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A Clay-Till Fabric: Its Character and Origin

1957; University of Chicago Press; Volume: 65; Issue: 3 Linguagem: Inglês

10.1086/626431

1537-5269

P. W. Harrison,

Climate change and permafrost

Three-dimensional fabric analysis of clayey tills in two ground- and end-moraine areas near Chicago, Illinois, is described. The fabric elements whose orientation was measured consist of disk-, blade-, and rod-shaped particles between 3 and 40 mm. in size and till joints. The particles measured were predominantly shale; the majority were 4-8 mm. in size. In samples of ground moraine and in samples of the cores of end moraines the disk- and blade-shaped particles tended to be preferentially imbricated upstream to former glacier-movement directions. In ground moraines the long axes of the blades tended to lie in the horizontal plane. Also in ground moraines the long axes of rod-shaped particles described a near-horizontal girdle (on a fabric diagram) but exhibited clear maxima dipping upstream to former glacier-movement directions. On the basis of kinematic analysis of the macrofabric, combined with several other lines of evidence (such as microfabric and englacial-debris studies), it is suggested that the bulk of the till fabric has been inherited from the transportational environment; i.e., it represents little-altered, remnant, englacial-debris fabric. The fabric-pattern maxima of disk- and blade-shaped particles are believed to define the vanished slip planes of the debris-charged basal zones of the glaciers which carried the till. Such relict slip planes are thought to be representative of distributive movements developed within heavily debris-charged basal zones of glaciers in response to extensive tectonic transport over roughly horizontal thrust planes or movement horizons. It is suggested that the bulk of the till was deposited by slow melting-out from the stagnant basal zones of the various glaciers which transported it. Deposition seems to have been confined in time almost entirely to the deglaciation phase of glaciation. Many till fabrics in end-moraine districts are related to postdepositional solifluction activity. This is determined by their similarity to known "mudflow" fabrics and by the fact that their preferred-orientation directions show slope control. Frost heave and disturbance by plants and man are responsible for modification of some of the fabrics found.