CONSTRAINTS ON THE PRESERVATION OF DIAMICT FACIES (MELT-OUT TILLS) AT THE MARGINS OF STAGNANT GLACIERS

This paper reviews the formation and preservation of diamict facies (melt-out till) generated in situ by the downwasting of stagnant ice at the margins of continental glaciers. Melt-out tills are commonly envisaged as being the product of the top (supraglacial) or bottom (subglacial) melt of stagnant debris-rich glacier ice but such deposits are relatively uncommon in modern glacier environments where they are thin, laterally discontinuous and have a low preservation potential. Theoretical considerations of the distribution of englacial debris in continental ice sheets show that only the glacier margin is likely to contain the necessary volumes of englacial debris for melt-out to be a significant diamict-forming process. However the application of thaw-consolidation theory to the formation of such deposits shows that substantial porewater pressures and self-sediment deformations can be induced during melt-out, especially in matrix-rich debris. The classic melt-out mechanism widely invoked in the literature since 1875 and involving passive in situ aggregation from thawing englacial debris can be seen to operate under restricted conditions. Diamict facies produced by melt-out are likely to form spatially disjunct and laterally discontinuous elements within complex and ice marginal stratigraphics; the melt-out process is not a mechanism whereby regionally extensive diamiet(ite) units can be deposited across glaciated basins. © 1990.