Characterization of pebble fabrics in modern terrestrial glacigenic sediments

Abstract

ABSTRACT Pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers, where sedimentary processes can be observed or inferred with relatively little ambiguity. Over 100 samples from contemporary environments illustrate fabrics characterizing melt‐out till, deformed and undeformed lodgement till, sediment flow deposits and ice slope colluvium. Lodgement till fabric variability is related to the two‐layer structure of these sediments; a structureless, friable upper layer with low shear strength and high consolidation coefficient, overlying a very compact material of horizontal platy structure. Fabric strength (assessed by eigenvalue analysis) is weaker and pebble dip is more dispersed in the upper structureless horizon. Stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon. Alternatively, upper horizon fabrics may be characteristic of all recently‐deposited lodgement tills, with stronger fabrics developing at depth by dewatering and consolidation. There is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt‐out tills, through undeformed and deformed lodgement tills, to sediment flow deposits and ice slope colluvium. There is, however, considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills. Fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some Quaternary glacial sediments. Relatively strong fabrics characteristic of melt‐out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments.