, fissile structure common Clasts Matrix supported Abraded, anvil shaped to subrounded Varied lithologies Can include local rip-ups Occasional bullet stones (stoss-and-lee clasts) Clast clusters common Strong consistant clast fabric Most clast a-axes parallel to ice flow Clast a-axes inclined up glacier Some a-axes transverse to flow Gentle upglacier imbrication of a-b planes common
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Bedding Scours and lag horizons common Tabular subhorizontal sand and silt stringers Shoestring channel deposits Planed N channel deposits Multiple clast pavements possible Structures Folding and faulting (low angle reverse), fissile structure Jointing Associations Architecture Associated with striated bedrock or shear deformation of underlying units Landforms Till plains, drumlins, proximal side of end moraines Basal meltout Processes Associated with warm stagnant ice Heat derived only from geothermal gradient Tends to be uniform but limited
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Accumulation rates <1 cm per year Basal ice stratification and structures preserved Preferred orientation of clast a-axis in ice preserved
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lecture_10 - LECTURE 10 - DIAMICTONS Nomenclature erratics...

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