Typologies of genetic tills in the Sierra de Gredos (Spanish Central System)

Abstract

Despite the fact that tills are crucial to the reconstruction of former glaciations, they are the most difficult deposits to interpret and classify. The only glacial diamictons that can be classified as true tills are those from subglacial environments because diamictons produced supraglacially are mass-flow deposits. In order to reconstruct the genesis of tills, it is critical to understand the processes taking place at glacier beds. The processes of deformation, melt-out, flowage, sliding, lodgement, meltwater reworking, and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members on a continuum of till types. The end members range from glaciotectonically folded and faulted stratified material to texturally homogeneous diamicton. Because subglacial processes vary both spatially and temporally, tills or complex till sequences contain superimposed signatures of transportation and deposition at the ice–bed interface. Till classification must reflect the range of products encompassed by the subglacial till production continuum and therefore, we can presently unequivocally identify: (i) glaciotectonite (rock or sediment that has been deformed by subglacial shearing/deformation but retains some of the structural characteristics of the parent material); (ii) subglacial traction till (sediment deposited by a glacier sole either sliding over and/or deforming its bed, the sediment having been released directly from the ice by pressure melting and/or liberated from the substrate and then disaggregated and completely or largely homogenized by shearing); and support the theoretical case for (iii) melt-out till (sediment released by the melting of stagnant or slowly moving debris-rich glacier ice and directly deposited without subsequent transport or deformation).