Nature and Rate of Basal Till Deposition in a Stagnating Ice Mass, Burroughs Glacier, Alaska

Abstract

Rates of basal till deposition ranging from 0.5 to 2.5 cm year-" have been obtained from the Burroughs Glacier in southeast Alaska. Unlike minimum rates of till deposition which are derived by dividing the thickness of till deposited by the length of time the area was ice covered, these values are believed to more closely represent an actual rate of deposition. Because of the emergence of hills during deglaciation, a change in ice flow direction of up to 90 0 has occurred near the southeast terminus. Since this change is recorded by maps and photographs dating to 1892, a rate of change of ice flow direction can be estimated. Till fabric measurements and till composition at two or three depths in the till at seven localities reflect this change. Estimates of the rate of till deposition are obtained by assuming that the fabric azimuth represents the ice flow direction at the time the till was deposited. Most till deposition seems to have taken place during late stages of deglaciation since at two locations fabric of tiiI just above bedrock or a paleosol records a post-1892 flow direction. The rate obtained by dividing the average thickness of till by the number of years the area was ice covered is an order of magnitude lower than those calculated above.

Quaternary glacial deposits: Implications for interpretation of Proterozoic glacial deposits

“…The bedded (Virkkala, 1952) or stacked (Moran, 1971) ortho-tills and the subglacial tills with vertically changing strong pebble fabric (Mickelson, 1973) probably combine both above processes of deposition: first lodging of debrisrich basal ice, but in sheets of various thickness rather than particle-by-particle, followed by melting out of these sheets, beginning with their base. Such a till may be called lodged melt-out till.…”

“…Such a till may be called lodged melt-out till. This kind of deposition has been described from present-day glaciers in Svalbard by Boulton (1970) and from Alaska by Mickelson (1973). Lavrushin's (1980) theoretical basal exfoliation appears to be similar.…”

Submarginal debris transport and till formation in active temperate glacier systems: The southeast Iceland type locality

“…(2011) identification of potential supercooling grain size signatures in ice-marginal deposits. The very localized preservation, as indicated by the discontinuous outcrop in section Sk1, is likely the result of a zone of stratified basal ice facies having been overprinted by a subglacial traction till, as recorded by the overlying fissile to massive diamicton, similar to the scenario reported from Alaska by Mickelson (1973) and Ham and Mickelson (1994). This would have slowed the melt rate and release of any meltwater by groundwater seepage.…”